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ABSTRACTS Number 92

International Conference on Island Arc-Continent Collisions

The Macquarie Arc Conference and field trips

13-26 April 2009 Orange, New South Wales, Australia

International Conference on Island Arc-Continent Collisions

The Macquarie Arc Conference and related field trips 13-26 April 2009 Turners Vineyard Orange, New South Wales, Australia

Compilers: Glen R.A. & Martin C.

ABSTRACTS Number 92 ISSN Number 0729 01IX

Geological Society of Australia Abstracts No 92

ISSN Number 0729 01IX

Preferred citation for this volume:

Glen, R.A. & Martin, C. (Compilers) 2009. International Conference on Island-Arc Continent Collisions: The Macquarie Arc Conference, April 2009, Geological Society of Australia Abstracts No. 92,

Copies of this publication may be obtained from: The Business Manager Geological Society of Australia Incorporated Suite 61, 104 Bathurst Street Sydney NSW 2000 or online at: http://www.gsa.org.au/bookshop.html IGCP 524

2009 International Conference on Island Arc-Continent Collisions: Macquarie Arc Conference, Orange, New South Wales, Australia With thanks to all our sponsors

frontiers new south wales NEWCREST MINING LIMITED

ALKANE RESOURCES

LTD

NEWMONT ASIA PACIFIC

BARRICK C O W A L

neweold

. I GOLDEN CROSS RESOURCES

i i m RANGDTT

/VLS LaboratorLi

C r o u p

ANALYTICAL CHEMISTRY & TESTING SERVICES

MINERAL

EXPLORATION

Geological Society of Australia Abstracts No 92

elcome to the Macquarie Arc conference, an international conference with associated field trips that, from 13-26 April 2009, will examine, argue about and debate the processes, geometries and durations involved in the collisions between island arcs and continental margins; and the variety of, and controls on, mineral deposits that are associated with these collisions.

This conference is part of International Geoscience Program (IGCP) project 524 (leaders Dennis Brown Barcelona, Spain and Chi-Yue Huang Tainan,Taiwan) titled:

Arc-continent collision: benefiting human society through an enhanced understanding of plate tectonic processes.

The conference is also sponsored scientifically by the International Lithosphere Program (ERAS-Earth Accretionary Systems (in space and time) chaired by Peter Cawood). The committee welcomes you to NSW and the city of Orange. Orange is a dynamic inland city well known for its good food and high altitude wines. Orange is also a major centre for mining and exploration in NSW, being surrounded and underlain by Ordovician rocks of the Macquarie Arc that host major gold and copper deposits. Welcome again to Turners Vineyard and Conference centre for a convivial and dynamic meeting. We hope you enjoy getting acquainted with the Macquarie Arc, its rocks and mineral deposits, and the agricultural products derived from it. Your conference committee: Dick Glen (convener) Cameron Quinn Ian Percival Carol Simpson Elaine Power

IGCP 524

Shahn Moseley (organiser) Isabella Mee

April 2009

2009 International Conference on Island Arc-Continent Collisions: Macquarie Arc Conference, Orange, New South Wales, Australia

IGCP PROJECT 524 The aim of IGCP 524 is the study of processes that occur in zones of collision or accretion between volcanic island arcs and continental margins, in both fossil and active settings.

One of the key areas of basic research among earth scientists are processes that occurred, and are occurring today, along the boundaries of the tectonic plates that make up Earth s lithosphere. Of particular importance are the processes of tectonic accretion (the addition of material) along constructive plate boundaries. Tliese processes have occurred throughout geological time, and still occur today. One of the principal mechanisms of accretion, which leads to continental crustal growth, occurs when an intraoceanic volcanic arc that formed above a subduction zone (where one plate slides beneath another) collides with the margin of a continent. The proposed project aims to enhance the basic understanding of this important plate tectonic process and by doing so provide an advanced model that can be used to societies benefit. Zones of arccontinent collision are producers of much of the worlds primary economic wealth, especially in the form of minerals. The proposed project will provide key new data and insights that will help to advance the understanding of how, when, and where volcanic-hosted mineral deposits form and are preserved, thereby helping to develop models for mineral exploration. Zones of active arc-continent collision are among some of the most populated in the world, as well as among the most seismically active. They are, therefore, zones of high geological risk for people and infrastructure. IGCP project 524 will provide an enhanced tectonic model for arc-continent collision zones that can be used to better understand the risks posed by seismic shaking or landslides. Understanding the geological processes that take place in the Earth s lithosphere is therefore not only of importance for our understanding of how collisional orogens evolve and how the continental crust grows, but it is also of significant importance in understanding how its mineral wealth is formed and preserved, and what the geological risks are for those who live along these active plate boundaries.

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Macquarie Arc Conference 2009:

SUMMARY PROGRAM

MONDAY 13 APRIL Registration desk opens at 3pm. Icebreaker session starts at 5.30pm

TUESDAY 14 APRIL — FIELD DAY 1 Introduction: What is the Macquarie Arc? (Dick Glen). One-day trip to northern part Molong Volcanic Belt: Fairbridge Volcanics — Reedy Creek Limestone — Cheesemans Creek Formation and Copper Hill Complex and deposit (courtesy Golden Cross Resources)

WEDNESDAY 15 APRIL — FIELD DAY 2 One-day trip to central part Molong Volcanic Belt: Cargo Volcanics — Bowen Park Limestone — Malachis Hill Formation

THURSDAY 16 APRIL — FORMAL PRESENTATIONS DAY 1 FRIDAY 17 APRIL — FORMAL PRESENTATIONS DAY 2 SATURDAY 18 APRIL — FORMAL PRESENTATIONS DAY 3 SUNDAY 19 APRIL — FORMAL PRESENTATIONS DAY 4 MONDAY 20 APRIL — FIELD DAY 3 One-day trip to southern part Molong Volcanic Belt: Cadia Valley Deposits (courtesy Newcrest Mining Limited) — Angullong Volcanics Ashburnia Group

TUESDAY 21 APRIL — FIELD DAY 4 One-day trip to Junee-Narromine Volcanic Belt: Nelungaloo Volcanics, Yarrimbah Formation, Northparkes Group — Northparkes Mine (courtesy NPM)

WEDNESDAY 22 APRIL — POST CONFERENCE MINE VISIT One-day post conference trip to Cowal Mine via Cargo Deposit (courtesy Cowal Gold Mine and Max Rangott)

WEDNESDAY 22 APRIL TO SUNDAY 26 APRIL Six-day post-conference field trip through Ordovician terranes and granites of the Lachlan Orogen in southern NSW

Macquarie Arc Conference 2009: Program: Presentations, Day 1 8.00am

Thursday, 16th April

Registration desk opens Welcome and opening address

9.00am

Principles

9.40am

Principles

10.20am Principles

4 0 C R A W F O R D , MEFFRE & DANYUSHEVSKY

Modern intra-oceanic island arcs: a template for the interpretation of ancient volcanic arcs in fold belts

40 ROSENBAUM

Tectonic response to anomalous subduction and arc-continent collision

2 0 CASEY & D E W E Y

The ophiolite enigma resolved?

10.40am

Discussion — 1 0 minutes

10.50am

Morning tea — 20 minutes

11.10am Pampean

2 0 ESCAYOLA & VAN STAAL

The age and significance of the Puncoviscana Formation with respect to Neoproterozoic to Cambrian tectonic evolution of the Proto-Andean margin of Gondwana

11.30am Ross-Delamerian 4 0 B R A D S H A W

Ross-Delamerian arc-continent collision in Antarctica and New Zealand with some comparisons with other Austral examples

12.10pm Tasmanides

20 GLEN

The bipolar character of the Tasmanides of eastern Australia

12.30pm Delamerian

20 FODEN

The Delamerian Orogen: the last of the Pan Africans

12:50pm 1.50pm

Lunch — 1 hour Delamerian

2.10pm

Delamerian

2.30pm

Delamerian

2.50pm

DelamerianLachlan

2 0 G R E E N F I E L D , MILLS, MUSGRAVE & GILMORE

Cambrian arc-continent collision during the Delamerian Orogen — evidence from the Koonenberry Belt, northwest NSW

2 0 M U S G R A V E & GREENFIELD

Transition between the Delamerian and Lachlan orogens: geophysical definition and geodynamic implications

2 0 G I B S O N , MORSE, NAYAK & IRELAND

Arc-continent collision and terrane accretion in western Tasmanides: insights from basement studies along Australia's southern rift margin

2 0 CAYLEY

3.10pm 3:20pm 3.40pm

Exotic crustal block accretion to the eastern Gondwana margin in the Late Cambrian —Tasmania, the Selwyn Block, and implications for the Cambrian-Silurian evolution of the Ross, Delamerian and Lachlan orogens

Discussion — 1 0 minutes Afternoon tea — 20 minutes Macquarie Arc

4.00pm

Macquarie Arc

4.20pm

Macquarie Arc

4.40pm

2 0 C R A W F O R D , MEFFRE, SOUIRE, Ordovician magmatic evolution of the Macquarie Arc, NSW BARRON & FALLOON 20 SIMPSON

Recognition of volcanic centres in ancient island arc systems: examples from the Ordovician Macquarie Arc, NSW, Australia

2 0 S A E E D , GLEN, O U I N N , BELOUSOVA & GRIFFIN

Basement to the Ordovician Macquarie Arc, Lachlan Orogen, NSW: constraints from zircon data

Macquarie Arc

2 0 O U I N N & GLEN

A new supra-subduction zone rift model for the eastern Lachlan Orogen, south-eastern Australia: implications of intimately related siliciclastic turbidite and volcanic packages

5.00pm

Macquarie Arc

2 0 FORSTER & DOWNES

Lead isotope systematics of Ordovician to Early Silurian porphyry and skarn deposits

5.20pm

Macquarie Arc

2 0 K E M P & BLEVIN

Evolution of Macquarie Arc magmas from Hf-0 isotope and trace element systematics of zircon: tectonic and metallogenic implications

5.40pm

Discussion and refreshments Dinner

Macquarie Arc Conference 2009: Program: Presentations, Day 2 8.00am

Friday, 17th April

Registration desk opens

8.30am

Macquarie Arc mineralisation

4 0 C O O K E , HARRIS & ZUKOWSKI

Gold and copper deposits of the Macquarie Arc, NSW

9.10am

Macquarie Arc mineralisation Macquarie Arc mineralisation

2 0 F o x , HARRIS, COOKE, COLLETT & FAURE

Controls on the formation of the Cadia East alkalic porphyry Au-Cu deposit NSW: potential reactivation of early basin structures

9.30am

H A R R I S , PERCIVAL, ALLEN, COOKE, TOSDAL, MCMILLAN, DUNHAM & COLLETT

9.50am

Macquarie Arc mineralisation

2 0 W A S H B U R N G R O O M , HARRIS & TOSDAL

10.10am

Macquarie Arc mineralisation

D A V I D , BURRELL & COIANIZ

Inverted submarine basins hosting the Cadia Valley porphyry ore deposits, NSW: fundamental controls on system architecture

Architecture of the Silurian sedimentary cover sequence in the Cadia porphyry Au-Cu district, NSW, Australia: implications for post-mineral deformation Geology of the Copper Hill porphyry deposit Discussion — 1 0 m i n u t e s

10.30am 10.40am

Morning tea — 20 minutes

11.00am

Tasmanides

4 0 COLLINS

Petrological/geochemical tracking of geodynamic process: theTasmanide retreating orogenic system, eastern Australia

11.40am

Lachlan Orogen

20 W Y B O R N & WILLIAMS

12.00pm

Lachlan Orogen

2 0 LENNOX

Metamorphics from Geehi Gorge, Upper Indi and Bethanga and the enigma of anomalously high geothermal gradients in the Lachlan Orogen Understanding the Wyangala Batholith, eastern Lachlan Fold Belt

1.20pm

New England Orogen

2 0 P H I L L I P S & OFFLER

1.40pm

New England Orogen New England Orogen Palaeogeographic linkages

Lunch — 1 hour

12.20pm

2.00pm 2.20pm 2.40pm

2 0 MURRAY & OFFLER 20

PISAREVSY, CAWOOD, LEITCH New Enqland Orocline in the Late Paleozoic: qeoloqical and paleomaqnetic constraints & NEMCHIN

20 PERCIVAL 20 TAIT

3.50pm

Palaeozoic palaeogeography: the Variscan, Alpine and Lachlan fold belts

Afternoon tea — 20 minutes

3:10pm

4.30pm

Palaeontological and palaeogeographical aspects of the Macquarie Arc

Discussion — 1 0 m i n u t e s

3.00pm 3.30pm

Source and transportation of high-pressure exotic blocks in serpentinite melange, Peel-Manning Fault System, southern New England Fold Belt Unravelling the tectonic setting of Devonian sequences in the New England Orogen

Palaeogeographic linkages Principles

20 MURPHY

CLIFT

The conundrum of Pangea: a genetic linkage between the Appalachian and Terra Australisorogens?

The relative roles of arc magmatism, arc accretion and tectonic subduction erosion in balancing the mass of continental crust Discussion a n d r e f r e s h m e n t s Dinner

Macquarie Arc Conference 2009: Program: Presentations, Day 3 8.00am

Saturday, 18th April

Registration desk opens

8.30am

Caledonides

40 D E W E Y , M A N G E & RYAN

9.10am

Caledonides

20 R Y A N

Fast-track Barrovian metamorphism, arc-continent collision and Connemara

9.30am

Caledonides

20 D R A U T , CLIFT, AMATO, BLUSZTAJN & SCHOUTEN

Arc-continent collision and the formation of continental crust: geochemical and isotopic records from the Irish Caledonides

9.50am

Americas

20 O L I V E I R A

Arc-continent collision in the Palaeoproterozoic Rio Itapicuru Greenstone Belt, Sao Francisco Craton, Brazil

Ordovician arc-continent collision in the Appalachian/Caledonian Orogen

Discussion — 1 0 m i n u t e s

10.10am M o r n i n g t e a — 20 m i n u t e s

10.20am 10.40am

Americas

40 V A N S T A A L & ZAGOREVSKI

Arc development, VMS mineralization and collisional tectonics in the northern Appalachians

11.20am

Americas

20 R O T A S - A G R A M O N T E , GARCI'A-CASCO, KRONER, CARRASQUILLA-ORTIZ, ITURRALDE-VINENT & MILLAN-TRUJILLO

The Mabujina Amphibolite conundrum (central Cuba): a case of metamorphosed root zone of an island arc, or an exotic pre-arc basement?

11.40pm

Americas

20 D A L Z I E L

Mesozoic 'Rocas Verdes' marginal basin, southernmost Andes: tectonic settings and implications for interpretations of older arc-back arc systems

12.00pm

Urals

20 P U C H K O V

Chronology and geometry of arc-continent collisions

Lunch — 1 hour

12:20pm 1.20pm

Urals

40 BROWN

Arc-continent collision in the southern Urals: insights into the growth and destruction of the continental crust

2.00pm

Urals

40 H E R R I N G T O N & ROBERTS

Formation ofVMS deposits in oceanic arcs: are some linked to arrested subduction resulting from arc-continent collision? Discussion — 1 0 m i n u t e s

2.40pm A f t e r n o o n t e a — 20 m i n u t e s

2:50pm 3.10pm

Cordillera

40 T O S D A L , M O R T E N S E N , HARRIS, BISSIG & HART

Construction ofanalkalic porphyry Au-Cu province in British Columbia: Triassic and Early Jurassic magmatism, amalgamation, and accretion of offshore island arcs to North America

3.50pm

Cordillera

Orogenic gold in the Cordilleran Accretionary Orogen of western North America: changing stresses in the fore-arc and back-arc

4.30pm

GOLDFARB

Discussion a n d r e f r e s h m e n t s Dinner

Macquarie Arc Conference 2009: Program: Presentations, Day 4

Sunday,! 9th April

Registration desk opens

8.00am 8.40am

New Guinea

DAVIES

9.20am

New Guinea

CLOOS

10.00am New Guinea 10.20am 10.30am 10.50am Asia 11.10am Asia

20 CORBETT

Geology of New Guinea Arc-continent collision followed by lithospheric delamination in New Guinea: implications for magma generation and Cu-Au mineralization Tectonic/structural control to Papua New Guinea Au-Cu mineralisation

Discussion — 1 0 minutes Morning tea — 20 minutes 20

H E R R I N G T O N , SCOTNEY, Isotopic evidence for progressive magma contamination in arc volcanism during arc-continent collision and the temporal ROBERTS, BOYCE & HARRISON association with massive sulphide deposits on Wetar Island (Banda Arc)

2 0 A L V A R E Z - M A R R O N , BROWN, SCHIMMEL & Y - M W u

Deformation of the continental crust in Taiwan arc-continent collision: structure and seismic energy release

11.30am Asia

SCHOLL

11.50pm Australia

K O R S C H , KOSITCIN, & CHAMPION

Australian island arcs through time: geodynamic implications in the Archean and Proterozoic

12.10pm Africa

DE W A E L E , HORSTWOOD, PITFIELD, THOMAS, K E Y , RABARIMANA, RAFAHATELO, RALISON & RANDRIAMANANJARA

The architecture of the"Betsimisaraka Suture Zone"; a record of oceanic arcs and associated metasedimentary successions between the "Indian"and "African" parts of Madagascar

12:30pm 1.30pm Asia

20 WALSHE

1.50pm

Asia

2.10pm

Asia

20 WILLIAMS & WYBORN

2.30pm

Asia

Lunch — 1 hour BARNICOAT

G O L D F A R B & HART

4.00pm 4.40pm 5.00pm

Links between Earth degassing and Late Ordovician Cu-Au metallogenesis in the Macquarie Arc, eastern Lachlan Orogen, NSW Arcs, magmas and ore-forming fluids Is the theory of plate tectonics an impediment to successful mineral exploration? Gold in Asia: accreted Paleo-Tethyan arcs and deformed continental margins

Discussion — 1 0 minutes

2.50pm 3:00pm 3.20pm

Tectonic consequences of the end-on collision of an intra-oceanic arc with a continent — the example of the orthogonal plunge of the Aleutian arc into the Kamchatka subduction zone

Afternoon tea — 20 minutes Central Asian Orogenic Belt

Central Asian Orogenic Belt Central Asian Orogenic Belt

40 XIAO

Late Permian to mid-Triassic continent-arc collision in theTien Shan: implications for the architecture of central Asia

2 0 WAINWRIGHT, T O S D A L , FORSTER, KIRWIN, LEWIS & WOODEN

The Devonian and Carboniferous arc in the Oyu Tolgoi area of the Central Asian Orogenic Belt, southern Mongolia

KRONER, DEMOUX, ALEXEIEV, The Central Asian Oroqenic Belt: what can we learn from field-related aeochronological studies?

ROJAS-AGRAMONTE, WLNDLEY & JLAN

Discussion, refreshments and meeting close Dinner

Geological Society of Australia Abstracts No 92

CONFERENCE SOCIAL FUNCTIONS (All meals are included in the conference fee)

Monday,

April

Welcome Icebreaker (BBQ) at Turners Vineyard

Wednesday, 1 A p r i l

Dinner at Sisters Rock restaurant, Borrodell Winery and Orchard

Remaining meals will be held in the Lucknow Valley Room at Turners Vineyard Tuesday, Thursday, Friday,

April April April

Saturday,

April

Conference Dinner

Sunday,

April

End of talks dinner

Monday,

April

Tuesday,

April

Please check the daily program on the noticeboard for meal times and any other changes.

IGCP524

2009 International Conference on Island Arc-Continent Collisions: Macquarie Arc Conference, Orange, New South Wales, Australia

CONFERENCE SPONSORS

Principal sponsor

i frontiers new south wales The Geological Survey of NSW (GSNSW) is the state's premier geoscience agency. It provides information and advice to the government, the mineral exploration and mining industry and the community on the state's geology, mineral resources, exploration highlights and their impact on land use planning. The GSNSW assesses the state's mineral and energy resources and geology, and provides a comprehensive information framework to: • optimise responsible mineral exploration and development • inform resource, land use and infrastructure planning. GSNSW programs have been augmented by a three year (2008-2011), $16.5m extension to the New Frontiers initiative, to fast-track the provision of new geoscience data and promote petroleum and mineral exploration investment in the state.

Geological Society of Australia Abstracts No 92

Gold sponsors

ALKANE RESOURCES

LTD

Alkane is a multi commodity explorer and miner which has been focused on the Macquarie Arc in the Central West of NSW for many years. In 1996 the Company developed the Peak Hill Gold Mine on the oxidised cap of a high sulphidation epithermal gold deposit within Ordovician andesitic rocks. The mine produced 153,000 ounces from a heap leach operation and a 450,000 ounce moderately refractory gold resource lies beneath the oxide ore. In 2002 Alkane discovered several gold deposits (Wyoming-Caloma) near Tomingley, 14 km north of Peak Hill. While these deposits are hosted by the same andesitic sequence, they are of an orogenic style. Nearly IMoz have been defined and these deposits to date which will advance to production mid 2010. Exploration in joint venture with Newmont at the Moorilda Project near Orange identified a large low grade gold system at the McPhillamys prospect within Silurian felsic to intermediate volcanic and intrusive rocks. This mineralisation extends over a strike length of 600 metres and up to 200 metres in width, typified by a core intersection of 366 metres grading 1.86g/t gold. Exploration is continuing to further test this new environment for gold deposits in NSW. As part of the multi commodity approach, the company has an advanced feasibility study in progress for the development of the Dubbo Zirconia Project, located 30km south of Dubbo. The project is based upon a very large in-ground resource of the metals zirconium, hafnium, niobium, tantalum, yttrium and rare earth elements, and is hosted in a Jurassic trachyte intrusive. Development of the DZP would involve the construction of a world first processing facility which would produce a number of products used in the electronics, catalyst, advanced ceramics, special alloy and glass, rechargeable batteries, permanent magnet and nuclear power industries.

IGCP524

2009 International Conference on Island Arc-Continent Collisions: Macquarie Arc Conference, Orange, New South Wales, Australia

NEWCREST MINING LIMITED

Newcrest is Australia's largest gold producer and one of the world's top 10 gold mining companies by production, reserves and market capitalisation. Our workforce of around 5,000 is based around the Pacific Rim. The company's current activities include six operating mines: • Cadia Valley Operations, comprising Cadia Hill open pit mine and Ridgeway underground mine (NSW, Australia) • Telfer open pit and underground mines (WA, Australia) • Gosowong underground mine (Indonesia) • Cracow underground mine (NSW, Australia). Newcrest focuses on internal (organic) and external growth opportunities and our strategy is to select quality, large-scale, near-term, long-life projects. Our investment in future growth increased substantially to $591 million during the half year ending December 2008, with spending on major projects in Australia, Papua New Guinea and Indonesia, including Hidden Valley and the Cadia East, Ridgeway Deeps, and Kencana underground developments. Exploration spending in the second half of 2008 doubled to $61 million compared with a year earlier, with ongoing projects in Australia, Fiji, Papua New Guinea, Indonesia and the Americas. It is Newcrest's vision to be 'Miner of Choice' for our people, our investors and the communities in which we work, by leading the way in innovation, safety and sustainability and providing opportunities for our people.

Geological Society of Australia Abstracts No 92

NEWMONT ASIA PACIFIC Newmont Mining Corporation is a leading gold producer with operations on five continents. Newmont is also engaged in the exploration for and acquisition of gold properties in some of the world's best gold districts. Employing approximately 34,000 employees and contractors worldwide, Newmont operates core assets in North America, South America, Australia, Indonesia, and Ghana, with new mine projects currently being developed. Our mines also produce copper and silver. Newmont is committed to high standards and leadership in the areas of environmental management and health and safety for its employees and neighboring communities Founded in 1921 in New York City, Newmont has been trading on the New York Stock Exchange (NYSE) since 1925. In addition to the NYSE, Newmont trades on the Australian and Toronto stock exchanges. Newmont is headquartered in Denver, Colorado. We invite you to explore Newmont's world of gold. Newmont owns or has an interest in three Australian gold mining operations and is nearly finished with constructing the expanded Boddington gold mine 81 miles (130 kilometers) southeast of Perth, Australia. We also operate the Martha mine open pit and Favona underground mines in Waihi, New Zealand.

IGCP524

2009 International Conference on Island Arc-Continent Collisions: Macquarie Arc Conference, Orange, New South Wales, Australia

Silver sponsors

Based in Hobart, Tasmania, CODES is widely regarded as the world's leading centre for ore deposit research; building a reputation for excellence in fundamental through to applied research, with direct industry applications. It is currently engaged in 38 research projects in 16 countries, employing a research team of 45 people, plus 79 postgraduate students and 15 technical and support staff. Activities include postgraduate training that produces a flow of world-class personnel for the Australian and international minerals industry. Its research is multidisciplinary and encompasses igneous petrology, geochemistry, melt/ fluid inclusion research, volcanology, structural geology, tectonics, geophysics, ore petrology and geometallurgy. It has produced over 180 research reports to industry in the past 3 years and is now the leading academic group to publish in Economic Geology. Highly productive worldwide collaborations have been developed with 11 major industry partners, plus a host of joint research initiatives with 34 institutes and universities - 17 in Australia and 17 overseas. CODES is based at the University of Tasmania, with satellite facilities, known as nodes, at the University of Queensland, University of Melbourne, Australian National University, CSIRO Exploration and Mining, the University of British Columba (Canada) and the Colorado School of Mines (USA).

Geological Society of Australia Abstracts No 92

BARRICK COWAL Barrick is the world's pre-eminent gold producer, with a portfolio of 27 operating mines, advanced exploration and development projects located across five continents, and large land positions on the most prolific and prospective mineral trends. The company also has the largest reserves in the industry, with 138.5 million ounces of proven and probable gold reserves, 6.4 billion pounds of copper reserves and 1.03 billion ounces of contained silver within gold reserves as at December 31,2008. The company's Australia Pacific business unit is comprised of 10 mines: six in Western Australia, one each in New South Wales, Tasmania and Queensland and the Porgera mine in Papua New Guinea.ln 2008, Barrick produced 7.66 million ounces of gold at a cash cost of $443 per ounce. In addition, the company produced 370 million pounds of copper at atotal cash cost of $1.19 per pound. For 2009, Barrick is targeting gold production of 7.2 -7.6 million ounces and copper production of approximately 375 -400 million pounds. Total cash costs are expected to be $450 -$475 per ounce of gold and $1.25 -$1.35 per pound of copper.The company has a successful track record of mine development, having completed the construction of theTulawaka, Lagunas Norte and Veladero mines in 2005, the Cowal mine in early 2006, and the re-opening of the Ruby Hill mine in early 2007. Barrick has been progressing a new generation of projects that advanced significantly in 2008: Buzwagi in Tanzania, Cortez Hills in Nevada and Pueblo Viejo in the Dominican Republic. Barrick has the gold mining industry's strongest and only 'A'rated balance sheet, which positions the company well to fund its project pipeline and seize other attractive exploration and acquisition opportunities as they may arise.Barrick shares are traded on the Toronto and New York stock exchanges.The company's vision is to be the world's best gold company by finding, acquiring, developing and producing quality reserves in a safe, profitable and socially responsible manner.For more information, visit www.barrick.com

IGCP 524

2009 International Conference on Island Arc-Continent Collisions: Macquarie Arc Conference, Orange, New South Wales, Australia

Nickel sponsor

newgold Bronze sponsors

GOLDEN CROSS RESOURCES

/VLS L a b a r a t a r q G r o u p ANALYTICAL CHEMISTRY & TESTING SERVICES

Satchel insert sponsors

IIME RANGOTT

MINERAL

EXPLORATION

Geological Society of Australia Abstracts No 92

IGCP 524

I g

CONTENTS

J. Alvarez-Marron, D. Brown, M. Schimmel & Y-M Wu

A.C. Barnicoat

J.D. Bradshaw

.27

D. Brown

.31

John F. Casey, and John F. Dewey..

.32

R.A.Cayley

.33

Peter 0. Clift, Paola Vannucchi, Jason Phipps Morgan

IVI.CI00S

W.J. Collins..

.40

D. R. Cooke, A. C. Harris and W. Zukowski.,

.42

G.J.Corbett

A. J. Crawford, S. Meffre & L.V. Danyushevsky.

.50

A. J. Crawford, S. Meffre, R. J. Squire, L. M Barron and T. J. Falloon

CQ ^

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B. de Waele, M.S. A. Horstwood, PE.J. Pitfield, R.J. Thomas, R.M. Key, M. Rabarimana, J-M. Rafahatelo, V. Ralison T. Randriamananjara... 56 lanW.D.Dalziel

V.David, P. Burrell&G.Coianiz

Hugh L. Davies

John Dewey, Maria Mange, and Paul Ryan

Amy E. Draut, Peter D. Clift, Jeffrey M. Amato & Jerzy Blusztajn and Hans Schouten

Monica Escayola and Cees van Staal

John Foden

David B. Forster & Peter M. Downes

N. Fox, A.C. Harris, D.R. Cooke, D. Collett & K. Faure

S o CO cq

Geological Society of Australia Abstracts No 92

G. M. Gibson, M. P. Morse, G. Nayak & T. R. Ireland

R A. Glen

R. A. Glen, A. J. Crawford, I. G. Percival, D. Cooke, S. Meffre, R. Scott, L M. Barron

R.J.Goldfarb

R.J.Goldfarb&CJ. R. Hart

J.E. Greenfield, KJ. Mills, R.G. Musgrave & PJ. Gilmore

A.C. Harris, I.G. Percival, C.M. Allen, D.R. Cooke, R.M. Tosdal, C. McMillan, P.D. Dunham, & D. Collett

R.J.Herrington & S.Roberts

R. J. Herrington, P. M. Scotney, S. Roberts' A. J. Boyce & D. Harrison A.I.S.Kemp&P.L Blevin

90 92

R. J. Korsch, N. Kositcin & D. C. Champion

A. Kroner, A. Demoux, D. Alexeiev, Y. Rojas-Agramonte, B.F. Windley & P. Jian

P.G. Lennox

J.B. Murphy, R.D. Nance, & P.A. Cawood,

C. Murray, &R. Offler

101

R. J. Musgrave & J. E. Greenfield

102

E.ROIiveira

104

LG. Percival

106

G. Phillips &R. Offler

108

S.A. Pisarevsky, P.A. Camod, E.C. Leitch, A. Nemchin

109

V.N.Puchkov

Ill

C. D. Quinn, R.A.GIen

113

Y. Rojas-Agramonte, A. Garcia-Casco, A. Kroner, S. Carrasquilla-Ortiz, M. A. Iturralde-Vinent & G. Millan-Trujillo

115

G. Rosenbaum

118

PaulD. Ryan

119

A. Saeed, R. A. Glen, C. D. Quinn, E. Belousova, & W. Griffin

120

David W.Scholl

121

CJ. Simpson

124

J.A.Tait

126

R.M. Tosdal, J.M. Mortensen, A.C. Harris, T. Bissig & C. Hart

128

Cees van Staal and Alex Zagorevski

131

A.J. Wainwright, R.M. Tosdal, C.N. Forster, D.J. Kirwin, P.D. Lewis & J.L Wooden

132

J.LWalshe

134

M.W. Washburn Groome, A.C. Harris, R.M. Tosdal

137

N. Williams & L.A.I Wyborn

139

L.A.I.Wyborn& N.Williams

141

W.J.Xiao

143

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2009 International Conference on Island Arc-Continent Collisions: Macquarie Arc Conference, Orange, New South Wales, Australia

DEFORMATION OF THE CONTINENTAL CRUST IN TAIWAN ARC-CONTINENT COLLISION: STRUCTURE AND SEISMIC ENERGY RELEASE J. Alvarez-Marron', D. Brown^ M. SchimmeP & Y-M Wu^ ^Institute de Ciencias de laTierra"Jaunne Almera", CSIC, Barcelona, Spain ^ Department of Earth Sciences, National Taiwan University,Taipei,Taiwan

An important question regarding arc-continent collision tectonics is the behaviour of the continental crust as it enters into the subduction zone and collision progresses. With the onset of collision, a fold and thrust belt develops either above a discrete subhorizontal basal detachment generally at the base of the platform sedimentary cover, or by crustal failure along steeply dipping faults commonly influenced by pre-existing basement faults hence involving nearly the entire crust. The Taiwan orogen has been proposed as the type example of a thrust belt developed above a basal detachment. However, in the last years a growing amount of seismicity data that shows nearly the whole crust beneath Taiwan is seismically active, opens the descrete detachment model into question. Here we present an analysis of seismicity data converted to energy release, combined with detailed structural mapping in an attempt to constrain which model best fits Taiwan. In Central Taiwan, between 24.25« and 23.75« latitude, the maximum cumulative energy released by earthquakes beneath the Western Foothills occurs at 8-12 km, below which there is only minor amounts of energy release. This area corresponds to the Peikong Basement High. Eastward, beneath the Hsiiehshan Range, there is high energy release to around 18-20 km depth, with the maximum energy released along

the western flank of the range. Below around 20 km depth there is only minor energy release, but it continues to the Moho at around 45 km. The eastern flank of the Hsiiehshan Range is marked by a drop in energy release, coinciding with the Lishan Fault. In the field, the Lishan Fault is marked by an area of high strain that appears to have a vertical dip. The Central Range has had only minor energy release. Field based studies in Central Taiwan indicate that the energy release maps can be very well correlated with major structural features in the field. For example, along the western flank of the Hsuehshan Range the Shuangtung and Shuilikeng thrusts appear to be associated with a cloud of energy release suggests that these faults dip steeply eastward. These faults have associated, pronounced hangingwall and footwall folds whose geometry is also suggests that they dip steeply eastward. Preliminary cross sections indicate that there is a significant uplift of the Mesozoic rift-related basin rocks across the area, suggestive of inversion of pre-exisitng faults. The integration of our field and seismicity data better fits with a model of a short detachment at the front of the thrust belt, whereas and whole crustal deformation appears to be taking place beneath the Hsuehshan Range and eastward. In this area it is difficult to identify a descrete basal detachment.

Geological Society of Australia Abstracts No 92

ARCS, MAGMAS AND ORE-FORMING FLUIDS A.C. Barnicoat^ ^ Geoscience Australia, G P O Box 378, Canberra ACT 2601, Australia

A diverse range of mineralisation, including porphyry and epithermal deposits, intrusion-related gold and other metal deposits, iron oxide-coppergold (lOCG) deposits and orogenic gold deposits all have spatial and temporal links to crustal growth and magmatic arcs. Furthermore, all of these deposit types are associated with fluids containing H2O, CO2 and NaCl in varying proportions. In all cases, it can be argued that magmas are one of the key source of hydrothermal fluids for these mineral systems, and that subduction processes are critical to controlling fluid chemistries, the metal-bearing capabilities of the fluids and depositional processes. The differences in typical/bulk fluid chemistries between deposit types can be explained in part by differences in the P-T conditions of fluid segregation from its magmatic source. The most significant control here is the pressure at which fluid forms from the magma as this has a strong effect on fluid CO2/H2O values. This is clearly exemplified by the rare occurrence of readily detectable CO2 in deep porphyry systems (Rusk et al., 2004). On the other hand, fluid CI contents, which strongly influence its base metal carrying capacity, are very sensitive to the bulk composition of the magma. Only some subduction-related magmas are fertile, however, and the differences do not seem to be due solely to variations in effectiveness of metal deposition processes. So what controls the volatile content of the magmas? Isotopic and other evidence, in particular for S and CI, shows (unsurprisingly) that the greater contents of these elements in arc magmas compared to other melts is due to contributions from subducted materials, although there may be additional, lower crustal sources of CI. Variations in the budget of volatiles subducted may thus play

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a role in controlling the chemistry of magmas and associated hydrothermal fluids, but variations within individual arcs suggests that again this is not the entire story. Arc magmatism typically involves complex, mixed magmas derived by a combination of the differentiation of mantle partial melts and the melting of lower or middle crustal rocks (e.g. Heinrich et al., 2005; Annen et al, 2006). As mafic melts have the capacity to carry large quantities of volatiles, and are normally derived from the suprasubduction zone mantle, this suggests that variations in the chemistry of the mafic melt entering the crust and hence crustal magma chambers plays an important role controlling the volatile budget of granitoids associated with mineralisation. Ore formation is hence controlled by metasomatism, melting and mixing processes above subducting slabs, and these are highly complex (Nikolaeva et al., 2008), and metallogenic outcomes are probably hard to predict in detail without field data. Later magmatic events in areas where metasomatised mantle developed during early magmatic events may be more readily predicted using remote techniques. REFERENCES ANNEN C . , BLUNDY J . D . & SPARKS R . S . J . 2 0 0 6 . Journal

of Petrology, 47, pp. 505-539. HEINRICH C . A . , HALTER W . , LANDTWING M . R . & PETTKE

T. 2005. Geological Society (London) Special Publication, 248, pp. 247-263. NIKOLAEVA, K . , GERYA, T . V . & CONNOLLY, J . A . D . 2 0 0 8 .

Physics of Earth & Planetary Interiors, 171, pp. 336-356 RUSK B . G . , REED M . A . , DILLES J . H . , KLEMM L . M . & HEINRICH C . H . 2004. Chemical Geology, 2 1 0 , pp.

173-199.

2009 International Conference on Island Arc-Continent Collisions: Macquarie Arc Conference, Orange, New South Wales, Australia

ROSS-DELAMERIAN ARC-CONTINENT COLLISION IN ANTARCTICA AND NEW ZEALAND WITH SOME COMPARISONS WITH OTHER AUSTRAL EXAMPLES J. D. Bradshaw Department of Geological Sciences, University of Canterbury, Private bag 4800, Christchurch, 8020, New Zealand

Delamerian Orogen. The close similarities between the rock assemblages and sequence of events in the two arcs strongly suggest that they are the result of the same collision event.

Belts of magmatic rocks with the characteristics of intra-oceanic volcanic arcs occur within present day continents and can reasonably be attributed to arccontinent collision. Such events are easily conceived and drawn within the framework of plate tectonics, although the actual processes and results are diverse. Three basic models can be identified, i) where the arc and the continent are above the subduction zone and collision is due to the closure of a marginal sea or back-arc basin, ii) where the continent is on the subducting plate and collision is with the fore-arc or subduction complex and iii) where the arc appears exotic and abuts a truncated margin and neither forearc nor back-arc rocks can be identified.

The Bowers Terrane is in tectonic contact with granites and metamorphic rocks of the active Cambrian Gondwana margin (Wilson Terrane of figure 1). The oldest unit in the Bowers arc assemblage is conglomerate dominated by boninitic clasts that pre-dates the main arc assemblage. The arc assemblage is Middle Cambrian and includes abundant volcanic rocks with intra-oceanic arc affinities plus basic volcanic rocks with MORB type chemistry (Rocchi et al. 1998). Sediments ranging from conglomerate to siltstone sourced from within the arc are common and inter-finger with quartzofeldspathic turbidites and rare conglomerate derived from Gondwana continental crust. Carbonate massflows and olistoliths, presumably derived form the

This review will focus on the collision of the Cambrian Bowers arc with the active continental Gondwana margin in northern Victoria Land (Antarctica), and the related Devil River arc of New Zealand, both components of the Cambrian Ross-

172E

50 km

Robertson Bay Terrane

Buller Terrane Takaka Terrane Devonian granitoids

. Anatoki Fault

Bowers \ Terrane

shear zone

Westport

100 km

Bowers Terrane Robertson Bay Terrane Wilson 'terrane'

Millen Shear Zone (mixed Robertson Bay & Bowers terrane rocks)

Devonian Granitoids Fig. 1. The location of the Bowers Terrane and Takaka Terrane: the Bowers Terrane is sutured to the Gondwana margin (Wilson Terrane) to the west and thrust over rocks of western Lachlan Fold Belt type, (Robertson Bay Terrane) in the Millen Shear zone. The Takaka Terrane is thrust west over Lachlan type rocks (Buller Terrane) along the Anatoki Fault.

Geological Society of Australia Abstracts No 92

margins of bathymetric highs, also occur. In the Late Cambrian, waning volcanism is associated with the development of a regressive sedimentary succession, with mass-flow emplaced marine conglomerate, and culminates in a thick red-bed' fluvial conglomerate. The conglomerates contain clasts from both the volcanic arc and the contemporaneous continental margin arc, suggesting that both margins were uplifted in the early Late Cambrian. The filling of the back-arc basin marks the collision of the arc and continent. High-pressure mafic metamorphic rocks are developed in a narrow suture zone between the back-arc basin and the contemporaneous continental margin arc (Rocchi et al. 2003) In New Zealand, Cambrian arc rocks lie in a number of tectonic slices in the western part of the Takaka Terrane, immediately east of the Anatoki Fault, the terrane boundary(Fig. 1). Initial boninitic conglomerates are succeeded by the main arc to back-arc assemblage (Devil River arc) that shows the same lithologies and the same pattern of events as the Bowers Terrane of northern Victoria Land. Quartzo-feldspathic sandstones with Gondwana affinities are confined to one fault slice, but are cut by dikes of Devil River arc type (Munker & Cooper, Bowers Terrane, NVL

Takaka Terrane, NZ

Leap Year Gp Lockett Cgl Dead Goat C

Haup Gp Christmas pCgl Junction Fm. Limestone ^ O O j Volcanics

HI °H"o°l Polymictic conglomerate

Mafic conglomerate. Non-marine conglomerate

Fig. 2. Comparison of Cambrian successions in the Bowers and Takaka Terranes.

1999), indicating that they were deposited in close proximity to the arc. The mixed sedimentary and volcanic arc assemblage again culminates with red-bed conglomerates. Recent research shows that large boulders of sedimentary and magmatic rocks characteristic of the Gondwana margin are present in

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the conglomerates together detritus from the volcanic arc and these data require the continent to be close by (Gutjahr et al. 2006). The position of a suture in New Zealand, however, cannot be established and evidence of continental rocks is limited to Ross-Delamerian granitic orthogneiss in central Fiordland (Gibson & Ireland, 1996) The close similarities of the successions (Fig. 2) strongly suggest that the two regions were adjacent parts of the same margin. In New Zealand, however, the change from marine to fluviatile sedimentation takes place at or near the Middle Cambrian to Late Cambrian boundary, earlier than the corresponding change in Antarctica. Arc accretion in the Late Cambrian of northern Victoria Land is late relative to peak orogenic activity further south in the Transantarctic Mountains but is consistent with the sinistral oblique convergence model widely accepted for this sector. In the context of this model, an initial closure of the back-arc basin in the south coupled with a northward progradation of non-marine sedimentation would place New Zealand south of northern Victoria Land in the Cambrian (Fig. 3). Both accreted arcs became sites of post-accretion passive margin type quartzite and carbonate sedimentation in the Ordovician and Silurian. The change in tectonic setting was achieved before the end of the Cambrian in New Zealand but cannot be accurately dated in Antarctica. Both arcs and succeeding passive margin rocks are now tectonically juxtaposed against Lachlan Fold Belt rocks of'western Victorian' aspect. In detail, however, their post-Cambrian tectonic history is very different. The Bowers arc remains adjacent to the Cambrian Gondwana margin to the west and is thrust over younger turbidites of the Robertson Bay Terrane on the eastern margin (Fig. 1). In contrast, the New Zealand Devil River arc probably separated from the Gondwana margin soon after accretion was displaced to the outer margin of the Lachlan belt before the Permian. This displacement and that of a number of other Ross-Delamerian fragments in the Ross Sea and West Antarctica is probably related to marginal sea development and subduction zone roll-back in the Ordovician and Silurian (Bradshaw, 2007). At present, the arc assemblage is thrust westwards over Ordovician turbidites and graptolitic shales (Buller Terrane) at its western limit and the eastern margin is tectonically truncated.

2009 International Conference on Island Arc-Continent Collisions: Macquarie Arc Conference, Orange, New South Wales, Australia

Coarse sediments (non-marine and marine)

Fig.3 Schennatic illustration of the suggested filling of the back-arc basin fronri the south in Middle and early Late Cannbrian tinnes. New Zealand would lie near the southern edge of the block and northern Victoria Land near the northern nnargin (Bradshaw et al. in press)

At least two other arc/back-arc accretion events can be clearly identified on the Gondwana-Pacific margin. The Permian Brook Street arc of New Zealand lies close to the edge of the Lachlan Fold Belt with a suture that subsequently became the site of an elongate mainly Mesozoic batholith. Neither fore-arc nor back-arc rocks are developed and the arc is buried by non-volcanic Late Permian and Jurassic sediments (Mortimer et al. 1999). The western suture was probably along a major zone of pre-Permian tectonic erosion that removed much of the Takaka Terrane and possibly equivalents of the New England Fold belt. The eastern margin is over-thrust by a Triassic-Jurassic fore-arc basin assemblage. In Patagonia, the Rocas Verdes ophiolites represent the floor of a back-arc basin that developed in the Late Jurassic and Early Cretaceous (Stern & DeWit, 2003). Mafic rocks cut the original continental margin and the presence of a volcanic arc along

the western (Pacific margin) is indicated by the provenance of sediments. To the east lies a broad zone of continental rifting characterised by bimodal volcanism. The basin closed in the mid-Cretaceous by collision and is marked by thrusting of back-arc basin rocks on to the continental margin. Development from initial rifting to closure occupied approximately 40 and 50 million years. The site of the western volcanic arc now appears to be occupied by a large granite batholith beyond which a well developed Mesozoic accretionary complex, with exotic elements, attests to the subduction of Permian and Mesozoic ocean floor. The above examples show the wide range of events that might be classified as arc-continent collision. Detailed study of adjacent sedimentary and igneous belts and regional structure is necessary to determine the development of arc and back-arc basin rocks found within a continental context.

Geological Society of Australia Abstracts No 92

REFERENCES BRADSHAW, J . D . 2 0 0 7 . USGS OF-2007-1047, short

research paper 059; doi:10.3133/of 2007-1049.srp059. BRADSHAW, J . D . , GUTJAHR M . , WEAVER S . D . , BASSETT

K . N . (in press). Cambrian intra-oceanic arc accretion to the austral Gondwana margin: constraints on the location of proto-New Zealand. Dun Mtn Ophiolite

Australian Journal of Earth Sciences.

GIBSON G . M . & IRELAND, T . R . 1996. Geology. 24, pp. 1087-1090.

GUTJAHR, M . , BRADSHAW, J . D . , WEAVER, S . W . , MUNKER, C. & IRELAND, T . R . 2 0 0 6 . Journal of the Geological Society, London. 163, 9 9 7 - 1 0 1 0 . MORTIMER, N . , GANS, P., CALVERT, A . AND WALKER, N . 1999.

Island Arc, 8, pp. 4 0 4 - 4 2 5 .

MUNKER, C. & COOPER, R . 1 9 9 9 . NZ Journal Geology and

Geophysics 42, pp. 4 1 5 - 4 4 5 .

ROCCHI, S., CAPPONI G . , CRISPINI L., DI VINCENZO G . , GHEZZO C., MECCHERI M . & PALMERI R . 2 0 0 3 . Terra

Antartica 9, pp. 1 4 5 - 1 4 8 .

ROCCHI, S., TONARINI, S., ARMIENTI, P., INNOCENTI, F . & MANETTI, P . 1998. Tectonophysics, 284, pp. 2 6 1 - 2 8 1 . STERN, C. & DE WIT, M . 2003. Geological Society (of London) Special Publication 218, pp. 1-19. Fig. 4. The Brook Street arc and adjacent rocks, southwest New Zealand (Mortimer et al 1999).

-520

i Arc-Trench/^

-540

Back-arc Basin -560 Fig 5. Distribution of the main tectonic elements in Patagonia in Cretaceous times. The Rocas Verdes forms the floor of the back-arc basin (From Stern & De Wit, 2003)

IGCP 524

2009 International Conference on Island Arc-Continent Collisions: Macquarie Arc Conference, Orange, New South Wales, Australia

ARC-CONTINENT COLLISION IN THE SOUTHERN URALS: INSIGHTS INTO THE GROWTH AND DESTRUCTION OF THE CONTINENTAL CRUST D. Brown Institute de Ciencias de laTierra"Jaume Almera", CSIC, Barcelona, Spain

The Southern Urals of Russia contain what is arguably one of the best-preserved examples of an arc-continent collision in any Paleozoic orogen. The arc-continent collision history recorded in the rocks of the Southern Urals began in the Early Devonian with the onset of intra-oceanic subduction and the formation of the Magnitogorsk Arc and ended with its collision with the margin of Laurussia during the Late Devonian. The Laurussia margin consisted of a basement that was composed predominantly of rocks of Archean and Proterozoic age that, by the time of arc-continent collision, was overlain by Cambrian, Ordovician, Silurian, and Devonian sediments interpreted to have been deposited in rift-related grabens on the continental slope and rise, and on the shallow marine platform. The Magnitogorsk Arc consists of Early to Late Devonian island arc volcanic rocks and overlying volcaniclastic sediments. Arccontinent collision led to the development of an accretionary complex that includes shallowly and

deeply subducted continental margin rocks, ophiolite fragments, and sediments that were deposited in a foreland-basin setting. High-pressure rocks derived from the leading edge of the continental margin indicate that it was subducted to a depth of between 70 km (eclogite assemblages) to 120 km (micro diamonds). It is estimated that the volume of continental crust that was subducted and lost to the mantle was approximately one third of the volume that was added to the Laurussia margin by the accretion of the Magnitogorsk arc. The geochemistry of the Magnitogorsk Arc volcanic rocks, the structure of the arc-continent collision accretionary complex and the forearc, the high-pressure rocks beneath and along the suture zone, the mafic and ultra-mafic ophiolitic material, and the syn-tectonic sediments show that the Paleozoic tectonic processes recorded in the Southern Urals can be favorably compared with those in currently active settings such as the west Pacific.

Geological Society of Australia Abstracts No 92

THE OPHIOLITE ENIGMA RESOLVED? John F. Casey, and John F. Dewey University of Houston

Ophiolites and ophiolite complexes have been recognized as having an oceanic affinity or origin since the classic work of Ian Gass in the 1950 s on the Troodos Complex. A problem has been that the term ophiolite has included a very diverse range of meanings from obscure slivers of mafic and ultramafic rocks of doubtful origin in orogenic belts to large obducted slabs with the full range (Coleman, 1972), from base to top, of Iherzolite/ ariegite, harzburgite, dunite, gabbro, sheeted dyke complex, pillow basalts, and sediments, commonly with a two-pyroxene mafic granulite as a thin aureole attached to the base of the complex. Large obducted ophiolite slabs are mainly Early Ordovician and midCretaceous. The principal enigma of these obducted slabs is that they clearly must have been generated by some form of organized sea-floor spreading/plateaccretion, such as may be envisioned for the oceanic ridges yet they have a calc-alkaline arc affinity with boninites (high-temperature/low-pressure, high Mg and Si andesites), which suggest a forearc origin. Many ophiolites have complexly-deformed associated assemblages that suggest fracture zone/transform geology, which in turn has led to models involving the nucleation of subduction zones on fracture zones/ transforms. Hitherto, arc-related sea-floor-spreading has been considered to be either pre-arc (fore-arc

IGCP 524

boninites) or post arc (classic Karig-style back arc basins that split arcs). We propose a new model with syn-arc boninites that involves a stable ridge/ trench/trench triple junction, the ridge being between the two upper plates. The direction of subduction must be oblique with a different sense in the two subduction zones and the oblique subduction cannot be partitioned into trench orthogonal and parallel strike-slip components. As the ridge spreads, new oceanic lithosphere is created, the arc and forearc lengthen, and a syn-arc ophiolite complex is generated that ages along arc-strike; a distinctive diachronous boninite/arc volcanic stratigraphy develops. Dykes in the ophiolite are normal to the trench as are magnetic anomalies in the "back-arc" basin. Boninites are generated in the fore-arc under the aqueous, low pressure/high temperature, regime at the ridge above the dehydrating slab. The mafic protolith, garnet/two pyroxene, aureole is generated in and sliced from the subducting slab and attached to the base of the overriding lithosphere at about 1000°C, twelve million years from the ridge axis, where the SSZ ophiolite is about ten kilometers thick, at which thickness the ophiolite is buffered by the subducting slab. Obduction of the SSZ ophiolite with its subjacent aureole occurs whenever the oceanic arc faces and s close to a continental margin.

2009 International Conference on Island Arc-Continent Collisions: Macquarie Arc Conference, Orange, New South Wales, Australia

EXOTIC CRUSTAL BLOCK ACCRETION TO THE EASTERN GONDWANA MARGIN IN THE LATE CAMBRIAN — TASMANIA, THE SELWYN BLOCK, AND IMPLICATIONS FOR THE CAMBRIAN-SILURIAN EVOLUTION OF THE ROSS, DELAMERIAN, AND LACHLAN OROGENS R. A. Cayley GeoScience Victoria, Victoria Department of Primary Industries. GPO Box 4440 Melbourne, Victoria 3001, Australia

Extended Abstract

Reconstructions of the tectonic evolution of eastern Gondwana in the Cambrian-Devonian, when the Tasmanides developed in southeastern Australia and the Ross Orogen formed in Antarctica, are based in part on correlations of Palaeozoic geology between these two regions. These correlations have received a recent boost from the acquisition, in 2006, of deep seismic data across the western Lachlan Orogen in Victoria (Cayley et al, in prep.). This has supported the Vice' model scenario involving convergence between the Australian craton and an outlier of Proterozoic continental crust, proposed for the evolution of the western Lachlan orogen (Cayley et al., 2002). A clearer understanding of western Lachlan evolution allows previous correlations with Tasmania and with Antarctica to be re-evaluated. This paper summarizes the Delamerian-Lachlan transition in western Victoria and previous attempts at correlation between northern Victoria Land (Antarctica), Tasmania and Victoria. It concludes that most previously published correlations have shortcomings, and introduces a new model that addresses these, and explains some of the major evolutionary differences seen between the Delamerian and Lachlan orogens in Australia, and the Ross Orogen in Antarctica. Although recent correlation of Palaeoproterozoic terranes between Australia and Antarctica (southern Eyre Peninsula, Australia with Cape Hunter, George V Land, Antarctica; Oliver & Fanning, 1997) appear to provide good constraints on Gondwana configurations west of the 'Tasman Line', reconstructions further east have been more difficult. Various Gondwana reconstructions over the years that correlate Australian Tasmanides geology with that seen in Antarctica (North Victoria Land) have produced a range of convincing, yet apparently contradictory results—western Tasmania with North Victoria Land (Griffiths, 1974; Laird et a/., 1977;

Laird, 1981; Burrett & Findlay, 1984, Weaver et a/., 1984); North Victoria Land with western Victoria (Harrington, 1979; Weaver et a/., 1984; Stump et a/., 1986; Flottmann et al, 1993); western Tasmania with western Victoria (Burrett & Findlay, 1984; Crawford et al., 1996; Direen & Crawford, 2003); central Victoria with western Tasmania and western Victoria (Crawford et al, 1988; Cayley et al., 2002). The Antarctic-mainland Australia fit seems the most convincing, with westward subduction beneath the East Antarctic Craton at 500 Ma (Weaver et al, 1984; Gibson & Wright, 1985; Borg et al, 1987; Flottmann & Kleinschmidt, 1991) developing paired metamorphic belts with the Gondwana craton to west (Grew et al, 1984), closely matching inferences for western Lachlan margin in Victoria at this time (eg. Flottmann et al, 1993; Miller et al, 2005). But how do the other correlations fit in? The Selwyn Block concept (Cayley et al., 2002) appears a further complication. The Selwyn Block model for the western Lachlan Fold Belt has solved several long-debated problems in west-Victorian geology and provides a crucial link to Tasmania, yet introduces a paradox that needs explanation. How is it possible for different regions of Proterozoic/ Early Palaeozoic crust to both show evidence of involvement in Delamerian orogenesis—far western Victoria and the Selwyn Block— when they are clearly widely separated, across strike, by a region of oceanic affinity (the Bendigo Zone) which contains oceanic igneous crust and sediment that completely spans the age of Delamerian orogenesis yet has no evidence of any involvement in it? There is a clear need for a new tectonic model for Gondwana in the Early Cambrian-Early Devonian that can reconcile all previous correlations. This can be achieved by interpreting Tasmania, together with its northern extension the Selwyn Block, and possibly including the Surgeon Island Terrane of NVL, as

Geological Society of Australia Abstracts No 92

an exotic microcontinent(s) of unknown origin, but not directly derived from the adjacent Australian Proterozoic craton as the ages are different. Prior to the middle Cambrian, it lay embedded in Early Cambrian oceanic crust of the Palaeopacific ocean outboard of the eastern Gondwana margin. The Tasmania microcontinent was carried westwards towards this margin, drawn by subduction of this Early Cambrian oceanic crust beneath the eastern margin of Gondwana beginning in the Middle Cambrian. The arrival of the Tasmanian microcontinent into the west-dipping subduction system active at that time is marked by emplacement of the ophiolite succession of the Early Cambrian mafic-ultramafic complexes seen in western Tasmania (Berry & Crawford, 1988). The buoyant Tasmanian microcontinent stalled subduction, and is possibly the cause of a marked hiatus in accretion in the Late Cambrian along much of the length of the eastern Gondwana margin (Berry & Crawford, 1988; Cox et al., 2000). This hiatus is marked by widespread post-collisional calc-alkaline volcanism along the length of the formerly active subducting margin, including the Mount Stavely Volcanics and Licola Volcanic Group in Victoria, the Mount Read Volcanics in Tasmania, and the Glascow Group in NVL, and possibly the Mount Wright Volcanics in NSW. In Tasmania and Australia, this event marks the end of the Delamerian Orogeny at 500-495 Ma, and the beginning of an interval of nearly 50 My when there is no evidence of strong deformation or accretion to the margin. Antarctica, although directly along strike from Australia, has a different structural history in this Ordovician interval. The transient hiatus caused by the collision of the Tasmanian microcontinent is clearly marked in the Bowers Terrane, but ongoing convergence is recorded as progressively eastyounging Ordovician Ar/Ar dates in metamorphic mica accompanying folding and cleavage formation across the width of the adjacent Robertson Bay Terrane (Dallmeyer & Wright, 1994). The different history suggests that subduction-accretion recommenced along part of the east Gondwana margin south of the Tasmanian microcontinent but did not immediately recommence to the north. This difference can be resolved by interpreting a right-lateral transform fault, active from 495 Ma—450 My, separating the southwestern margin of the accreted Tasmanian microcontinent from

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the progressively shortening accretionary prism of the RBT farther south. A conjugate structure may mark the deeply-buried northwestern margin of the Selwyn Block beneath the Bendigo Zone in Victoria. Reconstructions of Gondwana require that such a structure would extend deep into Antarctica in the vicinity of George V Land. A pre-existing transform in this general vicinity has been postulated to explain the separation of Laurentia in the Late Neoproterozoic (eg. Fig. 365, Veevers, 2000). This scenario is speculative, but major strike-slip movements of continental material away from zones of collision are well documented worldwide (Burke & Sengor, 1986), and variations on these themes have been applied to the Tasmanides (eg. Glen et al, 1992). Ordovician left-lateral movement on a northeast trending transform bounding the north-western aspect of the Tasmanian microcontinent may have been triggered by plate reorganisation following collision, or have been in response to ongoing accretion in NVL. Displacement of the order of 300400km magnitude along it can explain how a portion of the eastern Gondwana margin that includes the Tasmania microcontinent came to lie some distance east of the rest of the Gondwana margin in Australia by the Mid-Ordovician—ie. outboard of the presentday Stawell and Bendigo zones—effectively placing the proto-Bendigo Zone into an intraplate position without deforming it or significantly influencing its sedimentary history. By the end of the Middle Ordovician, palaeogeographic constraints show that the Bendigo Zone was effectively sandwiched between the Gondwana margin and the Tasmanian microcontinent (VandenBerg et a/., 2000). Ordovician easterly 'tectonic escape' of the Tasmanian microcontinent can explain the apparent conundrum of a common Delamerian collision history between western Victoria and the Selwyn Blocks despite these regions now being widely separated across strike by a part of the palaeopacific ocean (the proto Bendigo Zone) that had no involvement in this collision. By the Middle Ordovician, the Ar/Ar data of Dalmeyer & Wright (1994) suggest that most of the RBT had been accreted to the eastern Gondwana margin in Antarctica, so that ongoing subduction beneath this accreted margin had moved a few hundred km east of where it was in the Cambrian. This eastward retreat continued to the point where, by the Middle Ordovician, the system essentially outflanked the adjacent Tasmanian microcontinent.

2009 International Conference on Island Arc-Continent Collisions: Macquarie Arc Conference, Orange, New South Wales, Australia At this point the convergent subducting system was able to re-estabhsh as a single, simple, westdipping system up the whole length of the eastern Gondwana margin. In Australia this event may be marked by the appearance of the Macquarie Arc in the Mid Ordovician, and the return of regional-scale convergence in front of, and behind, the arc. This formed the Lachlan Orogen. This tectonic model can explain the transfer of Tasmania, the Selwyn Block and the undeformed oceanic crust of the Bendigo Zone from the lower plate Paleo-Pacific setting they occupied in the Cambrian to an upper-plate position—possibly located hundreds of kilometres behind (west of) the Macquarie Arc and the active eastern Gondwana plate margin—that they occupied from the end of the Middle Ordovician onwards. This scenario can also explain the episodic nature of tectonism characteristic of the Australian Tasmanides— especially the protracted hiatuses in deformation evident from palaeogeographic studies—and why this is not replicated in the Ross orogen along strike. Lachlan orogenesis driven by the Macquarie Arc accretionary system can explain the timing of shortening in the intraplate parts of the orogen such as the Bendigo Zone, where no local evidence for active plate margins exists at this time. Deformation of Lachlan age is not seen west of the Robertson Bay Terrane in NVL, or in exposed Precambrian shield rocks farther west, and yet along strike in Victoria it is clear that a few hundred km of east-west convergence occurred across the width of the Bendigo Zone, mainly in the Silurian (Gray et al., 2006). In Victoria the Moyston Fault and buried western Selwyn Block margin converge towards a triple-point near Cape Otway on the southern Victoria coast. Such convergence of Vice jaws' deforming the intervening Bendigo Zone (Cayley et al., 2002) suggests that shortening may have intensified towards this point. Rather than invoking large amounts of unseen crust of Lachlan age in Antarctica, convergence between the Selwyn Block/ Tasmania and the Australian part of the Gondwana margin to the west can be fully accommodated by invoking -170 km of right-lateral movement along an east-west oriented transform fault extending west from a fault-tip located near the Cape Otway triplepoint. Such a structure can fully account for Silurian shortening measured in the Bendigo Zone without any need for large areas of shortening of similar

age south of the transform, directly along-strike in Antarctica, or any need to offset Tasmania and the Selwyn Block. Evidence for a fault in the position of this postulated transform does exist in the form of the curved east-west structure along which Australia and Antarctica separated in the Mesozoic. This new tectonic model provides a mechanism that explains the hiatus that separates the Delamerian and Lachlan orogenic cycles in Australia, and also explains why this hiatus is not seen so clearly in the Robertson Bay Terrane of the Ross Orogen in Antarctica. It greatly simplifies the tectonic history of the east Gondwana margin in the Cambrian by restoring all rocks of known Delamerian affinity into a single line along the eastern Gondwana margin at the time of Delamerian accretion—from south to north: North Victoria Land, western Tasmania, the Selwyn Block, the Grampians-Stavely Zone and, further north, the Mt Wright region of western NSW. The accretion of Tasmanian microcontinent(s) provides the mechanism for disrupting simple subduction along the Eastern Gondwana margin in the Middle Cambrian. Post-collisional calc-alkaline magmatism occurred along the length of the margin, and is very discrete in time and lateral extent, and highly distinctive in chemistry. Post-Cambrian collision plate reorganisation involving northeast 'tectonic escape' of the Tasmanian microcontinent explains why the present-day distribution of these rocks appears much more complex. Transform faults developed as subduction resumed shortly after the Cambrian in Antarctica (to form the Robertson Bay Terrane), and during subsequent formation of the Lachlan Fold Belt in Australia, achieve this redistribution. Evidence for these structures exists, as they were subsequently reactivated to control the geometry of the separation of Australia and Antarctica. The main Australia-Antarctica rift probably reactivated transform weaknesses formed or reactivated during Benambran intraplate convergence, but as the new rift reached eastern Gondwana this simple west-east separation failed to propagate into Tasmania-Selwyn Block crust. A likely reason is that the east-west transform fault active during Silurian Bendigo Zone shortening and exploited by rifting likely terminated close to the Moyston Fault-Selwyn block triple-point—there is no geometrical need that this fault ever penetrated the interior of the Tasmanian microcontinent—Tasmania

Geological Society of Australia Abstracts N o 92

and the Selwyn block share a c o m m o n deformation history and appear to have been firmly embedded into the Tasmanides since the Silurian. After Bass rifting failed, ongoing spreading between Australia and Antarctica stepped south, by-passing Tasmania. It achieved this by utilising north-south trending structures subparallel to the former Delamerian Gondwanan suture as transform faults—an example is the Sorrell Fault. REFERENCES

FLOTTMANN, T . , GIBSON, G . M . , & KLEINSCHMIDT, G . 1993.

G^o/ogy, 21, pp. 319-322. FLOTTMANN, T . & KLEINSCHMIDT, G . 1991. Geology,

19, p p .

45-47. GIBSON, G . M . , & WRIGHT, T . O . 1985. Nature,

315, pp.

480-483. GLEN, R . A . , SCHEIBNER, E & VANDENBERG, A . H . M . 1992.

Geology 20. pp. 795-798. GRAY, D . R . , WILLMAN, C . E . & FOSTER, D . A . 2 0 0 6 .

BERRY, R.F. & CRAWFORD, A.R. 1988. Australian

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of Earth Sciences, 35, pp. 523-533.

Australian Journal Of Earth Sciences, 53, pp. 329-341. GREW, E . S . , KLEINSCHMIDT, G . & SCHUBERT, W . 1984.

BORG, S . G . , STUMP, E . , CHAPPELL, B . W . , MCCULLOCH,

Geologisches Jahnbuch, B60, pp. 253-264.

M . T . , WYBORN, D . ARMSTRONG, R . L . & HOLLOWAY, J . R .

1987. American Journal of Science, 278. pp. 127-169.

GRIFFITHS, J . R . 1974. Nature,

BURKE, K . & SENGOR, C . , 1986. The

HARRINGTON, H.J. 1979. Journal of the Geological Society

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Crust. Washington DC. American Geophysical Union (Geodynamics Series) 14, pp. 41-53.

of Australia

249, pp. 336-338.

26. pp. 276-277.

LAIRD, M.G. 1981. Journal of the Royal Society of New BURRETT, C . F . & FINDLAY, R . H . , 1984. Nature

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425-438.

72?>-726. LAIRD, M . G . , COOPER, R . A . , & JAGO, J . B . 1977.

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CAYLEY, R . A . , TAYLOR, D . H . , VANDENBERG, A . H . M &

265, pp. 107-110.

MOORE, D.H. 2002. In: Central Victoria: The Selwyn Block and its tectonic implications. Australian Journal of Earth Science. 49, pp. 225-254

MILLER, J . M C L . , PHILLIPS, D . , WILSON, C . J . L . & DUGDALE,

L.J. 2005. Australian Journal of Earth Sciences 52, pp. 921-940.

CAYLEY, R . A . , KORSCH, R . J . , MOORE, D . H . , COSTELLOE, R . D . , NAKAMURA, A . , WILLMAN, C . E . , RAWLING, T . J . ,

OLIVER, R . L . , & FANNING, C . M . 1997. The

MORAND, V . J . , SKLADZIEN, P . B . , & O'SHEA P . J . I n p r e p .

Region: Geological Evolution And Processes, pp. 163-172

Australian Journal of Earth Sciences.

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STUMP, E . , WHITE, A . J . R . , & BORG, S . G . 1986. Earth

CRAWFORD, A . J . , BUCKLAND, G . L . & VANDENBERG, A . H . M .

1988. Cambrian. In: Douglas, J.G. & Ferguson, J.A. (Eds.) Geology of Victoria, Edition, pp. 37-62. Geological Society of Australia, Victorian Division, Melbourne.

And

Planetary Science Letters, 79, pp. 348-360. VANDENBERG, A . H . M . , WILLMAN, C . E . , MAHER, S . , SIMONS, B . A . , CAYLEY, R . A . , TAYLOR, D . H . , MORAND, V . J . , MOORE, D . H . & RADOJKOVIC, A . 2 0 0 0 . In: V i c t o r i a .

CRAWFORD, A . J . , DONAGHY, A . G . , BLACK, L . P . & STUART-

Geological Survey Of Victoria Special Publication.

SMITH, P.G. 1996. Australian Institute of Geoscience Bulletin, 20, pp. 97-102.

VEEVERS, J . J . 2000. In: Veevers, J.J. (Ed.) Gemoc Press, pp.

DALLMEYER, R . D . , & WRIGHT, T . O . 1992. Tectonics,

325-343. 11, p p .

437-448 DIREEN, N.G., & CRAWFORD, A. J . 2003. Australian

of Earth Sciences, 50, pp. 491-502

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WEAVER, S . D . , BRADSHAW, J . D . , & LAIRD, M . G . 1984.

Earth And Platentary Science Letters, 68, pp. 128-140. Journal

2009 International Conference on Island Arc-Continent Collisions: Macquarie Arc Conference, Orange, New South Wales, Australia

THE RELATIVE ROLES OF ARC MAGMATISM, ARC ACCRETION AND TECTONIC SUBDUCTION EROSION IN BALANCING THE MASS OF CONTINENTAL CRUST Peter D. Clift\ Paola Vannucchi^ Jason Phipps Morgan^ ^School ofGeosciences, Meston Building, University of Aberdeen, Aberdeen, AB24 3UE, United Kingdonn ^Dipartinnento di Scienze dellaTerra, Universita degli Studi di Firenze,Via La Pira,4,50121 Firenze, Italy ^Departnnent of Earth and Atnnospheric Sciences, 3140 Snee Hall, Cornell University, Ithaca, NY 14853, USA

Global mass balancing estimates suggest that every year ca. 1.7 km^ of trench sediment and 1.3 km^ of tectonically eroded forearc basement are subducted at least as deep as the magmatic roots of island arcs and active continental margins. Nd isotope balances then indicate that only around 20% of this mass is recycled into the arc magmatism, requiring deep recycling of large volumes of continental crust at a rate that would recycle the entire mass of the continents in only 1.8 Ga. In addition, smaller volumes of continental crust are lost via the subduction of passive margins in continental collision zones and due to lower crustal delamination events, such as in North China Craton. In order to balance this loss, crust is generated in large igneous provinces (LIPs) and active margins. We estimate that no more than ca. 1 km^ is formed in oceanic LIPs and that continental LIPs are volumetrically insignificant. As a result arc magmatism must average net production of around 3.4 kmVyr if the volume of the crust is to be maintained over long periods of time. A long-term rate of arc production of 3.4 kmVyr would imply a mean net magmatic production rate of 85 kmVm.y./ km for the global arc system. 3.4 kmVyr is a net rate

that does not include melts added to the crust and then delaminated as dense cumulates. This rate is a little more than seismically derived estimates for net production rates for the oceanic Aleutian and Mariana arcs, but these figures do not account for the expected losses via subduction tectonic erosion in these systems, so that net rates would be higher and are within error of the 85 kmVm.y./km we estimate. Oceanic arc bears a disproportionate part of the crustal production compared to their total length since melting is high in margins of rapid convergence in the western Pacific and where the crustal lid is thin. Consequently arc accretion is anticipated to be an efficient process, a prediction that is borne out by first order crustal mass balancing in the Taiwan collision zone. Although oceanic arc crust is too mafic and LREE-depleted to be a simple continental crustal analogue we argue that the tectonics and magmatism of the arc accretion process, coupled with subsequent chemical weathering that tends to selectively destroy mafic minerals, will shift the bulk crustal composition towards continental end member values over long periods of geologic time.

Geological Society of Australia Abstracts No 92

ARC-CONTINENT COLLISION FOLLOWED BY LITHOSPHERIC DELAMINATION IN NEW GUINEA: IMPLICATIONS FOR MAGMA GENERATION AND CU-AU MINERALIZATION M. Cloos Department of Geological Sciences, Jackson School of Geosciences, University ofTexas at Austin, Austin,Texas, USA, 78712

The formation of the island of New Guinea has long been recognized as the product of a Cenozoic arccontinent collision. Geologic studies in the Central Ranges of the Papua Province of Indonesia (near Puncak Jaya, 4884 m) has revealed field and timing relationships which, combined with mechanical considerations, leads to a refined model for the tectono-magmatic effects of collisional orogenesis.

of the western highlands from about 4 to 2 Ma (Sapiie and Cloos, 2004). The ~ 3 Ma Grasberg (MacDonald and Arnold, 1994) and the 1.4 Ma Ok Tedi (Van Dongen et al., 2008) porphyry copper-gold deposits formed along the axis of the collision-generated mountain belt during the latest Pliocene.

Collisional Delamination

The geology of the Ertsberg District has become well known since 1990. Intrusions into the strata forming km-scale folds near the Ertsberg District were mostly small dikes, sills, and plugs that were emplaced between 4.4-2.6 Ma (McDowell et al., 1996). Most of these plutons are distinctly potassic ranging from hornblende and biotite-bearing medium to high K trachyandesites to trachytes with a groundmass rich in orthoclase (McMahon, 1994a, b). The Grasberg Igneous Complex (GIC) crosscuts the axis of a large fold and was emplaced between left-lateral strikeslip faults that trend subparallel to the regional structural grain created by the folded strata (Sapiie and Cloos, 2004). The Grasberg Cu-Au orebody, an extraordinary porphyry copper-type system (~2 x 10^ tons of ore grading at 1 wt.% Cu and 1 g/t Au), was emplaced at -3.0 Ma. The upper part of the GIC is a cone-shaped body tapering from -1800 m diameter at surface elevation of -4000 m to -800 m diameter at a depth of 1 km. The GIC is the product of three main phases of magmatic activity (MacDonald and Arnold, 1994): 1) the early and outer Dalam intrusion and associated volcanics, 2) the central Main Grasberg plug and 3) the multistage Kali dikes, which form a wedge-shaped mass that is largely unmineralized in the southeast part of the GIC. The GIC is the upper part of a low-relief, maar-type, caldera complex that has the characteristics of typical porphyry coppertype systems (Paterson and Cloos, 2005 a, b).

The generation of the Central Ranges of New Guinea can be explained by the plate tectonic process of collisional delamination (Cloos et al., 2005). Northwards dipping subduction of the oceanic end of the Australian plate began prior to 20 Ma at a Mariana-type subduction zone that generated part of the Melanesian island arc. Sediment accretion began after continental rise and slope strata reached the trench (Warren and Cloos, 2007). Massive offscraping occurred as shelf strata (Kembelangan and New Guinea Limestone groups) entered the system. The top of the accretionary/pre-collision complex became widely emergent at -12 Ma (Quarles and Cloos, 2005). Collisional jamming of the subduction zone by the underthrusting of thick Australian crust-capped plate began at ~8 Ma as evident from the detachment and southward displacement (10-20 km) of the large block of basement forming the core of the Mapenduma anticline (Weiland and Cloos, 1996). The oceanic end of the Australian plate did not stop subducting and dangle, but rather broke off. Subterranean plate rifting between 8 to 3 Ma along western New Guinea generated magma by adiabatic decompression melting of asthenosphere that upwelled into the gap along with varied amounts of melt generated in extended lithospheric continental mantle (Housh and McMahon, 2000). The upwelling also caused a rapid isostatically-driven vertical uplift of 1-2 km of the orogenic belt. The tear in the subducted end of the Australian plate propagated from west to east at a speed of about 150 km/m.y. (Cloos et al., 2005). Strike-slip transform faulting was active in the core

IGCP 524

Porphyry Copper Deposit Formation

Tectonic Implications Porphyry copper ore deposits are found near sites of subduction around the Pacific basin. Giant deposits

2009 International Conference on Island Arc-Continent Collisions: Macquarie Arc Conference, Orange, New South Wales, Australia involve the solidification of stock and batholith magma chamber systems (Cloos, 2001). Deposits in Arizona and along the Andes are Cu-Mo systems whereas several in the west Pacific (e.g., Grasberg) and British Columbia are Cu-Au systems. While both types form along convergent margins, the origin of the chemical dichotomy is unexplained (Cloos and Housh, 2008). In both "normal" subduction zones and collisional orogenic belts, the primary source of magma is the asthenosphere. The differing abundance of Mo and Au is explained as due to a difference in the secondary melt source materials. Cu-Mo systems episodically form in "normal" subduction zone arcs during times when the locus of magmatism is migrating. These magmas are enriched in Mo because they include a significant amount of melted oceanic sediment. Cu-Au systems form during episodes of collisional delamination. These magmas are enriched in Au because they contain a significant component derived from metasomatically enriched lithospheric mantle that underwent decompression melting during collisional delamination.

REFERENCES

MCDOWELL, F . W . , MCMAHON, T . P . , WARREN, P . Q . , &

CLOOS, M. 1996, Journal of Geology, 104, pp. 327-340. MACDONALD, G . D . & ARNOLD, L.C., 1994, Journal

Geochemical Exploration, 50, pp. 143-178. MCMAHON, T . P . 1994a, International Geology Review, 36,

pp. 925-946. McMahon, T. P., 1994b, International Geology Review, 36, pp. 820-849. PATERSON, J . T . , & CLOOS, M. 2005a, In: Porter, T.M., ed.. Super Porphyry Copper and Gold Deposits: A Global Perspective, PGC Publishing, Adelaide, pp. 303-319. PATERSON, J . T . , & CLOOS, M. 2005b, In: Porter, T.M., ed.. Super Porphyry Copper and Gold Deposits: A Global Perspective, PGC Publishing, Adelaide, pp. 321-345. QUARLES VAN UFFORD, A., & CLOOS, M . 2005,

American

Association of Petroleum Geologists Bulletin, 89, pp. 119-140. SAPIIE, B., & CLOOS, M . 2004, Geological

Society

America Bulletin: 116, pp. 277-293. VAN DONGEN, M . , WEINBERG, R . F . , TOMKINS, A . G . , &

CLOOS, M., 2001, International Geology Review, v. 43, p. 285-311. CLOOS, M . , & HOUSH T . B . 2008, In: Spencer, J. E., and Titley, S. R., eds., Circum-Pacific Tectonics, Geological Evolution, and Ore Deposits: Tucson, Arizona Geological Society Digest 22, pp. 235-244.

ARMSTRONG, R.A., 2008, PacRim Congress 2008: The Pacific Rim: Mineral Endowment, Discoveries and Exploration Frontiers, Extended Abstracts, Australasian Institute of Mining and Metallurgy Publication Series No. 11/2008, pp. 397-400. WARREN, P . Q . , & CLOOS, M . 2007, International

Geology

Review, 49, pp. 520-553. CLOOS, M . , SAPIIE, B., QUARLES VAN UFFORD, A., WEILAND, R . J . , WARREN, P . Q , & MCMAHON, T . P . 2005,

Geological

Society of America Special Paper 400.

WEILAND, R. J . & CLOOS, M. 1996, Geological Society of

America Bulletin, 108, pp. 1438-1449.

HOUSH, T . , & MCMAHON, T . P . 2000, Lithos, 50, pp.

217-239.

Geological Society of Australia Abstracts No 92

PETROLOGICAL/GEOCHEMICAL TRACKING OF GEODYNAMIC PROCESS: THE TASMANIDE RETREATING OROGENIC SYSTEM, EASTERN AUSTRALIA WJ. Collins School of Earth & Environmental Sciences, Jannes Cook University,Townsville, Queensland, 45811, Australia

The vast Paleozoic Tasman Orogenic System (TOS) is a series of outboard younging (retreating), turbiditegranite orogens comprising the inboard Delamerian (530-500 Ma), medial Lachlan (490-350 Ma), and outboard New England (350-240 Ma) orogens. It was subjected to long-term extension, interrupted by short-lived contractional events. The extensional events produced a series of extensive turbidite-filled, backarc basin sequences, separated by isolated, coeval oceanic arc remnants that become successively younger outboard. The intermittent contraction events generated a complex array of short-lived deformation zones intruded by plutonic belts that also generally young outboard, forming narrow foldthrust belts, usually some 20-60 Ma after turbidite deposition. Four major extensional episodes are recognized as a repeated tripartite lithological association, each of which initiated the Delamerian, western Lachlan, eastern Lachlan and New England orogens. These associations generally consist of (i) inboard S-type granite, (ii) outboard oceanic arc, and (iii) intervening, turbidite-filled backarc basin. S-type granites herald the formation of new, outboard oceanic arc and extensional backarc systems, following thickening of the pre-existing, sedimentdominated backarc basin. S-type plutonism is

IGCP 524

triggered by renewed arc magmatism following contraction, when hot mafic magmas are intruded into and mixed with the thickened backarc crust once slab retreat is re-established. With ongoing extension during retreat, the crust becomes progressively thinned, the sedimentary contribution is diminished, and I-type granites intrude the interarc rifts and proximal backarcs. With progressive slab retreat, the backarcs become distal and the granites become A-type. Because the outboard oceanic arc progressively moves outboard at the leading edge of the orogenic system, it is rarely preserved. The petrological evolution is mimicked through basalt compositions and isotopic evolution. The arcbackarc transition with S-, I-, to A-type granite is also evident as decreasing Sr/Nd, La/Nb, Sr/Ce, and similar LILE/HFSE ratios that indicate a progressive loss of slab flux character in the basalts, as the slab retreats. Nd wholerock and Hf isotopic ratios in granite-hosted zircon show a rapid change to crustal character following backarc closure, then a gradual change to depleted mantle character, also with progressive slab retreat. The evolutionary isotopic pattern is repeated with each contraction-extension cycle. These distinctive geochemical/petrological cycles are the hallmark features of backarc opening and closing in retreating accretionary orogens.

2009 International Conference on Island Arc-Continent Collisions: Macquarie Arc Conference, Orange, New South Wales, Australia

(a)

First stage rollback (crustal extension)

(c)

initial second stage rollback (post-contraction magmatism)

^^^

Second stage rollback (First tripartite association) <S>

<S>

^^^^^ ^

Model for S-type granite generation in tlie Tasmanides. (A). Slab rollback induces arc retreat and produces an oceanic backarc basin which fills with turbidites. Arrival of anomalously buoyant crust induces flat subduction and transient crustal thickening (B). Once the anomaly is subducted, slab rollback and arc magmatism recommences, and the thickened backarc crust is melted to produce S-type granites. The tripartite association begins (C), but is not fully developed until stage (D). 1 = S-type granite and HTLP complex; 2 = outboard arc; 3 = backarc basin. Ongoing backarc extension is not associated with S-type magmatism.

Geological Society of Australia Abstracts No 92

GOLD AND COPPER DEPOSITS OF THE MACQUARIE ARC, NSW D. R. Cooke'* A. C. Harris' and W. Zukowski' 'ARC Centre of Excellence in Ore Deposits, University ofTasnnania, Private Bag 79, Hobart,TAS 7001, Australia

the metallogenic evolution of the Macquarie Arc places the various deposit types within this tectonomagmatic evolutionary framework.

Introduction The Ordovician to Early Silurian Macquarie Arc has a metallogenic association typical of modernday oceanic island arcs. Economically, the most significant ore deposits are the alkalic porphyry gold-copper deposits of the Cadia and North Parkes districts (Table 1). The arc also contains calc-alkalic porphyry gold-copper deposits, skarns, high sulfidation gold - (copper), and quartz-pyritecarbonate-base metal-style epithermal gold deposits (Figure 1; Table 1).

Quartz - carbonate - pyrite - base metal epithermal gold deposits The Lake Cowal Volcanic Complex, a middle Ordovician volcanic arc succession, is part of the Junee-Narromine belt and hosts three economically significant gold deposits. Endeavour 42 (E42), Endeavour 41 (E41) and Endeavour 46 (E46). E42 contains 63.5 Mt @ 1.22 g/t gold (Table 1, Fig 1) and is the newest mine in the Macquarie Arc, having recently been brought into production by Barrick Gold Corporation. It occurs close to the E39 porphyry system (Miles and Brooker, 1998) within a N-trending corridor that contains similar

Crawford et al. (2007a) and Glen et al. (2007) documented four phases of magmatism occurring during the evolution of the Macquarie Arc. Gold mineralisation (± copper) is known to have been associated with phases 3 and 4, and possibly also with phase 2 magmatism. The following review of

Mineral Deposit of the Macquarie Arc Legend

^Peak Hill

^ ^ ^ H

Ordovician volcanic & Intrusive rocks

Copper Hill

. M

Silica-saturated aikaiic porphyry Au-Cu

133 M t @ 0.28 g/t Au 0.32 % Cu

Calc-alkalic porphyry Au-Cu

High sulfidation epithermal Au

Quartz-pyrlte-carbLS epithermal Au

Skarn

113 M t @ 1.29 g/t Au 0.11 % C u

North Parkes 153 Mt @ 0.46 g/t Au 1.03 % C u

Cadia 33°oos

Condoboiin O

2.245 Mt @ 0.52 g/t Au 0.15 % C u

Cowai 129 Mt @ 0.77 g/t Au

Marsden

6//

76,7 Mt 0.3 g/t Au 0.5 % Cu

Junction Reefs Gidginbung 9.1 M t @ 2.4 g/t Au

2.4 Mt @ 3.25 g/tAu

Cadia Skarns 42 M t @ 0.38 g/t Au 0.48 % Cu

100 km

Figure 1. Location of the alkalic porphyry gold-copper districts of New South Wales, together with selected Ordovician calc-alkalic porphyry copper- gold, epithermal gold and skarn deposits (nnodified after Holliday et al., 2002, and Cooke et al., 2007).

IGCP 524

2009 International Conference on Island Arc-Continent Collisions: Macquarie Arc Conference, Orange, New South Wales, Australia District / Deposit

Au (g/t)

Cu (%)

Reference http://www.newcrest.com.au/ Oct 08

Alkalic porphyry deposits - Cadia district Cadia Hiir

417.0

0.5

0.13

Cadia Quarry*

37.0

0.4

0.23

http://www.newcrest.com.au/ Oct 08

Ridgeway*

157.0

0.8

0.39

http://www.newcrest.com.au/ Oct 08

Cadia East*

1,634.0

0.5

0.13

Gooleys*

44m @ 1.3 g/t Au, 0.55% Cu,

http://www.newcrest.com.au/ Oct 08 Smith etal.2004

Alkalic porphyry deposits - NorthParkes district E26'

65.3

0.39

1.37

E2r

18.6

0.61

0.71

Lickfold et al. 2003a

E48*

33.4

0.59

1.04

Wolfe 1994

E31N*

6.6

0.39

0.35

Jones 1985

E3r

6.8

0.02

0.66

Jones 1985

E28*

8.1

0.04

0.35

Jones 1985

£27*

14.4

0.73

0.71

E37W*

No data

Smith etal.2004

E22N*

No data

Smith etal.2004

E20*

No data

Smith etal.2004

E26S*

No data

Smith etal.2004

Veedas*

102m @ 0.47% Cu, Au < 0.1 g/t

Smith etal.2004

Hopetoun Gold*

No data

Lye etal.2006

Brazen*

No data

Lye etal.2006

GRP314*

No data

Lye etaL2006

Lickfoldetal. 2003a

Lickfoldetal. 2003

Calc-alkalic porphyry deposits Copper Hill*

133

0.28

0.32

Marsden*

76.7

0.3

0.5

E43*

490m @ 0.19 %Cu,

Smith etal.2004 Smith etaL2004

http://www.goldencross.com.au/ Oct 08 http://www.newcrest.com.au/ Oct 08

Mandamah*

206m (S) 0.51 g/t Au, 0.37 % Cu,

Cullingarai*

50m @ 0.76 g/t Au, 0.53 % Cu,

Smith etal.2004

The Dam*

167m @ 1.0 g/t Au, 0.7 % Cu,

Smith etal.2004

Cargo*

3.7

1.24

http://www.goldencross.com.au/

Big Cadia*

34.0

0.40

0.50

Little Cadia*

8.0

0.30

0.40

Hollidayetal.2002

Junction Reefs*

2.4

3.25

Gray etal. 1995

0.77

http://www.barrick.com/ Oct 08

Skarn deposits http://www.newcrest.com.au/ Oct 08

Low sulfidation gold deposits E42 (Cowal)*

129

E41 (Cowal)*

No data

Bywater et al. 2004

E46 (Cowal)*

No data

Bywater et al. 2004

High sulfidation gold deposits PeakHiir

11.3

1.29

0.11

Masterman et al. 2002

Gidgingbung / Temora'

8.7

2.40

Smith etal. 2004

Table 1: Reserves and resources for the principal Ordovician gold-copper ore deposits of the Macquarie Arc. * - mine. # - prospect). Modified after Cooke et al. (2007).

Geological Society of Australia Abstracts No 92

auriferous veins at the E41 and E46 prospects. The gold deposits are hosted by an early Ordovician subaqueous volcano-sedimentary succession (Phase 1 magmatism) that has been intruded by multiple sills and dykes. The intrusions range in age from -460 to -440 Ma. The E42 system consists of gold-rich quartzcarbonate-pyrite-base metal sulfide veins associated with sericite-carbonate-pyrite alteration. Pyrite, sphalerite and galena are the most abundant base metal sulfide minerals. E42 appears to belong to the pyrite-quartz and carbonate-base metal subdivisions of epithermal gold deposits as defined by Corbett and Leach (1998). Such deposits appear to form at greater depths (1-2 km) than typical low sulfidation epithermal systems. Some workers (e.g., Bywater et al., 2004) consider the Cowal gold deposits to belong to the orogenic (mesothermaF) class of ore deposits. Epithermal and porphyry styles of mineralisation and alteration occur in the Endeavour 41 (E41) gold prospect (Zukowski et al, 2007). Gold is associated with quartz-pyrite and quartz-sphalerite-carbonate veins. Alteration facies include potassic, calc-potassic, phyllic and propylitic mineral assemblages. The nature and distribution of these assemblages has been controlled by rock types, structure and geometry of the mineralised zones. Early hydrothermal alteration produced actinolite-magnetite-albite-chlorite and garnet-epidote-carbonate assemblages. Synmineralisation alteration facies are characterised by K-feldspar, sericite (muscovite/illite), chlorite, epidote and arsenopyrite, whereas late alteration comprises epidote-carbonate-prehnite. The hydrothermal system evolved from early high-temperature actinolite-magnetite-albite-chlorite and garnet alteration and vein facies, characteristic of an alkalic porphyry environment, to assemblages more typical of an epithermal style gold deposit (e.g. quartz, carbon-ate, chalcedony, adularia, gold, sphalerite, galena and illite). The paragenetic history of the E41 gold prospect appears to record the transition from deep to shallow-level magmatic-hydrothermal activity, and implies unroofing of the system synchronous with mineralisation. E42, E41 and E46 are all partially hosted within a large diorite sill, the Muddy Lake diorite, which was emplaced at 455.9 ± 5.6 Ma (Bastrakov, 2000). The E42 deposit was inferred to have formed at 439 ± 1 Ma by Perkins et al. (1995), based on ^^Ar/^^Ar dating of sericite. Strickland (2005) reported an age of 448

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± 4 Ma zircon age for a post-mineralisation vesicular dyke from E42, raising doubts about the veracity of the sericite age. The exact age and magmatic affinity of the Cowal gold deposits therefore remains uncertain. They may have formed in association with phase 2 magmatism (circa 455 Ma) or phase 4 magmatism at the culmination of Macquarie Arc magmatic activity (circa 440 Ma). Alteration features at E41 point to a possible alkalic porphyry affinity, possibly implying a 440 Ma timing of ore formation (Zukowski et al., 2007). Calc-alkalic porphyry copper-gold deposits Phase 3 magmatism in the Macquarie Arc produced a widespread but relatively small volume suite of intrusive rocks around 450 - 445 Ma. Calc-alkalic copper-gold porphyry deposits were emplaced during a time of regional uplift, erosion and limestone deposition, possibly associated with an arc-reversal event (Glen et al., 2007). The principal calc-alkalic deposits are Copper Hill, Cargo and Marsden (Table 1, Figure 1). Other sub-economic calc-alkalic porphyry copper-gold systems occur at Mandamah, The Dam and Culingerai in the Rain Hill district, and E39 and E43 in the Cowal district. Copper Hill was the site of the first production of copper in NSW (1845), and is currently being explored by Golden Cross Resources Ltd. A resource of 133 Mt @ 0.28 g/t gold, 0.32 % copper has been identified. Mineralisation is associated with an adakite-like suite of medium-K calc-alkalic rocks. Copper Hill is an unusual porphyry copper-gold deposit in that palladium is enriched over intervals of tens of metres in some drill holes. Locally, grades of up to 0.9 ppm palladium have been detected in association with zones that contain > 5 g/t gold. The Marsden calc-alkalic porphyry copper-gold deposit was discovered in 1997 by Newcrest Mining Ltd. It contains 76.7 Mt @ 0.3 g/t gold, 0.5 % copper. Miineralisation is hosted in the Marsden intrusive complex, which is covered by 100m of clay-rich transported alluvium and has been truncated at depth by a low-angle thrust fault which has juxtaposed Ordovician and Devonian rocks. Although no geochronological data are currently available for Marsden, it is inferred to be associated with phase 3 magmatism based on geochemical affinities between the mineralising intrusions at Marsden and the Copper Hill suite.

2009 International Conference on Island Arc-Continent Collisions: Macquarie Arc Conference, Orange, New South Wales, Australia Alkalic porphyry gold-copper deposits The Cadia and Northparkes districts contain several major silica-saturated alkalic porphyries (Table 1; Figure 1). A total of four alkalic porphyry gold-copper deposits have been identified at Cadia (Ridgeway, Cadia Hill, Cadia East and Cadia Quarry/Cadia Extended). These deposits include the largest and highest grade copper-gold deposits in the Macquarie Arc (Cadia East and Ridgeway, respectively). The characteristics of the Cadia district are described by Harris et al. (this volume), Fox et al. (this volume) and Washburn et al. (this volume). The first porphyry-related copper-gold mineral occurrence in the Parkes district was discovered in 1976 (Jones, 1985). Subsequent exploration led to the discovery of several other mineralized centers in the area, including the Endeavour 22, 26, 27, and 48 deposits (E22, E26, E27, and E48), which have a total combined resource of 131.7 Mt @ 1.12 % copper and 0.51 g/t gold. In November 1992, the North Broken Hill Peko Limited board approved the Northparkes project to mine and process coppergold ore from E22, E26 and E27. The Northparkes project commenced in 1993 with open pit mining and processing, firstly of oxide, and later sulfide ores at E22 and E27. Underground production (via block caving) commenced at E26 in 1995. Approximately 160,000 tonnes of copper is produced annually. Hydrothermal alteration at the Northparkes porphyry copper-gold deposits is typically restricted to within -750 m of the quartz monzonite intrusive complexes and occurs in both the intrusive and volcanic rocks. Early albite alteration was followed by widespread biotite-magnetite alteration of the volcanic wallrocks (Heithersay and Walshe, 1995; Lickfold et al., 2003). Orthoclase and sericite selvage alteration associated with high grade copper-gold mineralisation in quartz stockwork and sheeted veins then overprinted the sodic and early potassic assemblages. Early, pervasive biotite-magnetite alteration assemblage has the greatest aerial extent of all alteration assemblages except for more distal propylitic assemblages (chlorite-carbonate-hematitefluorite-pyrite). A core of late stage, deep-seated pervasive sericite-carbonate-albite alteration assemblage occurs in the E48 intrusive complex, extending to depths of >1 km below the surface (Wolfe, 1994). It is associated with late stage high grade copper-gold-arsenic mineralisation (Hooper et al., 1996). Similar deep-level sericite alteration zones

occur in the core of the E26, E27 and E22 deposits Lickfold et al., 2003). This alteration assemblages originates from high temperature, magmatichydrothermal fluids (Harris and Golding, 2002). Most of the available geochronological data for the Northparkes and Cadia districts indicates that the ore deposits are intimately associated with phase 4 magmatism (circa 443 - 437 Ma; Lickfold et al., 2003, 2007; Wilson et al., 2007), and that the deposits formed during the culiminating phase of magmatism and deformation in the Macquarie Arc. Wilson et al. (2007) also noted some geochronological evidence for earlier alkalic-style magmatism at Cadia, circa 455 Ma, possibly indicating some alkalic activity during phase 2 magmatism.

High sulfidation gold-copper deposits Four Ordovician HS-style alteration zones in the Junee-Narromine Volcanic Belt: Peak Hill, Gidginbung (Temora), Dobroyde and E35. They comprise highly deformed zones of advanced argillic alteration, characterised by pyrophyllite, alunite and muscovite. Their ages are controversial. Many workers consider them to have formed during phase 4 magmatism circa 440 Ma (e.g., Perkins et al., 1995), although some workers (e.g., Allibone et al. 1995; Allibone, 1997, 1998) have advocated a younger (Devonian) syn-deformation age for HS mineralisation. The most intensively studied HS deposit in the Macquarie Arc is Peak Hill. Gold was first discovered on Peak Hill in 1889. By 1917, approximately 0.6 million ounces of gold was extracted from the upper 150 m of the system. More recent exploration and mining by Alkane Resource Ltd led to a further 153,000 ounces being recovered from the oxide resource with the sulfide resource (11.27 Mt @ 1.29 g/t gold and 0.11 % copper) remaining largely intact. Gold mineralisation at Peak Hill is atypical of highsulfidation epithermal deposit, whereby ore (>5 g/t gold) is located on the margin of a core of advanced argillic alteration assemblages. The core of the deposits lacks extensive zones of residual, vuggy quartz, and is instead dominated by a pyrophyllite (± diaspore) alteration zone -350 m wide and at least 550 m long, which grade outwards through paragonite-muscovite to kaolinite with chloriteepidote at the margins (Masterman et al, 2002; Squire et al., 2007). Several stages of overlapping

Geological Society of Australia Abstracts No 92

gold-copper-bearing veins, breccias and replacements (quartz - pyrite ± barite - paragonite - muscovite) occur and are related to wall-rock reaction that neutralized acid fluid. Despite young ages (409.3 ±1.9 Ma), sulfide-bearing hydrothermal alteration assemblages have a distinctive Pb isotope data (Carr et al., 1995) that implies fluids came from a Late Ordovician to early Silurian source magma (Squire et al., 2007). The pyrophyllite-rich, quartz deficient ore zones at Peak Hill have been interpreted as the root zone of a high sulfidation deposit (Masterman et al., 2002). Another possible interpretation is that the deposit originally contained lower temperature (i.e. shallower level) clays such as kaolinite and dickite, but the clays were metamorphosed to pyrophyllite during the Devonian Tabberabberan Orogeny. Skarn deposits

There are several mineralised skarns known from the Molong belt, including the Big Cadia and Little Cadia copper-gold-iron skarns in the Cadia District, which are intimately associated with phase 4 magmatism and alkalic porphyry deposit formation. Big Cadia was mined historically as an iron (± copper) resource, and still contains a significant resource of copper and gold mineralisation (34 Mt @ 0.5% copper and 0.4 g/t gold). The Molong Belt also hosts the Junction Reefs and Browns Creek gold skarns, both of which were mined in the past twenty years for gold. Conclusions

The porphyry, epithermal and skarn deposits of the Macquarie Arc are comparable to the ore deposits that form in modern oceanic island arc settings. Most of the ore deposits, and the largest and most well-endowed formed in association with phase 4 magmatism during the Benambran Orogeny. This period of deformation marked the culmination of magmatism in the Macquarie Arc, and was the critical event for mineralisation. An earlier arc-reversal event (circa 450 - 445 Ma) produced small volume adakitic intrusive complexes that are associated with the calc-alkalic copper-gold porphyry deposits throughout the Macquarie Arc. Limited evidence for mineralisation associated with an as-yet undetermined tectonic event during phase 2 magmatism (460 - 450 Ma) has been determined from Cowal and possibly also from the Cadia

IGCP 524

district. Further work, including more detailed geochronological investigations of the ore deposits and their associated intrusive rocks, is required to resolve the controls on the earliest mineralising events in the Macquarie Arc. REFERENCES

ALLIBONE, A. 1997. Australian Journal of Earth Sciences 44, pp. 727-742. ALLIBONE, A . H . 1998. Mineralium Deposita 33, pp. 495-512. ALLIBONE, A . H . CORDERY, G. R., MORRISON, G. W . , JARIETH, S. & LINDHORST, J . W . 1995. Economic Geology 90, pp. 1570-1604. BASTRAKOV, E. 2000, Unpublished Ph.D Thesis, Australia

National University.

BYWATER, A., WILLIAMS, S., MCINNES, P. & DIJKMANS, 2004. pp. 77-82. Geological Society of Australia Abstracts,

74.

CARR, G.R., DEAN, J.A., SUPPEL, D . W . , AND HEITHERSAY, P.S. 1995, Economic Geology 90, pp. 1467-1505. CooKE, D.R., WILSON, A.J., HOUSE, M . J . , WOLFE R.C., WALSHE, J.L., LICKFOLD, V., & CRAWFORD, A . J . 2007, Australian Journal of Earth Sciences 54, pp. 4 4 5 - 4 6 3

CORBETT, G . J . & LEACH, T . M . 1998. Society of Economic

Geologists Special Publication 6.

CRAWFORD, A.J., MEFFRE, S., SQUIRE, R . J . , BARRON, L . M . & FALLOON, T . J . 2007a. Australian Journal of Earth

Sciences 54, pp. 181-214.

CRAWFORD, A.J., COOKE, D . R . & FANNING, C . M . 2007b.

Australian Journal of Earth Sciences 54, pp. 243-271. GLEN, R.A., CRAWFORD, A . J . & COOKE, D . R . 2007a.

Australian Journal of Earth Sciences 54, pp. 465-479. HARRIS, A.C. & GOLDING, S.D. 2002. Geology 30, pp. 335-338.

HEITHERSAY, P.S. & WALSHE, J . L . 1995, Economic Geology 90, pp. 1506-1532. HOLLIDAY, J . R . , WILSON, A.J., BLEVIN, P.L., TEDDER, L J . , DUNHAM, P . D . & PFITZNER, M . 2002. Mineralium Deposita

37, pp. 100-116.

JONES, G . J . 1985. Economic Geology 80, pp. 591-613. LICKFOLD, V., COOKE, D.R., SMITH, S.G. & ULLRICH, T . D .

2003.

Economic Geology 98, pp. 1607-1636.

LICKFOLD V., COOKE, D.R., CRAWFORD, A . J . & FANNING, C . M . 2007. Australian Journal of Earth Sciences 54, pp. 417-444.

2009 International Conference on Island Arc-Continent Collisions: Macquarie Arc Conference, Orange, New South Wales, Australia

D. 2002. Society of Economic Geologists Newsletter 51, pp.

STRICKLAND, T.J., 2005, Unpublished BSc. Thesis, University Of Tasmania.

Earth Sciences 45, pp. 837-847.

MILES, I.N. & BROOKER, M . R . 1998. Australian Journal of

WILSON, A.J., COOKE, D.R., STEIN, H.J., FANNING, C.M., HOLLIDAY, J.R., & TEDDER, I.J., 2007, Economic Geology 102, pp. 3-26.

PERKINS, C., WALSHE, J.L. & MORRISON, G. 1995.

WOLFE, R.C., 1994, UnpubUshed B.Sc. (Honors) Thesis,

MASTERMAN, G.J., WHITE, N.C., WILSON, C.J.L. & PAPE,

1-16.

Economic Geology 90, pp. 1443-1466. SMITH, S., MOWAT, B. & SHARRY, M. 2004. Geological Society ofAustralia, Abstracts 74, pp. 51-62. SQUIRE, R.J., HERRMANN, W., PAPE, D. & CHALMERS, D. I.,

2007. Mineralium Deposita 42, pp. 489-503.

Hobart, Australia, University Of Tasmania.

ZuKOwsKi, W., COOKE, D.R., DEYELL, C.L., & MCINNESS,

P., 2007, In: Andrew, C.J., et al. (Eds.), Digging Deeper:

Proceedings of the 9th Biennial SGA Meeting, Dublin

2007, Irish Association For Economic Geology, Dublin, Ireland, l,pp. 403-406.

Geological Society of Australia Abstracts No 92

TECTONIC/STRUCTURAL CONTROL TO PAPUA NEW GUINEA AU-CU MINERALISATION G J . Corbett Consultant, PO Box 282 Wi I lough by, NSW, Australia

The setting and structural controls to many porphyry and epithermal Au-Cu occurrences in Papua New Guinea (PNG) reflect development within a complex oceanic-continental plate collision zone (figure 1, Corbett, 2005, Corbett and Leach, 1998). In many instances magmatic arc intrusion-related ore systems are localised by steep dipping arc-parallel structures, locally as terrain boundaries, or arcnormal deep crustal transfer structures, which tap deep magma sources (Corbett, 1994). Intersections of these two trends are important settings for ore systems (Porgera, Yandera, Frieda River), although mineralisation may extend along either trend, and structures display activity over protracted periods of time. At the Yandera porphyry Cu-Mo prospect, arcparallel aligned mineralisation is dissected by arcnormal faults. Pronounced uplift and erosion in the collision setting has been important to expose at the surface, the youthful (1.2 m.y.) Ok Tedi porphyry CuAu mineralisation which must have formed at a depth of at least 1 km. While porphyry Cu-Au deposits (Panguna, Ok Tedi, Frieda River, Yandera) which developed at deep crustal levels host bulk tonnage lower Au grade mineralisation (with Cu and local Mo), the epithermal deposits formed at higher crustal levels may contain bonanza Au grades (and Ag) within dilatant portions of host structures (Corbett and Leach, 1998; Corbett, 2005). Au-rich alkaline rocks of the Pliocene-Recent LihirTabar island arc are considered to have been derived by the remelting of previously melted oceanic crust as a result of the reversal of subduction following the Miocene closure of the south facing subduction as it became blocked by the thick Otong Java plateau oceanic crust. Here, mineralisation within the porphyry-epithermal Au transition is controlled by regional deep seated NS arc-normal structures, and best mineralisation locally occurs in the steeper dipping portions of listric faults developed during sector collapse of the Luise volcano, which triggered ore formation. A series of NNE trending arc-normal transfer

IGCP 524

structures localise intrusion-related ore systems in the fold-thrust collision zone of mainland PNG and continue into West Papua (Grasberg), particularly at the intersections of major arc-parallel structures. Important examples include Porgera, where much of the early low sulphidation epithermal carbonate-base metal Au mineralisation is aligned within the transfer trend, while the arc-parallel Roamane fault hosts later bonanza grade low sulphidation epithermal quartz Au-Ag style Zone VII mineralisation. Thrust erosion is interpreted to have initiated the later higher crustal level epithermal mineralisation and the nearby Mt Kare Au mineralisation is considered to represent the thrust-off top of Porgera. Arc-normal transfer structures are also recognised at Wafi, Ok Tedi, Frieda River, Yandera and Bilimoia. At Wafi a major transfer structure localises overprinting mineralisation. The early porphyry Cu-Au is upgraded by the diatreme-flow dome hosted high sulphidation Au-Cu system, which evolves to marginal low sulphidation carbonate-base metal Au mineralisation, where higher Au grade better metallurgy ores are recognised. At Bilimoia, a corridor of major arc-parallel structures, formed as part of the Markham Fault (a terrain suture, figure 1), host porphyry and deep level epithermal low sulphidation quartz-sulphide style Au mineralisation over a strike distance of some 20 km. Similarly, the Frieda-Nena mineralisation is localised by a splay formed at the intersection of the arcparallel Fiak-Leonard Schultz Fault and arc-normal Ok Tedi transfer structure. Here, a block faulted 10 km long corridor hosts Cu-Au mineralisation developed at different crustal levels, varying from the deep eastern Horse-Ivaal Cu-Au porphyry systems, to Nena shallow level high sulphidation epithermal AuCu mineralisation at the western portion of the trend. The Morobe Goldfield is localised within the major intra-arc extensional Bulolo Graben (Figure 1) where Pliocene felsic volcanics and diatreme-flow dome complexes overlie basement metamorphic

2009 International Conference on Island Arc-Continent Collisions: Macquarie Arc Conference, Orange, New South Wales, Australia and granitic rocks. Here, some low sulphidation epithermal Au-Ag occurrences are localised within graben bounding structures (Hamata, Hidden Valley), while others occur within intra-graben structures such as the Escarpment Fault (Wau), especially at the intersection with cross structures (Kerimenge, Edie Creek). Best mineralisation occurs within steeper dipping portions of graben-related listric faults (Hidden Valley) and immediately adjacent to diatreme breccia pipes (Kerimenge, Wau, Edie Creek). As typical of the pronounced vertical zonation in low sulphidation epithermal u mineralisation, bonanza Au grades are best developed at the highest elevation (Edie Creek) in a region of deeply dissected topography. The Tolukuma classic chalcedony-ginguro style low sulphidation epithermal Au-Ag banded quartz vein system occurs within a graben bounding structure separating volcanic and basement metamorphic rocks. Normal and strike-slip fault movement have localised bonanza Au bearing ore shoots within dilatant fault portions. Similarly, the Umuna Lode low sulphidation carbonate-base metal epithermal Au mine at Misima Island lies within a fault jog formed by a component of strike-slip movement on

regional arc-parallel faults. At Woodlark Island the Kulumadau group of mineral occurrences extend along a major horst bounding structure, while the Busai mineralisation occurs as flat dipping lower Au grade tension veins constrained between steep structures which locally host bonanza Au grade lodes. On the Papuan Peninsular where southward collision has obducted oceanic crust onto the basement mainland rocks, sub-economic Cu-Au mineralisation occurs in flatter dipping portions of reverse faults and Ni occurrences are under investigation.

REFERENCES CoRBETT, G.J., 1994, in Rogerson, R., ed., Geology, exploration and mining conference, June 1994, Lae, Papua New Guinea, proceedings: Parkville, The Australasian Institute of Mining and Metallurgy, pp. 57-70. CoRBETT, G.J., 2005, Geology and Mineral Potential of Papua New Guinea: Ed. A. Williamson & G. Hancock, Papua New Guinea Department of Mining. CoRBETT, G . J . , AND LEACH, T . M . , 1998, Special Publication 6, Society of Economic Geologists.

Figure 1: Locations of mineral occurrences discussed superimposed upon the tectonic elements of Papua New Guinea, from Corbett (2005).

Geological Society of Australia Abstracts No 92

MODERN INTRA-OCEANIC ISLAND ARCS: A TEMPLATE FOR THE INTERPRETATION OF ANCIENT VOLCANIC ARCS IN FOLD BELTS A. J. Crawford', S. Meffre' & L.V. Danyushevsky' 'ARC Centre of Excellence in Ore Deposits (CODES), University ofTasnnania, Private Bag 79, Hobart,Tas. 7001, Australia.

Introduction

This presentation reviews the tectonic development and petrological and geochemical evolution of island arcs from the point of view of facilitating the identification and interpretation of ancient island arcs incorporated in fold belts as a result of plate collisions. We focus on intra-oceanic arcs of the Western Pacific region, in particular, those arcs in the SW Pacific (Tonga-Kermadecs, Fiji, Vanuatu, Hunter Ridge), and the well studied Bonin-Mariana Arc. Subduction Initiation and Boninites

Many Western Pacific intra-oceanic arcs were initiated around 50-55 Ma, probably in response to major changes in the global plate circuit following India-Asia collision. Boninites are a key feature of the forearc regions of those arcs that were initiated at or very close to active spreading centres (Eocene Loyalty arc in the SW Pacific), or where an active ridge was subducted broadly parallel to the newly initiated subduction zone (Bonin-Mariana Arc). Where subduction was initiated beneath an abnormally young and hot oceanic plate (S Fiji Basin oceanic crust beneath the actively spreading N Fiji Basin crust around 7-8 Ma), boninites were erupted adjacent to the spreading centre on the over-riding plate. Subduction beneath young oceanic lithosphere further from the ridge on the over-riding plate produced low-Ti magnesian arc tholeiites and highMg andesites. The dominant metasomatic agent responsible for triggering partial melting of the hot, very refractory, shallowest oceanic crust in such ridge subduction settings is tonalitic or trondhjemitic magma derived from slab melting. Petrographically distinctive low-Ca boninite magmas are the defining feature of hot subduction initiation, and may occur along >1000km of newly formed plate boundary. These erupt in a deep submarine, extensional setting to form an extensive tract of new supra-subduction zone lithosphere which replaces the foundering oceanic lithosphere, and ultimately forms the forearc region of a new island arc. With time, the subducting

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oceanic crust gradually develops a down-dip motion, leading to eventual stabilization of a typical intraoceanic arc magmatic system 100-300km from the trench. Being at the 'front' of such arc intra-oceanic arc systems, the boninitic terrain is the likely collider that will overthrust any passive margin arriving on the subducting plate. It is hardly surprising, therefore, that the volcanic sections of many Phanerozoic ophiolites are boninitic, and do not represent normal spreading ridge-generated oceanic crust. Most boninitic suites, especially low-Ca boninites, are dominated by glassy, autobrecciated lavas with the petrographically distinctive phenocryst assemblage olivine-clinoenstatite-(ortho)enstatite and very Cr-rich, Al-poor chromite. The value of ophiolitic' boninitic suites in ancient fold belts is that they provide both a relative polarity of subduction that preceded collisional emplacement of the boninitic forearc, as well as useful information about the age and thermal structure of the oceanic crust involved at subduction initiation. They may also well record major episodes of global plate boundary reorganisation, such as the 55-50Ma suites from around the Western Pacific margin, including those from the Bonin-Mariana forearc. Cape Vogel in Papua New Guinea, and in New Caledonia. Not all intra-oceanic arcs develop in response to major changes in the global plate circuit. For example, more localised reorganisation or initiation of subduction may result from collision of a major submarine plateau with a trench, such as that involving the collision of the Ontong Java Plateau with Solomons arc between -25 and lOMa. This collision produced a flip in the orientation of subduction, with W-directed subduction of Pacific Oceanic crust being replaced by E-dipping subduction of Oligocene oceanic crust of the South Fiji (backarc) Basin. The new subduction regime led to production of the Vanuatu intra-oceanic island arc. Boninites are absent in such cooF arc initiation settings and the forearc region will consist of the

2009 International Conference on Island Arc-Continent Collisions: Macquarie Arc Conference, Orange, New South Wales, Australia oceanic lithosphere substrate upon which the new arc is constructed. Volcano Spacing and Parental Arc Magmas Within <10 m.y. of subduction initiation, W Pacific intra-oceanic island arcs appear to stabiHze their magmatic systems such that the volcanic front occurs around 100-150km above the Benioff Zone. Early near-trench lithospheric spreading type magmatism that generated boninites in the Bonin-Mariana arc was replaced by focussed or point source' magmatism which with time evolved into individual volcanoes of the proto-Mariana Arc. The spacing of volcanoes in intra-oceanic arcs varies from a remarkably constant 90km between each of the seven southernmost volcanoes in the Vanuatu arc, through typical values of 40-70km for other intra-oceanic arcs (eg., Solomons, S Sandwich arc), to values between 20 and 35km for arcs with a history of repeated splitting and construction of new arcs upon the rifted remains of earlier arc and backarc basin crust (eg., Mariana, N Tonga arc, Izu-Bonin arc). However, even in arcs built on thick continental crust, such as the Central Volcanic Zone in Chile, for which peak frequency between volcanoes is 10-30km, there is still a strong suggestion that the spacing between magmatic systems being provided from the mantle wedge is 75-110km. This spacing likely reflects gravitational (Raleigh Taylor) instability in the mantle wedge and appears unrelated to plate convergence velocity. Gradual (probably episodic) thickening of new arc crust via magmatic additions leads to development of volcano clusters and a progressive diminution of the volcano spacing. Between arc volcano edifices in arcs with the maximum volcano spacing (eg., S Vanuatu arc), it is unlikely that volcaniclastic aprons from adjacent arc volcanoes will coalesce, so crust along the axial volcanic between volcanoes is probably the oceanic crust upon which the young arc was constructed. As arcs mature and arc crust thickens, reduced spacing between arc volcanoes and the development of volcano clusters leads to overlapping volcaniclastic aprons. As most W Pacific arcs develop in systems characterised by trench rollback and significant extension in the over-riding, arc-bearing plate, extensional basins between volcanoes and behind the volcanic axial chain receive mass flow volcaniclastics derived from sector collapse of arc volcanoes, and significant vitric ash component from subaerial eruptions.

There is little argument now that the parental magmas of most arc magmas are high-Mg picritic (olivine-rich) basalts with possibly as much as 20% MgO. In most well developed intra-oceanic arc systems, however, strong fractionation of olivine and clinopyroxene occurs in subarc magma chambers, such that erupted magmas are rarely more magnesian than 8%MgO, Many arc basalt magmas eventually crystallise plagioclase, leading to widespread eruption of high-Al island arc basalts. Histograms of Si02 content of intra-oceanic arc lavas reflect very well the evolutionary development of the arc. For example, in the young Vanuatu arc and the entirely submarine Hunter Ridge arc, such a histogram shows a strong peak in the basalt - basaltic andesite range, with andesites poorly represented and a small dacitic peak reflecting occasional caldera eruptions from stalled magma chambers. Notably magnesian basalts (>8% MgO) are well represented. In intra-oceanic arcs that have a longer developmental history and thicker crust, such as the N Tongan or Mariana arcs, basalts are relatively evolved, and the main Si02peak is in the andesite range, with relatively common but still subordinate more felsic lavas. Mature arcs, especially those built on rifted continental crust such as the NE Honshu arc or Sumatra, are dominated by andesites and dacites, with abundant and often large volume rhyolitic eruptions. Thus in ancient arc systems accreted into continental crust during fold belt development, histograms of Si02 content are an excellent indicator of the maturity of the arc involved. An often overlooked characteristic of arc magmatic products in the overwhelming occurrence of fragmental volcanic rocks relative to coherent lava flows. This is in part due to the hydrous, potentially explosive nature of many arc magmas relative to their intraplate counterparts, but also because of the propensity of arc stratovolcanoes to sector collapse, and of arc calderas to erupt catastrophically. This observation derives largely from the exposed islands of intra-oceanic arcs, but it needs to be kept in mind that such exposed islands may represent perhaps as little as 10% of a single arc volcanic edifice, and that this may be the most evolved and thus most explosive part of the magmatic system. Dredging of the submarine cones along the Late Miocene Hunter Ridge between Vanuatu and Fiji has yielded dominant basaltic lavas, with lava breccias and volcaniclastic units far less well represented than in subaerial parts of the adjacent Vanuatu arc.

Geological Society of Australia Abstracts No 92

Arc - Backarc Basin Relations Most Western Pacific arc systems involve significant trench rollback and episodic arc rifl:ing and development of backarc basins, usually subparallel to the arc axial chain. Rollback may be impeded by the arrival of a significant bathymetric ridge at the trench, so that rotation of the arc platform is pinned down and unzipping' of the arc cannot proceed beyond the collision zone. If such a collision involves the almost orthogonal arrival of a submarine ridge composed of discrete volcanic cones (eg., Louisville Ridge colliding with the N Tongan arc, or the Marcus Necker Ridge colliding with the N Mariana arc), localised shoaling of the forearc and forearc erosion may occur. Where more substantial positive features such as oceanic plateaux or microcontinental ribbons like the Lord Howe Rise arrive at a trench, significant shoaling of the forearc will occur, coupled with development of fiexural basins astride the arc axial chain. In the central Vanuatu arc, collision of the West Torres Massif submarine plateau produced massive forearc uplift, behind which the north and south Aoba fiexural basins developed, and the immense basaltic shield volcanoes of Aoba and Ambrym grew above deep mantle-tapping faults. Both these volcanoes contain abundant, strikingly olivine-rich picritic lavas, presumably due to enhanced opportunity for parental magmas to erupt little modified in this setting. Where backarc basins open subparallel to the arc volcanic axial chain, such as the Mariana and Tonga-Kermadec arcs, early backarc basin lavas are arc-like basalts. This may be because their mantle source regions are at this earliest stage of rifting and spreading still in proximity of slab-derived fluids implicated in typical arc magmatism. Alternatively, it may be because the developing backarc ridge system effectively pirates the arc volcano flux (such as at the southern end of the Valu Fa Ridge in the Lau Basin, where this ridge has apparently pirated magma from the nearby Ata arc volcano, so that basalts erupted at this unambiguous spreading centre are typically arc-like in major and trace element composition. As continued spreading widens the backarc basin, basalts erupted at backarc spreading centres approach typical mid-ocean ridge basalt compositions. Arclike basalts also occur at the eastern end of the Woodlark Basin spreading centre, which is on the plate currently subducting to the northeast beneath the northern Solomons arc. The explanation of this

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peculiar occurrence is that the mantle involved in regional extension and backarc basin magma genesis in this complex setting was part of the mantle wedge of an earlier island arc, and thus modified by ingress of slab-derived metasomatic components. In ancient arc-backarc basin systems accreted into fold belts, arc-like basalts that erupted at a backarc spreading centre should be easily identified as such because of their association with abundant (perhaps sheeted) dykes, that reflect this extensional tectonic setting. Terminal Arc Magmatism and Post-Collisional Magmatism The Fijian arc system offers an outstanding picture of the tectonic and magmatic development of an intra-oceanic arc following the demise of arc magmatism. Oligo-Miocene arc lithosphere of the Fijian section of the long Vitiaz Arc was rotated anticlockwise away from the trench facing subducting Pacific crust, probably commencing around 12-10 Ma. This corresponds in time with the hard docking of the Ontong Java Plateau and arrival of the complex Melanesian Border Plateau at the Vitiaz Trench that led to the flip in subduction polarity beneath the northern half of the Vitiaz Arc. Anticlockwise rotation of at least 11S"" of Fijian arc lithosphere resulted in its fragmentation, as well as a short period (7-3Ma) of subduction of S Fiji Basin oceanic crust, producing the Hunter Ridge arc. This fragmentation generated linear, often orthogonally organised extensional zones or 'lineaments' through the Fijian arc lithosphere that are demarcated by major volcanoes, such as Raki Raki and Tavua along the northern part of Viti Levu. These volcanoes erupted dominantly basaltic lavas of shoshonitic and high-K calc-alkaline affinity, mainly in the period 5-3Ma. These post-subduction, extensionrelated high-K and shoshonitic mafic lavas appear to form the K-rich end of the compositional spectrum defined by the earlier arc-related basalts, andesites and related rocks. Isotopic data for the shoshonitic lavas indicate no continental input into their source region, which appears to have undergone the same supra-subduction zone metasomatism as arc lavas of the preceding Vitiaz Arc magmatism. We interpret the generation of these lavas as extension-related decompression of subarc lithosphere, which, when extension accelerates, leads to breakup of the arc lithosphere and nucleation of a new backarc basin spreading centre.

2009 International Conference on Island Arc-Continent Collisions: Macquarie Arc Conference, Orange, New South Wales, Australia Post-collisional magmatism in Western Pacific settings is widespread but not well documented. Following S-directed emplacement of the massive forearc ophiolites' of Papua New Guinea, major shoshonitic volcanoes such as Mts Lamington and Trafalgar have been built on the ophiolite', but appear to be unrelated to contemporaneous subduction. Similarly, following diachronous Late Eocene - Oligocene emplacement of the giant New Caledonian forearc ophiolites in a belt that can be trace from New Caledonia to Northland (NZ), submarine, post-collisional shoshonitic (basalt and andesite) volcanoes were erupted in early Miocene in a belt to the west of the ophiolite (Loyalty Iss). In pre-Tertiary fold belt settings, post-collisional lava sequences may be mistakenly identified as subduction-related arc volcanics. Their occurrence in generally linear belts dominated by typical orogenic series' (basalt-andesite-dacite-rhyolite) lavas imparts a very arc-like character to such suites. It is critical that regional geological evidence should complement geochemical data in determining the arc vs postcollisional ancestry of calc-alkaline and shoshonitic suites in fold belts. One such valuable line of evidence is the composition of detrital chromites in sandstones interstratified with fold belt calc-alkaline lavas. The 500 Ma Mt Reid Volcanics in western Tasmania were long regarded as a subduction-related arc suite until the discovery in interbedded sandstones of very Crrich, Al-poor detrital chromites of obvious boninitic derivation demonstrated that these volcanics postdated emplacement and erosion of the 514 Ma boninitic ophiolite that outcrops in the same region.

Adakites Lavas termed adakites occur in numerous arc systems. Despite their petrographically being typical hornblende andesites to dacites, adakites have peculiar geochemical features (essentially high Sr, low Y and low heavy REE) that suggested derivation via partial melting of abnormally hot, young, subducted oceanic crust. In several arc systems for which adakites were initially described (not including, ironically, the type area in the Aleutian arc), regional tectonic settings (ie., subduction of young, hot oceanic lithosphere) are appropriate for this petrogenetic model. However, adakites have been increasingly frequently recognised in arcs lacking such tectono-magmatic ingredients (and even in some locations manifestly unrelated to subduction). This has not stopped regular claims of ancient adakite suites relating to subduction of young oceanic crust. Recent studies have shown that some adakites may be due to partial melting of eclogitic or amphibole eclogite lithologies in fold belt basement packages. Probably more widespread still are broadly adakitic lavas that have compositions appropriate to mid- or deep crust fractionation of otherwise typical calcalkaline arc lavas, with the fractionation scheme dominated by amphibole and involving apatite. As such, these adakite-like lavas record compressional episodes in which arc lavas are less successful at moving relatively rapidly to eruption, or to low-P subvolcanic differentiation sites where typical olivinecpx-plagioclase-FeTi oxide fractionation imparts the usual calc-alkaline character of such lava suites.

Geological Society of Australia Abstracts No 92

ORDOVICIAN MAGMATIC EVOLUTION OF THE MACQUARIE ARC, NEW SOUTH WALES A. J. Crawford^ S. Meffre\ R. J. Squired L. M Barron' and T. J. Falloon' ^ ARC Centre of Excellence in Ore Deposit, (CODES), University ofTasnnania, Private Bag 79, Hobart,Tas. 7001, Australia. ^ School of Geosciences, Monash University, Vic 3800, Australia. ^ 7 Fairview Place, St Ives, NSW 2075, Australia.

The first recorded igneous activity of the nowfragmented Macquarie Arc in central New South Wales began in Early Ordovician (LancefieldianBeendigonian) times. In the westernmost JuneeNarromine Volcanic Belt, mafic to intermediate lavas of the Nelungaloo Volcanics, intruded by monzonite at 481 Ma, pass up into sandstone, conglomerate and siliceous siltstone of the Lancefieldian to early Bendigonian Yarrimbah Formation. Further east, in the northern Molong Volcanic Belt, the Mitchell Formation consists of volcaniclastic conglomerate, sandstone and siltstone with minor lavas and is succeeded conformably by volcaniclastic granule conglomerate, sandstone and siltstone of the middle-late Bendigonian Hensleigh Siltstone. This earliest phase of volcanism in the Macquarie Arc is dominated by high-K calc-alkaline to shoshonitic evolved basalt, basaltic andesite and andesite, but felsic lavas with Si02 >63% are absent. Age-corrected £ Nd values for five lavas range from +6.2 to +7.8, indicating an absence of any old continental crustal component in the petrogenesis of these volcanic rocks. The high-K calc-alkaline to shoshonitic nature but mantle isotopic compositions of these Early Ordovician lavas recall post-collisional lavas in modern intra-oceanic arc settings. We have no record of any earlier arc magmatism but high-K calc-alkaline and shoshonitic lavas dominate the later stages of the slightly older (SOOMa) postcollisional Mt Read Volcanics in western Tasmania and correlates in western Victoria and south of the Broken Hill Block in western New South Wales.

values (+6.9 to +7.8) and volcanic facies suggesting eruption in an intra-oceanic arc stratovolcano; lavas in the fault-bounded Parkes Volcanics in the Junee-Narromine Volcanic Belt are compositionally identical to those in the Cargo block, suggesting that similar Phase 2 Middle Ordovician arc-type lavas may underlie the Cowra Trough; (ii) medium- to high-K dioritic to monzodioritic intrusions in the Narromine and Cowal Igneous Complexes of the Junee-Narromine Volcanic Belt have ages that cluster in the 470-460 Ma interval, and intrude presumed Phase 1 lavas and volcaniclastics; and (iii) in all three main volcanic belts. Middle Ordovician lavas range from medium-K to dominantly high-K calc-alkaline compositions with a clear trend to shoshonitic compositions late in the Phase 2 magmatic episode. Phase 2 units in the Molong Volcanic Belt (lower Blayney, Byng and lower Fairbridge Volcanics) and Rockley-Gulgong Volcanic Belt (Rockley and lower Sofala Volcanics) are dominated by significantly more unfractionated high-MgO lava compositions than contemporaneous lavas in the Cargo block or JuneeNarromine Volcanic Belt, suggesting that rifting of the arc had occurred by this time, and that the main extensional zone lay along the eastern side of the Macquarie Arc. Identical compositions of unusual shoshonitic ultramafic lavas in the Byng Volcanics of the Molong Volcanic Belt and the Rockley Volcanics of the Rockley-Gulgong Volcanic Belt provide strong evidence that these volcanic belts were once contiguous and were disrupted during SilurianDevonian opening of the Hill End Trough.

Early Ordovician (Phase 1) magmatism in the Macquarie Arc was followed by a magmatic hiatus of - 9 million years, between late Bendigonian and early Darriwilian (i.e. between ca 475 and ca 466 Ma). Resumption of magmatism in the Middle Ordovician produced Phase 2 rocks, recorded by three major rock suites: (i) medium-K calc-alkaline lavas in the Cargo block (Molong Volcanic Belt) have primitive eNd

Phase 3 magmatism in the Macquarie Arc is represented by a widespread but relatively small volume magmatic event, dominated by shallow intrusive rocks of the Copper Hill Suite, emplaced in the Eastonian-Bolindian, between 456 and 441 Ma. These distinctive porphyritic dacites and associated holocrystalline diorites and granodiorites show medium-K calc-alkaline compositions, and their

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2009 International Conference on Island Arc-Continent Collisions: Macquarie Arc Conference, Orange, New South Wales, Australia emplacement was intimately linked to an episode of regional uplift, erosion and limestone deposition in the Junee-Narromine Volcanic Belt and western Molong Volcanic Belt. Phase 4 magmatism extended from late Eastonian or Bolindian until Early Silurian time, and was dominated by relatively evolved (compared with Phase 2 lavas) shoshonitic lavas until the end of the Bolindian and porphyries in the Early Silurian. Collision-related shut-down of the arc, and initiation of arc extension and dismemberment.

occurred around 438 Ma in the latest Ordovician. Post-arc magmatism during the Early Silurian is represented by high-Th, high-Nb lavas of the shoshonitic Nash Hill Volcanics in the JuneeNarromine Volcanic Belt, and Alaskan-type zoned ultramafic intrusions of the Fifield complexes farther west. The latter were emplaced through deformed Ordovician turbidites of the Girilambone Group and their radiogenic isotope signatures show strong crustal involvement.

Geological Society of Australia Abstracts No 9 2

THE ARCHITECTURE OF THE "BETSIMISARAKA SUTURE ZONE"; A RECORD OF OCEANIC ARCS AND ASSOCIATED METASEDIMENTARY SUCCESSIONS BETWEEN THE "INDIAN" AND "AFRICAN" PARTS OF MADAGASCAR. B. de Waele^ ^ M.S.A. Horstwood^ P.EJ. Pitfield^ R J . Thomas^ R.M. Key^ M. Rabarimana^ J-M. Rafahatelo^ V. Ralison^ T. Randriamananjara^ ^SRK Consulting, 10 Richardson Street, West Perth, WA6005, Australia ^NERC Isotope Geosciences Laboratory, Keyworth, Nottingham NG12 5GG, UK ^British Geological Survey, Keyworth, Nottingham NG12 5GG, UK ^Projet de Gouvernance des Ressources Minieres, Madagascar tPreviously working for the British Geological Survey

Madagascar is made up of three Archaean crustal fragments; the 2.5 Ga-old Antananarivo Domain in the west, and the 3.2+2.5 Ga-old Antongil and Masora Domains in the east. During the late Neoproterozoic East African Antarctic Orogen, a series of Neoproterozoic terrains were juxtaposed with these cratonic assemblages; in the north the -750-720 Ma-old Bemarivo Domain docked with the Antongil and Antananarivo Domains, while to the south a series of late-Neoproterozoic Domains (Vohibory, Ondroyen, Anoysen) were accreted to the Antananarivo Domain. The accretion of the Bemarivo Domain against both the Antananarivo and Antongil Domains shows that these latter two domains were juxtaposed by -520 Ma, the timing of peak metamorphism related to the docking event (e.g. Buchwaldt et al. 2003). In between the Antongil-Masora and Antananarivo Domains, a series of granulite-grade graphite-bearing metasedimentary units and intrusives occur, in which abundant small pods of mafic and ultramafic rocks are recognised (e.g. Collins, 2006 and Raharimahefa & Kusky, 2009). The graphitic nature and presence of mafic-ultramafic pods, broadly aligned along a north-south trend within the metasedimentary units prompted various authors to interpret these units as part of a suture zone, the Betsimisaraka Suture Zone (see Collins, 2006 and references therein). However, this interpretation was underpinned only by the interpretation of these granulite-facies lithological assemblages to comprise an ophiolitic melange, and by detrital zircon U-Pb SHRIMP data on two samples, one collected south of the Masora Domain (one zircon analysis with age 811 Ma) and one just

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south of the Antongil Domain (seven analyses between 832 and 709 Ma, ColHns et al, 2003). In this paper we present additional detrital zircon U-Pb Laser Ablation Multi-Collector Inductively Couple Plasma Mass Spectrometry and detrital zircon U-Pb Sensitive High Resolution Ion MicroProbe data for twelve samples of metasedimentary units between the Antananarivo and Antongil-Masora Domains. The Manampotsy Group occurs to the west and north of the Masora Domain. It is comprised of a series of paragneiss sequences making up six formations, broadly from south to north the Ampasary, Perinet, Sakanila, Ambatondrazaka, Sasomanangana and Bealanana Formations. The relative position of these within the group is ambiguous due to the intense deformation, and it is therefore possible that several of those are laterally equivalent. We collected data for three of those Formations, the Ampasary, Perinet and Bealanana Formations. The Manampotsy Group is dominated by quartzofeldspathic gneisses, interpreted to have been derived from felsic volcanoclastic protoliths, but also contains mafic paragneisses with biotite, hornblende, garnet, graphite and sillimanite as well as minor calc-silicate units. Lenticular pods of mafic and ultramafic rocks are a characteristic feature of the group, and its outcrop pattern largely overlaps with the Betsimisaraka Suture Zone. Three samples from the Ampasary Formation immediately west of the Masora Domain show a dominant contribution from Mesoarchaean sources (3.2-3.0 Ga) with minor modes at 2.7 and 2.5 Ga. The maximum age of deposition is given by the youngest

2009 International Conference on Island Arc-Continent Collisions: Macquarie Arc Conference, Orange, New South Wales, Australia

analysis at -780 Ma, which is in agreement with the sparse data from Colhns et al. (2003). Two felsic metavolcanic units from the Perinet Formation north of the Masora Domain yielded ages of 800 and 790 Ma, while one paragneiss gave a nearunimodal population at 840 Ma, interpreted to reflect a dominant volcanic mode. No zircons older than 840 Ma were recognised suggesting that the Perinet Formation was deposited aside an oceanic arc. Ages on orthogneiss units within the Perinet Formation overlap with the maximum depositional age bracket of 840-790 Ma, suggesting that they form part of the base of these oceanic arcs. Metamorphic zircon rims at 560 Ma provide a minimum depositional age for the Perinet Formation. One quartzite and three paragneisses were dated of the Bealanana Formation west of the Antongil Domain, and all show near-unimodal age distributions indicating derivation from active arcs with ages between 830 and 770 Ma, similar to the age range observed for the Perinet Formation. As was the case for other units of the Manampotsy Group, no older zircons were recognised, in keeping with the interpretation that the Bealanana Group, too, represents the volcaniclastic product of active oceanic arcs. Metamorphic zircon rims in the Bealanana Formation are dated at -510 Ma, giving a slightly younger minimum depositional age than the successions further south. The Ambatolampy Group is a patchily preserved supracrustal package that rests tectonically on rocks of the Antananarivo Domain. It occurs to the west of the Manampotsy Group, from which appears to always be separated by Archaean orthogneisses of the Antananarivo Domain. The Ambatolampy Group is, however, lithologically quite similar to the Manampotsy Group, and is comprised of various felsic and mafic paragneisses and quartzites. Three quartzites of the Ambatolampy Group gave detrital age patterns dominated by modes at 2.7 and 2.5 Ga, suggesting source terranes within the Antananarivo Craton. One sample yielded an additional important mode at 1060 Ma,

corresponding to the age of the regionally restricted Dabolava Suite further west (e.g. Tucker et al., 2007). The youngest zircon has an age of 1056 Ma, providing the maximum age of deposition of the group. Abundant metamorphic (high U) zircon rims indicate an overprint at 560-540 Ma, providing a minimum age estimate. The detrital age data obtained as part of this study indicate the deposition of sedimentary and volcaniclastic successions along the margins of the Antongil-Masora and Antananarivo Domains, as well as deposition of volcaniclastic-dominated successions alongside oceanic arcs well away from these Archaean crustal domains. The detrital age modes in the Ampasary Formation clearly show derivation from the Masora Domain, but going west, this Archaean contribution disappears, to give way to zircon exclusively derived from active volcanic arcs between 840 and 770 Ma. These Neoproterozoic oceanic arc successions can be recognised along a NS-oriented belt corresponding to the Betsimisaraka Suture Zone, and which is decorated with lenses of maficultramafic rocks, which could represent preserved parts of an ophiolitic melange. To the west of this, the Ambatolampy Group contains detrital zircons only reflecting source terrains from the Antananarivo Domain. The metasedimentary successions of central Madagascar therefore present a compressed cross sectional view of the ocean that existed between the Indian and African blocks prior to the assembly of Gondwana. REFERENCES

BUCHWALDT, R. TUCKER R.D. & DYMEK R.F. 2003. American Mineralogist, 88, pp. 1753-1768. COLLINS A.S., KRONER A., FITZSIMONS I.C.W. & RAZAKAMANANA T. 2003. Tectonophysicsy 375, pp. 7 7 - 9 9 . COLLINS A.S. 2006. Gondwana Research, 9, pp. 3-16. RAHARIMAHEFA T. & KUSKY T . M . 2009. Gondwana Re-

search, 15, pp. 14-27.

TUCKER R.D., KUSKY T.M., BUCHWALDT R. & HANDKE M.J. 2007. Gondwana Research, 12, pp. 3 5 6 - 3 7 9 .

Geological Society of Australia Abstracts No 92

MESOZOIC 'ROCAS VERDES' MARGINAL BASIN, SOUTHERNMOST ANDES: TECTONIC SETTING AND IMPLICATIONS FOR INTERPRETATION OF OLDER ARC-BACK ARC SYSTEMS Ian W. D. Dalziel Institute for Geophysics, Jackson School of Geosciences, University of Texas at Austin

The existence of the Late Jurassic-Early Cretaceous 'Rocas Verdes' fossil marginal basin of the southernmost Andes was demonstrated over thirty years ago. Its tectonic setting along the convergent western boundary of the South American plate and adjacent to the South Atlantic seafloor spreading centre has several implications for understanding of supra-subduction zone arc-back arc basin systems globally, particularly those of pre-Mesozoic age.. First, the existence of the Rocas Verdes basin demonstrates that Marianas-type (steep) and Chilean-type (shallow) subduction zones have not always been characteristic of the western and eastern Pacific Ocean basin, respectively, as they are at present. Indeed there may have been alternation of the two types across the ocean basin. Second, extension and the initial formation of oceanic crust in the Rocas Verdes marginal basin preceded seafloor spreading in the South Atlantic Ocean basin. The Rocas Verdes rifting may have propagated from the opening Weddell Sea into a zone of extension above the Pacific margin subduction zone of proto-Andean Gondwanaland. Thus the associated magmatism apparently was not directly related to supra-subduction zone processes.

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Third, the mid-Cretaceous closure of the marginal basin and the inversion of its floor in the initiationofe compressional Andean orogenesis took place after the initial phase of seafloor spreading in the South Atlantic Ocean basin. Rather, it coincides with the speed up of the westward motion of the South American plate relative to the Atlantic Ocean-Indian Ocean hotspot reference frame. It appears that the initiation of spreading in an ocean basin behind the marginal basin merely added to the area of the overriding plate, it was forward motion of that plate towards the downgoing oceanic lithosphere that resulted in the collapse of the marginal basin under compression at the converging plate boundary. The tectonic setting of the Rocas Verdes basin in time and space, well documented by the seafloor spreading record in Mesozoic times, therefore suggests that the regional tectonic significance of pre-Mesozoic arc-marginal basin systems identified in the geologic record may be open to several equally viable interpretations.

2009 International Conference on Island Arc-Continent Collisions: Macquarie Arc Conference, Orange, New South Wales, Australia

GEOLOGY OF THE COPPER HILL PORPHYRY DEPOSIT, CENTRAL WEST NSW V. David^*, P. BurrelP & G. Coianiz^ ^OZ Minerals Limited, level nine, 31 Queen street, Melbourne, Victoria, 3000. ^ Exploration services Pty Ltd, PO Box 31, Cowra, NSW, 2794. ^ Golden Cross Resources, 22 Edgeworth David Avenue, Hornsby, NSW 2077.

Copper Hill was the first copper mine in New South Wales with production commencing in 1845 and lasting until 1931. Recognition that Copper Hill was one of several porphyry systems in eastern Australia led to active exploration programs throughout the 1970 s and early 1980 s by numerous exploration companies. Copper Hill deposit contains 133 million tonnes at an average grade of 0.32% copper and 0.28 grams per tonne gold at a 0.2% copper cut-off grade.

system are interpreted to provide extensive post porphyry mineralisation offsets, locally placing mineralised porphyries overlying barren volcanic rocks. The southwestern margin of the Copper Hill deposit is delineated by the Western Fault Zone which displays possible scissor displacement in a probable left lateral sense, up-lifting the northeastern block (Copper Hill) and down-throwing the south western block (Vale Head).

The Copper Hill Cu-Au porphyry deposit is one of several mineralised intrusive porphyry complexes, associated with rocks of Late Ordovician age (Fairbridge Volcanics), emplaced in the Molong Zone of the Macquarie volcanic arc. At Copper Hill deposit, a microdiorite-quartz diorite-tonalite/dacite complex of low-K calc-alkaline suite intrudes basaltic andesite country rocks (volcaniclastics and lava, porphyry). At the northwestern margin of the porphyry system, intrusions of dacite into limestone have formed a small skarn deposit at Little Copper Hill.

Prograde hydrothermal alteration of initial K-Na alteration has been overprinted by subsequently propylitic (quartz-chlorite-magnetite-chalcopyritebornite style, with rare epidote) or phyllic (quartzsericite-carbonate-pyrite-chalcopyrite) alteration assemblages (Ashley, 2006). A partly concentric alteration zonation was recognised by Torrey & Burrell (2006):

The NNW trending porphyry Cu-Au mineralisation at Copper Hill, which is up to 800 m long, up to 450 metres wide and extends to a depth of over 400 metres, is characterised by overprinting, multiphase porphyry intrusions with associated varying alteration, vein and mineralisation styles, which are overprinted by telescoped epithermal alteration and minerahsation (Corbett, 2006). The pre-mineral intrusions have undergone passive mineralisation by mineralisers and both intrusions are overprinted by post-mineral barren intrusion of fresh tonalite/dacite, which diluted mineralised system The main structural fabric is characterised by NNW trending, easterly flat dipping normal (listric) faults. Differential movement along listric structures was accommodated with the development of steep ENE trending faults, accompanied by strike-slip and dip slip movement. However, the flat dipping normal (listric) faults responsible for unroofing the porphyry

1. Pre-mineral porphyry bodies and andesitic volcanic rocks, on the margins of the system exhibit an epidote-chlorite-calcite±haematite assemblage (ECC) and at the contact between the volcanic and intrusive rocks, a biotite-magnetite hornfels (BMT). The dominant sulphide mineral is pyrite, which occurs as disseminations and veinlets occupying 1-5% of the rock. 2. Intra-mineral porphyry bodies at the centre of the system display a pervasive sericite-chlorite-calcite assemblage (SCC) with disseminated and veinlet pyrite and minor chalcopyrite. 3. Surrounding the core of the Copper Hill hydrothermal system is a zone of pervasive sericitechlorite-magnetite alteration (SCM), which hosts several zones of intense sheeted "M" veins and stockwork "B" veins, which contain centreline chalcopyrite and locally bornite. This zone hosts the highes grades of copper and gold, generally, over 0.5g/t gold and 0.5% copper. 4. The south western portion of the hydrothermal

Geological Society of Australia Abstracts No 92

system is characterised by a pervasive sericite-silicaclay (kaolinite) assemblage (SSC or argillic), which hosts open, quartz-chalcocite-pyrite veinlets and chalcocite disseminations. This zone is interpreted as a deeper development of the supergene zone adjacent to the major fault (Western Fault Zone). Carbonate-dominant veins are observed to cross-cut smoky quartz-magnetite veins and also host minor chalcopyrite mineralisation. In general, mineralisation is characterised by porphyry-style quartz-magnetite sheeted veins and stockwork related to an early phase of magmatic, potassic alteration. Mineralisation occurs mostly within quartz veins of the M (laminated quartzmagnetite-chalcopyrite-bornite), B (centrally terminated combination of quartz with pyritechalcopyrite-bornite infill) and C (overprinting pyrite-chalcopyrite-bornite fill breccias) vein styles, as described in the geological literature. These veins contain chalcopyrite, pyrite, gold and rarely, the palladium mineral merenskyite. At the north-eastern margins of the mineralised system weak Pb-Zn mineralisation (Boundary zone) occurs, indicating typical porphyry metal zonation. In summary. Copper Hill is a large, multiphase porphyry system, in which well-mineralised, early porphyry phases have been intruded and disrupted

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by a series of weakly mineralised, intra-mineral porphyry bodies and unroofed by normal listric faults. Mineralisation formed during early intrusive stages was subsequently disrupted and assimilated into later, less mineralised intra-mineral porphyries (Torrey & Burrell, 2006). This has resulted in high grade material being diluted, leading to the formation of a large, but relatively low grade porphyry deposit. Potential remains for the discovery of additional in situ high grade remnants (apophyses) of the early mineralisation or in the discovery of mineralisation concealed by later tectonic offsets, by a combination of listric NNW striking faults and downthrown blocks, bounded by ENE faults. REFERENCES: ASHLEY, P. 2006. Petrographic report on the drill core and

drill chips from the Copper Hill Project, Molong, Centralwest NSW, l-112p CoRBETT, G. 2006. Comments on the geology, arising from a brief inspection of the Copper Hill Project, New South Wales, Australia. OCR report pp. 1-26. TORREY, C & BURRELL, P. Geology and mineralisation at Copper Hill area. Mines and Wines Conference , Cessnock.

2009 International Conference on Island Arc-Continent Collisions: Macquarie Arc Conference, Orange, New South Wales, Australia

GEOLOGY OF NEW GUINEA Hugh L. Davies' Earth Sciences, University of Papua New Guinea, PO Box 414, University NCD, Papua New Guinea. ^Currently seconded to the PNG Mineral Resources Authority

Introduction New Guinea lies across the northern margin of Austraha and has the outhne of a giant bird flying westward (Fig.l). It is the second largest island in the world, 2200 km long and up to 750 km wide, and one of the most mountainous with peaks to almost 4900 m above sea level. A central mountain range runs the length of the island and is bounded to the north by lesser mountain ranges and plains, and to the south by a broad plain. The Mamberamo and Sepik rivers drain the north side of the central range, and the Digul and Fly rivers drain the south. Beyond the southern plains a broad shallow shelf extends to the Australian coast. Other shorelines are steeper and some are bounded by deep sea trenches (Fig. 2). Small ocean basins lie to the northeast and southeast and a great submarine plateau (Ontong

Java Plateau) lies to the extreme northeast, beyond the islands of the Bismarck Archipelago. Smaller submarine plateaus lie south of the bird s tail. Politically, the island is divided between the independent state of Papua New Guinea (PNG) in the east and Indonesia in the west, with a boundary that coincides, for the most part, with the 141®E meridian. The western half was known as Irian Jaya and is now known as Papua and Western Irian Jaya (Western Irian Jaya is the bird s head and neck). The population of PNG is 6 million and is dominantly Melanesian. The population of the Indonesian provinces is 2.1 million and comprises 60-70% indigenous Melanesian and 30-40% migrants from Java, Sulawesi and Ambon.

Geological Setting

Figure 1. Physiographic map of New Guinea. OJP Ontong Java Plateau, EP Eastern Plateau, PP Papuan Platform. Seafloor topography from Smith, W. H. F., and D. T. Sandwell, Global seafloor topography from satellite altimetry and ship depth soundings, Science 277:1957-1962 (1997); http://topex.ecsd.edu/marine_topo/mar_topo.html. (Drawn by R. Betuela; copyright R. Betuela and H.

Davies.)

Geological Society of Australia Abstracts No 92 New Guinea is at the interface between the northward-moving Australian plate and the WNWmoving Pacific Plate (Fig. 2). The resultant motion is convergence at a rate of 110 mm/yr on an azimuth close to 070o. Convergence has led to a succession of collisions of the Australian craton with the microcontinents and volcanic islands of the Pacific and with fragments of the craton that had been separated from the craton and then docked again. While the southern half of the island was always part of the Australian continent, the northern part has been built up by successive collisions. In geological terms, the southern part is autochthonous and the northern part allochthonous, being made up of accreted terranes.

deep sea trenches, and transform faults and onshore by thrust, extensional and strike-slip faults, and folds. Earthquakes are located on the microplate boundaries (Fig. 3) and volcanic activity is associated with the deep sea trenches and spreading ridges.

Convergence of the Pacific and Australian plates has caused the development of a number of microplates; these are bounded offshore by spreading ridges.

Measurements by Stevens et al. (2002) showed that the bird s head is moving WSW at a rate of 93 mm/yr. This is almost the same motion as the Pacific Plate (110

11A H I CHgiocene diorltic intrusion and volcsnics 11 H Nelogerte dioritie inlrusioos 10A 03 sepilc CofDplex 10 Paleogene to mdi Mo icene arc-type wolcanlcs 8 m Cretaceous and Md i -Eocene oceanfloorbasalts Mo icene limestone and sediments (Nl. NB, FR) 8 H C>phwtile

nil Umestone ie H I Teco l mte - Sorong f««lt jone

Neotectonics In northwestern New Guinea the oblique convergence between the Australian and Pacific plates is accommodated in three ways: by left-lateral strike-slip motion on the fault systems that connect the Bismarck Transform in the east with the Sorong Fault in the west; by subduction at the New Guinea Trench; and by transpressional folding and faulting in the fold belt and in the Mamberamo Basin.

Mesozoic and Cenorioc metamorptci rocks

5 E3 Papuan Fold Belt

68 1 1 Mesozoci to md i del Mo i cene sediments 3B [ 1 Papuan Basin Mesozoic to Cenozoic sediments 6A [ 3 to Mo i cene sediments SB CD Aure Fold Belt (aocrelionary prism In part) 5A S3 Lenflflufu FoW &etl

Figure 2. Geological map of New Guinea. AB Am Basin; AFB Aure fold belt; B Bougainville; BB Bintuni Basin; BK Biak; BT Bismarck Sea Transform; C Cyclops Mountains; CB Cenderawasih Bay; PR Finisterre Range; G Gauttier Range; GR Grasberg Mine; KT Kilinailau Trench; L Lihir Island (mine); LFB Lengguru Fold Belt; M Manus; MB Manus Basin; Ml Misool; MT Manus Trench; MU Mussau; NB New Britain; OT OkTedi; P Porgera; PT Pocklington Trough; R Rabaul; SB Salawati Basin; SF Sorong Fault; ST Seram Trench; T Timor Trough; TTTrobriand Trough; W Wau; WA Waipona Basin; WB Woodlark Basin; WM Wamena; WN Wandamen Peninsula; WO Waigeo; WT Weyland Thrust; Y Yapen. (Drawn by R. Betuela; copyright H. Davies and R. Betuela.)

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2009 International Conference on Island Arc-Continent Collisions: Macquarie Arc Conference, Orange, New South Wales, Australia

130°

Figure 3: Earthquakes stronger than M 5 in the period 1963-2004. Focal depths as follows: Red <50km, yellow <100 km, green <200 km, blue <300 km, purple <400 km, brown <500 km, grey >500 km. (Map byEmile Okal.)

mm/yr) and suggests that the western part of the New Guinea Trench is locked intermittently. This motion has caused the opening of Cendrawasih Bay, the development of Waipona Basin, and the development of the Lengguru Fold Belt. The lithosphere of the Caroline Sea is subducted at the New Guinea Trench. Seismic tomography shows the subducted slab to dip at a shallow angle and to extend beneath the island of New Guinea to near the line of the south coast (Tregoning and Gorbatov, 2004). If this interpretation is correct then the igneous activity in the Papuan Basin fold belt, including the Grasberg and Ok Tedi intrusive rocks, can be seen as slab-related, rather than related to slab break-off as was suggested by Cloos et al. (2005). A shallow-dipping slab that is partly coupled to the upper plate also would explain the transfer of convergent motion for 400 km from the line of the New Guinea Trench to the southern front of the fold belt. In northeastern New Guinea, collision between the Finisterre and Sarawaged ranges and the Bismarck volcanic arc causes uplift of the north coast of the Huon Peninsula at (averaged) rates of 1-3 mm/yr (Chappell, J., 1974, Geological Society of America Bulletin 85:553-570). Study of raised coral terraces

on the peninsula yielded a high-quality record of fluctuations in sea level during the Late Quaternary. The same convergence causes the Finisterre mountain mass to ride southward and results in down-warping of the northern end of the Papuan peninsula, which is subsiding at a rate of 5 mm/yr. In eastern New Guinea, active sea floor spreading in the Woodlark Basin is advancing westward and causes north-south extension of the mainland and adjacent islands. One result is the emergence in the Pliocene of domes and half-domes of metamorphic rocks by low-angle extensional faulting in the islands and on the mainland (Abers, et al, 2002, Nature 418:862-865). Another is the opening of small rift basins offshore. Spreading within the last 1.2 Ma has caused the separation of Misima Island from a position adjacent to Woodlark (Muyua) Island (Taylor, B., et al., 2002, Nature 374:534-537).

Economic Aspects Oil is produced from the Salawati Basin and a large volume of gas is to be developed in the Bintuni Basin beneath Bintuni Bay. In PNG oil and gas are produced from structures in the fold belt. Copper and gold are produced from major mines at Grasberg and Ok Tedi, and gold from Porgera and Lihir

Geological Society of Australia Abstracts No 92 Island. Gold at Hidden Valley, near Wau, will start production in 2009, Gold and massive base metal sulfides associated with seafloor hydrothermal activity in the eastern Bismarck Sea are at advanced exploration stage. Much of this abstract for Macquarie Arc Conference 2009 is drawn from Davies (2009).

737-752. MAHONEY, J., FITTON, G., & WALLACE, P. & LEG 1 9 2 SCI-

ENTIFIC PARTY. 2001. JOIDES Journal

27(2) pp. 2 - 6 .

PARRIS, K. 1996. Central Range Irian Jay a Geology Compilation 1:500 000 scale geological map. Jakarta, P.T. Freeport Indonesia. PIGRAM, C.J., & DAVIES, H.L. 1987. BMR Journal

Relevant literature

Austra-

lian Geology and Geophysics 10 pp. 193-211.

CLOOS, M . , SAPIIE, B., QUARLES VAN UFFORD, A., W E I LAND, R.J.,WARREN, P . Q . & MCMAHON, T . P . 2 0 0 5 .

P. A. 2002, Australian Journal of Earth Sciences 49. pp.

Geologi-

QUARLES VAN UFFORD, A., & CLOOS, M . 2 0 0 5 .

Ameri-

cal Society of America Special Paper 400.

can Association of Petroleum Geologists Bulletin 89 pp.

DAVIES, H.L. 1990. In: G.J. and Z. Carmen, eds.. Petroleum Exploration in Papua New Guinea: Proceedings of the First Papua New Guinea Petroleum Convention, Port Moresby,

STEVENS, C . W . , MCCAFFREY, R., BOCK, Y . , GENRICH, J.F.,

pp. 2 4 5 - 2 6 9 .

DAVIES, H.L., 2009. In: Gillespie, R. & Glague, D., eds..

Encyclopedia of Islands. Berkeley, University of California, pp. 6 5 9 - 6 6 5

Dow, D.B. 1977. Bureau of Mineral Resources, Australia, Bulletin 201. D o w , D . B . , ROBINSON, G . P . , HARTONO, U . , & RATMAN, N .

1988. Geology of Irian Jay a. Geological Research and Development Centre, Indonesia, in cooperation with Bureau of Mineral Resources, Australia. HILL, K . C . , KENDRICK, R . D . , CROWHURST, P . V . , & G o w

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119-140.

PuBELLiER, M. & SURABAYA, C. 2002. American

cal Union Geodynamics Series 30 pp. 87-99

TREGONING, P., & GORBATOV, A . 2 0 0 4 . Geophysical

Geophysi-

Re-

search Letters 31, L13608, doi:10.1029/2004GL020190.

2009 International Conference on Island Arc-Continent Collisions: Macquarie Arc Conference, Orange, New South Wales, Australia

ORDOVICIAN ARC-CONTINENT COLLISION IN THE APPALACHIAN/CALEDONIAN OROGEN John Dewey ^ ^ Maria Mange ^ and Paul Ryan ^ ^ University College Oxford, ^ UC Davis, ^ UC Galway

The mid-Ordovician Humberian/Grampian Orogen of Newfoundland and the Caledonides in the British Isles, like the Miocene Bismarck Orogen of New Guinea, and the early Cretaceous Nevadan Orogen of the Sierra Nevada, probably developed by the collision of a supra-subduct ion zone (SSZ) ophiolite/ oceanic arc (s) with a continental margin followed by a flip in subduction polarity, leading to the addition of oceanic arc complexes to the edges of continents and, hence, continental growth. In the Grampian Orogen, imminent collision is heralded by a switch from mafic to silicic magmatism, fore-arc/successor basins preserve clastic records of collisional events and unroofing of the obducted SSZ ophiolite and underlying metamorphic complexes, ancient zircons from subducted crust appear in the post-collisional arc, and the crust was returned to normal thickness, mainly by extension. The preservation of low-grade rocks in these collisional zones may have been the result of four factors. First, subduction systems commonly show a general subsidence of the overriding lithosphere resulting from the colder negative buoyancy of the subducting slab(s). Second, the subducting, thinned and stretched, continental margins probably contain substantial amounts of rift-related mafic igneous rocks, which if converted to eclogite during continental thickening, would contribute to depression of the orogen and reduce erosion. Third, the 12 km-thick obducted arc/suprasubduction-zone ophiolite/arc nappes had an average density of about 3200 kg.m•^ beneath which the evolving orogens were depressed below sea level. Fourth, the Grampian orogen in western Ireland, and probably the Bismarck and Nevadan Orogens,

enjoyed a short (2 Myr) period of late-orogenic extensional denudation, when only very recentlygenerated staurolite-bearing garnet amphibolites were drawn up beneath an extensional detachment(s) to contribute a pulse of detritus, as the ophiolite/arc hanging wall was drawn down. Subduction flip led to extensional collapse and, probably, delamination/ detachment of the eclogitised Laurentian root, which would have generated uplift of the Grampian core from which the high-level obducted sheet was withdrawn. Collision, polyphase deformation, Barrovian metamorphism, erosion of the obducted arc/SSZ ophiolite nappe, subduction flip, extensional collapse, and the establishment of a continental margin arc with retro-charriage, occurred in less than 20 Myr (478-460 Ma) synchronously with the accumulation of 6 km of clastic sediments a hanging wall basin, the South Mayo Trough, whose detrital heavy minerals and detrital white mica ages record these events precisely. The rapid Barrovian metamorphism (475-468 Ma) of the Dalradian footwall cannot have resulted from crustal thickening and thermal relaxation. Rather, it resulted from the rapid advection of heat into the footwall from the hot obducting arc/ophiolite, probably by the syn-nappe lateral injection of large volumes of mafic magma (Connemara mafic/ultramafic suite). Newfoundland and the Caledonides diffier in that, in the former, the hanging wall arc/ophiolite with a complex precollisional, transtensional, structural/magmatic history is superbly preserved whereas, in the latter, the footwall Barrovian complex is better developed and exposed.

Geological Society of Australia Abstracts No 92

ARC-CONTINENT COLLISION AND THE FORMATION OF CONTINENTAL CRUST: GEOCHEMICAL AND ISOTOPIC RECORDS FROM THE IRISH CALEDONIDES Amy E. Draut', Peter D. Clift', Jeffrey M. Amato' & Jerzy Blusztajn and Hans Schouten^ ^152Torrey Pine Terrace, Santa Cruz, CA 95060, U.S.A. adraut@usgs.gov ^School of Geosciences, University of Aberdeen, Aberdeen AB24 SUE, United Kingdonn ^Department of Geological Sciences, New Mexico State University, Las Cruces, NM 88003, U.S.A. "^Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, U.S.A.

ABSTRACT

Collisions between island-arc terranes and passive continental margins are thought to have been important in the formation of continental crust throughout much of Earth s history. Magmatic evolution during this stage of the plate-tectonic cycle is evident in several areas of the Ordovician Grampian-Taconic Orogen in Ireland: the South Mayo and Connemara terranes and the Tyrone Igneous Complex. Upper and middle crust exposures in these areas record the collision of an intraoceanic island arc with the Laurentian passive continental margin ca. 475 Ma. Geochemical changes accompanying collision included an increase in the degree of light rare earth element (LREE) enrichment and increasingly silicic, evolved volcanism; these changes are due partly to incorporation of continental material into magmatism (reflected in Nd isotopic composition) and partly to crystal fractionation. Notably, LREE enrichment in syn-

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collisional volcanism and silicic intrusions exceeds that of the Dalradian (Laurentian) continental material, which would have been thrust under the colliding forearc and potentially recycled into the magmatic roots of the arc. This implies that substantial crystal fractionation, in addition to magmatic mixing and assimilation, accompanied the formation of new crust in the GrampianTaconic Orogeny, with melt percentages as low as -2%. Because similar super-enrichment of orogenic melts is observed elsewhere in the Caledonides in the British Isles (Scotland) and Newfoundland, the addition of new, highly enriched melt to this accreted arc terrane was apparently widespread spatially and temporally. Such super-enrichment of magmatism, especially if accompanied by loss of corresponding lower-crustal residues, supports the theory that arccontinent collision plays an important role in altering bulk crustal composition toward typical values for ancient continental crust.

2009 International Conference on Island Arc-Continent Collisions: Macquarie Arc Conference, Orange, New South Wales, Australia

THE AGE AND SIGNIFICANCE OF THE PUNCOVISCANA FORMATION WITH RESPECT TO NEOPROTEROZOIC TO CAMBRIAN TECTONIC EVOLUTION OF THE PROTOANDEAN MARGIN OF GONDWANA. Monica Escayola^'^ and Cees van Staal ^^^ CONICET (National Research Council of Argentina). 601 Booth St. Ottawa K1S 4H7, ON, Canada, e-mail: nnescayol@NRCan.gc.ca ^^^ Geological Survey of Canada. 625 Robson Street, Vancouver, V6B 5J3 BC, Canada.e-nnail: cvanstaa@NRCan.gc.ca

We present a new tectonic model for the PampeanTilcarian accretion of the Arequipa-AntofallaWestern Pampia (AA-WP) ribbon continent to the Proto-Andean margin of Gondwana represented by the Amazonia and Rio de La Plata cratons, based on our studies of the Puncoviscana Formation and adjacent units in northern and central Argentina. A compilation of existing detrital zircon ages of the Puncoviscana Formation, and correlative units along strike in the Pampean orogenic belt to the south, combined with our new U-Pb SHRIMP zircon ages of Puncoviscana Formation, which are based on recently discovered felsic tuffs and mafic volcaniclastic rocks 531±3.5 Ma) in the unit s type locality suggests that the Puncoviscana Formation mainly represents an Early Cambrian arc-trench gap to foreland basin succession formed during east-directed closure of a late Neoproterozoic oceanic back-arc basin. The backarc basin, which probably remained relatively narrow, initially had opened behind an east-facing -650-570 Ma island arc (Eastern Pampia arc), built upon the rifted, leading edge of the AA-WP The c. 531±3.5 Ma felsic tuffs are interpreted to represent the products of a new, short-lived Early Cambrian magmatic arc built upon the composite Proto-Andean margin, following Late Neoproterozoic, soft-accretion of

the Eastern Pampia arc and a subduction polarity reversal. Puncoviscana Formation conglomerates and mafic volcanics previously interpreted as early riftrelated deposits are better interpreted as late-orogenic basin fills and/or were deposited after basin closure. Our new U-Pb zircon age of the post-collision Canani Tonalite (516±4.6 Ma), which intruded into Tilcarian deformed Puncoviscana Formation rocks in the north westernmost part of Argentina in the Puncoviscana type locality, combined with the existing 529-517 Ma zircon ages for post-collision peraluminous granites and tonalites in the Eastern Pampean Ranges to the south indicates that the synorogenic Puncoviscana basin formed between 540 and 517 Ma, progressively cannibalizing its orogenic hinterland over time. In addition, the Tilcarian and Pampean orogenies represent the same event. We suggest that AA-WP rifted-off from Laurentia between 700 and 650 Ma, shortly after Amazonia s departure during Rodinia s break-up. We emphasize that it is the departure of AA-WP, not Amazonia that opened lapetus in the Late Neoproterozoic. We also suggest that the Ganderia terrane in the northern Appalachians, originally formed an extension of the AA-WP, but returned later to Laurentia during lapetus' closure.

Geological Society of Australia Abstracts No 92

THE DELAMERIAN OROGEN: THE LAST OF THE PAN AFRICANS John Foden Geology and Geophysics, University of Adelaide, Adelaide, SA 5005, Australia

The Cambrian aged South African Saldanian-, Antarctic Ross- and Australian Delamerian- Orogens formed a continuous belt along the Pacific margin of the then newly assembled Gondwana supercontinent (the start of Peter Cawood s Terre Australis orogen). These contain magmatic rocks that provide the first evidence for Pacific subduction. This initiation of subduction is the consequence of stress transfer to the outboard trailing edge of the newly assembled supercontinent and is the culmination of a progressive "westerly" accretion of Gondwana. This occurred in the Early to Mid Cambrian on completion of Pan African convergence, deformation and subduction along the sutures between eastern and western Gondwanan continental fragments. Prior to this, Neoproterozoic- to Early Cambrian-aged rocks in eastern Australia were formed in a passive margin and recorded dispersion of Rodinia with consequent opening of the proto-Pacific. Our U-Pb and Rb-Sr geochronology shows that in the South Australian (Adelaide Fold Belt) domain of the Delamerian Orogen, convergent orogenesis commenced at 514±3Ma and persisted for ~24my until 490±3Ma, terminated by rapid uplift, cooling and extension in association with post-tectonic magmatism. Our assembly of new and published U-Pb and Ar-Ar geochronology from the entire Ross-Delamerian belt shows that although both the Delamerian and Ross have a synchronous late magmatic and terminal cooling history, the Ross commenced its convergent orogenic history at about 540Ma. This was 25my before Delamerian deformation commenced. The Cape-Saldanian orogeny commenced even earlier (~560Ma). From the Late Neoproterozoic through to the Early Cambrian, eastern Australia was still in extension (or transtension), with the opening of the Kanmantoo Basin and associated anorogenic, largely mafic magmatism in SA and the deposition of the Kara beds and associated volcanics. This basin received sediment from the already exposed Ross Orogen to the south.

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The simultaneous first occurrence of strain fabrics and subduction-related magmatism (including boninite (in Tasmania and the SA Murray Basin), granite and andesitic volcanism) at ~514Ma in New Zealand, Victoria, South Australia, NSW (Mt Wright volcanic - Ponto Group) and Tasmania implies that the Delamerian Orogeny was driven by ridge-push forces transmitted on the initiation of westwarddipping subduction. Subsequent eastwards slab rollback at 490Ma occurred when the new slab had reached the 650Km discontinuity, resulting in upper plate extension and anorogenic "Basin and Rangestyle" magmatism in South Australia and Tasmania (Mt Read belt). The delayed on-set of subduction in the Australian sector of the margin implies that westward motion of the Australian portion of eastern Gondwana continued to be accommodated during the late Early Cambrian by either subduction or deformation in either the Mozambique or at the northern end of the S. Prince Charles Mountains Prydz Bay suture. The transition from Delamerian convergent deformation to apparent extension at 490Ma is associated with the invasion of the middle crust by granite and mafic magmas and with the development of apparently extensional migmatite complexes (Core Complexes ?). These are best illustrated by outcrops on the south coast of Kangaroo Island. This post-Delamerian phase is associated with rapid uplift and exhumation, in SA delivering 3-4 kbar metamorphic complexes to the surface at the same time as producing A-type granite and somewhat potassic monzogabbro - norite complexes (Black Hill). These imply new influx of hot asthenosphere, with Nd-isotopic compositions and lithophile trace element characteristics suggesting crustally contaminated mantle sources. In Tasmania at the same time, very rapid exhumation produces the Jukes and Owen conglomerates and there is also evidence for rapid exhumation, uplift and cooling in the Ross and Saldanian (Cape) orogens. These features suggest lithospheric delamination with attendant upper

2009 International Conference on Island Arc-Continent Collisions: Macquarie Arc Conference, Orange, New South Wales, Australia mantle contamination. Abundant eclogite xenoliths transported to the surface by Jurassic kimberlite in the Adelaide Fold Belt north of Adelaide may provide samples of this delaminated orogenic keel. In South Australia, the Delamerian orogen s history and behaviour closely mirrors the earlier histories of the various Pan African / Koongan orogenic belts that facilitated the accretionary growth of Gondawana. Across eastern Africa and the Arabian shield numerous, relatively short-lived orogens were terminated by rapid uplift and intruded and sutured by A-type granites. If these are indeed a hallmark of delamination, then it seems likely the Gondwanan accretion may have left a significant footprint in the form of an enriched mantle anomaly.

Geological Society of Australia Abstracts No 92

LEAD ISOTOPE SYSTEMATICS OF ORDOVICIAN TO EARLY SILURIAN PORPHYRY AND SKARN DEPOSITS 'David B. Forster & 'Peter M. Dowries ^ Geological Survey of New South Wales, NSW Department of Primary Industries, PO Box 344, Hunter Region Mail Centre, NSW 2310, Australia.

The Ordovician to Early Silurian Macquarie Arc encompasses a variety of mineralising processes and includes major porphyry-style deposits at Cadia, Northparkes and Copper Hill. However, the nature and evolution of lead incorporated into deposits associated with this arc has received little study subsequent to the work of Carr et al. (1995). Those authors established a plumbotectonic model for the Lachlan Orogen. Here, we use lead isotope data from the CSIRO lead isotope database and new analyses for sulfides from porphyry and skarn mineralisation, that were precipitated as part of the mineralising event, to fingerprint potential source reservoirs of gold and copper for Macquarie Arc. Figure 1 summarises the available lead isotope dataset for the Cadia district. Copper Hill, Cargo and Goonumbla. We indentify several populations from vs data shown in Figure 1. Initial ratio data from Cadia and Goonumbla plot close to the mantle growth curve of Carr et al. (1995) and are similar to other Late Ordovician to Early Silurian porphyry-related deposits. These data include galena from Cadia Quarry (and therefore represent intitial ratios) with lead model ages consisent with the age of mineralisation at Cadia Quarry (438.2 ± 2.7 Ma by ^®Ar/''Ar method — Forster et al. 2004). The Goonumbla skarns contain slightly less evolved (older) lead, consistant with the interpretation that lead and other metals were derived from the Late Ordovician host sequence. Most of the Ordovician-Early Silurian deposits include data with ratios > 18.25. These results represent apparently "young" radiogenic lead, and are not initial ratios. Calculation of Pearsons r for results for radiogenic samples from Cadia (r = 0.96) indicates that all ore sulfides at Cadia were: 1) related to a single source, the Cadia Intrusive Complex, 2) formed during a single event that occurred around 438 Ma, and 3) that subsequent tectonic events did not contribute significant lead and other metals.

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Data from Cadia, Cargo and all analyses from Copper Hill provide evidence for even older (less evolved) lead isotope reservoirs <18.2). These data strongly suggest that lead included in the Macquarie Arc was sourced from one or more mantle-derived reservoir(s) that may be as old as Pre-Cambrian (Fig. 1). Carr et al. (1995) noted that most Ordovician porphyry and skarn systems contain old lead, but neither the results nor tectoncic implications were discussed in detail. These data are consistent with the presence of old oceanic crust and/or "depleted" mantle that underwent partial melt segregration to form that crust. Crawford et al. (2007) proposed that melting of old oceanic crust in the volcanic sequence accounts for the geochemistry of mineralised intrusions at Copper Hill. Blevin (2002) argued that long-lived subduction and mantle metasomatism could account for the potassic, LILEenriched and fundamentally calc-alkaline affinity of many magmatic rocks of the Macquarie Arc. Our data support these interpretations and provide additional evidence for protracted subduction associated with the Macquarie Arc. Some results from Cadia and Copper Hill have 207p,^/204p,^ vs ' ' ' P h m h values that deviate from pure mantle values and thus are anomalous for what was then an interoceanic arc (see Fig. 1). Results for Copper Hill are equivocal with only a weak signature of crustal-derived lead. However, results from Cadia suggest a significant input of lead derived from old crustal reservoirs. Some of these reservoirs may also be as old as the Pre-Cambrian. These data could indicate that sources of old, crustal lead (i.e. old sialic crust) were present at the time of mineralisation during the Late Ordovician to Early Silurian. Incorporation of lead derived from crustal sources during the mineralising event at -440 Ma at Cadia (Forster et al. 2004), is consistent with tectonic reconstructions by Crawford et al. (2007), which suggest that accretion of the allocthonous Macquarie Arc with the east Gondwandan continent occurred at this time. Alternatively, or additionally, given the

2009 International Conference on Island Arc-Continent Collisions: Macquarie Arc Conference, Orange, New South Wales, Australia

low abundance of lead in these deposits (several ppm or less), lead sources from the mantle may have failed to overhwhelm the signature of trace lead associated with fine oceanic sediments sourced from Gondwana. The lead isotope signature of distal sediments should reflect the dominant sources of sediment in the palaeo oceanic basin (Christensen et al. 1997; Yu 2002). This latter interpretation is favoured given that samples containing old crustal lead from Cadia are: 1) from skarns; 2) distal to the interpreted sources of mineralising fluids; and 3) contain a very low abundance of lead. Permission to include lead isotope analyses from previously unpublished studies by the CSIRO Exploration and Mining is acknowledged. Published with permission of the Director, Geological Survey of New South Wales, NSW Department of Primary Industries — Mineral Resources.

REFERENCES

BLEVIN P.L. 2002. Mineralium

Deposita, 37, pp. 87-99.

CARR O.K., DEAR J.A., SUPPEL D.W. & HEITHERSAY P.S.

1995. Economic Geology, 90, pp. 1467-1505.

CHRISTENSEN J.N., HALLIDAY A.N., GODFREY L.V., HEIN, J . R . & READ D.K., 1997. Science 111, no. 5328, pp. 913-918. CHUNJIANG YU (2002). Unpublished MSc Thesis. Washington State University. CRAWFORD A.J., MEFFRE, S., SQUIRE R.J., BARRON L.M. &

FALLOON, T.J., 2007. Australian Journal of Earth Sciences 54, pp. 181-214.

FORSTER D.B., SECCOMBE P.K. & PHILLIPS D., 2004.

Economic Geology 99, pp. 761-788.

15.70 n Analytical Precision

15.60 -

15.50 -

Age of Mineralisation Lead

Old Mantle Lead

15.40 17.70

17.85 A + V

18.15

18.00

Cadia - Big Cadia Cadia - Newcrest data Unnassigned) Cadia - Little Cadia

O • •

206pb/204pb

Cadia - Ridgeway Cadia - Cadia Quarry Copper Hill

18.30

18.45

18.60

O Copper Hill - CSIRO data <S> Little Copper Hill - CSIRO data • Cargo - CSIRO data

Open symbols represent samples with radiogenic lead

Figure 1. Plumbotectonic model for the Lachlan Fold Belt after Carr et al. 1995. Shows data from Newcrest 1998 and data from this study.

Geological Society of Australia Abstracts No 92

CONTROLS ON THE FORMATION OF THE CADIA EAST ALKALIC PORPHYRY AU-CU DEPOSIT NSW: POTENTIAL REACTIVATION OF EARLY BASIN STRUCTURES N. Fox^* A.C. Harris\ D.R. Cooke^ D. Collets & K. Faure^ M R C Centre of Excellence in Ore Deposits, University of Tasmania, Private Bag 79, Hobart,TAS 7001, Australia. ^ Newcrest Mining Ltd, Level 9 600 St. Kilda Road, Melbourne VIC 3004, Australia ^ GNS Science, 30 Gracefield Road, Lower Hutt 5010, New Zealand

The Cadia East deposit occupies a mineralised zone 2km in strike length, 600m in width and over 1500m in vertical extent. Total resources exceed 1.8 billion tonnes averaging 0.5 g/t Au and 0.30% Cu. With 28 Moz of contained Au and more than 5.5 million tonnes of Cu, Cadia East is the largest known concentration of metal in a single deposit in the Macquarie Arc. Mineralisation is spatially and temporally associated with a swarm of narrow Early Silurian, alkalic monzonite and monzodiorite dykes that intruded the Ordovician Forest Reefs Volcanics, a thick succession of volcanic (lavas and sub-volcanic intrusions) and associated clastic rocks (debris flow volcaniclastics, sandstones, siltstones and minor limestones). The distribution of alteration and mineralisation in the Cadia East orebody reflects the palaeohydrology of the hydrothermal system which was strongly influenced by host rock permeability and a protracted history of structural reactivation. Sheeted quartz - calcite - bornite - chalcopyrite ± molybdenite veins, oriented sub-parallel to E-SE trending monzonite intrusions define a high grade resource at depth (Wilson, 2003; Wilson et al., 2007). Pervasive biotite - orthoclase - albite - magnetite ± actinolite alteration envelopes surround these veins forming a zone of calc-potassic alteration. Propylitic (epidote - magnetite - hematite - chlorite) alteration extends laterally away from this calc-potassic core (Wilson, 2003). Distinctive hematite dusting of hydrothermal feldspars occurs throughout the propylitic alteration at Cadia East, a characteristic feature of many alkalic porphyry Au-Cu deposits (HoUiday and Cooke, 2007). Disseminated chalcopyrite ± molybdenite mineralisation and associated biotite - tourmaline alteration occurs in the higher levels of the deposit and is overprinted by an unusual texturally destructive quartz - orthoclase - albite - sericite - calcite ± tourmaline alteration assemblage. The

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broadly stratabound nature of these alteration styles implies that hydrothermal fluid flow was lithologically controlled and strongly influenced by host rock permeability. The broad lateral and vertical extent of this pervasive, high-level feldspar-stable alteration blanket led Holliday and Cooke (2007) to suggest that this may be analogous to the vast zones of near-surface advanced argillic alteration common in calc-alkalic porphyry deposits (referred to as lithocaps by Sillitoe, 1995). The elongated geometry of the Cadia East ore body is unusual for porphyry-style Cu-Au deposits. Orebearing veins at the adjacent Ridgeway deposit are centred on narrow pencil porphyry stocks. This concentrically zoned deposit geometry contrasts the E-W oriented monzonite dikes and associated sheeted veins of the Cadia East deposit. The concentric and radial fracture patterns and intrusion geometries at Ridgeway are attributed to magmatic processes, whereas the linear, more elongate form at Cadia East appears to be more structurally controlled (e.g., Tosdal and Richards, 2001). Geologic cross-sections, isopachs and 3D models of the Cadia East deposit reveal that bedded volcaniclastic sandstones, siltstones, and limestones thicken towards basin bounding faults defining E-W oriented half-grabens. Adjacent submarine volcanic centres located along these structures formed the source to the volcanogenic sediments infilling this basin. NW-SE directed compression during the early stage of the Benambran Orogeny reactivated the basin-bounding faults with a dextral strike-slip sense of displacement. Emplacement of monzonitic dykes and associated high-grade veins may have exploited localised dilational sites in these fault zones. Alternatively, monzonite emplacement may have occurred during the relaxational phase of the Benambran Orogeny in an extensional stress

2009 International Conference on Island Arc-Continent Collisions: Macquarie Arc Conference, Orange, New South Wales, Australia regime (Glen et al., 2007). Extension may have caused emplacement of dikes and sheeted veins and a pronounced elongation of the Cadia East orebody. Stable isotope evidence suggests that hydrothermal fluids were dominantly magmatic derived, and there is little evidence to indicate the involvement of external fluids in the hydrothermal system at Cadia East. Magmatic-derived hydrothermal fluids caused high-grade Cu-Au mineralisation at depth immediately adjacent to the intrusions. Ascent of these fluids into overlying permeable strata resulted in disseminated low-grade Cu ± Au mineralisation in a broadly stratabound replacement style. Development of the shallow, lithologically controlled Cu-Au mineralisation and quartz-feldsparmuscovite alteration were strongly influenced by the original basin architecture. Lateral fluid flow of dominantly magmatic-derived fluids through the upper, permeable parts of the hydrothermal system contrasts strongly with fracture controlled fluid flow at depth, and implies a near-surface environment of mineralisation. Work is on-going to test whether lateral fluid flow occurred in a submarine or subaerial setting. The geometry of the Cadia East orebody is in part inherited from the initial basin architecture in which the Ordovician volcano-sedimentary host rocks were deposited. Emplacement of fluid saturated monzonitic dikes and associated sheeted veins was facilitated by reactivation of the earlier basin

bounding structures during the Benambran Orogeny. At Cadia East, intrusion of ore-related monzonite dykes occurred during the Early Silurian. It appears that the extension that occurred during the relaxation phase of the Benambran Orogeny was instrumental in the emplacement of monzonitic dykes and CuAu mineralisation. This contrasts with many other porphyry deposits of similar size to Cadia East, where compressional tectonic regimes prevailed during porphyry emplacement and mineralisation (Cooke et al., 2005).

REFERENCES

cooKE D . R . , ROLLINGS P . & WALSHE J . L. 2005. Economic Geology, 100, pp. 801-818. GLEN R . A . MEFFRE S. & SCOTT R . J . 2007.

Australian

Journal of Earth Sciences, 54, pp. 385- 415. HOLLIDAY J . R . & COOKE D . R . 2007. In: Milkereit B. ed. Proceedings of Exploration 07: Fifth Decennial International Conference on Mineral Exploration: Toronto, Canada, pp. 791-809.

TOSDAL R. M . & RICHARDS J . P. 2001. In: Richards J. P.

& Tosdal R. M. eds.. Structural Controls on Ore Genesis; Reviews In Economic Geology Volume 14: Littleton, Society of Economic Geologists, pp. 157-181. WILSON A . J . 2003. Unpub. PhD thesis, University of

Tasmania. WILSON A . J . , COOKE D . R., HARPER, B . J . & DEYELL, C.

2007. Mineralium Deposita, 42(4), pp. 465-487.

Geological Society of Australia Abstracts No 92

ARC-CONTINENT COLLISION AND TERRANE ACCRETION IN WESTERN TASMANIDES: INSIGHTS FROM BASEMENT STUDIES ALONG AUSTRALIA'S SOUTHERN RIFT MARGIN G. M. Gibson^ M. P. Morse\ G. Nayak^ & T. R. Ireland'

^Geoscience Australia, PO Box 378, Canberra, ACT 2601, Australia, ^Australian National University, Canberra, ACT2002, Australia Datasets acquired by Geoscience Australia for the purpose of better understanding the basin history and hydrocarbon potential of Australia's southern continental margin have proved equally well suited to studies concerned with basement structure in SE Australia, and crustal architecture beneath the western Tasmanides in particular. This segment of the orogen has been further subdivided into the Delamerian and western Lachlan fold belts which, together, constitute one of the most complete and best preserved records of late Neoproterozoic-early Paleozoic tectonism and terrane accretion along the former Pacific margin of Gondwana (Glen, 2005). Temporal equivalents of these rocks occur in Antarctica and, from seismic reflection studies, are known to extend offshore beneath Australia's Southern Rift System (SRS) where they form basement to sedimentary rift basins of Jurassic-early Cainozoic age (Blevin & Cathro, 2008). Basin geometry and evolution in the SRS reflect successive stages of rifting during Gondwana breakup as well as a strong basement control best delineated in geophysical data interpretations that make greater use of the analytical signal (phase and amplitude). Most obvious in the aeromagnetic images are NW- and north-trending crustal-scale terrane boundaries and shear zones inherited from the underlying pre-Mesozoic Tasmanides and which subdivide the former convergent margin into its constituent structural domains. A few basement structures are common to both Australia and Antarctica, and can be traced along strike to the initiation points of ocean transform faults that not only constrain palaeogeographic reconstructions of the Gondwana margin but reduce the uncertainty with which terranes of similar age in SE Australia and Antarctica can be successfully correlated. The best constrained reconstruction (Willcox & Stagg, 1990; Royer & Rollet, 1997; Hill & Exon, 2004) places northern Victoria Land (NVL), along with formerly contiguous parts of the South Tasman Rise, west of Tasmania thereby aligning Cambrian island arc assemblages in the Bowers (NVL) and Grampians-Stavely terranes (Finn

IGCP 524

et al., 1999), and multiply-deformed continental margin sequences farther west in which west-verging, cratondirected structures predominate (Wilson terrane and Glenelg-Kanmantoo zones) (Flottman et al., 1993; Squire & Wilson, 2005). Continental margin sequences share a common detrital zircon signature (Ireland et al., 1998), preserve a record of metamorphism under intermediate, followed by low, P/T conditions, and were extensively intruded by Cambro-Ordovician granites during the later stages of Delamerian-Ross orogenesis. A major shear zone hosting 516 Ma granitic orthogneiss cuts across the earliest recognised structural fabrics along the western margin of the Delamerian orogen, indicating that this event commenced earlier in Australia than previously recognised and could extend back to 540560 Ma as in Antarctica (Goodge, 2002). Geochemical and isotopic signatures from the granites and their host rocks further indicate that the former continental margin is widely underlain by crust of Proterozoic age (Borg et al., 1987; Handler et al., 1997). Deformation in the Ross orogeny was initially driven by collision of the Gondwana margin with a west-facing island arc (Bowers terrane) during the course of which sections of the passive margin sequence were subducted to mantle depths and metamorphosed under UHP conditions along with their underlying subcontinental lithosphere, now represented by 500 Ma eclogite and sheared ultramafic rocks caught up along the boundary (Lanterman fault zone) between the Wilson and Bowers terranes in NVL (Ghiribelli et al, 2001). The corresponding boundary in the Delamerian orogen is a belt of poorly exposed but highly magnetic and locally intensely sheared ultramafites and serpentinites (Dimboola subzone) occupying a comparable structural position between the Glenelg and Grampians-Stavely zones in Victoria (Moore, 2006). Aeromagnetic images indicate that this belt shares the same NNW trend as the Stavely arc and continues offshore beneath the Otway Basin before terminating against the northsouth-trending Avoca fault. The western boundary of the Dimboola subzone is the Escondida fault (Moore,

2009 International Conference on Island Arc-Continent Collisions: Macquarie Arc Conference, Orange, New South Wales, Australia 2006), one of several east-dipping, crustal-scale shear zones imaged in seismic reflection profiles for this part of Victoria (Korsch et al, 2002). Following collision with island arc rocks, the Gondwana margin evolved from passive to convergent, and subduction was re-established in the opposite sense (west-dipping) driving further arc magmatism in the newly accreted Bowers-Stavely terranes, back-arc extension and renewed 490-480 Ma granite magmatism in the Glenelg and Wilson terranes, and partial exhumation of ultramafites and 500 Ma UHP rocks along the intervening crustal boundary. Quartz-rich turbidites floored by Proterozoic mafic crust in western Victoria (Stawell zone) occupy a fore-arc position and are separated from high P/T metamorphic rocks (including blueschists) of the associated accretionary prism to the east (Bendigo zone) by the Avoca fault (Foster & Gray, 2000). This fault truncates the Moyston fault and other NNW-trending structures in the Grampians-Stawell zone and is the more important crustal boundary. It extends offshore into the Shipwreck Trough and Sorell Basin, south of which it merges into a major transform fault (Tasman fracture zone) along which Australia finally separated from Antarctica (Royer & Rollet, 1997). No corresponding structure or accretionary wedge is evident in NVL although the Stawell zone is readily correlated with the Robertson Bay terrane which similarly consists mainly of quartz-rich turbidites and basaltic rocks. West-directed subduction ceased following choking of the subduction channel by continental crust preserving an earlier record of Meso- to late Neoproterozoic tectonism (Tyennan and Selwyn blocks; Cayley et al., 2002). This crust probably rifted off the continental margin at some time during or subsequent to Rodinia breakup at 830 Ma and on entry to the subduction zone was subjected to further metamorphism, including the development of 500 Ma eclogite and other intermediate P/T rocks now exposed in Tasmania (Meffre et al., 2000). Together with their overlying cover sequences, these rocks were subsequently buried beneath Cambrian ophiolites brought about by another reversal in subduction polarity followed by west-directed thrusting and further docking of outboard terranes from the east (Crawford and Berry, 1992). These events culminated in the mid-Devonian Tabberabberan orogeny and accretion of the Macquarie arc. Exhumation of high grade rocks in Tasmania occurred on extensional structures before final accretion of the eastern terranes.

REFERENCES BLEVIN J . & CATHRO D. 2008. Australian Southern Margin Synthesis, Project GA707, Client Report to Geoscience Australia by FrOG Tech Pty Ltd. BORG S . G . , STUMP E . , CHAPPELL B . W . , MCCULLOCH. M . T . , WYBORN D . , ARMSTRONG R . L . & HOLLOWAY J . R . 1987.

American Journal of Science, 287,127-169. CAYLEY R . A . , TAYLOR D . H . , VANDENBERG A . H . M . &

MOORE D.H. 2002. Australian Journal of Earth Sciences, 49, 225-254. CRAWFORD A . J . & BERRY R . F . 1992. Tectonophysics,

214,

37-56. FINN C . A . , MOORE D . H . , DAMASKE D . , MACKEY T . 1999.

Geology, 27, 1087-1090. FLOTTMAN T . , GIBSON G . M . & KLEINSCHMIDT G . 1993.

Geology, 21, 319-322. FOSTER D.A. & GRAY D.R. 2000. Annual Reviews of Earth

Planetary Science, 28,47-80. GHIRIBELLI B., FREZZOTH M - L . & PALMERI R . 2002. Euro-

pean Journal of Mineralogy, 14, 355-360. GLEN R.A. 2005. Geological Society of London Special Publication, 246, 23-96.

GOODGE J.W. 2002. Royal Society of New Zealand Bulletin, 35, 61-74. HANDLER M . R . , BENNETT V . C . & ESAT T . M . 1997. Earth

and Planetary Science Letters, 151, 61-75. HILL P . J . & EXON N . F . 2004. American

Geophysical

Union

Monograph Series, 151, 19-42. IRELAND T . R . , FANNING C . M . , GIBSON G . M . & PREISS W . V .

1998. Geology, 26, 243-246. KORSCH R . J . , BARTON T . J . , GRAY D . R . , OWEN A . J . &

FOSTER D.A. 2002. Australian Journal of Earth Sciences, 49, 1057-1075. MEFFRE S., BERRY R . F . & HALL M . 2000. Australian

Jour-

nal of Earth Sciences, 47, 971-985. MOORE D.H. 2006. Geological Society of Australia Extended Abstracts, 82. ROYER J - Y & ROLLET N. 1997. Australian Journal of Earth Sciences, 44, 543-560. SQUIRE R . J . & WILSON C . J . L . 2005. Journal

of the

Geologi-

WILLCOX J . B . & STAGG H . M . J . 1990. Tectonophysics,

173,

cal Society, London, 162, 749-761.

269-281.

Geological Society of Australia Abstracts No 92

THE BIPOLAR CHARACTER OF THE TASMANIDES OF EASTERN AUSTRALIA RA.GIen^ ^Geological Survey of New South Wales, NSW Department of Primary Industries, PO Box 344, Hunter Region Mail Centre, NSW 2310, Australia

Because the Tasmanides of eastern Australia were never affected by a continent-continent collision, they represent an excellent region in which to study the processes and geometries that develop during formation of accretionary orogens. The history of the Tasmanides began with Rodinia split up, beginning -750 Ma, followed by passive margin formation until -520 Ma, and convergent margin formation thereafter until 220 Ma (middle Triassic).Younger convergent margin tectonism is now preserved east of the Tasman Sea. The - 3 0 0 million years of convergent margin tectonism is largely represented by long periods of sedimentation, extension and magmatism interspersed by short orogenies, some of which can be attributed to collision/accretion of arcs with back arc basins or collisions of arcs or forearcs with the continental margins. Analysis of the Tasmanides is carried out in two ways: On a temporal ("pre-orogenic") basis by recognition of tectonic cycles or supercycles that are bounded by orogenies; and on a spatial basis by the recognition and analysis of five orogenic belts (and a Permian-Triassic internal foreland basin system) that focus on structural architecture and timing of deformation — the latter displaying a general west to east younging in the ages of their main deformations. There are competing ideas about the locations, criteria and natures of boundaries between these orogens, especially where the acquisition of new geophysical data shows truncations of previously invisible trends but of course no data on ages of deformation (e.g. the eastern boundary of the Delamerian Orogen; southern boundary of the Thomson Orogen).

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The bipolar nature of the Tasmanides, a narrow northern part (North Queensland Orogen, northern New England Orogen) and very much wider southern part (Delamerian Orogen, Lachlan Orogen, New England Orogen)(Figure 1), needs to be taken into account in discussing its accretionary nature. The northern part of the Tasmanides was developed adjacent to Precambrian core of Gondwana. It was developed on Precambrian crust and the vertical stacking of Cambrian to Carboniferous stratigraphy shows little sign of continental growth. The greater width of the southern part is marked by an eastwards stepping out of geological units and supra-subduct ion zone elements that reflects rollback of the palaeoPacific plate (Figure 1). Overlying this are two other processes: rifting of older fragments from the older craton/Gondwana margin and their incorporation as 'inliers' in more outboard parts of the Tasmanides; and the shuffling of major Ordovician turbidite terranes along the Gondwana margin, increasing their east-west width. Resolution of this bipolar character of the Tasmanides requires understanding of geological relations unfortunately concealed beneath Mesozoic strata of central Queensland. However, it suggests the presence of major segmentation in both Gondwana and palaeo-Pacific plates. Published with permission of the Director, Geological Survey of New South Wales, NSW Department of Primary Industries.

2009 International Conference on Island Arc-Continent Collisions: Macquarie Arc Conference, Orange, New South Wales, Australia

granites intruding

Hunter-Bowen Queensland

Orogen

'ilan but with younger intrusives

nly late Devonian Triassic It with earlier Cambrianiarly Devonian histories

Brisbane

wcastle Sydney

Tasman Sea l-S line Neoproterozoic Cambrian, Cambrian deformation

Tabberabberan Benambran Tasmania Delamerian

Lachlan Orogen Mainly Ordoviclan to Carboniferous, some Cambrian

FIGURE 1. TASMANIDES OF EASTERN AUSTRALIA showing approxinnate changes with tinne in locations (present positions shown) of supra-subduction zone elennents identified by cycle / supercycle nannes.

Geological Society of Australia Abstracts No 92

KEY FEATURES OF THE MACQUARIE ARC, EASTERN LACHLAN OROGEN: A 2007 SUMMARY R. A. G\en\ A. J. Crawford^ I. G. PercivaP, D. Cooked S. Meffre^ R. Scott^ L. M. Barron' ^ Geological Survey of New South Wales, NSW Department of Primary Industries, PO Box 344, Hunter Region Mail Centre, NSW 2310, Australia. ^ ARC Centre of Excellence in Ore Deposits, University of Tasmania, Private Bag 79, Hobart,Tas. 7001, Australia. ^ Geological Survey of New South Wales, NSW Department of Primary Industries, Londonderry Geoscience Centre, 947-953 Londonderry Road, Londonderry, NSW 2753, Australia.

This paper summarises key features of the largely andesitic Macquarie Arc as recognised at the end of 2004 from a joint CODES and GSNSW ARCcompany funded project that was subsequently published in 2007 in AJES 54, number 2/3. Key features of the Macquarie Arc are the recognition that its evolution encompasses almost the whole Ordovician and the Llandovery part of the Silurian; its relationship to coeval craton-derived sedimentary Ordovician terranes that constrain its tectonic setting; the nature and timing of its accretion into the growing Lachlan Orogen; its subsequent segmentation by Silurian-Devonian extension; and the rich endowment of gold and copper, mainly in porphyry deposits. Evolution of the Macquarie Arc occurred in four magmatic phases. Phase 1, Early Ordovician calcalkaline andesitic to basaltic magmatism was developed on older igneous crust and was followed after a poorly defined ~9 myr hiatus by calk-alkaline to shoshonitic Phase 2 magmatism in the Middle Ordovician. Phase 2 magmatism was terminated in the western part of the arc by a second hiatus marked by uplift and erosion before subsidence and development of a carbonate platform in the early Late Ordovician (-455-450 Ma). Phase 3 is represented by intrusive rocks that overlap in age with this hiatus. Phase 4 shoshonitic magmatism is divided into an extrusive interval ( - 4 5 6 - 4 4 3 Ma) followed by the emplacement of porphyries at - 4 4 0 - 4 3 7 Ma that is synchronous with deposition of Early Silurian sediments unconformably above Phase 4 arc rocks. Establishment of a subduction vector is complicated by the absence of volcanogenic forearc basin sediments or of a subduction complex from the

eastern Lachlan Orogen. However, the synchronicity between hiatuses in arc evolution and changes in sedimentation patterns in identical flanking passive margin terranes, comprising Early and Middle Ordovician quartz-rich-turbidites overlain by Late Ordovician black shales, suggest the Macquarie Arc lay on the Gondwana plate. In the Early and Middle Ordovician, the arc lay above a west-dipping subduction zone with local rifting in the east. There was a reversion to east-dipping subduction during Phase 3. In the Late Ordovician, the arc was either under transtension or had returned to west-dipping subduction. Accretion of the Macquarie Arc to the 'backarc basin' quartz-rich sedimentary terrane and its incorporation in the growing Lachlan Orogen occurred in the multi-stage Benambran Orogeny in which two short compressive/transpressive deformations at - 4 4 3 Ma and 430 Ma were separated by a basin-forming event synchronous with the shallow emplacement of mineralised and barren porphyries into dilational sites cutting deformed volcaniclastic arc rocks. The presence of a quartzrich sedimentary terrane outboard of the arc is attributed its tectonic shunting along the plate margin, beginning in the early Late Ordovician as recorded by switchover from turbidite to black shale deposition. In a west-dipping subduction zone model, arrival of this sedimentary terrane would have acted as a collider to terminate subduction and to generate compressive stresses in upper plate. Published with permission of the Director of the Geological Survey of New South Wales, NSW Department of Primary Industries.

* This paper will be given before the start of the field trip on 14th April.

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2009 International Conference on Island Arc-Continent Collisions: Macquarie Arc Conference, Orange, New South Wales, Australia

OROGENIC GOLD IN THE CORDILLERAN ACCRETIONARY OROGEN OF WESTERN NORTH AMERICA: CHANGING STRESSES IN THE FORE-ARC AND BACK-ARC R.J. Goldfarb^ 'U.S. Geological Survey, Box 25046, Mail Stop 973, Denver Federal Cer^ter, Denver, CO 80225-0046, USA

Orogenic gold deposits are widespread adjacent to the massive batholith complexes that form the subductionrelated continental magmatic arc of the Middle Jurassic to Eocene Cordilleran orogen along the western edge of North America (Fig. 1). The deposits formed between ca. 150 and 50 Ma, mainly within accreted oceanic arcs and overlapping flysch basins that define the accretionary margin to the craton (Goldfarb et al, 2008). It is this broad process of arc-continent collision, followed by evolution to a more transform margin that appears essential to development of major gold resources within Cordilleran-type orogens. The most productive (i.e., >40 Moz lode and >65 Moz placer Au) part of the northern Pacific Rim margin has been the goldfields of the Sierra foothills province in central CaHfornia, which includes the Mother Lode belt, and Alleghany and Grass Valley districts. Oceanic arcs and overlying turbidites that form the terranes of the foothills were accreted to the North American craton in a series of transpressional events between the Permian and 166 Ma (Snow and Scherer, 2006; Ernst et al., 2008). The allochthonous volcanic and sedimentary rocks range in age from mainly Carboniferous through Middle Jurassic. To the north, the foothills are offset 200 km to the west, where correlative rocks are exposed in the Klamath Mountains of northern California and southern Oregon. The gold deposits occur adjacent to regional fault zones that represent the sutures between the accreted oceanic terranes. They are specifically localized in zones of minimum principal stress and dilation, which include conjugate shears, dilational jogs, granitoid pressure shadows, lithologic contacts, and competency contrasts. Gold formation occurred during two episodes within the accreted arc rocks of the Sierran-Klamath active margin. Along much of the length of the Sierra foothills, gold was deposited between 134 and 115 Ma, but the highly productive Grass Valley district, near the northern end of the province, was generated sometime between 152 and 143 Ma (Marsh et al., 2008; Snow et al., 2008). The latter range also characterizes gold lodes of the Klamath Mountains (Elder and

Cashman, 1992). Almost all gold pre-dates the ca. 120-80 Ma emplacement of the Sierra batholith, the major Andean arc developed a few tens of kilometers seaward of the goldfield along the boundary between the innermost allochthonous blocks and the craton margin. The younger gold episode overlaps a distinct 140-120 Ma magmatic lull in the fore-arc, whereas the older event is coeval with a period of widespread fore-arc plutonism. Regional metamorphism and deformation of the arc terranes were widespread over the duration of ore formation. The older gold event correlates with the still poorly understood latest Jurassic east-west offset of the Sierra foothills and Klamath Mountains. Dickinson (2008) associates the offset with a complex style of fore-arc normal faulting due to Pacific basin plate reorganization. Ores formed along major fault zones on both the east (Klamath) and west (Grass Valley) sides of a displaced belt of highly tectonized melange (e.g., Eastern Hayfork-Calaveras Complex terrane). A change in Farallon-North America plate convergence, likely related to a plume-induced major change in Pacific basin plate configuration (Goldfarb et al., 2007), corresponds to the younger gold event, just prior to onset of batholith emplacement. Associated stress changes initiated dextral strike-slip events along terrane-bounding faults, such as the Melones and Bear Mountain systems, and facilitated the hydrothermal activity leading to gold deposition. The Juneau gold belt in southeastern Alaska presents an analogous spatial setting to that of the Sierra foothills; however, the temporal association between gold and batholith formation is contradictory. The deposits are hosted in a series of terranes with a complex geological history along a contractional to transpressional part of the North American margin (Gehrels, 2000; McClelland and Mattinson, 2000). Rocks along the most landward part of the gold belt, within the Yukon-Tanana terrane are interpreted as middle to late Paleozoic continental margin sequences and overlying Carboniferous-Permian arc rocks, which were rifted from the continent during latest

Geological Society of Australia Abstracts No 92

Paleozoic to early Mesozoic. During this time they were amalgamated with Permo-Triassic clastic, mafic volcanic, and carbonate rocks along their western margin. The two terranes were accreted back onto the miogeocline by Early Jurassic, with the fartraveled Wrangellia-Alexander Paleozoic-Triassic oceanic arc colliding along their seaward margin by Middle Jurassic. Development of the Late JurassicEarly Cretaceous Gravina flysch basin, reflected a transtensional event between the Yukon-Tanana/Taku and Wrangellia-Alexander blocks. Both the footwall and hangingwall zones of steeply dipping thrust faults between Yukon-Tanana and Taku, and Taku and Gravina, became sites of later gold-veining (Miller et al, 2000). All gold deposits along the 200 km length of the Juneau gold belt formed between 56 and 53 Ma, in both shear and tensional vein systems (Miller et al., 1994). The largest resources were concentrated in structurally favorable igneous bodies within a few kilometers of the regional thrust faults; these included Triassic gabbros in the Taku terrane (AJ deposit) and mid-Cretaceous, subduction-related dioritic rocks emplaced into the Gravina belt (Treadwell deposit) and Yukon-Tanana terrane (Kensington deposit). A massive subduction-related batholith, the northern Coast batholith, was mainly emplaced between 70 and 50 Ma and a few kilometers inland of the gold deposits. Regional metamorphism and widespread deformation occurred within the fore-arc rocks from 70-55 Ma. The timing of gold formation in the continental margin fore-arc corresponds temporally to a welldefined major change in plate kinematics in the northern Pacific Basin (Goldfarb et al., 1991). The resulting shift from a mainly contractional to transpressional continental margin regime was associated with initiation of dextral transpression along the terrane-bounding thrust systems and uplift of the batholith and adjacent rocks. The resulting seismic events along the newly established strike-slip regime defined the gold event 15 m.y. after the onset of magmatic arc development. In contrast to the above two gold provinces, gold ores of the Bridge River district in southern British Columbia are situated in accreted oceanic arc rocks, but a few tens of kilometers landward of the subduction-related batholith. The deposits occur along steep thrust faults between the Early Carboniferous to Middle Jurassic arc rocks of the Bridge River terrane and the Late Triassic to mid-Cretaceous arc rocks

IGCP 524

of the Cadwalladar terrane. The two arcs were likely amalgamated before final Early Cretaceous accretion to the continental margin (Umhoefer and Schiarizza, 1996). Deformation, including thrusting, continued in the accreted rocks until about 90 Ma, during final collision of the more seaward Wrangellia oceanic arc. The southern Coast batholith was emplaced as the major continental arc between the Wrangellia and Bridge River terranes throughout the period of deformation. New argon geochronology of the gold veins in the Bridge River district indicate formation at ca. 6864 Ma (Hart et al., 2008), approximately 25 million years subsequent to the final stages of batholith emplacement. The gold event correlates with the onset of dextral strike-slip motion along the regional fault systems landward of the Coast batholith. The dextral slip along the complex Yalakom fault system, which is also the original suture between the Cadwalladar blocks and an inland turbidite basin, was initiated by oblique northward subduction of the Kula plate in latest Cretaceous (Umhoefer and Miller, 1996). This event was likely responsible for the large dilational zone along the older thrust faults that is recognized in the area of the major gold resource (Bralorne-Pioneer deposit) in the district. Although allochthonous oceanic arcs host most of the Jurassic-Eocene gold endowment of the Pacific margin of the Cordilleran orogen, the superimposed continental magmatic arc and the near-trench accretionary prism may, in certain tectonic settings, provide additional scenarios that favor formation of orogenic gold deposit districts. In south-central Alaska, the ca. 66 Ma gold deposits of the Willow Creek district are hosted by the 74-67 Ma southern Talkeetna Mountains batholith (Goldfarb et al, 1997, 2008). This part of the continental margin batholith was emplaced into the Jurassic Talkeetna arc, an extensive oceanic magmatic system built upon the allochthonous Wrangellia terrane and then accreted by the end of the Jurassic. A component of regional dextral strike-slip, localized between the Jurassic oceanic rocks and younger Talkeetna Mountains batholith, was responsible for development of the gold ores within the latter intrusive complex during its initial stages of uplift. Closer to the trench, and only a few million years later, progressive subduction of an Eocene slab window along 2000 km of the northern Pacific rim, was responsible for the development of a series of goldfields in the accretionary prism

2009 International Conference on Island Arc-Continent Collisions: Macquarie Arc Conference, Orange, New South Wales, Australia

between 60 and 50 Ma (Chugach Mountains, Kenai Mountains, Chichagof district). Both settings for these deposits, within the continental arc itself and within the accretionary prism, lack the great gold endowment that is observed within the immediate fore-arc and back-arc settings. Observations from the North American Cordillera indicate that the most favorable location for significant gold resources in an active margin is likely to be 100-200 km inland of the trench and within a few tens of kilometers of the continental arc. This area of crustal thickening represents the orogenparallel belt of maximum heat, as is supported by the voluminous intrusive bodies that define the batholiths. In the northern part of the Cordillera, batholith emplacement pre-dates gold formation, whereas to the south, most of the gold deposition clearly pre-dates subduction-related igneous events. The critical factor is the occurrence of a changing stress regime and resulting seismically-induced hydrothermal events (e.g., Sibson et al., 1988) along transpressionally-reactivated terrane sutures within the hotter parts to the orogen; timing of igneous events has no spatial relation to ore genesis. All gold ores within the oceanic arcs formed 35-80 m.y. after accretion. Nevertheless, the sulfur and gold content of these marine rocks likely play a key role in defining the eventual gold resource in the epigenetic deposits (Goldfarb et al., 1997, 2005).

HART G. J. R., GOLDFARB R. J., ULLRICH T. D., & FRIEDMAN R. 2008. British Columbia Ministry of Energy, Mines, and Petroleum Resources, 2008-1, pp. 47-54. MARSH E. E., GOLDFARB R. J., KUNK M. J., GROVES D. I., BIERLEIN F. P. & GREASER R.A. 2008. Arizona Geological

Society Digest, 22, in press.

MCCLELLAND W . C. & MATTINSON J. M. 2000. Geological Society of America Special Paper, 343, pp. 159-18. MILLER L. D., GOLDFARB R. J., GEHRELS G. E. & SNEE L.

W. 1994. Geology, 22, pp. 203-206.

MILLER L. D., STOWELL H. H. & GEHRELS G. E. 2000. Geological Society of America Special Paper, 343, pp. 193-212. SIBSON R. H., ROBERT F. & POULSEN K. H. 1988. Geology, 16, pp. 551-555.

SNOW C. A., BIRD D. K., METCALF J. & MCWILLIAMS M . 2008. International Geology Review, 50, pp. 503-518. SNOW C. A. & SCHERER H. 2006. International

Review, 48, pp. 4 6 - 6 2 .

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UMHOEFER P. J. & MILLER R. B. 1996. Tectonics, 15, pp.

545-565

UMHOEFER P. J. & SCHIARIZZA P. 1996. Geological Society of America Bulletin, 108, pp. 7 6 8 - 7 8 5

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DICKINSON W . R. 2008. Geosphere, 4, pp. 329-353.

ELDER D. & CASHMAN S. M . 1992. Economic Geology, 87, pp. 1795-1812. ERNST W . J., SNOW, C. A. & SCHERER H. H. 2008. Terra Nova, 20, pp. 394-413. GEHRELS G.E., 2000. Geological Society of America Special Paper, 343, pp. 213-234. GOLDFARB R.J., BAKER T., DUBE B., GROVES D. I., HART C. J. R. & GOSSELIN P. 2005. Economic Geology, 100th An-

niversary Volume, pp. 407-450.

GOLDFARB R. J., HART C. J. R., DAVIS G. & GROVES D. I.

2007. Economic Geology, 102, pp. 341-345.

GOLDFARB R. J., HART G. J. R. & MARSH E. E. 2008. Ari-

zona Geological Society Digest, 22, in press.

GOLDFARB R. J., MILLER L. D., LEACH D. L., & SNEE L. W .

1997. Economic Geology Monograph, 9, pp. 151-190.

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J. 1991, Nature, 354, pp. 296-298

Figure 1: Orogenic gold provinces of the Cordilleran orogen and the major continental margin batholirth complexes.

Geological Society of Australia Abstracts No 92

GOLD IN ASIA: ACCRETED PALEO-TETHYAN ARCS AND DEFORMED CONTINENTAL MARGINS R.J. Goldfarb^&C.J. R. Hart^ ^U.S. Geological Survey, Box 25046, Mail Stop 973, Denver Federal Center, Denver, CO 80225-0046, USA ^Mineral Deposit Research Unit, Dept. of Earth and Ocean Sciences, University of British Colunnbia, 6339 Stores Road, Vancouver, B.CV6T1Z4, Canada

Giant gold deposits are now being exploited throughout central Asia and are being evaluated throughout much of eastern Asia. The high degree of prospectivity reflects, in part, favorable geological settings mainly resulting from the Paleozoic-early Mesozoic suturing of the Siberia, Kazakstania, North China (NCC), South China (SCC), and Malaya crustal blocks along the northern and eastern edges of the Tethys Ocean, with collision of auriferous oceanic arcs to various margins of the amalgamating Precambrian blocks. In addition, the late Mesozoic circum-Pacific plate reorganizations were critical for the unusual scenario of formation of Phanerozoic gold in Precambrian rocks. Despite abundant Archean and Paleoproterozoic rocks in the cratonic blocks of Asian continent, important Precambrian greenstone gold deposits are unlikely to be present. This reflects the fact that exposed Precambrian rocks are mainly the highgrade metamorphic basement and thus typically representative of crustal depths beneath those most likely to host gold ores. The oldest gold ores in Asia are the ca. 750-200 Ma orogenic and epithermal/ porphyry deposits, which are spread across the terranes of the geologically complex Central Asian Orogen. Orogenic gold deposits of the Yenisei Ridge Province (ca. 750 Ma Olimpiada, Sovetskoe) and East Sayan region (ca. 450 Ma Zun Holba) are a part of the broader late Neoproterozoic-early Paleozoic Cordilleran-style orogenic belt occurring along the southwestern Siberian craton. The belt defined by arc accretion along the edge of the ancient Panthalassic Ocean. Subsequent tectonism along the northern Paleo-Tethys Ocean active margin resulted in additional orogenic gold deposits forming at midcrustal levels in accreted arcs and flysch basins during: (1) Ordovician accretionary orogenesis along the margin of the Kazakhstan microcontinent (ca.

IGCP 524

450 Ma Vasilovskoe, Bestobe), (2) similar DevonianCarboniferous events along the active edges of the Siberia craton in the northern Altaids and Patom Highlands (ca. 345 Ma Sukhoi Log, Irokinda), and (3) late Paleozoic terrane accretion onto the amalgamating Siberia-Kazakhstan active continental margin due to closure of the Ob-Zaisan marginal basin. Subsequent Permian strike-slip along terrane sutures of this latter margin was responsible for a continental-scale gold episode (Muruntau, Kumtor, Bakyrchik, Saidu, Olon Ovoot). Some middle Paleozoic orogenic gold deposits, however, on the northern margin of the NCC (Wulashan) may have formed when the block was still close to the northeastern Cimmerian margin of Gondwana and were transported with the block across the PaleoTethys Ocean; they are now located immediately south of the Solonker suture between the NCC and the Central Asian Orogen. In addition to the post-accretionary orogenic gold deposits, the Paleozoic accretionary events included addition of both pre- and post-accretionary porphyry and less widespread VMS deposits to the active margins of the Siberian and Kazakhstan blocks. The early Paleozoic accretionary event along Kazakhstania led to emplacement of oceanic arc rocks containing auriferous porphyry copper deposits such as the Taldy Bulak. The allochthonous arc terranes of the Rudny Altai contain Cambrian, Ordovician (Maikain), and Devonian (Lenionogorsk) gold-bearing polymetallic VMS deposits. Latest Devonian-Carboniferous epithermal and gold-rich porphyry deposits are world-class targets in the preserved shallow levels of both oceanic (Oyu Tolgoi) and continental margin (Almalyk, Tuwu, Kounrad) arc rocks of the Turkestan Ocean, a marginal basin of the northern Paleo-Tethys Ocean that closed during the Devonian to Early Permian along the southern side of Uzbekistan to southern Mongolia. Other

2009 International Conference on Island Arc-Continent Collisions: Macquarie Arc Conference, Orange, New South Wales, Australia important orogenic gold ores in the orogen developed during final closure of the Amurian seaway between Siberia and the NCC in the Jurassic (Boroo). The oldest Asian gold systems related to PaleoPacific Ocean events are those associated with the diachronous Permian to mid-Mesozoic closure of the Mongol-Okhotsk Ocean. Deformation within the turbidites of the basin was associated with formation of Middle to Late Jurassic orogenic gold ores in northern Mongolia (Boroo) and central to eastern Transbaikal (Darasun, Tokur). Early Cretaceous rifting within the basinal sediments and adjacent marine volcanic arc rocks generated shallow intrusions and large epithermal gold deposits at ca. 130-120 Ma (Bamsk, Pokrovskoe, Baley). Arc collision and subsequent extension, also during the Jura-Cretaceous but along the Siberian continental margin further to the north, was associated with orogenic (Natalka) and epithermal (Dukat, Julietta) gold formation throughout northeastern Russia. The Triassic collision between the NCC and SCC was responsible for orogenic gold deposit formation (Baguaniao) throughout terranes of the suture within the western part of the Qinling fold belt. Although their tectonic evolution is still very poorly understood, the reactivated margins of the SCC block contain: (1) Permo-Triassic [?] orogenic deposits to the west (Boka) and south (Hainan Island), (2) Jurassic Fe-Cu-Au skarns (Chengmenshan, Wushan, Tonglushan) and related porphyries to the northeast (lower Yangtze River), and (3) Mid-Mesozoic [?] Carlin-like deposits to the northwest (Laerma) and southwest (Jinfeng). The orogenic gold deposits likely correlate with subduct ion/accretion episodes

along the trailing margins of the SCC, as it was being amalgamated with Asia, whereas the other deposit types indicate subsequent local extensional events in past SCC carbonate-rich platform sequences. Major Early Cretaceous gold provinces in the eastern parts of the southern Siberia, North China (Jiaodong, Qinling), and South China (Jiagnan) cratons represent orogenic gold targets restricted to uplifted, high-grade metamorphic core complexes; these are globally unique in that Phanerozoic gold ores are hosted by Archean-Paleoproterozoic rocks. The giant Cretaceous Ontong-Java mantle plume in the southern Pacific was likely the cause of ca. 125 Ma northern Pacific basin plate reorganization, subsequent reactivation of major fault systems, and this associated remarkable >2000-km-long string of orogenic gold systems that developed along the eastern Asia margin at this time. The uncommon scenario of orogenic gold hosted by high-grade metamorphic rocks requires an exotic reservoir for the hydrothermal systems in eastern Asia because Precambrian rocks were devolatilized billions of years prior to ore formation. Devolatilization of oceanic crust and/or overlying sediment of the Izanagi (±Kula) plate, subducted below the highgrade crustal rocks subsequent to erosion of their Precambrian keels, best explains the Cretaceous ores. The youngest economically important gold ores in Asia are those of the young Pacific arcs, such as the deposits in northeastern Taiwan (Chinkuashih). Neotectonics along the Taiwan-Luzon volcanic arc were particularly favorable for the establishment of these ca. 1 Ma shallow metalliferous systems.

Geological Society of Australia Abstracts No 92

CAMBRIAN ARC-CONTINENT COLLISION DURING THE DELAMERIAN OROGEN EVIDENCE FROM THE KOONENBERRY BELT, NORTHWEST NEW SOUTH WALES. J.E. Greenfield^ KJ. Mills^, R.G. Musgrave^ & PJ. Gilmore^ ^ Geological Survey of New South Wales, NSW Department of Primary Industries, PO Box 344, Hunter Region Mail Centre, NSW 2310, Australia. ^ Geological Survey of New South Wales, NSW Department of Primary Industries, 32 Sulphide Street, Broken Hill, NSW 2880, Australia.

Geological mapping and geological-geophysical modelling of the upper crust in the Koonenberry Belt has provided new insights into the history of a Cambrian volcanic arc system termed the Mount Wright Arc (Scheibner 1972; Sharp and Buckley 2003). The Koonenberry fold-thrust belt is a linear package of Neoproterozoic to Palaeozoic rocks now positioned against the northeastern edge of the Mesoproterozoic Curnamona Province. The geological history of the belt involves Neoproterozoic break-up of the Rodinia supercontinent followed by the development of the Mount Wright Arc outboard of the eastern margin of the newly formed Gondwana landmass (locally Curnamona Province). Subsequent strong deformation during the Late Cambrian Delamerian Orogeny culminated in the cessation of major arc volcanism and the thrusting of volcanic arc elements Against/onto the Curnamona Craton. The Neoproterozoic Grey Range Group, which underlies the Cambrian arc elements, consists of shallow shelf marine sedimentary rocks (Kara Formation) and intercalated alkaline volcanic rocks and related intrusives (Mount Arrowsmith Volcanics). This cover sequence is interpreted to be related to intracratonic rifting during the breakup of the Rodinia supercontinent (Crawford et al. 1997). This rifting event led to the strongly extended oceanic crustal setting that would culminate in Cambrian subduction and volcanism along the Mount Wright Arc. The Mount Wright Arc is associated with three early to mid Cambrian lithostratigraphic groups. Marine turbidites of the Teltawongee Group have a conformable base on the Neoproterozoic Grey Range Group, and thicken to the east, outboard of the Mount Wright Arc. The Gnalta Group includes calc-alkaline bimodal volcanic rocks of the Mount Wright Volcanics (extrusives dated at -510 Ma; Black 2007), interpreted to represent the volcanic

IGCP 524

component of the arc (Mills 1992). However, only a small area of these volcanics is exposed, and the majority of the volcanic pile is interpreted to lie under the Bancannia Trough, a rift-sag basin immediately west of the Koonenberry Belt. To the east of the exposed Gnalta Group is the Ponto Group, a deep marine sedimentary package that includes E-MORB tholeiitic lavas (Bittles Tank Suite) and felsic tuffs. The distal airfall tuffs give a mean age of 511.1 ± 1.7 Ma (Black 2005). The chemistry of the Bittles Tank Suite and pelagic depositional environment of the host sedimentary rocks are consistent with a fore-arc setting within the Cambrian arc system. The Delamerian Orogeny (-504-497 Ma) deformed these early to mid Cambrian arc elements. Structural interpretation suggests there was an initial co-axial shortening stage followed by sinistral transpression and oroclinal folding around the Grasmere Knee Zone. Scheibner and Basden (1998) interpreted the Delamerian Orogeny in the Koonenberry Belt to be a result of eastward-dipping subduction of oceanic crust inboard of the Mount Wright Arc causing westward overthrusting of the Middle Cambrian rocks onto the Gondwana margin, forming the Delamerian Highlands. In their scenario the Koonenberry Belt was interpreted as a collection of exotic Neoproterozoic microcontinents (terranes). However geophysical image reconstruction reveals a jigsaw fit across the Bancannia Trough, as well as a coincident structural fabric that has been rotated ~ 20° about a possible Euler pole to the northwest of the Koonenberry Belt. It is interpreted that this rotation was a result of rifting in the early Cambrian, causally related to westward-dipping subduction and the development of the Mount Wright Volcanic Arc in a transitional intracratonic to oceanic setting.

2009 International Conference on Island Arc-Continent Collisions: Macquarie Arc Conference, Orange, New South Wales, Australia Published with the permission of the Director, Geological Survey of New South Wales, NSW Department of Primary Industries.

MILLS KJ.,

1992. Tectonophysics, 2 1 4 , 57-68.

ScHEiBNER E . 1 9 7 2 . Quarterly Notes Geological Sur-

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SCHEIBNER E . a n d BASDEN H . e d . 1 9 9 8 .

BLACK, L.P. 2005. Geological Survey of New South Wales Report GS2005/745.

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BLACK, L.P. 2007. Geological Survey of New South Wales Report.

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G e o l o g i c a l S o c i e t y o f Australia A b s t r a c t s No 9 2

INVERTED SUBMARINE BASINS HOSTING THE CADIA VALLEY PORPHYRY ORE DEPOSITS, NEW SOUTH WALES: FUNDAMENTAL CONTROLS ON SYSTEM ARCHITECTURE A.C. Harris^* I.G. PercivaP, C.M. Allen^ D.R. C o o k e \ R.M. T o s d a l ^ C. McMillan^ P.D. D u n h a m ^ & D. Collett^ ^ ARC Centre of Excellence in Ore Deposits, University ofTasnnania,TAS, Australia. ^ Geological Survey of New South Wales, Departnnent of Prinnary Industries, NSW, Australia ^ Research School of Earth Science, Australian National University, Canberra, Australia ^ Mineral Deposit Research Unit, University of British Colunnbia, Vancouver, BC, Canada ^ Newcrest Mining Linnited, Cadia Valley Operations, NSW, Australia

Ordovician rocks of the Macquarie Arc include packages of arc-related calc-alkalic to shoshonitic volcano-sedimentary sequences that were deposited over a 50 m.y. history (Glen et al. 2007). In the Cadia Valley, Late Ordovician to earliest Silurian alkalic magmatism was associated with the emplacement of over 50 Moz of Au (Holliday et al. 2002; Wilson et al. 2003; Cooke et al. 2007). Knowledge of the 3D architecture of the volcanic sequences in this district has proven critical to interpreting the palaeogeography, and has provided insights into the volcanic setting of the world-class porphyry Cu-Au deposits at Ridgeway, Cadia Hill and Cadia East. Unravelling the facies architecture of this ancient volcanic succession and associated intrusions has also provided important constraints on the deformation history of the district. Ordovician volcaniclastic rocks of the Cadia Valley are dominated by thick sequences of feldspathic sandstones that are intercalated with, and overlain by, volcanosedimentary breccias with associated lavas, sills and dykes. These rocks are interpreted to have been deposited in an active submarine sedimentary basin. The nature, distribution, and geometry of volcanic and volcaniclastic rocks indicate that volcanic eruptions evolved from effusive to mixed effusive and explosive styles. An early phase of mafic volcanism (Mt Pleasant Basalt Member) was characterized by small volume lavas erupted during the aggradation of arc-derived submarine turbidites of the Weemalla Formation. The Forest Reefs Volcanics and its precursor the Weemalla Formation record a major provenance change from sand and silt derived from distal volcanic centres, to breccias

IGCP 5 2 4

sourced from more proximal extra- and intra-basinal mafic to intermediate volcanic edifices. Volcanism evolved as a relatively low-relief, multiple-vent submarine volcanic complex. The vents comprised mafic to intermediate lava flows, cryptodomes and subvolcanic intrusions (dykes and sills). Stacked lava sequences, including hyaloclastites, massive lavas and their reworked equivalents, are up to 1 km thick, forming significant intra-basinal topography. Erosion and irregular unconformities, including one that is extensive across the district, developed late in the basin history. Explosive volcanism occurred during the late stages of Forest Reefs Volcanic deposition, resulting in the deposition of tuffaceous sediments rich in accretionary lapilli and juvenile clasts. The tuffaceous rocks are interpreted to have formed from sustained subaerial phreatomagmatic eruptions. Good textural preservation of these air-fall deposits, combined with coexisting shallow water faunal assemblages, imply that arc-related volcanism became locally emergent prior to the onset of porphyry-style mineralisation in the latest Ordovician to Early Silurian. The volcanosedimentary strata was tilted and deformed synchronous with and after the intrusion of Cu-Au mineralized porphyry monzonite complexes. The preservation of these ancient porphyry Cu-Au deposits occurred because of post-emplacement processes that exhumed, eroded and, most critically then buried the partly eroded ore deposits beneath Silurian rift-sag conglomerates, sandstones and siltstones (Groome et al. this volume). Our reconstruction of the Cadia district reveals that volcanism, porphyry intrusions and related

2009 International Conference on Island Arc-Continent Collisions: Macquarie Arc Conference, Orange, New South Wales, Australia mineralisation were focused along major structural zones in an active sedimentary basin. This environment evolved in a larger foreland basin system within the Macquarie Arc. In the Cadia Valley, episodic magmatism was a fundamental component of the Late Ordovician-Early Silurian porphyry mineralising events (Wilson et al. 2007). Porphyry emplacement occurred during periods of basin inversion and relaxation. Basin architecture, thermal and deformation history and hydrology all influenced the distribution of hydrothermal alteration and mineralisation assemblages. The nature and distribution of host rocks with contrasting permeability and reactivity profoundly affected the distribution of Cu and Au resources in the district. Orogenesis, including basin inversion, not only helped localize these world-class porphyry ore deposits, but also dictated the tectonic and surficial processes that ultimately lead to their preservation.

REFERENCES C o o K E D . R . , WILSON A.J., HOUSE M . J . , W O L F E R . C . , W A L S H E J.L., LICKFOLD V . , CRAWFORD A . J . 2 0 0 7 .

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Geological Society of Australia Abstracts No 92

FORMATION OF VMS DEPOSITS IN OCEANIC ARCS: ARE SOME LINKED TO ARRESTED SUBDUCTION RESULTING FROM ARC-CONTINENT COLLISION? R. J. Herrington^* & S. Roberts' ^The Natural History Museum, Cromwell Road, London, SW7 5BD, UK. ^School of Ocean and Earth Science, National Oceanography Centre, University of Southampton, Southampton, S014 3ZH, UK

The tectonic settings of many VMS deposits are well defined and classic examples are found forming at modern spreading ridges, which includes oceanic ridges, thickened oceanic crust, sedimented oceanic ridges and continental margin rifts. Modern and ancient arcs are also host to a range of VMS deposits of diverse mineralogies and styles. Rifted arc settings provide some of the best (and largest) examples of the deposit type in the ancient record namely in such settings as: nascent arcs - ophiolites; primitive volcanic arcs; complex oceanic arcs and mature volcanic arcs, with the VMS deposits forming in a range of environments within these structures. In this review, we note a specific association of VMS development and the process of arc-continent collision, well defined in two different examples of the Paleozoic Magnitogorsk Arc of the southern Urals and the Pliocene volcanics of the Banda Arc, Southeast Asia and potentially unrecognised elsewhere. The Paleozoic intraoceanic Magnitogorsk arc of the southern Urals is host to at least 80 VMS deposits VMS deposits are found in fore-arc, arc and rifted arc environments across the arc sequence, with the metal contents apparently reflecting the detailed lithostratigraphic assemblage hosting the deposits (Herrington et al. 2002). The VMS deposits formed in a relatively short time period between 400 and 385Ma, the host volcanic packages temporally well constrained by the biostratigraphic record. The Magnitogorsk arc docked with the passive margin of Eastern Europe with a record of the incursion of the East European Continental Margin eastwards into the subduction zone at the latest around 380 Ma, marked by the high-pressure metamorphism of subducted continental crust in the subduction zone which is later exhumed westwards (Brown et al. 2006). This event coincides with the climax of arc-volcanism with related VMS formation in the overlying arc sequence (Herrington et al. 2002), with the gap between peak

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VMS formation and the high-pressure metamorphism of the continental margin at depth down the subduction zone potentially less than 5Ma. The Pliocene volcanic Banda Arc of Southeast Asia contains the gold-rich VMS deposits of Wetar Island (Scotney et al. 2005). Like the case of the Magnitogorsk arc, there is a strong temporal link between the development of the VMS mineralisation and the arrested subduction process caused by arc-continent collision (Scotney et al. submitted). In the Wetar region the attempted subduction of the Australian Continental margin has similarly resulted in the cessation of subduction, with compression which now may be accommodated along the southward dipping Wetar Thrust, which may be a prelude to arc reversal under the Australian continental margin. In both cases, the host rocks to the mineralization tend to be andesites and basaltic andesites which are characterized by enrichments in Ba, Sr, Rb, negative Nb anomalies and low Ti, Y and Zr compared to N-MORB (Holland, 2004, Elburg et al. 2005). Furthermore, in both cases there may be evidence for a crustal influence in the geochemical and isotopic signature of the host volcanic sequences. On Wetar, there is a marked increase in contamination of the magmatic rocks which host the sulphide deposits from subducted continental-sourced material from the downgoing slab, the progressive contamination indicated by Sr, Nd and HeVHe^ studies (Elburg et al. 2005, Scotney et al. submitted). data from pyroxene phenocrysts in the host lavas at the goldrich Balta Tau VMS deposit in the Urals show more radiogenic signatures suggesting that some of the Sr derived from the subducted slab. A sedimentary/ lower crustal component is also suggested for the host rocks at Wetar (Scotney et al. submitted). Pb isotope data for galena from the Balta Tau deposit

2009 International Conference o n Island A r c - C o n t i n e n t Collisions: M a c q u a r i e Arc Conference, O r a n g e , N e w S o u t h Wales, Australia and from the host lava sequence are consistent with a crustal component to Pb in the magmas and sulphides (Tesalina et al. in review). The formation of these two gold-rich V M S systems may have been the direct result of the arrested subduction process and potentially other arccontinent collision environments may be prospective for this deposit type. This possibility will be discussed.

REFERENCES BROWN, D . , SPADEA, P., PUCHKOV, V . , ALVAREZ-MARRON, J., HERRINGTON, R., WILLNER, A . , HEZTEL, R. & GOROZHANINA, Y. 2 0 0 6 ,

Earth-Science

Reviews 79, pp. 2 6 1 - 2 8 7 ELBURG, M . A . , FODEN, J. D . , VAN BERGEN, M . J. &

ZuLKARNAiN, I. 2005. Joumd

ofVolcanology

and Geother-

mal Research 140, 2 5 - 4 7 . HOLLAND, N.G., 2004, Unpubl. PhD Thesis, University of

Southampton.

ScoTNEY, P . M . , ROBERTS, S., HERRINGTON, R.J., BOYCE,

A.J. & BURGESS, R. 2005. Mineralium

Deposita 40, 7 6 - 9 9 .

Geological Society of Australia Abstracts No 92

ISOTOPIC EVIDENCE FOR PROGRESSIVE MAGMA CONTAMINATION IN ARC VOLCANISM DURING ARC-CONTINENT COLLISION AND THE TEMPORAL ASSOCIATION WITH MASSIVE SULPHIDE DEPOSITS ON WETAR ISLAND (BANDA ARC) R. J. Herrington'* P. M. Scotney\ S. Roberts^ A. J. Boyce' & D. Harrison^ ^The Natural History Museum, Cromwell Road, London, SW7 5BD, UK. ^School of Ocean and Earth Science, National Oceanography Centre, University of Southampton, Southampton, S014 3ZH, UK ^Isotope Geosciences Unit, SUERC, East Kilbride, Glasgow, G75 OQF, UK ^Schreiner Strasse 59, CH-8004, Zurich, Switzerland

Whole-rock and data from volcanic rocks supported by ^He/^He data from sulphides and sulphates in mineralized rocks on the island of Wetar, Indonesia indicate a variable contribution of assimilated crustal material or sediment sourced from the subducted Australian craton to the south, consistent with other Banda arc lavas to the east of Flores. Our data support the idea of progressive arc lava source contamination, with Wetar Island showing the most extreme examples of crustal assimilation in the region. The increased continental contamination is recorded during the Pliocene (Zanclian to Piacenzian) during distinct magmatic events between 5 and 4 Ma, and at 2.4 Ma where ratios in unaltered lavas, with whole-rock data ranges between 5.7 and 9.6%o, increase from 0.707484 to extreme radiogenic values of 0.711068. The earlier of these magmatic events is important in the generation of the hydrothermal systems responsible for the polymetallic sulphide and barite mineralisation recorded on Wetar (Scotney et al. 2005). Samples collected from orebodies exhibit ^He/^He ratios of between 0.5-1.4 R/RA consistent with data reported from nearby Romang. The later of these magmatic events and extrusion of dacites (2.39 ± 0.14 Ma) coincides with the arrival of the Australian Continental margin and its subsequent locking to the outer Banda arc. The incorporation of continental material into the source region below the Wetar Island edifice results in the extrusion and intrusion of the most isotopically contaminated arc rocks anywhere in the Banda arc. Evidence of arc lava contamination with continental and or sedimentary material is documented in numerous arc systems e.g.: Sulawesi; Tyrrhenian

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Sea; Luzon-Taiwan arc and the Sunda Arc. Within the inner Banda arc and Ambon arc of Indonesia, magmatic rocks show mineralogical and geochemical evidence for the assimilation of continental and or sedimentary material with cordierite-bearing ambonites present on Seram and Ambon (van Bemmelen, 1949; Abbott & Chamalaun, 1981; Honthaas et a/., 1999); cordierite bearing andesites recovered from the Lucipara ridge (Honthaas et al.y 1998) and cordierite-bearing magmatic rocks reported on Wetar Island (Kavalieris, 1994). In addition to these petrological characteristics, contamination and assimilation is also documented isotopically. For example, high ®^Sr/®^Sr and low coupled with low Rc/Ra and high lead isotope ratios characterize the inner volcanic Banda arc (Vroon et aU 1996; Elburg et aU 2004). In particular, geochemical data from magmatic rocks in the Banda arc suggests the involvement of sediments in the generation of melts. The development of significant, gold-rich VMS deposits precisely at the onset of entry of the continental margin into the subduction zone (expressed by the extreme arc lava contamination) is highly significant and suggests that this tectonic setting elsewhere may be prospective for such deposit types.

REFERENCES

Geological Resources Development Centre Special Publication 2, pp. A B B O T T , M J . & CHAMALAUN, F . H . 1 9 8 1 . 253-268. ELBURG, M . A . , V A N BERGEN, M . J . & FODEN, J . D . 2 0 0 4 .

Geological Society of America 32(1), pp. 4 1 - 4 4 . HONTHAAS, C . , REHAULT, J., M A U R Y , R . C . , BELLON, H . , HEMOND, C . , M A L O D , J., C O R N E E , J., V I L L E N E U V E , M . ,

2009 International Conference on Island Arc-Continent Collisions: Macquarie Arc Conference, Orange, New South Wales, Australia

N. 1998. Tectonophysics 298, pp. 2 9 7 - 3 1 7 .

VAN BEMMELEN, R.W., 1949. Government Printing Office, The Hague.

HONTHAAS, C . , M A U R Y , R . C . , PRIADI, B . , BELLON, H . &

V R O O N , P . Z . , V A N BERGEN & M . J . , FORDE, E . J . , 1 9 9 6 .

C O T T O N , }., BURHANUDDIN, S., GUILLOU, H . & ARNAUD,

Geological Society (London) Special Publication 106, pp.

C O T T O N , J., 1 9 9 9 .

KAVALIERIS, I., 1994. Unpub Int Company Rpt.

445-453.

S c o T N E Y , P . M . , R O B E R T S , S., HERRINGTON, R . J . , B O Y C E ,

A.J. & BURGESS, R., 2 0 0 5 . Mineralium

Deposita 40,

pp. 76-99.

Geological Society of Australia Abstracts No 92

EVOLUTION OF MACQUARIE ARC MAGMAS FROM HF-0 ISOTOPE AND TRACE ELEMENT SYSTEMATICS OF ZIRCON: TECTONIC AND METALLOGENIC IMPLICATIONS A.I.S. Kemp^ & P.L. Blevin^ ^ School of Earth and Environmental Sciences, James Cook University,Townsville, QLD 4811, Australia. ^Geological Survey of New South Wales, NSW Department of Primary Industries, PO Box 344, Hunter Region Mail Centre, NSW 2310, Australia.

The association of oxidised, alkali-rich arc magmas with giant Cu-Au deposits is well established, yet the origin of the chemical enrichment, how this links to ore metal endowment, and the cause of the variable metallogenic fertility along and across arcs all remain unclear. For example, the heavily mineralised Macquarie Arc of NSW is characterised by an unusually high proportion of high-K to shoshonitic magmatism throughout its 45 million year history. Such magmas can be derived through: 1. delayed melting of enriched mantle, 2. metasomatism of the mantle wedge via fluids or melts contributed by the down-going oceanic crust and/or subducted sediment, or 3. crustal contamination during emplacement. Evaluating the relative importance of these processes, and how they relate to the metal, volatile and S endowment of the shoshonitic magmas, is hampered by the intrinsic hydrothermal alteration of intrusive porphyries, particularly mineralised suites. This complicates the retrieval of primary chemical information from these rocks. Such information is available, however, from robust mineral archives such

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as zircon, which can withstand intense alteration in the mineralising environment and whose isotope stratigraphy preserves the record of magmatic evolution. Here, we report the initial results of an in situ chemical (REE) and micro-isotopic (U-Pb, Lu-Hf, I8q^I6q^ investigation of zircon crystals from intrusive porphyries of the Macquarie Arc, both mineral-laden and barren, using ion microprobe and laser ablation mass spectrometry. Samples include rocks from the Goonumbla (485-440 Ma), Copper Hill (457 Ma) and Cadia (440 Ma) intrusive complexes. The data are used to reconstruct the chemical, relative oxidation state and isotope evolution of the host magmas, to reveal how the porphyries formed, how their sources and the processes that formed them changed or interacted in space and time, and how that information can help to understand the metallogenic fertility of the arc. The implications of the data for the tectonic evolution of the Macquarie Arc and its relation to the juxtaposed Lachlan Orogen are discussed.

2009 International Conference on Island Arc-Continent Collisions: Macquarie Arc Conference, Orange, New South Wales, Australia

AUSTRALIAN ISLAND ARCS THROUGH TIME: GEODYNAMIC IMPLICATIONS IN THE ARCHEAN AND PROTEROZOIC R. J. Korsch, N. Kositcin & D. C. Champion Onshore Energy and Minerals Division, Geoscience Australia, GPO Box 378, Canberra, ACT 2601, Australia

Beginning in the Archean, the continent of Austraha evolved to its present configuration through the accretion and assembly of several smaller continental blocks and terranes at its margins. Australia usually grew by convergent plate margin processes, such as arc-continent collision, continent-continent collision or through accretionary processes at subduction zones. The accretion of several island arcs to the Australian continent, through arc-continent collisions, played an important role in this process, and the geodynamic implications of some Archean and Proterozoic island arcs recognised in Australia will be discussed here. The operation and extent of modern-style plate tectonics in the Archean is controversial, though subduction and terrane accretion models have been proposed for most Archean cratons, including both the Yilgarn and Pilbara Cratons of Western Australia. These models are equivocal, however, as data are at best fragmentary, ambiguous and open to various interpretations. Possibly the best example of an Archean island arc (or primitive continental arc) is preserved within the Mesoarchean (3120 Ma) Whundo Group in the Sholl Terrane, West Pilbara Superterrane (Smithies et al. 2005). The Whundo Group is represented by an essentially continuous succession of mafic and intermediate volcanic rocks that include calcalkaline basalt, andesite and dacite, tholeiitic basalt, boninite-like rocks, highMg andesite and, towards the top of the succession, adakite and Nb-enriched basalt. The Whundo Group is thought to have accreted to the northwestern margin of the East Pilbara Terrane some time between 3120 and 2950 Ma. Younger, Neoarchean, island arc terranes, and associated accretion, have also been proposed for the Yilgarn Craton. A number of geological terranes have been recognised in the eastern Yilgarn Craton, which, based principally on geochemistry, have been interpreted to represent ca. 2720-2700 Ma oceanic arcs built on older rifted arcs and continental fragments (Barley et al. 2007; Gee & Swager 2008). An

island arc environment has also been suggested for 2715 to 2690 Ma mafic to intermediate magmatism (diorite and andesite) within the Saddleback greenstone belt in the southwest Yilgarn Craton (McCuaig et al. 2001). In the early Proterozoic, in the Central Zone of the Halls Creek Orogen, northern Western Australia, the Tickalara Metamorphics have been interpreted to represent an island arc, with a minimum age of -1865 Ma (Sheppard et al. 1999; Tyler & Sheppard, 2006). The subduction zone was inferred to be east dipping and closure of the ocean and accretion of the island arc to the Western Zone of the Halls Creek Orogen occurred at -1850-1845 Ma. SHRIMP U-Pb zircon geochronology suggests that arc magmatism appeared to have operated for at least 20 million years, and using a typical modern day convergence rate of 5 cm y r \ this would imply that about 1000 km of oceanic crust has been subducted. In the southwest Gawler Craton in southern Austraha, the St Peter Suite (1620-1608 Ma), of juvenile I-type calcalkaline tonalite to granodiorite, possibly represents island arc magmatism that developed well outboard of the current Gawler Craton, due to a subduction zone located south of the Gawler Craton, which dipped to the south under lithosphere that is now preserved in Antarctica (Swain et al. 2008). It is likely that subduction ceased at about 1608 Ma when the St Peter Suite magmatic arc collided with the Gawler Archean nuclei. In the Musgrave Province in central Australia, age and geochemical constraints are poor due to later overprinting tectonic events, but felsic orthogneisses with a maximum age of -1607 Ma, possibly represent juvenile felsic crust that was emplaced though subduction-related processes into an oceanic island arc (Wade et al. 2006). A south-dipping subduction zone operated from at least ca 1607 Ma to the north of the Musgravian island arc, with an oceanic backarc (marginal sea) to the south of the island arc. Consumption of the oceanic crust led to the suturing

Geological Society of Australia Abstracts No 92

of the island arc to the North Austrahan Craton, with the Chewings Event in the southern Arunta Region (having zircon met amorphic ages from about 1594 Ma to 1562 Ma) being located in a backarc setting north of the main collision zone. Following initial contact, polarity of the subduction zone flipped, with the oceanic crust of the marginal sea being consumed in a north-dipping subduction zone that dipped beneath the Musgravian island arc, which was being sutured to the North Australian Craton. Arc magmatism continued in the Musgrave Province until the marginal sea was consumed and the South Australian Craton collided with the expanded North Australian Craton. Subduction of oceanic crust probably commenced at about 1607 Ma and possibly ceased at about 1565 Ma; using an average plate motion of 5 cm yr ^ a total of about 2100 km of oceanic crust would have been consumed at the south- and north-dipping subduction zones. Thus, the interpretation of several rock units in the Archean and Proterozoic as fossil island arcs has led to the development of geodynamic models which suggest that Australia evolved to its present configuration through the accretion and assembly of several smaller continental blocks and terranes at its margins, due to ocean-basin closures and arccontinent collisions.

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REFERENCES

BARLEY M. E., BROWN S. J., KRAPEZ B., CASSIDY K. F., CHAMPION D. C. & KOSITCIN, N. 2007. Geoscience Australia, Record, 2007HA, 14-17. GEE M. A. M. & SWAGER C. Editors 2008. Precambrian Research, 161,1-199. McCUAIG T. C., BERN M., STEIN H., HAGEMANN S. G., McNAUGHTON N. J., CASSIDY, K. F., CHAMPION D. C. & WYBORN L., 2001. Geoscience Australia, Record, 2001/37, 453-455. SHEPPARD S., TYLER I. M., GRIFFIN T. J. & TAYLOR W. R. 1999. Australian Journal of Earth Sciences, 46, 679-690. SMITHIES R. H., CHAMPION D. C., VAN KRANENDONK M. J., HOWARD H. M. & HICKMAN A. H. 2005. Earth and Planetary Science Letters, 231, 221-237. SWAIN G., BAROVICH K., HAND M., FERRIS G. & SCHWARZ M. 2008. Precambrian Research, 166, 283-296. TYLER 1. M. & SHEPPARD S. 2006. Geoscience Australia, Record, 2006/16,51-52. WADE B., BAROVICH K., HAND M., SCRIMGEOUR I. 8c CLOSE D. 2006. Journal of Geology, 114, 43-63.

2009 International Conference on Island Arc-Continent Collisions: Macquarie Arc Conference, Orange, New South Wales, Australia

THE CENTRAL ASIAN OROGENIC BELT: WHAT CAN WE LEARN FROM FIELD-RELATED GEOCHRONOLOGICAL STUDIES? A. Kroner', A. Demoux\ D. Alexeiev^ Y. Rojas-Agramonte\ B.F. Windley^ & P. Jian^ ^Institut fur Geowissenschaften, Universitat Mainz, 55099 Mainz, Gernnany, and Beijing SHRIMP Centre, Departnnent of Geology,

Chinese Acadenny of Geological Sciences, 26 Baiwanzhuang Road, Beijing 100037, China.

^Geological Institute, Russian Acadenny of Sciences, 7 Pyzhevsky per, Moscow 109017, Russia. ^Departnnent of Geology, University of Leicester, Leicester LEI 7RH, UK ^Beijing SHRIMP Centre, Departnnent of Geology, Chinese Acadenny of Geological Sciences, 26 Baiwanzhuang Road, Beijing 100037, China.

The Central Asian Orogenic Belt (CAOB) is one of the largest orogenic systems on Earth. The eastern parts of the belt, within southern Siberia and Mongoha, are dominated by accretion of arcs and microcontinets from S to N during the Neoproterozoic and Paleozoic. Amalgamation of several arcs and microcontinents in the western part during the Cambrian and Ordovician led to formation of the composite continent of Kazakhstan, which was incorporated into Eurasia after collisions with Siberia, Baltica, and Tarim during the mid- Carboniferous to Permian. It has been suggested that 50 % of the CAOB consists of juvenile material, largely generated in the Palaeozoic and implying an exceptional crustal growth rate. We contest this model on the basis of field relationships, zircon geochronology and Nd isotopic systematics and argue that crustal growth occurred over a much longer period of 800 Ma, from about 1050 Ma to -250 Ma. The earliest history of ocean opening is recorded by -1020-1050 Ma ophiolites in southern Siberia. The next younger events farther S in Mongolia are exemplified by the evolution of -850-570 Ma arcophiolite terrains, such as Shishkid, Bayankhongor, Khantaishir and Dariv, followed by early Palaeozoic subduction/accretion and suturing during which some of the arc terrains were metamorphosed to granulitefacies. Although there are undisputed juvenile arc terranes, Precambrian zircon xenocrysts and negative 8values in arc assemblages of Mongolia and Nd(t) Kazakhstan document involvement of pre-existing continental crust as old as 3.9 Ga.

The most common rocks in modern trenches are clastic sediments derived by erosion of older arcs or old continental margins. Also, along-axis currents bring clastic sediments from eroded rocks even thousands of kilometres away, as from southern New

Zealand to Indonesia. Subducted old clastic sediments probably make up a significant component of the lower crust of the CAOB. Partial melting of such lower crust would be expected to incorporate xenocrystic zircons into crustal melt granites. This need not mean that the granites belong to continental margin arcs, although xenocrystic continental margin granites are also present elsewhere. Overall the CAOB records the formation of southwestPacific style small forearc and back-arc ocean basins that probably evolved between island arcs and microcontinents during the period -1050 to -250 Ma and were closed during continuous accretion between the Neoproterozoic and Palaeozoic. During this time the southward-growing southern margin of the Siberian craton always faced an open ocean. An active margin also developed in the Russian, Mongolian and Chinese Altay during the Devonian and early Carboniferous, although this margin has no major allochthonous terranes, and only small accretionary wedges evolved there. The beginning of collision of Kazakhstan with Siberia, Baltica and Tarim is dated at -320 Ma, -315 Ma, and -300 Ma, respectively, based on such features as cessation of marine sedimentation, initiation of foreland basins and/or the age of regional unconformities. Structural patterns in the East Kazakhstan orocline indicate that bending occurred during the late Carboniferous to early Permian as a result of opposing movements of Siberia and Tarim, which squeezed Kazakhstan during the latest phases of collision. Final closure of the Palaeo-Asian ocean was diachronous and occurred from the late Carboniferous and early Permian in the western Tianshan to late Permian and early Triassic in the East when the North China Craton (NCC) was attached to the CAOB.

Geological Society of Australia Abstracts No 92

A significant volume of predominantly felsic volcanic rocks in northern and central Mongolia is likely derived from remelting of older crust. We also found numerous Archaean to Neoproterozoic detrital zircons in Mongolian arc-derived clastic sediments, suggesting an origin from continental sources. These rocks are unlikely to have formed in an intraoceanic environment, and we favour a Japan-type setting. We caution against the sole use of Nd isotopic systematics to estimate crustal growth rates, and previous crustal growth models for these regions may require substantial revision. The presence of zircon xenocrysts in ophiolitic and arc-related gabbros as well as in subduction-related eclogites is a phenomenon not only found in the CAOB and poses the general question whether these grains reflect recycling in the upper mantle after sediment subduction. Another unexplained phenomenon in many igneous rocks of Mongolia and Chinese Inner Mongolia is the presence of idiomorphic late Triassic zircons although no major igneous event of this age is known from this region. Previous models derived many Precambrian crustal fragments in the CAOB from Siberia or the northern margin of Gondwana. Our Mongolian zircon age pattern, based on magmatic, xenocrystic and detrital grains, is characterized by a predominance of ages in the range 500-2100 Ma. The younger ages, up to about 700 Ma, may be derived from cannibalistic reworking of crustal components formed during the early stages of arc formation in the CAOB. Marked age groupings between 800 and 1100 and 1300-2100 Ma are distinctly different from the age patterns observed in Siberia and northern Gondwana. In particular there is increasing recognition of Grenville-age basement fragments, and overall there is a remarkable similarity with the age pattern in the Tarim Craton. This craton may have supplied much of the detrital material now found in both the Mongolian arcs and the Tianshan belt of NW China. Geological maps for many parts of the CAOB, particularly in Kazakhstan, Kyrgyzstan, NW China and Mongolia show most medium- to highgrade metamorphic complexes as Archaean to Palaeoproterozoic basement, whereas new single zircon ages for some of these complexes are as young as Carboniferous and document high-grade events associated with arc accretion and subduction. Some metamorphic terranes, such as in the northern Tianshan of Kyrgyzstan, have Neoproterozoic ages.

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and it is uncertain at present whether these reflect accretion/collision events in the early history of the CAOB or constitute exotic blocks derived from outside the Palaeo-Asian Ocean. One of the major unresolved problems in the CAOB is the original geometry and extent of individual terranes, sutures and major structural boundaries and their correlation. Palaeogeographic and stratigraphic analysis suggest coherent tectonic domains, i.e. allochthonous blocks, arcs, and microcontinents, which range in length from less than 1000 to 1500 km on average and rarely reach a maximum of -2500 km. The arcs are generally characterized by relatively short periods of activity, which were not synchronous in different arc systems. Cessation of arc volcanism in many regions can be correlated with closure of adjacent deeper marine basins and thus most likely reflects episodes of accretion and collision. Ridge subduction and collision-induced subduction polarity reversals probably also occurred. The above features point at an analogy with the present southwest Pacific where the complex geometry and evolution of plate boundaries leads to rapid changes in the size, orientation and length of tectonic elements (e.g.. Hall, 2009). Rotations about vertical axes in many parts of the CAOB are proven by palaeomagnetic data on Palaeozoic rocks. These reflect both large-scale oroclinal bending such as in the East Kazakhstan orocline, and domino-style rotations within broader belts dominated by strike-slip motion, like in the late Palaeozoic Tianshan. It is extremely unlikely that the broad E-W orientation of most CAOB terranes is an original feature, and rotations, accretion/ collision, and escape tectonics likely have modified the original geometry to such an extent that largescale correlation in many cases appears doubtful. A further complication arises from the fact that arcs and accretionary prisms are most vulnerable to destruction and disappearance (e.g., Scholl & von Huene, 2009) and the record now preserved is therefore fragmentary. The broad-brush compilations of large areas of arcs or terranes (Sengor et al. 1993; Badarch et al, 2002) might once have been useful to give a first order idea of rock distribution, but they give a misleading impression of what actually happened in the CAOB. Most so-called terranes contain numerous fragments of arcs, ophiolites, accretionary wedges etc, many

2009 International Conference on Island Arc-Continent Collisions: Macquarie Arc Conference, Orange, New South Wales, Australia of which have not yet been mapped out, and what we see today are only fragments of the originals. For example, in southern Bayankhongor a belt of arc rocks is only a hundred metres wide in places. Overall more has been subducted and tectonically eroded than has been accreted, as seen in modern accretionary orogens (e.g., Scholl & von Huene, 2009). The occurrence of large microcontinents both in the West and East of the CAOB, which were dominated by passive margin sedimentation during the Neoproterozoic, consistent younging of ophiolites from N to S in the East, numerous non-synchronous and short-lived arcs, as well as multiple nonsynchronous sutures make the single-arc model of Sengor et al. (1993) unlikely for the evolution of this huge orogenic domain, and we favour a development similar to the present SW Pacific.

REFERENCES BADARCH G . , CUNNINGHAM W . D . & W I N D L E Y B . F . 2 0 0 2 .

Journal of Asian Earth Sciences, 21, pp. 87-110. HALL R. 2009. In: Cawood P.E. & Kroner A. (eds.) Accretionary orogens in space and time. Geological Society of London, Special Publications, in press. S C R O L L D . W . & V O N H U E N E R . 2 0 0 9 . In: C a w o o d P . E . &

Kroner A. (eds.) Accretionary orogens in space and time. Geological Society of London, Special Publications, in press. SENGOR A . M . C . , NATAL'IN B . A . & BURTMANN V . S . 1 9 9 3 .

Nature, 3 6 4 , pp. 2 9 9 - 3 0 7 .

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UNDERSTANDING THE WYANGALA BATHOLITH, EASTERN LACHLAN FOLD BELT P.G. Lennox^ ^ School of Biological, Earth and Environmental Sciences,The University of New South Wales, Sydney 2052

The late Early Silurian(?) usually foliated Wyangala Batholith consists of 30 S- 65%), I- (-30%) and A- (2%) type plutons which intruded deformed Ordovician metasediments and volcaniclastics passively along pre-existing faults within the Eastern Lachlan Fold Belt. Microstructural, potential field, various dating studies and field work over the last twenty years in collaboration with numerous colleagues and students have shown that plutons within the Wyangala Batholith ascended via up to three steps to depths of 6-12km. The plutons were brought closer to the surface during the Tabberabberan or Kanimblan events and finally unroofed with removal of 3-4km of overburden postCarboniferous. Prior to ascent of the batholith, the Benambran event caused the development of poorly exposed almost east-west folds and thrusts and also rarely overprinting almost north-south folds and thrusts (?) in the Ordovician sequences. The exposed plutons are usually elongated north-south in plan view and sheet to wedge-like in cross section and were intruded passively along pre-existing weaknesses with

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formation of flow and tectonic fabrics. Some plutons have rotated about east-west or north-south axes during the Tabberabberan and Kanimblan events which brought them from their emplacement depths closer to the surface. There is evidence of southside up and west-side up movement of plutons and enclosing country rocks plus dextral rotation in plan view of one of the plutons. The Ar-Ar dating of gouge in regional faults transecting the Wyangala Batholith indicates some faults ceased movement at the time of pluton emplacement while others were active post the Kanimblan event. Recent AMS studies on the Wyangala Granite suggest the ultramylonite zones on its more intensely strained eastern margin are deformed quartz-epidote veins and not more deformed phases of the granite (Lennox et al 2008).

REFERENCES

LENNOX P.G., DE WALL H., CZARNOTA K. & WHITE L. 2008 Geotectonic Research 95/1, pp. 97-99.

2009 International Conference on Island Arc-Continent Collisions: Macquarie Arc Conference, Orange, New South Wales, Australia

THE CONUNDRUM OF PANGEA: A GENETIC LINKAGE BETWEEN THE APPALACHIAN AND TERRA AUSTRALIS OROGENS? J.B. Murphy/ R.D. Nance/ & P.A. Cawood/ ^Dept. of Earth Sciences, St. Francis Xavier University, Antigonish, Nova Scotia, B2G 2W5, Canada ^Dept. of Geological Sciences, Ohio University, Athens, Ohio 45701, USA ^University of Western Australia, Tectonics Special Research Centre, 35 Stirling Highway, Crawley, WA 6009, Australia

Growing evidence that the repeated amalgamation and dispersal of supercontinents have affected the Earth s evolution since the end of the Archean has led many researchers to propose the existence of a supercontinent cycle (Worsley et a l , 1984; Nance et al., 1986). However, the mechanisms responsible for these events are unclear. Following supercontinent breakup, there are two geodynamically distinct types of ocean; interior oceans formed between the dispersing continents with lithosphere whose age is younger than the time of supercontinent breakup, and an exterior ocean that surrounded the supercontinent prior to breakup and, consequently, is dominated by lithosphere that is older than the time of breakup. In order to evaluate geodynamic models for supercontinent formation, it is essential to determine which of these two types of ocean is consumed during supercontinent amalgamation. Although much of the evidence needed is destroyed by subduction, vestiges of oceanic lithosphere are preserved in mafic complexes accreted to continental margins prior to terminal collision. Constraints on the age of the mantle lithospheric sources that gave rise to these accreted complexes can be derived from Sm-Nd isotope systematics. Mafic terranes accreted in orogens that terminated in the formation of the Late Neoproterozoic supercontinent Pannotia, have Sm-Nd T D M (depleted mantle) model ages between ca. 1.2 and 0.71 Ga. These ages imply that much of the oceanic lithosphere that was subducted and recycled to yield these complexes was formed before the ca. 755 Ma breakup of the previous supercontinent, Rodinia (i.e. TDM>TR). These mafic complexes are therefore vestiges of oceanic lithosphere that formed within the peri-Rodinian (Mirovoi) ocean, such that Pannotia was formed by the closure of an exterior ocean (extroversion). The formation of Pangea is primarily recorded in the origin and evolution of the Paleozoic oceans (lapetus.

Rheic, Paleotethys) between Laurentia (ancestral North America), Baltica (northwestern Europe) and northern Gondwana (South America-West Africa), whose closure resulted in its amalgamation. These oceans were formed in the Paleozoic after the ca. 550 Ma breakup of Pannotia. Uncontaminated mafic rocks from both oceans that have eNd values close to depleted mantle values at their respective times of emplacement show closely matching crystallization and depleted mantle model ages that do not exceed the age of rifting (i.e. T D M < TR). This indicates ®that the oceanic lithospheric source of these suites was generated after the rifting of Pannotia, such that Pangea was formed by the closure of interior oceans (introversion). The above analysis suggests that Pangea and Pannotia were assembled by fundamentally distinct geodynamic processes. Recent theoretical and geodynamic models ebate whether plate tectonics is primarily driven by the top-down cooling affects of subduction, which stimulates convection in the mantle, or whether plate motions are the surface manifestation of mantle convection (Zhong and Gurnis, 1997; Anderson, 2001). Although the assembly of Pannotia is broadly consistent with topdown geodynamic models, the better documented record of Pangea assembly runs counter to such models. At the same time that Laurentia, Baltica and Gondwana separated to form the lapetus Ocean (ca. 550 Ma), subduction zones in the oceanic domains at the leading edges of the dispersing continents were already well established, as is the case for the 18,000 km Terra Australis orogen, which preserves a continuous record of subduction from 580 Ma until 230 Ma (e.g. Cawood, 2005; Cawood and Buchan, 2007). According to conventional top-down geodynamic models, slab pull forces associated with this

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subduction should have resulted in the migration of the dispersing continents towards these subduction zones, resulting in the amalgamation of an extroverted supercontinent. Instead, however, the motion of the dispersing continents reversed when subduction commenced in the relatively newly formed lapetus and Rheic oceans between Laurentia, Baltica and Gondwana. Subduction in the modern world preferentially removes the oldest, coldest and most negatively buoyant oceanic lithosphere, a selectivity generally considered fundamental to plate tectonics and applicable to plate geodynamics in the past. However, this scenario is not applicable to the formation of Pangea. Opening of the Rheic Ocean at ca. 490 Ma closely followed the onset of subduction in the lapetus Ocean and the convergence of Laurentia and Baltica. At ca. 460 Ma, the Rheic Ocean began to subduct, initiating the convergence of Gondwana with Laurentia and Baltica that would ultimately give rise to Pangea. During the assembly of Pangea, therefore, subduction was not only initiated within the new Paleozoic oceans, but the rates of subduction of this relatively young lithosphere must also have overpowered those of the already well established subduction zones within the exterior ocean. The mechanisms responsible for the amalgamation of Pangea are enigmatic. To a first order, we know where A Supercontinent

• Ocean ridge - Subduction zone

Breakup

Transform fault Coliisionai orogeny

C Drift

Mafic terranes A Interior arc * Exterior arc

Figure 1. Stages in the breakup of a supercontinent, the fate of old oceanic lithosphere that surrounds the supercontinent before breakup (exterior ocean) and the creation of relatively new oceanic lithosphere between the dispersing blocks (interior ocean).

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and when, but not why. At the heart of this debate is a lack of understanding of the forces that initiate the subduction process. Likewise, the documented evolution of Pangea highlights fundamental gaps in our understanding of the processes responsible for its amalgamation. To understand the processes leading to the formation of Pangea, we need to investigate the geodynamic linkages between the evolution of the interior Rheic Ocean and the penecontemporaneous evolution of the exterior Paleopacific ocean. REFERENCES ANDERSON, D.L. 2001, Science 293, pp. 2016-2018. CAWOOD, P A . 2 0 0 5 . Earth Science Reviews 69, pp. 249-279. CAWOOD, P.A. & BUCHAN, C. 2 0 0 7 , Earth Science Reviews 82, p p . 2 1 7 - 2 5 6 . MURPHY, J.B. & NANCE, R . D . 2 0 0 3 , Geology 31, pp. 873-876. MURPHY, J.B. & NANCE, R . D . 2 0 0 8 . Geology 38, in press. NANCE, R.D., WORSLEY, T . R & MOODY, J.B. 1986, Geology 14, p p . 5 1 4 - 5 1 8 . WORSLEY, T.R., NANCE, R . D . & MOODY, J.B. 1984, Marine Geology 58, pp. 3 7 3 - 4 0 0 . ZHONG, S. & GURNIS, M . 1997. Earth Interactions 1,

pp. 1-18.

2009 International Conference on Island Arc-Continent Collisions: Macquarie Arc Conference, Orange, New South Wales, Australia

UNRAVELLING THE TECTONIC SETTING OF DEVONIAN SEQUENCES IN THE NEW ENGLAND OROGEN C. l\/lurray\ & R. Offler' ^Department of Natural Resources and Mines, GPO Box 2454, Brisbane, Qld, Australia. ^ Departnnent of Earth Sciences, School of Environnnental & Life Sciences, University of Newcastle,Newcastle, NSW Australia 2308.

Sequences of Early to Late Devonian age that were originally located east of and subsequently accreted to the Gondwana margin, are exposed throughout the New England Orogen (NEO). The tectonic setting of the rocks in these sequences has been the subject of considerable debate and controversy for many years. In the last decade, as a result of detailed field studies and geochemical investigations of meta-basalts, a better understanding of the setting of many of the sequences has arisen. This has led to the development of a new model for the Devonian history of the NEO. Our studies have revealed that intra-oceanic island arc and back arc basin (BAB) settings dominate throughout the Devonian based on discriminant diagrams involving immobile elements and comparison of elemental ratios with present day basalts from various settings. Further, they show that basalt compositions become more arc-like to the west suggesting that the subduction zone dipped to the east throughout this period rather than to the west as is often stated. As well they indicate that calc-alkaline basalts become more common in the Late Devonian reflecting greater maturity of the arc. Many of the samples considered to have been formed in a BAB have a mixture of MORB and arc-like characteristics, others are almost entirely MORBlike. The arc-like features are due to the presence of

a subduction component in the basaltic magma, the amount of which is controlled by the distance from the arc. Those samples with essentially MORB-like compositions are thought to have been formed at spreading centres well away from the arc. Accretion of the intra-oceanic arcs to the margin of Gondwana occurred in the Frasnian at -380 Ma, causing local deformation, blockage of the subduction zone and subduction zone flip. Subsequently, a continental margin arc formed above the westdipping subduction zone about 100 km west of the present position of the Devonian intra-oceanic arcs, and was active until the end of the Carboniferous. The forearc basin associated with this continental arc was built over oceanic crust carrying the eroding Devonian arcs. Remnants of this oceanic crust are now exposed along the Peel and Yarrol Faults, and form a large ophiolite sheet near Rockhampton. The rocks associated with the Carboniferous arc record much higher (La/Sm)N and (Nb/Zr)N ratios than their Devonian counterparts. In the southern part of the NEO, ages obtained from inherited zircons extracted from Carboniferous rocks suggest that magmas required for this extended period of magmatism were most likely derived from a Middle Devonian felsic igneous basement.

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TRANSITION BETWEEN THE DELAMERIAN AND LACHLAN OROGENS: GEOPHYSICAL DEFINITION AND GEODYNAMIC IMPLICATIONS R. J. Musgrave' & J. E. Greenfield' ^Geological Survey of New South Wales, NSW Department of Primary Industries, PO Box 344, Hunter Region Mail Centre, NSW 2310, Australia

Between the Cambrian Mt Stavely-Mt Wright arc system of the Delamerian Orogen in western Victoria and northwestern New South Wales, and the Ordovician turbidite pile and Macquarie Arc of the Benambran Cycle in the Lachlan Orogen (Glen 2005), lies an intervening zone with a disputed affinity. Expressed in outcrop as the Stawell Zone in western Victoria, Devonian basins and Mesozoic/ Cenozoic platform sediments obscure this zone in New South Wales, leading to disputes about its geometry, continuity, and assignment (e.g. Glen 1992; VandenBerg et al. 2000; Cayley et al. 2002; Miller et al. 2005). Assembly of a suite of new geophysical data, coupled with a program of potential field modelling, has allowed the recognition of structural elements in the transition zone, prompting a reinterpretation of the early stages of evolution of the Tasmanides. Review of the aeromagnetic data enabled Hallett et al. (2005) to extend the Stawell Zone north into NSW. Increasing cover thickness obscured the continuation of the trace of the Moyston Fault, the northwestern boundary of the Stawell Zone, but new imagery based on the magnetic tilt filter shows that the Stawell Zone curves to wrap around the northern flank of the Hay-Booligal Zone, until it is terminated by the Bootheragandra Fault, a terrane boundary that divides the structurally complex western half of NSW from the simpler, dominantly N - S striking Lachlan Orogen to the east. By using gravity, upwardly-continued magnetics, and reflection seismics to image the deeper crust, and teleseismic tomography to reveal the lithospheric mantle, the surface traces of these features can be projected to depth. In Victoria, a wedge of the Cambrian Mt Stavely arc system is obliquely underthrust below the Stawell Zone. Near the NSW-Victorian border this underthrust is terminated by a leftlateral accommodation zone, which also displaces the Cambrian arc; north of this point, the arcuate segment of the Moyston Fault is vertical, implying that this part of the Stawell Zone has been extruded

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by escape tectonics, induced by the collision of the Hay-Booligal Zone — probably a microcontinental fragment rifted from the Proterozoic craton — with the Cambrian arc as part of the Delamerian Orogeny. Clockwise rotation of the central part of the arc (the Menindee Block) accompanied the collision. East of the Hay-Booligal/Stawell complex, the Bootheragandra Fault dips eastwards, and is overthrust by a wedge of deformed Ordovician turbidites of the Girilambone-Wagga Terrane. Further north, the Koonenberry Belt includes a Cambrian rifted arc (analogous to the modern Central Volcanic Zone of New Zealand) and accretionary wedge/fore-arc elements initially deformed in the middle Cambrian Delamerian Orogeny. Accretionary features visible in the geophysical imagery include localised deformations over what may be subducted seamounts. Abutting the Koonenberry Belt to the north-east is the Thomson Orogen, which overthrusts the Delamerian margin, and appears to have incorporated the outlying parts of the Koonenberry fore-arc. Late Ordovician deformation of the Benambran Orogeny in the Thomson Orogen propagates with diminished intensity into the Koonenberry Belt. This resembles the history of the Stawell Zone in Victoria, in which Delamerian structural elements have been reworked by the Benambran Orogeny. Emerging from beneath the overthrust edge of the Thomson Orogen, structural features of the Delamerian accretionary system — including perhaps the frontal thrust — can be faintly discerned in the tilt-filter imagery. Combining these observations suggests the following tectonic history: • Late Neoproterozoic continental sliver rifting on Rodinian margin, yielding microcontinental fragments including the Hay-Booligal Zone; • Middle Cambrian arc volcanism, rifting, and accretion, extending from the continental margin (Koonenberry) to open ocean (Mt Stavely);

2009 International Conference on Island Arc-Continent Collisions: Macquarie Arc Conference, Orange, New South Wales, Australia •

•

Late Cambrian collision of Hay-Booligal continental fragment, arc displacement and bending, escape tectonics in Stawell Zone, culmination of Delamerian Orogeny; Subduction steps eastwards a to west-dipping system under the Macquarie Arc, with backarc basin and turbidite sedimentation in Girilambone-Wagga terrane; Late Ordovician polarity flip of Macquarie Arc; accretion of Girilambone-Wagga; rotation of northern part of arc to form Thomson Orogen (with triangular back-arc behind); Early Silurian Benambran Orogeny, representing collision of eastward dipping Macquarie Arc subduction with Hay-Booligal fragment and arcarc collision of Thomson accretionary margin with older Koonenberry Belt (Mt Wright Arc).

Post-Benambran Tasmanide tectonics were simpler, dominated by retreat and advance of relatively long, linear, and continuous subduction zones and by strike slip translations along the margin (Collins 2002; Glen 2005), operating in a regime controlled by the downwelling side of a major mantle convection system (Collins 2003). Intriguingly, the earlier Delamerian to Benambran phase, involving microcontinental fragments and complex arc reversals and rotations, resembles in microcosm the Cenozoic tectonics of the Southwest Pacific (Crawford et al. 2003), which developed after Australia rapidly translated across an existing subduction channel (Gurnis et al. 1998), interrupting the mantle sink and exposing the margin to a plume system (Sdrolias et al. 2003; Muller et al. 2001). Neoproterozoic fragmentation of the Australian margin may have followed a similar event, and the complex geometry of the Delamerian to Benambran phase persisted until the microcontinents were "swept up" by accretion, and long, linear subduction was favoured by the establishment of penetrative cold antiplumes through the mantle.

REFERENCES CAYLEY, R.A., TAYLOR, D . H . , VANDENBERG, A . H . M . &

MOORE, D.H. 2002. Australian Journal of Earth Sciences, 4 9 , pp. 2 2 5 - 2 5 4 . COLLINS, W . J . 2 0 0 2 . Tectonics,

21,1024,

10.1029/2000TC001272.

COLLINS, W.J. 2003. Earth and Planetary Science Letters, 2 0 5 , pp. 2 2 5 - 2 3 7 . CRAWFORD, A.J., MEFFRE, S. & SYMONDS, P.A. 2 0 0 3 .

Geo-

logical Society of Australia Special Publication 22 & Geological Society of America Special Paper 372, pp. 383-403. GLEN, R.A. 1 9 9 2 . Tectonophysics,

2 1 4 , pp. 3 4 1 - 3 8 0 .

GLEN, R.A. 2005. Geological Society, London, Special Publications, 246, pp. 2 3 - 9 6 . GURNIS, M . , MULLER, R . D . & MORESI, L. 1 9 9 8 .

Science,

279, pp. 1 4 9 9 - 1 5 0 4 . HALLETT, M . , VASSALLO, J., GLEN, R. & WEBSTER, S. 2 0 0 5 .

Geological Survey of New South Wales, Quarterly Notes No. 118. MILLER, J.MCL., PHILLIPS, D., WILSON, C.J.L. &

DUGDALE, L.J. 2005. Australian Journal of Earth Sciences, 5 2 , pp. 9 2 1 - 9 4 0 . MULLER, R.D., GAINA, C., ROEST, W . & HANSEN, D.L.

2001. Geology, 29, pp. 2 0 3 - 2 0 6 . SDROLIAS, M . , MULLER, R . D . & GAINA, C. 2 0 0 3 .

Geologi-

cal Society of Australia Special Publication 22 & Geological Society of America Special Paper 372, pp. 343-359. VANDENBERG, A . H . M . , WILLMAN, C.E., MAHER, S., SIMONS, B.A., CAYLEY, R.A., TAYLOR, D.H., MORAND, V.J., MOORE, D . H . & RADOJKOVIC, A. 2 0 0 0 . The Tasman

Fold

Belt System in Victoria, Geological Survey of Victoria Special Publication.

Published with the permission of the Director, Geological Survey of New South Wales, NSW Department of Primary Industries.

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ARC-CONTINENT COLLISION IN THE PALAEOPROTEROZOIC RIO ITAPICURU GREENSTONE BELT, SAO FRANCISCO CRATON, BRAZIL E. P. Oliveira

Institute of Geosciences, P.O. Box 6152, University of Cannpinas-UNICAMR 13083-970, Cannpinas, SR Brazil The Rio Itapicuru Greenstone Belt (RIGB) is a major gold producer in the Serrinha Block of the Sao Francisco Craton (Fig. 1). Recent geological mapping along the western boundary of the RIGB with the basement provide superb field relations in support of a model in which an Archaean continental plate, locally represented by the 2980-3080 Ma Santa LuzJacurici gneiss-migmatite complexes and probably also the 2560-2660 Ma Caraiba gneiss complex, collided with a Palaeoproterozoic arc complex represented by ca. 2145 Ma metabasalts of the RIGB intruded by both calc-alkaline and trondhjemitic plutons (ages in the range 2142-2127 Ma). Before collision the Archaean plate was thinned and probably became a magma-poor passive continental margin as suggested by the occurrence of mafic dykes, marble, calc-silicate rocks, and transitional ophiolites (?). In this scenario, the RIGB metabasalts may be interpreted as the oceanic crust of the passive continental margin because its geochemistry is indistinguishable from basalts of the ocean-continent transition. Subduction of the continental margin was probably towards the east (present-day position) and may have involved slab breakoff in the time interval of 2110-2105 Ma when K-rich tonalite, granodiorite, lamprophyres and syenite plutons emplaced along the basement-greenstone transition. Because the K-rich plutons do not occur along the entire length of the collision zone, slab breakoff appears to have

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been laterally discontinuous. At north, the collision zone and exhumed rocks of the subducted passive continental margin are represented by west-verging folded banded gneisses composed of deformed migmatites interleaved with retrogressive garnet amphibolites (metamorphosed mafic dykes with garnetiferous core and amphibole-rich margin), peridotite complexes, granitic orthogneisses and remnants of sedimentary rocks (marble, graphite and biotite schists). To the south, the collision zone might have been overturned (east to north-verging folds) during late 2080-2070 Ma continent-continent collision/reworking. Biotite paragneisses, sillimanitegarnet-biotite schists and quartzites, which are not intruded by mafic dykes, overlay the deformed passive continental margin rocks in structural conformity. These metasedimentary rocks were intruded by the slab breakoff-related K-rich plutons and other collisional granites; on the basis of their Nd model ages (mostly younger than 2.4 Ga) they were possibly deposited on a fore-arc basin. Younger 2080 Ma-old adakitic and calc-alkaline dacites may represent another continental margin arc. Orogenic gold mineralizations (ca. 2050 Ma) are currently mined but no deposit are known along the arccontinent transition zone.

2009 International Conference on Island Arc-Continent Collisions: Macquarie Arc Conference, Orange, New South Wales, Australia 39.5°

38.5°

Figure 1: A) The Sao Francisco Craton in South America, with location of the Serrinha Block; B) The Serrinha Block and its basement and greenstone belts.

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PALAEONTOLOGICAL AND PALAEOGEOGRAPHICAL ASPECTS OF THEMACQUARIEARC I.G. Percival Geological Survey of New South Wales, NSW Department of Primary Industries, WB Clarke Geoscience Centre, 947-953 Londonderry Road, Londonderry, NSW 2753, Australia.

The contribution of palaeontology to tectonic reconstructions of arc settings is generally restricted to providing age constraints on timing of pre- and post-collisional stratigraphy. Structural complexity will often have destroyed or distorted fossils beyond recognition, but where they have been satisfactorily preserved, fossils can provide valuable information on water depth, palaeogeography and palaeobiogeography, in addition to biostratigraphy. This approach has been used with considerable success over the past decade in analysing the 40 Myr geological history of the Macquarie Arc in central New South Wales. Microfossils, chiefly conodonts, and biostratigraphically useful macrofossils such as graptolites enable subdivision of the Ordovician into biozones as little as 0.5 Myr in duration. Where both conodonts and graptolites are present, combination of their zonal schemes can often achieve a precision (in relative terms) much finer than the errors on the best absolute isotopic dating methods. The combination of biostratigraphic and isotopic dating was instrumental in establishing a rigorous framework for correlating stratigraphy across the Macquarie Arc in the recent study by Percival & Glen (2007), thereby constraining the geochemical model involving four phases, each restricted to a specific interval. Crucial evidence of discontinuities in the evolution of the arc, one extending over 9 Myr, was derived from study of conodont biostratigraphy. That particular hiatus, which was not observable in outcrop due to poor exposure in critical areas, was subsequently found to coincide with a significant change in geochemical signature from Phase 1 to Phase 2. Late Ordovician intrusions and volcanogenic rocks with radiometric age-dates were able to be plotted precisely against the biostratigraphically-constrained sedimentary units in this interval, thus constraining the Eastonian magmatic hiatus between Phases 2 and 4, which is a key aspect of the currently accepted model for evolution of the Macquarie Arc.

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Interpretation of relative water depth provides useful evidence of uplift or subsidence during arc evolution. Detailed study of brachiopod-dominated marine communities in Lower to Middle Palaeozoic sedimentary rocks by palaeontologists over the past four decades has led to recognition of a spectrum of Benthic Assemblages (BA) that extend from intertidal communities at the shoreline (BA 1) to deepwater base of slope and ocean basin faunas (BA 6-7). In the Macquarie Arc, Benthic Assemblages observed in strata deposited during the Eastonian magmatic hiatus reveal a series of fine-scale transgressions and regressions superimposed on an overall deepening trend (Webby & Percival 1983; Percival & Webby 1996). The profile of Benthic Assemblages interpreted from these rocks also demonstrates a series of massflow events resulting in allochthonous deposition of limestones from the shelf edge and upper slope into basinal graptolitic clastics in the late Eastonian. Steep gradients typical of volcanic islands accentuate this process in modern arc environments, and provide an analogy for the Macquarie Arc. Proximity of tectonic plates in the past can be gauged from the study of biogeographic affinities. Benthic faunas, i.e. those with adult populations tethered to, or crawling along the sea floor, frequently have pelagic (floating) or nektic (swimming) larval stages with a relatively limited range. Thus some idea of relative closeness of different geographic regions may be interpreted from the degree of similarities of their benthic faunas at genus or (more rarely) species level. Brachiopods and trilobites, which are often used in such analyses, reveal close faunal connections between the Macquarie Arc and several of the terranes now forming Kazakhstan (Webby et al. 2000). These two regions are separated by considerable distances in many palaeogeographic reconstructions that rely purely on palaeomagnetic constraints. Biogeographical studies provide a means to refine palaeolatitudinal estimates, which are unable to be resolved by palaeomagnetism.

2009 International Conference on Island Arc-Continent Collisions: Macquarie Arc Conference, Orange, New South Wales, Australia

The Macquarie Arc also displays significant faunal affinities with the Klamath Mountains and parts of Alaska that in the Ordovician formed terranes along the west coast of Laurentia. These links are suggested by similarities in brachiopods and gastropods, which imply presence of a series of islands or seamounts (almost all subsequently subducted or otherwise destroyed) across the intervening Palaeo-Pacific Ocean in order to permit migration of larval forms across vast oceanic distances. Potential candidates for these island remnants may exist in the form of Late Ordovician allochthonous limestones now incorporated in the accretionary complex of the western New England Orogen; other isolated limestone deposits of equivalent age and similar faunal content known from central Queensland may have had a similar tectonic setting. Surprisingly, despite their relatively close proximity today, there is considerably lower similarity in evidence between Late Ordovician faunas of the Macquarie Arc and contemporaneous fossils of the Delamerian continental margin exposed in Tasmania. Only one of sixteen trilobite species is in common between the two regions, and there is no commonality of genera or species of free-swimming coiled nautiloids (Tarphyceratids) whose modern day relatives (Pearly Nautilus) are known to range across large areas of the Pacific. This suggests that either Tasmania and the Macquarie Arc were separated by

much greater distances in the Ordovician, or else there was an intervening physical barrier such as a deep oceanic trench, or very strong oceanic currents. These ideas bring concepts of modern day faunal dynamics to the study of arc-continent collisions that haven't previously been explored and probably should be. REFERENCES

PERCIVAL, LG. & GLEN, R.A. 2 0 0 7 . Australian Earth Sciences, 54, 1 5 3 - 1 6 5 .

Journal of

PERCIVAL, LG. & WEBBY, B . D . 1996. Historical Biology, 11, 171-185. WEBBY, B.D. & PERCIVAL, LG. Lethaia, 16, 2 1 5 - 2 3 2 . WEBBY, B.D., PERCIVAL, I.G., EDGECOMBE, G . D . , COOPER R.A., VANDENBERG, A . H . M . , PICKETT, J.W., PoJETA, J., PLAYFORD, G., WINCHESTER-SEETO, T., YOUNG, G.C., ZHEN, Y-Y., NICOLL, R.S., Ross, J.R.P. & SCHALLREUTER R. 2 0 0 0 . Memoir of the Association of Australasian Palaeontologists, 23, 6 3 - 1 2 6 .

This paper is a contribution to IGCP Project No. 503: Ordovician Palaeogeography and Palaeoclimate, and is published with permission of the Director, Geological Survey of New South Wales, NSW Department of Primary Industries — Mineral Resources.

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SOURCE AND TRANSPORTATION OF HIGH-PRESSURE EXOTIC BLOCKS IN SERPENTINITE MELANGE, PEEL-MANNING FAULT SYSTEM, SOUTHERN NEW ENGLAND FOLD BELT G. Phillips^ & R. Offler^ ^Discipline of Earth Sciences, School of Environnnental & Life Sciences, University of Newcastle, NSW 2308, Australia.

In the southern New England Fold Belt (SNEFB) of New South Wales Australia, exotic blocks of varying composition, age, metamorphic grade and size occur in serpentinite mdange of the Peel-Manning Fault System (PMFS). The PMFS is a fundamental structural element separating Upper Silurian to Carboniferous arc and forearc basin sequences of the Tamworth Belt from Silurian to Lower Carboniferous sequences of the accretionary-subduction Tablelands Complex. It also contains fragments of a Cambrianaged (c. 530 Ma) dismembered ophiolite of supra subduction origin. Recent geochemical studies indicate that the protoliths of the exotic blocks are dominantly andesitic to basaltic, and to a lesser extent, boninitic to granitic in composition. Additionally, they have magmatic affinities varying from calc-alkaline to tholeiitic, which indicates that they formed in different tectonic settings, namely mid-ocean ridge, continental and intra oceanic island arcs and within oceanic plates. In the northern PMFS, U-Pb dating of zircons from plagiogranite indicate a crystallisation age of c. 530 Ma, whereas in the southern PMFS, U-Pb zircon ages from vary from 536 to 421 Ma for meta-diorite, hornblende cumulates and gabbro. By contrast, metamorphic U-Pb and K-Ar ages varying from 536 to 291 Ma have been obtained from eclogite, blueschist and amphibolite. Although several geochemical, metamorphic, structural and radiogenic studies of the exotic blocks have been carried out, few have dealt with their source, or, modes of exhumation. For example, were the exotic blocks derived from the Macquarie Arc of the Lachlan Fold Belt, or, did they form in an outboard supra subduction zone system? These questions are addressed in this paper. Eclogite blocks have MORB-like compositions that indicate they formed during subduction of basalt derived from a mid-ocean ridge located off

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the eastern margin of Gondwana. The blueschists also dominantly have MORB-like compositions indicating that they too formed at a mid-ocean ridge and were subsequently subducted and exhumed. In contrast, some blueschists and amphibolites have shoshinitic affinities, indicating they probably formed in a continental arc setting. One possible explanation for the occurrence of these rocks is the subduction of a continental arc comprising shoshonitic compositions along the eastern margin of Gondwana. This proposal is somewhat supported by the proximity of late Ordovician calc-alkaline, and shoshonitic rocks of the neighbouring Macquarie Arc. However, rocks of the Macquarie arc formed in an intra oceanic arc setting and record CNd values of +8.15 to +4.94, which are considerably less enriched than CNd values obtained from the exotic blocks (+2 to -4.8). Furthermore, blueschist blocks characterised by this magmatic signature have a Middle Ordovician metamorphic age and are derived from a high-pressure host of probable Cambrian age. Thus the conclusion is that exotic blocks observed in the serpentinite melange are derived from sources unrelated to the Macquarie Arc, and were probably derived from outboard arcs originally east of the Gondwana margin. In combination, two major tectonic events are suggested to be responsible for the exhumation of exotic blocks to higher crustal levels in the PMFS. The first event took place during the middle Permian Hunter-Bowen Orogeny when rocks of the accretionary wedge were thrust over the forearc basin. Thrusting was dominantly accommodated along the Peel Fault, which according to gravity and magnetic studies dips steeply to the east. Subsequently, a change in plate vector orientation in the Early Triassic(?) brought about reactivation of the PMFS causing major sinistral movement. Further transportation of the exotic blocks took place at this time.

2009 International Conference on Island Arc-Continent Collisions: Macquarie Arc Conference, Orange, New South Wales, Australia

NEW ENGLAND OROCLINE IN LATE PALEOZOIC: GEOLOGICAL AND PALEOMAGNETIC CONSTRAINTS S.A. Pisarevsky'^^ P.A. Cawood^ B.C. Leitch^ A. Nemchin^ ^School ofGeoSciences,The University of Edinburgh, Grant Institute, The King's Buildings, West Mains Road,

Edinburgh EH9 3JW,UK

^School of Earth and Environment, University of Western Australia, 35 Stirling Highway, Crawley, 6009, WA, Australia ^ Departnnent of Environnnental Sciences, University ofTechnology, Sydney, G.RO. Box 123, Broadway NSW 2007, Australia ^WASM, Curtin University, Bentley, WA, 6102, Australia

Oroclines are map-view bends of originally quasilinear lithospheric elements. The doubly vergent New England orocline lies within the Eastern Australian segment of the Terra Australis Orogen and developed during late Paleozoic to early Mesozoic Gondwanide orogeny that extended along the Pacific margin of Gondwana. Orogenesis commenced at around 310 Ma with termination of activity along the long established magmatic arc. Over the next 40 m.y. the region underwent pulses of compression, including oroclinal bending, extension and basin formation, and crustal melting and S-type granite emplacement, prior to establishment of a new magmatic arc located within the old subduction complex at around 270 Ma. This arc was active until around 230-220 Ma and was associated with further deformation and metamorphism of variable intensity. The present position and the shape of the orogen probably reflects foreshortening and westward displacement during oroclinal bending of the upper crustal lithosphere between pre-310 Ma and post-270 Ma rather than a dramatic shift in the position of the subduction zone within an asthenospheric reference frame. Our work is concentrated on the switching in the location of the magmatic arc and the development of the New England orocline. The orocline is doubly vergent with the southern and northern segments of the orocline show counterclockwise and clockwise rotation respectively of around 180®. The orocline deformed an arc assemblage consisting of a western magmatic arc, an adjoining forearc basin and an eastern subduction complex. Assuming an original linear trend for the system, oroclinal bending has resulted in at least 50% shortening in the New England region from an original length of at least 1200 km. This has in

part been accommodated by some 300 km of lateral displacement of the arc onto the Gondwana foreland. The doubly vergent nature of the orocline with the northern and southern segments related to dextral and sinistral convergent regimes respectively, has led to contrasting models of formation. We resolve these conflicting kinematic settings with a model involving buckling about a vertical axis due to northward translation of the New South Wales (New England) segment of the arc system against the Queensland segment that is pinned relative to cratonic Gondwana. Northward motion was driven by coupling between the Gondwana and Pacific plates. A new pulse of magmatic arc activity commenced around 270 Ma and formed a linear belt emplaced mostly within the pre-existing subduction complex in New England, cutting across the orocline and extending into the older forearc and magmatic arc in Queensland. This model requires a detachment within the lithosphere above which oroclinal motion and foreland displacement took place. The spatial correspondence of oroclinal bending with earlier extension and crustal melting suggests that thermal weakening of the crust may have facilitated development of a crustal decollement above which the orocline formed. Westward motion of the orocline onto the foreland during buckling and vertical axis rotation may have been buttressed by the rigid oceanic lithosphere of the Pacific plate restricting eastward motion of the arc system as it thickened. Paleomagnetic data from parts of the New England are adequate in quality, but not in quantity, so their paucity precludes an unequivocal tectonic model for the orocline. However, available paleomagnetic data support the proposed model.

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Geological Society of Australia Abstracts No 92 Table 1. Paleomagnetic poles from blocks of the New England Orogen.

Terrane,

Object

Reference

dp/dm

Plat

Age (Ma)

Kullatine Fm

14.3

139.3

13.4/13.4

-43

318-315

L.Visean

35.7

53.2

9.1/14.7

-25

326-336

M-L.Visean Ignimbrite

72.8

48.4

13.4/19.2

-35

329±1

Hunter Valley Volcanics

73.0

34.0

21.0/21.0

-35

345-326

Paterson Toscanite

73.0

327.0

4.0/5.0

-50

330-326

E-M.Visean Ignimbrite

64.9

78.1

5.5/9.2

-22

Geeve etal., 2002

N.Tomworth

Mean pole*

57.4

317.8

6.9/6.9

-62

330-300

N.Tomworth

Mean pole"

49.9

349.1

13.6/13.6

-66

300-260

N.Queensland

Connors Volcanics

46.0

280.0

13.0/16.0

-45

320-310

Rockwood Volcanics

55.0

312.0

22.0/25.0

-55

300-250

Clark, 1994

Texas Orocline

Alum Rock

5.3

301.7

17.3/17.3

Hastings

Paleopole

Schmidt etal., 1994 2

Myall Geeve etal., 2002

Gresford Geeve etal., 2002

Gresford Luck, 1973

Gresford Irving, 1966

Rouchel

Clark, 1994 10 11

N.Queensland

332±2 342±3

295-284

Auborg etal., 1994

(unrotated) •Resultnos 6681,8589-8592,8817-8822,9203-9205 - lAGA Paleomagnetic Database (McElhinny & Lock, 1996; Pisarevsky, 2005). ''Resultnos 6682, 7739,9206 - lAGA Paleomagnetic Database Table 1 shows paleomagnetic poles from the New England segment of the Terra Australis Orogen. Poles vary in ages between terranes, so any paleogeographic interpretation of them (including our interpretation) is only permissive, not conclusive. Schmidt et al. (1994) and Geeve et al. (2002) suggested the rotations of 80-120° of Hastings, Myall, Gresford, and Rouchel blocks with respect to the cratonic Australia. We think that taking into account apparent paleolatitudinal difference between these blocks (Table 1) such interpretation may be an oversimplification. Most of these apparent rotations could be accommodated by crossing paleo-meridians during generally "straight" drift in high latitudes (see our reconstructions at 340-270 Ma.) Such an approach explains the difference of paleolatitudes calculated from paleomagnetic inclinations between blocks and cratonic Australia. Proposed reconstructions do not contradict paleomagnetic data, but other models are also permissive. To establish paleopositions of cratonic Australia and Antarctica, we used Gondwanan paleopoles from Table 3 of McElhinny et al. (2003.)

IGCP524

REFERENCES AUBORG, C., KLOOTWIJK, C.T. & KORSCH, R.J. 1994. Austr. GeolSurv.Org.

Record

1994/58.

CLARK, D.A., 1994. C S I R O Aust

Expl

Mining

Rpt

6C.

GEEVE, R.}., SCHMIDT, P . W . & ROBERTS, J. 2002. Journal

the Geophysical Research, 107(B9), p. 2196. IRVING, E. 1966. Journal of the Geophysical Research, 71, pp. 6 0 2 5 - 6 0 5 1 .

LUCK, G.R. 1973. Geophysical Journal of the Royal Astronomic

Society,

32, pp. 3 5 - 5 2 .

MCELHINNY M . W . & LOCK J. 1996. Surveys in

Geophysics,

17, pp. 5 7 5 - 5 9 1 . MCELHINNY, M . W . , POWELL, C.MCA & PISAREVSKY, S.A. 2003.Tectonophysics,

3 6 2 , pp. 4 1 - 6 5 .

PISAREVSKY S.A. 2 0 0 5 . EOS transactions,

8 6 ( 1 7 ) , p. 170.

SCHMIDT, P.W., AUBOURG, C., LENNOX, P.G. & ROBERTS, J.

1994. Australian Journal of Earth Sciences, 41, pp. 547-560.

2009 International Conference on Island Arc-Continent Collisions: Macquarie Arc Conference, Orange, New South Wales, Australia CHRONOLOGY AND GEOMETRY OF ARC-CONTINENT COLLISIONS V.N, Puchkov^ ^ Geological Institute, Ufimian Science Centre, 450 000 K. Marx st, 16/2 Ufa Russia A theoretical approach to an arc-continent coUision process involves chrono- and geometrical aspects, closely interconnected. One of the first questions to solve is how to differ between subduction and collision, how to say where and when one changes to another, and when the latter ends finally. In a modern, active system (e.g. Taiwan: Yuan-Hse Lee, 2008), a topography, seismic CDP profiling, tomography, and analysis of distribution of earthquake foci can be sufficient to indicate the moment when the arc couples with the margin. In ancient foldbelts a determination of a subductionto-collision transition comes from two sources of information: sedimentary process and HP-LT or UHP metamorphism. At the onset of collision, an arc accretionary complex starts to be compressed against a continental slope and gets uplifted; flysch sediments spill from the eroded uplift to a continental domain and a flysch piggy-back basin propagates onto the continent. This moment can date the collision. Another, parallel way is an isotopic dating of HP-LT and UHP metamorphism. However it must be kept in mind that as it is shown at the example of the Southern Urals (Brown et al, 2006; Puchkov, 2009 and references therein), the whole spectrum of dates of the exhumed metamorphic rocks in the suture zone ranges from relict Proterozoic zircon ages and the age of granulitic metamorphism in upwelling mantle (garnet pyroxenites originated at 1.5-2 Gpa and 800nOO^'C) to final closure of Ar-Ar system in phengites of eclogite-glaucophane schists at 350®C. But even the latter date corresponds to a position of the schists at a depth of ca. 10 km; tracing of the further uplift of the schists to the surface needs also fission-track dates and also information on its erosional contacts with overlying sedimentary rocks. An additional evidence for an appearance of HP-LT rocks at the Earth s surface is given by a mineralogical study of flysch (Willner et al., 2002,2004). Nevertheless the analysis of the whole array of the data can show when the subduction of the continental edge ends and an exhumation starts. In some cases a direct data

on presence of exhumed high-pressure continental rocks, e.g. metagranites (Chopin, 2003) or arkoses give an additional information for the time of a final subduction of the continental margin. The end of an arc-continent collision is marked by a jamming of a subduction zone against a continental margin followed by a slab break-up which in active systems can be reflected in seismic profiles and sesmotomographic images. In ancient systems the latter event can be determined judging by a contemporary volcanism, which looses subductional features and resembles a rift type (Kosarev et al.,2006). The arc-continent collision is accompanied by a formation of a series of continental-vergent thrusts at the continental margin, arranged in a certain chronological order. The mechanism of thrusting works in accordance with the theory of a triangular zone (Jones, 1991) and therefore the law of a tectonostratigraphical sequence says: the higher thrust, the older (opposite to N. Stenon s law). The ophiolite obduction starts with a delamination of a mantle slice at the inner slope of subductional trench (or an outer slope of a forearc), followed by an underthrusting of pelagic bathyal and then shallowwater shelf sediments of the passive margin with flysch overlying them. The geometry of the arc-continent collision on the Earth s sphere (in plan) can be at some extent approximated by an Euler theorem. Euler's fixed point theorem states that any motion of a rigid body on the surface of a sphere may be represented as a rotation about an adequately chosen rotation pole, called an Euler pole. The theorem is widely used to describe the motions of rigid lithospheric plates. However an arc does not behave always as a rigid body. It may be easily distorted, and at places where it is bent, every differential part of the arc can have its own Euler pole, and in this case positions of the neighbouring poles are changing incrementally. This process can be approximated as simultaneous rotations of many small blocks. The plastic deformation model is also true for some cases of more large-scale oroclinal

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deformations, e.g. Kazakhstanian (Abrajevitch et al., 2008), or Karpatian (Horvath et al., 2006) which are supported by structural and/or paleomagnetic studies. An arc seldom collides with a continent in a single stage. The reason for it is that the outline of an arc is never complementary to an outline of an opposite continental margin with its promontories and embayments. Often the margin and the arc are not even roughly parallel, so their collision is oblique. In some cases, a single arc can collide with two or more continental masses. Such situations, which can be called collectively an obstacle tectonics, result finally in a horizontal deformation of the arc in the first hand, because it is the weakest of the colliding lithospheric blocks. That is why the arc-continent collision can be a diachronous, step-wise process, its chronology strongly depending on its geometry. As often as not, only one flank of an arc collides with a continent, another flank waiting for its turn to collide and to close a triangular oceanic gap left between the arc and the continent (Taiwan, Sunda arc, Urals). In some other cases, arcs develop in narrow spaces between closely arranged and approaching continental blocks, so the arcs become bent, distorted, and collide partially, in a conform, accommodating mode (Caribbean, Calabrian, Alpine-Carpatian cases) (Harangi et al., 2006; Horvath et al., 2006, Meschede, Frish, 1998). The study of diachronous, step-wise arc-continent collisions shows that ensuing folding and orogenies are not moment al, and not subjected to a strict global rhythm as it is supposed by many H. Stille s followers, but take a comparatively long time and are strongly conditioned by regional situations, such as an outline of a continental margin, its own movement and rotation, its proximity to a subduction zone, and a geometry of a collision as a whole.

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REFERENCES ABRAJEVITCH, A., VAN DER VOO, R., BAZHENOV, M . L., LEVASHOVA N . M . , & MCCAUSLAND P.J.A. 2008.

Tectonophysics, 45, pp. 561-76.

BROWN, D., SPADEA, P., PUCHKOV, V., ALVAREZ-MARRON, J., HERRINGTON, R., WILLNER, A.P., HETZEL, R., GOROZHANINA, Y., & JuHLiN, C., 2006. Earth-Science Reviews 79, pp. 2 6 1 - 2 8 7 CHOPIN, C. 2003. Earth and Planetary Science Letters 212,

pp. 1-14

JONES, P. B. Quantitative geometry of thrust and fold belt

structures. AAPG, Tulsa, Ocla. 1991.

HARANGI, S., DOWNES, H., & SEGHEDI, I., 2006. Geol Soc.

Lond. Memoir, vol. 32, pp. 1 6 7 - 1 9 0 .

HORVATH, F., BADA, G., SZAFIAN, P., TARI, G., ADAM A. & CLOETINGH S. 2006. Geological Society, London, Memoirs, 32, 1 9 1 - 2 0 6 . KOSAREV, A . M . , PUCHKOV, V.N. & SERAVKIN, LB. 2006.

Lithosphere, 1. pp. 1-31

MESCHEDE, M . & FRISCH, W . 1998. Tectonophysics, 296. pp. 269-291 PUCHKOV, V. N. 2009. Tectonophysics, doi: 10.1016/j. tecto.2009.01.014 WILLNER, A. P., ERMOLAEVA, T., GOROZHANINA, Y. N., PUCHKOV, V. N., ARZHAVITINA, M . , PAZUKHIN , V . N., KRAMM, U . & WALTER, R. 2002.. AGU Geophys. Monograph Ser., 132, pp. 1 8 3 - 2 0 9 . WILLNER, A.P., WARTHO, J.-A, KRAMM, U . & PUCHKOV,

V.N., 2004 GeolMag, 141, pp. 161-172.

YUAN-HSI LEE, CHIEN-CHI-CHEN, TSUN-KWEI LIU, HSIN-CHENG-HO, HSUEH-YU LU & W E I L o , 2 0 0 8 .

International Conference of Arc-Continent Collision IGCP524. Tainan, Taiwan, pp. 11-12.

2009 International Conference on Island Arc-Continent Collisions: Macquarie Arc Conference, Orange, New South Wales, Australia

A NEW SUPRA-SUBDUCTION ZONE RIFT MODEL FOR THE EASTERN LACHLAN OROGEN, SOUTH-EASTERN AUSTRALIA: IMPLICATIONS OF INTIMATELY RELATED SILICICLASTIC TURBIDITE AND VOLCANIC PACKAGES C. D. Quinn, R.A.GIen Geological Survey of New South Wales, NSW Department of Primary Industries, PO Box 344, Hunter Region Mail Centre, NSW 2310, Australia

A part of the Ordovician palaeo-Pacific margin of east Gondwana is exposed in the eastern Lachlan Orogen, south-eastern AustraHa. Here, the margin comprised three broadly coeval lithological associations (or terranes; Glen 2005): 1) ultramafic to intermediate volcanic and intrusive rocks with quartz-absent volcaniclastic sandstones, limestones, conglomerates, shales and siltstones, all attributed to the intra-oceanic Macquarie Arc; 2) siliciclastic turbidites, chert, black shale packages with extremely rare and localised pillow basalts, and 3) ophiolites (with MORE-like geochemical affinity) with fine-grained sediments such as cherts. Much of the volcanic and intrusive component of the Macquarie Arc package preserves mediumto high-K, calc-alkaline chemistry with relative depletion of Nb, high positive epsilon Nd values, and other chemical features that are consistent with intra-oceanic arc systems (e.g. Crawford et al. 2007). As a consequence, the eastern part of the Lachlan Orogen has been interpreted in terms of intra-oceanic arc accretion (Glen et al. 2007; Meffre et al. 2007). However, there is a dearth of features such as an accretionary wedge, obducted ophiolite or appropriate geothermal gradient that can be related to an accreted intra-oceanic arc and proximal subduction system. Further, the volcanic chemistry is varied with unfractionated high-MgO chemistries represented. Crawford et al. (2007) suggested that these may also be consistent with an intra-arc, hence supra-subduction zone, rift setting. The Macquarie Arc package is flanked on both sides by siliciclastic turbidite + chert + black shale packages of identical age and internal stratigraphy (Glen 2005). The relationship between the Macquarie Arc and these siliciclastic turbidite and black shale packages has been an enduring problem. Glen (2005)

and Meff're et al. (2007) proposed an entirely tectonic relationship based on distal coeval deposition of siliciclastic turbidite and volcaniclastic components. This was implied by a lack of mixing, unique provenance and temporal coincidence, particularly in the early Darriwillian (latest stage of the Middle Ordovician) before a shift to black shale deposition in the siliciclastic packages that was coeval with the later stages of volcanism (Glen 2005). Recent work has provided greater discrimination of the spatial and temporal linkages between these packages as well as direct evidence of provenance mixing. The volcanic and volcaniclastic episodes attributed to the Macquarie Arc can be divided into two major phases separated by a hiatus. The earliest corresponds to Phase 1 and the second major phase incorporates Phases 2, 3, and 4 of Percival and Glen (2007). Temporal and transitional relationships between these major phases and adjacent coeval siliciclastic packages would be expected in two critical intervals: the Early Ordovician, ca. Lancefieldian (La3)Bendigonian (Be2), and the latest Middle to earliest Late Ordovician, ca. Darriwillian (Da2)-Gisbornian (Gi2). These intervals correspond to the initiation of volcanism and coincident shifts in sedimentation patterns in the adjacent siliciclastic packages. The earliest major phase (Phase 1) is constrained below early Ordovician (La3-Be2) conodont (Paracordylodus gracilis a n d Oepikodus evae) b e a r i n g siltstones

(Percival and Glen 2007) and is not extensively exposed. This volcanic phase coincided with chert, black shale and extremely rare pillow basalt deposition within the turbidite package, constrained by chert layers bearing the same conodont fauna (Percival 2007). The earhest part of the second major phase (Da2-3) in the Macquarie Arc comprised allochthonous shelf limestones and deep marine sediments with no significant volcaniclastic component. The

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Geological Society of Australia Abstracts No 92

beginning of volcanism is constrained by conodont fauna in limestones and cherts, as well as analytical geochronology, to within an interval post-dating the very latest middle Ordovician (ca. Darriwillian3-4: Pygodusserra Zone) to the earliest Late Ordovician (Gisbornian). We favour an earliest Late Ordovician (Gisbornian) beginning for the bulk of this volcanism on the basis of a developing dataset. At this time (Da3-Gi2), a transition is recorded within the turbidite package from rhythmicallybedded turbidites (Adaminaby Group) to sandstonelaminated shale with sporadic lenticular quartzites and bedded chert (cf. Sunlight Creek Formation: VandenBerg et al. 1991), before the addition of thin (ca. l - 2 c m ) volcaniclastic layers and rare debris flows to bedded cherts immediately below the volcanics of the southern (Kiandra) belt of the Macquarie Arc. Mixing occurred within the Da3-Gi2 package and is manifest in recently recognised quartz + plagioclase + pyroxene + amphibole-bearing debris-flow lithologies within the bedded cherts, and (previously recognised) interlayered pyroxene-bearing, quartzabsent volcaniclastic lithologies (Orth et al. 1995). Away from the Macquarie Arc, the introduction of black shale (bearing graptolite Nemagraptus gracilis [Vandenberg et al.l991]) to the turbidite packages occurred in the earliest late Ordovician (Gisbornian 1) with the cessation of all siliciclastic sedimentation in favour of black shale before the introduction of graptolite Climacograptus bicornis hence Gisbornian 2 (Vandenberg et al.l991). This transition was coincident with increased volcanism in the later major phase of the Macquarie Arc (Glen 2005). If the stratigraphic relationships described above reflect those of the entire orogenic system, then these observations indicate that the two major phases of Macquarie Arc volcanism coincided with systematic and transitional orogen-wide shifts in sedimentation patterns (after Vandenberg et al. 1991; Orth et al. 1995 and Glen 2005) with now-recognised local mixing of volcaniclastic and siliciclastic material within a transitional sequence. The occurrence of identical siliciclastic packages both inboard and outboard of

114

IGCP524

the Macquarie Arc demands either lateral repetition (Glen 2005; Meff're et al. 2007) or rifting of a once contiguous package. These stratigraphic relationships between the Macquarie Arc and adjacent turbidite + black shale + chert packages, the chemical spectrum of the Macquarie Arc package and the coeval ophiolite with chemistry approaching that of MORE, may be better reconciled in an intra-oceanic, suprasubduction zone rift system that developed within an extensive passive-margin turbidite sequence throughout the Ordovician. Modification of the prevailing tectonic model from an accreted intra-oceanic arc system to an evolving supra-subduction zone rift has significant implications for expected styles of mineralisation and, particularly, the structural controls on mineralised porphyry intrusions. Published with permission of the Director of the Geological Survey of NSW, NSW Department of Primary Industries.

REFERENCES CRAWFORD, A . J . , M E F F R E , S., SQUIRE, R . J . , BARRON, L . M .

& FALLOON, T.J. 2007. Australian Journal of Earth Sciences^ 5 4 , pp. 1 8 1 - 2 1 4 .

GLEN, R.A. 2005. Geological Society (London) Special Publications, 246, pp. 2 3 - 9 6 . M E F F R E , S., SCOTT, R . J . , GLEN, R . A . & SQUIRE, R . J . 2 0 0 7 .

Australian Journal of Earth Sciences, 54, pp. 363-383. PERVICAL LG. 2007. Geological

GS 2007/856.

Survey of NSW Report No

PERCIVAL I.G. & GLEN R.A. 2007. Australian

Journal

Earth Sciences, 54, pp. 143-165. VANDENBERG, A . H . M . , N O T T , R . J . & GLEN, R . A . 1 9 9 1

(cover 1 9 9 2 ) . Geological Survey of Victoria Report 90. O R T H , K . , VANDENBERG, A . H . M . , N O T T , R . J . & SIMONS, B .

A. 1995. Geological Survey of Victoria Report 100.

2009 International Conference on Island Arc-Continent Collisions: Macquarie Arc Conference, Orange, New South Wales, Australia

THE MABUJINA AMPHIBOLITE CONUNDRUM (CENTRAL CUBA): A CASE OF METAMORPHOSED ROOT ZONE OF AN ISLAND ARC, OR AN EXOTIC PRE-ARC BASEMENT? Y. Rojas-Agramonte^'^^ A. Garcâ-Casco^ A. Kroner', S. Carrasquilla-Ortiz^ M. A. Iturralde-Vinent" & G. Millan-Trujillo' Înstitut fur Geowissenschaften, Universitat Mainz, 55099 Mainz, Gernnany. E-nnail: rojas@uni-nnainz.de Înstitute Superior Politecnico Jose Antonio Echeverria, Avenida 114 No 11901 entre 119y 127, Marianao, c. p. 19390, Habana,Cuba. ^Departannento de Mineralogia y Petrologia, Fuentenueva s/n, Universidad de Granada, 18002-Granada, Spain ^Museo Nacional de Historia Natural, Obispo no. 61, Plaza de Arnnas, La Habana 10100, Cuba Înstituto de Geologia y Paleontologia, Via Blanca y Linea del Ferrocarril, San Miguel del Padron, CP 11 000, Ciudad Habana. Cuba

The Geology of Cuba, the largest island of the Greater Antilles, is representative of the orogenic belt fringing the northwesternmost margin of the Caribbean plate. It can be characterized as a mixed tectonic terrane made of continental margin, volcanic arc, ophiolitic and subduction complexes formed as a result of collision between the leading edge of the Caribbean plate and Caribeana in the latest Cretaceous to early Tertiary and by oblique convergence with the North American plate during Tertiary times (Iturralde-Vinent, 1998; Garcia-Casco et al., 2008). In this process, continental (passive margins) and oceanic (ophiolites and intra-oceanic arcs) units were detached from the Pacific realm, Caribeana and the Caribbean and North America plates (IturraldeVinent, 1998; Garcia-Casco et al, 2008). Several intra-oceanic arc sequences are recognized in the Cuban orogenic belt: primitive pre-mid Albian island arc tholeiites, late Albian-Campanian calc-alkaline (Iturralde-Vinent, 1998) and Paleogene sequences; the latter is only developed in eastern Cuba (IturraldeVinent, 1998, Rojas-Agramonte et al., 2004). Most tectonic models for the Caribbean region accept a Pacific origin for the Caribbean Plate (Pindell et al., 2005; 2006; Pindell and Kennan 2009 and references therein). According to this model the Caribbean lithosphere formed in the eastern Pacific, west of an east-dipping subduction zone in pre-Aptian time that triggered island arc tholeiitic magmatism (Donnelly et al., 1990; Pindell et al., 2006). During AptianAlbian time (114-125 Ma), a polarity reversal is postulated to have occurred, and a new west-dipping subduction zone was established that consumed

Proto-Caribbean (i.e., Atlantic) lithosphere and triggered island arc, mostly calc-alkaline, magmatism (Pindell et al., 2005). The new arc was established on top of the primitive arc and started to move roughly eastwards into the proto-Caribbean oceanic basin until the subduction-collision of the Caribbean plate with the Caribeana ridge of the Protocaribbean Sea took place in the latest Cretaceous-early Tertiary (Garcia-Casco et al., 2008). This stage was followed by renewed subduction, and final diachronous collision of the leading edge of the Caribbean plate with the continental margins of the North and South American continents during Tertiary times (Pindell et al., 2005). Arc-derived volcanic rocks of Cretaceous age are widely developed all along Cuba and occur in tectonic contact above or below the Northern Ophiolite Belt (west-central and eastern Cuba, respectively) and above the Guaniguanico, Pinos and Escambray metamorphic terranes (Iturralde-Vinent, 1998). The Cretaceous arc in Central Cuba has a consistent stratigraphy across strike, with oldest and deepest rocks in the south (including the Mabujina Amphibolite Complex) and younger rocks in the north. The ~5-10 km wide Mabujina Amphibolite Complex (MAC), considered the deepest exposed section of the CVA and its oceanic basement in Cuba (Somin and Millan, 1981; Somin,1993; IturraldeVinent, 1998), occurs structurally above the HP metamorphic Escambray terrane, which outcrop as a tectonic window, and below the Cretaceous Volcanic Arc (CVA). The MAC is characterized

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by foliated to blastoporphyritic orthoamphibolite, metabasalt and metatuff as the dominant lithologies. The complex includes a variety of intrusive rocks, including hornblendite, metagabbro, garnetbiotite-hornblende orthogneiss, biotite-hornblende orthogneiss, metatrondhjemite, metadiorite and metatonalite (Somin and Millan 1981; Grafe et al, 2001; Blein et al., 2003). Unfoliated tonaliticgranitic rocks and associated pegmatites locally crosscut this metamorphosed sequence, mainly in the northern part close to the contact with the non metamorphosed Cretaceous volcanic arc rocks. Geochemical and isotopic data from the igneous protoliths of the MAC are interpreted as island-arc rocks derived from a depleted mantle source with a sedimentary component (Blein et al., 2003 and unpublished data). We undertook a single zircon geochronological study of 14 gneissic and intrusive rocks of the MAC in order to constrain the evolution of arc magmatism in this region. A SHRIMP zircon age of 132.9±1.4 Ma was obtained for a trondhjemitic gneiss from the Jicaya River. On the basis of field relationships this age represents the oldest phase of granitoid magmatism in this area and probably in the entire Caribbean region. Another trondhjemitic orthogneiss collected near the previous sample gave an age of 123.9 Ma, and a further trondhjemitic gneiss has an age of 112±2.1 Ma with one inherited zircon at 1045±17 Ma. Four variously foliated and deformed meta-granitoids from different areas in the complex yielded ages between 93.8±0.5 and 92.8±0.7 Ma, with zircons from two samples showing inheritance at 315, 471, 903 and 1059 Ma. Two foliated granitoids from the eastern part of the complex provided ages of 89.3±0.45 and 87.2 Ma, whereas three unfoliated samples from the central part of the massif gave ages of 88.7±0.7 to 87.0±0.6 Ma. Finally, two unfoliated samples from the central and eastern part of the complex gave ages of 84.2±0.8 and 83.1±0.8, respectively. These ages suggest that the main phase of metamorphism in the MAC probably occurred at ca. 92 Ma but continued until ca. 87 Ma (in the eastern part). At the moment it is difficult to propose a precise tectonic reconstruction showing the position of the MAC in middle Cretaceous times. Following the models of Pindell and Kennan (2001), Pindell et al. (2005, 2006), and Iturralde-Vinent (2006) our age data suggest that the Mabujina protholiths formed somewhere in the Pacific realm in pre-middle

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Hauterivian time (pre -133 Ma). The occurrence of inherited Precambrian and Palaeozoic zircons in the plutonic rocks suggests a relatively proximal setting close to cratonic areas (N or S America). Alternatively, the zircons may represent recycling of sediments into the mantle via subduction and incorporation into rising slab melts (e.g., Shimoda et al., 1998). At the moment the occurrence of zircon xenocrysts remains an unexplained phenomenon. Our data for (meta)granitoid rocks in the MAC, ranging in age from -132 to 83 Ma, suggest that plutonism within Mabujina Complex occurred during the building stage of Cretaceous island arc formation. If this magmatism was continuous and related to a single subduction zone, it would imply that subduction of the Protocaribbean started as far back in time as the Hauterivian. However, evidence for Hauterivian island arc building from elsewhere in the Caribbean is scarce or non-existent. While most evidences indicate ca. 120 Ma for the onset of subduction of the Protocaribbean, some volcanic rocks from Dominican Republic, Jamaica and Cuba (e.g, Los Ranchos, Los Pasos Fms.) have been dated as either pre-Aptian (based in fossils and stratigraphic position Garcia-Delgado, et al., 1998) or mid-upper Aptian (110-118 Ma, based on U-Pb zircon dating in Los Ranchos Fm.; Kesler et al., 2005; Escuder Viruete et al, 2006). In addition, if the MAC represents indeed the metamorphosed deepest exposed section of the CVA in Cuba, it will be difficult to explain a polarity reversal event during Aptian-Albian times (114-125 Ma) because it would imply that the axis of the newly developed arc (with opposite polarity) would spatially coincide with the older arc. On the other hand, if a polarity reversal indeed occurred and onset of subduction of the Protocaribbean started at ca. 120 Ma, then the MAC may represent an exotic arc terrane upon which the CVA was build. Based on current tectonic reconstructions of the Caribbean, this hypothesis would imply the transfer of fragments of the Pacific arc to the hanging wall of the Protocaribbean arc, as suggested for fore-arc subduction complexes in central Cuba and Guatemala (Pindell et al., 2005, Garcia-Casco et al., 2006). The age of transfer, and probably of metamorphism of the old gneisses and of the protoliths of the amphibolites, would be 120-110 Ma, conflicting with previous proposals (Graffe et al, 2001). Clearly, more work is needed to resolve this conundrum.

2009 International Conference on Island Arc-Continent Collisions: Macquarie Arc Conference, Orange, New South Wales, Australia REFERENCES

GRAFE, F.; STANEK K . P.; BAUMANN, A . ; MARESCH, W . V . ;

BLEIN, O . ; GUILLOT, S.; LAPIERRE, H . ; M E R C I E R D E LEPI-

HAMES, W . E.; GREVEL, C . ; & MILLAN, G . 2 0 0 1 . Journal

NAY, B.; LARDEAUX, J. M . ; MILLAN TRUJILLO, G . ; CAMPOS,

Geology.

M . & GARCIA, A . 2 0 0 3 . Journal

of Geology, 1 1 1 , 8 9 - 1 0 1 .

DONNELLY, T . W . ; BEETS, D . ; CARR, M . J . ; JACKSON, T . ;

ITURRALDE-VINENT, M . A . , 1 9 9 8 . Acta

Geologica

Hispanica, 33, 9-56.

KLAVER, G . ; LEWIS, J.; MAURY, R.; SCHELLENKENS, H . ;

ITURRALDE-VINENT, M . A . , 2 0 0 6 , International

SMITH, A . L . ; W A D G E , G . & WESTERCAMP, D . ; 1 9 9 0 .

Review 48(9) 7 9 1 - 8 2 7 .

Geological Society of America, Boulder, CO,

Geological Society of America Bulletin 117, 9 8 7 - 9 9 5 .

ESCUDER V I R U E T E J., DIAZ D E NEIRA A . , HERNAIZ HUERTA P . P . , MONTHEL J., GARCIA SENZ J., JOUBERT M . , LOPERA E . ULLRICH T . , FRIEDMAN R . , MORTENSEN J., PEREZ-ESTAUN a . , 2 0 0 6 . Lithos 9 0 1 6 1 - 1 8 6 .

C . & RODRIGUEZ VEGA, A., 2 0 0 6 . Geologica

Acta, 4 , 6 3 - 8 8 .

GARCIA-CASCO, A . ; ITURRALDE-VINENT, M . & PINDELL, J.,

Geology Review, 50, 7 8 1 - 8 0 9 .

GARCIA-DELGADO, D . E.; DELGADO DAMAS, R.; MILLAN TRUJILLO, G . ; DIAZ D E VILLALVILLA, L.; SUKAR SASTROPUTRO, K.; LLANES, I.; BERNAL, L ; ROJAS Y . ; PEREZ PEREZ, C . ; DIAZ O T E R O , C . ; FURRAZOLA BERMUDEZ, G . ; PENALVER, L.; GARCIA CADIZ, L; PARDO, M . ; SUAREZ, V . ; & DUANI, E., 1 9 9 8 . Memorias 1:263-266.

PINDELL, J.; KENNAN, L.; MARESCH, W . V . ; STANEK, K . P.; DRAPER, G . & HIGGS, R . 2 0 0 5 . Geological

America Special Paper 394, 7-52.

Society of

PINDELL, J.; KENNAN, L.; STANEK; K . P . ; MARESCH, W . V . ; &

GARCIA-CASCO, a . , TORRES-ROLDAN, R . L., ITURRALDEVINENT, M . A . , MILLAN, G . , NUNEZ CAMERA, K . , LAZARO,

Geologia.

Geology

KESLER, S . E . , CAMPBELL, I . H . , ALLEN, C H . M . , 2 0 0 5 .

pp. H, 3 3 9 - H , 374.

2 0 0 8 . International

109,615-631.

del III Congreso Cubano de

DRAPER, G . 2 0 0 6 . Geologica

Acta 4 , 3 0 3 - 3 4 1 .

ROJAS-AGRAMONTE, Y . ; NEUBAUER, F.; KRONER, A . ; W A N , Y . S.; LIU, D . Y . ; GARCIA-DELGADO, D . E . & HANDLER, R . 2 0 0 4 . Chemical

Geology. 213, 3 0 7 - 3 2 4 .

SHIMODA, G . ; TATSUMI, Y . ; NOHDA, S.; ISHIZAKA, K . JAHN, B . M . , 1 9 9 8 . Earth and Planetary

Science Letter, 160,

479-492. SOMIN, M . L. 1 9 9 3 . Petrol. Miner.

18-31 (in Russian)

SOMIN, M . ;

(in Russian).

Tect. Tbilisi, Metsniereba.

MILLAN, G . 1 9 8 1 . Nauka,

Moscow,

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TECTONIC RESPONSE TO ANOMALOUS SUBDUCTION AND ARC-CONTINENT COLLISION G. Rosenbaum^ ^School of Earth Sciences,The University of Queensland, Brisbane, QLD 4072, Australia.

The arrival of density anomalies at subduction zones is responsible for dramatic changes in the dynamics and kinematics of the whole subduction system. Subductable anomalies (Cloos, 1993), such as aseismic oceanic ridges and hotspot tracks, locally increase the lithospheric buoyancy of the downgoing plate. In response, two major changes in the subduction system are expected. Firstly, increased buoyancy can flatten the dip of the subducting slab (Gutscher et al., 2000), modifying the structural and magmatic evolution of the overriding plate and terminating arc volcanism (Nur and Ben-Avraham, 1981; Rosenbaum et al., 2005). Secondly, kinematic modifications can occur due to a local decrease in rollback velocities at the area of ridge subduction. In retreating plate boundaries, this results in a progressive deformation of the subducted slab and the development of cusps (e.g. Aleutian-Kamchatka junction). A more detrimental effect to the subduction system is produced by local collisions of non-subductable material, for example, thick oceanic plateaux, volcanic arcs and continental blocks. Such collisions prohibit further subduction rollback, locally pinning the hinge of the subduction zone and tearing the slab into narrower segments. The effect of local collisional events on the evolution of a subduction system is well manifested in the Alpine-Mediterranean subduction-collision system. The current morphology of this orogenic belt, from Gibraltar to the Aegean Sea, comprised of a series of oroclines. Their formation is attributed to interactions between subduction rollback and local collisional events (Rosenbaum and Lister, 2004a). In Italy, for example, the incorporation of continental material at the subduction zone resulted in variations in rollback velocities along the strike of the subduction zone (Rosenbaum and Lister, 2004b). The process involved tearing and segmentation of the subducting lithospheric slab (Royden et al., 1987; Rosenbaum et al., 2008). Such deep (100-500 km) sub-vertical tear faults are recognised in seismic tomography results (Rosenbaum et al., 2008) and their development is consistent with kinematic reconstructions.

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An important aspect associated with tear faulting and slab breakoff is its role in controlling the temporal and spatial distribution of magmatism. Lithospheric tearing provides a pathway for asthenospheric upwelling that can trigger magmatism. In Italy and the Tyrrhenian Sea, the position of a large number of magmatic centres, including the non-subductionrelated volcanos of Mt Etna and Mt Vulture coincides with such tear faults (Rosenbaum et al., 2008). Magmatic activity induced by tear faulting was a transitional phase between subduction-related arc magmatism and post-collisional magmas related to slab breakoff (Gasparon et al., 2009).

REFERENCES CLOOS, M. 1993. Geological Society of America Bulletin, 105(6), pp. 715-737. GASPARON, M . , ROSENBAUM, G . , WIJBRANS, J. & MANETTI,

P. 2009. Journal of Geo dynamics, 47, pp. 30-38. GUTSCHER, M . - A . , SPAKMAN, W . , BIJWAARD, H . &

ENGDAHL, E.R. 2000. Tectonics, 19(5), pp. 814-833. NUR, A. & BEN-AVRAHAM, Z. 1981. In: L.D. K u l m , J.

Dymond, E.J. Dasch, D.M. Hussong & R. Roderick (Editors), Nazca Plate: crustal formation and Andean convergence. Geological Society of America Memoir, pp. 729-740. ROSENBAUM, G . , GASPARON, M . , LUCENTE, P . P . , PEC-

CERILLO, A. & MILLER, M.S. 2008. Tectonics, 27, TC2008, doi: 10.1029/2007TC002143. ROSENBAUM, G . , GILES, D . , SAXON, M . , BETTS, P . G . ,

WEINBERG, R. & DUBOZ, G. 2005. Earth and Planetary Science Letters, 239, pp. 18-32. ROSENBAUM, G. & LISTER, G.S. 2004a. In: A.J. S u s s m a n

and A.B. Weil (Editors), Orogenic curvature: Integrating paleomagnetic and structural analyses. Geological Society of America Special Paper, pp. 41-56. ROSENBAUM, G. & LISTER, G.S. 2004b. Tectonics, 23(1), TC1013, doi:10.1029/2003TC001518. ROYDEN, L., PATACCA, E. & SCANDONE, P. 1 9 8 7 . 15, p p . 7 1 4 - 7 1 7 .

Geology,

2009 International Conference on Island Arc-Continent Collisions: Macquarie Arc Conference, Orange, New South Wales, Australia

FAST-TRACK BARROVIAN METAMORPHISM, ARC-CONTINENT COLLISION AND CONNEMARA. Paul D. Ryan Earth & Ocean Sciences, National University of Ireland Galway, Galway, Ireland, paul.ryan@nuigalway.ie

Advances in radiometric age dating have allowed, with some degree of precision, the resolution of Harrovian pressure-temperature-time (PTt) curves associated with collisional orogeny. Many first-cycle metamorphic rocks in such sytems show similar, clockwise PT loops. However, they may look very different in PTt space, with the rates of heating, the length of the metamorphic maximum and the rates and duration of cooling and exhumation differing. Such variations give insights into the tectonic evolution of the orogen. One problem highlighted by such studies is that simple crustal thickening is too slow to explain the prograde portion of some PTt loops. This contribution uses numerical modelling to investigate the time constraints on generating Harrovian conditions in a refrigerated continental margin in the foot-wall of an arccontinent collision. The geological scenario being investigated is that of the Ordovician Grampian Orogeny believed to be a result of arc-continent collision between the Laurentian margin and the Lough Nafooey-Tourmakeady island arc in which, remarkably, both the hanging and the foot wall of the orogen are preserved. A coincidence of timing between these two rock suites - the metamorphic complex of Connemara and the arc complex of

South Mayo respectively - allows us to define the entire PTt loop for the orogen. Collision leading to amphibolite facies conditions in Connemara apparently took no more than 10 Ma between 485 Ma and 475 Ma. This was followed by rapid isothermal uplift between 475 Ma and 470 Ma, believed to be associated with subduction flip. Exhumation and cooling to greenschist facies conditions had taken place by 460 Ma with migmatitic gneisses being unconformably overlain by Lower Silurian sediments by <440 Ma. A major problem in explaining this metamorphic history is the rapid rate of heating of the continental margin. Possible explanations for this high rate of heating are modelled including: crustal thickening during arc-continent collision; variation in initial thermal structure and heat productivity of both the hanging and footwalls; advection of heat by emplacement of magma; and different rates of overthrusting. This analysis suggests that in a collisional setting regional Harrovian metamorphic conditions in the footwall can be developed more rapidly by overthrusting a hot hanging wall than by crustal thickening. The geological feasibility of applying such a model to the thermal evolution of Connemara is discussed.

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BASEMENT TO THE ORDOVICIAN MACQUARIE ARC, LACHLAN OROGEN, NSW: CONSTRAINTS FROM ZIRCON DATA A. Saeed/ R. A. Glen/'" C. D. Quinn/ E. Belousova/ & W. Griffin/ ^ National Key Centre Geochemical Evolution and Metallogeny of Continents, Departnnent of Earth & Planetary Sciences, Macquarie University, Sydney NSW 2109 Australia ^Geological Survey of New South Wales Departnnent of Primary Industries PO Box 344 Hunter Regional Mail Centre, NSW 2310 Australia.

The Macquarie Arc is an intraoceanic island arc that was accreted to the Lachlan Orogen in the Early Silurian Benambran Orogeny and is now flanked by Ordovician quartz-rich turbidites ±black shales. The arc evolved in four phases from early Ordovician through to the Early Silurian, and was subsequently split into several structural belts by Silurian and Devonian extension/transtension. Zircons from three phases were analysed for U-Pb and Hf in order to constrain the evolution of the arc and provide information on the nature of its substrate.

(Hf data range from +14.6 to +8) with a very small number of older grains. Another sample is swamped by old grains between 560 and 610 Ma (with a spread of eHf from +4 to -25), as well grains around 1000 Ma, 1550-1650 Ma and older scattered Archaean zircons. One sample of the phase four Swatchfield Monzonite produced a single magmatic peak at 444 Ma and eHf from +9.43 to -0.47 or less with some lesser values down to - 4 4 The second sample didn t show any magmatic zircons, but older grains at 562 Ma, between 900-1300,1600 and 1700-2000 and 2800 Ma.

Phase one volcaniclastic sandstones (Mitchell Formation) give a dominant zircon peak at 490-507 Ma, inferred to represent the magmatic event. Older zircons occur at 560-600 Ma, -1050 Ma, 1780 Ma and extend back to the Archaean. 472 Ma ages contradict younger stratigraphic constraints. Magmatic zircons have eHf values ranging from 0 to - 5 . In contrast, the older 560-600 Ma zircons show a spread of eHf values (-7 to -25). A sample of the overlying Hensleigh Formation at the top of phase one indicates a main (inferred magmatic) peak at - 4 9 9 Ma (Hf =0), with scattered older zircons.

We interpret the zircon populations as reflecting the variable mixing of mantle-derived juvenile zircons with older zircons extending back to the Archaean. Possibilities for the 500-600, 1000-1300 Ma and zircons grains as old as the Archaean include either sedimentary input into the trench (considered unlikely) or underlying igneous or sedimentary rocks in the melt zone that would have undergone local melting during passage of mantle-derived magmatic melts, with resultant mixing of zircons. Older zircons might have been derived from igneous substrate of the arc (based on the geochemistry of A. J. Crawford, Glen et al. 2007) possibly rifted from the Delamerian Orogen during rollback at - 5 0 0 Ma (e.g. Glen and Crawford 2005), or from Early to Middle Ordovician quartz rich turbidites similar to those that flank the Macquarie Arc at the surface and which underlie the arc either depositionally or structurally.

Samples from phase two are represented by the Cargo Volcanics and Fairbridge Volcanics. Andesite from the Cargo Volcanics show a dominant age peak at 462 Ma, with eHf values of +15 to +10. A lower Fairbridge Volcanics sample shows two main populations, 455460 Ma (Hf=+12 to +10) (too young) and 470-475 Ma (Hf=+14 to +9) (older than thought if magmatic) with a small population of too young zircons and a small scattering of older zircons. The onset of the magmatic hiatus above phase 2 is represented by the Basal Ranch Member which contains zircons with dominant ages of 460-465 Ma inferred to have been derived from underlying Cargo Volcanics) as well as older zircons with age peaks of 485-490 and 495-505 Ma. From phase four, one sample of the Rockley Volcanics contains a single peak of magmatic zircons at 453 Ma

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Glen and Quinn publish with the permission of the Director, Geological Survey of New South Wales, NSW Department of Primary Industries.

REFERENCES

GLEN, R. A. & CRAWFORD A. J. 2005. Abstracts of the

Geological Society of Australia, 81, pp. 113-114.

GLEN, R. A., CRAWFORD A. J, PERCIVAL, I. G. (2007).

2009 International Conference on Island Arc-Continent Collisions: Macquarie Arc Conference, Orange, New South Wales, Australia

TECTONIC CONSEQUENCES OF THE END-ON COLLISION OF AN INTRA-OCEANIC ARC WITH A CONTINENT — THE EXAMPLE OF THE ORTHOGONAL PLUNGE OF THE ALEUTIAN ARC INTO THE KAMCHATKA SUBDUCTION ZONE. David W. Schoir^ ^Department of Geology and Geophysics, University of Alaska Fairbanks ^Mailing address: MS 999, U.S. Geological Survey, 345 Middlefield Rd. Menio Park, California, 94025, USA

INTRODUCTION

Along the western margin of the Pacific region, colHsions at four subduction zones are underway between an offshore arc massif and a continental margin. From south to north, these collision zones (CZs) are the Melanesian arc system with Papa New Guinea, the Luzon arc with SE Asia at Taiwan, the Izu-Bonin arc with SW Japan (Tanzawa CZ), and, in the northwestern corner of the Pacific Basin, the Aleutian arc with continental Kamchatka (KAT CZ) (Fig.l). The setting of the southern Melanesian-PNG and Luzon-Taiwan CZs are similar in that the arc massif resides in the upper plate, collisional contact is a progressive, migrating broadside, convergence is presently slow, and coastal orogenesis is spectacular. The tectonic setting of the two northern CZs differ in that their arc massif resides on the subducting lower plate, convergence is slow (Tanzawa) to high

(KAT), entrance of the arc into the subduction zone is effectively end-on, and coastal mountain building is modest. The KAT collision is further distinguished by a high convergence speed (-80 km/ myr) and orthogonal plunge of the far western or Komandorsky sector of the Aleutian massif into the Kamchatka SZ (Fig 1). The KAT CZ has remained generally fixed, or has migrated only slowly along the margin, during the late Cenozoic. EVOLUTION OF THE SETTING OF THE KAT COLLISION ZONE

Ar-Ar dating and paleomagnetic data document that, for problematic reasons, the Aleutian SZ formed in the early Eocene (-50 Ma) as a westward continuation of the older (Mesozoic), E-W trending SZ bordering southern Alaska (Fig. 2; Scholl, 2007). It is hypothesized that the new Aleutian SZ extended westward roughly half way across the north Pacific

160E

Figure 1: Late Cenozoic tectonic setting of the KAT (Kamchatka-Aleutian) continent-oflfshore arc collision zone 121

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North American Plate

NE RUSSIA

ALASKA

Shirshov Ridge

Bering Microplate

Connectiion

1000 km

Figure 2: Eocene formation of the Aleutian subduction zone, arc massif, and westward transform connection to the Kamchatka subduction zone before swing north along its now-defunct Shirshov sector to connect with the northwestern corner of the Pacific Basin (Fig. 2). Within -10 myr, magmatism above the new SZ system compiled the 200-300km wide, ~30-km-thick arc massifs of the thenconnected Aleutian and Shirshov Ridges (Figs.l and 2). Together they cordoned off the NW Pacific to form the natal, SZ- surrounded Bering Sea Basin. The principal plates involved were the North American (NAM) and Pacific (PAC) plates and, possibly linked to the extrusion of the Alaska interior toward the new offshore SZ complex, the Bering Sea microplate between them (Fig. 2). Figure 2: Eocene formation of the Aleutian subduction zone, arc massif, and westward transform connection to the Kamchatka subduction zone Birthing of the E-W Aleutian SZ forged a new, offcontinent PAC-NAM boundary that extended as far seaward as the Aleutian-Shirshov connection, beyond which the boundary continued westward as a transform shear to tectonically connect with the Kamchatka SZ (Fig. 2). In response to backarc spreading, convergence along the N-S-striking Shirshov SZ ended in the early Miocene (~ 20 Ma), but remained active beneath the E-W trending Aleutian SZ to nourish the continued and on-going magmatic growth of the Aleutian Arc massif.

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Along the modern Aleutian SZ, the angle of PACNAM convergence progressively decreases westward toward Kamchatka (Fig. 1). This kinematic link has apparently prevailed since the Eocene (Fig 2). The far western or Komandorsky sector of the Aleutian arc massif is presently a right-lateral transform plate boundary that intersects the Kamchatka SZ at the eastward jutting Cape Kamchatka Peninsula. The principal transform is the NW-striking, seismically active Bering-Kresta shear zone. It runs along the base of the sector s northern or Bering Sea side and orthogonally enters the SW-NE striking Kamchatka SZ at Cape Kamchatka Peninsula (Fig.l). GPS data document that the island-crested sector is moving northwestward toward Kamchatka at virtually the speed of the Pacific plate. The Komandorsky sector is thus effectively attached to the Pacific plate and vectored along the Bering-Kresta shear zone to eventually collide end-on with Kamchatka. The structural width of the sector, which is westernmost of several large blocks of arc massif moving toward Kamchatka, is on the order of 100 km. TECTONIC CONSEQUENCES OF END-ON ARCCONTINENT COLLISION The NW end of Komandorsky sector lies -150 km seaward of the Kamchatka Trench, and thus is not

2009 International Conference on Island Arc-Continent Collisions: Macquarie Arc Conference, Orange, New South Wales, Australia presently colliding with the Eurasian continent, but will so in ~2 myr. The Cape Kamchatka Peninsula and more inland Kumroch thrust belt structurally and physiographically register the cumulative impact of past end-on collisions of blocks of arc massif (Geist and Scholl, 1994). Inland of the KAT collisional area, the axis of active volcanic volcanism shifts inboard - 6 0 km, a deflection that probably in part reflects the subduction of the far western Aleutian massif beneath the Kamchatka forearc. Mafic basement rock of Late Cretaceous age are exposed at Cape Kamchatka Peninsula, but the oldest dated igneous rocks so-far collected from the Aleutian and Shirshov arc massifs are - 4 6 Ma, thus at least 30 myr younger. Past events of arc-continent collision apparently did not result in the frontal accretion of arc sectors but rather their westward subduction beneath the submerged forearc.

Differing, frontal accretion of middle and upper crustal rocks does occur at the end-on, suborthogonal, coUision of the Izu-Bonin arc with SW Japan (Tanzawa CZ). The settings of the two arccontinent collision zones are tectonically unlike in that the arc-accreting Tanzawa CZ is characterized by slow convergence, low dip-angle of subduction, and a wide (250 km), laterally continuous incoming arc massif. In contrast, the non-accreting KAT CL is distinguished by high convergence, high dip-angle of subduction, and a narrow (-100 km), laterally discontinuous incoming arc massif

REFERENCE G E I S T AND S C R O L L , 1 9 9 4 , Tectonics,

13, 5 3 8 - 5 6 0 .

SCROLL, D. W., 2007. American Geophysical Union, Monograph 172, 3-35. Australian Journal of Earth Sciences 54, pp. 1 6 7 - 1 7 9 .

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RECOGNITION OF VOLCANIC CENTRES IN ANCIENT ISLAND ARC SYSTEMS: EXAMPLES FROM THE ORDOVICIAN MACQUARIE ARC, NSW, AUSTRALIA CJ. Simpson Geological Survey of New South Wales, NSW Department of Primary Industries, PO Box 344, Hunter Region Mail Centre, NSW 2310, Australia

Interpretation of volcanic facies in ancient island arc systems can be problematical due to the constraints imposed by overlying younger rocks, tectonic fragmentation and variable degrees of hydrothermal alteration, metamorphism and weathering. Identification of facies or facies associations that constitute major volcanic centres in ancient island arc settings is of importance since many porphyry CuAu-(Mo) deposits are related to the intrusive phases of these centres. However, identification of ancient arc volcanoes is frequently inferred rather than demonstrated and the precise tectonic affiliation (i.e. main arc, inter-arc basin, back-arc basin, seamounts) relies more on geochemical attributes than on mapped relationships. In the case of the Ordovician intra-oceanic Macquarie Arc in the Lachlan Orogen in the central west region of New South Wales, our understanding of the volcanic facies architecture is limited, in part due to poor exposure and a lack of systematic facies mapping. As a consequence, only a small number of volcanic centres have been confidently recognised and described, although a much larger number have been inferred. Comparisons with volcano size and spacing and the variety of volcanic facies developed in modern island arc chains such as the Izu-Bonin-Marianas Arc and Tonga-Kermadec Arc, as determined from drilling and high-resolution imaging, has proved critical in the reconstruction of these ancient centres. In turn, well-documented ancient volcanic centres offer a cross-sectional view of facies relationships that can rarely be determined in modern island arcs. The early development of the Macquarie Arc (Lancefieldian to Bendigonian) is preserved only in two small areas (Nelungaloo Volcanics in the JuneeNarromine Volcanic Belt and Mitchell Formation in the northern Molong Volcanic Belt), where the succession consists of mainly high-K calc-alkaline basalt to andesite lavas and/or high-level sills that are intercalated with thick units of volcanic conglomerate

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and polymictic volcanic breccia and minor finer grained sedimentary rocks. Both areas have been interpreted as volcanic edifices or the proximal volcaniclastic apron adjacent to volcanic islands (Simpson et al. 2005; Glen et al. 2007), on the basis of the coarse grainsize and textural immaturity of the volcaniclastic facies. However, their spatial position in the architecture of the Early Ordovician arc is unknown. The Middle to Late Ordovician extrusive part of the Macquarie Arc (Darriwilian to BoHndian) is much more extensive in its development and a large number of volcanic centres has been inferred. Among the more convincing examples, is the preEastonian Cargo volcanic centre in the southern Molong Volcanic Belt (Figure 1) that is composed predominantly of massive and pillowed basalt and andesite lavas, sills and dykes, less voluminous dacite, conspicuous hyaloclastite and pillow breccia and minor intercalated turbiditic volcaniclastic horizons (Simpson et al. 2007). Erosion of the uplifted edifice is recorded by thick beds of volcanic conglomerate and sandstone that were reworked in a high-energy subaerial or shallow marine setting and which unconformably overlie the lava-dominated pile. The conglomeratic deposits are in turn unconformably overlain by the thick Eastonian limestone package that marks cessation of volcanism in this area. The Cargo volcanic centre has many attributes in common with large, oceanic arc volcanoes, including size (>25-30 km basal diameter), dominance of coherent lavas and shallow intrusive rocks, lenticular geometry, marked vertical and lateral facies variations and medium K calc-alkaline geochemistry. A small basaltic seamount of late Darriwilian to late Gisbornian age, composed of massive and pillowed lavas, has been described in the lower part of the Forest Reef Volcanics near Cadia (southern Molong Volcanic Belt) by Squire & McPhie (2007). Other possible centres of a similar age have been proposed to the north (e.g. around Molong and probably including the Copper Hill host succession, and north

2009 International Conference on Island Arc-Continent Collisions: Macquarie Arc Conference, Orange, New South Wales, Australia

of Wellington) on the basis of significant volumes of pillow lavas, lava breccia and associated volcanic conglomerate and sandstone. Convincing examples of latest Ordovician volcanic centres have been described in the upper part of the volcanic successions at Parkes and Cadia that host world-class porphyry Cu-Au deposits (Simpson et al. 2005; Squire & McPhie 2007). In both cases, the preserved sequence has been interpreted as a proximal subaqueous volcaniclastic apron developed on the flanks of a nearby substantial volcanic island. Large bodies of porphyritic trachyandesite were emplaced within the volcaniclastic apron as a series of sills and lava domes. In the case of the Parkes area, the volcaniclastic apron (Wombin Volcanics) includes thick units of polymictic volcanic breccia that contains megablocks of densely welded ignimbrite and coherent lava, both of trachyte composition.

The presence of subaerially erupted material within the apron sequence is further confirmation of the existence of a nearby volcanic island. Published with the permission of the Director, Geological Survey of New South Wales, NSW Department of Primary Industries REFERENCES G L E N , R . A . , C R A W F O R D , A . J . , P E R C I V A L , I . G . &: BARRON,

L.M. 2 0 0 7 . Australian Journal of Earth Sciences, 54,

pp.167-179.

SIMPSON, C . J . , G A S , R . A . F . & A R U N D E L L , M . G . 2 0 0 5 .

Australian Journal of Earth Sciences, 52, 8 6 3 - 8 8 6 .

SIMPSON, C . J . , S C O T T , R.J., G R A W F O R D , A . J . & M E F F R E S.

Australian Journal of Earth Sciences, 54, pp. 3 1 5 - 3 5 2 . SQUIRE, R.}. & M C P H I E , J. 2 0 0 7 . Australian Journal of Earth Sciences, 54, pp. 2 7 3 - 2 9 2 .

island summit caldera small ancillary vent

feeder & more crystalrich sills and dykes

subaqueous re-sedimented pyroclastics

volcaniclastic conglomerate

subaerial pyroclastics

pebbly sandstone, polymictic volcanic breccia

massive sparsely porphyritic andesite

siltstone, mudstone

re-sedimented hyaloclastite, pillow breccia

moderately porphyritic andesitic lava, dykes, sills, minor associated peperite

D •^

F i g u r e 1. S c h e m a t i c c r o s s - s e c t i o n o f a n o c e a n i c arc v o l c a n o b a s e d o n t h e lateral a n d v e r t i c a l v a r i a t i o n s in v o l c a n i c facies n n a p p e d w i t h i n t h e M i d d l e t o Late O r d o v i c i a n C a r g o Volcanics, M a c q u a r i e Arc.

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PALAEOZOIC PALAEOGEOGRAPHY: THE VARISCAN, ALPINE AND LACHLAN FOLDBELTS J. A.Tait School of Geosciences, Grant Institute, Kings Buildings, University of Edinburgh, Edinburgh EH9 3JW, Scotland.

The main elements controlling and driving Palaeozoic palaeogeography are Laurentia, Baltica, Gondwana and Siberia which formed as a result of the break-up of the Proterozoic supercontinent Rodinia. Reconfiguration of the various cratons in Neoproterozoic times resulted in formation of the supercontinent Gondwana, which comprised S America, Africa, Arabia, India, Antarctica and Australia. Consolidation of this large continental mass was completed by the latest Cambrian, and by Early Permian times, had collided with the now amalgamated N America and Eurasian plates to form Pangaea. However, and despite the numerous studies carried out over the last 30 years, the palaeogeography of this supercontinent for the intervening Palaeozoic remains controversial. This is due largely to questions concerning the palaeomagnetic dataset and the apparent polar wander path for Gondwana. While it is generally accepted that in Cambrian times, the palaeo-south pole was situated in northern Gondwana (N Africa), and that by the Carboniferous

Gondwana had moved across the south pole such that by the Late Carboniferous/Early Permian the palaeosouth pole was in Antarctica (Fig.l), the movement of Gondwana and the apparent polar wander path (i.e., the track of the south pole across the continent) remains disputed. Two end member models similar to those originally proposed by Morel and Irving (1978) to accommodate the different palaeomagnetic data sets have been proposed and remain relatively unchanged (Fig.l). These two contrasting models indicate either gradual northward movement of Gondwana from Late Ordovician through to Early Carboniferous times (path-X, Fig. la), or a much more complex path implying much more rapid movement of the continent and repeated collision of the north African margin of Gondwana with Europe/N America (see Tait et al., 2000 for discussion). The differences between these two models are dependant upon whether or not palaeomagnetic data derived from Palaeozoic sequences of the Lachlan foldbelt of Australia are taken into account. Various

Permian Carb Devonian Silurian Ordovician

Figurel. The two APW paths (i.e., the relative movement of the palaeosouth pole over the Gondwana Supercontinent) which have been proposed for Gondwana, (a) the more conservative X-path and (b) the more complex Y-path which takes into account data from the Lachlan Foldbelt (after Tait et al., 2000).

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2009 International Conference on Island Arc-Continent Collisions: Macquarie Arc Conference, Orange, New South Wales, Australia composite paths and models providing intermediary scenarios ranging between the two end-member models have been proposed. Nevertheless, the problem concerning the Australian data set and whether or not data from this region can be used to constrain Gondwana remains controversial. Clearly, therefore, our understanding and interpretation of the Palaeozoic structural and tectonic evolution of the Lachlan Foldbelt of eastern Australian has huge implications for our understanding of European and N American Palaeozoic palaeogeography, and possible vice versa. The Variscan fold belt of Europe resulted from the collision of North Africa with Baltica and Laurentia and the intervening Gondwana derived microplates in early Palaeozoic times. Convergence and intracontinental deformation lasted for some ISOMy with the closure of at least four oceanic basins - the lapetus, Tornquist, Rheic and GaliciaMassif Central Basins - with associated deformation corresponding to the Caledonian, Variscan and to some extent the Alpine orogenies. Thus, the earlier structures within Variscan Europe have been heavily overprinted or even destroyed by the younger events making determination of the pre-collisional plate configurations difficult. However, combining the most recent palaeomagnetic, geological, and palaeobiogeographical information have led to significant improvements in our understanding of this orogenic belt and demonstrate that it comprises an assemblage of terranes which originated from the northern margin of Gondwana and gradually, through Palaeozoic times, moved northwards and accreted onto the southern margin of Baltica/ Laurentia in the Late Palaeozoic.

The Alpine foldbelt of southern Europe is more complicated. The palaeogeography of the various pre-Variscan terranes remains unclear due to strong Alpine overprinting and deformation. Nevertheless, faunal and facies studies of the Proto-Alpine terranes, primarily the Eastern Alps, show they were clearly distinct from Gondwana assemblages (Schonlaub 1992) suggesting that these terranes did not remain adjacent to the northern margin of Gondwana. Recent palaeomagnetic data confirm this model and suggest that the Proto-Alpine terranes formed a tectonically discrete microplate which rifted from the Gondwana margin in the Early Palaeozoic and collided with the southern Eurasian margin in Carboniferous times. These palaeomagnetic data, obtained from the Eastern Alps of Austria, are perhaps the most significant for the Lachlan foldbelt! The different palaeogeographic reconstructions, palaeomagnetic work carried out in the Variscan, Alpine and Lachlan foldbelts, and the implications of the data from Lachlan foldbelt on our understanding of European palaeogeography will be presented and discussed REFERENCES MOREL, P. & IRVING, E. 1978. Journal of Geology, 86, pp. 535-56L

SCHONLAUB, H.P. 1992. Jahrbuch der Geologischen BundesanstalU

135, pp. 3 8 1 - 4 1 8 .

TAIT, J. A. 2000. Geological Society of (London) Special Publication,

179, p p . 2 1 - 3 4 .

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CONSTRUCTION OF AN ALKALIC PORPHYRY AU-CU PROVINCE IN BRITISH COLUMBIA: TRIASSIC AND EARLY JURASSIC MAGMATISM, AMALGAMATION, AND ACCRETION OF OFFSHORE ISLAND ARCS TO NORTH AMERICA R.M. Tosdal\ J.M. Mortensen^ A.C. Harris^ T. Bissig^ & C. Hart^ ^Mineral Deposit Research Unit, University of British Colunnbia, Vancouver, BCV6T1Z4, Canada. ^ ARC Centre of Excellence in Ore Deposits, University of Tasmania, Private Bag 79, Hobart,Tas. 7001, Australia.

The Cordilleran collage of British Columbia (BC) contains the largest concentration of alkalic porphyry Cu-Au deposits and prospects in the world, stretched over the 1500 km strike length of the Late Triassic and Early Jurassic magmatic arc that composes the inboard (present coordinates) Quesnel and outboard Stikine terranes. Whereas individual porphyry CuAu deposits and districts may not yet be comparable in size to the immense Late Ordovician Cadia complex in the Lachlan Orogen of New South Wales (Australia), the sheer number of mineralised centers and their areal distribution suggests a unique set of geologic, magmatic, and tectonic events operated over very large distance, in stark contrast to the comparatively smaller area of the Lachlan.

The Late Triassic magmatic arc Marine volcanic and derivative rocks dominate the Late Triassic marine arc complexes of the Quesnel and Stikine terranes. These rocks form the geographically distinct Nicola, Takla, and Stuhini groups, from south to north, in British Columbia. Submarine basaltic to andesitic augite and/or plagioclase phyric lavas and associated volcaniclastic rocks dominate the groups, with less common more felsic varieties and subaerial deposits locally forming important components. A common characteristic over the entire strike length of the Nicola, Takla, and Stuhini groups is the presence of augite (± plagioclase) phyric basalt and derivative monomictic volcanic-dominated sedimentary breccia and conglomerate representing fringing volcaniclastic aprons on the flanks of the marine volcanos. The Nicola group in southern British Columbia, the best known of the groups, represents a nearly complete cross section through a marine arc (Mortimer, 1987). An eastern belt is dominated by distal volcanic and sedimentary facies derived from augite porphyritic basalt and andesite, whereas

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the central belt contains more proximal deposits, including coherent lavas of similar composition as in the sedimentary rocks of the eastern belt. The western belt consists principally of flow, pyroclastic and volcaniclastic rocks ranging from basalt to rhyolite. Locally, the Nicola group consists of a lower volcano-sedimentary unit containing black argillite, limestone and volcanic-derived sedimentary rocks, a middle submarine augite phyric basalt and basalt breccia domain and an upper partially subaerial polymictic volcaniclastic unit. A similar spatial and stratigraphic understanding of the Takla and Stuhini groups is lacking, due to principally to poor access and consequent lack of detailed study. Three compositionally distinct groups of volcanic rocks are recognized (Mortimer, 1987; Barrie et al., 1992; Logan & Mihalynuk, 2005). Group 1 comprises porphyritic augite basalt and picrite that are generally shoshonitic in composition and at least locally around the alkalic porphyry system contain feldspathoidal phenocrysts. Group 2 consists of augite- and plagioclase-phyric basalt and andesite belonging to the low K calc-alkaline series. Group 3 is composed of petrographically heterogeneous andesite and basalt of tholeiitic to transitional affinity. These compositional groups are aerially overlapping, but group 3 rocks generally occur in the western belt whereas group 2 and 1 rocks are most abundant in the central and eastern belt. Shoshonitic lavas from group 1 are stratigraphically above group 2 and 3 rocks, where stratigraphic relations are known. Feldspathoidalbearing rocks also form the top of many volcanic sections, just beneath a major unconformity with Early Jurassic rocks. Subvolcanic intrusive complexes generally have similar compositions as the volcanic rocks. However, large batholiths formed late in the magmatic evolution of the arc. Notable from a metallogenic viewpoint are large zoned calc-alkaline plutonic

2009 International Conference on Island Arc-Continent Collisions: Macquarie Arc Conference, Orange, New South Wales, Australia complexes (e.g. Guichon Batholith) that have associated Cu-Mo porphyry deposits (Highland Valley and Gibralter), and the generally slightly younger complexly altered, locally feldspathoidalbearing, silica undersaturated to slightly saturated alkalic diorite to monzonitic batholiths (Iron Mask and Mt. Polley) which show a transition from anhydrous to hydrous phases at about the time of formation of the alkalic porphyry Cu-Au systems (Leuck et al., 1994). Pyroxenite is a common but a relatively minor component of the alkalic complexes. Some of the alkalic porphyry related magmatic rocks appear be associated with eruptive products particularly where feldspathoidal phenocrysts are present, but largely the batholiths appear unconnected chemically to the dominantly tholeiitic to transitional basaltic volcanic rocks of the Nicola, Takla, and Stuhini groups. The porphyry deposits A paired belt of calc-alkaline and alkaline porphyry deposits hosted in the Nicola and Takla group rocks in Quesnel terrane of southern and central BC has long been recognized (Barr et al., 1976). Calcalkaline porphyry systems are not known in Stuhini group rocks of the Stikine terrane. The calc-alkaline porphyry Cu-Mo deposits are generally older, being ~ 215 to 210 Ma (Mortensen et al., 1995), whereas the alkalic porphyry Au-Cu deposits are younger and formed during two discrete temporal events. The older episode is the most widespread, and formed between 210 and 200 Ma (Mortensen et al., 1995; Logan et al., 2007). Included within the older episode are two mineralizing events at Galore Creek (Schwab et al., 2009), Red Chris, Mt. Polley, deposits around the Iron Mask batholith, and Copper Mountain. Just south of Mt. Polley, calc-alkaline Cu-Mo-Au porphyry formation appears a few million years after emplacement of the alkalic porphyry Au-Cu deposits (Schiarizza & Friedman, 2009); none of these newly recognized Early Jurassic deposits are yet economic. A hiatus of 10 to 15 million years separates the widespread Late Triassic alkalic porphyry Cu-Au systems from the more the areally restricted Early Jurassic (180-190 Ma) Lorraine and Mt. Milligan silica saturated alkalic porphyry Au-Cu deposits in north central British Columbia (Mortensen et al., 1995). The time gap corresponds to a major Cordilleranlong shortening event that also marks a magmatic transition from dominantly tholeiitic volcanism to dominantly calcalkaline volcanism of the Early to

Middle Jurassic Hazelton - Rossland arc and a major deformation event (e.g. Monger et al, 1992). Tectonics setting The Quesnel and Stikine terranes represent marine island arc systems that are now separated by the Cache Creek terrane, which is considered to have formed to the west of the marine arc and represent the accretionary prism and forearc sedimentary rock sequences. Unlike the rifted Ordovician to early Silurian arc of the Lachlan orogen, these terranes are considered to represent distinct segments of a large and continuous marine island arc system. Two main models are proposed to explain how the two terranes became juxtaposed on opposite sides of the Cache Creek terrane. One model proposes the translation of Stikinia along dextral strike-slip faults from a more southern location to the current location outboard of the Cache Creek terrane (Wernicke and Klepacki, 1988). The second and more generally accepted model proposes a major oroclinal bending of the marine arc, and the trapping of the Cache Creek terrane between two parts of a laterally continuous marine island arcs (Mihalynuk et al., 1994; Nelson & Colpron, 2007). In this scenario, one might envision the paleotectonic setting of the Quesnel and Stikine arcs to be reminiscent of the island arc systems (e.g. Indonesian archipelago) of the southwestern Pacific or the Aleutian chain west of the Alaska Peninsula. Regardless of how the terranes arrived at their present juxtaposition, they formed some distance offshore from stable North America in more southerly latitudes, and were amalgamated to North America in the Middle Jurassic, with final closure of the intervening Slide Mountain ocean between 180 and 170 Ma. Emplacement of the Early Jurassic Lorraine and Mt. Milligan silica saturated alkalic Cu-Au systems is broadly contemporaneous with the onset of final amalgamation, suggesting a analogous tectonic setting to the formation of alkalic systems in Papua New Guinea and Tabar-Lihir-Tanga-Feni volcanic chain (Mclnnes and Cameron, 1994) arcs where collision and choking of a subduction zone facilitated the rise and emplacement of deeply sourced alkalic magmas. A similar understanding of the tectonics of the Late Triassic alkalic porphyry Cu-Au deposit is unknown. It is generally accepted that the Quesnel and Stikine terranes are fragments of originally continuous arc systems. Furthermore, alkalic porphyry Cu-Au deposits are scattered over a 1500 km strike length.

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and were emplaced in a short time frame between 210 and 200 Ma, mostly after the end of widespread tholeiitic to transitional magmatic activity and after formation of slightly older calc-alkaline porphyry Cu-Mo deposits. The Late Triassic alkalic porphyry Au-Cu deposits thus formed at the end of a long period of magmatism and broadly preceded a major period of uplift, erosion, and deformation, an environment common to porphyry Cu deposits worldwide. No evidence for a large-scale collision and cessation of subduction, as is the case for Papua New Guinea, is known. Furthermore, the long strike extent of the Late Triassic porphyry Cu deposits seems to preclude a local collisional event such as the choking of subduction zones by an oceanic plateau. Instead, the evidence for shortening and magma cessation requires a different explanation for the shift from tholeiitic and calcalkaline magmatism to a short duration of alkalic magmatism. Perhaps it is as simple as the far field effect recording reorganization of the offshore subduction zones that accompanied the arrival and interaction of the oceanic and island arc terranes with cratonal North America that heralded the initial stages of closure and obduction of the Slide Mountain ocean and onto western North America (Nelson & Colpron, 2007). REFERENCES BARR, D.A., F o x , P.E., NORTHCOTE, K.E., & PRETO, V . A .

LOGAN, J.M, MIHALYNUK, M . G . , ULLRICH, T., & FRIED-

MAN, R.M., 2 0 0 7 . BC Ministry of Energy and Mines and Petroleum Resources, Geological Fieldwork 2006, Paper 2007-1. pp. 9 3 - 1 1 6 .

LUECK , B.A., & RUSSELL, J.K. 1994. BC Ministry of Energy

and Mines and Petroleum Resources, Geological Fieldwork 1993, Paper 1994-1, pp. 3 1 1 - 3 1 5 .

MCINNES, B. I. A. & CAMERON, E. M . 1994. Earth and

Planetary Science Letters, 122, pp. 1 2 5 - 1 4 1 .

MIHALYNUK, M . G., NELSON, J., & DIAKOW, L. J. 1994. Tectonics, 13, pp. 5 7 5 - 5 9 5 . MONGER, J. W . H., & NINE OTHERS, 1992. Geologi-

cal Society of America, Geology of North America, G-2,

pp. 491-531.

MORTENSEN, J.K. GHOSH, D.K., AND FERRI, F. 1995. Cana-

dian Institute of Mining, Metallurgy and Petroleum, Special Volume 46, pp. 1 4 2 - 1 5 8 .

MORTIMER, N . 1987. Canadian Journal of Earth Sciences, 24, p p . 2 5 2 1 - 2 5 3 6 . NELSON, J., & COLPRON, M . 2 0 0 7 . Geological Association of Canada Special Publication 5. pp. 7 5 5 - 7 9 2 . SCHWAB, D.L., PETSEL, S., OTTO, B.R., MORRIS, S.K., WORKMAN, E.E., & TOSDAL, R . M . 2008. Arizona

Geological Society Digest 22, [in press].

ScHiARizzA, P. & FRIEDMAN, R. 2 0 0 9 . Roundup 0 9 ,

1976. Canadian Institute of Mining and Metallurgy 25,

pp. 22-23.

BARRIE, C . T . 1993. Journal of Geochemical Exploration 4 8 ,

WERNICKE, B. & KLEPACKI, D . W . 1988. Geology, 16, pp. 461-464.

pp. 359-367.

pp. 225-258.

LANG, J.R., LEUCK, B., MORTENSEN, J.K., RUSSELL, J.K., STANLEY, C.R., & THOMPSON J.F.H. 1995. Geology 23,

pp. 451-454.

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ARC DEVELOPMENT, VMS MINERALIZATION AND COLLISIONAL TECTONICS IN THE NORTHERN APPALACHIANS Cees van Staar and Alex Zagorevski^, ^Geological Survey of Canada, 625 Robson Street, Vancouver, BC,V6B 5J3 ^ Geological Survey of Canada, 601 Booth Street, Ottawa, ON, K1A 0E8 Canada. The northern Appalachians represent part of an extensive Paleozoic, accretionary-collisional orogen (Appalachian-Caledonian mountain belt) that formed in response to closure of the lapetus and Rheic oceans. It comprises a collage of Early Paleozoic peri-Laurentian and peri-Gondwanan oceanic suprasubduction zone (SSZ)- and microcontinental ribbons. The SSZ terranes represent infant arc, extensional arc and backarc settings, whereas the microcontinents were rifted-off from the Laurentian (Dashwoods) and Gondwanan (Ganderia, Avalonia and Meguma) margins. Some of the continental ribbons (e.g. Ganderia) became the basement to island arcs during the Cambrian and Ordovician (e.g. Penobscot arc). True oceanic crust formed at intra-oceanic spreading centers far removed from subduction zones and continental margins is generally not preserved, except as thin basaltic slivers derived from clipped-off seamounts. The Appalachian Orogen is rich in volcanic hosted massive sulphide deposits (VMS), because most of the accreted arc terranes formed in an extensional environment, which are sites of enhanced heat flow and hydrothermal circulation. VMS deposits mainly formed between c. 515 and 460 Ma, during subduction related to the closure of the c. 3000 km wide main lapetan oceanic tract, which at c. 480 Ma was and separated the peri-Laurentian and periGondwanan arc terranes. On the Laurentian side, big VMS deposits (Cu-Zn) occur in the boninite-rich Lush Bight and Baie Verte oceanic tracts, which formed during subduction initiation at c. 510 and 490 Ma respectively, and in the Middle Ordovician continental Roberts Arm-Buchans-Red Indian Lake arc belt (Cu-Zn-Pb). On the Gondwanan side, polymetallicVMS deposits occur in the CambrianEarly Ordovician Penobscot arc/backarc basin system and the Middle Ordovician Popelogan/Vitoria arcTetagouche/Exploits backarc basin system. VMS

deposits seem to form particularly in incipient rifts characterized by high magma production compared to extension rates. Sequential accretion of the oceanic and continental arc terranes to Laurentia or to one another caused numerous short-lived collisional events between the Late Cambrian and end of the Silurian. The most notable are the Taconic (Notre Dame arc-Laurentia and Red Indian arc-Popelogan/Victoria arc), Salinic (Laurentian Sahnic arc-Ganderia), Acadian (laurentian Acadian arc-Avalonia). The Taconic closure of the main lapetan tract culminated in the arc-arc collision of peri-cratonic Robert s ArmBuchans-Red Indian Lake and Penobscot-Victoria arc/backarc terranes, which were sutured along the Red Indian Line during the Late Ordovician (c. 450 Ma). Following the closure of the main lapetan tract, convergence continued through closure of the vestiges of lapetus, which included the wide (10001500 km) Tetagouche-Exploits backarc basin situated behind the Popelogan/Victoia arc and the oceanic seaway (Acadian seaway) that separated Avalonia from Ganderia. Closure of these oceanic seaways first caused the Early Silurian Salinic Orogeny at c. 430 Ma and subsequently initiated the Acadian Orogeny at c. 421 Ma. The latter event thus heralds the final closure of all lapetus-related oceanic lithosphere. Subduction responsible for the Acadian orogeny involved a shallow-dipping slab, which forced the overriding plate (composite Laurentia) into compression rather than extension. The latter setting explains the scarcity or absence of VMS deposits during this time. Instead orogenesis was accompanied by important gold mineralization in the central portion of the Canadian Appalachians. At least some of the gold may be associated with magmatism generated during two slab break-off events in the Early-Late Silurian and Early Devonian respectively.

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THE DEVONIAN AND CARBONIFEROUS ARC IN THE OYU TOLGOl AREA OF THE CENTRAL ASIAN OROGENIC BELT, SOUTHERN MONGOLIA A J . Wainwright R.M. Tosdal\ C.N. Forster^ D.J. Kirwin^ P.D. Lewis" & J.L. Wooden' ^Mineral Deposit Research Unit, University of British Colunnbia, Vancouver, BCV6T1Z4, Canada '4011 Pipeline Road, Blaine, WA 98230, USA ^Ivanhoe Mines, 51/278 Muang Ake,Tannbon Iak6, Annphoe Muang, Pathunntani, Bangkok,Thailand 12000 "15715 Mountainview Drive, Surrey, British Colunnbia, V3S 0C6, Canada ' Departnnent of Geological and Environnnental Sciences, Stanford University, Stanford, CA, USA

The Central Asian Orogenic Belt (Badarch et al., 2002; Buchan et al, 2002; Helo et al, 2006), also referred to as the Altaids (Sengor and NataFin, 1996) extends across Asia for 5000 km, is 1000 to 2000 km wide and is situated between Precambrian cratons; the Siberian craton to north and the Tarim and North China cratons to the south (Badarch et al., 2002; Dobretsov et al, 1995). The tectonic collage is an assemblage of microcontinental blocks and mobile belts of different ages including relicts of island arcs, Precambrian continental crust and Neoproterozoic to Paleozoic ophiolites (Buchan et al., 2002). In general, the ages of the orogenic complexes within the belt decrease from north to south away from the Siberian craton margin (present coordinates) (Kovalenko et al., 2004). To the west, other orogenic terranes, the Baikalides and the Pre-Uralides, separate the Central Asian Orogenic Belt from the Baltica craton (Yakubchuk, 2004). Yet another mobile belt to the south, termed the Manchurides, makes up the northern part of the North China craton (Sengor and NataFin, 1996). There is considerable debate regarding the Paleozoic tectonic evolution of Central Asia. Contradictory explanations for the current geometry have arisen due to differing approaches to explaining the origin of slivers of Precambrian crust and Neoproterozoic to Paleozoic ophiolites found within the tectonic collage. Sengor and NataFin (1996) proposed that a single, giant subduction-accretion complex was deformed during oroclinal bending. This structure was subsequently dismembered by strike-slip faulting in order to generate the current geometry. In this model, Precambrian crust was rifted from a combined Siberia-Baltica craton west of the Tuva-Mongol and Kipchak volcanic arc systems. Conversely, it has been

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proposed that the Central Asian Orogenic Belt grew by subduction and accretion of multiple oceanic basins and by development of individual magmatic arcs (e.g. Buchan et al., 2002). In this scenario, Precambrian material would have been derived from the Gondwana supercontinent on the east side of a Paleoasian ocean, in addition to the Siberia and Baltica cratons. The two models differ mainly in that one suggests a prolonged and steady period of subduction-accretion, followed by large-scale deformation of a single giant magmatic arc complex, whereas the latter suggests punctuated accretion of multiple individual arc terranes and microcontinents. During the Paleozoic, rocks that now comprise the South Gobi region of Mongolia underwent accretionary episodes that assembled a number of island and continental margin magmatic arcs, rifted basins, accretionary wedges and continental margins (Sengor and NataFin, 1996; Lamb and Badarch, 1997). Notable amongst the juvenile island arc magmatic complexes are the Oyu Tolgoi porphyry Cu-Au deposits, which are located within a sequence of Devonian rocks in the Gurvansayhan Terrane (Badarch et al. 2002; Helo et al, 2006; Wainwright, 2008), which is part of the larger assemblage of volcanic arc complexes that trend through western China and the Altai Mountains in western Mongolia across to northeastern Mongolia. The Gurvansayhan Terrane consists of highly deformed accretionary complexes and volcanic arc assemblages. The structure of the terrane is complex and dominated by imbricate thrust sheets, dismembered blocks, mdanges and high strain zones (Badarch et al., 2002). In fact, the Oyu Tolgoi porphyry Cu deposits were exhumed and then tectonically buried beneath Devonian or older

2009 International Conference on Island Arc-Continent Collisions: Macquarie Arc Conference, Orange, New South Wales, Australia marine mafic rocks within a few million years of their formation in the latest Devonian (Lewis, 2006; Wainwright, 2008). Several Carboniferous volcanic and volcaniclastic rock sequences depositionally overlie the shortened Devonian rocks, and these are in turn affected by polyphase folding prior to intrusion of post-kinematic Upper Carboniferous granitoids, and still younger Permian bimodal basalt-peralkaline granitoids (Kovalenko and Yarmolyuk, 1995; Wainwright, 2008). Whole rock geochemistry along with zircon U-Pb and trace element geochemistry place some constraints on the evolution of the Devonian and Carboniferous arcs in the southern Gobi Desert. The igneous suites in the Oyu Tolgoi district were erupted or intruded between -390 Ma and -320 Ma, and thus span - 7 0 m.y. of arc activity. Overall, rocks range in composition from basalt-gabbro to rhyolite-granite and are remarkably similar to one another in most geochemical characteristics. Exceptions include the presence of high-Nb basalts as well as adakite-like compositions for late-mineral intrusions in the Late Devonian. All suites appear to have been derived from depleted mantle in an intra-oceanic volcanic arc with that ranges from +3 to +6. Lead isotopic compositions show little difference between ore-stage magmas and magmas not associated with the porphyry Cu-Au deposits. There is in the radiogenic isotopic data little evidence for the presence of significant old continental crust beneath the arc, although sparse xenocrystic and inherited zircons with ages ranging from Proterozoic to Silurian (-1634 Ma, -1104 Ma, -783 Ma, -501 Ma, - 4 6 3 Ma, - 4 4 1 Ma, and -418 Ma) indicate that older crust or sediments were encountered by the ascending magmas. In detail, shallow crustal intrusions at the Oyu Tolgoi porphyry Cu-Au deposits are associated with high-K calc-alkahne monzonite that show a decrease in Sr/Y ratios and changes in La/Yb through time from -372-Ma, the time of major porphyry formation, to - 3 6 6 Ma, when weakly mineralised granodiorite intrusions cut the extant porphyry complex. Fractionation and cooling trends in intrusive rocks are complicated by mixing of magmas from discrete reservoirs, with distinct Th/U, Yb/Gd, Eu^/Eu^^ and C e J C e / ratios as well as Hf and Y concentration in zircon. Overall, the chemical changes recorded in the whole rock and zircons suggest that, despite the overall similarity

of the igneous rocks, magmatism fundamentally changed during and shortly after the mineralization event at -372 Ma. Based on the data from the Oyu Tolgoi area, the arc system apparently formed outboard from an adjoining continental craton and was shortened several times before final amalgamation in the Permian, and is thus consistent with either a single giant arc or multiple discrete arcs. However, the xenocrystic zircons suggest that the arc must have been located near a continental mass such that sparse Proterozoic to Silurian zircons became incorporated into the magmas but far enough so that significant old radiogenic crust was not incorporated in the magmas as the arc evolved. Major porphyry formation appears to correspond to the time just prior to significant shortening and exhumation.

REFERENCES BAD ARCH, G., DICKSON C . W . & WINDLEY, B.F. 2 0 0 2 .

Journal of Asian Earth ScienceSy 21, pp. 8 7 - 1 1 0 .

BUCHAN, C., PFAENDER, J., KROENER, A., BREWER, T.S., TOMURTOGOO, O., TOMURHUU, D., CUNNINGHAM, D., & WINDLEY, B . F . 2 0 0 2 . Chemical Geology, 192, pp. 2 3 - 4 5 . DOBRETSOV, N.L., SOBOLEV, N . V . , SHATSKY, V.S., COLEMAN, R . G . & ERNST, W . G . 1 9 9 5 . The Island Arc, 4 , pp. 2 6 7 - 2 7 9 . HELO, C., HEGNER, E., KROENER, A., BAD ARCH, G., TOMURTOGOO, O., WINDLEY, B.F. & DULSKI, P. 2 0 0 6 .

Chemical Geology, 111, pp. 2 3 6 - 2 5 7 .

KOVALENKO, V . I .

YARMOLYUK, V . V . 1995. Economic

Geology, 90, pp. 5 2 0 - 5 2 9 .

KOVALENKO, V . I . , YARMOLYUK, V . V . , KOVACH, V . P . , KOTOV, A.B., KOZAKOV, I.K., SALNIKOVA, E.B. &

LARIN A . M . 2 0 0 4 . Journal of Asian Earth Sciences, 2 3 , pp. 6 0 5 - 6 2 7 . LAMB, M . A . & BADARCH, G . 1997. International Geology

Review, 39, pp. 3 4 2 - 5 7 6 .

LEWIS, P . D . 2 0 0 5 . Geological Society of America Abstracts

with Programs, 37, 7, 97.

SENGOR, A . M . C . & NATAL'IN, B.A. 1996. Annual Review of

Earth and Planetary Sciences, 24, pp. 2 6 3 - 3 3 7 .

WAINWRIGHT, A.J. 2 0 0 8 . Ph.D. dissertation. The

University of British Columbia.

YAKUBCHUK, A. 2 0 0 4 . Journal of Asian Earth Sciences, 2 3 , pp. 7 6 1 - 7 7 9 .

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LINKS BETWEEN EARTH DEGASSING AND LATE ORDOVICIAN CU-AU METALLOGENESIS IN THE MACQUARIE ARC, EASTERN LACHLAN OROGEN, NSW J. L.Walshe 'CSIRO Exploration and Mining, PO Box 1130, Bentley WA6102, Australia.

Introduction Metal transport and deposition capacities of mineral systems are closely linked to propagation of redox and related physico-chemical gradients. Redox gradients are intrinsic to the mineral systems, originate in the deep-Earth and are linked to the degassing history of the Earth. The evolution of gradients within mineral systems largely reflects the interplay of deep-Earth anhydrous fluids with the Earth s hydrous outer layers. Such models imply links between the formation of the Earth s resources across time and space, secular changes in architecture and geochemistry of the planet over some 4.5 billion years of evolution and Earth phenomena such as mass extinction events, global anoxia and climate change. The ~ 440 Ma Cu-Au and Au mineral systems of south-eastern Australia (Foster et al, 1998; Perkins et al., 1995; Vos et al., 2007) provide an example being coeval, or broadly so, with one of the five great extinction events of the Phanerozoic (Sheehan, 2001) and long-lived glaciations of the Late Ordovician (Saltzman and Young, 2005) although there is no record of glaciation in the Lachlan Orogen.

Characteristics of deep-Earth anhydrous fluids: Learnings from Archean Au deposits Recent studies of ~ 2.65 Ga Au systems of the Yilgarn Craton, Western Australia, have emphasised the significance of oxidized anhydrous (CO2 ± SO2) and reduced anhydrous (H2 ± CH4 ± N2 ± HCl) fluids as well as hydrous fluids in deposit formation. carbonate for deposits of the Eastern Yilgarn lie within the range of mantle CO2 and values of primary anhydrite and celestine are consistent with precipitation from mantle derived anhydrous SO2 - CO2 fluids at temperatures of ~ 400 - 500 The ultimate source of CO2 ± SO2 fluids was probably the sub-continental lithosphere as evidenced by the SO2 discharge during island arc volcanism. In contrast to CO2 inclusions, reduced CH4 fluids in Late Archean Au deposits have elevated

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values (up to -50,000), Cl/'^Ar and ''Ne/'^Ne values indicating an independent origin for reduced fluids from deep crustal ± mantle sources (Walshe and Kendrick, 2009). H2-CH4 fluids could be generated during serpentinisation reactions in the mantle wedge (and/or lower crust). Thermodynamic studies, coupled with inclusion mineralogy of diamondiferous kimberlites and lamproites, suggest they also dominate at depths greater than 300 to 400 km. A reduced or hydridic fluid, possibly sourced from depths > 300 to 400 km within the Earth will be highly unstable in the crustal environment. If armoured from hydrous fluids or minerals, at least transiently, there is potential for significant migration into or through the crust.

Links between Cu-Au metallogenesis in the Eastern Lachlan and Earth degassing: Learning to read the signals of productive systems What are the signals, preferably observable at the terrain-scale, that indicate the presence of productive systems. Arc magmatism, particularly plutonism, may be taken as a proxy for flux of oxidized fluids from the sub-continental lithosphere. Four groups of porphyries were emplaced during the ~ 50 million year punctuated history of the Macquarie Arc from earliest Ordovician to earliest Silurian (Glen et al., 2007). Cu-Au mineralisation is known to be associated with groups 3 and 4 that were intruded in the late Ordovician; at ca 453 to 450 Ma and ca 444 - 437 Ma respectively. Some mineralization may be associated with group 2 porphyries of Middle Ordovician age. Global positive excursions of values for marine carbonate, commonly temporally associated with ice-ages and mass-extinction events are taken as the proxy for reduced (H2 ± CH4), deep-Earth fluids. Such shifts in are related to a secular decrease in the amount of dissolved inorganic carbon (DIG) in the marine reservoir, driven by high biological productivity and preferential sequestering of ^^G

2009 International Conference on Island Arc-Continent Collisions: Macquarie Arc Conference, Orange, New South Wales, Australia to organic carbon or methane clathrate in marine seiments. Methanogens provide a biological pathway for low temperature reduction of CO2 to CH4 and can grow with CO2 and H2 as their sole energy sources. Sustained increases in the supply of H2 through Earth degassing would fuel the growth of methanogens and shift the redox balance in the hydrosphere and atmosphere towards more reduced conditions, draw down CO2 levels and promote ice-house conditions. Positive excursions of for marine carbonate occurred globally in the Late Ordovician at ~ 455 Ma and at end Ordovician at ~ 445 Ma (Bergstrom et al., 2009; Saltzman and Young, 2005), with minor positive excursions in the Middle Ordovician. Significant positive excursions of for marine carbonate also occurred in the Silurian (Cramer and Saltzman, 2005). The times of strongest redox gradients and greatest potential for metallogenesis will be times of coincident flux of reduced and oxidized fluids. The most favourable times in the Ordovican appear to have been in the Late rather than Middle Ordovician, despite the development of the Macquarie Arc and inferred flux of oxidized fluids during four major phases of magmatism over ~50Ma, from earliest Ordovician to earliest Silurian. The times of metallogenesis seem to correspond broadly with times of overlap of magmatism with positive excursions of in marine carbonate around ~ 455 Ma and ~ 445 Ma.

Geodynamic constraints on open and closed systems and timing of deposit formation Events such as positive excursions of for marine carbonate and/or magmatism are unlikely to coincide precisely with metallogenesis. Such events record times of fluids dispersion rather than the fluid focusing required for formation of mineral deposits. An empirical observation is that in well endowed terranes there is an absence of volcanic activity or a transition from volcanic activity to limited plutonic activity at the time of mineralization. Examples include the porphyry deposits of northern and central Chile and New Guinea, the Eocene Au and Cu-Au deposits of northern Nevada and Utah. This is taken as evidence of transition from overall extension to compression or increased rates of compression (convergence) with the implication that magma chambers and other suitable potential reservoirs in the rock mass were loaded, sealed and, at least

transiently, acted as reservoirs of fluids (melts, brines, volatiles). Terrain inversion and uplift, commonly triggered by far-field plate and stress re-orientation, combined with localized extension are likely controls on fluid release in the system. In the Eastern Lachlan Orogen, most of the available geochronological data for the Northparkes and Cadia districts indicates Cu-Au deposition are intimately associated with phase 4 magmatism at ca 443 - 437 Ma (Cooke et al., 2009; Cooke et al., 2007; Glen et al., 2007) and that most deposits formed during the culminating phase of magmatism and deformation in Macquarie Arc. Equally, the absence of such an event in the Silurian-Devonian, periods marked by coeval S and I type volcanism and plutonism, is consistent with the absence of world class deposits in the eastern Lachlan at this time.

Architectural constraints on pathways of deep-Earth fluids The most likely mechanisms for rapid advection of hydridic (H2 ± CH4) fluids through the mantle and crust are transient, deep-lithospheric blasts of gas, or Verneshots (Morgan et al., 2004). Tears in slabs, cross-arc structures and times of plate re-orientation, slab-rollback and slab-foundering are important architectural and dynamic factors that help create pathways for the deep-Earth fluids into the uppercrust. Arguably, such factors played an important role in the distribution of the major provinces of the Tasmanides. The control of the Lachlan Tranverse Zone on the Ordovician Cu-Au Province and the Cobar Field is one example. The cross-arc structural control on the Permo-Carboniferous Gold Province and associated magmatism is another.

Summary Comment on the Exploration Challenge Learning to read the presence of productive mineral systems, particularly at the terrain to global scales, remains a challenge. The "5 Question" description of the mineral system explicitly highlights the need to understand the system in space and understand the temporal evolution of the system at all scales. Issues of "source" "transport" and "trap" (Q3, Q4 and Q5) are addressed within the context of the architectural and geodynamic history (Q1 and Q2) of the whole system, taking account of information from the deposit to the terrain and global scales.

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Here it is suggested that it is possible to utilized knowledge of the magmatic history of terrains, coupled with knowledge of the chemical evolution of the hydrosphere and atmosphere, to infer when deep-Earth fluid fluxes were sufficient to sustain physico-chemical gradients in mineral systems. The additional constraints from knowledge of terrain scale architectures and geodynamic histories should permit favourable exploration domains within terrains to be identified. REFERENCES BERGSTROM, S.M., X u , C., SCHMITZ, B., YOUNG, S., JIA-YU, R. & SALTZMAN, M . R . 2009, Geological Magazine

146, pp. 1-11. COOKE, D.R., HARRIS, A.C. & ZUKOWSKI, W . 2009, this volume. COOKE, D.R., WILSON, A.J., HOUSE, M.J., WOLFE, R.C., WALSHE, J.L., LICKFOLD, V. & CRAWFORD, A. J., 2007,

Australian Journal of Earth Sciences

54, pp. 445-463.

CRAMER, B.D., & SALTZMAN, M . R . 2005, Palaeogeography,

Palaeoclimatology,

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Palaeoecology

219, pp. 333-349.

FOSTER, D.A., GRAY, D.R., KWAK, T.A.P. & M.B. 1998, Ore Geology Reviews 13, pp. 2 2 9 - 2 5 0 . GLEN, R.A., CRAWFORD, A.J. & COOKE, D.R. 2007, 54, pp. 4 6 5 - 4 7 9 .

Australian Journal of Earth Sciences

MORGAN, J.P., RESTON, T.J. & RANERO, C.R. 2004, 217, pp. 2 6 3 - 2 8 4 .

Earth and Planetary Science Letters

PERKINS, C., WALSHE, J.L. & MORRISON, G. 1995, pp. 1 4 4 3 - 1 4 6 6 .

Economic Geology 90,

SALTZMAN, M.R., & YOUNG, S.A. 2005, Geology 33, pp. 109-112. SHEEHAN, P.M. 2001, Annual Review of Earth and Planetary Sciences 2 9 , p . 3 3 1 - 3 6 4 V o s , I.M.A., BIERLEIN, P.P. & HEITHERSAY, P.S. 2007, Mineralium Deposita 42, pp. 5 1 5 - 5 2 2 . WALSHE, J.L. & KENDRICK, M . A . 2009,19th VM, Goldschmidt Conference — Challenges to our Volatile Planet, 2009.

2009 International Conference on Island Arc-Continent Collisions: Macquarie Arc Conference, Orange, New South Wales, Australia

ARCHITECTURE OF THE SILURIAN SEDIMENTARY COVER SEQUENCE IN THE CADIA PORPHYRY AU-CU DISTRICT, NSW, AUSTRALIA: IMPLICATIONS FOR POST-MINERAL DEFORMATION M.W. Washburn Groomed A.C. Harris^ R.M. Tosdal^ ^Mineral Deposit Research Unit, University of British Columbia, 6339 Stores Rd, Vancouver, BC,V6T1Z4, Canada. ^ ARC Centre of Excellence in Ore Deposits, University ofTasnnania, Private Bag 79, Hobart,Tas. 7001, Australia, t Newcrest Mining Ltd., Cadia Valley Operations, Cadia, NSW 2800, Australia.

The world-class alkalic porphyry Au-Cu deposits of the Cadia District include the Cadia Hill, Ridgeway, and Cadia East systems and contain a combined total in-situ resource of 39.4 Moz Au 6.96 Mt Cu (Newcrest Mining Ltd., annual report, 2008). Mineralization in the district occurs along a 6km long northwest-striking trend and is associated with Late-Ordovician to Early-Silurian monzonite intrusions emplaced during the final phase of Macquarie Arc magmatism at the end of the Benambran Orogeny (Holliday et al, 2002; Wilson et al., 2003). Subsequent post-collisional extension and later renewed contraction resulted in the structural dismemberment of the mineralized system. Understanding the post-mineralization structural dismemberment of the Cadia District porphyry system is necessary for constraining district-to-deposit scale models and putting the mineralized system into a tectonic context. Detailed facies and structural reconstruction of syn- and postintrusive sedimentary and volcanic rocks was used to investigate the timing of deformation relative to mineralization in the Cadia District. Rocks of the Cadia district can be divided into Ordovician-Early Silurian "basement" and Late Silurian Waugoola Group sedimentary cover sequence. The Ordovician basement can be subdivided further into the Weemalla Formation, the Forest Reefs Volcanics, and intrusive rocks of the Cadia Intrusive Complex. The stratigraphy in the Cadia District preserves a complex history of evolution from a foreland setting to an active volcanic arc complex in the Late Ordovician, to an extensional basin setting in the Silurian. These major cycles of deposition, punctuated by periods of uplift and erosion (notably during the Early Silurian and the Devonian) correspond to regional tectonic events (Glen et al., 2007). Post mineralization,

intrusions of the Cadia Intrusive Complex and the Forest Reefs Volcanics were uplifted and eroded to various levels across the Cadia district. This phase of uplift is preserved as a depositional hiatus between the Ordovician basement rocks and the Waugoola Group sedimentary cover sequence. Variable uplift and erosion is recorded by differences in the relative stratigraphic position of the Forest Reefs Volcanics immediately below the unconformity. Stratigraphic relationships in the Ordovician Forest Reefs Volcanics suggest that NW- striking and N-striking regionally-significant faults, including the curviplanar, N-striking, moderately west-dipping reverse faults of the Cadiangullong system, developed early in the district history. Major regionally extensive N- and NW- striking faults are recognized throughout the Macquarie Arc, and significant mineralization is localized at the intersection of these long-lived deep-crustal structures (Glen et al, 2007). During the late Silurian, the partially-exhumed porphyry systems were buried beneath the Waugoola Group sedimentary cover sequence, which is generally preserved in the footwall of the Cadiangullong fault system. Macrofossils recognized in rocks of the Waugoola Group in the Cadia District include orthid brachiopods and graptolites (e.g. testograptus testis) that constrain the age of the Waugoola Group in the Cadia District to the Late Wenlock (426-424 Ma) (Rickards et al., 2001). The Waugoola Group is a typical rift-sag sequence. Deposition was initially controlled by local faultbounded basins which then transitioned to a gradually shallowing marine environment as local topography was overwhelmed by sedimentation, and represents the drowning and dismemberment of the Macquarie Arc during rifting in the late Silurian. In the Cadia District, the Waugoola Group is divided into a

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basal unit, a lower siltstone-dominant succession, and an upper sandstone-dominant succession. The sandstone-dominant upper unit is divided into interbedded siltstone and sandstone, of varying proportions (siltstone- or sandstone- dominant) and overlying massive sandstone. The basal unit varies across the district. Locally boulder conglomerate, calcirudite limestone, or redbeds occur at the unconformity. Different packages at the base of the Silurian sedimentary cover sequence in the Cadia district reflect different depositional environments at the onset of Waugoola Group sedimentation. Changes in the basal unit across the district provide insight into postmineral uplift and erosion, and the resultant paleogeography. Stratigraphic relationships observed in the Silurian Waugoola Group cover sequence suggest that basin geometry was controlled by preexisting faults and significant topography influenced lithological distribution. Relief persisted into the later stages of Waugoola Group deposition, and was defined by N- and NE- striking extensional faults, forming a horst-and-graben style topography. Basin-bounding faults that controlled distribution of the Waugoola Group were subsequently inverted during the Devonian Tabberabberan Orogeny, offsetting the unconformity between basement and cover by up to 300m vertically. In the Waugoola Group, Devonian shortening was accommodated by a complex network of minor meter-scale to tensof-meters-scale gently-dipping thrusts that strike parallel to major underlying faults and associated folds. This deformation event was also expressed as

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reverse separation on basement penetrating faults and slip along the basement-cover unconformity. DiflFerences in the relative strength of basement and cover rocks as well as irregularities in the shape of the unconformity produced complexity at the interface. During thrusting, basement fault blocks were displaced and rotated relative to one another, which tilted the cover sequence down to the west. Steep basement faults flatten approaching the unconformity surface, and shortening is accommodated along multiple detachments including the unconformity and along low angle bedding-parallel faults. Faults and folds in the Waugoola Group are similar in orientation to related faults in the Ordovician basement rocks. Additionally, the intensity of deformation in the sedimentary cover rocks increases approaching major basement structures, a relationship that can be used to identify blind basement faults. REFERENCES GLEN, R . A . , CRAWFORD, A J . , COOKE, D . R . , 2 0 0 7 .

Australian Journal of Earth Sciences, 54, pp. 465-479. HOLLIDAY, J.R., W I L S O N , A.}., BLEVIN, P.L., TEDDER, I.J.,

DUNHAM, P.D., PFITZNER, M., 2002. Mineralium

Deposita

37, pp. 1 0 0 - 1 1 6 . RICKARDS, R.B., PERCIVAL, I . G . , SIMPSON, A.J.,

WRIGHT, A.J., 2001. Proceedings of the Linnean Society of New South Wales, 123, pp. 173-191. W I L S O N , A.J., COOKE, D . R . , H A R P E R , B.L., 2 0 0 3 .

Geology and the Bulletin of the Society of Economic Geologists, 98, pp. 1637-1666.

Economic

2009 International Conference on Island Arc-Continent Collisions: Macquarie Arc Conference, Orange, New South Wales, Australia

IS THE THEORY OF PLATE TECTONICS AN IMPEDIMENT TO SUCCESSFUL MINERAL EXPLORATION? N. Williams^ & L.A.I Wyborn^ ^Geoscience Australia, GPO Box 378, Canberra, ACT, Australia

In 1968 the theory of plate tectonics was formulated and quickly accepted by geoscientists around the world. The theory has revolutionised our understanding of modern geology and enabled us to take a holistic approach to the understanding of the formation of various rock types in the present world and the relationships between them. Interestingly the rise in our understanding of geological processes that plate tectonic theory has generated coincides with a worrying decline in mineral exploration success around the world. Given the great advances in the geosciences catalysed by plate tectonics, why haven't these advances improved the efficiency and effectiveness of mineral exploration, particularly in greenfields exploration terrains? There are many possible answers to the question and the evolution of thought concerning the geology and exploration history of the copper and gold mineralised Lower Palaeozoic of the Lachlan Fold Belt offers some useful insights the apparent disconnect between the growing scientific success of plate tectonics and failure of mineral exploration. Gold and copper have long been exploited in the Lachlan Orogen and most of the newly discovered deposits were at localities known to the early prospectors by the end of the century. Modern exploration in the Macquarie Arc began in the mid 1960 s when Anaconda Australia Inc., noting that the geological environment had some similarities to areas hosting porphyry copper deposits, began exploring for low grade high tonnage porphyry-style mineralisation that would be suitable for open pit mining. Other companies followed and Geopeko had the first success with E22 and E27 North Parkes in 1977. Initially the mineralisation was assumed to be of VMS type, but subsequently was identified as porphyry type. BHP Gold, now Newcrest, initially targeted the Cadia area to find more gold for their mill at Browns Creek. Later the porphyry potential was recognised and Cadia Hill was discovered in 1992. Both companies then had a string of successes

with Geopeko finding E26 (1979), Cowal (1988), and North Parkes E48 (1992), whilst Newcrest located Cadia East (1994), Ridgeway (1996), Cadia Far East (1996). The last significant discovery was Marsden (1997). Success has also been limited outside the Northparkes and Cadia districts. Most of the deposits were located using empiricallybased exploration strategies targeting copper and/ or gold anomalies and geophysical anomalies. Some deposits do not outcrop and were found by deep drilling undercover (Marsden, E48 and Ridgeway). Knowing the tectonic setting does not seem to have been part of the targeting strategy and discovery papers for both the North Parkes and Cadia areas note that the interpretation of the tectonic setting was equivocal. Plate tectonics is very successful in interpreting the geology of the modern day environment, but the application to past environments in some areas is often controversial. As the application of plate tectonic theory has not increased the success rate of exploration, several questions can be raised. 1) Are certain deposit types necessarily unique to a particular tectonic setting? Exploration for porphyry deposits has long targeted convergent island- and continental-arc settings. However, porphyry Cu-Mo-(Au) deposits in southern and western China are localised contrasting tectonic settings: one is a subduction environment, the other is a post-collisional environment related to rapid plateau exhumation and regional extension. The chemistry of the magmas and the deposit geology of both types appear identical. Another example, the Olympic Dam Deposit, discovered in 1975, became the type example of a new deposit type, the Iron Oxide Copper Gold (lOCG) type. There is no unique tectonic setting for lOCG s, and the tectonic setting operating at the time of formation of Olympic Dam is still under debate. 2) Does plate tectonics as we understand it today, operate back in the past?

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Several compilations have noted that the dominance of certain ore deposit types changes with time. Although some of these changes can be attributed to increasing exhumation as the crust becomes older, denudation rates do not explain all of these differences. It is possible some crustal processes that operated in the past do not have modern counterparts, and that these exceptions may in fact enhance some ore forming processes. For example, the Proterozoic of central and eastern Australia exceptionally high heat flow. Averaging 50-80 mW/m^, these values are 2-3 times the continental average. The area of anomalously high heat flow also contains the majority of Australia's world class ore deposits (Olympic Dam, Broken Hill, Mount Isa, McArthur River, and Jabiluka). Such anomalously hot crust would have different thermomechanical properties: it would be weaker, more susceptible to far field deformation stresses and less likely to subduct. More importantly the anomalously high heat flow would promote the circulation of hot fluids over longer periods of time and assist in the formation of these supergiant deposits. 3) The length scale of many plate tectonic features can be orders of magnitude larger than the size of most deposits. Many plate tectonic processes considered important to ore formation are measured on scales of hundreds, if not thousands of kilometres (eg, slab roll back, ridge subduction.

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arc reversals). In contrast, the diameter of most of the mineralised porphyries in the Macquarie Arc is less than 200m. Positively recognising these supposedly key features in past environments is also subjective. There is no doubt that future exploration in the Macquarie Arc will become increasingly difficult and will be predominantly undercover. At the province scale we need to focus more on developing a predictive minerals systems approach that combines an understanding of the fluids carrying the metals and how they will react with different potential host rocks. We need to be able to predict what mineral assemblages will result from interaction of these fluids with a greater variety of potential host rocks and then predict their different spectral, geophysical and geochemical signatures. Further research into the tectonic setting operating at the time of formation of the porphyries of the Macquarie Arc is unlikely to assist exploration undercover. For the theory of plate tectonics to be relevant to mineral exploration, we need to be aware that the tectonic setting may not be unique for a particular deposit type. Perhaps we should also be searching for those exceptions to the modern plate tectonic paradigm in the geological record and elucidating alternate relationships between mineral systems and tectonics to provide explorers with less ambiguous and more useful inputs to mineral exploration models.

2009 International Conference on Island Arc-Continent Collisions: Macquarie Arc Conference, Orange, New South Wales, Australia

METAMORPHICS FROM GEEHI GORGE, UPPER INDI AND BETHANGA AND THE ENIGMA OF ANOMALOUSLY HIGH GEOTHERMAL GRADIENTS IN THE LACHLAN OROGEN L.A.I. Wyborn^&N. Williams^ ^Geoscience Australia, GPO Box 378, Canberra, ACT, Australia

The Bethanga area (10 kms east of Albury) and the Geehi Gorge and Upper Indi areas (Mount Kosciuszko region) were affected by regional low-pressure, hightemperature (high-P, Low-T) metamorphism that occurred in the Lachlan Orogen during the late Ordovician to Silurian Benambran Orogeny. Due to the diverse lithologies present, these areas provide important constraints on the Pressure-Temperaturetime (P-T-t) path of this orogeny Both areas also provide insights into the generation of the widespread Silurian-Devonian S-type magmatism. The Bethanga area occurs on the western edge of the Wagga Omeo Metamorphic Belt, whilst the Geehi Gorge and Upper Indi areas straddle the eastern margin of the Wagga Omeo Metamorphic Belt, the Macquarie Arc and the western margin of the Adaminaby Superterrane. In both areas, metamorphic assemblages are dominated by low-P, high-T assemblages which are typical of the regional metamorphism in the central and eastern Lachlan Orogen. In some previous interpretations, this metamorphism has been referred to as regional contact metamorphism'. In the Bethanga and the Geehi Gorge areas staurolite and garnet are recorded in rocks that are more iron-rich than elsewhere in the Lachlan Orogen and when combined with other mineral assemblages, the pressure estimates are higher than previously recorded. In both areas, andalusite is stable in lower amphibolite grade rocks, whilst in upper amphibolite grade rocks K-feldspar is stable with sillimanite. Kyanite has not been recorded and the prograde metamorphic path goes below the andalusite-kyanite-sillimanite triple point and the P-T-t path is anticlockwise. The timing of the metamorphism is best constrained in the Geehi Gorge area where mafic volcanics and volcanics of the Upper Ordovician (Gisbornian to Eastonian) Kiandra Group and black shales of the uppermost Ordovician (Bolindian) Bendoc Group have been metamorphosed to upper amphibolite grade. In the Tantangara area and in the Upper Indi area

there is a metamorphic unconformity between Upper Ordovician and lowermost Silurian rocks and rocks of Middle Silurian age. In the Geehi Gorge area foliated amphibolite-grade metamorphic rocks have been intruded and displaced by S-type granites, which have been foliated by later deformation. This puts the timing as post-Ordovician to earliest Silurian sedimentation, but pre the main S-type granite event, giving a narrow time interval of <10 Ma for the metamorphism to occur. In the Geehi Gorge, Indi and Bethanga areas the metamorphic isograds do not usually follow the outline of exposed granite intrusions. In some uppermost amphibolite grade localities the sediments show evidence of local melting and the volumetrically minor Geehi Granodiorite and the Bethanga Gneiss can be sourced from nearby Ordovician quartz-rich turbidites. The remainder of the S-type granites on either side of the Macquarie arc are similar in composition, and similar to many other S-types in the Lachlan Orogen. These cannot be derived from the exposed Upper Ordovician quartz-rich turbidites as they are too low in GaO, Na20, Sr and Pb. Although through a complex mixing process, mafic rocks of the Macquarie Arc could provide CaO, Na20, Sr they cannot provide Pb: the mafics could also provide too much Ni, Gr and MgO. Further, the majority of granite xenoliths are of quartzofeldspathic sediments and there are very few xenoliths of mafic igneous or volcaniclastic origin. The preferred interpretation is that the S-type granites are formed by melting of pre-Upper Ordovician sediments that are more enriched in feldspar and hence CaO, Na20, Sr and Pb. Rocks of this composition occur at Davies Plain in the Upper Indi area as granulite and charnokite inclusions in the Silurian (?) Elkins Creek gabbro. These inclusions contain cordierite + hypersthene ± K-feldspar ± spinel assemblages which are also interpreted to result from low-P, high-T metamorphism. Geothermal gradients calculated from metamorphic assemblages in both the Bethanga and Geehi Gorge areas and the granulites from Davies Plain are

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estimated to be 40® to 70 ®/km. Calculations on the conditions of melting at the source of the S-type granites from the Canberra and Cooma areas also require a similar high geothermal gradient. Thus the low-P, high-T event today extends E-W over SOOkms and there is no evidence of a paired metamorphic belt that is focussed on the Macquarie Arc. In fact in the Geehi Gorge Area the low-P, high-T metamorphic rocks extend some 15 kms to the east of the Gilmore Fault Zone, which has frequently been interpreted as the eastern edge of the metamorphics of the Wagga Omeo Metamorphic Belt. Metamorphic rocks also extend at least 5 kms to the east of the Macquarie Arc. The anomalously high geothermal gradients over such a wide area and the anti-clockwise P-T-t paths are atypical of modern collision zones. The Lachlan Orogen is not unique with this problem as most thermo-mechanical models of modern orogenic collision processes, including those of island arcs, have been unable to reproduce the extensive low-P, high-T conditions observed in other Palaeozoic and Precambrian orogens without invoking special processes than can be difficult to test (e.g., mantle plumes, lithospheric delamination, shear heating along the subduction contact). Another enigmatic feature in the Geehi Gorge-Indi area is the velocity/depth profile of the crustal structure determined from seismic refraction profiling. Under the Snowy Mountains this profiling shows a depth to the Moho of 45 to 52 km today. As to when this thickening occurred is conjectural, but if the Benambran Orgeny was the main crustal thickening event then this thickness would been achieved by the Upper Ordovician to Silurian. At the time of metamorphism there could also have been at leastl5 kms of rock above the amphibolite-grade metamorphics in the Geehi Gorge area, making a total crustal thickness of 60 to 70 kms. Such crustal thicknesses are atypical of west Pacific island arcs. Further, if these high geothermal gradients of 40 to 70 ®/km existed in the Ordovician in the upper 20 km of crust, then steady state conductive models cannot be applied, as they would imply wholesale melting in the lower crust and upper mantle: there is no evidence of this. Generating the source of the high geothermal gradient in the Lachlan Orogen has long been problematic. Advection of magmatic heat has been proposed, but yet the greatest development of the low-P, high-T metamorphism is where the Ordovician magmatism is thinnest and least developed, whilst middle to upper Silurian gabbros appear to post date the metamorphic

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peak. Radiogenic heat has also been proposed as a heat source, but this modelling could not generate the required heat without heat input from mafic magmatics. The organic-rich black shales of the Bendoc Group could provide the solution to the anomalously high geothermal gradients that need to be constrained within the upper crust. These shales were deposited on top of the Upper Ordovician quartz-rich turbidites and occur extensively throughout the Wagga Omeo Metamorphic Belt and Adaminaby Superterrane. Such organic rich sediments would be characterised by low thermal conductivities, which would be lower by a factor of 2 compared with other common sediment types. It is hypothesised that these shales could act as a thermal blanket and create anomalously high geothermal gradients in the upper crust similar to those in the Bethanga and Geehi areas. Geothermal gradients of up to 60®/km are recorded in rocks beneath the Cooper Basin (Australia) and these appear localised in the upper crust. Black shales of the extent that is observed in the Lachlan Orogen are not common in modern environments although black shales in the southern Lachlan Orogen have been interpreted as part of a global oceanic anoxic event. Hence greater consideration should be given to the thermal effects of black shales as drivers of anomalously high geothermal gradients in other lower Palaeozoic and even Precambrian collision belts. This widespread low-P, high-T metamorphic event in the Lachlan Orogen has two key economic implications. Firstly, as the main S-type, Siluro-Devonian Granite Event is dominated by low temperature granites they show little sign of magmatic fractionation and mineralisation. The exception is a suite of fractionated, higher temperature granites along the western edge of the Wagga Omeo Belt which are associated with tin mineralisation. Secondly, the potential for orgogenic gold deposits is high. One interpretation of orogenic gold deposits is that they form from dewatering of the sediment pile during greenschist facies metamorphism and many are located at or above the brittle ductile transition. The occurrence of widespread low-P, high-T metamorphism across the Lachlan Orogen would mean that the brittle ductile transition would occur at shallower depths, and be extensive. This agrees the widespread Au occurrences in the Lachlan Orogen and also opens the prospect for further new lode gold discoveries such as Wyoming and McPhillamys deposits (central west NSW) and Lockington (northcentral Victoria).

2009 International Conference on Island Arc-Continent Collisions: Macquarie Arc Conference, Orange, New South Wales, Australia

LATE PERMIAN TO MID-TRIASSIC CONTINENT-ARC COLLISION IN THE TIEN SHAN: IMPLICATION FOR THE ARCHITECTURE OF CENTRAL ASIA W J . Xiao State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Acadenny of Sciences, Beijing 100029, China.

The Altaids is one of the largest accretionary orogenic collages in the world with the highest rate of Phanerozoic continental growth and significant metallogenic importance (Fig. 1) (§engor et al. 1993; Jahn et al. 2000; Yakubchuk et al. 2001). It is widely accepted that subduction-related orogenesis of the

Altaids started in the late Precambrian and gradually migrated southward (present coordinates). However, it is uncertain when and how the building of the Altaids was finally completed. This paper discusses the late Permian to mid-Triassic geological history of the several periods of accretion and collision of archipelago systems lying between the Tarim continent and accreted arcs along southern Siberian continental margin. In this contribution the various tectonic units in the Tien Shan of the southern Altaids are described, which is subdivided into eastern and western parts according to their different tectonic features (Fig. 2).

Fig. 1 Schematic map showing the position of the Tien Shan (modified after 5engor et al. 1993).

—•

International border

Kyrgyzstan Middle a Tien Shan

Kashi

1 Chinese Northern Tien Shan

1 '

1 Kokshaal-Kumishi ' subduction-accretion complex

1 Mostly Kyrgyzstan Northern Tien Shan

1 Altaid collages

1 Mostly Kyrgyzstan Middle Tien Shan

' •• 1 Pamirs

Precambrian rocks

Paleozoic accretion direction

Fig. 2 Schematic tectonic map showing the tectonic elements of the Tien Shan.

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Geological Society of Australia Abstracts No 92 Eastern Tien Shan The eastern Tien Shan was an archipelago that was characterized by (a) the Harhk-Dananhu subduction system with a northerly S-dipping polarity in the north; (b) southerly N-dipping subduction system beneath the Hi-Central Tien Shan arc in the middle; and (c) the South Tien Shan ocean against Tarim in the south (Xiao et al 2004) (Fig. 3).

During the Devonian to Early Carboniferous, N-dipping subduction led to the Harlik-Dananhu arc and the Kanggurtag forearc basin/accretionary complex. In the Early to Mid-Carboniferous, the magmatic front associated with the N-dipping subduction beneath the Dananhu-Harlik arc migrated southwards, forming the Yamansu arc constructed upon the Kanggurtag accretionary

Fig. 3 Schematic tectonic map showing the tectonic elements of the eastern Tien Shan (Xiao et al. 2004).

Carboniferous Dananhu ^ ^ ^ ^ ^ N

^^^^ Ocetn

^ |}'

Occan V

^ ^

Kanggurtag accretionary wedge

/ Late Carboniferous to Early Permian Dananhu

Central T.en Shar,

Fig. 4 Tectonic evolution of the eastern Tien Shan (modified after Xiaoeto/. 2004).

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forearc. By the Late Carboniferous the DananhuHarlik arc was attached northwards to the southern Siberian margin. In the latest Carboniferous to endPermian multiple accretion-collision events left wide suture zones in the south that include the ophiolitestrewn Kokshaal-Kumishi accretionary complex (Fig. 4), resulting in considerable lateral enlargement of the Siberian continent (Xiao et al 2004).

Western Tien Shan The western Tien Shan records an ideal collision between the southern Siberian active margin to the north and the Tarim passive margin to the south (Fig. 5). A continuous accretionary complex (Kokshaal-Kumishi) is defined here along the southern part of the Tien Shan, which separates a complicated huge subduction-related orogenic collage (southern part of the Altaids) to the north from southerly Tarim continent.

2009 International Conference on Island Arc-Continent Collisions: Macquarie Arc Conference, Orange, New South Wales, Australia Correlation cross borders between China and Kyrgyzstan and Kazakhstan and tectonostratigraphic analysis unravels a general southward and oceanward accretion as recorded by the juxtaposition of ophiolites, slices of ophiolitic mdanges, cherts, island arcs, seamounts, olistostromes, blueschists, and turbidites, which are mainly Paleozoic in age, with the youngest age constraint being the end-Permian to early Triassic (Gao et al 1995; Li et al 2005; Zhang et al 2007; Xiao et al 2009a; Xiao et al 2009b). The northern orogenic collage is characterized by three Paleozoic island arcs (Northern Hi, Issyk Kul, and Chatkal) which may have amalgamated into a composite arc by either oroclinal bending or collision in the Paleozoic. The docking of the southerly Tarim and Karakum cratons to this complicated late Paleozoic accretionary complex occurred in the end-Permianearly Triassic, leading to a unique orogenic process along the western segment of the Altaids (Fig. 6).

Discussion Regarding when and how the building of the Altaids was finally completed, some researchers proposed that the Altaids was built in a way of oroclinal bending and strike-slip duplexing of a single, long-lived arc in the Paleozoic (§eng6r et al. 1993), while some thought that the architecture of the Altaids was characterized by amalgamation of multiple terranes including several micro-continents either in the Paleozoic, or in the early Triassic (Mossakovsky et al 1993; Ruzhentsev & Mossakovskiy 1996; Buslov et al 2003) The Tien Shan orogen offers an ideal place to address these controversial points. First, as documented above, both archipelago (multiple terrane amalgamation) and oroclinal bending and strike-slip duplexing (single arc) played an important role in the architecture of the Altaids. Even recent paleomagnetic data showed that large-scale rotations did occur in Kazakhstan (Van der Voo et al 2006; Fig. 5 Tectonic map of the western Tien Shan.

Late Carboniferous to End-Permian i-Central Tien Shan Arc

South Tien Shan

Tarim Craton

Ocean

Kokshaal-Kumishi accretionary wedge

Fig. 6 Tectonic evolution of the western Tien Shan.

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Abrajevitch et al 2007), tectonostratigraphic analysis indicated that there were also some collisional events in these complicated arc systems (Windley et al. 2007), thus suggesting a possible archipelago paleogeography before collision. Second, concerning the final amalgamation time, the Tien Shan orogen has shown clearly the youngest geological record extended to the end-Permian to mid-Triassic. However, a huge accretionary orogen like the Altaids which extends from several thousands of kilometres would have much complicated suturing history along strike, which should not be simply interpreted by using the suturing criteria from collisional orogens like the Himalayas.

Acknowledgements

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Russian Geology and Geophysics, 44, pp. 49-75. GAO, J., HE, G . Q . , Li, M . S . , XIAO, X . C . & TANG, Y . Q . 1995.

146 IGCP 524

pp. 1 8 1 - 1 9 3 . LI, Y.J., SUN, L.D., W u , H.R., WANG, G.L., YANG, C.S. &

PENG, G.X. 2005. Chinese Journal

of Geology, 40, 2 2 0 - 2 2 6 .

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WINDLEY, B.F., ALEXEIEV, D., XIAO, W . KRONER, A.

Thanks are due to many MSc and PhD students whose field and laboratory work provided many elements on which the present synthesis was built, in particular Q.G. Mao, X.P Long, K. Wong, and K.D. Cai. I am indebted to B. Windley, A. Kroner, C.M. Han, C. Yuan, H.L. Chen, M. Sun, J.Y. Li, J. Gao, A.M.C. §eng6r, D. Alexeiev, and V.S. Burtman for collaborations and discussions. Funds from the Chinese State 973 Program (2007CB411307) and the Natural Science Foundation (40725009,40523003) are gratefully acknowledged. This paper is a contribution to ILP (ERAS, Topo-Central Asia), IGCP 473,480 and 524.

BAZHENOV, M.L. 2 0 0 7 . Tectonophysics,

JAHN, B.-M., W u , F.-Y. & CHEN, B. 2000.

& COLE, A. 2001. Society of Economical Geologists Newsletters, 46, pp. 7-14. ZHANG, L.F,. AI, Y.L., Li, X . P . , RUBATTO, D., SONG, B., WILLIAMS, S., SONG, S.G.D., E. & L i o u , J.G. 2 0 0 7 . 9 6 , pp. 2 6 6 - 2 8 0 .

Lithos,