Abstracts No.66: 15th Victorian Universities Earth Sciences Conference, 2001 by GSAustralia

Geological Society of Australia

ABSTRACTS Number

15th Victorian Universities Earth Sciences Conference La Trobe University September 2001

15th Victorian Universities Earth Sciences Conference

La Trobe University September 2001

A meeting organised by the postgraduates of the Department of Earth Sciences, La Trobe University

Geological Society of Australia Abstract Volume Number 66

ISSN 0729 011X ©

Geological Society of Australia Incorporated, 2001

Citation for this volume: Storkey, A.C. (editor) 2001. 15th Victorian Universities Earth Sciences Conference, September 2001. Geological Society of Australia Abstracts 66, 54p.

Example of citations for papers in this volume: Ansell, H.L.E., 2001. Ichnofossils from the Early Permian of the Bacchus Marsh, Victoria In: Storkey, A.C. (editor). 15th Victorian Universities Earth Sciences Conference. Geological Society of Australia Abstracts 66, pp. 9.

Copies of this publication may be obtained from: The Business Manager, Geological Society of Australia Incorporated, Suite 706 Thakral House, 301 George Street, Sydney NSW 2000, Australia

The Organising Committee of the 15th Victorian Universities Earth Sciences Conference, would like to thank the following sponsors for their kind support:

Australian Society of Exploration Geophysicists

Esso Australia Pty Ltd

Geological Society of Australia (GSA)

Geological Society of Australia (VIC Division) i

Geological Survey of Victoria

GSA - Specialty Group in Economic Geology

Specialist Group for Geochemistry, Mineralogy, & Petrology

GSA - Specialty Group in Geochemistry, Mineralogy and Petrology

GSA - Specialty Group in Solid Earth Geophysics

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LaTrobe University - Faculty of Science,Technology and Engineering

LA TROBE ^ U N I V E R S I T Y

LaTrobe University - Department of Earth Sciences

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M ONASH U N I V E R S I T Y Monash University School of Geosciences

National Centre for Petroleum Geology and Geophysics (NCPGG)

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Table of Contents Sponsors of the 15th VUESC Conference Organising Committee

i x

Symposium: Igneous Geology and Volcanology

Distribution characteristics of the Cerro Galan Ignimbrite and the implications for eruption and collapse of the Cerro Galan Caldera, NW Argentina Hildyard, S.C., Cas, R.A.F. & Viramonte J.G

Volcanic stratigraphy and origin of the Wandsworth Volcanic Group Purdy, D.J. & Gust, D.A

Granite diversity in the Buckrabanyule district, northwest Victoria Cairns, C.P

An overview of basaltic volcanism in the Werribee Plains, Newer Volcanic Province, Vic: products and frequency of eruptions Hare, A.G., Cas, R.A.F. & Musgrave, R.J

Symposium: Sedimentary Systems

Geochemical and Nd isotopic constraints on provenance of sediments in the Officer Basin Wade, B.P., Barovich, K.M. & Hand, M

Ichnofossils from the Early Permian of the Bacchus Marsh, Victoria Ansell, H.L.E

Facies architecture and depositional controls: Early Cretaceous Eumeralla Formation, Eastern Otway Ranges, Victoria Noll, C.A. & Hall, M

10 v

Seismic interpretation and basin analysis of the Colac area, eastern Otway Basin, Victoria O'Brien, T. & Hall, M

A small mammal assemblage from Bullock Creek, Australia Schwartz, L

A petrographic and isotopic study of the Late Neoproterozoic Nuccaleena Formation, Flinders Ranges, SA Smith, H.D., Gammon, P.R., McKirdy, D.M. & Lemon, N.M

Symposium: Crustal and Economic Geology

Reinterpreting the tectonic significance of the Garze-Litang Suture Zone, East Tibet Reid, A. & Wilson, C

Ultramylonites in the Palaeoproterozoic Strangways Metamorphic Complex, central Australia: a record of terrane working Riewers, M. & Hand, M

Post-Palaeoproterozoic thermotectonic evolution of the northern western Australian Shield Weber, U.D., Kohn, B.P., Foster, D.A., Nelson, D.R. & Gleadow, A.J.W

Geology, structure and fluid characteristics of the quartz reefs and indicators in the Kingower District Bowron, K.M., Jackson, P. & Fleming, P.D

The Proterozoic structural evolution of the Eldee West Region Neilson, I.E., Lister, G.S. & Betts, P.G

Skarn alteration associated with pegmatites in the Harts Ranges, central Australia Lavery, P.E., Buick, I.S., Hand, M. & Storkey, A.C vi

Interaction of plume-derived melts and metasomatised lithosphere in the Archean: the 2.9-2.7 Ga Bellingwe Greenstone Belt Bolhar, R., Hergt, J. & Woodhead, J

Late stage thermal histories of two core complexes, Basin and Range, USA: new insights from (U-Th)/He thermochronometry Carter, T.J., Kohn, B.P., Gleadow, A.J.W., Foster, D.A., Belton, D.X. & Woodhead, J.D

Structural and fractal analysis of the Doughboy Breccia, Olary Domain, South Australia Clark, C. & James, P

Proterozoic tectonothermal evolution of the Robertson River area, Georgetown Inlier, North Queensland Hansen, D. & Boger, S

Petrogenesis of Early Palaeozoic mafic rocks associated with intracratonic rifting in central Australia Lee, M.J., Hand, M. & Foden, J.D

Low-temperature history of the central Fennoscandian Shield using apatite fission track and (U-Th)/He thermochronometry Lorencak, M., Kohn, B.P. & Gleadow, A.J.W

The structure, stratigraphy and economic potential of the Pilbailey area, eastern goldfields Western Australia Potter, J

New tectonic model for the opening of the Tyrrhenian Sea (central Mediterranean) Rosenbaum, G., Lister, G. & Duboz, C

Kinematics and dynamics of opening of the north Fiji back-arc basin Schellart, W.P., Jessell, M.W. & Lister, G.S

vii

A metamorphic perspective on the evolving pattern of deformation in the southern Adelaide Fold Belt Webb, G.P. & Hand, M

Symposium: Climate and the Environment

Interaction of upper ocean heat content with sea surface temperature in the equatorial Pacific Bettio, L.M., Power, S. & Walsh, K

Geochemical and thermal characterisation of the Paralana Hot Springs, northern Flinders Ranges, South Australia Long, N.,McPhail, D.C., Brugger, J. & Plimer, 1

A luminescence chronology of east-west orientated linear dunes in southeastern Australia Smith, B

Modelling stable water isotopes in the ocean surface Brown, J. & Simmonds, 1

A geochemical investigation of the derelict Conrad Mine, northern NSW Mercuri, M

Sea surface temperature variability in the tropical Indian Ocean Ansell, T. & Meyers, G

Quantification of groundwater discharge of salt in a local groundwater system, using isotopic techniques Bennetts, D.A

The factors influencing linear dune orientation in Australia Hope, P., Simmonds, I., & Bowler, J.M

viii

Decadal climate variability in the Pacific region Sae-Koe, S

Environmental impact of a waste rock dump in the Tooma River catchment, Kosciusko National Park, NSW Kress, J. & McPhail, D.C

South Australia's Murray Basin stratigraphy and its relevance to the Waikerie Salt Interception Scheme Wall, M., Bone, Y., Hopkins, B., and Telfer, A

Symposium: Geophysical and Analytical Methods

Numerical modelling of the Mise-a-la-Masse near-miss scenario Carey H., Heinson G. & Sexton M

Morphology and structure of clay minerals investigated by Transmission Electron Microscopy (TEM) and Atomic Force Microscopy (AFM) Duong, L., Kloprogge, T., Bostrom, T. & Frost, R

Pluton geometry and emplacement mechanisms in the Gawler Craton: insights from potential field data sets and structural geophysics McLean M.A & Betts, P.G

A new palaeomagnetic pole from the mid-late Eocene 'Older Volcanics' Sawyer, M

Geophysical methods for fresh water detection in saline groundwater environments Barrett, B.E., Hatch, M. & Heinson, G

A new approach to near surface mapping of groundwater salinity: application of fast sampling EM surveying methods Bennetts, D.A. & Hatch, M

51 ix

Measuring continental margin escarpment retreat in the 21st century Dimas, V.A

Pb-Pb stepwise leaching (PbSL): a new geochronological tool for high-grade polymetamorphic terranes Tonelli, M., Woodhead, J. & Hergt, J

Index of Authors

Conference Organising Committee Conference Convenor

Silvana Santomartirio

Abstract Selection Organiser

Gresley Wakelin-King

Editor

Angela Storkey

Sub-Editors

Anna Klein Silvana Santomartino

Sponsorship Committee

Mike Power Patricia Lavery Silvana Santomartino

Web Page Design

Jeff Keetley Darren Bennetts

Audiovisual

Graeme Luther Mike Power

Catering

La Trobe University Union

General Support

Anna Klein Daniel Palmowski Christian Ihlenfeld Sue White X

Geological Society of Australia Abstracts no. 66

15th Victorian Universities Earth Sciences Conference, La Trobe University, September 2001

SYMPOSIUM: IGNEOUS GEOLOGY AND VOLCANOLOGY

Igneous Geology and Volcanology

DISTRIBUTION CHARACTERISTICS OF THE CERRO GALAN IGNIMBRITE AND THE IMPLICATIONS FOR ERUPTION AND COLLAPSE OF THE CERRO GALAN CALDERA, NW ARGENTINA 1

Hiidvard. S.C.. 1Cas, R.A.F. & 2Viramonte J.G.

department of Earth Sciences, Monash University, Clayton VIC 3800, Australia lnstituto Geonorte, Facultad de Ciencias Naturales, Universidad Nacional de Salta, Buenos Aires 177, Salta, Argentina

The eruption of the large volume (1000 km3+) Cerro Galan Ignimbrite, at approximately 2.0 Ma, produced the Cerro Galan caldera, one of the largest known calderas of the Central Andes, NW Argentina (26Q00'S 67Q00'W). The caldera is an elliptical structure, 35 x 20 km with its longest axis orientated N-S parallel to the regional structural trends. A resurgent dome in the caldera centre consists of 1400 m of welded intracaldera ignimbrite and reaches a peak elevation of 6100 m. Detailed data on the internal stratigraphy and facies characteristics of the Cerro Galan Ignimbrite has allowed reinterpretation of the collapse model for the Cerro Galan caldera, traditionally interpreted as a classic example of symmetrical plate or piston collapse. The Cerro Galan Ignimbrite consists of two main depositional units that form a single compound cooling unit. The lower unit has a relatively even distribution and thickness (7-12 m, distal facies) around the caldera. However, the upper unit has an asymmetric distribution, with the thickest outcrops (150m+) to the east and north of the caldera. The upper unit progressively thins around the caldera, and to the south, the exposure is sparse and extremely limited. Both depositional units are thought to be the product of the same eruption with the initial trigger possibly the result of a change in the regional stress regime, which allowed ascent of a separate magma batch to mix with the dominant homogenous magma. The initial eruption deposited the lower (basal) Cerro Galan unit sourced from a vent thought to be located on the eastern margin of the caldera. Subsequently, there was a sufficient time break (hours to days) to the eruption allowing deposition of a fines-depleted unit (ash cloud surge) that separates the upper and lower flow units. The asymmetric distribution of the upper Cerro Galan unit suggests the (main) pyroclastic density current was initially directed to the east and north as the vent propagated around the caldera, unzipping the caldera, in an anticlockwise motion (E-N-W-S). This produced a collapse with a possible trapdoor like configuration. Key words: Cerro Galan caldera, collapse model, asymmetric distribution, large volume ignimbrite.

Geological Society of Australia Abstracts no. 66 15th Victorian Universities Earth Sciences Conference, La Trobe University, September 2001

VOLCANIC STRATIGRAPHY AND ORIGIN OF THE WANDSWORTH VOLCANIC GROUP Purdv. D.J. & Gust, D.A. School of Natural Resource Sciences, Queensland University of Technology, P.O. Box 2434, Brisbane OLD 4001, Australia Within the New England Orogen during Late Permian to Early Triassic, large volumes of dominantly silicic volcanics were deposited in terrestrial and shallow marine environments. Exposed throughout southern OLD and northern NSW, these now form the Wandsworth Volcanic Group, comprising the Wallangarra Volcanics, Emmaville Volcanics, Drake Volcanics and Dundee Rhyodacite. Field mapping in the Wallangarra area combined with petrographic, geochemical and phase chemistry analyses shows that the Wallangarra Volcanics are comprised of at least two subaerial ignimbrite facies with a total thickness exceeding 500m. Facies one ignimbrites are rhyolitic (72-77 wt % Si0 2 ) and contain phenocrysts of sanidine (50%), quartz (16%), oligoclase (29%) and biotite (4%). This facies generally consists of finegrained, ash-rich (60%), moderately to very strongly welded ignimbrites with minor air fall interbeds. Small, dominantly ignimbritic lithic clasts (1%) are a very minor component. In comparison, facies two ignimbrites are trachydacitic (65-71 wt% Si0 2 ) with significantly higher biotite content (13%), a higher abundance of more calcic plagioclase (andesine - 66%) and low sanidine (17%) and quartz (3%). These ignimbrites are crystal (39%) and pumice (17%)-rich and strongly to very strongly welded. They contain a variety of sparse, small to medium-sized lithic clasts and some minor air fall interbeds. A general lack of large lithic clasts throughout relatively homogeneous stratigraphies indicates deposition of both facies in a medial to distal environment. From a regional perspective, the Wallangarra Volcanics along with the Emmaville Volcanics and Dundee Rhyodacite show a high-K calcalkalic trend. The Drake Volcanics which lie >40km to the east of these units includes a greater mafic component and possibly shows a calkalcalic trend. This arrangement suggests a setting similar to the central Andean continental margin where thick continental crust causes extensive fractionation of magmas and the development of large calderas and widespread ignimbrite sheets. Key words: New England Orogen, Late Permian volcanism, rhyolitic ignimbrite, volcanology.

Igneous Geology and Volcanology

GRANITE DIVERSITY IN THE BUCKRABANYULE DISTRICT, NORTHWEST VICTORIA Cairns, C P . Department of Earth Sciences, La Trobe University, Bundoora VIC 3086, Australia The geology of the Buckrabanyule District provides invaluable information about the northern segment of the Stawell Zone due the identification of several new metaluminous and peraluminous granites. The metaluminous granites exhibit a variation in the oxidation state of biotite while biotites in peraluminous granites are frequently reduced. True granites appear rare and the majority of the felsic igneous intrusives straddle the boundary between granodiorite and tonalite. Granitic intrusion appears synchronous with local deformation at least in the north of the Stawell zone. The contact aureole is essentially schistose with the recognition of up to four deformational events. Therefore it seems probable that a major deformational event occurred at approximately 400Ma since granite ages in western Victoria are well established. An intimate relationship exists between many of the newly identified granites and localised mafic lithologies which exhibit a strong spatial relationship with the contact area between adjacent plutons. An extensive episode of fluid infiltration throughout the district is best observed in these mafic lithologies. The Avoca Fault, interpreted to occur in the east of the district, may have played a significant role in the evolution of the Buckrabanyule Intrusive Complex. Key words: granites, metaluminous, peraluminous, mafic magma, deformation, Stawell Zone.

Geological Society of Australia Abstracts no. 66 15th Victorian Universities Earth Sciences Conference, La Trobe University, September 2001

AN OVERVIEW OF BASALTIC VOLCANISM IN THE WERRIBEE PLAINS, NEWER VOLCANIC PROVINCE, VIC: PRODUCTS AND FREQUENCY OF ERUPTIONS 1

Hare. A.G.. 1Cas, R.A.F. & 2 Musgrave, R.J.

department of Earth Sciences, Monash University, Clayton VIC 3800, Australia Department of Earth Sciences, La Trobe University, Bundoora VIC 3086, Australia

Although the Newer Volcanic Province (NVP) is classified as an active province, little is known volcanologically about it, including the frequency and volume of eruptions, the distribution of lava types, flow dimensions etc. These parameters are investigated in this study in the Werribee Plains (-2,000 km2) which is located in the east of the late Tertiary- Quaternary intra-plate basaltic NVP. Several small point source eruption centres are considered to have fed the majority of the lava flows exposed at the surface. The eruption centres are scoria cones and low-angle lava shields and are considered to have formed by fissure-fed eruptions in which the magma output was focussed to a point source after a couple of days. Inflated pahoehoe sheet-flows are the most abundant type of lava flow fed from the eruption centres, representing - 5 0 % of all diamond drill core logged, suggesting a substantial proportion of the lavas were tube-fed. In general the lava flows are relatively thick compared with Hawaiian lavas: pahoehoe sheetflows average 6.5 m, S-type pahoehoe lobes average 0.6 m, aa flows average 24 m, and ponded lavas average 11 m in thickness. The sequence is up to 120 m thick locally, and can be subdivided into distinct eruption packages (the products of a given eruption), which are generally separated by weathered basalt and/ or paleosols. In order to correlate eruption packages between drill holes a magnetostratigraphy has been identified. Eruptions have occurred infrequently over the span of three different polarity intervals, (tentative age constraints: - 3 - 1.4 Ma, although radiometric ages are yet to be determined). Further correlations can be made using the secular variation stratigraphy combined with geochemical data. This study adds to our understanding of volcanism in plains-basalt provinces, which have largely received little attention (compared with ocean island basalt and flood basalt provinces). Key words: basalt, lava, volcanology, magnetostratigraphy, Newer Volcanic Province.

Geological Society of Australia Abstracts no. 66 15th Victorian Universities Earth Sciences Conference, La Trobe University, September 2001

SYMPOSIUM: SEDIMENTARY SYSTEMS

Sedimentary Systems

GEOCHEMICAL AND Nd ISOTOPIC CONSTRAINTS ON PROVENANCE OF SEDIMENTS IN THE OFFICER BASIN Wade. B.P.. Barovich, K.M. & Hand, M. Department of Geology and Geophysics, The University of Adelaide, Adelaide SA 5005, Australia The intracratonic Neoproterozoic to mid-Palaeozoic Officer Basin is one of a number of basins that together formerly comprised the Centralian Superbasin. The Officer Basin is bound to the north by the Mesoproterozoic Musgrave Block, and to the south by the Achaean-Mesoproterozoic Gawler Craton. It was separated from the remainder of the Centralian Superbasin during the late Neoproterozoic-early Palaeozoic Petermann Orogen (-560-540 Ma), which exhumed the Musgrave Block rapidly and formed the basin's northern margin. Major and trace element data from mid-Neoproterozoic to early-Silurian Officer Basin shales, siltstones, and sandstones obtained from drillholes, suggest a comparatively uniform sediment source over time. In addition, initial eNd values suggest that the Gawler Craton was the principal sediment source region for much of the basin history. The absence of Musgrave Block derived sediment input is surprising given both the magnitude of Musgrave Block exhumation during the Petermann Orogeny, and the basin morphology, which points to a well-developed Petermann foreland. One possibility for the absence of Musgravian-derived syn-orogenic sediments is that the Musgrave Block was covered by an extensive sediment blanket. Erosion of this blanket during the initial stages of the Petermann Orogeny filled relatively low-volume forelands with supracrustals, with subsequent synorogenic sediment transported across the filled foreland, and out of the Officer Basin. Another possibility is that the system of east-west dextral strike-slip faults that link deep grabens within the Musgrave Block provided an extremely efficient system that transported sediment eastward out of the system, bypassing the Officer Basin. This could account for the presence of 1100 Ma Musgrave type detrital zircons found in Petermann-aged sedimentary rocks in the Adelaide Fold Belt. Key words: Officer Basin, Nd isotopes, Provenance, Centralian Superbasin, Petermann Orogeny.

Geological Society of Australia Abstracts no. 66 15 th Victorian Universities Earth Sciences Conference, La T r o b e University, September 2001

ICHNOFOSSILS FROM THE EARLY PERMIAN OF THE BACCHUS MARSH, VICTORIA Ansell, H.L.E. School of Ecology and Environment,

Deakin University, Rusden Campus, 662 Blackburn Road, Clayton VIC 3168,

Australia

Ichnofossils from the Early Permian deposits of Bacchus Marsh, Bald Hill, Victoria, are described. The ichnofossils have been tendatively identified to the ichnogenus level, and Scalarituba (Weller 1899) Scolicia (De Quatrefages 1849), and Torrowangen (Webby 1970) have been recognised. They are preserved in cream coloured sandstones and light to dark brown ferruginous siltstones. Previous studies by Bowen (1959) and Crowell & Frakes (1971) use the evidence of varves to indicate that the depositional environment could have been lacustrine, proximal to the sea. However, the 'varves' are bioturbated by Scolicia and Torrowangen, and together the ichnofossils and lithology indicate a shallow marine to deltaic environment. Scolicia and Torrowangen occur in the same ichnofacies whereas Scalarituba occurs by itself. The tracemakers of the ichnofossils described, were probably formed by an annelid like organism. They are all internal trails. Key w o r d s : Ichnofossils, Early Permian, deltaic, Bacchus Marsh, Victoria.

Sedimentary Systems

FACIES ARCHITECTURE AND DEPOSITIONAL CONTROLS: EARLY CRETACEOUS EUMERALLA FORMATION, EASTERN OTWAY RANGES, VICTORIA Noll. C.A. & Hall. M.

Department of Earth Sciences, Monash University, Clayton VIC 3800, Australia The Skenes Creek - Wongarra region, on the eastern margin of the Otway Ranges, around 160 km southwest of Melbourne, hosts spectacular coastal outcrop of the Aptian-Albian Eumeralla Formation. The stratigraphic framework comprises mudstone dominated overbank and lacustrine successions and high flow regime, fluvially derived sandstones. Lacustrine deposits are characterised by the anomalously thick (>1000 m) Skenes Creek Coal Measures, a sequence comprising massive and laminated mudstone, abundant thin coal seams, siltstone, interlaminated siltstone and mudstone, and minor fine-grained, planar laminated and massive sandstone. Overbank sequences, up to 50 m thick, also comprise thinly bedded sandstone and mudstone, with common laminated sheet-like (LS) architectural elements. A high-energy, low-sinuosity, braided fluvial system also operated within the basin at this time, and comprises intraformational and exotically derived conglomerate, as well as massive, planar laminated, trough and tabular cross-bedded and ripple laminated sandstone. Individual channel units are upward fining, and were deposited in channels up to 80 m wide and typically less than 10 m deep. Fluvial architecture of the braided system is dominated by multistorey channel elements (CH) that comprise channel sand bodies (SB), downstream (DA) and minor lateral accretion (LA) elements, and are predominantly bound by fourthand fifth-order bounding surfaces. Although regional palaeocurrrent trends indicate drainage from east to west, local palaeocurrents are predominantly unidirectional and trend to the southwest, probably reflecting the control on fluvial drainage by active, northeast trending faults such as the Skenes Creek Fault. Accumulation of the Skenes Creek Coal Measures within the hanging-wall of this fault is thought to result from the structural isolation of lacustrine conditions within a predominantly high-energy fluvial system. Petrologically the Eumeralla Formation is dominated by compositionally immature andesitic and felsic volcaniclastics, with an average Q:F:L ratio of 20:8:72. The assemblage is texturally submature as a result of having been deposited within a cold or periglacial environment. The fluvial system was therefore partially controlled by high aggradation rates associated with an eruption-induced source to the east but was also locally controlled by intrabasinal, northeast trending faults, which defined axial-through drainage, channel density and distribution, and isolated lacustrine environments. Key words: fluvial architecture, braided river, stratigraphic framework, Eumeralla Formation, Otway Basin.

Geological Society of Australia Abstracts no. 66 15th Victorian Universities Earth Sciences Conference, La Trobe University, September 2001

SEISMIC INTERPRETATION AND BASIN ANALYSIS OF THE COLAC AREA, EASTERN OTWAY BASIN, VICTORIA O'Brien. T. & Hall. M. Department of Earth Sciences, Monash University, Clayton VIC 3800, Australia Near Colac in the northeast, onshore part of the Otway Basin, up to 7 km of Early Cretaceous (Aptian and Albian) Otway Group fluvio-volcanic sediments were deposited in a series of NE-ENE trending half grabens. Thick coal seams cause continuous, high amplitude reflections in the middle part of an otherwise monotonous seismic stratigraphy. These Early Cretaceous sediments dip uniformly south and thicken towards the generally north-dipping faults bounding the half grabens. Erosion during the Late Cretaceous resulted in the removal of up to 2 km of the older sediments This was followed by the deposition of 600m of restricted to open marine Tertiary sediments. In the north of the area the Tertiary sediments are deeper and more flat-lying, while to the south they form large, postMiocene anticlines above reversed, Early Cretaceous normal faults. Isopach maps and cross-sections constructed for various stages of basin development illustrate the extent of the fault growth during the deposition of the Otway Group and the amount of structural inversion experienced before and during Tertiary deposition as well as the amount of erosion and uplift which has occurred. The coal seams are most likely to be the only effective petroleum source rocks in the area, but are now over mature as a result of deep burial in most areas and a high geothermal gradient in the Early Cretaceous. The only traps present are small structural traps in the Tertiary which were formed to late to trap the hydrocarbons which were migrating during the Cretaceous. Key words: Otway Basin, hydrocarbons, seismic.

Sedimentary Systems

A SMALL MAMMAL ASSEMBLAGE FROM BULLOCK CREEK, AUSTRALIA Schwartz, L. Department of Earth Sciences, Monash University, Clayton 3800 VIC, Australia Bullock Creek is a fossil site in the Northern Territory, part of a group of Oligo-Miocene sites that includes the Riversleigh World Heritage area, and the South Australian fossil sites. This poster examines the biodiversity of the small mammals at Bullock Creek. A more complete understanding of the biodiversity of this site is considered important due to its position as the most northerly Tertiary fauna known on the Australian continent. Stage-of-evolution biochronology, the only dating method available for these deposits, also requires as complete a faunal list as possible. The Camfield Beds, in which the Bullock Creek Local Fauna is found, were deposited in a fluvio-lacustrine environment, with vertebrate fossils concentrated in coarse, channel-fill deposits. Preliminary mapping of the fossil bearing strata indicates that there may be a stratigraphic succession of sites. Examination of the microvertebrates is ongoing, but the presence in the fauna of forms not previously known from this locale or in this time period, suggests that previous interpretations of the age of Bullock Creek may be incorrect. The accepted view that Bullock Creek is younger than Middle-Miocene Riversleigh faunas and much younger than the South Australian Kutjumarpu Local Fauna seems unlikely, given the close similarities in faunal composition. In particular, the macropodines, thylacines and zygomaturines shared between them all support a close temporal relationship of these sites, indicating that they probably represent a similar time interval. Key words: Micro-mammals, Bullock Creek, Camfield Beds, biochronology.

Geological Society of Australia Abstracts no. 66 15th Victorian Universities Earth Sciences Conference, La Trobe University, September 2001

A PETROGRAPHIC AND ISOTOPIC STUDY OF THE LATE NEOPROTEROZOIC NUCCALEENA FORMATION, FLINDERS RANGES, SA 1

Smith. H.D.. 1Gammon, P.R., 1McKirdy, D.M. & 2Lemon, N.M.

department of Geology and Geophysics, The University of Adelaide, Adelaide SA 5005, Australia National Centre for Petroleum Geology and Geophysics, The University of Adelaide SA 5005, Australia

The Nuccaleena Formation of the Adelaide Fold-Thrust Belt is a thin, laterally persistent dolostone that abruptly overlies the glacial Elatina Formation in the Flinders Ranges and Stuart Shelf of South Australia. Its preservation, possibly the best of all Neoproterozoic cap carbonates, dictates that this is a key formation in testing the Snowball Earth controversy. From the study area at Parachilna Gorge, central Flinders Ranges, 42 samples were taken at - 3 0 cm intervals through the stratigraphy. Sedimentary structures include the much-discussed "teepees" (previously attributed to seafloor cementation), which are overlain by growth faults. Each fault is in structural continuity with the axial plane of the underlying "teepee" until about 1 m above its apex, where the fault soles into a listric decollement along shale laminae. Growth fault movement suggests that the "teepee" caused gravitational instability in unconsolidated overlying sediment, and that the "teepees" were at least 5 m (compacted distance) below the seafloor when moving. Petrographic analysis of the dolostone revealed three stages of carbonate veins and sedimentary textures suggestive of early diagenesis: 1.) minor fibrous dolomite is usually associated with layer-parallel movement along clay seams; 2.) abundant, clay-rich, blocky euhedral dolomite, infilling all fractures, especially growth-fault sheet fractures and the myriad tiny, curvilinear clayrich "brown veins" throughout the dolostone; 3.) moderately common, blocky, clear anhedral calcite, restricted to re-brecciated stage 2 veins. All veins are isotopically distinct. The Nuccaleena dolostone matrix is crystalline dolomite, with a patchwork texture of finer, more clay-rich, and coarser, clay-poor, crystalline areas. This indicates the matrix has been recrystallised, and that the clay-rich stage 2 veins are indicative of physical removal of clay particles from the host rock into the veins whilst large enough pores still existed. Microprobe and isotopic analysis of microdrilled samples confirm that the matrix dolostone is chemically and isotopically equivalent to the stage 2 vein dolomite. Thus, contrary to the Snowball Earth hypothesis, this cap carbonate does not retain its primary depositional C-isotope signature. Key words: isotopic analysis, petrography, Nuccaleena, teepee, Neoproterozoic, cap carbonate, Snowball, veins.

Geological Society of Australia Abstracts no. 66 15th Victorian Universities Earth Sciences Conference, La Trobe University, September 2001

SYMPOSIUM: CRUSTAL AND ECONOMIC GEOLOGY

Crustal and Economic Geology

REINTERPRETING THE TECTONIC SIGNIFICANCE OF THE GARZE-LITANG SUTURE ZONE, EAST TIBET Reid. A. & Wilson, C. School of Earth Sciences, The University of Melbourne, Parkville VIC 3010, Australia While the present spectacular topography of the Tibetan Plateau is a function of the northward propagation of India into Eurasia, it is recognised that the Eurasian continent, far from being an amorphous mass, preserves a long history of continental accretion. In east Tibet, two of the major continental blocks, the Songpan Ganze Fold Belt and the Yidun Volcanic Arc are separated by the GarzeLitang Suture Zone. This zone of sheared mafic volcanics, mudstone, ultramafic and brecciated limestone, is generally considered to record the late-Triassic suturing of these two blocks. However, stratigraphic data and new structural evidence suggests that this melange zone is not a 'suture' between two unrelated blocks and that the presence of the melange in the deformed sedimentary pile is a function of late-Triassic deformation. Stratigraphically both the Songpan Garze Fold Belt and Yidun Arc record Palaeozoic passive margin sequences, overlain by a monotonous sequence of Triassic flysch, with intercalated arc volcanics in the Yidun Arc. Deformation in the Songpan Garze Fold Belt, is predominantly chevron style folding in the upper pile with a progressive increase in intensity with stratigraphic depth and proximity to an inferred regional decollement. In the Yidun Arc, deformation is likewise characterised by upright chevron style folds and associated thrusting. The strongest deformation observed in the Yidun Arc is S2 parallel layer transposition at the western margin. These structural features are consistent with regional late-Triassic south-west directed shortening across both the Songpan Garze Fold Belt and the Yidun Arc. Based on these observations, the Garze-Litang melange zone is reinterpreted to be a zone of back-arc/oceanic crust brought into the overlying sedimentary pile during the late-Triassic deformation, forming what is here termed the Garze-Litang Shear Zone. Key words: fold and thrust belt, suture zone, Tibet.

Geological Society of Australia Abstracts no. 66 15th Victorian Universities Earth Sciences Conference, La Trobe University, September 2001

ULTRAMYLONITES IN THE PALAEOPROTEROZOIC STRANGWAYS METAMORPHIC COMPLEX, CENTRAL AUSTRALIA: A RECORD OF TERRAIN REWORKING Rieuwers. M. & Hand, M. Department of Geology and Geophysics, The University of Adelaide, Adelaide SA 5005, Australia The Palaeoproterozoic Strangways Metamorphic Complex (SMC) forms the core of the southeastern Arunta Block in central Australia. The complex consists of multiply deformed granulite-grade metasediments and meta-igneous rocks that record at least two regional metamorphic events over the interval 1780-1715 Ma. These events were in large part responsible for creating the metamorphic architecture of the central Arunta Block. The pervasive granulite-facies structural fabrics in the SMC have been overprinted by at least two generations of ultramylonites that range in width from sub-cm to around 30 meters. Both generations of ultramylonite dip steeply to the northeast and east and contain steep-plunging stretching lineations. The first generation of ultramylonites records west and southwest-directed thrusting and contain mediumpressure granulite assemblages that define the ultramylonitic fabric. The compressional ultramylonites have been overprinted by a northeast-down normal-sense system. The ages of the two systems are unknown, however the contrast in kinematic style and overprinting relationships suggest that they may have formed during two different tectonothermal events. This notion is supported by the presence of prograde reaction textures in the extensional ultramylonites. Conceivably, the compressional shear zones are linked to the terminal stages of convergent Palaeoproterozoic highgrade deformation (Goscombe, 1992). However, the extensional shear zones show a regional eastward increase in metamorphic grade from amphibolite to granulite facies toward the Cambro-Ordovician Harts Range Metamorphic Complex, which in part preserves a late Cambrian rift sequence that was deposited on the Strangways basement complex (Buick et al., 2001). This increase in metamorphic grade is accompanied by an increase in the abundance and scale of the extensional ultramylonites. Both these observations suggest that the extensional ultramylonites may be linked to the high-T Cambro-Ordovician continental rifting event centered on the Harts Range region, and be the expression of Palaeozoic extensional deformation in the Proterozoic basement in central Australia. Key words: ultramylonites, amphibolite to granulite facies, Strangways Metamorphic Complex, Arunta Block.

Crustal and Economic Geology

POST-PALAEOPROTEROZOIC THERMOTECTONIC EVOLUTION OF THE NORTHERN WESTERN AUSTRALIAN SHIELD 1

Weber. U.D.. 1Kohn, BP., 2Foster, D.A., 3Nelson, D.R. & 1Gleadow, A.J.W.

Australian Geodynamics Cooperative Research Centre, School of Earth Sciences, The University of Melbourne, Parkville VIC 3010, Australia department of Geological Sciences, University of Florida, Gainesville, FL 32611, USA 3

Geological Survey of Western Australia, 100 Plain Street, East Perth, WA 6004, Australia

We report the results of an Âr/Âr (16 K-feldspars, 2 secondary muscovites) and apatite fission track (AFT) thermochronological study of the northern Western Precambrian Shield of Western Australia. Broadly distributed Âr/Âr data suggest slow cooling from either -1800-1600 Ma or -1200-1000 Ma, depending on the sample. Most of the samples give minimum ages -500 Ma, which suggests either a small amount of reheating and minor argon loss at that time, or slightly more rapid cooling. AFT data reveal cooling between 290-180 Ma and confined horizontal mean track lengths range between 11.5-14.3 jiim with standard deviations falling between 1.1-2.2 jim. Forward modelling of timetemperature history paths from AFT data reveal a late Carboniferous to early Permian regional cooling episode and minor late Mesozoic cooling. Assuming that the average present day geothermal gradient of -18±2° C.km'1 was prevalent since the late Palaeozoic, then a minimum of -75° C of cooling predicted by the AFT modelling suggests overall denudation of -3.7-4.6 km of section since that time. Together with the 40Ar/39Ar K-feldspar data the results suggest an overall regional cooling since the Palaeoproterozoic from -350-400° C. Proterozoic and Phanerozoic basins, that partly continue offshore, contain up to -15 km of predominantly clastic sediments and are likely to have been depocentres for much or at least some of the detritus derived from the denudation inferred by the thermochronological data. Forward modelling of 40Ar/39Ar K-feldspar data will reveal whether thermal relaxation following a heating episode may also have been part of this cooling history. The post-Palaeoproterozoic thermotectonic history presented here complements previous geochronological data (U-Pb zircon SHRIMP, Âr/Âr dating of hornblende, muscovite, biotite) revealing the early history of the Western Shield. Key words: Âr/Âr dating, apatite fission track dating, thermochronology, Western Australia, timetemperature history, Proterozoic, Phanerozoic.

Geological Society of Australia Abstracts no. 66 15th Victorian Universities Earth Sciences Conference, La Trobe University, September 2001

GEOLOGY, STRUCTURE AND FLUID CHARACTERISTICS OF THE QUARTZ REEFS AND INDICATORS IN THE KINGOWER DISTRICT Bowron. K.M.. Jackson, P. & Fleming, P.D. Department of Earth Sciences, La Trobe University, Bundoora VIC 3086, Australia The Kingower district has produced a significant number of large gold nuggets including the Blanche Barkly (54.2kg) and the Hand of Faith (27.2kg). The origin of the gold has been long debated, however, much mention has been made in the literature of gold occurring on indicator structures. These structures are discrete, thin, vertical fault lines that contain quartz, and commonly, graphitic seams. The indicators intersect subhorizontal quartz veins called flatmakes. Flatmakes are planar tension gashes which spur off main vertical reefs that are up to 30 meters wide and may be up to 1km long in the Kingower district. The appearance of the quartz is variable with three types defined; milky white massive quartz, mottled dark grey massive quartz and translucent sugary textured quartz. The mottled dark grey quartz always occurs in the flatmakes in the proximity of intersections with indicators. Thin section studies have shown that some quartz have undergone multiple deformation as evidenced by microstructures such as multiple generations of subgrains, stylolitic boundaries and remarkably different concentrations of fluid inclusions from one grain to the next in the same sample. Other quartz are undeformed. Decrepigrams from the different quartz types show contrasts in fluid inclusion populations that can be related to microstructures. Simple quartz generally has a simple pattern of peaks while multiply deformed quartz has a more complex pattern with multiple peaks representing the trapping of fluids of differing properties, coincident with deformational events. All quartz from the area appears to have a common peak at approximately 430°C, which is a regional trait. High grade gold mineralisation was recently discovered nearby to the project area, in similar structures to those observed at Kingower, with gold occurring at a flatmake-indicator intersection. Key words: gold, indicators, flatmakes, quartz, deformation, fluid inclusions, decrepigram, Kingower district.

Crustal and Economic Geology

THE PROTEROZOIC STRUCTURAL EVOLUTION OF THE ELDEE WEST REGION Neilson. I.E.. Lister, G.S. & Betts, P.G. Australian Crustal Research Centre, Department of Earth Sciences, Monash University, Clayton VIC 3800, Australia The Proterozoic structural and geometrical evolution and the process involved of the north western region of Broken Hill, the Eldee West structure, are investigated using potential field interpretation in conjunction with structural mapping. The Eldee West region underwent poly deformation resulting in complex geometry formed by the overprinting of recumbent and upright folds. Four periods of deformation have been preserved in metasedimentary and igneous rocks of the Sundown and Paragon Groups. D1 and D2 deformations occurred under amphibolite-granulite facies conditions and both gave rise to sub-horizontal shallow dipping fabrics (S1 and S2). The first fabric (S1) is observed as a bedding parallel foliation, defined by quartz, biotite, K- feldspar and sillimanite. No other structures related to F1 were observed. The second generation fabric (S2) is dominantly horizontal to shallowly inclined. S2 is defined by sillimanite and white micas. S2 is axial planar to recumbent to inclined isoclinal F2 folds. The third generation folds (F3) are associated with an axial planar cleavage, (S3), defined by biotite minerals. F3 folds are open and upright, shallow plunging folds with axial surfaces trending NE - SW. Late retrograde shear zones have developed adjacent to D3 developed shear zones and overprint S3. The S4 fabric is defined by the spaced cleavage defined by biotite. S4 is axial planar to F4 trending EW upright folds and overprints S3. There are two types of pegmatitic melt intruding into the metasedimentary pile. Both contain quartz, with minor K- feldspar and white micas. The first melt occurs parallel to S0/S1. The second melt crosscuts the S1 schistosity plane, and is folded by recumbent F2 folds. Migmatitic textures were observed locally close to large coarse grained granites. Both melts were only observed within the Sundown Group. The overprinting relationship has resulted in a dome and basin geometry for the Eldee West Structure. These dome and basins are highly non-cylindrical and may have formed during sheath folding or by fold interference as suggested previously by other workers. Key words: Eldee West, Broken Hill, structure, Sundown Group.

Geological Society of Australia Abstracts no. 66 15th Victorian Universities Earth Sciences Conference, La Trobe University, September 2001

SKARN ALTERATION ASSOCIATED WITH PEGMATITES IN THE HARTS RANGES, CENTRAL AUSTRALIA 1

Laverv. P.E.. 1Buick, I.S., 2Hand, M. & 1Storkey, A.C.

Department of Earth Sciences, La Trobe University, Bundoora VIC 3086, Australia

Department of Geology and Geophysics, The University of Adelaide, Adelaide SA 5005, Australia

The Palaeozoic Harts Range pegmatites occur throughout the stratigraphic units of the Harts Ranges of central Australia. The stratigraphy of the Harts Ranges can be divided into the Palaeoproterozoic Entia Gneiss complex, mostly inter-layered quartzofeldspathic gneiss and mafic amphibolite, and the Neoproterozoic/Palaeozoic Irindina and Brady Gneiss Complexes dominated by supra-crustal pelitic and calcareous assemblages along with significant mafic amphibolite. The mineral assemblage of the pegmatites is directly related to the host lithology, muscovite rich in the Irindina and Brady Complexes contrasting with mica poor pegmatites in the Entia Complex. The stable isotope geochemistry of the pegmatites varies systematically with the host rock type as well with higher silicate whole rock oxygen isotope signatures in the micaceous pegmatites and lower signatures in the quartz-feldspar dominated Entia pegmatites. The mineralogical and stable isotope evidence points to a local partial melt model for the genesis of these pegmatites. In contrast to the local controls on lithology and stable isotope geochemistry, the formation of skarn alteration assemblages along contacts with, or proximal to, the pegmatites is regionally compositionally homogeneous. Small variations in alteration pattern exist in the form of haloed zonation or cm-scale layered zonation, but overall, the alteration style does not vary throughout the region. The metasomatic pattern is characterised by layers or zones of grossular and/or andradite garnet, clinopyroxene and epidote which correspond respectively to zones of decreasing temperature. Whereas the metasomatic style is homogeneous throughout the Harts Range region, not all pegmatites have been metasomatised. This is thought to be evidence for the preferential channelisation of late fluids. Textural evidence supports the argument that the metasomatism postdates the pegmatite formation. Key words: skarn, pegmatite, metasomatism, stable isotope geochemistry, Harts Ranges.

Crustal and Economic Geology

INTERACTION OF PLUME-DERIVED MELTS AND METASOMATISED LITHOSPHERE IN THE ARCHAEAN: THE 2.9-2.7 GA BELINGWE GREENSTONE BELT Bolhar. R.. Hergt, J. & Woodhead, J.

School of Earth Sciences, The University of Melbourne, Parkville VIC 3010, Australia Ultramafic to mafic igneous rocks from five stratigraphic units in the 2.9-2.7 Ga Belingwe Greenstone Belt were analyzed for trace elements (by Laser Ablation ICP-MS) and Sm-Nd isotopic compositions. Two distinct groups of rocks can be distinguished: (1) Samples from the 'incompatible element enriched' group show trace element features consistent with variable incorporation of crustal material. This is also supported by correlations between trace element ratios and initial Nd isotopic compositions. (2) The 'unfractionated to depleted' group of samples shows characteristic and ubiquitous High Field Strength Element (HFSE) depletions relative to neighboring Rare Earth Elements (REE). A variety of petrogenetic processes can be invoked to generate trace element patterns with slightly depleted LILE and LREE, and relative depletions in Nb, Ta, P and Ti, and thus must be tested in detail. Given strong similarities of the Belingwe rocks to mafic rocks generated in Phanerozoic intra-plate environments, and distinct differences from modern oceanic arc rocks in terms of incompatible element systematics, an arc environment is omitted. The coexistence of crustally contaminated rocks and rocks formed in an intra-plate environment can be used to argue for a continental intra-plate origin, probably in the form of rift-related magmas. Fractionation by high-pressure phases, source contamination by sediment recycling, shallow-level crustal contamination and recycling of subduction-modified oceanic crust can all be ruled out on the basis of incompatible trace element ratios and simple trace element modelling. HFSE/HFSE vs. La/SmPM relationships are suggestive of two-component mixing, with both end-members slightly depleted in incompatible elements, but strongly fractionated in HFSE relative to each other. HFSE fractionation may point to a petrogenetic scenario where asthenospheric melts interacted with lithospheric mantle in the presence of fluids. It is argued that the Belingwe igneous rocks acquired their geochemical characteristics by interaction of plume-derived melts with pre-existing crustal material and metasomatised lithospheric mantle, probably in an intra-continental rift setting. Key words: Belingwe Greenstone Belt, Plume-Lithosphere Interaction, continental intra-plate, Archaean, Th-U-Nb systematics.

Geological Society of Australia Abstracts no. 66 15th Victorian Universities Earth Sciences Conference, La Trobe University, September 2001

LATE STAGE THERMAL HISTORIES OF TWO CORE COMPLEXES, BASIN & RANGE, USA: NEW INSIGHTS FROM (U-Th)/He THERMOCHRONOMETRY 1

Carter. T.J.. 1Kohn, B.P., 1Gleadow, A.J.W., 2Foster, D.A., 1Belton, D.X. & 1Woodhead, J.D. 1

School of Earth Sciences, The University of Melbourne, Parkville VIC 3010, Australia 2 Department of Geology, University of Florida, Gainesville, Florida 32611, USA

The recent development of (U-Th)/He apatite thermochronometry has opened up new possibilities for determining thermal histories down to temperatures lower than any other known thermochronometer. For a cooling rate of 10°C/m.y. the closure temperature of helium in apatite is - 7 5 ± 5°C, although this varies with grain size. Knowledge of the thermal histories for rocks at these temperatures enables the characterization of near-surface tectonic processes in much greater detail than previously possible. The technique makes use of 4He produced during alpha decay of U and Th within apatite. By measuring the amount of 4He, U and Th within apatites from a given sample an apparent "He age" can be defined which records the time at which the sample cooled below ~75°C. This method has been applied to two metamorphic core complexes in the Basin and Range province of southwestern USA. Miocene extension in this area resulted in the rapid unroofing of mid-crustal rocks to the surface along low-angle normal faults. Obtaining cooling ages from samples along transects parallel to the transport direction provides estimates of lower plate exhumation rates. Preliminary data from the Harcuvar Mountains core complex, Arizona, broadly supports the rapid exhumation rate previously obtained from apatite fission track data, but a two-stage cooling history may also be applied, which differs markedly from previous interpretations. Completion of further U & Th determinations, along with extra samples from critical localities, will enable more confident evaluation of the two stage cooling model. Data from the Chemehuevi Mountains core complex shows a less consistent trend, with the northern transect showing a good correlation between apparent He age and sample position, whereas the southern transect bears no clear relationship. Key words: (U-Th)/He, low temperature thermochronology, Basin and Range province, metamorphic core complex.

Crustal and Economic Geology

STRUCTURAL AND FRACTAL ANALYSIS OF THE DOUGHBOY BRECCIA, OLARY DOMAIN, SOUTH AUSTRALIA Clark. C. & James, P. Department of Geology and Geophysics, The University of Adelaide, Adelaide SA 5005, Australia The Olary Domain of the Curnamona Province, a major Proterozoic terrain located within eastern South Australia and western New South Wales, is an example of a region that has undergone multiple brecciation events. The Doughboy breccia located in the northern Outalpa Inlier is the result of one of these brecciation events. It is an elongate body stretching 1.5 km from southwest to northeast and is approximately 0.5 km wide. The breccia consists of albitic (Na-plagioclase + quartz ± magnetite ± K-feldspar) clasts supported by a fine-grained matrix of biotite + quartz + plagioclase + K-feldspar ± actinolite ± magnetite. The breccia is surrounded by psammopelitic metasediments and is core to a macroscopic F2 syncline, which plunges 53 towards 226. The breccia also exhibits a strongly developed S2 fabric oriented 061/87 NW. Structural analysis of the surrounding metapelites shows an earlier ST fabric parallel to S0, which is interpreted to be the result of large scale recumbent folding during D v A third generation of folding has also been observed causing plunge reversals of F2 folds and dome and basin style interference patterns. Fractal analysis performed on clasts from the Doughboy breccia and on the nearby Cathedral Rock breccia reveal significant differences in the fractals for particle roughness (Fd) and particle size distribution (Fs). These differences can be attributed to the differing mechanisms of brecciation. The fractals obtained from the Doughboy breccia (Fd = 1.069, Fs = 1.701) suggest that the mechanism for brecciation was tectonic comminution, which indicate that it occurred during D v The fractals obtained from Cathedral Rock (Fd = 1.112, Fs = 1.373) indicate that fluid played a greater role in the brecciation process. Collectively, results obtained from structural and fractal analyses may shed light on the mechanism and timing of brecciation and may also be used to quantitatively compare the origins of different breccias in the same region. Key words; Olary Domain, breccia, structural analysis, fractal analysis.

Geological Society of Australia Abstracts no. 66 15th Victorian Universities Earth Sciences Conference, La Trobe University, September 2001

PROTEROZOIC TECTONOTHERMAL EVOLUTION OF THE ROBERTSON RIVER AREA, GEORGETOWN INLIER, NORTH QUEENSLAND Hansen. D. & Boger, S. Australian Crustal Research Centre, Department of Earth Sciences, Monash University, Clayton VIC 3800, Australia The Georgetown Inlier has previously been interpreted as a LP-HT terrane typical of many Australian Proterozoic terranes. However, new field and petrographic observations, and thermal modelling in THERMOCALC suggests otherwise. The study recognised two Proterozoic fabrics and metamorphic events within the Robertson River area. The earliest fabric S1 is preserved as relict hinges in Quartzdomains of the dominant schistosity (S2), and as inclusion trails at a high angle to the external fabric within garnet porphyroblasts. A metamorphic event late- to post-S1 resulted in the growth of garnet + quartz + muscovite ± chlorite ± biotite. The second and more pervasive fabric S2 was found to be a composite schistosity derived from the overprinting of an earlier foliation S1. This fabric forming event was associated with the growth of the peak metamorphic mineral assemblage comprising staurolite + garnet + biotite + muscovite + quartz, a typical Barrovian metamorphic assemblage. Based on this peak assemblage, modelling undertaken using THERMOCALC derived P-T estimates of 600°C at 6 kbars. This estimate of pressure is in keeping with pressure estimates for other Barrovian terrains, but is - 2 kbars greater than previous estimates for the area and greater than the pressure usually attributed to LP-HT metamorphic terranes. The presence of abundant inclusion trails continuous with the external schistosity suggests that the peak assemblage post-dates the main fabric forming event responsible for producing S2. Andalusite variably replaces staurolite and matrix muscovite, while garnet rims appear to be replaced by biotite ± chlorite. The overprinting of staurolite by andalusite implies decompression of up to 4 kbars occurred late in the thermal history. The final metamorphic overprint is the growth of sillimanite, typically over biotite. Sillimanite is not aligned within the fabric but statically overgrows aligned biotite laths, suggesting that a post-D2 thermal pulse pushed the rocks into the sillimanite stability field. This thermal pulse is attributed to the intrusion of abundant late- to post-D2 granites dated at 1550 Ma. The deformation, metamorphism and the intrusion of granites is interpreted to be the result of a west-dipping subduction zone under the north Australian continent at this time. Keywords: metamorphism, THERMOCALC, P-T estimates, Robertson River Area, North Queensland.

Crustal and Economic Geology

PETROGENESIS OF EARLY PALAEOZOIC MAFIC ROCKS ASSOCIATED WITH INTRACRATONIC RIFTING IN CENTRAL AUSTRALIA Lee. M.J.. Hand, M. & Foden, J.D.

Department of Geology and Geophysics, The University of Adelaide, Adelaide SA 5005, Australia The rift-related Cambo-Ordovician Harts Range Metamorphic Complex in the eastern Arunta Block contains a suite of early Ordovician meta-basaltic rocks (the Harts Range Dykes) that were emplaced synkinematically into low-angle regional-scale extensional detachment zones. The early Ordovician extension was coeval with the deposition of the Larapinta Group in the Amadeus Basin, and was associated with development of the Larapintine Seaway, which linked Australia's eastern margin to the Canning Basin. Early dykes and sills have been recrystallised to transitional granulite-grade assemblages and were emplaced coevally with garnetiferous megacrystic granite, whereas later dykes contain relic igneous textures and amphibolite-grade metamorphic assemblages. Major element variation of the Harts Range Dykes is consistent with an evolutionary path dominated by olivine, and to a lesser extent clinopyroxene and plagioclase fractionation. Ti0 2 and Y contents fall along a fractionation trend corresponding to that exhibited by N-type MORB. Although the tectonic setting is clearly identified, the geochemical data suggests that magma generation occurred in a number of different tectonic environments including MORB, within-plate basalts, island arc tholeiites and volcanic arc basalts. The later dykes exhibit a LREE depleted pattern. Non-parallel, REE patterns suggest that dyke samples were derived from different mantle sources and that the melt compositions have been modified by variables other than fractional crystallisation. La and Sm contents support the view that there are two distinct sources, which follow different fractional crystallisation paths. The presence of positive Th and negative Nb anomalies aid claims that crustal contamination of the mantle source has occurred. Earlier dykes have eNd values -11.48 to -0.02 suggesting crustal contamination, while the later dykes have positive eNd values associated with depleted signatures. Extremely high ^Sr/^Sr values also favour claims that earlier dykes produce trends inherent of more enriched mantle sources. The eNd values and variations in Nd and Sr concentrations in the Harts Range Dykes provide evidence for isotopic heterogeneity or multiple magma injections over time. Key words: Cambro-Ordovician rifting, meta-basaltic dykes, central Australia, eNd.

Geological Society of Australia Abstracts no. 66 15th Victorian Universities Earth Sciences Conference, La Trobe University, September 2001

LOW-TEMPERATURE HISTORY OF THE CENTRAL FENNOSCANDIAN SHIELD USING APATITE FISSION TRACK AND (U-Th)/He THERMOCHRONOMETRY Lorencak. M.. Kohn, BP. & Gleadow, A.J.W. School of Earth Sciences, The University of Melbourne, Parkville VIC 3010, Australia The application of temperature-sensitive chronological methods provides an unique approach for unravelling the lower temperature history of cratons, which can not be detected by other analytical methods. We present results of a pioneering study, in which apatite fission track (AFT) analysis and the relatively new apatite (U-Th)/He thermochronometry have been applied together on a craton. While the AFT method provides useful data in the range of ~120-60°C, the apatite (U-Th)/He system is most sensitive between ~80-45°C, with closure temperature dependent on grain size and cooling rate. AFT ages from crystalline rocks of the Fennoscandian Shield, southern Finland, vary widely from 360-1000 Ma. Preliminary apatite (U-Th)/He results on gabbroic basement rocks yield ages of -300 Ma, while those from granitic gneisses are not reproducible, most likely due to the presence of U and Th-rich microinclusions. Using the measured AFT parameters, t-T paths have been modelled and these suggest lengthy residence in the partial annealing zone and slow exhumation to the surface for eastern Finland, whereas cooling to near-ambient conditions around 600 Ma followed by reheating to ~70-80°C post -550 Ma is indicated for western and southern Finland. The range of AFT data is probably the result of variations in apatite chemistry, sedimentary cover and heat flow/heat generation. Modelled t-T paths probably reflect burial under a Caledonian foreland basin, whose thermal signature has previously been detected in Sweden, to the west. Our results suggest that this signature can also be traced across southern Finland, some 300 km further east. While reported AFT ages have been completely reset in Sweden during Caledonian burial, the Finnish samples exhibit only partial annealing of the AFT clocks. The lower temperature sensitivity of the apatite (U-Th)/He system permits the unique time-space resolution of shallower burial in southern Finland and provides additional constraints on the AFT numerical models. Key words: thermochronology, apatite fission track, (U-Th)/He thermochronometry, Fennoscandian Shield, Finland.

Crustal and Economic Geology

THE STRUCTURE, STRATIGRAPHY AND ECONOMIC POTENTIAL OF THE PILBAILEY AREA, EASTERN GOLDFIELDS WESTERN AUSTRALIA Potter, J.R.

Department of Earth Sciences, La Trobe University Bundoora VIC 3086, Australia Detailed mapping and petrography were carried out to decipher the stratigraphy and structural history of a portion of a greenstone belt within the late Archaean, Kalgoorlie terrain. The area lies on the boundary between the Coolgardie and Kambalda domains, on the edge of Lake Lefroy, 10km south of Kambalda. Two separate stratigraphies have been recognised which are separated by a major regional fault interpreted as the domain boundary. Generally a broad succession of igneous rocks occurs interlayered with and overlain by metasediments. The igneous rocks occur as flows, sills and dykes and range from ultramafic komatiites at the base to felsic volcanics at the top. The metasediments are an immature clastic sequence ranging from mudstone to sandstone and overall the environment of deposition becomes shallower towards the top. Multiple episodes of deformation have been recognised although minor episodes are difficult relate to the major events. The entire sequence was faulted and sheared during early extensional tectonics, then later folded and sheared during compressional events. At least four episodes of deformation have been recognised although more are likely to exist. The first deformation is recognised only in thin section and inferred faults. Deformation prior to the first recognised event may have occurred but obliterated by later metamorphism and shearing. Many of the early faults have been reactivated during later tectonic episodes. Hydrothermal overprinting and possible gold mineralisation occurred late, after the second event; this is evident from the quartz vein distribution and alteration assemblages associated with late stage structures. The peak metamorphic grade was upper greenschist which occurred during the first and second recognised deformations. Palaeoproterozoic dykes intrude the sequence; these are undeformed and ummetamorphosed. Key words: Archaean, tectonics, compressional, extensional, Kalgoorlie Terrain, alteration, deformation, Coolgardie Domain, Kambalda Domain.

Geological Society of Australia Abstracts no. 66 15th Victorian Universities Earth Sciences Conference, La Trobe University, September 2001

NEW TECTONIC MODEL FOR THE OPENING OF THE TYRRHENIAN SEA (CENTRAL MEDITERRANEAN) Rosenbaum. G.. Lister, G. & Duboz, C. Australian Crustal Research Centre, Department of Earth Sciences, Monash University, Clayton VIC 3800, Australia Coeval back-arc extension in the Tyrrhenian Sea and crustal shortening in the Apennines characterise the Late Miocene tectonic evolution of the central Mediterranean. Compilation of multidisciplinary (i.e., palaeomagnetic, seismic, structural, magmatic and metamorphic) data shows that this tectonic activity has been accompanied by (1) an eastward migration of the volcanic arc; (2) counterclockwise rotations of Apennine units and clockwise rotations of Maghrebides (Sicilian) units; and (3) subduction of heterogeneous lithosphere beneath the Apennines. All these tectonic processes occurred in a wide deformation zone that define the convergent boundary between Africa and Europe, which during the opening of the Tyrrhenian Sea, was characterised by relatively slow convergence rates. Earlier models suggested that E-W extension in the Tyrrhenian Sea was associated with extrusion of tectonic wedges due to N-S convergence of Africa and Europe. However, the amount of strain in the Tyrrhenian-Apennine system was considerably higher (factor of -10) than the actual shortening derived from the Africa-Europe convergence. Thus, it is unlikely that the opening of the Tyrrhenian Sea has been controlled by extrusion tectonics. In alternative models, the Tyrrhenian back-arc extension was linked to an eastward subduction rollback of a continental Adriatic lithosphere. We argue that subduction of a continental lithosphere would not be sufficient to generate the gravitational instability required for subduction rollback. Thus, in the revised model, extension in the Tyrrhenian Sea was associated with subduction of oceanic basins that existed within the Adriatic plate. Deep-sea sedimentary formations within the Apennines (e.g., Lagonegro and Molise) are possibly associated with these oceanic basins. In the Late Miocene, the arrival of these basins into the subduction zone triggered the gravitational instability required for slab rollback. Thus, the Tyrrhenian Sea was opened during episodes of subduction rollback, while Apennine units rotated anticlockwise until they were eventually overthrust on top of Adriatic units. Key words: Tyrrhenian Sea, Apennines, central Mediterranean, subduction rollback, tectonic reconstruction.

Crustal and Economic Geology

KINEMATICS AND DYNAMICS OF OPENING OF THE NORTH FIJI BACK-ARC BASIN 1

Schellart. W.P.. 1Jessell, M.W. & 2Lister, G.S.

Epsilon Laboratory, Australian Crustal Research Centre, Department of Earth Sciences, P.O. Box 28E, Monash University, Clayton VIC 3800, Australia

In most intra-oceanic back-arc basins extension is perpendicular to the arc. Thus individual spreading ridges run approximately parallel to the arc. In the North Fiji back-arc Basin, however, some ancient and active spreading ridges strike approximately 70-90° to strike of the New Hebrides Arc. Furthermore, the high angle spreading ridges migrated towards the south during the asymmetric opening of the North Fiji Basin. We interpret the orientation of the high angle spreading ridges to be related to the asymmetric opening of the back-arc basin around a hinge point, where these high angle ridges form close to the hinge point. The asymmetric opening is the result of the southward increase in hinge-line retreat of the subducting Australian lithosphere. The migration of the high angle spreading ridges is most likely related to subduction of the West Torres Plateau along the New Hebrides Trench. This led to localised collision between the two plates, stopped rollback of the subducting slab in the NW corner of the New Hebrides Arc and stopped extension and shearing in the NW corner of the North Fiji Basin. Continuing rollback of the southern part of the subducting plate led to continuing back-arc extension in the rest of the North Fiji Basin. Here, active extension was separated from the non-extending NW corner by a zone striking at a high angle to the New Hebrides Arc, i.e. the Hazel Holme Extensional Zone. Furthermore, impingement of the d'Entrecasteaux Ridge into the overriding plate led to local deformation and fragmentation of the arc. We have simulated the structural development of the New Hebrides Arc and the North Fiji Basin with scaled analogue models, where the results are similar to the structural pattern observed in the North Fiji Basin and the New Hebrides Arc and support the hypothesis outlined above. Key words: North Fiji back-arc Basin, New Hebrides Arc, hinge-line retreat, extension, analogue modelling.

Geological Society of Australia Abstracts no. 66 15th Victorian Universities Earth Sciences Conference, La Trobe University, September 2001

A METAMORPHIC PERSPECTIVE ON THE EVOLVING PATTERN OF DEFORMATION IN THE SOUTHERN ADELAIDE FOLD BELT Webb. G.P. & Hand. M. Department of Geology and Geophysics, The University of Adelaide, Adelaide SA 5005, Australia Foreland-directed (northwest-vergent) shear zones within metapelitic sequences along eastern Dudley Peninsula on Kangaroo Island in the southern Adelaide Fold Belt, preserve a petrologic record of sequential cordierite, andalusite, fibrolite, staurolite, and chlorite growth (+ biotite, muscovite, plagioclase, quartz, and ilmenite) during progressive deformation associated with the late Cambrian Delamerian Orogeny. This petrologic record is essentially identical to the metamorphic evolution recorded in the apparent thermal aureole of a syn-tectonic granite at Petrel Cove, on southern Fleurieu Peninsular. P-T pseudo-sections in the system KFMASH, calculated for representative bulk compositions, show that the sequence of mineral reactions records a pressure increase of around 1kbar (from ~3kbar) during cooling from around 580°C. Heating at low pressure (<3kbar), under high geothermal gradient conditions is implied from the early growth of cordierite. The pressure increase is attributed to footwall burial during northwest-directed thrusting toward the Gawler Craton. The initial stages of burial are indicated by the growth of syn-kinematic andalusite. Continued burial with cooling is implied by the growth of postkinematic staurolite-chlorite assemblages. This 'anticlockwise' P-T path implies that the foreland-directed shear zones underwent burial during their evolution, rather than exhumation as would be expected if the sequence of thrust initiation was directed toward the foreland. Rather the P-T evolution of the Dudley Peninsula shear zones implies that subsequent thrusts were initiated in a hinterland-ward (eastward) direction. Conceivably the evolving pattern of strain localisation in the southern Delamerian Fold Belt is controlled by the location of granite emplacement within the high grade parts of the orogen to the east and northeast, rather than the dynamics of a critically tapered wedge. If this hypothesis is correct, it implies that the age of thrust initiation along the western side of the orogen is older than deformation in the hinterland. Key words: Kangaroo Island, southern Adelaide Fold Belt, shear zone, foreland deformation, P-T pseudo-sections, 'anticlockwise' P-T paths.

Geological Society of Australia Abstracts no. 66 Victorian Universities Earth Sciences Conference, La Trobe University, September 2001

SYMPOSIUM: CLIMATE AND THE ENVIRONMENT

Climate and the Environment

INTERACTION OF UPPER OCEAN HEAT CONTENT WITH SEA SURFACE TEMPERATURE IN THE EQUATORIAL PACIFIC 1

Bettio. L.M.. 2Power, S. & 13Walsh, K.

School of Earth Sciences, University of Melbourne, Parkville VIC 3010, Australia 2 Bureau of Meteorology, Melbourne, Australia 3 CSIRO, Division of Atmospheric Research, Melbourne, Australia

El Nino (La Nina) events are characterised by an anomalous warming (cooling) of Sea Surface Temperatures (SSTs) in the tropical Eastern Pacific, and an anomalous cooling (warming) of SSTs in the western Pacific. The 1997-98 El Nino (EN) was one of the strongest on record. Most of the current models used for the prediction of EN failed to predict the strength of this event. This uncertainty delayed official warnings until April-May, 1997. Alternative indicators of the approach of an EN event could strengthen existing forecasting systems. Here we examine upper ocean heat content as an indicator of imminent EN and La Nina (LN) events. Upper ocean heat content is an indication of the movement of the thermocline which affects SST in the waters above. It also shows the movement of oceanic waves such as Kelvin and Rossby waves, thought to play an important part in the development and cessation of EN and LN events. This project uses data from a Coupled Ocean Atmosphere model from the Bureau of Meteorology. This model was run for 200 years, of which the last 100 years are examined here. Investigated in this project is the general behavior of the upper ocean heat content in the months leading up to EN and LN events. Of interest is the identification of a state of upper ocean heat content that is common for each event. An EOF analysis is performed to see if the heat content fluctuates on EN-LN timescales. Also of interest is the interaction between upper ocean heat content and SST, and how far back a statistically significant relationship can be found. Key words: El Nino, La Nina, ocean model.

Geological Society of Australia Abstracts no. 66 15th Victorian Universities Earth Sciences Conference, La Trobe University, September 2001

GEOCHEMICAL AND THERMAL CHARACTERISATION OF THE PARALANA HOT SPRINGS, NORTHERN FLINDERS RANGES, SOUTH AUSTRALIA 1

Long. N.. 1McPhail, D.C., 12Brugger, J. & 3Plimer, I.

department

of Earth Sciences, Monash University, Clayton VIC 3800, Australia 2 Museum of South Australia, Adelaide SA, Australia School of Earth Sciences, Melbourne University, Parkville VIC 3010, Australia

The Paralana Hot Springs in the Northern Flinders Ranges are the only active hot spring remaining along the Paralana fault. The Paralana Fault has been the locus for hydrothermal activity, as shown by the epithermal precious metal, Cu-Fe and Fe-U deposits along the fault. The underlying granitic rocks have high concentrations of radioactive elements, which result in anomalously high heat flow values in the region. The main purpose of this study is to determine the primary source of the water and heat in the hot springs. This is important in considering the possibility for further hydrothermal mineralisation within the area and to gain an understanding of local hydrothermal processes. Water samples from the hot springs and surrounding cold springs and groundwater bores were collected and analysed for their chemical characteristics (temperature, pH, dissolved oxygen, electrical conductivity, major, minor and trace elements) and their isotopic (hydrogen, oxygen and carbon) composition. In addition, gas emanates from the hot springs in steady streams and this was sampled and analysed for its major constituents and stable isotopic ratios. Charcoal traps were used to sample and subsequently analyse for radon at the springs. Heat flow calculations and geothermometry are used to estimate the most recent water-rock interaction temperature, and to consider possible inputs from radioactive decay and movement along a dislocated geothermal gradient. Water flows from the springs at 16L/s. Temperatures range from 57°C in the springs, to 35°C downstream. The hot spring waters are neutral (pH 7-8), and have low element concentrations (Electrical Conductivity: 2.00-2.15 mS/cm - TDS 1350 mg/L) in comparison to the cold springs and groundwater bores in the area. However, fluorine (5 ppm), molybdenum (33 ppb), tungsten (11 ppb), cesium (16 ppb) and rubidium (200 ppb) concentrations were comparatively high in the spring water. The spring water is a poor carrier of uranium, but the groundwaters in the granites are highly enriched in uranium (300-600 ppm). Dissolved oxygen and Eh electrode measurements are inconsistent but both measurements indicate the water is below atmospheric oxygen saturation. 5 18 0 and 5D values show that the hot spring water has a meteoric water signature. Radon concentrations range from 11000 Bqm"3 at the springs, to 200 Bqm'3 downstream. Preliminary geothermometric calculations indicate that the most recent water-rock interaction temperature is between 50 - 85°C (Si geothermometers) and 200°C (Na-K-Ca geothermometer). Preliminary interpretations suggest that the low element concentrations and elevated temperatures could be the result of meteoric water circulating at shallow levels in an area of high local heat flow. However, the large volume of consistent water flow from the springs is at odds with the low and sporadic rainfall in the region. Key words: Flinders Ranges, hot springs, hydrothermal activity, geothermometry, isotopes.

Climate and the Environment

A LUMINESCENCE CHRONOLOGY OF EAST-WEST ORIENTATED LINEAR DUNES IN SOUTHEASTERN AUSTRALIA Smith, B. Department of Earth Sciences, La Trobe University, Bundoora VIC 3086, Australia Six Optically Stimulated Luminescence (OSL) ages have been obtained from an E/W linear dune core in the Woorinen Formation, southeastern Australia. The ages have been obtained using single and multiple grains of quartz. The core chosen for analysis is located in a National Park that has never been cleared for grazing, and hence post European erosion of the topsoil is minimal. The optical ages from the upper 2.0m of the core fall between 20-40ka, and indicate that the last major period of dune activity occurred during this interval. Two OSL ages of 128 ± 48ka (2.5m) and 193 ±70ka (3.0) indicate that previous episodes of dune activity are also evident. Lengthy periods of dune stability persisted since 20ka, and between 125-40ka. Thirty-three luminescence ages from linear dunes in southeastern Australia have been pooled, and these indicate two periods of major dune activity between 15-45ka, and 125-135ka. These episodes of dune activity are broadly co-incident with periods leading up to, and overlapping with glacial maxima. During these times, palaeoclimates were most likely cooler, drier and windier than at present. The single-grain ages indicate that neither partial bleaching nor surface to depth bioturbation since initial deposition have not affected the age estimates in the Woorinen Formation. Key words: luminescence dating, dunes, palaeoclimate.

Geological Society of Australia Abstracts no. 66 15th Victorian Universities Earth Sciences Conference, La Trobe University, September 2001

MODELLING STABLE WATER ISOTOPES IN THE OCEAN SURFACE Brown. J. & Simmonds, I. School of Earth Sciences, University of Melbourne, Parkville VIC 3010, Australia Stable water isotopes provide an important source of information about past climates through reconstructions of past sea surface temperature and salinity from isotopic ratios in coral skeletons. Such coral isotopic records have recently been used to reconstruct long term records of El Nino-Southern Oscillation (ENSO) as well as to constrain estimates of sea surface temperatures at the last glacial maximum. However uncertainties in the coral isotopic record exist due to the fact that changes to both the local sea surface temperature and the local precipitation-evaporation balance can alter the coral isotopic ratio. Hence modelling of the isotopic signal at the ocean surface allows comparison of the surface isotopic distribution for different climate scenarios and the separation of sea surface temperature and water flux balance effects. The Melbourne University Atmospheric General Circulation Model has been fitted with an isotopic tracer scheme which allows the isotopic ratio to be calculated at each stage of the hydrological cycle. A simple slab ocean surface layer has now been added to this model to enable the isotopic ratio in the ocean surface to be determined interactively. Including only the vertical moisture flux balance produces a global surface isotopic distribution which matches many of the large scale meridional features of the observed isotopic distribution. Inclusion of river runoff and snow melt leads to a better representation of coastal isotopically light ocean surface ratios. Preliminary model runs indicate that such a simple ocean surface isotopic tracer scheme can capture the isotopic signal associated with changes in the moisture flux balance such as shifts in precipitation patterns due to ENSO. Key words: water isotopes, climate modelling, palaeoclimate, coral, ENSO.

Climate and the Environment A GEOCHEMICAL INVESTIGATION OF THE DERELICT CONRAD MINE, NORTHERN NSW Mercuri, M.D.

Earth Sciences, University of New England, Armidale NSW 2351, Australia The Conrad Mine in northern New South Wales has been periodically mined from 1890 until the 1950's. The deposit is hosted within a felsic granitoid with primary ore minerals galena, arsenopyrite, sphalerite, chalcopyrite and stannite. When mining ceased, the mine workings, infrastructure and waste dumps were abandoned. The environmental impact of the mine has been investigated by analysing the source of contamination (mine water, waste rock, tailings and slag material), and affected media including stream sediments, soil, surface water and vegetation. The aim of the project is to determine major sources of pollution, characterise the extent of heavy metal contamination in the local area and propose short and long term management options for the site. The decomposition of primary sulphide minerals in waste material leads to the formation of leachate with high levels of metals such as As, Cd, Cu, Fe, Mn, Pb and Zn (7 ppm As, 167 ppm Zn). Mine waters and erosion of waste have polluted creek water and stream sediments in Borah Creek and Maids Creek downstream of the mine site for 3 kilometres to Copeton Dam. Water and sediment quality have been severely compromised with respect to current guideline values for contamination. Soil has been contaminated by the erosion and deposition of contaminated waste material and partly by dust generated from dumps, with extreme levels of arsenic (average 1730 ppm As) and elevated levels of Cu, Pb, Zn and Ag compared with background values (average 49 ppm As). Some native flora species are observed to be metal tolerant, readily colonising areas considered to be highly contaminated. Certain species are shown to be bio-accumulators of As, Pb, Cu and Zn and are deemed to be important in final the final rehabilitation of the site. The potential for waste material to produce acid through the oxidation of sulphide minerals has been determined by conducting net acid generation tests, shake flask experiments and leach column tests. These will provide a quantitative determination of acid production by waste material and metal availability to the environment. Key words: heavy metals, arsenic, geochemistry, mine wastes, derelict mines.

Geological Society of Australia Abstracts no. 66 15th Victorian Universities Earth Sciences Conference, La Trobe University, September 2001

SEA SURFACE TEMPERATURE VARIABILITY IN THE TROPICAL INDIAN OCEAN 1Ansell. T. & 2Meyers, G. 1

School of Earth Sciences, University of Melbourne, Parkville VIC 3010, Australia 2 CSIRO, Division of Marine Research

A study of sea surface temperature (SST) variability in the tropical Indian Ocean is presented. The tropical Indian Ocean is a particularly interesting region and SST here varies on a number of different timescales. There is a vigorous seasonal cycle and intraseasonal, semiannual and interannual signals are also prominent. Expendable bathythermograph observations (XBT) along line 1X12 (Fremantle to the Red Sea) and results from a tropically enhanced ocean general circulation model (OGCM) are used to examine an upper ocean heat budget of the region. The aim is to further the understanding of upper ocean heat content and SST variability here. We also examine the influence of the semiannual Wyrtki Jets, which occur during the transition periods of the Asian Monsoon. In particular we consider how these jets influence SST both locally and remotely in the southeast Indian Ocean and explore the possible implications for Australian rainfall variability. Key words: Tropical Indian Ocean, sea surface temperatures.

Climate and the Environment QUANTIFICATION OF GROUNDWATER DISCHARGE OF SALT IN A LOCAL GROUNDWATER SYSTEM, USING ISOTOPIC TECHNIQUES. Bennetts, D.A.

Department of Earth Sciences, La Trobe University, Bundoora VIC 3086, Australia The Willaura catchment in Western Victoria has suffered a substantial increase in salinisation since the 1930's, threatening its highly productive agricultural land. In order to quantify the extent and duration of salinisation, groundwater flow mechanisms have been studied using a multi-faceted isotopic approach. Stable isotope analyses of groundwaters reveal light (meteoric) waters (5 18 0 -4%o to -6%o) in recharge zones (Grampians Range and large lakes) and heavier enriched waters in discharge areas (-4%o to 0%o), particularly the two major springs at Gellerts Swamp and Boggy Creek. The enrichment along the flow path is probably due to the progressive addition of diffuse recharge from the overlying ground surface; this water has a heavy oxygen isotope composition due to evaporation within the soil profile, which also results in very high salinities in the soil water. Tritium analysis reveals young groundwaters (10-15 years old) close to Grampians recharge areas, and older groundwaters (50 years and older) along flow paths. Apparently young waters (2-3 years) at the two major springs probably reflect the mixing of two different water bodies of separate ages. Strontium isotopes (^Sr/^Sr) were used to source these water bodies; groundwater in Cainozoic basalt is less radiogenic (0.70882) than that in the underlying Cambrian-Ordovician basement (0.7123). Mass balance calculations show that the spring at Gellerts Swamp (2ML/day) has 85% of its water derived from underlying Cambrian-Ordovician basement and 15% from the overlying basalt, whereas for Boggy Creek (1.5ML/day) the situation is reversed (70% from basalt, 30% from Palaeozic basement), owing to the much larger basalt recharge area for the latter spring. The basalt groundwater is more dilute than that in Palaeozoic basement, with substantially lower levels of the inert tracers Br and CI; mass balance calculations using these species validate the Sr isotope results. The groundwater in the basalt is likely to be relatively young, due to its lower salinity and shorter flow paths, and predominantly diffusely recharged through the soil overlying the basalt. The basement groundwater is older, and recharged relatively rapidly through the thin soils of the Grampians slopes and the permeable sediments underlying the large lakes in the area. Along its flow path it receives slow, diffuse recharge through overlying low permeability soils, progressively increasing the salinity of the water and enriching its stable isotope composition. The sub-surface relatively impermeable greenstone belts within the basement directly influence the groundwater flow paths, by deflecting groundwaters recharged from lacustrine recharge areas through conductive fault zones to discharge springs, as shown by geophysical mapping using shallow highresolution electromagnetics (nanoTEM). Salinity profiles from soils and sediments across the area show that groundwater recharge represents - 2 % of rainfall, and lake sediments represent the major salt storage and most important area of salt export. Salt budget calculations indicate that salt export through the springs is at least 4 times greater than input from rainfall; this disequilibrium was probably caused largely by draining and diversion of inflow into the large lakes, reinforced by clearing of native vegetation. At current rates it will take 10-20 years to remove the salt in the lake sediments (along with the high salinity groundwaters), and re-establish equilibrium between salt input and output within the Willaura catchment, thereby substantially reducing groundwater salinity through much of the area. Key words: Willaura, salinity, hydrogeology, stable isotopes, radiogenic isotopes, oxygen, deuterium, strontium, tritium, nanoTEM.

Geological Society of Australia Abstracts no. 66 15th Victorian Universities Earth Sciences Conference, La Trobe University, September 2001

FACTORS INFLUENCING LINEAR DUNE ORIENTATION IN AUSTRALIA Hope. P.. Simmonds, I., & Bowler, J.M. School of Earth Sciences, The University of Melbourne, Parkville VIC 3010, Australia Australia has many regions of linear dunes in its interior. Linear dunes lie broadly in the direction of the mean wind that formed them. Thus the annual mean of the surface winds in a region should determine the direction of the dunes, given the local topographic features. There are corollaries to this basic idea. The speed of the wind is one modifying factor, as the amount of sand shifted is proportional to the cube of the wind, and thus stronger winds are more important to dune formation. Another modification is the wetness of the surface. A wet surface will require far stronger winds to move any sand at all. Thus the growth of dunes will have a seasonal component related to the rainfall regime in the area. The last 20 years of Australian surface winds and soil moisture are used to assess the possible dune orientations from the above constraints. These directions are then compared to the dune extensions occurring today, as well as relict linear dunes. These relict dunes are believed to have formed in a cooler, drier climate, as may have been evident during the last glacial maximum. The above analysis will also be carried out on the results from simulations of the last glacial maximum with the Melbourne University general circulation model. Key words: Australia, dune orientation, last glacial maximum, general circulation model.

Climate and the Environment

DECADAL CLIMATE VARIABILITY IN THE PACIFIC REGION Sae-Koe, S. School of Earth Sciences, The University of Melbourne, Parkville VIC 3010, Australia Examples of the decadal variability in the Pacific region are the large-scale change in the winter circulation pattern over the North Pacific during the mid-1970s and the ENSO-like mode pattern in the global sea surface temperature during the 1990s. Several authors have attempted to identify the mechanisms involved. At present several possible mechanisms in the Pacific Ocean have been put forward. One is called 'the positive feed back' was first suggested by Tim P. Barnett and Mojib Latif in 1994. This mechanism is driven from the midlatitudes of the North Pacific. The sea surface temperature (SST) anomalies there drive changes in the trade wind system that alter the east-west slope of the tropical thermocline. Also in 1994, Kevin Trenberth suggested that the decadal variations in the mid-latitude circulation are driven by the variations of the tropical Pacific SST. This project also aims to study the decadal variability in the Pacific region by using the CSIRO coupled ocean-atmosphere general circulation model. The ability of this model to simulate the above mechanisms is still being tested. Preliminary results show that the CSIRO model does not show a time difference between SST changes in the tropics and the midlatitudes. This might suggest that the decadal variation in the tropics and the midlatitudes could occur simultaneously without one causing the other. However, the result is not conclusive and be further assessed in the future. Key words: decadal variability, Pacific, positive feed back.

Geological Society of Australia Abstracts no. 66

15th Victorian Universities Earth Sciences Conference, La Trobe University, September 2001

ENVIRONMENTAL IMPACT OF A WASTE ROCK DUMP IN THE TOOMA RIVER CATCHMENT, KOSCIUSKO NATIONAL PARK, NSW Kress. J. & McPhail, D.C.

Department of Earth Sciences, Monash University, Clayton VIC 3800, Australia In Co-ordination with Environmental Protection Authority NSW, Department of Land and Water Conservation NSW, Co-operative Research Centre for Freshwater Ecology and the Snowy Mountains Hydro-electric Authority, with funding from Agriculture, Fisheries and Forestry Australia. Severe pollution events in the Tooma River, located in the Kosciusko National Park, NSW, have resulted in depleted fish and invertebrate populations. Past surveys in the Tooma River catchment have targeted a waste rock dump on Deep Creek, a Tooma River tributary, as a likely source of pollution. Although the dump appears to consist mainly of granitic rock that is likely to be environmentally benign, pyrite has been identified in some rocks on the dump's surface and in the Tooma -Tumut tunnel, which was constructed as part of the Snowy River Hydro-electric Scheme and is the source of rock in the dump. The presence of pyrite suggests that acid drainage or other contamination could be generated within the dump. Three irrigation experiments on the dump were undertaken between during April 2001, in order to simulate rain events that may flush potential contamination into Deep Creek and the Tooma River. The fish and invertebrate populations, streamflow and water chemistry of Deep Creek were monitored before, during and after the experiments. No disruptions to fish or invertebrate populations were observed. Preliminary hydrologic budget calculations indicate that some of the water flowing through the dump may enter the groundwater system underlying the creek. During the experiments, chemical changes in water downstream of the dump are evident, including an increase in electrical conductivity from 20 juS/cm to 35 |LiS/cm, a decrease in dissolved oxygen from 9 mg/L to 7 mg/L and an increase in sulphate concentration from 1mg/L to an average of 2.5mg/L, with one site adjacent to the dump recording a concentration of over 70mg/L. pH and temperature remained approximately constant at 6.2 and 10°C, respectively. Element concentrations remained low; however, some (e.g., Cu and Al) neared ANZECC recommended guidelines for freshwater quality. The environmental impact of the dump is being estimated by using the measured element concentrations and streamflow to calculate mass loadings to the Tooma River. Key words: environmental geochemistry, water-rock interaction, waste rock piles, spoil dumps, acid drainage, environmental impacts.

Climate and the Environment

SOUTH AUSTRALIA'S MURRAY BASIN STRATIGRAPHY AND ITS RELEVANCE TO THE WAIKERIE SALT INTERCEPTION SCHEME 1

Wall. M.. 1Bone. Y., 2Hopkins, B., and 2Telfer, A.

department of Geology and Geophysics, The University of Adelaide, Adelaide SA 5005, Australia 2 Australian Water Environments, Norwood, SA 5067, Australia River Murray salinity levels have been reduced by the Waikerie Salt Interception Scheme. This consists of (a) installing groundwater-monitoring piezometers, (b) drilling production bores to intercept saline groundwater and (c) pumping it to a disposal basin at Stockyard Plains. Groundwater flow is controlled by major aquitard and aquifer lithologies in the underlying stratigraphy. Thorough understanding of the stratigraphy is mandatory. Results show relatively uniform thickness of the Tertiary Murray Supergroup sediments in the Ramco-Qualco area. Borehole cuttings show that Murray Supergroup sediments are identifiable by their lithological properties, fossil assemblages and downhole geophysics. The lowermost unit, the highly fossiliferous Lower Mannum Formation (dominant aquifer) contains abundant echinoids and no Operculina, the large benthic foram common in the Upper Mannum Formation (aquifer). Hard bands in the Upper Mannum Formation give strong peaks on the neutron log. The clay-rich Finniss Formation (major aquitard) contains sparse aragonitic fossils compared to the overlying calcite-rich, fossiliferous Glenforslan Formation (saline-water aquifer), which is rich in Lepidocyclina. The Cadell Marl (aquitard) is recognizable in gamma logs as sharp narrow peaks. The Bryant Creek Formation is a gypsiferous limestone (minor aquifer). The indurated North-West Bend Formation (minor aquifer) contains abundant, large oyster shells. It accounts for circulation loss when drilling. The quartzose Loxton Sands (major aquifer) are bright orange-yellow and contain few fossils. The red-brown quartzose Blanchetown Clay (aquitard - often poorly) has scattered calcareous lenses, and is recognized within the gamma log. The surficial Woorinen Sands are fine-grained, quartz-rich dune sands, restricted to the flood plain. Lower Murray Group Aquifer refers to the Mannum Group and Upper Murray Group Aquifer to Glenforslan Fm and above. Aquifer and aquitard properties vary spatially. Production bores pump from the Lower Murray Group Aquifer. Piezometers monitor the entire sequence. The Waikerie Salt Interception Scheme is successful. Key Words: Murray Basin, Salt Interception Schemes, stratigraphy, salinity, aquifer, aquitard.

Geological Society of Australia Abstracts no. 66 15th Victorian Universities Earth Sciences Conference, La Trobe University, September 2001

SYMPOSIUM: GEOPHYSICAL AND ANALYTICAL METHODS

Geophysical and Analytical Methods

NUMERICAL MODELLING OF THE MISE-A-LA-MASSE NEAR-MISS SCENARIO 1

Carey H.. 1Heinson G. & 2Sexton M.

department of Geology and Geophysics, The University of Adelaide, Adelaide SA 5005, Australia 2 Normandy Mining Limited, Wayville SA 5034, Australia Mise-a-la-Masse (MALM) is an inexpensive method for electrical resistivity exploration of subsurface conductivity contrasts. The principle of the MALM method to measure surface potential variations, when current is injected into a buried conducting body. The goal of the project is to investigate the scenario where the current electrode is not in direct contact with conductive mineral; the "near miss" scenario. The near miss scenario is of interest as measured equipotentials frequently delineate a subsurface anomaly, while MALM theory predicts a homogenous Earth response. The Project is in association with Normandy Mining Limited, which has provided MALM data from a copper VMS deposit to aid in this investigation. A variety of methods have been used to model the response of a near miss scenario, including 3D Finite Element Method forward modelling, 2.5D Finite Element Method inversion of existing data and image source reproduction method using existing data. 3D FEM modelling allows for accurate constraint of a modelled ore-body, and easy comparison of calculated data to the original MALM data. 2.5D FEM inversion enable a cross section of the ore-body to be calculated from the MALM data acquired by Normandy. The image source reproduction method delineates the mineralisation by generating correlation coefficients for source points, which outline the body when contoured. An association between the depth of the electrode, its distance from the ore-body and the resulting surface potentials can be developed for current electrodes that are not in contact with the conducting body. By determining this association, the placement of current electrodes in boreholes that do not intersect mineralisation can be optimised such that they still provide data on the mineralisation. Key words: Mise-a-la-Mass, Finite Element Method, near miss, VMS, anistropy.

Geological Society of Australia Abstracts no. 66 15th Victorian Universities Earth Sciences Conference, La Trobe University, September 2001

MORPHOLOGY AND STRUCTURE OF CLAY MINERALS INVESTIGATED BY TRANSMISSION ELECTRON MICROSCOPY (TEM) AND ATOMIC FORCE MICROSCOPY (AFM) 1

Duong. L., 2Kloprogge, T., 1Bostrom, T. & 2Frost, R.

Analytical Electron Microscopy Facility Unit, School of Natural Resource Sciences, Queensland University of Technology, P.O. Box 2434, Brisbane QLD 4001, Australia School of Natural Resource Sciences, Queensland University of Technology, P.O. Box 2434, Brisbane QLD 4001, Australia

Clay minerals belong to the phyllosilicate group, which are distinguished by a layered structure composed of polymeric layers of one or two Si0 4 tetrahedral sheets linked to one or both sides of a sheet of (Al, Mg, Fe)(0, OH) 6 octahedra. The most popular clay minerals are kaolinite and smectite. The special atomic structure of the smectite clay minerals with the exchangeable water molecules and the charged interlamellar surfaces is responsible for special properties useful in applications such as catalysts for the oil industry, absorption of heavy metals and organic molecules in the environment, etc. The methods of transmission electron microscopy and atomic force microscopy offer the opportunity to get detailed information at high-resolution of the structural surfaces of the clay minerals. Using a combination of the TEM and AFM analytical techniques one can obtain detailed information about the morphology and structural changes taking place during the preparation of (modified) clays. This paper will discuss 1) the principle of the TEM and AFM methods, 2) the techniques of sample preparation and 3) some applications for studying clay minerals. Key words: TEM, AFM, smectite, kaolinite, clay.

Geophysical and Analytical Methods

PLUTON GEOMETRY AND EMPLACEMENT MECHANISMS IN THE GAWLER CRATON: INSIGHTS FROM POTENTIAL FIELD DATASETS AND STRUCTURAL GEOPHYSICS McLean M.A & Betts, P.G. Australian Crustal Research Centre, Monash University, Clayton VIC 3800, Australia At -1595-1585 Ma, a large episode of mantle up-welling beneath the Gawler Craton resulted in the extrusion of the Gawler Range Volcanics, the formation of the Olympic Dam Ore Deposit and the emplacement of the anorogenic Hiltaba Suite Granitoids. Large areas of this magmatic province lie beneath younger cover sequences. Aeromagnetic and gravity datasets can be used to delineate this magnetic province beneath these cover sequences. This study investigates, using aeromagnetic and gravity interpretation and modelling, pluton emplacement mechanisms by examining the pluton geometries and their relationships with the surrounding structures. A large number of major shear zones overprint each another in the Central Gawler. Major Archaean-aged east-west trending shear zones are spatially and structurally associated with the Mesoproterozoic Hiltaba Granitoid Suite. This suggests that these shears were important conduits during pluton emplacement. The distribution of granites are largely controlled by these shears. The validity of these hypotheses is being tested with the aid of forward geophysical modelling using Noddy. Noddy allows us to determine modelling geometries and to generate synthetic potential field responses from the model to compare with the survey data. Key words: Hiltaba, Gawler Craton, aeromagnetic, Pluton Emplacement, Shear Zones.

Geological Society of Australia Abstracts no. 66 15th Victorian Universities Earth Sciences Conference, La Trobe University, September 2001

A NEW PALAEOMAGNETIC POLE FROM THE MID-LATE EOCENE 'OLDER VOLCANICS' Sawyer, M.R. Department of Earth Sciences, La Trobe University, Bundoora VIC 3086, Australia Substantial debate over the trajectory and rate of motion of the Australian Tertiary Apparent Polar Wander Path (APWP) has occurred over the last fifteen years. Differing views concerning the applicability of the Geocentric Axial Dipole Hypothesis and the comparative reliability of igneous and sedimentary poles from south east Australia have created two APWP's with markedly different rates of motion for the last 100Myr. Hot-spot data tends to support a uniform rate, while true polar wander and non-dipole components in the Earth's magnetic field were arguments introduced to explain a more complex rate of motion. Doubts have been expressed over the accuracy of Australian igneous poles, with the majority of poles collected during the 1960's using old, less reliable palaeomagnetic techniques. With a K/Ar age of approximately 42 Ma, conveniently located in the contentious area of the Australian Tertiary APWP, the 'Older Volcanics' from the Mornington Peninsula have been used to determine the new pole. Twenty individual lava flows, identified in the field by outcrop characteristics and in the laboratory by petrographic analysis, were sampled such that when results from all flows are averaged, the time difference between flows will eliminate the variation in palaeomagnetic directions caused by secular variation. Using these results, a new pole will be created meeting modern quality criteria. A comparison will be made between this new pole and the two existing APWP's in an attempt to determine whether the Australian Tertiary APWP was uniform or more complex, and whether or not true polar wander and non-dipole components in the Earth's magnetic field can be substantiated. Key Words: Apparent Polar Wander Paths, palaeomagnetic poles, Older Volcanics, Mornington Peninsula.

Geophysical and Analytical Methods

GEOPHYSICAL METHODS FOR FRESH WATER DETECTION IN SALINE GROUND WATER ENVIRONMENTS 1 Barrett. B.E.. 2Hatch. M. & 1Heinson, G.

department of Geology and Geophysics, The University of Adelaide, Adelaide SA 5005, Australia Zonge Engineering and Research Organisation (Aust) Pty Ltd, Welland, Adelaide 5007, SA, Australia

The effectiveness of ground based and non-invasive geophysical tools for the delineation of fresh water bodies in saline ground-water environments has been tested over a known fresh water lens. DC Resistivity, Transient EM and Frequency domain EM were tested for their quality of results and time effectiveness in acquisition on private property approximately 15km East South East of Waikerie. Frequency domain EM measurements were found to be the fastest, and DC resistivity the slowest. DC resistivity data was acquired in the dipole-dipole arrangement and was inverted using finite difference and finite element smooth 2D modelling, with a greater weighting put on models with more horizontal continuity. Transient EM data was acquired with the Zonge Engineering NanoTEM system. Contouring adjacent 1D smooth models has produced 2D sections that have been compared with the DC resistivity data. Inversions for the frequency domain EM data have not been completed. The lens formation is due to infiltration of meteoric water where near surface karstification and porosity is high. Results have consistently shown two resistive bodies centred laterally in the two perpendicular profiles. The most resistive of these bodies is a surface feature, and is larger than the deeper resistor. This is thought to be a resistor due to the karstification and porosity. The deeper resistor is thought to be the fresh water body. Results from this study present geophysics as a useful tool for both fresh water detection and water resource monitoring in saline ground water environments. Key words: DC Resistivity, Transient EM, Frequency domain EM, fresh water lens, saline ground water.

Geological Society of Australia Abstracts no. 66 15th Victorian Universities Earth Sciences Conference, La Trobe University, September 2001

A NEW APPROACH TO NEAR SURFACE MAPPING OF GROUNDWATER SALINITY: APPLICATION OF FAST SAMPLING EM SURVEYING METHODS 1

Bennetts. D A & 2Hatch, M.

Department of Earth Sciences, La Trobe University, Bundoora VIC 3086, Australia Zonge Engineering and Research Organisation (Australia) Pty Ltd, Adelaide, SA

The Zonge manufactured NanoTEM system offers high resolution mapping of shallow sub-surface (<100m) conductivity, and its fast turn off electromagnetics allow for rapid sampling rates. A site in the Willaura catchment in Western Victoria, Australia, was surveyed along a transect crossing a major saline spring (Boggy Creek) and following north-south groundwater flow lines. The local geology comprises Cainozoic basalt overlying a deformed Palaeozoic basement of Cambrian-Ordovician sandstones and shales with infaulted belts of Cambrian metavolcanics (greenstone). NanoTEM time domain measurements were made using a transmitter loop of sides 20m and a centrally placed receiver loop of sides 5m. One hundred and twenty measurement sites 20m apart along a 2.5km transect were made in less than fifteen hours. Data quality was exceptionally good in low resistivity ground; resistivities varied from >25ohm-m to <1 ohm-m. Deeper resistive units (>30m depth) coincide with and have similar geometric features to linear greenstone belts interpreted from airborne magnetic data. Resistive units at 10m depth match the occurrence of fresh basalt in bore logs. A slightly more conductive unit apparent above the basalt probably represents a perched water table in the regolith overlying basalt; fresh water is found in bores at <2m depth in this area. Below the basalt is a conductive feature corresponding to saline groundwaters within the Palaeozoic basement. Convergence of the fresh and saline groundwaters is evident at the discharge point of Boggy Creek; the preferred pathway of the basement groundwaters appears to be directed by an underlying inferred greenstone belt. This convergence supports isotopic evidence of younger diffuse recharge mixing with older saline basement water at Boggy Creek. This study shows that NanoTEM can be highly effective at mapping small-scale changes in lithology, lithological porosity and fluid composition, providing an insight into salinisation mechanisms. Key words: NanoTEM, electromagnetics, Willaura, salinity, hydrogeology.

Geophysical and Analytical Methods

MEASURING CONTINENTAL MARGIN ESCARPMENT RETREAT IN THE CENTURY Dimas. V.A. School of Earth Sciences, University of Melbourne, ParkviHe VIC 3010, Australia The aim of this project is to quantify rates of escarpment retreat using in-situ produced cosmogenic isotopes and low temperature thermochronologic data. Major escarpments are primary morphotectonic features formed by the process of continental rifting. They are characteristic topographic features of many continental margins around the world. The origin, behaviour and preservation of continental margin escarpments have been highly controversial over the last century. The SE Australian escarpment is a prime example and has played an important role in the debate. The combination of cosmogenic isotope analysis of quartz with (U-Th)/He and fission track analysis of apatites offers a powerful opportunity to finally test the range of conflicting models of escarpment evolution. Cosmogenic nuclides (eg. & are produced in-situ in bedrock and surficial materials by interaction of highly energetic cosmic-rayderived particles with nuclei in rock forming minerals. The concentration of these cosmogenic nuclides tells us about 'dwell time' in the attenuation zone (top few metres of surface) from which we can determine the rate of denudation. This technique has been successfully used in determining the rate of erosion along the southern African continental margin using in-situ bedrock. An extension of this technique is measuring erosion rates from stream sediment material within a specified drainage catchment area in SE Australia, where estimates will be made on material that is already in transit. Helium diffusion in U- and Th-rich minerals (eg. apatite) is able to provide constraints on the thermal history of a sample. (U-Th)/He is a new tool for measuring the low temperature thermal history of the upper crust. The low closure temperatures of helium dating of apatites provides us with a valuable thermochronometer. The method of apatite He thermochronometry gives insight to the cooling of apatites from through to --40° C. By combining these two techniques with already existing fission track data this will help in further constraining the model. Key words: escarpment retreat, erosion rate, cosmogenic isotopes, catchment basin, low temperature thermochronology, (U-Th)/He, apatite fission track.

Geological Society of Australia Abstracts no. 66 15th Victorian Universities Earth Sciences Conference, La Trobe University, September 2001

Pb-Pb STEPWISE LEACHING (PbSL): A NEW GEOCHRONOLOGICAL TOOL FOR HIGH-GRADE POLYMETAMORPHIC TERRANES Tonelli, M.. Woodhead, J. & Hergt, J. School of Earth Sciences, The University of Melbourne, Parkville VIC 3010, Australia Staurolite and garnet are important phases in areas of medium to high-grade metamorphism. Temperatures experienced by metamorphic rocks in such conditions commonly exceed 600 °C, leaving UPb, Pb-Pb and Th-Pb as the only widely employed geochronometers. Pb-Pb dating of individual metamorphic minerals directly associated with distinctive metamorphic grades has proven to be a powerful tool for deciphering the history of metamorphic activity within a polymetamorphic terrane. Unfortunately, such dating has often been treated with scepticism due to a range of potential obstacles to accurate and precise thermochronometry of rock-forming minerals including: 1. 2. 3. 4.

Low parent/daughter ratios, which limit the isotopic spread obtained on ^ P b / ^ P b vs ^ P b / ^ P b diagrams Sensitivity of the (U-Th)Pb-Pb systems to post-intrusive disturbance (i.e. hydrothermal alteration, thermal overprinting) Uncertainties in the closure temperatures of (U-Th)Pb-Pb systems in minerals Frequent presence of U-and/or Th-rich inclusions (zircon, monazite) in the host phase, which can greatly influence its radiogenic Pb budget and apparent age.

PbSL is a new silicate digestion technique based on sequential acid treatment which allows the selective recovery of radiogenic and common Pb components from a mineral, making single-phase Pb-Pb dating possible. Our PbSL data were obtained using a Nu plasma multi-collector ICP-MS with thallium internal standard normalization. The Pb isotopic-progressions of successive leach steps are plotted in conventional uranogenic ( ^ P b / ^ P b vs ^ P b / ^ P b ) and thorogenic ( ^ P b / ^ P b vs ^ P b W b ) Pb diagrams. PbSL derived ^ P b / ^ P b trends, together with petrographic observations, reveal that staurolite hosts ^i-size inclusions of an opaque phase; the garnet is essentially inclusion-free. The slopes defined by our data yield apparent ^ P b / ^ P b isochron ages of 1323 ± 11 Ma and 874 ± 34 Ma (two age components in the staurolite), and 1599 ± 2.5 Ma (garnet). All experiments recorded leach spectra with progressively increasing radiogenic Pb recovered with subsequent leach steps of increasing acid strength and duration. Key Words: PbSL (Pb-step leaching), closure temperature.

Index of Authors A

Ansell, H Ansell, T

Barrett, B Bennetts, D Bettio, L Bolhar, R Bowran, K Brown, J.

Cairns, C Carey, H Carter, T Clark, C

Dimas, V-A Duong, L

Hansen, D Hare, A Hildyard, S Hope, P

Kress, J

Lavery, P Lee, M Long, N Lorencak, M

9 39

McLean, M Mercuri, M

50 40, 51 34 22 19 37

Neilson, 1 Noll, C

O'Brien, T

Potter, J Purdy, D

4 46 23 24

Reid, A Riewers, M Rosenbaum, G

52 47

Sae-Koe, S Sawyer, M Schellart, W Schwartz, L Smith, B Smith, H

25 5 2 41

Tonelli, M

21 26 35 27

Wade, B Wall, M Webb, G Weber, U

48 38

20 10

28 3

16 17 29

42 49 30 12 36 13

8 44 31 18

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