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Geological Society of Australia ABSTRACTS number 35

MID- TO LOWER- CRUSTAL METAMORPHISM AND FLUIDS CONFERENCE

MOUNT ISA, JULY 26 - AUGUST 1,1993.

MID- TO LOWER-CRUSTAL METAMORPHISM AND FLUIDS CONFERENCE, MOUNT ISA

JULY 26 - AUGUST 1, 1993 presented by

AUSTRALIAN METAMORPHIC STUDIES GROUP Specialist Group in Geochemistry, Mineralogy & Petrology, Geological Society of Australia, SGTSG, IGCP 304 (Lower crustal metamorphism) and IGCP 291 (Metamorphic fluids and mineral deposits)

in association with

MONASH UNIVERSITY AUSTRALIA

JAMES COOK UNIVERSITY

CONFERENCE PROGRAMME

Monday 26th of July 9.00-9.15 am

Opening Address and Welcome Session

1: Low P metamorphism,

P-T-t paths,

plutonism,

anatexis

9.15

Kurt Stiiwe. Michael Sandiford Magmatism, temperature dependent rheologies and shear heating, or what might PT paths really look like?

9.35

Scott Mildren. Mike Sandiford, Annette Bingemer Heat refraction and "unconformity-related" contact metamorphism, with application to the Mount Painter region in the Adelaide Fold Belt.

9.55

Mike Sandiford Time scales, rates and mechanics of deformation and metamorphism with application to the southern Adelaide Fold Belt.

10.15

Bas Hensen. Bo Zhou Reliability of reaction textures as guides to metamorphic histories.

10.35-10.55 Morning Tea 10.55

Yasuhito Osanai. Bas Hensen, Yoshikuni Hiroi, Massaki Owada Metamorphic P-T evolution of sapphirine-quartz bearing granulite from central Sri Lanka

11.15

Jiirgen Reinhardt Sequential growth of Al2SiC>5 polymorphs: Documentation of a complete anticlockwise P-T loop in the central Mount Isa Inlier

11.35

Scott Johnson. Ron Vernon Stepping stones and pitfalls in the determination of an anticlockwise P-T-t deformation path: the low-P, high-T Cooma Complex, Australia.

11.55

Chris Mawer, J. Clemens, G. Stephens Granites: Melts, Magmas, Migration and Metals

12.15-2.00

Lunch and Posters

2.00

Michel Ballevre. Ronan Hebert Medium to low-pressure metamorphism and magma emplacement during deformation of an island arc - the late Precambrian Cadomian Belt (Brittany, France)

2.20

Jorn Kruhl. S. Paterson, T.Brudos Deformation and contact metamorphism during pluton emplacement: an example from the White-Inyo Mountains (California)

2.40

Masaaki Owada.. Yasuhito Osanai, Hiroo Kagami Crustal anatexis and magma process in the island-arc deep-crust, Hidaka metamorphic belt, northern Japan.

3.00

Karin Ehlers. Roger Powell, Kurt Stiiwe The determination of cooling rate histories from garnet-biotite equilibria.

3.20

Ian Scrimpeour Adjacent apparent isobaric cooling and isothermal decompression textures in granulites from Else Platform, East Antarctica: implications for the interpretation of inferred P-T paths.

3.40-4.00

Afternoon Tea

Session 4.00

2: Fluids,

metamorphism,

and

mineralization,

part

Vic Wall Metamorphic fluids and mineralization

4.20

Neil Phillips Revolatilization by auriferous fluids. The start to a century of gold mining at Kalgoorlie.

4.40

Kevin Blake The Kiirunavaara magnetite deposit: A high temperature magmatic-hydrothermal deposit.

5.00

Peter Sorionen-Ward. Matti Pajunen, Hugh O'Brien An overview of deformation, granite intrusion, metamorphism and mesothermal gold mineralization in the late Archean Hattu schist belt, eastern Finland.

Tuesday 27th of July Session

3: Fluids,

metamorphism,

and

mineralization,

part

9.15

Bruce Hobbs Mechanical, fluid flow and thermal coupling in exploration models.

9.35

Alison Ord. Bruce Hobbs Dynamic coupled fluid flow and deformation in numerical models.

9.55

Stephen Cox. Shuqing Zhang, Mervyn Paterson The dynamics of permeability changes and fluid migration in metamorphic environments.

10.15

Nick Oliver. Alison Ord Mechanisms of fluid flow in the mid- to upper crust.

10.35

Tania Aslund. Ian Cartw right, Nick Oliver Fluid flow asociated with metasomatism of the Revenue Granite and aureole: source, geometry and timing. Mary Kathleen Fold Belt, Mt Isa Inlier, Queensland, Australia

10.55-11.15 Morning Tea 11.15

Tadao Nishiyama. Naoko Sato, Takeru Yanagi Fluid flow and alkali-silica metasomatism within a serpentine melange from a glaucophanitic metamorphic terrain - the Nagasaki metamorphic rocks, southwest Japan

11.35

Andrew Barker Evolution of fluids in ductile out-of-sequence thrusts in the Caledonides from Arctic Norway

11.55

Annette Bingenter. Mike Sandiford Assessment of fluid infiltration in an 'unconformity related' contact metamorphic terrain: Arkaroola, northern Flinders Ranges.

12.15

Nick Cook Composition and source of fluids in metamorphosed non-marine evaporites, Olary Block, South Australia

12.35-2.00

Lunch

iii

2.00

Robin Oliver Alex Christ Talc formation in the Mount Lofty Ranges, South Australia - a review.

2.20

Ian BuicL Ian Cartwright Pre-regional metamorphic fluid flow in metacarbonates from the Reynolds Range, central Australia.

2.40

Ian Cartwright Ian Buick Stable isotope and mineralogical constraints on channelled fluid infiltration in Wollastonite-bearing calc-silicates: Reynolds Range, Central Australia.

3.00

Neil Adshead Mineralization and alteration styles at the Osborne deposit, NW Queensland.

3.20

Trevor Beardsmore The role of metamorphic fluids in the pedogenesis of Mount Dore-style breccia-hosted copper - gold deposits in the eastern Mount Isa Inlier, Queensland.

3.40-4.00

Afternoon Tea

4.00

Patrick Williams. Geoffrey de Jong, Trevor Verran Evolution of Na-K-Fe-Si metasomatism and mineralization associated with the Cloncurry Fault, SE Mount Isa Inlier: A comparison with Kiruna-Olympic Dam type systems

4.20

Geoffrey deJong Syn- to late-metamorphic Na metasomatism, southeast Mount Isa Inlier

4.40

Michael Rubenach Petrogenesis of a variety of metasomatic lithologies west of Mt Isa

5.00

Wanfu Huang. Michael Rubenach Structural controls on syntectonic metasomatic tremolite and tremolit-plagiocalse pods in the Molanite Valley, Mount Isa, Australia

CONTENTS Cover

page

Conference Programme

Contents

Seminar Abstracts Kurt Stiiwe and Michael Sandiford: Magmatism, temperature dependent rheologies and shear heating, or what might PT paths really look like?.

Scott Mildren, Mike Sandiford and Annette Bingemer: Heat refraction and "unconformityrelated" contact metamorphism, with application to the Mount Painter region in the Adelaide Fold Belt.

Mike Sandiford : Time scales, rates and mechanics of deformation and metamorphism with application to the southern Adelaide Fold Belt.

Bas Hensen and Bo Zhou: histories.

Reliability of reaction textures as guides to metamorphic

Yasuhito Osanai, Bas Hensen, Yoshikuni Hiroi and Massaki Owada: Metamorphic P-T evolution of sapphirine-quartz bearing granulite from central Sri Lanka Jiirgen

Reinhardt: Sequential growth of Al2SiC>5 polymorphs: Documentation of complete anticlockwise P-T loop in the central Mount Isa Inlier

a 8

Scott Johnson and Ron Vernon: Stepping stones and pitfalls in the determination of an anticlockwise P-T-t deformation path: the low-P, high-T Cooma Complex, Australia.

Chris Mawer, J. Clemens and G. Stephens: Metals

Granites: Melts, Magmas, Migration and

Ronan Hebert and Michel Ballevre: Medium to low-pressure metamorphism and magma emplacement during deformation of an island arc - the late Precambrian Cadomian Belt (Brittany, France)

Jorti Kruhl, 5. Paterson and T.Brudos: Deformation and contact metamorphism during pluton emplacement: an example from the White-Inyo Mountains (California)

Masaaki Owada„ Yasuhito Osanai and Hiroo Kagami: Crustal anatexis and magma process in the island-arc deep-crust, Hidaka metamorphic belt, northern Japan.

Karin Ehlers, Roger Powell and Kurt Stiiwe: The determination of cooling rate histories from garnet-biotite equilibria.

lan Scrimgeour: Adjacent apparent isobaric cooling and isothermal decompression textures in granulites from Else Platform, East Antarctica: implications for the interpretation of inferred P-T paths.

Neil Phillips:

Revolatilization by auriferous fluids. The start to a century of gold mining at Kalgoorlie.

Kevin Blake:

The Kiirunavaara magnetite deposit: A high temperature magmatichydrothermal deposit.

Peter Sorjonen-Ward, Matti Pajunen and Hugh O'Brien: An overview of deformation, granite intrusion, metamorphism and mesothermal gold mineralization in the late Archean Hattu schist belt, eastern Finland.

Alison Ord and Bruce Hobbs: Dynamic coupled fluid flow and deformation in numerical models.

Stephen Cox, Shuqing Zhang and Mervyn Paterson: The dynamics of permeability changes and fluid migration in metamorphic environments.

Nick Oliver and Alison Ord: Mechanisms of fluid flow in the mid- to upper crust.

Tania Aslund, Ian Cartwright and Nick Oliver: Fluid flow asociated with metasomatism of the Revenue Granite and aureole: source, geometry and timing. Mary Kathleen Fold Belt, Mt Isa Inlier, Queensland, Australia

Naoko Sato, Tadao Nishiyama and Takeru Yanagi: Fluid flow and alkali-silica metasomatism within a serpentine melange from a glaucophanitic metamorphic terrain - the Nagasaki metamorphic rocks, southwest Japan

Andrew Barker: Evolution of fluids ift ductile out-of-sequence thrusts in the Caledonides from Arctic Norway

Annette Bingemer and Mike Sandiford: Assessment of fluid infiltration in an 'unconformity related' contact metamorphic terrain: Arkaroola, northern Flinders Ranges.

Nick Cook: Composition and source of fluids in metamorphosed non-marine evaporites, Olary Block, South Australia

Robin Oliver and Alex Christ: Talc formation in the Mount Lofty Ranges, South Australia - a review.

lan Buick and lan Cartwright: Pre-regional metamorphic fluid flow in metacarbonates from the Reynolds Range, central Australia.

Ian Cartwright and Ian Buick: Stable isotope and mineralogical constraints on channelled fluid infiltration in Wollastonite-bearing calc-silicates: Reynolds Range, Central Australia.

Neil Adshead: Mineralization and alteration styles at the Osborne deposit, NW Queensland.

Trevor Beardsmore: The role of metamorphic fluids in the petrogenesis of Mount Dorestyle breccia-hosted copper - gold deposits in the eastern Mount Isa Inlier, Queensland.

Patrick Williams, Geoffrey de Jong and Trevor Verran: Evolution of Na-K-Fe-Si metasomatism and mineralization associated with the Cloncurry Fault, SE Mount Isa Inlier: A comparison with Kiruna-Olympic Dam type systems

Geoffrey deJong: Syn- to late-metamorphic Na metasomatism, southeast Mount Isa Inlier

Michael Ruhenach: Petrogenesis of a variety of metasomatic lithologies west of Mt Isa

Wanfu Huang and Michael Rubenach: Structural controls on syntectonic metasomatic tremolite and tremolit-plagiocalse pods in the Molanite Valley, Mount Isa, Australia

6 j

Posters

Abstracts

Mark Anderson: P-T-t trajectories through the Caledonian nappes of N.Norway: a clue to complex burial and exhumation histories in collisional orogens.

Ian Buick: Post-regional metamorphic fluid flow in marbles form the Reynolds Range, Central Australia

Paul Gow and Victor Wall: Mid-Proterozoic Fe-metasomatism on the Stuart Shelf, South Australia.

Penny King; The Jerangle Metamorphic Complex: Relationships to granites, structures and fluids.

Jorn Kruhl and Torsten Huntemann: Dry high-temperature shear in mid-crustal rocks.

vii

Helen Lang and Timothy Grover: Low-P metamorphism in coastal Maine, USA: anticlockwise P-T paths from garnet zoning.

Hassina Mouri,Michel Guiraud, Jean-Robert Kienast and Khadidja Ouzegane: Al-Mg granulites of the Ihouhaouene (Mole In Ouzzal, Hoggar-Algeria) - an example of phase relationships in the KFMASH system and melt absent equilibria.

Sance Mundondo: Partial melting and local water activity, [a(H20)], control during the formation of granulite facies assemblages in the Einasleigh Metamorphic Belt, Northeastern Australia.

Nick Post: The structural and metamorphic evolution of the Woolanga Bore area, northeast Strangways Range, Central Australia.

Michael Raith, Stefan Pierdzig, Stephan Hoernes and Raymond Rakotondrazafy : The phologopite mineralisations in the Berataka Belt of southern Madagascar: a spectacular example of channelised fluid flow and fluid-rock interaction

Jurgen Reinhardt: Textural constraints on timing relationships between metamorphism and deformation in the west-central Mary Kathleen Fold Belt, Australia.

Kazuyuki Shiraishi, Yoichi Motoyoshi, Yoshikuni Hiroi, David Ellis and C. Fanning : Provenance of the Cambrian Lutzow-Holm Complex, East Antarctica

Itta Somaia: Structural evolution of the Lake Julius area.

Peter Sorjonen-Ward: history, alteration and gold mineralization in the Lapland GreenstoneStructural Belt, Finland

Kurt Stuwe and Michael Sandiford: Episodicity in metamorphism, deformation and magmatism in low-P, high-T terrains

Kerry Turnock: Deformation and metamorphism in the southern Halls Creek Mobile Zone, East Kimberley, Western Australia.

Dave Young: Microstructural evolution during deformation and metamorphism - the vortex, Mount Isa Inlier, Queensland, Australia.

Sylvia Zakowski, Nick Oliver and lan Cartwright: The structural, igneous and metamorphic evolution of the Mount Lofty Ranges, South Australia

viii

Magmatism, Temperature Dependent Rheologies and Shear Heating, or What Might PT Paths Really Look Like ?

Kurt Stuwe and Michael Sandiford, Department of Geology and Geophysics, Adelaide University, GPO Box 498, Adelaide, SA5001, Australia Pressure temperature time (PTt) paths of metamorphic rocks have been an important tool for geologists to interpret the tectonic evolution of metamorphic terranes. PT paths that show evidence for decompression during the cooling history were often interpreted to originate from erosion driven exhumation of rocks buried in orogenic belts similar to the European Alps, whilst PT paths that show compression during the cooling history have invoked a range of tectonic models, for example, rapid crustal thickening during cooling from an advective heating processes. In brief, considerable tectonic significance has been awarded towards the interpretation of the shape of PT paths. However, a number of external factors may govern apparent or real pressure changes inferred from metamorphic textures that are independent of the tectonic evolution of the terrane. These factors may include the effects of deviatoric stress changes in media with strongly temperature dependent rheology, the superposition of events in orogens with repeated magmatic activity and the effects of shear heating. In this talk we concentrate on the effects of temperature dependent rheology on the apparent pressure changes in a deforming medium. Whilst large deviatoric stresses in mid-crustal levels are implied by many mechanical models and field observations, their contribution to metamorphic PT paths has received little attention since the debate of tectonic overpressure. We show that in media with a power law rheology, in which the maximum resolved shear stress has an inverse exponential dependence on temperature, the destruction of the deviatoric stress field around large intrusions may result in significant decompression near the metamorphic peak at geologically reasonable strain rates. Moreover, for a specified strain rate and temperature evolution PT paths may vary from "clockwise" to "anticlockwise" merely as functions of vertical distance from the heat source (Figure 1). In as much as there is mounting independent evidence supporting the notion that the crust can support deviatoric stresses in the range of x ~ 100 - 200 MPa at temperatures of 400-500°C, and that the shear strength of the crust is strongly temperature dependent in the range 400-800°C, these results suggest that caution should be taken in the tectonic interpretation of PT paths involving decompression of some 100 MPa or less. Indeed, rather than reflecting the influence of erosion or tectonic denudation, the common evidence for near isothermal decompression of about 50-100 MPa in low-P high-T terrains may reflect deviatoric stress regimes of 100-200 MPa during deformation of the mid-upper crust.

Figure 1.: The "clockwiseness" of PT paths defined as the rate of pressure change at the metamorphic temperature peak, (dP/dt)Tmax kbar/Ma, plotted against vertical distance across a terrain heated by a single intrusion event. The rheological behaviour of the intruded medium is assumed to be governed by power law and able to support deviatoric stresses up to 200MPa at 400°C. The shaded area indicates the heat source. The two curves are for two different strain rate evolutions which are characterised by: (a) the majority of deformation occurring after the metamorphic temperature peak and (b) the majority of deformation occurring near the metamorphic temperature peak. The asymmetry of the curves above and below the intrusion arises from the assumption of an initial geotherm with lower background temperatures at shallow crustal levels. m

Heat Refraction and "Unconformity-Related" Contact Metamorphism, with application to the Mount Painter region in the Adelaide Fold Belt. S C O T T M I L D R E N , M I K E SANDIFORD, A N N E T T E Department

of Geology and Geophysics,

BINGEMER

University of Adelaide, GPO Box 498, Adelaide,

Australia

W e model the temperature distribution resulting from spatial variations in thermal conductivity and heat production appropriate to deformed basement-cover contacts with conductivity ratios of 1.5 to 3.5. Basement-cover interfaces are modelled as simple sinusoidal fold geometries using wavelengths of the order of 25 to 150 km and amplitudes of 5 to 20 km with heat refraction resulting in lateral temperature variations up to 100° C and gradients around approximately 13°C km-1.

T h e maximum effect is associated with structures of wavelength approximately 85 km. The heat refraction

process may have implications for the origin of unusually high lateral temperature gradients associated with high temperature low pressure metamorphism of the lower Adelaidean cover sequences of the Adelaide Fold Belt near the basal unconformity with mid-Proterozoic gneisses of the Mount Painter Block in the Northern Flinders Ranges. The metamorphic character of the region is defined by lateral temperature gradients between 12 to 35°C km"" 1 , reaching peak P T conditions of 510°C and 4 kbars in andalusite-cordierite bearing assemblages within cover sequence pelites near the basement contact. This raises the possibility that heat refraction can give rise to a unique type of regionally extensive, unconformity-related,

shallow crustal, contact metamorphism.

The thermal models provided here show

that heat refraction alone cannot account for the metamorphic signature without appealing to additional advective heat transfer as would accompany high temperature fluid fluxes. Evidence for extensive fluid infiltration is provided by the depletion of oxygen isotopes in the interior of carbonate units within the Adelaidean cover sequences by as much as 12%0 , strongly supporting the notion that the lateral temperature gradients, reflect in part, significant fluid fluxes focused by the heat refraction mechanism.

Heat refraction may play an important role in a wide range

of geological scenarios provided there exists a lateral variation in thermal conductivity.

For example, sedimentary

formations within the Cooper-Eromanga basin characteristically have a low bulk conductivity in substantial contrast with the more conductive underlying intracratonic basement.

Heat transfer simulations such as this may provide

additional insights for petroleum explorationists as to the thermal structure and hence hydrocarbon maturity within sedimentary basins.

Proposed future heat refraction research will incorporate real rather than inferred conductivity

variations within metamorphic sequences.

Timescales, rates and mechanics of deformation and metamorphism with application to the southern Adelaide Fold Belt Mike Sandiford, Dept.

Geology & Geophysics, University of Adelaide, GPO Box 498, Ade-

laide} Australia

The duration of deformation in orogenic belts is usually constrained indirectly, for example by absolute age determinations of igneous rocks intruded prior to and following the deformation. Such methods necessarily provide only an upper bound on the duration of deformation and in order to understand more precisely the controls on the distribution and rates of deformation in orogenic belts it is important to establish more direct methods.

Since deformation of

the lithosphere is heterogeneous over a wide range of scales one potentially direct method is provided by the metamorphic record of the thermal response to the evolving structure, for example the geometry of isotherms around large-scale fold structures. Both indirect and direct methods yield consistent estimates for the development the Karinya Syncline in the Mount Lofty Ranges in the southern Adelaide Fold Belt, constraining the duration of the deformation to < 3 Ma and < 1 Ma, respectively. Such constraints provide an important basis for examining the links between deformation, low-P, high-T metamorphism and magmatism. By analogy with arguments derived from simple thermo-mechanical models of lithospheric deformation that assume a temperature-dependant rheology, it is suggested that the deformation associated with the development of the Karinya Syncline was localised by advective heating related to ascent of granitic and, ultimately, mantle magmas. In-as-much as the process is driven by "active" mantle magmatism the localisation of crustal deformation in the Mount Lofty Ranges is viewed as a result, rather than a cause, of heating.

Reliability of reaction textures as guides to metamorphic histories Bas Hensen and Bo Zhou, Department of Applied Geology, UNSW, Kensington 2033

Thermal modelling of collisional and extensional tectonic regimes has led to an increased emphasis on the dynamic aspects of metamorphism. Because of this change in perspective it is now fashionable to determine Pressure-Temperature-time (P-T-t) paths, or at least P-T vectors, for metamorphic terranes. Reaction textures have been widely used to claim a large number of IBC (isobaric cooling) and ITC (isothermal decompression) paths for granulite facies, and lower grade, terranes. In most cases there are no time constraints on the formation of the textures used to derive the paths. It has been tacitly assumed by most workers that rocks habitually show textural evidence of re-adjustment to changes in pressure and temperature and that therefore the textures, and the derived sequence of events, can be attributed to a single metamorphic episode. This assumption may not be universally applicable. Reaction textures tend to be rare, even in very high grade rocks which have had an extended cooling history. This suggests that, more often than not, rocks cool back to ambient conditions without the development of any textural evidence for re-equilibration. Dry rocks in particular often seem to react only under special conditions such as deformation and/or fluid influx. Reaction textures are mostly found in association with structural features that suggest activation of reaction by deformation. Therefore it cannot be assumed that rocks carry a complete record of their P-T history and it is possible that reaction textures in a single rock record segments of P-T paths that are unrelated. The existence of poly-metamorphism has been documented for many pre-Cambrian terranes and, in the absence of proper age constraints, it is clearly inadvisable to assume that the textures of such old rocks have formed in a single metamorphic episode and record a single P-T path . Without geochronological evidence it is uncertain whether a particular texture has formed in response to heating or cooling. ITD textures (ie.: cordierite-orthopyroxene symplectites replacing garnet) may form by reheating at a lower pressure, if earlier cooling and decompression after a first metamorphic event passed unrecorded. Similarly IBC textures, such as the development of garnet coronas between orthopyroxene and plagioclase, may record reheating to a somewhat lower temperature in a second metamorphic cycle, entirely unrelated to the first. In the latter case it may be argued that the rocks most likely remained at depth between the thermal events and that isobaric cooling after the first metamorphism still applies. In the absence of reliable criteria to determine whether reactions took place in response to heating or cooling, age dating of metamorphic episodes is required to establish the P-T-t history of a terrane. One of the most promising techniques for this purpose is Sm-Nd dating using garnet. The application of this method to granulite facies rocks is limited only by the temperature at which REE-diffusion in garnet becomes sufficiently rapid to destroy any memory of an earlier metamorphic event. This "closure" temperature has been variously estimated at 600900°C. Preliminary results for poly-cyclic granulites from Antarctica indicate that reasonably sized garnets (>3-4 mm diam.) do retain a memory of an earlier metamorphic event, thereby providing evidence for a closure temperature in the upper part of the suggested range.

Metamorphic P-T evolution of sapphirine-quartz bearing granulites from central Sri Lanka Yasuhito Osanai. Department of Applied Geology, University of New South Wales, Kensington, 2033 Australia, Bastiaan J. Hensen, Department of Applied Geology, University of New South Wales, Kensington, 2033 Australia, Yoshikuni Hiroi, Department of Earth Sciences, Chiba University, Chiba, 263 Japan and Masaaki Owada, Department of Mineralogical Sciences and Geology, Yamaguchi University, Yamaguchi, 753 Japan. Sapphirine-quartz bearing granulites and related very-high-temperature metamorphic rocks have been found in the Highland Series near Talatu Oya, eastern Kandy, central Sri Lanka. The supra-crustal sequence around the Talatu Oya district is mainly composed of granulite-facies metapelites, amphibolite and charnockites with thin intercalations of marble and quartzite. The sapphirine-bearing very-high-temperature granulites (111901C-H) occur as lensoid or irregular shaped exotic blocks in marble layers. The rocks are layered, poorly foliated, silicapoor and Mg and Al-rich metapelites containing sapphirine (Spr), orthopyroxene (Opx), spinel (Spl), garnet (Git), kornerupine (Km), cordierite (Crd), sillimanite (Sil), gedrite (Ged), phlogopite (Phl)/biotite (Bt), quartz (Qtz) and plagioclase (PI); no single sample contains the full set of minerals. Minor constituents are ilmenite, rutile, pyrrohtite, pyrite, magnesite, apatite and zircon. On the basis of their mineralogy the rocks can be divided into two micro domains (max. 30 cm in diameter) , which appear to have equilibrated at different f02. Each domain contains compositional sub-domains with the following mineral associations: la: Spr-Opx-Grt-Phl-Pl lb: Spr-Opx-Krn-Phl-Pl lc: Spr-Opx-Grt-Spl±Crd-Phl-Pl (Bt and Spl inclusions in Opx, Spr and Grt; Grt and Bt inclusions in PI) 2a: Spr-Opx-Qtz-Spl-Crd-Ged±Phl (Spl in the matrix and as inclusion in Spr) 2b: Opx-Spl-Crd±Sil-Ged-Phl±Pl (Spl in the matrix and as incluisions in Opx) The boundaries between sub-domains la, b & c and 2a & b are gradational. Sub-domains la and lc are characterised by occurrence of highly-unusual euhedral oscillatory-zoned plagioclase, which suggests crystallisation from a melt. Fe-Mg minerals in domain 1 have lower Mg/(Fe+Mg) values (Spr: 0.77-0.86, Opx: 0.74-0.83, Grt: 0.50-0.63, Spl: 0.61-0.63, Krn: 0.83-0.88) than those of domain 2 (Spr: 0.89-0.91, Opx: 0.870.90, Spl: 0.72-0.81, Crd: 0.94-0.95). In the domain 1 biotite inclusions in porphyroblastic orthopyroxene, garnet, sapphirine and corroded biotite and garnet inclusions in oscillatory-zoned plagioclase suggest prograde metamorphism and/or melting reactions involving biotite and garnet. Sapphirine-aluminous orthopyroxene (7.4-8.0 wt% Al203)-garnet±spinel (Fe +/ Fe +=0.053-0.056) coexisted at the maximum P-T conditions. Aluminous-orthopyroxene, garnet and sapphirine are often rimmed by a symplectite of sapphirine, orthopyroxene (4.5-6.2 wt% AI2O3), spinel, and plagioclase, whereas in domain lc garnet has partial coronas of orthopyroxene (5.7-6.2 wt% AI2O3), spinel, and plagioclase. Kornerupine with orthopyroxene (7.5-8.8 wt% Al203)±phlogopite are believed to have formed later at the expense of garnet and sapphirine. 3

In domain 2a sapphirine-orthopyroxene (4.6-5.8 wt% Al203)-quartz±spinel (Fe +/Fe +=0.204-0.286) coexist. Domain 2b contains cordierite-spinel (Fe +/Fe +=0.022-0.122) torthopyroxene. Both sub-domains contain late gedrite and phlogopite. Fe +/Fe + ratios of sapphirines from domain 1, where spinel is only a minor component, have lower values (Fe +/Fe +=0.002-0.382) than those from domain 2 (Fe +/Fe +=0.357-0.922), where spinel occurs as major component and garnet is absent. Therefore we conclude that domain 1 equilibrated under relatively low-f02 conditions, whereas f02 was higher in domain 2. 3

Geothermobarometry suggests that mineral assemblages la and lc equilibrated at 900-1000 MPa and 900-1000 °C. Estimated P-T conditions and reaction textures in domain 1 are consistent with early isobaric heating and subsequent decompression around the quartz-biotite free invariant point in the petrogenetic grid of the KFMASH and FMAS systems. Kornerupine was produced in domain 1 at around 750 MPa and 800 °C. Assemblages 2a and 2b equilibrated at high temperature (c. 900-1000 MPa and 1000-1100 °C) around the sillimanite-biotite and orthopyroxene-biotite absent invariant points. The temperature estimate, based on the occurrence of sapphirine+quartz and sapphirine+orthopyroxene+quartz, may be a maximum value because the lower temperature limit of this assemblage may be extended under high-f02 conditions. The suggested clockwise P-T path, although consistent with the textural and compositional evidence, needs to be considered as tentatve at this stage, particularly because rocks in this area are known to have undergone three major heating events at 2300-2600,1000-1100 and 500-600 Ma respectively.

Sequential growth of AI Si0 polymorphs: Documentation of a complete anticlockwise P-T loop in the central Mount Isa Inlier 2

Jurgen Reinhardt, Institutfur Mineralogie, Ruhr-Universitat Bochum, D-4630 Bochum, Germany In the Mary Kathleen Fold Belt, a Proterozoic volcano-sedimentary sequence is exposed that experienced prograde low-pressure, high-temperature metamorphism. Detailed penological and structural studies have been carried out in the west-central Mary Kathleen Fold Belt in order to determine the pressure-temperature history and its relation to deformation. In this area, peak metamorphic temperatures of 580-640°C had been reached at pressures of about 4 kbar (Reinhardt, 1992). Aluminous metapelites are abundant as andalusite-sillimanite-muscovite-biotite schists or cordierite-andalusitesillimanite-biotite rocks, with intermediate to high Mg/Fe ratios. In these rocks, multiple generations of Al^iC^ polymorphs could be identified which allow to constrain, at least in general terms, the prograde and retrograde P-T path. The earliest generation of andalusite (andalusite-I) has a cordierite corona, indicating the reaction andalusite + chlorite + quartz <=> cordierite + H 0.

(1)

The andalusite must have formed from a lower-grade reaction, presumably from pyrophyllite + quartz. The bulk of the prograde andalusite crystallized at higher grade from the reactions chlorite + muscovite <=> andalusite + biotite + quartz + H 0, chlorite + muscovite + quartz » cordierite + andalusite + biotite + H 0,

(3) (4)

cordierite + muscovite «=> andalusite + biotite + quartz + H2O.

(5)

and

In contrast to andalusite I, andalusite from reactions (3) to (5) has abundant biotite inclusions. Second-generation andalusite II from reactions (3) and (4) is further characterized by very fine-grained inclusions that form foliation-lineation (SpLj) patterns quite different to those observed in the coarser-grained S2-L2 rock matrix. The porphyroblast-matrix relations indicate early growth with respect to syn-metamorphic deformation (D ; see also Reinhardt, this volume). 2

The latest prograde andalusite generation is a breakdown product of the assemblage cordierite + muscovite, according to reaction (5). Where andalusite in occurs in abundance (such as in Mg-rich schists), cordierite is strongly corroded and has a biotite corona Andalusite III overgrew a late-stage S matrix foliation. In the lowest-grade area, there are little or no visible effects of further flattening of the schist matrix around such porphyroblasts. 2

All prograde andalusite shows at least incipient transformation to sillimanite. In some samples from the highestgrade domains of the study area, the conversion has gone to completion. Here, sillimanite may also have grown directly from reaction (5). The andalusite-sillimanite transformation is partly paramorphic-topotaxial. For the PT-X relationships, the presence of Fe has to be considered (andalusite 0.94 -1.94 wt%, sillimanite 0.59 -1.14 wt% Fe20 ). This shifts the andalusite-sillimanite reaction to higher temperatures and increases the variance of the system (Kerrick & Speer, 1988). 3+

A retrograde polymorphic transformation was not observed in any sample, which means that the bulk of the aluminous schists provide no information on the retrograde P-T history. However, retrograde Al2Si0 species do occur in some of the cordierite-bearing rocks. Retrograde hydration of cordierite schists resulted in a breakdown of cordierite to aggregates of randomly oriented Al Si0 minerals, chlorite and quartz, according to the reaction 5

cordierite + H 0 <=> sillimanite, kyanite, or andalusite + chlorite + quartz. 2

(1')

Samples from the Rosebud and Little Beauty Synclines show all stages of cordierite breakdown, from incipient to complete. In the field, there are spectacular examples of coarse-grained kyanite-andalusite-chlorite-quartz rocks associated with late-stage fluid conduits. Inclusions of retrograde kyanite in retrograde andalusite-IV imply that kyanite formed earlier. One sample of massive retrograde andalusite contains abundant kyanite inclusions, but also a patch of coarse intergrown needles of sillimanite the texture of which does not compare with any of the prograde sillimanite varieties. This sillimanite is most likely the first retrograde aluminosilicate that formed from the rehydration of cordierite schist. Accordingly, the sequence of retrograde Al^iC^ polymorphs is sillimanite II kyanite andalusite IV. The close spatial association of these three polymorphs suggests that the fluid conduit had been active for a longer period of time. In summary, seven successive Al Si0 generations can be readily distinguished in this area. Due to the sluggishness of the polymorphic reactions and incomplete retrogression, it is not uncommon to find two or three Al Si0 polymorphs in a single sample. The sequence of Al Si0 species requires a complete anticlockwise P-T loop with prograde low-pressure, high-temperature metamorphism followed by a period of near-isobaric cooling and, finally, decompression (Fig. 1). 2

References: Kerrick, D.M. & Speer, J.A., 1988. Amer. J. Science 288,152-192. Reinhardt, J., 1992. Geol. Magazine 129,41-57.

T 400

500

600

700 °C

Fig. 1. Syn-D2 and post-D2 P-T path for the Rosebud Syncline (west-central Mary Kathleen Fold Belt). Shown are also experimentally determined reactions of the KMASH boundary system as well as reactions of the ASH system (solid lines; for references see Reinhardt, 1992). The position of the univariant KFMASH reaction (4) indicates the shift of reactions to lower pressures, with increasing Fe. A: early-D2 reactions; B: late-D2 reactions; C: retrograde path. Reactions (1), (3), (4), (5): see text; dotdashed line: reaction (1), calculated for a H 2 0 = 0.8; other reactions: (2): chlorite + muscovite + quartz <=> cordierite + biotite + H ^ ; (6): chlorite + quartz <=> cordierite + talc + H 2 0; (7): biotite + muscovite + quartz » cordierite + K-feldspar + H p ; (8): muscovite + quartz » K-feldspar + Al 2 Si0 5 + H 2 0; (9): pyrophyllitc + quartz <=> Al 2 Si0 5 + H 2 0. (From Reinhardt, 1992).

Stepping stones and pitfalls in the determination of an anticlockwise P-T-t-deformation path: the low-P, high-T Cooma Complex, Australia Scott Johnson. Ron Vernon, School of Earth Sciences, Macquarie University, Sydney, NSW, 2109, Australia This paper assesses microstructural and mineralogical evidence for piograde and retrograde metamorphic conditions in the Cooma Complex, which is located approximately 110 km south of Canberra in the Eastern Metamorphic Belt (Fig. 1). The complex shows a progressive isograd sequence, which from west to east is: chlorite, biotite, cordierite, cordierite-andalusite and cordierite-andalusite-K-feldspar, the grade increasing towards the Cooma Granodiorite (Fig. 1). The highest grade rocks contain sillimanite, and migmatites occur against the granodiorite. The metamorphic rocks of the complex can be conveniendy divided into schists, which occur to the west of the cordierite-andalusite-K-feldspar isograd, and high-grade gneisses, which occur to the east The schists and gneisses are largely pelitic and psammitic in composition.

Metamorphic rocks in the complex are of the low-pressure/high-temperature (LPHT) type, and show field and micros tructural evidence of a detailed prograde pressure-temperature-time-deformation (P-T-t-D) path (Fig. 2). It is interpreted as being part of an anticlockwise P-T-t-D path (Fig. 2), similar to those of some other LPHT metamorphic areas of the Australian Proterozoic and Palaeozoic. In contrast with some of these other LPHT areas, the Cooma Complex provides detailed information on the prograde, rather than the retrograde history. Evidence of the retrograde path is inconclusive, but is consistent with approximately isobaric cooling, as are available isotopic data on the Cooma Granodiorite, which indicate initially rapid cooling following attainment of peak temperatures. The retrograde path is inconsistent with either a clockwise P-T-t-D path involving rapid or even moderate decompression immediately postdating the peak of metamorphism, or a path in which the retrograde component simply reverses the prograde component, because both these paths should cross reactions forming cordierite from aluminosilicate, for which no evidence has been observed.

Determination of the deformational-metamorphic history of the complex is not straightforward and depends on careful and unbiassed examination of very critical samples. In Cooma such critical samples are relatively rare, and the evidence they contain may be easily misinterpreted. At Cooma this is emphasized by the fact that earlier studies (including our own!), despite being careful and detailed, have missed or misinterpreted some of the critical evidence. Evidence necessary for successful elucidation of the complete prograde, and part of the retrograde, deformational-metamorphic history in the complex includes: (1) sequentially grown porphyroblasts that can be timed relative to surrounding foliations; (2) partial replacement microstructures that provide relative timing of metamorphic reactions that cannot be timed relative to foliation development; (3) a tectonic marker foliation (S4 at Cooma) that allows correlation of foliations, and therefore porphyroblast growths, from one location to another, and (4) single samples containing all foliations, and all generations of porphyroblast growth within a single metamorphic zone. The latest two or three foliations should involve low strain accummulation, allowing relative timing relationships between foliations and porphyroblasts to be

unambiguously determined. Although less important than the above evidence, bedding (So) can be useful as a check that early foliations preserved as inclusion trails in porphyroblasts are tectonic foliations, rather than SQ.

One of the main conclusions of this paper is that, without these critical pieces of evidence, the earliest foliations may go unrecognized, relative timing of metamorphic mineral growths may be misinterpreted, and therefore an inferred P-T-t-D path may be incomplete or incorrect Though the presence of critical evidence provides the stepping stones to a correct P-T-t-D path, they still do not guarantee a correct path because there are numerous pitfalls regarding interpretation of the evidence that must first be negotiated. Correct interpretation may be difficult to impossible, depending on the clarity of timing relationships between foliations and porphyroblasts. Unambiguous timing relationships are rare, and confident interpretation over the entire range of foliations and porphyroblasts in a particular isograd sequence is only possible in exceptional circumstances. Incorporating all the above evidence in this study and re-interpreting previously misinterpreted porphyroblast timing relationships has allowed the determination of a more complete and correct P-T-t-D path.

Figure 1. Study area showing metamorphic isograds (approximate), location of the Cooma Granodiorite and a rough outline of the town of Cooma. Chi = chlorite, Bt = biotite, And = andalusite, Crd = cordierite, Ksp = K-feldspar. AMG numbers on border. 12

(a)

(b)

Figure 2. Two equally probable anticlockwise P-T-t paths for the schists (left path) and gneisses (right path). In (a), the reaction Crd + Ksp + H 0 = Bt + Als + Qtz is depicted as occurring after the rocks leave the sillimanite field, whereas in (b), this reaction is depicted as occurring before the rocks enter the sillimanite field. Reaction grid for the system KFMASH involving cordierite (Crd), aluminosilicate (Als), K-feldspar (Ksp), biotite (Bt), muscovite (Ms), chlorite (Chi), quartz (Qtz) and H 0. Reactions in quartz- or H 0deficient subsystems not included. 2

Granites: Melts, Magmas, Migration and Metals C.K. Mawer. Project Generation, MIM Exploration, Brisbane, Australia J.D. Clemens, Department of Geology, The University, Manchester, England G. Stephens, Department of Geology, The University, Manchester, England Voluminous generation of granite magmas, and thus effective crustal differentiation, seems viable only during high-temperature, fluid-absent metamorphism. Temperatures in excess of 850°C are necessary for generation of significant percentages of melt from fertile source rocks. Production of these melts will be highly non-linear, and will involve positive volume changes. This will lead to melt fracturing, and transport of melt and magma from generation sites through the resultant fractures. The highly water-undersaturated melts/magmas are effective transporters of heat, water and chemical components upwards through the crust. Granite magmas commonly ascend from fertile source regions to upper crustal emplacement sites through tens of kilometres of otherwise unremarkable crust. Passage of these granite magmas is rapid and thermally efficient, and voluminous plutons can be grown in geologically short periods of time. Granitic magmas ascend, in general, through propagating fractures as dykes; diapiric transport of such magmas through crustal sections is not viable on thermal or mechanical grounds, and there is an absence of geologic evidence for diapiric transport even in supposed 'type' localities. These facts are geodynamically significant. Granite magmas are trapped in dilatant sites. These sites are either within shear zones of apparently any kinematics (though zones with extensional or strike-slip kinematics, or their combinations, are preferred), or as laccolith-style plutons in strata with roughly horizontal layering, anisotropics and/or discontinuities. It is unclear which of the settings (shear zones, laccolith-promoting) is more common. When trapped, magmas will cool rapidly, exsolving late-crystallization pegmatitic and hydrothermal fluids; these will in turn fracture their way upwards from the pluton. If these fluids carry metals, or if metal-saturated hydrothermal fluids are generated from adjacent wall-rocks, thermal and physical considerations indicate that the metals will precipitate no more than several kilometres above, or within about 1-2 kilometres laterally from, the pluton/wall-rock contact. This is essentially irrespective of fluid chemistry, and is accentuated by suitably reactive wall rocks.

Medium- to low-pressure metamorphism and magma emplacement during deformation of an island arc: the late Precambrian Cadomian belt (Brittany, France). Ronan Hebert and Michel Ballevre, Laboratoire de Tectonique, Geosciences Rennes, Universite Rennes I, 35042 Rennes Cedex, France. The Cadomian orogeny, of late Proterozoic age, occured within the continental crust of Western Europe. Few areas escaped reworking during the Variscan orogeny, but the largest one is well exposed between the northern coast of Brittany and the Variscan North Armorican Shear Zone, France. There, three main domains are to be distinguished: 1. The Tregor domain consists of plutonic and volcanic rocks of calc-alkaline affinities and of Upper Proterozoic age (640-610Ma). 2. The Saint Brieuc domain is characterized by abundant basic volcanic and plutonic rocks, with minor sediments. 3. The Saint Malo domain is dominantly made of metasediments of presumed Upper Proterozoic age, which were submitted to an extensive fluid-present partial melting. This study focus on the tectonic and metamorphic history of the Saint Brieuc domain. Two main stages are recognized. 1. The major tectonic event took place between 590 and 570 Ma. It is characterized by an intense ductile deformation during the regional metamorphism and is associated to the intrusion of several mantled-derived plutons.The regional metamorphism presents a southward-increasing grade. To the North, metasediments from the Binic formation show only biotite. The Lanvollon formation is dominantly made of strongly deformed metabasalts, which contain plagioclase (An 10-30) + hornblende + quartz + ilmenite + epidote. Interbedded sediments are now staurolite-bearing micaschists. Estimated P-T conditions are 3-5 kbar and 550 ±50 °C. To the South, the Yffiniac formation displays a cumulate sequence from ultramafics up to gabbros. Small undeformed volumes present coronitic reaction rims around relics of magmatic phases, especially spinel + orthopyroxene symplectites around olivine. Deformed rocks are characterized by plagioclase + hornblende + garnet or clinopyroxene, depending on the bulk-rock chemistry. P-T estimations within these rocks are 8 ± 2 kbar, 700 ± 50 °C. Several plutons of gabbroic to dioritic composition and calc-alkaline affinity are intrusive within the Binic and Lanvollon formations. These rocks were deformed during the last stages of their crystallization history, revealing that their intrusion is contemporaneous 15

with the main ductile deformation of the Saint Brieuc domain. Contact aureoles around these plutons are more or less developed, depending on their depth of emplacement. 2. Late granitic and dioritic intrusions took place at around 540Ma. These intrusions are not deformed and develop contact aureoles within country-rocks. Complete recrystallization as well as new parageneses are observed within migmatitic metapelites and amphibolites at a very small distance from the Saint Brieuc pluton. Only retrograde transformations are observed with increasing distance from the contact. They imply extensive fluid flow. The Saint-Brieuc domain could represent a cross-section through an island arc of late Proterozoic age. Its low-P metamorphism results from the shortening of a young crust with initial high heat flow.

Simplified geological map of northern Brittany (Armorican Massif, Franoe).

B pluton,

Bint formation,

Lanvolbn formation,

< x Cadorrian post-tectonic pluton, x

Yffiniac formation,

>4 Variscan granitoids.

Cadorrian syrvtedonic

Deformation and contact metamorphism during pluton emplacement: an example from the White-Inyo Mountains (California) Jprn H. Kruhl, Institute of Geology and Paleontology, JW Goethe-University, D-6000 Frankfurt/MGermany, S.R.Paterson and T.C.Brudos, Dept. of Geological Sciences, USC Los Angeles, CA 90089-0740, USA. A considerable part of the White-Inyo Mountains (East-Central California is occupied by plutons of known or postulated Jurassic or Cretaceous age [1]. Amongst them, Joshua Flat and Anvil Pluton provide a good example of the interaction of metamorphism and deformation in the pluton contact aureole and the relationship between contact and regional metamorphism and deformation. 1) Locally, a weak low-temperature deformation occurred before pluton emplacement specifically in pelitic rocks. It is indicated by relics of a fine schistosity within cordierite porphyroblasts of the contact aureole. 2) Contact metamorphism interferes with a strong high-temperature solid state flow with approximately vertical transport. A steeply plunging strong foliation with a steep lineation is developed. Locally, the foliation is folded on the centimeter to meter scale. Fold axes are generally parallel to the lineation. However, deformation was not effective until high metamorphic temperatures during a late stage of pluton emplacement, (i) Diopside overgrows fine-grained randomly oriented quartz and carbonate and is itself overgrown by large garnet and idocrase crystals, (ii) The foliation and lineation always wrap these crystals and, therefore, postdate their formation, (iii) Foliation and lineation are figured by preferred dimensional and crystallographic orientations of quartz, carbonate, wollastonite, phlogopite, scapolite and andalusite. Specifically diopside and wollastonite are formed in pressure shadows of garnet and idocrase porphyroblasts. (iv) Quartzites show extreme grain-coarsening with basis-parallel subgrain boundaries in quartz and with c-maxima near the local elongation direction, (v) Wall-rock deformation during the late stage of pluton emplacement is indicated by isoclinally folded aplitic dykes which show igneous but not any solid state deformation textures, (vi) Flow structures are present and high-temperature deformation structures are absent within the pluton. 3) A post-pluton low-temperature deformation is indicated by the rotation of porphyroblasts between late schistosity planes and by kinking and fracturing of feldspars and by prism-parallel subgrain boundaries and small recrystallized grains of quartz. Low-temperature deformation is also present in the pluton margin and in late dykes in which indications of high-temperature deformation do not exist. It may be concluded that, within a regional stress field, deformation was extremely intensified in the pluton aureole during the late stage of intrusion, as a result of heat and fluid migration. The relatively rigid igneous body acted as buttress around which the metasedimentary wall rocks were deformed. On the basis of similar observations in other parts of the White-Inyo Mountains [2], models are discussed which relate large-scale regional deformation to the emplacement of plutons and their heat and fluid production. [1] Dunne, Gulliver, Sylvester (1978); Pazific Coast Paleogeogr. Symp. 2, Pazific Section SEPM, April 1978. [2] Paterson, Brudos, Fowler, Carlson, Bishop, Vernon (1991); Geology 19, 324-327. 17

Crustal anatexis and magma process in the island-arc deep-crust, Hidaka metamorphic belt, northern Japan

Masaaki Owada. Department of Mineralogical Science and Geology, Yamaguchi University, Yamaguchi City, Yamaguchi 753, Japan, Yasuhito Osanai, Department ofApplied Geology, the University of New South Wales, Kensington, New South Wales 2033, Australia, and Hiroo Kagami, Institute for Study of Earth's Interior, Okayama University, Misasa Town, Tottori 682-01, Japan The Hidaka metamorphic belt forms the axial zone of Hokkaido island, northern Japan. This metamorphic belt can be divided into two different crustal slabs, the Western Zone (Poroshiri Ophiolite) and the Main Zone, the latter thrust over the former. The boundary between the two zones is a large shear zone, named the Hidaka Main Thrust. The Main Zone consists of various metamorphic rocks and acidic to basic intrusive rocks, including Stype granitic rocks, and is considered to represent a series of overthrust crustal slabs derived from an island arc (Komatsu et al., 1989). The imbricated slabs of metamorphic rocks of the Main Zone of the Hidaka metamorphic belt show a range in metamorphic grade from granulite facies at low structual level to unmetamorphosed sedimentary rocks at the top. Osanai et al. (1991) have shown that the Main Zone of the Hidaka metamorphic belt underwent a clockwise PT-time path with maximum P-T conditions of 7.3kbar, 850X3 in the basal part (granulite unit). In the granulite unit, pelitic granulites include leucosome patches which consist mainly of orthopyroxene - plagioclase - quartz. Based on melting experiments for the pelitc granulite in water-sufficient system, the leucosome patch was formed by incipient anatexis at close to the highest P-T condition of the Main Zone (Osanai et al., 1992). Orthopyroxene-bearing tonalite (Opx tonalite) which intruds into the granulite unit shows S-type (peraluminous) chemical affinities. Osanai et al. (1991) interpreted that the Opx tonalite has been derived from peraluminous granitic magma generated by crustal anatexis of pelitic metamorphic rocks in deeper crust, based on its mineralogical and chemical nature, combined with result of the metamorphic condition. Many recent papers invoke the mantle derived basic magma underplating in order to explaine the heat source of crustal anatexis and infer mantle - crust interaction in deep crust. In the south-western part of the Main Zone of the Hidaka metamorphic belt, gabbro-diorite intrusions and the Opx tonalites intrude contemporaneously into the granulite unit.

In this presentation, we will determine the timing of anatexis using Rb-Sr whole rock dating for the pelitc granulite and the Opx tonalite, and then deal with the relationship between the gabbro-diorite and the Opx tonalite in order to understand the interaction between mantle and crustal magma in the deep crust in the Main Zone of the Hidaka metamorphic belt. We measured major, trace (Rb, Sr, Ba, Nb, Zr, Y, Cr and Ni), REE and Sr isotope ratio of the pelitic granulite, the Opx tonalite and the gabbro-diorite. Results summarize as follows; (1) The granulite unit consists of orthopyroxene - bearing amphibolite, garnet - cordierite - biotite gneiss (Grt-

Crd-Bt gneiss) and garnet - orthopyroxene - cordierite gneiss (Grt-Opx-Crd gneiss). The Opx tonalite usually associates with the Grt-Crd-Bt gneiss and the Grt-Opx-Crd gneiss. (2) On the Si02 versus oxides diagrams, the leucosome patch, the Grt-Crd-Bt gneiss and the Grt-Opx-Crd gneiss form a regression line. The synthetic melt and caluculated restite which are obtained by melting experiments for the Grt-Crd-Bt gneiss plot along this regression line. The Opx tonalite is enclosed within the field of the Grt-Crd-Bt gneiss and the leucosome. (3) The Grt-Opx-Crd gneiss is depleted in LIL comparing with the Grt-Cid-Bt gneiss. (4) Chondrite normalized REE abundances of the Grt-Cid-Bt gneiss plot between those of the Grt-Opx-Crd gneiss and the Opx tonalite. The Grt-Opx-Crd gneiss, the Grt-Crd-Bt gneiss and the Opx tonalite correspond to restite, source rock and anatexite, respectively, based on chemical characteristics of major, trace and REE. (5) The Opx tonalite, the leucosome, the Grt-Crd-Bt gneiss and the Grt-Opx-Crd gneiss yield a well-defined isochron with an age of 58Ma and initial ratio of 0.70569. This isochron indicates that the isotopic equilibration in the measured samples was onece isotopically homogenious, suggesting that anatexis occured concurrently with the granulite facies metamorphism, giving rise to the formation of the S-type tonalitic magma. (6) The fine-grained gabbro represents primary mantle derived basaltic magma, based on petrochemical features. Primordial mantle normalized trace element pattern for the fine-grained gabbro is plotted within the field of NandT-type MORB. (7) The major and trace element compositions and Sr isotope ratios of the gabbro-diorite relect the effect of fractonal crystallization combined with mixing of mantle (MORB affinity) - and crust (S-type granitic component) - derived melts. References: Komatsu, M., Osanai, Y., Toyoshima, T. and Miyashita, S., 1989, In: Yardley, B. W. D., Geological Society Special Publication, 43, 487-493. Osanai, Y., Komatsu, M. and Owada, M., 1991, Journal ofMetamorphic Geology, 9, 111-124. Osanai, Y., Owada, M. and Kawasaki, T., 1992, Journal ofMetamorphic Geology, 10, 401-414.

T h e Determination of Cooling Rate Histories from Garnet - Biotite Equilibria Karin Ehlers, Roger Powell, School of Earth Sciences, Vic 3052, Australia, and Kurt Stiiwe, Department

University of Melbourne,

Parkville,

of Geology & Geophysics,

G.P.O.

Box 498, University of Adelaide, SA 5001, Australia Dodson's solution [1,2] for the temperature-dependent diffusion equation may be directly used to determine cooling histories from geothermometrically-inferred closure temperatures (T c ), if centres of grains can be analysed. If a mineral assemblage, such as garnetbiotite has re-equilibrated during cooling, then the thermometrically-inferred temperature, here called the closure temperature, is a strong function of grain size, cooling rate (s), and diffusion parameters.

Thus different mineral pairs within one rock or thin section will

record different stages of the thermal history, depending on the garnet grain size. What one sees and measures from a thin section are apparent grain sizes, depending on the section position, and therefore apparent closure temperatures, given that most garnets display concentration profiles. Thermal histories may still be obtained if the effect of uncertainties on the section position are propagated to the closure temperatures and cooling rates. We have solved this statistical problem and evaluated 95% confidence intervals and expected mean values for closure temperatures and cooling rate. With the additional information of the grain size dependency on temperature, it can be predicted which garnets are the most likely ones to be cut near their centres. A Monte Carlo study is then used to demonstrate that different cooling histories will give distinct pattern of confidence intervals and expected means on a s — Tc diagram. We simulated cooling histories for which the cooling rate decreased/increased exponentially with temperature and for constant cooling rates. Each of those thermal histories give a distinct pattern, which cannot have resulted from another cooling history. We are therefore in a position to parameterize thermal histories of high grade metamorphic rocks. [1 ] Dodson M. H. (1986) Material Science Forum 7, 145-154 [2 ] Dodson M. H. (1973) Contr. Mineral, and Petrol. 40, 259-274

Adjacent apparent isobaric cooling and isothermal decompression textures in granulites from Else Platform, East Antarctica: implications for the interpretation of inferred P-T paths Ian Scrimgeour, Department of Geology and Geophysics, University of Adelaide, Adelaide, SA, 5001, Australia. Over the last decade, tectonic models for the evolution of orogenic belts have been based largely on the interpretation of P-T paths constructed by metamorphic geologists. As a consequence, the interpretation of inferred P-T paths as being either "clockwise" (near isothermal decompression after the metamorphic peak) or "anticlockwise" (near isobaric cooling or compression during cooling) has been a fundamental tool in classifying various high grade terrains. These interpretations have been made largely through the observation of what are regarded to be Classic decompression textures or cooling textures, and granulite terrains have often been subdivided into isobaric cooling (IBC) and isothermal decompression (ITD) granulites on this basis (e.g. Harley, 1989). 1

Else Platform, in the northern Prince Charles Mountains (NPCM) region of MacRobertson Land, forms part of the extensive Late Proterozoic low to medium pressure granulite terrain of East Antarctica. This terrain has long been considered to be a classic example of a regional granulite terrain which has undergone isothermal decompression (e.g. Harley & Hensen, 1990), due to the widespread occurrence of 'decompression textures' such as cordierite-bearing coronas and symplectites after garnet and sillimanite. However, in the NPCM, recent studies have documented 'classic' cooling textures in calc-silicates, suggesting that the NPCM may have a different P-T evolution to the rest of the Late Proterozoic terrain (Fitzsimons & Harley, in prep.). On Else Platform, pelitic lithologies preserve spinel-cordierite symplectites after garnet and sillimanite, and cordieriteorthopyroxene symplectites after garnet and biotite, and these indicate apparent near-isothermal decompression of around 2 kbars or morefrompeak conditions of 750-800°C and 7-8 kbars (Hand et al., 1992). However, in calcsilicate layers within the same sequence, calc-silicate lithologies preserve anorthite-calcite symplectites after scapolite, grossular coronas between scapolite, wollastonite and calcite, and calcite-quartz intergrowths replacing wollastonite, which indicate apparent near-isobaric cooling of at least 100°Cfrompeak conditions in the vicinity of 7 kbars and 800°C. Clearly, these layers within the same sequence could not have experienced such differing physical conditions during their retrograde history, and therefore one or both of these P-T paths must be incorrect A number of factors could account for the spatial association of pelitic decompression textures and calc-silicate cooling textures on Else Platform. Hand et al (1992) have proposed, largely through textural evidence, that the decompression textures on Else Platform record a second prograde granulite event, M2, which has overprinted a higher pressure Mi assemblage, resulting in apparent decompression. If this is the case, it may be that the pelites and calcsilicates merely record differing parts of the same metamorphic evolution, ie. the calc-silicates only record the cooling history from Mi, whilst the pelites record the lower pressure M2 overprint. Clearly, without age constraints, an overprinting metamorphic assemblage cannot be assumed to represent a continuous P-T evolutionfrompeak conditions. It is clear that granulite metamorphism at mid-crustal levels must involve significant advection of melts, and the crystallisation of these melts as well as partial melts of the country rock will necessarily result in the release of fluids. An increase in the activity of water will generally shift the stabilityfieldof cordierite to higher pressures, thus resulting in cordierite coronas around garnet which would appear to represent decompression. This increase in the activity of water would assist the formation of grossular coronas 21

between wollastonite, scapolite and calcite, but would otherwise not significantly affect the cooling history preserved in the calc-silicates. Another possible factor in the apparent decompression in the pelites is the influence of the deviatoric stress, following recent work by StOwe & Sandiford (submitted). It is likely that the quartz-rich pelites on Else Platform would be able to support higher deviatoric stresses than the calc-silicates, and this would result in a higher apparent pressure. Therefore it is possible that the weakening of the crust due to the intrusion of melts during crustal thickening could result in a component of apparent decompression as the pelites would be no longer able to support the deviatoric stress, and therefore this component of pressure would be removed. However this is unlikely to account for all of the apparent decompression. Irrespective of the reasons for the two differing apparent P-T paths, the most important point to this observation is that the 'decompression textures', when interpreted on their own elsewhere in East Antarctica, have been used to infer "clockwise" P-T paths and associated tectonic models (e.g. Nichols & Berry, 1991), whilst similar calcsilicates have been interpreted elsewhere to represent isobaric cooling from peak conditions. Clearly the fact that these textures are found together on Else Platform highlights the significant doubts on the confidence with which we can apply tectonic models to the apparent P-T paths preserved in many high grade metamorphic terrains Fitzsimons I.C.W. & Harley S JL. (in prep.). Garnet coronas in scapolite-wollastonite calc-silicates and their implications for granulite pressure-temperature paths: an examplefromthe northern Prince Charles Mountains, East Antarctica. Hand, M., Scrimgeour, I., & Wilson, C J.L., 1992. Metamorphic evolution of granulitesfromthe northern Prince Charles Mountains: implications for Late Proterozoic decompressional P-T paths in East Antarctica. Abstracts, 29th IGC, Kyoto, 2,584. Harley, S.L., 1989. The origin of granulites: a metamorphic perspective. Geol. Mag. 126, 215-247. Harley, S JL. & Hensen, B J., 1990. Archean and Proterozoic high-grade terranes of east Antarctica (40-80°E): a case study of diversity in granulite fades metamorphism. In: High-grade metamorphism, Brown, M. (ed.) Unwin Hyman, pp 320-370. Nichols G.T. & Berry R.F. 1991. A decompressional P-T path, Reinbolt Hills, East Antarctica. /. Met. Geol. 9, 257-266. Stiiwe & Sandiford, (submitted). On the contribution of deviatoric stresses to metamorphic P-T paths: An example appropriate to low-P, high-T metamorphism. Submitted to /. Met. Geol.

REVOLATILIZATION BY AURIFEROUS FLUIDS The start to a century of gold mining at Kalgoorlie G Neil Phillips. Key Centre in Economic Geology, James Cook University, Townsville, Qld 4811, Australia Kalgoorlie reached a milestone of 100 years of gold mining in 1993. All up, nearly 1300 tonnes of gold has been won from this very small area of Western Australia, and reserves of 190 million tonnes @ 2.5 ppm Au suggest that several more years of mining are ahead (Homestake, 1993).

The substantial database of geological information on the Golden Mile at Kalgoorlie and the extensive underground access available in the early 1980's provided an outstanding opportunity to document the 3dimensional results of intense fluid activity in this area of a few square kilometres (Phillips, 1986). Geochemical tracers of this fluid activity include Au, C0 2 , K and S, and data on the distribution of these elements has been collected from the start of mining (but in different forms for quite different reasons)!

Hydrothermal alteration and Archaean gold mineralization at Kalgoorlie are recorded throughout an area covering 3 km by 1 km, and extending to 1 km depth. In the thick Golden Mile Dolerite sill, alteration that overprints regional metamorphic assemblages is zoned from an outer chlorite zone (1 km thick) to an intermediate carbonate zone (100 m thick) to an inner pyrite zone (1 m thick), with the latter being confined to brittle - ductile shear zones. Gold is strongly concentrated in the pyrite zone. Fluid channelling was influenced by the regionally important Boulder - Lefroy Fault, stratigraphic capping by a less-permeable pelitic sequence, and numerous smaller shear zones that were preferentially developed in the thick dolerite.

The alteration mineralogy of magnetite, hematite, pyrite, siderite and other carbonates, combined with chlorite geochemistry, allows calculation of log^O^], log[a(H2S)] and log[a(CC>2)] profiles. The activities of H2S and C0 2 are poorly contrained but high in the shear zones. Around the shear zone, these activities are constrained by mineral assemblages and only change where the mineral assemblage changes. Within the shear zones, many elements have been mobile and the number of stable minerals is limited. Outside the shear zones, within the alteration halo, a majority of elements were immobile and assemblages contain many co-exisiting minerals (Phillips and Gibb, 1993).

In the shear zones, fluid migration was by infiltration guided by mica-rich intervals. Fluid migration outside the shear zones was by pervasive infiltration on a grain - boundary scale, and is a reverse analogue of devolatilization during metamorphism. There is no evidence for strong chemical trends across individual alteration zones that might suggest significant diffusion in response to concentration gradients. Fluid / rock ratios were low (i.e. rock buffering was dominant) except within the pyrite alteration zone. Progressive alteration appears to have been driven by fluid infiltration. This infiltration was accompanied by higher levels of gold and related elements.

A metamorphic source is inferred for the fluid on the basis of its temperature around 300°C, the low salinity, and H 2 0-C0 2 composition; and gold was likely to have been complexed with sulphur.

The Golden Mile was an exceptional site for mineralization due to a number of factors being favourably developed. These include the chemical and mechanical nature of the Golden Mile Dolerite, regional structural geometry and the inferred thermal history of the Kalgoorlie area. The re-emergence of the Golden Mile in the 1980's has led to extensive open pit operations that have temporarily curtailed many underground operations. However, with the deepest workings less than 1.5 km deep, the Golden Mile is still shallow by comparison with some Indian, Brazilian, southern African and Canadian greenstone gold deposits that continue to almost twice this depth. The 1980's has seen a spectacular performance by gold as an export commodity for Australia: from a production of 17 tonnes in 1980 to 241 tonnes in 1991. Much of the rise in production has focussed on Western Australia, and much of the knowledge, innovations and skills base has concentrated around Kalgoorlie. Hence Kalgoorlie provides a natural workshop for understanding the role of fluids and then transferring this knowledge through the gold industry. Homestake Gold of Australia Ltd, anouncement to stock exchange, 26 January 1993. Phillips, G N, 1986. Geology and alteration in the Golden Mile, Kalgoolrie. Econ. Geol., 81, 779-808. Phillips, G N and Gibb, H C F, 1993. A century of gold mining at Kalgoorlie. Economic Geology Research Unit, James Cook Univ., Contrib. 45, 68p.

Figure. Map of the Golden Mile area, Kalgoorlie, showing the regionally expensive actinolite zone, the thick chlorite alteration zone, the narrower carbonate alteration zones, and the auriferous pyrite alteration zones (from Phillips and Gibb, 1993. 24

The Kiirunavaara magnetite-apatite deposit: A high temperature magmatic-hydrothermal deposit Kevin Blqk? Department of Geology, University of Wales College of Cardiff, PO Box 914, Cardiff CF1 3YE, United Kingdom..

Now at: Geology Department and National Key Centre in Economic Geology, James Cook University of North Queensland, Townsville Q4811, Australia.

The Kiruna iron ores of northern Sweden are an Early Proterozoic example of magnetite-apatite deposits collectively termed the "Kiruna-type". The largest of these deposits, Kiirunavaara, forms a sheet-like body over 4km in length, has an average width of 100m and is proved to a depth of 2km. It dips 50-60° E within an alkalirich host rock sequence of high level intrusive and volcanic rocks ranging from syenites to rhyolites (with subordinate andesites). Primary magnetite and titanite indicate the magmas had relatively high initial oxygen fugacities. These silicate rocks are dominated by alkali feldspar, typically displaying antiperthitic textures indicative of deuteric alteration.

Field relationships and textural features indicate ore formation was a multiphase intrusive event closely associated in time to the emplacement of the host rocks. Hydraulic brecciation and cementation of the host rock contacts by magnetite and actinolite testifies to the release of a high temperature volatile phase. The resultant alteration is typically associated with the introduction of magnetite, apatite, actinolite and titanite, both as disseminated and vein/fracture-fill mineralization. Mobilization of the rare earth elements and yttrium in the host rocks suggests fluorine/chlorine-rich ore-forming fluids. Similar rare earth element characteristics of the ore, apatite and host rocks also indicate a common source.

Whole rock and mineral separate oxygen isotope ( 8 1 8 0 ) signatures between +6 and +9%o indicate alteration of the the host rock (during ore formation) was caused by a magmatic fluid with minimum equilibrium temperatures of ~600°C. There is no evidence to suggest significant involvement of sea or meteoric water. The geochemical characteristics of the sequence are of rocks formed in a continental, within-plate environment, and combined with 8 1 v a l u e s further suggests an arid environment.

The combined features of the Kiirunavaara host rocks and ore indicate a high level environment in which ore formed from high-temperature, magmatic-hydrothermal fluids genetically associated with the host rock magmatism.

An overview of deformation, granite intrusion, metamorphism and mesothermal gold mineralization in the late Archean Hattu schist belt, eastern Finland. Peter Soijonen-Ward, Matti Pajunen, Geological Survey of Finland, SF-02150 Espoo, Finland, and Hugh E O'Brien, Department of Geological Sciences, A/-20, University of Washington, Seattle WA 98105, USA. The late Archean Karelian Craton, which is exposed throughout much of eastern Finland and adjacent parts of the Russian Republic of Karelia, has behaved as a coherent crustal unit since about 2.7 Ga. The overall dimensions of the craton, as well as the ages and relative proportions of granitoids, gneisses and greenstones, may be compared with the Murchison and Southern Cross Provinces of the Yilgarn Craton, and the Wawa and Quetico Subprovinces of the Superior Province. Episodic extension and widespread platform sedimentation and volcanism throughout the early Proterozoic preceded compressive deformation and intense thermal reworking of the margins of the craton at about 1.9 Ga. Thus, the Belomorian Orogeny resulted in the amalgamation of a number of late Archean and early Proterozoic tectonic units and caused thrusting and high-grade metamorphic reworking along the northeast margin of the Karelian Craton. At about the same time, the southwest margin, which includes the type area for Eskola's mantled gneiss domes, was deformed in an Alpine-style orogeny culminating in the juxtaposition of the Karelian Craton with juvenile Svecofennian crust between 1.90 and 1.85 Ga. This overview is concerned with the Hattu schist belt, which is a 2.75 Ga sediment-dominated supracrustal sequence in the southwestern part of the Karelian Craton, where outcrop and geophysical evidence from early Proterozoic mafic dikes indicate that tectonic reworking of Archean structures has generally been minimal. A distinct Proterozoic thermal overprint has nevertheless been recognized in the widespead resetting of Rb-Sr mineral and whole-rock systems, in K-Ar mica data and in disturbances to oxygen isotopes. In contrast, zircon, monazite and sphene have all retained Archean U-Pb ages. The Hattu schist belt consists predominantly of felsic pyroclastic and epiclastic deposits and fine-grained pelitic sediments, with sporadic mafic and ultramafic intercalations; primary depositional features are commonly preserved. Conventional U-Pb isotopic ages from some of the lowest exposed pyroclastic units (2754±6 Ma) overlap statistically with those obtained from syntectonic granitoids (2746±6 Ma) thus indicating rapid crustal formation and deformation. No depositional basement to the Hattu schist belt has been identified, nor has any direct evidence for a wholly allochthonous origin been preserved. However, Nd-Sm data and discordance patterns in conventional zircon data indicate that older material is present in both the granitoids and detrital sediments; SHRIMP analyses have recently confirmed the existence of xenocrystic zircon between 3.1-3.2 Ga in a granodiorite with a magmatic emplacement age of 2757±4 Ma (SoijonenWard & Claou6-Long, in prep.). The involvement of older continental crust in the tectonic processes that deformed the Hattu schist belt is further attested to by the presence of some highly evolved granitoids, including tourmaline-muscovite leucogranites. Although zones of anomalous younging with respect to F folds have been recognized (Figure 1), it is not clear whether F, folds were initially upright or reclined. A progressive rather than episodic interpretation of deformation is preferred, based on the general geometrical congruence of overprinting phases and the relatively short time span between volcanism, deformation and granitoid emplacement Thus, younger structures appear to represent the partitioning of deformation into more discrete, high-strain zones, with an increasing component of constrictional strain related to continued granitoid intrusion. Where early structures were in appropriate orientations, it has been possible to establish a close, and in some cases, sequential relationship between tightening of folds, attenuation of fold limbs, development of shear zones and duplexes 2

with strike-slip displacements, and the propagation of new folds due to strain incompatibilities. This can result in either distinct overprinting fabrics, or else transposition and recrystallization of earlier fabrics, thus requiring care in defining sequential deformational phases. A close association between deformation and granitoid emplacement is demonstrated by a group of biotite tonalite plutons which appear, at the present erosion level, to represent the coalescence of swarms of semiconcordant dikes. In outcrop these are commonly concordant with lithological layering so that it is difficult to determine their initial attitude and timing, but at a regional scale they may truncate folds or wedge along fold limbs. Thermal contact aureoles have not been recognized, but narrow, highly-strained and hydrothermally altered and recrystallized carapaces are sometimes present. These may represent local contact strain phenomena, but may also be linked to larger shear zones that accommodated emplacement of larger intrusions by transferring country rocks along strike, or upwards. The lenticular geometry of some plutons is not attributed entirely to post-emplacement strain since reticulate vein networks predating foliation development retain consistent orientations over considerable areas. Although episodic and incremental magma emplacement at releasing bends or into dilatant sites between overlapping shear zones is a reasonable paradigm for some intrusions, it is more difficult to account for the largest pluton intruding the schist belt with this mechanism. Gold mineralization in the Hattu schist belt is currently being evaluated and appears to be connected with syntectonic hydrothermal alteration. Two major zones of potential have been recognized. The more northerly one contains the largest single deposit and shows the most obvious structural control, being situated at the backfolded footwall at the toe of a duplex developed in a mafic-ultramafic sequence detached from the underlying graywackes and iron formations. At a regional scale this zone resembles a strike-slip releasing bend, but if so, it has clearly had a contractional history superimposed upon it. Hydrothermal alteration is characterized by potassium addition rather than albitization, so that northwards from this zone, pelitic sediments, felsic pyroclastic deposits and granitoids are extensively replaced by quartz-sericite-microcline assemblages. The second mineralized zone in the southern part of the schist belt is hosted primarily by metasediments surrounding one of the biotite tonalite plutons. Alteration in this zone tends to be diffuse and dispersed throughout pervasively deformed sequences rather than being restricted to particular mechanically or chemically favourable horizons or vein systems. Therefore it is difficult to identify local structural controls on alteration and mineralization. Widespread garnet and local staurolite porphyroblasts overprint S, and appear to post-date gold mineralization, as well as extensive sericitization of andalusite. Microstructural evidence clearly indicates intense deformation and dynamic recrystallization of hydrothermally altered assemblages and matrix surrounding porphyroblasts so that by implication, gold mineralization in this area predated the regional thermal metamorphic peak. The only constraint available so far on cooling history comes from concordant U-Pb sphene and monazite ages some 40-50 Ma younger than zircon ages obtained for the same rocks. Therefore, it is not certain whether porphyroblast growth is related to prolonged progressive metamorphism that outlasted gold mineralization, instead of a distinctly later event. Analysis of stable garnet-biotite intergrowths indicated temperatures around 510-540°C have been consistently obtained, irrespective of calibration and garnet manganese contents. In several critical areas staurolite and kyanite have been found, with kyanite growth clearly superimposed upon sericitized andalusite porphyroblasts. Although microstructural relations favour an Archean age for this kyanite, but in view of the Proterozoic reheating, this interpretation may be contentious.

tourmaline-muscovite leucogranitoids homogenous, weakly deformed adamellites. granodiorites and quartz diorites Kuittila-type restite-poor biotite tonalites migmatites with abundant s c h i s t inclusions

predominantly mafic p y r o c l a s t i c s , pillow lavas and doleritic intrusions bimodal, ultramafic to rhyolitic intercalations dominantly felsic to intermediate volcanites

a n o m a l o u s D i domain

zone of intense ductile strain

D 2 fold v e r g e n c e

Au-mineralization

feldspathic epiclastic and pyroclastic deposits

Dynamic coupledfluidflowand deformation A. Ord and B.E. Hobbs, CSIRO, Division of Exploration and Mining, Floreat Park, Wembley, WA. 6014. Many mineral deposits are the result of rapid changes in fluid pressure or fluid properties with a resultant rapid change in deformation characteristics. Since fluid pressures and deformation are coupled for pressure sensitive constitutive relations, changes in deformation characteristics also lead to changes in fluid migration paths in a deforming medium. Examples here include the formation of vein hosted deposits during rapid venting events between lithostatic and hydrostatic fluid pressure regimes, boiling of hydrothermal fluids and dynamic pressure changes associated with dilatant faulting. Our understanding of the formation of ore deposits has evolved through predictions of fluid flow according to pore pressure changes as follows: 1. static, time independant, non-coupled analysis; 2. static, time independant, coupled analysis; 3. static, time dependant, coupled analysis. The time dependency here refers to allowed histories of, for example, permeability with volumetric strain. We investigate here a fourth stage of evolution, dynamic, time independant, coupled analysis, before again allowing various history-dependant changes. A finite difference grid is used to model a crust, formed of a simple, non-hardening, Mohr-Coulomb material. A fault is formed within the crust, and the whole is brought to equilibrium with fluid present. Shaking is then allowed to occur along the fault, and volume changes,fluidflow and changes in other conditions and properties are investigated. In another case, fluid flow changes are investigated, associated with the apparent 'boiling' of a fluid in an inclusion when the increasing fluid pressure reaches a critical point, this problem is the basis for investigations of zoning of hydrothermal vein deposits with time, and cross-cutting, brecciated and highly mineralised veins. The complexity of the problem increases through the problems studied: 1)fluidflowassociated with 'boiling' in a single inclusion within a Mohr-Coulomb material undergoing deformation; 2) similar to (1) but tailing' is now modelled for a vein of finite thickness cross-cutting the deforming medium; 3) similar to (2) but for a system of veins and the associated hydrothermal activity. The entire, highly complex problem of predicting fluid flow in a deforming, heterogeneous crust is approached incrementally through adding complexities, in boundary conditions and in material properties, to this simple model.

The dynamics of permeability changes and fluid migration in metamorphic environments

Stephen F Cox. Shuqing Zhang and Mervyn S Paterson, Research School of Earth Sciences, The Australian National University, Canberra, ACT 0200, Australia. The architecture and dynamics of fluid circulation in the Earth's crust are influenced by spatial and temporal variations in rock permeability and by the forces driving fluid migration. This paper outlines recent highpressure/high-temperature experimental studies, together with microstructural and field observations, that provide insights about the nature of crustal p e r m e a b i l i t y and controls on fluid migration during metamorphism. We highlight the time-dependence of crustal permeability, and the roles of active deformation and high fluid pressures in controlling its evolution. In doing so, we emphasise the importance of cyclic variations in shear stress and fluid pressure, and the associated competition between porosity-creation and porosity-destruction processes, in controlling the dynamics offluidmigration in actively deforming structures. Fluid Migration and Permeability

Fluids migrate through the crust in response to gravitational (buoyancy) forces and pressure gradients. These driving forces, in conjunction with the permeability distribution, control the geometry and magnitude of fluid flow. Laboratory measurements at high confining pressures indicate that in the absence of high fluid pressures the permeabilities of unfractured metamorphic rocks at depth in the Earth's crust should be less than 10' Time-integrated fluid fluxes greater than 10' m m' are unlikely to be achieved on the time-scales of regional metamorphism if permeabilities are less than 10~ m . However, fluid budget and mass balance calculations indicate that time-integrated fluid fluxes as high as lO^^m" , and transient permeabilities as high as 10" m may not be uncommon during regional metamorphism. These observations raise questions about the nature of permeability during metamorphism, how it varies in time and space, and what factors influence its evolution during crustal metamorphism. As permeability is sensitively dependent on the response of a rock mass to the stress regime, we discuss the nature of permeability firstly in isostatic stress regimes, and secondly during deformation. 2

Isostatic Stress Regimes

In high-temperature isostatic stress regimes, permeability is dependent on the degree of connectivity within a pore network whose topology is strongly influenced by surface energy effects. At low porosities, pore connectivity tends to be governed by the dihedral wetting angles between porefluidand minerals. For dihedral angles less than 60°, an interconnected network of fluid-filled channels develops along grain edges. At wetting angles greater than 60°, the equilibrium fluid distribution consists of isolated fluid-filled lenses on grain interfaces and grain edges, so pore connectivity tends to be lost at low porosities. The relationship between porosity and permeability in an isostatic stress regime is illustrated by the experimentally-determined evolution of these parameters during hot-pressing of calcite aggregates in the presence of argon pore fluid. This system has a dihedral wetting angle around 80°. At high porosities, connected porosity (<t>) is equal to total porosity (<|>), and permeability (k) is related to porosity by a relation of the form k « <(> , (1) where n has a value around 3. When porosity falls below 7%, the ratio <t>/<t> decreases substantially and permeability becomes much more sensitive to changes in porosity. A percolation threshold, at which <|> becomes zero, occurs at a porosity of 4%. The results indicate that, for fluid-mineral systems with dihedral angles greater than 60°, pore connectivity will be completely lost at porosities as high as several percent. For c

situations in which wetting angles are less than 60°, pore connectivity is maintained to somewhat lower total porosities. Recent experimental studies (Watson & Brenan, 1987; Laporte & Watson, 1991; Lee et al, 1991; Holness and Graham, 1991; Holness, 1992) have examined the wetting characteristics of a range of fluid compositions in quartz and calcite agggregates, and shown that dihedral angles, and hence pore connectivities and permeabilities, are particularly sensitive to fluid composition and fluid pressure. Significantly, in quartz-rich rocks at most midto lower-crustal conditions, and especially where CO2 is a component of the pore fluid, dihedral angles tend to be greater than 60°. Available experimental data suggest that under hydrothermal conditions, porosity reduction rates are sufficiently rapid that intergranular porosities greater than a few percent will have geologically short lifetimes (Cox & Paterson, 1991). Accordingly, quartz-rich rocks will tend to have porosities below the percolation threshold and will be aquitards in many static metamorphic regimes unless the development of supralithostatic fluid pressures promotes transient hydrofracture. Fracture-Controlled Flow During Deformation A wealth of field and microstructural evidence indicates that pore fluid distribution during metamorphism is probably controlled more by fracture porosity than by equilibrium pore geometries (Cox et al, 1987; Cox & Etheridge, 1989). The widespread distribution of extension veins in metamorphic terranes attests to the importance of high fluid pressures and active deformation in promoting the development of macroscopic fracture arrays. However, except adjacent to faults and shear zones, these fracture networks have poor connectivity and are unlikely to control pervasive fluid infiltration. Instead, pervasive fluid circulation is most likely governed by percolation through deformation-induced, grain-scale fracture porosity. The importance of deformation in controlling permeability is illustrated by the evolution of fracture porosity and permeability during experimental deformation of marble. The fluid transport properties are found to be very sensitive to strain and effective confining pressure. For example, during room temperature shortening at an effective pressure of 30 MPa, permeability increases by two orders of magnitude after only 3% shortening. This dramatic permeability increase is associated with the formation of a pervasive grain-scale microcrack network. At progressively higher effective pressures, permeability enhancement becomes less substantial because deformation occurs more by intracrystalline plasticity than by brittle processes. The experiments demonstrate the important role of fluid pressure in controlling permeability during deformation, and suggest that large fluid fluxes in the crust may be facilitated by quite low strain in near-lithostatic fluid pressure regimes. The results also highlight the importance of localised deformation (eg in active faults and shear zones) for controlling the architecture of fluid migration in tectonically active regimes. At elevated temperatures, fracture-healing and fracture-sealing processes compete with deformation-induced porosity-creation processes. The evolution of permeability during fracture healing in deformed marble is used to illustrate the time-dependence of fracture permeability and the rapidity with which fracture healing can reduce porosity below the percolation threshold at elevated temperatures. These results, together with the microstructure of crack-seal veins and experimental observations of crack-healing in quartz in hydrothermal environments (Brantley et al, 1990; Brantley, 1992), indicate that fracture lifetimes are likely to be extremely short at mid- to lower-crustal depths. Therefore, the time-dependence of permeability in deforming metamorphic rocks will be critically dependent on the dynamics of competition between crack growth and processes leading to the healing and sealing of cracks.

Dynamics of Permeability Variations and Fluid Migration in Actively Deforming Rocks As the balance between porosity growth and porosity reduction in actively deforming rocks is strongly dependent on variations influidpressure and stress difference, any coupling between fluid pressure and stress history will be an important factor influencing fluid pressure gradients and the dynamics of fluid migration. Coupled, timedependent fluctuations in shear stress and fluid pressure associated with the seismic cycle are used to illustrate how transient fluid pressure gradients and time-dependent changes in permeability can influence fluid migration, fluid mixing, and ore deposition in metamorphic environments. References Brantley, S L, 1992. Earth & Plan SciLett 113, 145-156. Brantley, S L, Evans, B, Hickman, S H & Crerar, D A, 1990. Geology 18,136-139. Cox, S F & Etheridge, M A, 1989. J Struct Geol 11, 147-162. Cox, S F & Paterson, M A, 1991. Geophys Res Lett 18, 1401-1404. Cox, S F, Etheridge, M A & Wall, V J, 1987. Ore Geology Revs 2, 65-86. Holness, M B, 1992. Earth & Plan Sci Lett 114, 171-184. Holness, M B & Graham, C M, 1991. Contr Mineral Petrol 108, 368-383. Laporte, D & Watson, E B, 1991. J Geol 99, 873-878. Lee, V W, Mackwell, S J & Brantley, S L, 1991. J Geophys Res 96, 10023-10037. Watson, E B & Brenan, J M. 1987. Earth & Plan Sci Lett 85, 497-515.

MECHANISMS OF FLUID TRANSFER IN THE MID- TO UPPER CRUST

OLIVER. Nicholas H. S.. Department of Earth Sciences, Monash University, Clayton, Australia, 3168 ORD, Alison, C.S.I.R.O. Division of Exploration and Mining, P.O. Box 54, Mt Waverley, Australia,

3165.

Mechanisms for kilometer-scale pervasive fluid flow during regional metamorphism remain problematic, despite documentation of regional petrological and stable isotopic changes accompanying Paleozoic regional metamorphism in the northeastern USA and elsewhere. Such giant hydrothermal cells require an inter-connected porosity and a large regional gradient in hydraulic head (possibly thermally controlled) that dominates over perturbations in hydraulic head due to local deformation. For this type of fluid flow, fluid flux rates calculated using petrologic or stable isotopic data are quite slow.

In contrast, dynamic or transient porosity inter-

connection and fluid flow accompanying deformation of heterogeneous rock suites should result in moderately to strongly channelized and locally quite rapid flow, of which there are several examples in the literature. The upper limit to the flow rates are those involved in dilatancy pumping during earthquakes, perhaps at rates on the order of metres/sec. How such types of flow could interact is more than difficult to determine.

Slow flow through a pervasive grain-boundary network in permeable rocks allows complete chemical and isotopic exchange between the rocks and the through-going fluid. In contrast, rapid flow in larger cracks with less permeable walls results in minimization of the degree of fluid-rock interaction. Variations on these two endmembers, along with examples of short-distance fluid flow, provide a basis for categorizing mid- to upper-crustal fluid flow, and perhaps thus allowing some rationale for understanding fluid flow mechanisms (Figure). This diagram describes fluid flow in terms of the metre-scale behaviour apparent from structural, petrological and stable isotopic studies in a range of tectonic settings. Degree of equilibration between fluid and rock Permeability Grain-scale Meso-scale fractures

Low Closed system ^ unfractured

Moderate 1

Scale dependant

Closed system unfractured

open system fractured reactive

open system fractured unreactive

High

open system pervasive/reactive closed system fractured

Detailed work in the Mary Kathleen Fold Belt in northern Australia reveals that long-distance peak-metamorphic amphibolite-facies fluid flow has occurred predominantly in specific channels, the location of which is clearly related to deformation. Geometric, stable isotopic and petrological studies of widespread calcite veins indicate a) the fluid was dominated by externally derived fluids with only a small local fluid contribution, b) fluid transfer occurred at kilometer scales, and c) the bulk of fluid flowed through individual fractures locally in excess of 5 meters wide, and brittle-ductile shear zones.

Although this may be an exceptional example of structurally

focussed fluid flow, it requires substantial local perturbations of hydraulic head at a scale that would destroy the integrity of any km-scale pervasive fluid flow pattern. Models for convection in the mid-crust have received only limited support, because of the difficulty of preserving interconnected cells under assumed lithostatic fluid

pressure gradients. However, recent papers by Ferry and co-workers (e.g. Ferry, 1992) imply a thermal control on long-distance fluid flow, without specifically suggesting convection.

Finite difference modelling may be used to simulate deformation in permeable continuua, and also in fractures (see Ord and Hobbs abstracts, this volume): coupling these two processes mathematically is decidedly non-trivial. Continuum models of deformation of relatively homogeneous permeable crust surrounding a large pluton adequately simulates the type of fluid flow pattern proposed by Ferry (1992) for giant mid-crustal hydrothermal systems, in which fluid slowly flows and equilibrates along gentle gradients in temperature and hydraulic head. This type of flow contrasts strongly with inferred rapid fluid transfer and minimal wallrock interaction at Mary Kathleen and for several other examples where deformation plays a controlling role at km-scales. Key controls on the difference between these two end-member scenarios may thus be the strain rate and the degree of homogeneity of the rocks being deformed and metamorphosed. Resolution of these types of problems may be improved using integrated field, petrologic/isotopic and finite difference modelling techniques, and has important implications for both local and regional analysis of mineralized hydrothermal systems.

Ferry, J. M., 1992. Regional metamorphism of the Waits River Formation, Eastern Vermont: delineation of a new type of giant hydrothermal system. J. Petrol. 33, 45-94.

Fluid flow associated with metasomatism of the Revenue Granite and aureole: origin, timing and spatial relations, Mary Kathleen - Duchess Fold Belt, Mt Isa Inlier, Queensland, Australia. Tania Aslund, Ian Cartwright, Nick Oliver: Department of Earth Sciences, Monash University, Clayton, Victoria 3168, Australia.

Oxygen and carbon isotope data indicate that the salt-rich fluids resulting in high temperature alteration of the Revenue Granite and aureole were derived from a magmatic source. Field relations and geochemistry suggest that these fluids circulated at least once through the granite-aureole system. This study has important practical implications for current theoretical models of fluid-flow around intrusions. The study area is located within the Mary Kathleen - Duchess Fold Belt, approximately 65 km SE of Mt Isa, NW Queensland. The country rocks consist of Proterozoic calcareous and evaporitic sediments of the Corella Formation, a part of the Eastern Succession of the Mt Isa Inlier. The formation is approximately 2km thick and comprises sediments deposited during the sag-phase of basin-forming rifting, between 1780-1760 Ma (Page 1983a,b). The lower part of the Corella Formation consists of large amounts of metasiltstones and minor calcsilicate rocks, intruded by doleritic sills and overlain by a thick sequence of calcsilicate rocks and marbles. The upper Corella Formation comprises volcanogenic sediments and pillow basalts interbedded with minor calcsilicates, pelites and quartzites. The calcsilicate rocks throughout the formation have a significant evaporite component. The region has undergone three main phases of deformation (Oliver et al. 1991): D\ subhorizontal extension (-1740 Ma); T>2 E " W compression that coincided with peak amphibolite facies metamorphism (1620-1550 Ma); and a later retrograde event or sequence of events associated with faulting. Typically, Dj fabrics consist of an originally subhorizontal foliation and a shallow, variably oriented stretching lineation. D2 fabrics include tight to isoclinal, variably plunging upright folds and a steep N-S trending foliation with a subvertical stretching lineation. The Revenue Granite was intruded as a sill into the lower-mid Corella Formation during D\ extension, probably at depths near 5km. The sill is broadly parallel to the regional N-S S2 foliation due to large-scale D2 folding. However, D\ fabrics dominate within the Granite and its surrounding aureole, where strong S\ and Li fabrics are only locally reoriented by F2. The western margin of the granite represents the roof zone of the sill; the basal zone of the sill is exposed along the eastern contact (fig.l). The host rocks immediately west of the granite (i.e. above) are dominated by amphibolitised dolerites. East of the sill (i.e. below) calcsilicate and metasiltstone are the dominant rock-types. Metasomatic alteration comprises albitisation and scapolitisation +/- pyroxene veining. Above the granite, conspicuous metasomatism is restricted to the dolerites (fig.l), which appear to have acted as fluid conduits. The dolerites presumably behaved in a more brittle manner than surrounding sediments, preferentially fracturing and thus channelling the salt-rich metasomatising fluids. Alteration within the granite is invariably associated with strong Di fabrics; fluids were probably channelled along zones of high ductile strain during D\. The individual minerals show high temperature (600-800°C) oxygen isotopic fractions indicating that there has been no resetting after the peak regional metamorphism (D2). The predominance of D] fabrics within the Revenue Granite and aureole suggests recrystallisation during D2 was negligible. Thus, presently observed isotopic signatures probably reflect D] rather than D2 processes. Whole-rock oxygen isotope data (8 1 8 0\yR, relative to SMOW) are presented in figure 2. Altered granite from the upper part of the sill typically has 5 18 0\VR-values that are at least 1 permil (%©) lower than those of the adjacent unaltered granite (9.1-9.5%© vs. 9.6-10.4%©, respectively). Similarly, hornfelsed sediments have lower 5 , 8 OwR- v alues (8.8-11.0%c) than that expected for typical pelites (14 to 15%©) and calcsilicates (>20%©). The metasomatised dolerites above the granite have higher 818OWR-values (8.5 to 9.7%©) than unaltered dolerite (6 to 7%c). Thus, alteration of a wide range of rock-types exhibit remarkably similar oxygen isotope values. Samples from the middle and lower portions of the sill, however, exhibit trends of increasing rather than decreasing 8^ 8 OwR" v a l u e s - T h e 5 1 8 0 \ V R " v a l u e s o f t h e altered granite are 1 to 1.5%© higher (11.0-11.2%©) than the adjacent unaltered granite. This is unexpected as the mineralogy of the alteration is identical throughout the system. It is possible that variations in temperature significantly altered subsequent 8 1 8 0-values and trends. The overall oxygen isotopic signature of the metasomatising fluids is comparable to that of granite, suggesting the fluids were either derived from or last equilibrated with granite. C-isotopes from altered rocks of the Revenue granite and aureole also give signatures indicative of a magmatic origin for the metasomatising fluids. 5 13 C-values obtained from scapolite range from -4.5 to -6.5%©, and fall within the carbonatite box' for juvenile or magmatically derived fluids (fig.3). By contrast, sediment-derived carbon characteristically gives a signature of approximately 0%©, and thus did not contribute significantly to the metasomatising fluids.

In conclusion, the Revenue Granite was intruded and metasomatised during Dj. The O- and C-isotopic signatures of the alteration within the granite and aureole suggest the salt-rich metasomatising fluids were derived from a granitic source. Significant fluid circulation occurred during metasomatism, which resulted in the redistribution of some components such as iron. Temperature variation within the granite-aureole system may account for the spatial variations in the oxygen isotopic ratios. This may be a function of changing the position and size of the postulated convection cell, with the locus of convection moving downwards as the granite cooled and crystallised from the top. References: Page, R. W. 1983a - Timing of superposed volcanism in the Proterozoic Mt Isa Inlier, Australia. Precambrian Research 21,223-245. Page, R. W. 1983b - Chronology of magmatism, skarn formation and uranium mineralization, Mary Kathleen Queensland, Australia. Economic Geology 78, 838-853. Oliver, N. H. S.; Holcombe, R. J.; Hill, E. J. and Pearson, P. J. 1991 - Tectono-metamorphic evolution of the Mary Kathleen Fold Belt, northwest Queensland: A reflection of mantle-plume processes? Australian Journal of Earth Sciences 38, 425-455.

Figure 1: schematic cross-section through the Revenue Granite showing distribution of oxygen isotope data

East

West 8.5-9.7%o

9.J-9.5%c 11-11.2%c

a 1 3 cr

8.8-11.0%o

~100m

'CARBONATITE — 51 BOX'

H2 I—6I—I—10I 1—I14 1 181—I—22I 1—h-

alteration Revenue Granite dolerite hornfels

9.6-10.4%

MKFB METASEDIMENTARY ^ — V CALCITE

Figure 3: C- and O- isotope data from scapolite

Figure 2: wholerock oxygen data ^ ^ ^ ^ typical dolerite

x xxxxxx x pyroxene-scapolite 'skarn' within amphibolite xx x x albite- or scapolite- bearing amphibolite xhornfelsed x x xxsediment x x^:

altered granite

(upper sill) xx

7.0

8.0

9.0

(lower sill) . xx unaltered granite

10.0 11.0 12.0 9 1 8 0 W R (%.) 36

typical Kpelite

13.0

14.0

15.0

Fluid flow and alkali-silica metasomatism within a serpentine melange from a glaucophanitic metamorphic terrain - the Nagasaki metamorphic rocks, southwest Japan. Naoko Sato, Tadao Nishivama, and Takeru Yanagi, Department of Earth and Planetary Sciences, Kyushu University, 33, Fukuoka 812, Japan. This paper reports fluid flow and alkali-silica metasomatism within and adjacent to a tectonic melange zone between serpentinite and crystalline schists from a glaucophanitic metamorphic terrain. Albitites sometimes associated with jadeitite widely occur in serpentinites from glaucophanitic metamorphic terrains in the world. The origin of albitites, however, has been still controversial. They have been considered either as metasomatic products during glaucophanitic metamorphism or as oceanic plagiogranites. We found clear evidence of transformation of metapelites and metabasites into albitites in the Nagasaki metamorphic rocks, southwest Japan, and clarified the nature of metasomatism based on isocon analysis ( Grant, 1986: Econ. Geol. 81, 1976-82. ). We have also examined the problem of selecting an isocon which has arisen in the practical application of the isocon analysis. The Nagasaki metamorphic rocks in the Nomo Peninsula consists of three lithologic units: the older metagabbro complex ( 450 - 580 Ma ), the serpentinite complex ( 90 Ma), and the crystalline schists unit ( 70 - 88 Ma ). They are in fault contact with one another, and tectonic melange zones develop along the contact between the serpentinite complex and the crystalline schists unit. Alkali amphiboles ( magnesioriebeckite and crossite ) and alkali pyroxene ( omphacite ) occur in metabasites from the serpentinite complex and also from the melange zone, although none of them occur in the crystalline schist unit. Miyazaki and Nishiyama ( 1989: Mem. Geol. Soc. Japan, 33, 217-236 ) defined three mineral zones in the crystalline schists unit based on mineral assemblages in metapelites: chlorite, garnet, and biotite zones in order of ascending metamorphic grade. The metamorphic conditions have been estimated as 450 - 510 °C and 7 - 10 kb for the biotite zone based on garnet - biotite geothermometer and plagioclase - biotite - garnet - muscovite geobarometer ( Miyazaki, pers. comm.). Albitites occur in the melange zone and also nearby the crystalline schists belonging to garnet zone. They show three types in mode of occurrence : 1) albitites replacing metapelites, 2) albitites as blocks in a serpentine matrix ( a serpentine melange ), and 3) albitites as blocks and as selvages around metabasite blocks in a melange with chlorite and chlorite - actinolite schists matrix (an ultramafic melange ). We have collected serial samples from albitites to the original rocks, determined their major chemistries with XRF, and examined mass balance by Grant's isocon analysis. We identified two stages of albitization in metapelites. The initial stage is characterized chemically by gains of AI2O3, Na20 and K2O with losses of MgO and CaO, and mineralogically by disappearance of actinolite and graphite with increment of albite and phengite. The advanced stage is characterized chemically by gains of AI2O3 and Na2<3 with losses of SiC>2 and K2O, and mineralogically by disappearance of phengite and quartz with increment of albite (Fig.l). These albitizations may be caused by the fluid derived from dehydration reactions between serpentinite and crystalline schists, which enhanced tectonic transposition of serpentinite complex by the mechanism of reaction-enhanced ductility. The fluid may have dissolved Na+ at the deeper level, migrated upwards through the melange zone, and reacted with metapelites and metabasites within and nearby the melange zone to form albitites at the shallower level. A serious problem has arisen at the practical application of isocon analysis, which evaluate gains and losses of components based on inert components, of which array in an isocon diagram is called " isocon In practical application, however, most isocon diagrams give two or three arrays of components through the origin, which makes it difficult to define the isocon. To resolve this problem, we have made three isocon diagrams for three serial samples, showing slightly different degree of albitization, plotted against the same protolith. In principle, these isocon diagrams should define the same isocon ( a straight line through the origin with the same slope ), therefore, comparison of these diagrams allows us to select the isocon within uncertainties of original compositions and chemical analyses. As shown in the superimposed isocon diagrams (Fig. 2), the isocon selected by this method gives the systematic change of mobile components according to the progress of albitization ( shown by arrows ), which validates our method.

metapelite

albitite

-T20 C

D1 0 2

ao E

40 60 G1 G2 G3 SAMPLE No

Fig. 1. Modal cross section of albitite replacing metapelite in the crsyalline schist unit nearby the melange zone. Metapelite mainly consists of albite, quartz, phengite and chlorite, whereas albitite mostly consists of albite with small amounts of chlorite, quartz and others and lacks phengite.

£ 100

0) o> 03 Q)

I I I I Nil

I I I ITIT

21-7G-1

•

21-7G-2

I I I Nil

21-7G-3

<0 O hi

™ 0.1

0.01 0.01

M l l l l

0.1

10 21-7A-1

100 wt%

Fig. 2. A superimposed isocon diagrams for three albitite samples plotted against the same protolith ( metapelite ). The isocon is selected to be common to the three samples, and defined as a narrow zone indicated by two thin lines. The arrow indicates the direction of albitization. Most mobile components show systematic change, upwards ( components gained ) or downwards ( components removed ) from the isocon, as the albitization proceeds.

EVOLUTION OF FLUIDS IN DUCTILE OUT-OF-SEQUENCE THRUSTS IN THE CALEDONIDES FROM ARCTIC NORWAY Andrew J. Barker, Department of Geology, The University, Southampton, S09 5NH, UK The 0se Thrust of northern Norway represents a major late Caledonian out-of-sequence thrust (Fig. 1), which duplicates a previously assembled nappe stack with inverted metamorphic gradient (Anderson et al., 1992). Phyllonites with well developed S-C-C fabrics are widespread in the hangingwall towards the basal contact. Retrogression associated with fluid movement along the 0se Thrust gave rise to retrogression of peak metamorphic (amphibolite facies) porphyroblastic phases in both footwall and hangingwall. The fact that chlorite pseudomorphs after garnet are not smeared out suggests that much of the retrogression post-dated thrust movement. Dominated by vein quartz, the 0se Thrust zone and adjacent footwall and hangingwall rocks record five structurally distinct vein types (stages) in a protracted history of fluid flow. In simple terms these can be grouped as: \ (peak metamorphic); V /V (late D ); V /V (Retrogression related post-D veins, crosscutting schistosity and C-surfaces at moderate or high angle). The proportion of different vein types varies considerably between footwall and hangingwall rocks, largely as a function of bulk rock composition and intensity of fabric development at different stages in the tectonometamorphic evolution. x

Preliminary fluid inclusion studies have revealed that the record of peak metamorphic fluids is largely lost due to decrepitation of inclusions but that the fluids present at this stage were dominantly aqueous. V /V veins contain H 0-C0 fluid inclusions whereas the V /V late-stage quartz associated with retrogression contain highly saline aqueous fluid inclusions with abundant daughter minerals. These are comparable to fluids described by Bennett & Barker (1992) associated with retrogression in early ductile thrusts from the same region. This indicates that irrespective of age, the various thrust zones within the upper allochthon of the Scandinavian Caledonides acted passively as important conduits for late-stage fluid flow and retrogression during uplift. 2

References: Anderson, M.W., Barker, AJ., Bennett, D.G. & Dallmeyer, R.D. (1992). A tectonic model for Scandian terrane accretion in the northern Scandinavian Caledonides. Journal of the Geological Society, 149, 727-741. Bennett, D.G. & Barker, A.J. (1992). High salinity fluids, the result of retrograde metamorphism in thrust zones. Geochimica et Cosmochimica Acta, 56, 81-95. 39

ESE

WNW ®0 OFOTEN SYNFORM

^W TORNETRASK

" ^vcW>> , a W E

H-

BALTIC SHIELD -j- TI-IRUSTS WITH PREVIOUS [Bj = BENNETT (PhD. 1991) FLUID INCLUSION STUDY : [G] = GIBSON (1990) UNDERGRADUATE PROJECT [M] = McBRIDE (1989) UNDERGRADUATE PROJECT METAMORPHIC GRADE OF NAPPES: ANCHI = ANCHIZONE Bt = BIOTITE

EARLY DUCTILE THRUSTS

Grt = GARNET

FRONTAL IMBRICATES

Stt = STAUROLITE

LATE OUT-OF SEQUENCE DUCTILE THRUSTS

Ky = KYANITE Sil = SILLIMANITE

NOT TO SCALE:

MAX. HORIZONTAL ~ 130km

MAX. VERTICAL ~ 7km

Assessment of fluid infiltration in an 'unconformity related contact metamorphic terrain: Arkaroola, northern Flinders Ranges 1

Annette Bingemer, Mike Sandiford

Department of Geology and Geophysics, University ofAdelaide, Adelaide 5050, Australia Neoproterozoic Adelaidean sediments which form cover sequences unconformably overlying the medium to high grade Midproterozoic Mt. Painter Inlier in the northern Flinders Ranges, preserve evidence for significant metamorphic fluid flow as well as metasomatism. Mapped sequences of metamorphic isograds in calcsilicates and pelites of the cover sediments indicate dramatic lateral temperature gradients of up to 35°C/km approaching the unconformity. The mineralogy, including widespread scapolitization of a variety of rock types, suggests multiple fluid infiltration phases associated with prograde, syn- to postkinematic lower amphibolite facies and retrograde greenschist facies metamorphism. Marbles within the cover sequence indicate peak temperatures of 520°C for P=3kbar (depending on fluid Xc02) defined by the transition tremolite - diopside. Andalusite- and cordierite-bearing assemblages in pelites of the cover indicate peak metamorphic PT-conditions of 510° C and <4 kbar. These high-temperature low-pressure conditions found in the Adelaidean cover sediments developed at a shallow crustal level and are unlikely to have been generated by conductive heat flow only. Thermal modelling by Mildren et al. (see this abstract-volume) reveals that heat refraction alone is insufficient to explain the observed thermal signature but may have been sufficient to induce the advective hydrothermal activity. Additional advective heat flux associated with high temperature fluids must have made an important contribution to the thermal perturbation in this area. The arrangement of the metamorphic isograds suggests that fluid flow has occured upward from the basement, based on the progressive metamorphic grade with decreasing distance to the basement-cover contact. In order to assess sources and pathways of the fluid flow events, carbon and oxygen isotope profiles across the Adelaidean carbonates have been analysed. While oxygen isotopic signatures with values of 21.4%o down to 10.8%o indicate a general depletion in S 0 with decreasing distance to the basement-cover contact, there are significant variations in 8 0 along profiles with values of 15.2%o in the centre down to 10.8 %o at the boundary layers on the margin of the marbles. Additional channelized fluid flow, focussed along structurally induced zones of high permeability (e.g. faults), is indicated by the sudden lowering of 8 0 signatures along strike. The general trends of isotopic alteration in 6 0 by as much as 12%o with increasing metamorphic grade and decreasing distance from the basement-cover contact, support the notion that enhanced fluid infiltration and up temperature fluid flow may have contributed to the anomalous thermal regime in this 'unconformity related' contact metamorphic terrain. 1 8

1 8

Composition and source of fluids in metamorphosed non-marine evaporites, Olary Block, South Australia Nick Cook, Department of Geology and Geophysics, University ofNew England, Armidale 2351, Australia. The Willyama Supergroup in South Australia comprises a sequence of Proterozoic metasedimentary and metaigneous rocks, within which are numerous small stratiform, stratabound and vein mineral deposits. Rock types include abundant albite-rich rocks (albitites and calcalbitites), calcsilicate rocks and impure marbles, in addition to "normal" metamorphosed clastic rocks. Recent data (Cook and Ashley, 1992) suggest that parts of the sequence, in particular the albitites, calcalbitites and possibly the calcsilicates and marbles, were formed in an evaporitic non-marine environment. The albitites are inferred to have formed through many processes, including the metamorphism of analcime-rich clastic or pyroclastic rocks and pre-, syn, and post-metamorphic hydrothermal alteration. Evidence for the presence of evaporites in the sequence includes the occurrence of hyper-saline fluid inclusions, an abundance of halogen-rich minerals, boron isotope data and pseudomorphs after probable evaporite minerals. Multiphase (solid, liquid and vapour) salinefluidinclusions are present in many of the calcareous rocks, mostly in quartz, clinopyroxene, fluorite, scapolite and apatite. The most common solid phase in the inclusions is halite, but some inclusions contain up to three, and in one case, four solids, of which anhydrite is tentatively identified. The high fluid salinities are inferred to have formed largely from the diagenetic and early metamorphic dissolution of evaporites (e.g. Behr and Horn, 1982), although in retrograde shear zones concentration of NaCl in thefluidphase is likely (e.g. Bennett and Barker, 1992). Microprobe analyses of hydroxyl-bearing minerals, for example scapolite, biotite, amphibole and titanite indicate them to be unusually enriched in CI and/or F. Calculated fluid halogen compositions indicate substantial variability between thin layers in the calcsilicate rocks and impure marbles. Mineral assemblages in the calcsilicate rocks indicate the minerals equilibrated with a I0W-XCO2 fluid, suggesting decarbonation reactions did not overwhelm the fluid composition, perhaps suggesting external buffering. This is however, incompatible with the inferred small-scale (cm) variation in CI and F in the fluid. Behr, H.J., and Horn, E.E. 1982. Fluid inclusion systems in metaplaya deposits and their relationship to mineralization and tectonics. Chemical Geology 37: 173-189. Bennet, D.G. and Barker, A.J. 1992. High salinity fluids: the result of retrograde metamorphism in thrust zones. Geochimica et Cosmochimica Acta 56: 81-95. Cook, N.D.J, and Ashley, P.M. 1992. Meta-evaporite sequence, exhalative chemical sediments and associated rocks in the Proterozoic Willyama Supergroup, South Australia: implications for metallogenesis. Precamhrian Research 56: 211-226.

Talc formation in the Mt Lofty Ranges, Sooth Australia - a review Robin L. Qlivyr and Alex Christ, Department of Geology and Geophysics, University of Adelaide, Adelaide 5000, Australia. Like many metamorphic minerals, talc would seem to have a variety of geneses, within the limitations of equilibrium pressure and temperature, dependent on how the pressure and temperature came to be and, also, on how the appropriate compositional environment was obtained. In South Australia there are a number of talc deposits, one, at Mt Fitton in the northern Flinders Ranges, being actively mined and producing about 10% of Australia's output This occurrence of talc, and/or tremolite, presumbly results from inteiraction of associated dolomitic marble and siliceous fluids. The talc deposits in the Mt Lofty Ranges outcrop most abundantly in the vicinity of Gumeracha and 20-25 kms to the north in the Pewsey Vale area, east of the Barossa Valley (Fig 1). Figure 1 shows also the deposits to be at the margin of metamorphic sillimanite grade terrane which is bounded to the west by a substantial north-south trending fault, parallel to the regional strike. Hosting the talc occurrences are metasediments of the Adelaidean (Late Proterozoic) sequence. In the Pewsey Vale area the field lithologies are: i) predominant bi-mica schist, with andalusite and sillimanite, ii) calc-silicate lenses, consisting of actinolitic amphibole ± epidote ± scapolite ± diopside ± K feldspar ± quartz, iii) quartzite, iv) albitite, v) talc lodes, 10-20 m long, lensing out along the strike, invariably surrounded by an "aureole" of the albitite and including disseminated albite throughout the lode (Offler, 1966; Christ, 1984). Still well and Edwards (1951) considered that talc (in the Gumeracha area) formedfrombiotite by reaction with soda rich solutions, thus: biotite + quartz + soda => albite + talc + potash + Fe, and there is some evidence for this. Peripheral to the albitite "aureole", mentioned above, mica schist with some talc and some albite and, in places, relict biotite surrounded by a talc corona, represents a transition betweeen the unaltered schist and the talcalbitite association (see Sillwell and Edwards, 1951; Offler, 1966). The presence of some vesuvianite with the talc-albitite represents an intermediate stage in the biotite-talc conversion. Networks and discrete crystals of rutile have formed from titanium released by the biotite breakdown, and iron from biotite.is presumably contained in scattered limonitised pyrite concentrations in the albitite. The above relationships represent a metasomatic loss of K or reduction in JIK20 from the initial rock assemblage similar to that described by Arnold and Sandiford (1990) to account for the development of cordieriteorthoamphibole in the Springton region to the east(Fig 1). The difference between the Springton and the Pewsey Vale rocks is the abundance of albitite in the latter apparently representing a high concentration of sodium in metamorphic fluids affecting that area.

Enhancement of potassium depletion is a likely consequence of the sodium concentration, expressed by such possible reactions as: KAl3Si3Oi0(OH)2 + 6Si(>2 +Na => 3NaAlSi30g + 2H+ + 2K+ and K(MgFe)3(AlSi)30io(OH)2 + Na => NaAlSi30g + K + 3(MgFe) + 2H+ both of which are apparent in the field Hydrothermal partial replacement of biotite by albite has been replicated in the laboratory (Christ, 1984). Fluid-solid relationships can be envisaged as depicted in Figure 2 (e.g. Barnes,1967). Higher temperatures are likely to favour a higher Na /K ratio in the metasomatising solutions, according to Orville (1962). It is thus suggested that the sodium derives from the neighbouring Kitchener Granite (Fig 1) which, itself, is strongly albitised. Fluid access via one or other of the north-south faults in the area (Fig 1) is also a possibility, an expression of the restricted distribution of advecting fluids similar to that in the Springton region referred to by Arnold and Sandiford (1990). An alternative explanation which may account for the genesis of some of the talc is suggested by the association of calc-silicate lenticular layers with the talc-albitite deposits in the rock sequence. Carbonate rocks have not been observed in the immediate area concerned but the Skillogalee Dolomite, which stratigraphically underlies the Woolshed Flat Shalefromwhich the bi-mica schists are derived, outcrops two to three kilometres to the southwest It is thus considered not unlikely that the calc-silicates associated with the talc-albitite deposits represent ferruginous, siliceous dolomites. The development of talc would therefore be a consequence of decarbonation of the dolomite during the regional metamorphism: 3 Dol + 4 Qz + H2O Tc + 3 Cc + 3C02 and, for lower partial CO2 pressures, 3Act + 3Cc + 3C02 => 6Cc + 4Qz + 5Tc (Fig 3). Variation in partial CO2 pressure could account for actinolite and talc being in relatively close proximity (10's of metres) yet seldom closer. +

References: Arnold, J. and Sandiford, M., 1990. Pedogenesis of cordierite-orthoamphibole assemblages from the Srington region, South Australia. Contr. Min. Pet 106,100-109. Barnes, H. L., 1967. Geochemistry of Hydrothermal Ore Deposits. Christ, A., 1984. The geology of the Tweedie Gully area. Hons, thesis (unpublished). Dymoke, P. and Sandiford, M., 1992. Phase relationships in Buchan fades pelitic assemblages: calculations with application to andalusite-staurolite parageneses in the Mt Lfty Ranges, South Australia. Contr. Min. Pet 110, 121-132. Evans, B. W. and Guggenheim, S., 1988. Talc, pyrophyllite and related minerals, in Reviews in Mineralogy, 19, 225-291. Offler, R., 1966. The structure and metamorphism of the Pewsey Vale area north-east of Williamstown. PhD thesis (unpublished). Orville, P. M., 1962. Alkali metasomatism and feldspars, in Norsk Geologisk Tidsskrift Bd 42,283-316. Stillwell, F. L. and Edwards A. B., 1951. Petrology of the Gumeracha talc deposits, in Talc Deposits in South Australia, Bull. 26, Geological Survey of South Australia, 31-49.

Biotite Zone Stauroliteandalusite Zone Fibrolitic sillimanite Zone Prismatic * \ s \ k/ \/ \/ sJ sillimanite Zone Migmatite Zone Basement inlier

Upper limit of stauroliteandalusite

Figure 1. Map of part of the Adelaide Fold Belt (from Dymoke and Sandiford, 1992) showing talc localities in relation to regional metamorphic zonation J 1 I I I I L - 300O+47TC+30H20- 15Tr+2Atg+60(X>2 -17Tc445Mao+45H20-2Atg445C02 480-

3 o o ET 8*1

I K-feldspar

K-mica

J Albite

^\Na-mica Pyrophyllile \ I 1A L1 0 1 2 3 L

°9

Nac/mHd

« 4

0 X (C0 ) at P=2000 bars Figure 3. Isobaric temperature-XCC>2 diagram showing the dependence of the relative stabilities of talc and trcmolite on variation of XCC>2 (from Evans and Guggenheim, 1988) 2

Figure 2. Preliminary equilibrium diagram in the system K20-Na20-Al2C>3-Si02-H20) at400°C and 1500 psi total pressure, with quartz present (from Barnes, 1967) 45

Pre-regional metamorphic fluid flow in metacarbonates from the Reynolds Range, central Australia. Ian S. Buick. Department of Geology, La Trobe University, Bundoora, Vic. 3083, Australia, and Ian Cartright, Department of Earth Sciences, Monash University,Clayton, Vic. 3168, Australia.. A major cause of uncertainty in studies of regional metamorphic fluid flow is the great number of processes that may contribute to the observed evidence for fluid-rock interaction. The careful integration of petrological, textural and isotopic evidence at a number of grades is therefore important if we are to understand the fluid history of a regional metamorphic terrain. In this contribution, we present evidence for early contact metamorphic fluid-rock interaction histories in two regionally extensive calcsilicate units from the Reynolds Range, in the Arunta Block of central Australia. The Reynolds Range (Fig. 1) occurs within the northern province of the Arunta Block of central Australia, and was multiply deformed and metamorphosed during the mid-Proterozoic (Dirks et al. 1991). Within the Reynolds Range, sediments of the Reynolds Range Group (RRG) contain two carbonate units that are particularly useful for evaluating fluid-rock interaction histories. The lower calcsilicate unit occurs at the base of the RRG and comprises siliceous marls and minor lenses of sandstone and limestone. The upper calcsilicate unit comprises a thick horizon of dolomite- and/or calcite-rich limestones. The main structural-metamorphic event to affect the RRG (D2-M2; DII-MII of Dirks & Wilson (1990)) post-dated an earlier high grade metamorphic-deformational cycle (Dl-Ml) that affected only basement to the RRG. Earliest (S2\) fabrics in the RRG pre-date contact metamorphic assemblages around pre-regional metamorphic granites. Contact metamorphic assemblages formed early in a possibly progressive event, M2i-2-D2i-2, that ended in penetrative deformation (S22), and pervasive regional metamorphism (M22). M22 metamorpism occurred at 4 to 5 kbar and ranged in grade from greenschist (ca. 350 to 400 °C) in the NW, to granulite (ca. 700 to 750°C) facies in the SE, of the Reynolds Range (Dirks et al. 1991). M22 isograds cut the range at near right angles (Fig. 1). The Lower Calcsilicate Unit: Massive, 1 to 15 cm-thick grandite (grossular-andradite) garnet-rich calcsilicates are a ubiquitous feature of impure calcareous marls in the lower calcsilicate unit. These layers crop out along a 40 kilometre strike-parallel section of the Reynolds Range (Fig. 2), in which a transition zone from regional (M22) middle amphibolite facies (550-600 °C) to granulite facies (700-750 °C) rocks is exposed. Across this transition, massive garnet layers do not display systematic changes in mineral assemblages (Grt (XAnd = 0.65-85)-Plg (XAII= 2-10)±Cpx±Cal±Qtz), mineral compositions, modal proportions (70-85% garnet), volume % of outcrop (10-20%), or stable isotope compositions. Moreover, garnet-rich layers are deformed by pervasive, peak-regional metamorphic fabrics (S22). Massive grandite garnet layers therefore pre-date regional metamorphism and appear to have been closed chemical systems during prograde M22 metamorphism. Mineral assemblages in

grandite garnet layers are consistent with their having formed through near-complete decarbonation of thin, impure marbles or calcareous marls. The stable isotope values of rare calcite inclusions (8 C= -4.20 to -0.8%* V-PDB; 8 O = 10.5 to 14.0%oV-SMOW) and bulk silicate fractions ( 5 O = 6.1 to 10.8 %o V-SMOW) of massive garnet layers are too low, however, to have formed through internally-buffered decarbonation reactions (Fig. 3). Fluid infiltration by water-rich fluids (XC02 < 0.1 at T ® 550-600 °C), most probably from a magmatic source, is required to explain inclusion assemblages and stable isotope values. It is most likely that fluid infiltration occurred during pre-M22 contact metamorphism, because the lower calcsilicate everywhere overlies a major pre-M22 granite (the Napperby Gneiss; WR 5 0 = 8-9%o). 13

l s

1 8

Upper Calcsilicate Unit: Calcite- and dolomite-rich marbles of this unit occur at all regional M22 grades. Greenschist fades marbles are characterised by coarse grained porphyroblasts of, and pseudomorphs after, intermediate scapolite (EqAn48_5g), which localy coexists with Ca-rich plagioclase (XAn = 84-97). Scapolite porphyroblasts a) pre-date regional fabrics, and b) are always partially pseudomorphed by peak M22 greenschist facies assemblages (Cal+Qtz +Mu±Ca-rich PlgiKfs). Scapolite porphyroblast compositions, and scapoliteplagioclase Na-Ca partitioning, are not consistent with a low temperature (i.e T < 400 °C) origin. The stability of the assemblage Scp(EqAn5g)-Cal-Plg(X =97) indicates a minimum temperature of 550 °C for the growth of scapolite-bearing assemblages, thus indicating an early high-T history that pre-dates regional greenschist facies metamorphism. Greenschist facies marbles (60-90 wt% carbonate) also display a range of carbonate stable isotope values ( 8 0 = 15.8 to 19.7 %o\ 8 C =-1.4 to 0.7 %o\ n = 57 samples; Fig. 4). Although the measured 8 ^ C values fall within the range expected for non-graphitic Proterozoic carbonates, the oxygen isotopic values are generally lower than likely limestones precursors. Since the RRG at these grades contains a large pre-M22 granite (the Conniston Orthoschist), we interpret both early scapolitisation of the marbles, and their low oxygen isotope values as resulting from pre-M22 contact metamorphism. The effects of contact metamorphism in other units at this grade is obscured by more pervasive syn-M22 rehydration. The heterogeneity of oxygen isotopes in these marbles at greenschist facies grades, and their generally 0-depleted nature, has obvious implications for studies of isotope trends with changing grade in regional metamorphic terrains. an

The preservation of pre-M22 contact metamorphic assemblages in both the lower and upper calcsilicate units may reflect limited infiltration of syn-M22 fluids into these units during prograde M22 metamorphism. Evidence for limited, channeled post-M22 fluid flow in the upper calcsilicate is presented in Cartwright & Buick (1993), and Buick & Cartwright (1993; both this volume). References Dirks, P.H.G.M., & Wilson,C.J.L. 1990. Journal of Structural Geology, 12, 651-665. Dirks! P.H.G.M. el ai 1991. Journal of Metamorphic Geology, 9, 641-661.

Figure 1: locality map, Reynolds Range

M 2 amphibolite

180

i = 1.006 180 a 2 = 1.012 a

Napperby Gneiss

•I

Lower Calcsilicate Reynolds Range Group-undifferentiated E3 Lander Rock beds EZJ Early M2 granitoids ED Pre-or-syn M-j granitoids —— M 4 shear zone

13C

40 - wt % caiat* "T

I I

Rayleigl Rayleigh Fractionation

Batch Fractionation

a == 1.0025

contact metamorphic assemblages in pelites

Figure 2: Distribution of Lower Calcsilicate Unit

Figure 3: Calcite stable isotope trends w.r.t. decarbonation trends for the model garnet-producing reaction 2Cal + Qtz + An = Gro + 2C02 CD

D a o

O (SMOW)

Figure 4: Carbonate stable isotopes, UCS,greenschist facies end of Reynolds Range 48

Stable Isotope and Mineralogical Constraints on Channelled Fluid Infiltration in Wollastonite-Bearing Calc-Silicates: Reynolds Range, Central Australia Ian Cartwright. VIEPS Department of Earth Sciences, Monash University, Clayton Vic. 3168, Australia, and Ian S. Buick, VIEPS Department of Geology, LaTrobe University, Bundoora Vic. 3083, Australia. Constraining the pattern of metamorphic fluid-rock interaction is important for studies of metasomatism, heattransfer, and deformation. Many parameters, including time-integrated fluid fluxes, the direction of fluid flow, and intrinsic permeabilities, may be calculated from changes in major or trace-element geochemistry, and/or changes in mineralogy. Marbles are ideal for the study of fluid-rock interaction as they readily undergo reaction with incoming fluids to form mineral assemblages that allow fluid compositions to be constrained. Additionally, within the normal range of pressures and temperatures for contact or regional metamorphism, certain minerals (e.g. wollastonite, vesuvianite, or grossular) are only generally formed by infiltration of water-rich fluids, and thus can be used to map fluid pathways. Here, we use marble mineralogy to document the centimetre- to metrescale variability of fluid-rock interaction from marbles in the Reynolds Range, central Australia, and to estimate variations in fluid fluxes and intrinsic permeabilities. The Reynolds Range (Fig. 1) is a multiply deformed and metamorphosed Proterozic terrain that forms part of the Arunta Block of central Australia. The Reynolds Range consists of a suite of metamorphosed pelitic and semipeltic sediments, the Lander Rock Beds, and early metagranitic intrusives that form the basement to the Reynolds Range Group metasediments. The Reynolds Range Group comprises a basal quartzite, metapelites, a lower calcsilicate unit that is laterally equivalent to the quartzite, and an upper calcsilicate unit (UCS) that is discussed here. The main structural-metamorphic event to effect the Reynolds Range Group, D2-M2, involved NE-SW shortening of the range to form tight to isoclinal NW-SE trending upright isoclinal folds with associated penetrative fabrics synchronous with medium to low pressure regional metamorphism (M22) that ranges in grade from greenschist (~400°C) in the northwest to granulite (~740±40°C) in the southeast, at a pressure of 400500MPa. The UCS unit compriss two main lithologies. The majority of the rock comprises a carbonate-rich marble unit, in addition there are concordant, centimetre-wide, fine-grained bands and lenses that contain less carbonate and which probably represent marl layers. Throughout most of the UCS, the marbles contain 60-90% calcite, 5-15% diopside, 0-10%) alkali feldspar, 0-10% phlogopite, and up to 11% quartz. The marls contain quartz+calcite+clinopyroxene+

plagioclase+K-Feldspar±epidote±dolomite. Wollastonite-grossular bearing

marbles occur in strike-parallel regions that are tens of metres wide and hundreds of metres in length which have relatively sharp margins with the surrounding calc-silicates. The wollastonite in these marbles is coarsly intergrown forming mats that are sometimes aligned within, but often overgrow, the regional S22 fabric, suggesting that wollastonite grew late in D2-M2. On the scale of an individual thin section there is great variability in the percentage of wollastonite (Fig. 2). Millimetre- to centimetre wide, often strike-parallel, zones that contain up to 70% wollastonite and no quartz alternate with zones that contain 50-60% calcite and <10% wollastonite that are still quartz bearing. In addition to the marl and marble layers, a third reaction zone

assemblage that does not exist outside the wollastonite zones is recognised in the wollastonite-bearing rocks. The reaction zone is often developed at the contact of marble and marl layers, it varies in width from a few millimetres to up to a centimetre and consists largely of grandite garnet with minor (<10%) calcite. At the edge of the reaction zones, garnet often overgrows the wollastonite in the marbles suggesting that the development of the reaction zone postdated, or outlasted, the formation of wollastonite. The mineral assembalges developped imply that on the tens-of-metres scale, strike-parallel flow of H20-rich fluids (XCO2 <0.1) formed wollastonite-bearing assemblages within marble layers, but not within interlayered marls. The mineralogy of the marbles outside the region of fluid infiltration suggests that more than -17% wollastonite cannot have been formed by simple reaction of calcite + quartz, implying that silica metasomatism also occured to form the zones with high wollastonite volumes. Application of advective transport models suggests that the alternation of wollastonite-rich and wollastonite-poor zones may be explained by timeintegrated fluid fluxes, and hence intrinsic permeabilities, varying by at least two orders of magnitude on a millimetre to centimetre scale. The distribution of wollastonite in thus provides a "map" of the fluid pathways and palaeo-permeabilities in the samples. Given the lack of internal layering, fluids were probably focussed through the marble within microfractures and the variations in intrinsic permeability may reflect variable fracture density. 5 18 0(Calcite) values for the wollastonite-bearing lithologies vary from 17.6 to 13.8%c, significantly lower than the average 5 1 8 0(Cc) value of high-grade Reynolds Range Upper Calcsilicate rocks (~17.1±1.1%0) or the values of the k-feldspar + diopside marbles away from the edges of that unit (16.3 to 18.7%o). 8 1 3 C(Cc) values for the wollastonite-bearing rocks vary from 0.7%© to as low as -5.9%©, S 1 3 C(Cc) values in the range 1 to -1 %0 are probably close to the original carbon isotope ratios of these rocks. The data show a number of significant features that must be explained by any models of fluid infiltration: 1) with the exception of a few samples, the marble layers in these rocks show a correlation between 5 13 C(Cc) and Wt% calcite, especially for samples with less than ~30Wt% calcite; 2) in the marbles there is also a correlation between 8 ^ 0 ( C c ) and Wt% calcite, with the lowest 8 1 8 0(Cc) samples having <20wt% calcite; 3) the reaction zone samples appear to lie on the same 5 13 C(Cc) vs Wt% calcite and 5 18 0(CC) vs Wt% calcite trends as the marbles, at the low-Wt%calcite end of the trends; 4) the marls have low calcite contents (<15 Wt%) but much higher 8 1 3 C(Cc) values (-2 to 1 %0) than marbles with similar calcite concentrations, the stable isotope composition of the marls in these rocks is not appreciably different from that of the marls in the Ksp + Di calcsilicates; 5) there is little correlation between 5 1 8 0 and the distance from the cordierite-quartzite boundary; 6) most of the lowest 5 1 3 C marbles (below -2%©), however, are found within 5m of the cordierite-quartzite contact, and, consistent with what was noted above, many of the marbles within 5m of this boundary have low calcite contents (<20%).

Figure 1. Summary geological map of the Reynolds Range showing locality of calcsilicates from this study.

M2 g r e e n s c h i s t

BHD Reynolds Range Group • Lander Rock beds Early M2 granitoids E 3 Pre-or-syn M i granitoids M4 shear zone Bi — y

M2 Isograd

Mineralogy

Marble i) Woll (0-20%)+Gr +Cc±Qtz+Di+Plg±Ksp ii) Woll (50-70%) +Gr+Cc+Di+Plg+Ksp

Fluid-Flux / Permeability Moderate High

Low / Zero Marl (Qtz+Plg+ Ksp+Di+Cc±Dol)

Arunta Block

Reaction Zone (Gr+Cc±Di)

Moderate High

Figure 2. Mineralogical zones in a typical thin section o f Wollastonite-zone calcsilicates, and the interpretation of fluid fluxes and permeabilities

Mineralization and alteration styles at the Osborne deposit, NW Queensland

Neil D. Adshead. Department of Geology and National Key Centre in Economic Geology, James Cook University, Townsville 4811, Australia.

The Osborne copper-gold deposit is located 195km south-east of Mount Isa. It is hosted by Mid Proterozoic metasediments of the Eastern Succession and is one of several significant base and precious metal deposits (others include Ernest Henry and Cannington) recently located under the Mesozoic cover flanking the eastern and southern margins of the Mount Isa Inlier. In the vicinity of Osborne there is a 30-40m blanket of flat-lying silcrete.

Quartzites and feldspathic quartzites are the dominant host rocks with two (or more?) metasedimentary ironstone units and rare pelitic horizons. The sub-centimetre scale banded ironstones are confined to the western half of the deposit and to the east are truncated by a major WNW trending fault. Comparison with other sequences in the region reveals similarities with the Mount Noma Quartzite of the Soldiers Cap Group. Pegmatite and metadolerite sheet intrusions are common and in the north-eastern part of the deposit grid there is a technically emplaced, 30m thick sliver of an altered olivine-orthopyroxene metagabbro(?) situated in the hanging wall. The area has a complex polyphase deformation history with evidence for early compression

followed by sillimanite grade metamorphism,

anatexis and regional sodic (-Ca-Fe)

metasomatism, all overprinted by the structurally-controlled mineralization and related alteration. Syn to post mineralization chloritic shears and Mesozoic extensional faulting further complicate the sequence. The quartzite sequence has been metamorphosed to sillimanite grade and areas with coarse granofels and partial melting are common. Rare pelitic bands deep (500m) in the south-east of the Osborne grid contain porphyroblasts of garnet ( A l n ^ , , Pyri6-24> SpessJ, cordierite and sillimanite accompanied by biotite and albite. Metamorphism has coarsened and deformed the relict sedimentary banding in the ironstones. Cummingtonite occurs as local granular or acicular concentrations within and below the banded ironstones and may reflect a magnesian silicate sedimentary ironstone facies. Amphibolite and metadolerite sheets cut across the ironstones and comprise prograde hornblende-oligoclase-titanite-magnetite with retrograde spots of epidote-actinolite. The constituents of altered metagabbro(?) include relict olivine, orthopyroxene and green pleonaste spinel in a mass of hornblende and pale chlorite.

The main sulphide-bearing intersections are in a siliceous, sheet-like body at the base of the strongly deformed upper ironstone which, in the north-west, is truncated by the Mesozoic unconformity and dips at between 10 and 50 degrees to the north-east and continues for at least 1100m. A spatially separate, nonironstone-related and prolate-shaped ore body/approximately 300m long by 70m wide, occurs 200m to the east of the ironstone-related mineralization and has its own characteristic mineralogical and alteration features, viz.,pyrrhotite-rich ore and a well-developed sericite halo.

Chalcopyrite is the only primary copper mineral. The accompanying gangue assemblage is dependent upon the host rock. At the base of the upper ironstone and in the eastern mineralization, massive silicification hosts the economic sulphides; the former with magnetite-pyrite whereas the latter is particularly pyrrhotiterich and has anomalous Co and Ni.The magnetite-quartz banded ironstones are generally poorly mineralized but where economic grades are attained, specular haematite, pyrite and granular magnetite accompany the chalcopyrite. Various accessory phases are present, including siderite, molybdenite, various Co-Ni sulphides, chlorite and muscovite, plus rare native silver.

Both replacement and brecciation textures are common in the massive silicification, with frequent relicts of banded ironstone plus chloritized and sericitized pegmatite, amphibolite and quartzite. The subordinate chalcopyrite-iron oxide mineralization within the ironstones also reflects a later, secondary overprint disrupting or truncating the planar or folded banding. Stringer chalcopyrite occurs below parts of the silica hosted mineralization and minor amounts of sulphide accompany carbonate and sericite in late veins. There is no evidence of sulphide deformation but there are important chlorite shears which appear to dismember and stack the silicification and ironstones into duplex structures.

There are several alteration styles at Osborne formed during different geological events and subject to lithological controls. A synmetamorphic albite-haematite granofelsic alteration, locally accompanied by epidote-calcite-andradite-titanite-pyrite-magnetite,

is overprinted

by mineralization-related

pervasive

sericitization often with pyrite-quartz-chlorite. Coarse biotite alteration, associated with albite-magnetitesulphides-monazite, is only locally developed and is concentrated directly below intervals of massive and mineralized silicification. The margins of the hanging wall metagabbro(?) show a similar potassium and LREE-enrichment. The cummingtonite "pockets" are frequently altered to talc with subordinate dolomite, apatite, haematite and pyrite as common associates.

Preliminary fluid inclusion data suggest high salinities and, coupled with the geochemical signature of the mineralization, ie.,elevated Cu-Au-Ag-Mo-Se-Hg-Bi-Te-Sn-W, is consistent with granite involvement. Recent drilling has identified granitic bodies just to the north and east of Osborne, and geophysical modelling of the Eastern Fold Belt suggests large, ca. 1500Ma. Williams-type intrusions occur to the south of the current Proterozoic outcrop.

Metal-bearing fluids were channelled along early, possibly reactivated regional-scale shallowly-inclined structures. Chemical interaction with banded ironstones and feldspathic quartzites/earlier albitic alteration was probably the major control on ore deposition. Equilibria in the metal-bearing fluid were rock-buffered, enabling continued sulphide precipitation due to the reducing potential of magnetite (producing specular haematite) and the hydrolysis of feldspar to sericite, biotite and chlorite, and also cummingtonite to talc.

THE ROLE OF METAMORPHIC FLUIDS IN THE PETROGENESIS OF MOUNT DORE-STYLE BRECCIA-HOSTED COPPER-GOLD DEPOSITS IN THE EASTERN MOUNT ISA INLIER, QUEENSLAND Trev J. Beardsmore1. Geology Department, James Cook University, Townsville 4811, Australia

Mount Dore-style breccia-hosted copper-gold deposits define a 70 kilometre-long, north-trending lineament from Kuridala (65 kilometres south of Cloncurry), southwards. The type deposit lies 130 kilometres south of Cloncurry, and a detailed study of it was undertaken to produce a metallogenic model applicable (with suitable modifications) to all deposits having this style.

The Mount Dore deposit occurs within steeply east-dipping quartz-muscovite schists and carbonaceous slates of the uppermost Maronan Supergroup structurally overlying meta-calcarenites, calcilutites, marbles and metabasalts of the Staveley Formation. The structural history includes early, subhorizontal (Dj) detachment of the Staveley Formation from older units, followed by upright, north-trending, tight to isoclinal folding (D^, accompanied by peak metamorphism in the lower to middle amphibolite facies (Jaques et al.y 1982). Northwesttrending corridors of D3 deformation are scattered across the region, and one of these passes through the Mount Dore orebody. Latest tectonism produced the Mount Dore Fault Zone, a moderately- to steeply east-dipping reverse fault-zone about 250 metres wide, which passes through Mount Dore and reactivates the D1 structure. The fault zone contains a thin sliver of uppermost Maronan Supergroup, sandwiched between footwall Staveley Formation and hangingwall (truncated) Mount Dore Granite. The granite is dated at 1510 Ma (Nisbet et al., 1983).

A complex history of brecciation and alteration is related to movement along the Mount Dore Fault Zone and to associated hydrothermal activity. Textural evidence points to post-D3 hydrothermal activity. Early alteration produced K-feldspar (or biotite), tourmaline, sericite and quartz. Later alteration produced carbonate (dolomite and calcite), apatite and chlorite. Primary sulphide mineralization is associated temporally with carbonate alteration, and comprises pyrite and chalcopyrite, with minor sphalerite and galena. Pyrite is early, and is replaced by the other phases. Chalcocite also clearly replaces earlier pyrite, but is restricted to shallow depths and probably formed by supergene processes.

Alteration, fluid inclusion and stable isotope geochemistry identify a primary deep-seated, hot (>500°C?), oxidized, C02-bearing, highly-saline (65-70 wt% salt) metamorphic or magmatic fluid containing K \ Na+, Fe2*, Ca2+, B, Si0 2 , H \ CI and possibly S0 2 . After initial separation and loss of an immiscible C0 2 -rich phase, the residual aqueous fluid became more dilute with time, probably by mixing with a cooler, lower salinity (<20 wt% salt), low-C0 2 fluid, possibly also of metamorphic origin. A model accounting for mineralization at Mount Dore

Present address: Geology Department, P.O. Box 414, University of Papua New Guinea, N.C.D., P.N.G.

invokes dilation and hydraulic biecciation during movement along the Mount Dore Fault Zone, where the fault intersects D3 "corridors" of shallowly-dipping bedding and S2 foliation. Early potassic and silicic alteration released ore metals (Cu, Pb, Zn, Ag, Co, U, Au) to the fluid from the host rocks at this time. Sulphide precipitation was controlled by sulphate reduction with carbon released from host Pyrite scavenged most of this, and later Cu-, Pb- and Zn-sulphides formed by scavenging of S from pyrite.

Limited data concerning other Mount Dore-style deposits (Mount Elliott, S.W.A.N., Hampden, Stuart) suggests they may have formed by similar processes, with appropriate modifications to suit the local conditions. These deposits apparently all formed during a single metallogenic event related to late tectonism in the eastern part of the Mount Isa Inlier. A speculative regional model proposes emplacement of at least one large allochthonous slab of Maronan Supergroup over the carbonate-evaporite successions of the Mary Kathleen Group. The latter passed highly saline, C02-bearing connate and prograde metamorphic fluids upwards into and along the decollement. Subsequent upright to inclined F2 antiforms may have ponded these fluids, allowing them to "stew" for some time in contact with relatively metal-rich lithologies in the overriding plate. Eventual release to higher crustal levels occurred only when these structures were breached during late-tectonic reverse faulting. Passing rapidly upwards along these structures, the fluids encountered local dilatant zones, where high fluid-rock ratios and rapidly changing physical and chemical conditions instigated extensive alteration and sulphide precipitation. Low salinity fluids of meteoric or upper-plate metamorphic derivation could have migrated into the dilatant zones when the deeply penetrating fault structures became available, and subsequently mixed with the saline fluids, perhaps initiating some styles of mineralization in the process.

Epigenetic mineralization across the Cloncurry Fold Belt (and perhaps the entire Mount Isa Inlier) appears to be the result of large-scale devolatilization of the crust during the waning stages of regional deformation and metamorphism. The characteristics of individual deposits depends on the combination of local factors a such as structure and lithologies available adjacent to these structures for leaching of metals.

REFERENCES

Jaques, A.L., Blake, D.H. and Donchak, PJ.T., 1982. Regional metamorphism in the Selwyn Range area, northwest Queensland. B.M.R. Journal of Australian Geology and Geophysics, v.7: 181-196.

Nisbet, B., Devlin, S.P. and Joyce, P., 1983; Geology and suggested genesis of cobalt-tungsten mineralisation at Mount Cobalt, northwestern Queensland. Proceedings of the Australasian Institute of Mining and Metallurgy, v.287: 9-17.

Evolution of Na-K-Fe-Si metasomatism and mineralization associated with the Cloncurry Fault, SE Mount Isa Inlier: A comparison with Kiruna-Olympic Dam type systems Patrick Williams. Geoffrey de Jong and Trevor Verran, Geology Department and National Key Centre in Economic Geology; James Cook University of North Queensland, Townsville Q4811, Australia. The "Cloncurry Fault" is a major lineament in the southeastern part of the Mount Isa Inlier. It had a complex history of reactivation including early ductile and late brittle phases with the latter occurring after the consolidation of the exposed portions of the major -1480-1510 Ma Williams-Naraku batholithic granite complex. Late metasomatism during the brittle phase activity on the fault is significant firstly because it preserves a record of strongly telescoped fluid activity during the unroofing of the "Isan" orogenic belt, and secondly because of the possible roles of the fluids in the metallogeny of this world class mineral province. Brittle-dilational and metasomatic features of the Cloncurry Fault system are particularly well-expressed in the northeastern part of the Selwyn Range some 50km SSE of Cloncurry. In this area the fault is exposed at a structural level close to the roof of the Williams-Naraku Batholith and undergoes a marked trend-change with its trace rotated from = 170° in the south to 160° further north. The fault is represented by a dominant steep NNWtrending fracture system developed parallel to older ductile fabrics. Slickenside lineations and vein and fracture orientations suggest most of the the dilational quartz infill occurred during a phase of sinistral strike-slip motion. The disturbed zone is several kilometres broad and also characterized by abundant subsidary fractures and veins most of which trend E or ESE. The major brittle features commonly exploit preexisting ductile shear zones and the system is the locus for a very large alteration system within which 10s to possibly 100s of cubic kilometres of rock have been significantly reconstituted. Alteration affected both the granites and the heterogeneous metasedimentary and meta-igneous country rocks which had previously undergone amphibolite facies (sillimanite to sillimanite + K feldspar) regional metamorphism. The duration of the alteration system and source(s) of the fluids are as yet very poorly-constrained. The alteration paragenesis (Table 1) from albite-fmagnetite stable assemblages to muscovite+hematite is compatible with progressively reduced temperatures and covariation of other conditions including f 0 and pH. Overprinting relationships commonly demonstrate that multiple alteration events occurred within the ranges of conditions responsible for individual assemblages. Albitization and K-feldspathization were caused by the passage of hypersaline fluids, affected all primary rock types in the altered zone and caused a major redistribution of iron in the system. Extreme elemental mobility is indicated by occurrences of hydrothermal Ti and REE minerals. The distinctive strong red colouration due to hematization ("red rock" alteration) is present in several associations that reflect significantly different physico-chemical conditions. The youngest stages of alteration are distinguished by coarsely crystalline hematite and/or barite which are invariably overprinted by cryptocrystalline to chalcedonic silica and drusy quartz. The latter have textures diagnostic of deposition under high level (epithermal) conditions. 2

The telescoping of the secondary assemblages mirrors variation with depth observed in the alteration systems of

the Kiruna district in Sweden that have recently been suggested to reflect the deeper equivalents of the sort of hydrothermal system responsible for the giant hematite breccia-hosted Cu-U-Au-Ag-REE deposit at Olympic Dam in South Australia. The constitutional similarities between these systems is further emphasized by the occurrence of quartz veins in the Cloncurry Fault zone that contain the distinctive and well-known assemblage of actinolite + magnetite + apatite as accessory phases.

To date very little mineralization has been recognized within the main part of the Cloncurry Fault system. One exception is the minor Cu occurrence at Mount Kalkadoon which is related to a subsidiary ESE-trending system of K-feldspar-bearing quartz veins with phyllic alteration selvages. The mineralization, like that at Olympic Dam, is strongly anomalous in Au+Ag+U+LREE and also carries elevated amounts of Co+Sn+As. Zn deposits at the Boorama and Maramungee prospects lie to the east of the main fracture/alteration system but are controlled by brittle features that are likely to be similar in age which cut comparatively reduced (graphite-bearing) metasedimentary rocks. Some pyritic epithermal breccias in the main part of the fault system are anomalous in Au, Sb and As suggesting that there is potential for economically-significant precious metal deposits.

Mineral

Association

Colour in Hand Samples

Estimated Temperature

Ab + Qtz ± Act ± Bt ± And ± Mag ± Ttn ± Ap

white, grey

>400-450°C

Ab + Qtz ± Act +Hem ± Mag ± Ttn ± Ep

pink, purple-red, red

Kfs + Qtz ± Hem ± Mag ± Rt ± Ms ± Ep

pink, red*

Kfs + Brt + Qtz + Hem ± Ms

rcd

Ms + Chi ± Hem ± Cal ± Py ± Ccp ± Ep ± Sid (in part associated with strong silicification)

pink,

Crystalline hematite

metallic grey red, yellow, grey

Chalcedony ± Py

300-400°C

red**

< 300°C

"Epithermal"

* varying to white in Fe-oxide free varieties * * varying to white or pale green in hematite-absent assemblages

Table 1: Outline sequence of post Williams Granite vein and replacement assemblages in the northeastern Selwyn Range.

Syn-to late-metamorphic Na metasomatism, southeast Mount Isa Inlier Geoffrey de Jong, National Key Centre in Economic Geology, James Cook University, Townsville Qld 4811, Australia.

Structurally controlled sodium metasomatism and associated brecciation covers 100s of km^ south of Cloncurry (Selwyn Range) of the Mount Isa Inlier. This alteration and breccia complex maybe the largest of its kind in the world, and has not yet been described in any detail. The alteration overprints the contact between the Maronan Supergroup in the east and the Doherty Formation in the west. The alteration and breccia complex is controlled by late syn to post D2 shearing and brittle structures that were active post D2 after emplacement of granite intrusions (1510-1480 Ma). This is in contrast with the Mary Kathleen zone where altered zones, with abundant calcite, are localized in areas adjacent to metadolerites. The Selwyn Range area can be divided into subzones, separated by shear zones interpreted to be Di thrusts that possibly form part of a terrain boundary fault system. East west drainage systems give good three dimensional insight in the alteration and breccia complex. Exposures in Maramungee Creek, in the eastern Selwyn Range, gives a good insight into the early (pre granite) alteration and breccias. The main foliation developed in the Cloncurry Selwyn zone is the S2, with a general north-south trend. Overprinting by younger deformations causes variations of this trend from NW-SE to NE-SW. The peak of metamorphism coincided with D2 (-1544 Ma ?) and reached the upper amphibolite facies. Fluids present during post-peak metamorphism were responsible for the alteration and were saline (16-38 equivalent NaCl wt %) and hot (400-650 °C), based on fluid inclusion data. Evaporitic sequences within the Corella Formation were possibly the source of this high saline fluid. The fluids were channelized in the zones of high strain, localized on the reactivated early fault. The fluid pressure reached at least the 300 MPa, based on hornblende geobarometry in the less altered rocks, and caused veining and brecciation concentrated around the Cloncurry Fault. The rocks that have "seen" the hot saline fluids are totally reconstituted with destruction of primary textures and mineralogy. In the vicinity of high level granite intrusions (-1500 Ma), the breccia complex is rebrecciated, with production of similar metasomatic mineral assemblages, suggesting the sodiumrichfluidsmay have been active for up to 40 million years. The breccias have pseudo-igneous texture, with the same metasomatic mineral assemblages as the altered clasts. The clast size varies from millimetre up to 10s of metres scale, and are in places totally altered. Relict schist textures can occasionally be recognized in some clasts. The clasts are subrounded, but this is due to reaction with the fluid, rather than due to transport.

The main alteration product is an albitic plagioclase (An < 8 %), with magnesium rich actinolite (Mg/Mg+Fe is 80-90 %) ± magnetite (±Ti) ± titanite. Diopside occurs in places, depending on the temperature and original host rock. Titanite and ilmenite common occupying dilatational veins and indicate high mobility of Ti. The main element that has been depleted is potassium. Much of the iron liberated by the mica destruction was reprecipitated as magnetite, and strongly redistributed within the alteration system. The shear zones are associated with veins that occur as two distinct sets, one which is layer parallel and the other subhorizontal. The layer parallel set occupies axial planes of the refolded folds in the shear zones. These veins consist of actinolite-tremolite centres ± ilmenite selvages and microcline and plagioclase envelopes. The amphibole fibres are aligned subparallel to the vein wall, indicating the veins are syntectonic. Subhorizontal veins are dominant adjacent to the shear zones, consisting of actinolite and diopside with microcline selvages. Scapolite alteration is abundant in the Corella Formation of the Mary Kathleen zone, but comparatively rare in the eastern Selwyn Range. Scapolite described from the Mary Kathleen zone (Oliver and Wall, 1987) has an intermediate mizzonite-mairalite composition. This contrasts with the scapolite found in the eastern Selwyn Range which are of two distinct types, a Ca-scapolite with approximately the same composition as the Mary Kathleen scapolite and a very Na-rich scapolite with a high CI content. The latter are found as selvages around oligoclase-quartz veins in the sodium metasomatic zones replacing plagioclase on the margins of strained metadolerites. This high Na and CI content is an indicator of highly saline fluids, a deduction which is supported by fluid inclusion evidence. Reference Oliver, N.H.S. and Wall, V.J., 1987. Metamorphic plumbing system in Proterozoic calc-silicates, Queensland, Australia. Geology, 15, 793-796.

PETROGENESIS OF A VARIETY OF METASOMATIC LETHOLOGIES WEST OF MOUNT ISA Michael J. Rubenach, Department of Geology, James Cook University, Townsville 4811, Australia. Rocks west of Mount Isa were subjected to low-P/high-T metamorphism (syn-D^, with an anticlockwise P-T-t path (Rubenach, 1992). Also present are a variety of regional metasomatic styles, some of which are quite unusual. The earliest recognized (syn- or pre-D,) affected metabasalts, especially those adjacent to early faults. Such rocks now consist of chlorite + quartz ±_ talc ±_ cummingtonite _+ anthophyllite in the cordierite zone, changing abruptly to cordierite + anthophyllite 4- quartz in the sillimanite zone. This alteration involved addition of Mg and removal of Ca and Na, and, on analogy with similar alteration in ocean-floor basalts, probably formed by interaction with circulating brines. Two varieties of metasomatic lithologies occur along a 7km belt of biotite-zone Bortala Formation. Massive tremolite has replaced pods of psammite, quartzite and marble in a shear zone in the east of this belt, while in the west tremolite-plagioclase rocks have replaced psammite adjacent to quartz veins. This occurred late in the D2 event, extending into the development of the subsequent shear zone (Huang & Rubenach, in press). The most unusual metasomatic type are cordierite-rich rocks which have preferentially replaced mica-cordierite schists immediately adjacent to boudinaged metadolerite. Assemblages are typically cordierite + quartz +. andalusite sillimanite +_ plagioclase, and are abundant along a belt 27Km long within the upper cordierite and lower sillimanite zone of the Eastern Creek Volcanics. Formation of these rocks require removal of K and generally the addition of Mg and Al. Another type consists of small-scale skara alteration of metabasic and calcsilicate rocks late in D2, assemblages including allanite and uraninite. The various syn-D2 metasomatic types are all associated with quartz veins and with fracturing accompanying ductile deformation. With respect to lsO, the metasomatic assemblages are generally in isotopic equilibrium with associated quartz veins. However, quartz from most non-metasomatized schist and quartzite has also been reset, giving values of 9-11 permil compared with 14-16 permil for the lower-grade equivalents east of the Mount Isa Fault. The preferred hypothesis for such large-scale downward resetting of 180 is circulation of fluids uptemperature. Such fluids from lower temperature regions interacted with locally-derived fluids to produce the metasomatic rocks. In the case of those in the Bortala Formation, the external fluids may have mixed with those in the shear zone, where calcite was being preferentially dissolved to form talc schists, to form the tremolite metasomatic rocks. In the case of the cordierite metasomatic rocks, the externally-derived fluids may have mixed with those derived from the terminal chlorite 4- quartz reaction in the nearby altered metabasalts. In all cases, structural control was critical regarding localization of the metasomatic rocks. Huang, W., & Rubenach, M. J., in press. Structural controls on the formation of tremolite and tremoliteplagioclase metasomatic rocks, Molanite Valley, Mount Isa, northern Australia. Rubenach, M. J., 1992. Proterozoic low-pressure/high-temperature metamorphism and an anticlockwise P-T-t path for the Hazeldene area, Mount Isa Inlier, Queensland, Australia. J. Metam. Geology, 10, 333-346.

STRUCTURAL CONTROLS ON SYNTECTONIC METASOMATIC TREMOLITE AND TREMOLITE-PLAGIOCLASE PODS IN THE MOLANITE VALLEY, MOUNT ISA, AUSTRALIA W. Huang and M. J. Rubenach Department of Geology, James Cook University, Townsville, Q 4811, Australia

ABSTRACT-ln the Molanite Valley west of Mount Isa, tremolite-bearing metasomatic pods, showing sharp contacts with enclosing rocks, occur over a strike length of 7.4 kilometres in the biotite-zone Bortala Formation. In relatively low-strain rocks of the western part of the valley, plagioclase-tremolite pods (PTP) have replaced psammite or mica schist surrounding the terminations of buck quartz veins, whereas in intensively sheared rocks comprising the eastern part of the valley, pods of medium to coarse-grained massive tremolite (MTP, commonly containing greater than 95% tremolite) have replaced marble, quartzite, and psammite. Talcchlorite schists, formed by dissolution of carbonates during shearing of marble, are associated with some of the massive tremolite pods. The direct relationships between structural domains and metasomatism suggest that migration of externally-derived fluids was controlled by fractures and the shear zone. The fluids, having migrated along fractures, interacted with wall rocks to produce plagioclasetremolite pods. In the shear zone, buck quartz veins, some tremolite-bearing, show no obvious geometric relationships to massive tremolite pods. It is postulated that the metasomatic fluids fluxed through shear domains and penetrated progressive shortening domains to replace the host rocks and form the massive tremolite pods. Both styles of metasomatic rocks developed from late S2 to early S3 during a progressive E-W shortening event, which corresponds to the regional metamorphism. The metasomatic rocks represent only a small part of widespread the metasomatism across the Mount Isa Inlier.

P-T-t trajectories through the Caledonian nappes of N. Norway: a clue to complex burial and exhumation histories in collisional orogens

M.W.Anderson. Department of Geological Sciences, University of Plymouth, Drake Circus, Plymouth PL4 8AA

The Caledonian nappe stack of Torms, northern Norway comprises two major nappe complexes. Each complex records an inverted greenschist to upper amphibolite facies metamorphic profile, related to initial nappe assembley along major in-sequence thrusts. Upper amphibolite facies structural units within each nappe complex record hornblende ^ A r / ^ A r ages ranging from 425 to 421 Ma. One of these units records nearly identical muscovite cooling ages, suggesting rapid nappe exhumation after peak metamorphism. However, muscovites from the other units yield 4 0 A r / 3 9 A r ages of ca. 395 Ma suggesting a protracted period of post-peak metamorphic cooling. Furthermore, fossil evidence from several interleaved units suggests that some nappes had yet to experience burial and greenschist-amphibolite facies metamorphism during this initial phase of lateCaledonian crustal evolution. A continuous underplating-exhumation-burial model is developed to account for this.

A feature of both major nappe complexes is the progressive younging of 4 0 A r / 3 8 A r muscovite age data downward through each of the inverted metamorphic profiles. Microthermometry on pseudomorphs after peak metamorphic porphyroblasts in the major thrust zones (Bennett& Barker, 1992) suggest retrograde temperatures of ca. 350°C, similar to those required for argon retention in muscovite. The preservation of soft pseudomorph assemblages indicates that retrogression post-dates intense shearing in the thrust zones, which acted as pathways for enhanced fluid flow. The pattern of muscovite age data is then most easily explained by relaxation of the 350°C crustal isotherm through technically thickened, but largely stable, Caledonian crust. At this stage, continued accretion of the orogenic wedge to the Baltoscandian margin was probably characterized by piggy-back style translation of the nappes above a low-angle basal decollement. Subsequent basement reactivation was accompanied by out-of-sequence thrusting which resulted in further interleaving of the nappe stack into two major nappe complexes. However, the repetition of muscovite age data downwards through both nappe complexes (producing a "saw-tooth" profile) suggests that this later thickening event developed in the upper levels of a relatively cold Caledonian crust and was therefore not accompanied by major late-stage perturbation of the crustal isotherms.

Post-regional metamorphic fluid flow in marbles from the Reynolds Range, central Australia. I a n S. Bni£k T Department of Geology, La Trobe University, Bundoora, Vic. 3083, Australia, a n d I a n C a r t w r i g h t , Department of Earth Sciences, Monash

University, Clayton, Vic. 3168, Australia. An uncertainty in studies of regional metamorphic fluid flow is the possible superposition of a number of phases of fluid-rock interaction. The careful integration of penological, textural and isotopic evidence at a number of grades is therefore important if we are to understand the fluid history of a regional metamorphic terrain. In this contribution, we present evidence for two episodes of high-temperature, post-regional metamorphic fluid-rock (F/R) interaction in a calcsilicate unit from the Reynolds Range, in the Arunta Block of central Australia.

The Reynolds Range (Fig. 1) occurs within the northern province of the Arunta Block of central Australia, and was multiply deformed and metamorphosed during the mid-Proterozoic. Within the Reynolds Range, sediments of the Reynolds Range Group (RRG) contain the upper calcsilicate unit, a laterally extensive, thick, carbonate-rich horizon that is particularly useful for evaluating fluid-rock interaction histories. The main structural-metamorphic event to affect the RRG (D2-M2) post-dated an earlier metamorphic-deformational cycle (Dl-Ml) that affected only the basement to the RRG. Earliest (S2i) fabrics in the RRG pre-date contact metamorphic assemblages around pre-regional metamorphic granites. Contact metamorphic assemblages formed early in a possibly progressive event, M2i-2-D2i-2, that ended in penetrative deformation (S22), and pervasive regional metamorphism (M22). M22 metamorpism occurred at 4 to 5 kbar and ranged in grade from greenschist (ca. 350 to 400 °C) in the NW, to granulite (ca. 700 to 750°C) facies in the SE, of the Reynolds Range (Dirks et al. 1991). M22 isograds cut the range at near right angles to the strike of lithological units (Fig. 1).

The upper calcsilicate unit (UCS) comprises interlayered dolomite- and calcite-rich marbles, with minor intercalations of carbonate-bearing marls, biotite-psammites, and semipelites. At the lowest (greenschist facies) M22 grades, UCS marbles have heterogeneous carbonate stable isotope values in the range: 8 1 8 0 = 15.8 to 19.7 %o (mean 8 1 8 0 = 17.7±l.l%c); 5 1 3 C =-1.4 to 0.7 %c (mean 6 1 3 C = 0.5±0.5%c; n = 57 samples). Low 8 1 8 0 values, and isotope heterogeneity, are attributed to the pre-M22 contact metamorphic history of the UCS (Buick & Cartwright, this volume). At M22 granulite facies grades, stratigraphically equivalent marbles display a similar range of carbonate stable isotope values i.e. (mean 8 1 8 0 = 17.3 ± 1.2%c, mean 8 1 3 C = -0.5 ± 0.5%c; n = 47 samples). Mineral assemblages in calcite- and dolomite-rich marbles locally indicate CC>2-rich fluids (Xco2> 0.8-0.95) under peak M2 2 P-T conditions e.g the assemblages Cal-Dol-Qtz-Kfs-Cpx-Bt in some calcareous marls; Cal-Dol-Fo-Spin-Plg, and CalDol-Fo-Spin-Cpx in dolomitic marbles. Taken together, the stable isotope evidence and the mineral assemblages imply little fluid-rock interaction along the prograde M2 2 P-T-t path.

Mineral assemblages and stable isotope values at the high grade end of the Reynolds Range have been further modified by two F/R interaction events: a) channelled fluid flow that during late M2 2 ; and b) later channelled fluid flow that is associated with major mid-Proterozoic (M4-D4) amphibolite facies shear zones. Late-M2 2 fluid flow occurs in

semi-continuous, strike-parallel zones that are kilometres in length and 10 to 100m wide (Fig. 2), and that are spatial associated with undeformed pegmatitic and/or quartz-rich aluminous segregations (assemblage: Qtz-Sill-Cd±Bt±Grt) Fluid flow zones partially, to completely, overprint peak-M22 granulite facies marbles, and contain assemblages that equilibrated under upper amphibolite facies conditions. A variety of late assemblages occur, calcite-rich maittes are transformed into wollastonite marbles and massive, metasomatic wollastonite+Ca-garnet-rich skarns (Cartwright & Buick, this volume), and dolomite-rich marbles are altered to clinohumite-bearing marbles, and masssive, metasomati. clinopyroxene- and hornblende-rich layers. Alteration haloes in psammites around the pegmatites contain texturally late, 5 to 15 cm diameter Fe-rich garnets, and unoriented sillimanite. Quartz-bearing semipelites are also transformed into quartz-absent, cordierite-orthoamphibolei spineliorthopyroxene gneisses on an outcrop-scale within the fluid Oo zones. Unoriented orthoamphibole assemblages overprint peak-M22 fabrics, and are sometimes cut by M4-D4 shear zones. Mineral equilibria constraints indicate the infiltration of water-rich fluids (Xc02 to 0.15 at T« 600 to . 650 °C). Dolomitic marbles (40-90 %calcite) have carbonate S 0 and 8 C values that are lowered by upto «5 and 3 %© (Fig. 3), respectively, from mean syn-peak M2 values; additional lowering of isotope values occurs in carbona poor calcsilicates. Stable isotope data are most consistent with near-isothermal infiltration-driven decarbonation. \ Across-strike profiles do not reveal systematic lowering of isotope values with distance; rather, the great heterogeneitj of isotope values, and the preservation of locally unaltered marbles, indicates that fluid flow was primarily channelled along lithological strike. This is consistent with penological evidence that product and reactant mineral assemblages are interlayered on across-strike scales of millimetres to several centimetres, but are laterally persistent along strike foi tens of metres. 0 0 5

1 8

1 3

Where M4-D4 Shear Zones cut the UCS, calcite-rich marbles are characterised by a) equilibrated assemblages (e.g. Cal+Qtz+Trem+Kfs+Czo+Plg±Bt±Cpx) that are not developed elsewhere (typical assemblage in these marbless M22 granulite facies grades is Cal+Qtz+Kfs+Cpx±Bt±Pl), and b), carbonate stable isotope values in the range 8180 = 16.6 to 24.2%© (mean 8 0 = 19.2±2.0%c), and 8 C = 0.2 to -1.6%© (mean 8 C = -0.5± 0.6%©). Although 8 C values are similar to those determined elsewhere in the UCS, 8 ^ 0 values are ~2%© higher, on average, than in UCS marbles at either high or low regional metamorphic grades (Fig. 3). Mineral assemblages constrain fluid compositions to the range 0.05 < Xco2 < 0.2, for estimated M4 P-T conditions of (5 kbar, 550-600 °C). Rare reaction textures (tremolite overgrown by hornblende) suggest that mineral assemblages in shear zones developed during prograde metamorphism. The observed shifts in 5180 values, relative to granulite facies equivalents, is most consistent with fluid flow down temperature gradients (Dipple & Ferry, 1992). 1 8

1 3

Although the two channelled fluid flow events have both involved infiltration of water-rich fluids, the petrological aw isotopic signatures of each event in the UCS are markedly dissimilar. In particular, 8 ^ 0 values have been altered in opposite directions. The presence of such spatially close zones in a regional metamorphic terrain is obviously important; superposition of fluid flow events that lower 5180 values, with those that raise 8 ^ 0 values may result i; stable isotope signatures that are little different from sedimentary precursors! References: Dipple, G.M., & Ferry, J.M. 1992. Geochim. Cosmochim. Acta, 56, 3539-3550 Dirks, P.H.G.M. el al 1991. Journal of Metamorphic Geology, 9, 641-661.

I C^ N

BB Reynolds Range Group E3 Lander Rock beds 722 Early M 2 granitoids Q Pre-or-syn M«j granitoids ev ^ M4 shear zone M22 greenschist

IfeJvHarverson granite

+ Sill

0) CO o "35 o (0 O 0 Q. CL D

Figure 1: locality map9 Reynolds Range

Marble Fluid Infiltration

Pelite Unit Quartzite Unit O

<0.2 < 0.1 >0.9 >0.9

• Fo-Spin-Plg-Cc-Dol

>0.9

Woll-Cc-Qtz

• C'hum-Fo-Spin-Cc-Dol • Fo-Spin-Cpx-cc-Dol Bt-Kfs-Qtz-Cpx-Cc-Dol

Fig. 2: Distribution of low X C 0 2 assemblages in post-M2 2fluid flow zones

granulite fades marbles outside of fluid flow zones

Post-peak M2 2fluid flow zones

m o o. o

Dol-rich Mbls

Cal-rich Mbls

M4-D4 shear zones

O (SMOW) Fig. 3:lsotope alteration in post-peak M22 fluid flow zones, and M4-D4 shear zones 65

Cal-Mbl in M4 shear zones

Mid-Proterozoic Fe-metasomatism on the Stuart Shelf, South Australia. Paul. A.Gow. Department of Earth Sciences, Monash University, Clayton 3168 Australia and Victor. J .Wall, Mount Isa Mines Exploration, P.O. Box 1042, Brisbane,4001, Australia. Overprinting relationships in some Mid-Proterozoic metasomatic Fe-oxide units of the central Stuart Shelf show that Fe-oxide production, and associated Cu-mineralisation, occurred as a two stage process. The first stage consisted of deposition of high temperature magnetite (up to hornblende hornfels facies), commonly associated with skarn-like assemblages involving combinations of pyroxene-amphibole-garnet-quartz. These assemblages are commonly present as vein networks, some of which show open space filling textures, or as replacements. Host rocks range from felsic volcanics to metasiltstones, and the style of magnetite varies from almost massive magnetite (approx. 90% by volume) associated with the highest temperature assemblages, to disseminated magnetite associated with lower temperature biotite+quartz assemblages and interpreted to be a halo to the massive magnetite. K-feldspar alteration may be present associated with magnetite. The second period of Feoxide deposition consisted of production of hematite-chlorite-quartz-pyrite-Cu-sulfide assemblages, with the hematite commonly as an alteration product of precursor magnetite. It is unclear whether further iron was introduced in this period or existing iron was remobilised to form hematite+chlorite. Zones of hematite alteration are commonly associated with brittle deformation textures indicating a fracture control on fluid flow during this second stage of alteration. Prime examples of the two stages of Fe-oxide production, and their overprinting relationships, exist in the Arcoona area of the central Stuart Shelf. Whole rock stable isotope analyses of rocks from this area indicate that two different fluids were associated with the two periods of Fe-oxide deposition. Magnetite-rich samples yield a consistant 'magmatic' 80 1 8 of 8 per mil., whilst samples showing evidence of hematite alteration have variable 80 1 8 values of between 0-7 per mil., depending on the degree of brittle deformation, fluid infiltration and consequent hematite alteration. Thus, a model for the genesis of Cu-bearing, Fe-oxide zones in the Arcoona area involves two stages; 1) production of high temperature, magnetite-rich, skarn-like assemblages. These zones of alteration are interpreted as forming in pluton roof zones, with the vein networks that are associated with the highest temperature assemblages forming as a result of high fluid pressures associated with intrusion and volatile release. Some of these alteration zones are localised along large scale sub-circular structures (with arc lengths up to 30 km) interpreted as ring fractures or faults. This stage of alteration crosscuts felsic units of the Gawler Range Volcanics, suggesting that granitoids from the Hiltaba Suite, which is the only known major intrusive event to post-date the Gawler Range Volcanics, are the most likely source of heat and/or fluid for this period of metasomatism. 2) overprinting of the high temperature, essentially unmineralised, magnetite-rich assemblages by oxidised fluids which may produce hematite-rich, locally Cu-bearing, assemblages. The localisation of these assemblages, and the spatial correlation with brittle deformation textures, suggest this period of fluid advection is fault controlled. Fe-oxide-rich zones from elsewhere on the Stuart Shelf show evidence of assemblages associated with an early high temperature fluid which have been overprinted by lower temperature hematite-rich assemblages (ie; Olympic Dam and Acropolis, Oreskes & Einaudi, 1992, and Oak Dam, Davidson & Paterson, 1993). We suggest that this two stage model for Fe-oxide deposition, evident from the Arcoona area, may have a bearing on the genesis of other mineralised Fe-oxide units on the Stuart Shelf and in similar regions.

References. Davidson, G.J. and Paterson, H.L., 1993. Oak Dam East: A prodigious uranium-bearing massive iron-oxide body on the Stuart Shelf., in Abstracts from the 2nd National Meeting, SGEG, Armidale, Geol. Soc. Aust. pp.18-19. Oreskes, N. and Einaudi, M.T., 1992. Origin of Hydrothermal Fluids at Olympic Dam: Preliminary Results from Fluid Inclusions and Stable Isotopes, Economic Geology, v.87, pp.64-90.

The Jerangle Metamorphic Complex: Relationship to granites, structures and fluids. P.L. King, Geology Department, Australian National University, Canberra ACT 0020.

The Jerangle Metamorphic Complex is situated about 5 km west of Jerangle, NSW, and 80 km SSE of Canberra, within the Lachlan Fold Belt. The complex contains: chlorite metasediments, biotite phyllites, andalusite-cordierite schists and minor felsic dykes (Richards, 1967; Slepecki, 1973; Richardson, 1979). On a regional scale, the metamorphic grade is not controlled by proximity to the adjacent I- and A-type granites and only limited areas of hornfels have developed. The Jerangle region has been repeatedly subjected to relatively high heat flow for a considerable amount of time as evidenced by magmatic activity in the Ordovician, Early Silurian-Early Devonian, Late Devonian (?) and Tertiary. The local history is summarised in the accompanying figure. I- and A-type granite emplacement post-dates metamorphism and no genetic relation is proposed between these intrusive and metamorphic rocks. In contrast, a genetic relationship may exist between the metamorphic rocks and minor felsic aplite dykes which possibly represent partially melted metasediments. Unusual features of the Jerangle Metamorphic Complex are: (1) The higher grade andalusite-cordierite schists are distributed in a complicated, discontinuous geometry over a narrow interval (maximum of ~3 km wide). This distribution is encountered both along and across the strike of the ubiquitous cleavage in the area and does not appear to be controlled simply by structural features (cf. Glen, 1992). Instead, andalusite formation is considered a function of variable metamorphic conditions close to the andalusite stability field with fluctuating fluid content. Involvement of fluids in andalusite development is favoured over compositional variation in the protolith because there is an abundance of hydrous minerals (Al-rich micas) and pressure-solution cleavage is ubiquitous. (2) Negligible contact metamorphism has developed adjacent to the Devonian granites and P-T conditions estimated for the A-type granite emplacement (P = 100-400 MPa, T = 750-800°C) are only slightly higher temperature than peak metamorphic conditions (P = 200 MPa, T = 600°C). (3) It is uncertain whether the higher grade metamorphic rocks are equivalents of the lower grade Ordovician sedimentary rocks. The development of the Jerangle Metamorphic Complex has a regional significance as the complex belongs to the NS trending "Eastern Belt' of high-grade metamorphic rocks defined by Vallance (1967). The Eastern Belt includes the Cooma Metamorphic Complex and lies adjacent to the I-S line. An unsolved problem in the Eastern Belt is whether or not the high-grade metamorphic rocks, S-type granites and felsic aplites represent equivalents of the adjacent Ordovician metasediments. Some authors suggest that some of these units are equivalents and that partial melting (perhaps with fluid interaction) has occurred (Flood & Vernon, 1978; Munskgaard, 1988; White & Chappell, 1988; Williams et a/., 1991; King, 1992). Those workers base their ideas on lithological, field, geochemical and isotopic data and have various propositions for the proximity of the high-grade rocks to the protolith. Other workers suggest that the high-grade rocks are not continuous into the lowgrade Ordovician rocks, and thus invoke uplift for the higher grade rocks, based on structural, geochemical and phase equilibria constraints (Glen, 1992; Ellis & Obata, 1992). The possibility of fluid involvement in the evolution of the Jerangle Metamorphic Complex and the lack of structural discontinuities indicates that there may be a range of processes operating to produce the high-grade metamorphism observed in the Belt. This may also have implications for the relatively rapid uplift rate along the I-S line following peak metamorphism. In order to further constrain these problems each complex requires more detailed field, structural, geochemical and isotopic work.

STRATIGRAPHY OF THE JERANGLE AREA

Reactivation of the Narongo Fault to produce the current fault scarp

5 N o

Intrusion of alkafine basalts

UJ o

Intrusion of tholeiitic dykes Intrusion of Sapling Flat Igneous Complex A-type granite and associated felsic rocks (355-358 Ma) P « KXMOOMPa, T « 750-800°C

Deformation & faulting (380Ma) Reactivation of N-S faults and NW faulting associated with widespread granite emplacement. ? Tilting of the Cullarin Block

uJ < S > 5S

Intrusion of l-type granites (413-425 Ma)

Limestone development-sedimentation • rhyolitic-andesitic volcanism: Deposition with increasing water depth from N to S in a marine basin setting (along the present day Narongo Fault)

o > UJ

cc < UJ t

Deformation with metamophic minerals wrapping around peak metamorphic assemblages. Upright, tight folding and N-S cleavage formation

z < 0c

D -J

Peak metamorphlsm Possible minor partial melting at depth P r 200 MPa, T = 600°C

Deformation producing schistosity Ui Crustal melting and intrusion of felsic dykes Intrusion of mafic dykes

M il I5

Deposition of quartz-rich sedimentary rocks, sourced from the S and/or SW and contain zircons which record magmatic / metamorphic ages in the ranges: 450-650 Ma, 800-1075 Ma and 1075-3350 Ma. Minor volcanogenic component from the inferred craton to the W and/or from the S.

z< o >

o o

(Data from Slepecki, 1973; Rowley. 1975; Richardson, 1979; Hayden, 1980; Williams et al., 1991; D.Wyborn, pers. comm., 1991; Abell, 1992; King, 1992)

REFERENCES Abell, R., 1992. Geology of Canberra 1:100 000 Sheet Area. BMR, Canberra. Ellis, D.J. & Obata, M., 1992. Trans. R. Soc. Edin.: Earth Sci., 83, 95-106. Flood, R.H. & Vernon, R.H., 1978. Geology, 6, 81-84. Glen, R.A., 1992. Tectonophys., 214, 341-380. Hayden, P., 1980. PhD thesis (unpubl.), ANU. King, P.L., 1992. BSc (Hons) thesis (unpubl.), ANU. Munskgaard, N.C., 1988. Aust. J. Earth Sci.t 35, 363-377. Slepecki, S., 1973. BSc (Hons) thesis (unpubl.), Sydney Uni. Richards, D.N., 1967. BSc (Hons) thesis (unpubl.), ANU. Richardson, S.J., 1979. Geology of the Michelago 1:100 000 Sheet- Expl. Notes. Geol. Surv. NSW, Sydney. Vallance, T.G., 1967. Medd. fra Dansk Geol. Forening, 17, 494-506 White, A.J.R. & Chappell, B.W., 1983. G. S. America Memoir, 159, 21-34. Williams, I.S., Chappell, B.W., McCulloch, M.T. & Crook, K.A.W., 1991. G. S. Aust. Abstr., 29, 58. 69

Dry high-temperature shear in mid-crustal rocks Jorn H. Kruhl and Torsten Huntemann, Institute of Geology and Paleontology, JW Goethe-University, D6000 Frankfurt/M., Germany During isobaric cooling of the Hercynian former lower continental crust of Calabria (S.Italy) in a mid-crustal level [1], mm-cm thick retrograde shear zones developed mainly within K-feldspar-quartz-sillimanite-garnet layers of pelitic units. Shearing occurred during higher amphibolite down to mid-greenschist facies conditions, (i) The composition of deformed and recrystallized perthitic feldspars indicates maximum temperatures of shearing of about 670°C. (ii) Shearing occurred partly within the stability field of sillimanite. (iii) K-feldspar and plagioclase recrystallized. (iv) In contrast to the host rocks, within the shear zone fine-grained recrystallized quartz shows preferred c-axis orientations typical of dominant prism-a glide, (v) Within narrow zones sillimanite is rarely altered to white mica and quartz, biotite exsolves Ti, and feldspars are fractured and rotated. In general, cooling and hydration reaction textures (e.g. biotite-sillimanite-quartz symplectites) are deformed, (vi) Garnet-biotite thermometry from these zones leads to temperatures of approximately 440°C. Infra-red spectroscopic measurements of K-feldspar host and recrystallized grains from the shear zone show that the relatively low water content, which can be interpreted as a consequence of the earlier granulite facies metamorphism, was not increased during shear zone formation. Moreover, dry conditions are indicated by the (meta)stability of sillimanite and K-feldspar under upper greenschist facies conditions of shearing. Regional aspects are: (i) Shear zone formation occurred subsequent to the isothermal decompression of the Hercynian lower crust of south Calabria [1]. (ii) The time period of shearing was at least 40 Ma. (iii) During this time an approximately horizontal tectonic transport with constant direction and sense of shear occurred along this cm-wide shear zone, according to the cooling ages related to the P-T path of the lower crustal section [2]. (iv) The kinematic of shearing corresponds to the regional kinematic during the post-Hercynian development of the former lower crustal section [3] and possibly reflects long-lasting motions at the boundary between the African and the European plate. In addition, the concentration of movements in narrow shear zones over a long period of time might be an indication that at mid-crustal levels former lower crustal rocks can act as a rigid block which in most parts is not able to deform plastically. [1] Schenk (1990), in: Salisbury & Fountain (eds.), Exposed Cross-Sections of the Continental Crust, 21-42, Kluwer Academic Publishers. [2] Schenk (1980): Contrib.Mineral.PetroUX 23-38. [3] Kruhl: The base of the lower-crust section in southern Calabria (S.Italy): Nappe kinematics during postHercynian times. J.Struct.Geoi (subm.)

Low-pressure metamorphism in coastal Maine, USA: anticlockwise P-T paths from garnet zoning Helen M. Lang, Department of Geology and Geography, West Virginia University, PO Box 6300, Morgantown, West Virginia 26506-6300, U.S,A., and Timothy W. Grover, Department of Geology and Geophysics, University of Calgary, Calgary, Alberta T2N 1N4, Canada.

Metamorphosed pelites in the Casco Bay area range from garnet grade to sillimanite grade via staurolite-andalusite and staurolite-sillimanite zones. Most rocks show evidence of only one metamorphic episode, however, rocks from one exposure contain spectacular coarse-grained muscovite pseudomorphs after staurolite (or andalusite) and abundant, fresh, porphyroblastic staurolite, indicating that at least locally, the rocks have experienced polymetamorphism. Metamorphic textures indicate that garnet, staurolite and andalusite grew sequentially during compressional deformation and that sillimanite and prograde muscovite pseudomorphs after andalusite and staurolite grew after major deformation. Metamorphic grade increases toward the east, but increase in grade is not directly associated with any exposed pluton. There are a few small elongate plutonic bodies just to the east of the highest grade pelitic rocks, but the only large pluton, the Late Devonian Waldoboro Pluton, is approximately 35 kilometers to the northeast. West, et al. (1992, GSA Abs. with Programs, v. 24, p. A289) report late Devonian and early Carboniferous Ar cooling ages for hornblende in metamorphic rocks from the Casco Bay area. Metamorphism in the Casco Bay area apparently occurred during the Devonian Acadian Orogeny and may have been related to Devonian plutonism.

The Casco Bay area is in the Coastal Lithotectonic Block, which is separated from low-pressure metamorphic rocks of western Maine by the Norumbega Fault Zone. The Norumbega Fault Zone is a major zone of dextral strike-slip deformation on which major deformation ceased prior to Late Devonian (Hubbard, et al., 1991, GSA Abs. with Programs, v. 23, p. A311). Rocks of the Casco Bay area contrast with rocks of western Maine, which have been studied by Guidotti, Holdaway and others (e.g., Holdaway, et al., 1988, Am. Mineral., v. 73, p. 20-47), in that metamorphism in western Maine was almost entirely post-kinematic and metamorphic zone boundaries are commonly closely associated with Devonian and Carboniferous plutons.

Progressive changes in metamorphic mineral assemblage in metapelites from the Casco Bay area are consistent with AFM-Mn relations predicted from the petrogenetic grid of Spear and Cheney (1989, Contrib. Mineral. Petrol., v. 101, p. 149-164). Phase relations indicate that temperatures near 600°C at 2-3 kilobars were reached by sillimanite zone rocks. Thermobarometry yields maximum temperatures of ~550°C, perhaps because of diffusional reequilibration of garnet rims.

Semi-quantitative X-ray maps of garnet composition have been collected for samples from different metamorphic zones and assemblages in the Casco Bay area (see Figure). The X-ray maps show garnets in these samples, especially the cores, to be particularly Mn-rich. X-ray maps from a garnet zone sample (H-159) show a core-to-rim decrease in Ca, Mn and Fe/Fe+Mg, and a core-to-rim increase in Mg and Fe. This zoning, particularly the decreasing Fe/Fe+Mg, indicates garnet growth with increasing temperature (~450-520°C) in the assemblage garnet+chlorite +biotite+muscovite+quartz (pressure change is indeterminate). Because textural evidence indicates that garnet, staurolite, andalusite and sillimanite grew sequentially in high-grade samples (Lang & Dunn, 1990, J. met. Geol., v. 8, p. 199-216), it is likely that garnet cores in high-grade rocks also grew in the presence of chlorite and biotite with increasing temperature. Isopleths for Fe/Fe+Mg in garnet in the assemblage garnet+biotite+aluminosilicate are nearly horizontal (independent of temperature) in the andalusite and sillimanite fields, and a decrease in Fe/Fe+Mg for garnet grown in this assemblage indicates an increase in pressure during garnet growth (Spear (in press), Metamorphic Petrology, MSA). Garnet zoning maps for two samples that contain garnet+biotite+andalusite with chlorite (H-9) or staurolite (SMP-10) show a decrease in Fe/Fe+Mg in garnet rims, which grew in the presence of biotite and andalusite, that indicates a late increase in pressure. In both of these samples, Mn is very high in the core and decreases to - 2 0 mole percent in the rim. Ca decreases from core to rim in both garnets; Mg and Fe both increase toward the rim, although their ratio decreases. The garnet (from sample H-9) shown in the accompanying figure is fractured and the fractures are filled with quartz. Concentration of Fe, Mg, and Mn, and Fe/Fe+Mg in garnet adjacent to the fractures is identical to that in the garnet rim, indicating that garnet along the fractures equilibrated with the matrix minerals at peak metamorphic conditions. The presence of quartz-filled fractures and fluid inclusions in garnet and quartz suggest that an aqueous fluid was present near the metamorphic peak and that it enhanced diffusional equilibration of garnet. Note that intricate zoning in Ca is preserved near the garnet rim, and that Ca, which diffuses much more slowly than Mn, Mg and Fe, does not show diffusional equilibration along fractures. It is possible that fluid-filled fractures may have communicated with the surface and that Pf\u\d may have been less than Frock-

Textural relationships within individual samples, changes in metamorphic assemblage with increasing grade and phase relations all indicate that rocks from the Casco Bay area experienced and increase in temperature at low (and relatively constant (?)) pressure. Garnet zoning confirms this inference and adds the additional constraint that pressure increased near the peak of metamorphism as andalusite + sillimanite grew. The metamorphic pressuretemperature path for rocks from the Casco Bay area was, therefore, an anticlockwise one. The pressure increase may have been due to crustal thickening by bulk inhomogeneous shortening that produced the dominant near-vertical foliation. Rapid heating of the area, which is suggested by thermal modeling (DeYoreo, et al., 1989, J. met. Geol., v. 7, p. 169-190), is confirmed by elongate, almost dendritic, garnet textures. Rapid cooling is suggested by preservation of steep gradients and intricate zoning patterns in Mn and Fe in garnet (see Figure).

Lang & Grover Garnet Element Maps (Mole Fraction) H-9 Bt-Grt-Chl-And Assemblage (light areas-high concentration, dark areas-low concentration) Image size: 3.0 x 3.6 mm

Ca(.07-.02)

Mn(.56-.20)

core rim

Fe/(Fe+Mg) (.93-.89)

Fe(.34-.63)

core nm

core rim

Al-Mg granulites of Ihouhaouene (Mdle In Ouzzal, Hoggar-Algeria): an example of phase relationships in the KFMASH system and melt absent equilibria. Hassina MOIJRI. Michel GUIRAUD, Jean-Robert KIENAST, URA. CNRS 736 Universites ParisVI-VII, Museum; and Khadidja OUZEGANE, 1ST - USTHB ALGER Introduction Many reaction textures in granulite facies rocks can be interpreted within the KFMASH system ([1], [2], [3], [4]). The KFMASH petrogenetic grid presented by Hensen and Harley (1990) [2] provides a good description of high temperature textures but is restricted to melt bearing systems. The Archaean Al-Mg granulites from Ihouhaouene (Mole In Ouzzal-Hoggar-Algeria) do not display any evidence of melting which has has led us to investigate a grid differentfromHensen and Harley's one and accounting for the metamorphic textures recognized in these granulites. Petrography- The primary metamorphic mineral assemblage in the Ihouhaouene granulites is composed of porphyroblasts of garnet (Grt) together with inclusions of sillimanite (Sil), biotite (Bt), orthopyroxene (Opx) and rare quartz (Qtz). All these minerals are also in equilibrium with perthitic feldspar (Kfs) and plagioclase (Pig) within the matrix. Rare rutile (Ru) occurs as inclusion in orthopyroxene. This early assemblage is replaced by symplectites and corona reaction textures involving cordierite (Crd) ± sapphirine (Spr) ± orthopyroxene ± spinel (Spl) ± perthitique feldspar ± plagioclase. Paragenetic analysis- All the observed assemblages can be represented within the CNKFMASVT chemical system, as we do not consider Na, Cr, Zn and Mn as independent components. Moreover, according to many authors ([1], [2], [3], [4]) H2O released from the breakdown of hydrous minerals enters in a melt phase under granulite-facies conditions. As there is no evidence for melting in the Ihouhaouene granulites, all the fluid elements H2O, F, CI, CO2 i. e (V) have been considered to be distributed between biotite and cordierite only. Therefore, mineral compositions have been projected from the excess phase plagioclase, K-feldspar, orthopyroxene, biotite and rutile onto the plane AF-AM-S (Fig. la). The compatibility diagrams (Fig. lb, c, d, e) account for all the observed reactions and show that reactions (Al, A2, A3, A4, A6) are continuous and that reaction (A5) is the only discontinous reaction within the system. The question of melt occurence- Assuming that the observed reactions are not influenced by independent components (Ca, Ti, CI, F, CO2), our rock textures can be interpreted within the framework of the KFMASH system. However, The KFMASH petrogenetic grid [2] available so far for high temperature granulite facies involves excess melt, which is neither proved nor required for representing our parageneses. Therefore, applying this grid to the studied rocks is bound to occurrence of degeneracy, that is all the phases are coplanar and melt is not involved in reactions albeit in equilibrium within the thermodynamic system. In order to check this assumption, bulk compositions of charnockitic gneisses (L) and granites (L') from In Ouzzal have been projected in the same compatibility diagram (Fig. 2b) as for granulites parageneses. Moreover in order to take into account H2O releasing when melt crystallizes, various H2O contents have been chosen [8]. Such projections lead to inconsistencies with textural observations: projection of charnockitic gneiss compositions precludes garnet-sillimanite equilibrium and projection of granitic composition precludes quartz-K-feldspar equilibrium. Whatever the water content of the melt, the observed parageneses are not in equilibrium with melt. 74

This diagram shows also that the univariant reaction A5 is a melt absent reaction within the KFMASH system. Therefore, although F, CI, and CO2 could stabilize melt within the natural system, the excess melt grid of Hensen and Harley cannot be used for the determination of the P-T evolution of the studied rocks. Petrogenetic grid for melt absent equilibria - From the previous study, the petrogenetic grid for this system must have melt absent invariant points. Another difference with Hensen and Harley's grid is that H2O in cordierite must be considered because analysis totals are less than 100% [7]. Therefore the parageneses are more accurately represented in the KFMASH system by considering projection from biotite instead of melt. Elements for building this grid are: (i) the paragenetic analysis shows that observed melt absent reaction (A5) Grt + Spr = Crd + Spl + Opx (L, Qtz, Sil) is stable and (ii) the following reactions: (Rl) Grt + Spr = Opx + Sil + Spl (L, Crd, Qtz) and (R2) Sil + Opx = Spr + Qtz + Grt (L, Crd, Spl), are degenerate and common to the two grids. Therefore, the invariant point (L, Qtz) is located on the reaction Rl line and the observed (L, Qtz, Sill) reaction emanates from this invariant point. Geometry of this grid is obtained using Schreinemakers' rules [10], however invariant points locations and reaction slopes in P-T space are not exactly known. Nevertheless, presence of water in cordierite brings some constraints on the grid: (i) hydrous cordierite (crdH) bearing reactions probably have slopes close to that of anhydrous cordierite of Hensen and Harley [2], and (ii) as hydrous cordierite bearing assemblages are stable at higher pressure than anhydrous ones [9] invariant points (L, Qtz) and (L, Spl) are stable at higher pressure than (Qtz, Bt) and (Spl, Bt) points of Hensen and Harley [2]. Therefore the invariant points (L, Qtz) and (L, Spl) are stable approximative^ in the range of pressure and temperature of 8-12 Kbar, and 900-1100°C. This grid (Fig. 3) accounts for all the observed reactions whose succession indicates a decompression path consistent with the P-T evolution proposed for Al-Mg granulites from "Mole In Ouzzal" ([11], [12], [13]). Bibliography

[1] J. A. GRANT, In: Ashworth J. R. (ed.), Migmatite. London, Blackie, 1985, p. 86-144. [2] B. J. HENSEN et S. L. HARLEY, In: Ashworth J. R., & Brown M. (eds.), High-temperature metamorphism and crustal anatexis. London, Unwin Hyman, 1990 , p. 19-56. [3] A. B. THOMPSON, Am. J. Sci., 282, 1982, p.1567-1595. [4] D. VIELZEUF et J. R. HOLLOW AY, Contr. Mineral. Petrol., 98, 1988, p. 257-276. [5] S. L. HARLEY , B. J. HENSEN et J. W. SHERATON, J. Metam. GeoL, 8, 1990, p. 591-613. [6] B. D. GOSCOMBE, J. Petrology, 33, 1992, p. 917-962 [7] T. ARMBRUSTER et F. D. BLOSS, Nature, 286, 1980, p. 140-141. [8] W. JOHANNES et F. HOLTZ, In: Ashworth, J. R. & Brown, M. (eds.), High-temperature metamorphism and crustal anatexis. London, Unwin Hyman, 1990 , p. 87-71. [9] R.C, NEWTON, J. Geol., 80, 1972, p. 398-420. [10] E. An. ZEN, U. S. GeoL Surv. Bull., 1225, 1966, 56 pp. [11] P. BERTRAND, K. OUZEGANE et J.- R. KIENAST, J. Metam. GeoL, 7, 1992, p. 17-91. [12] J.- R. KIENAST et K. OUZEGANE, GeoL J., 22, 1987, p. 57-79. [13] K. OUZEGANE, These de doctoral d'etat. Univ. Paris 06 , N° 87-27, 1987,433 pp. [14] B. J. HENSEN et D. H. GREEN, Contr. Mineral. Petrol. , 38,1973, p. 151-166. t

CKFMASVT +Bt Si +Plg Jfr ^Plan de projection

M H(<a

AIMg + AIFe

c )

A1 - Sll + Qtz + (Opx) = CrdH Qtz

A2 - Grt + Qtz = (Opx) + CrdH A3 - Sil + (Opx) = CrdH + Spr CrdH

Grt

A5 - Grt + Spr = CrdH + Spl +(Opx) *) A6 - Grt = CrdH + (Opx)

d) A4 - Grt = Spr + CrdH + (Opx)

Fig. 1 Projection from: Bt, Kfs, Opx, Pig, Ru. (a), projection plane within the tetrahedron Fe-Mg-Al-Si; (b, c, d, e), compatibility diagrams showing observed reactions (A1 to A6) in Ihouhaouene Al-Mg granulites. Arrows indicate decreasing in AlMg/(AlMg+AlFe) in garnet. KFMASH system +Bt +Spr +Opx

FeAl

Fig. 2. Projection from: Bt, Opx, Spr. (a) projection plane within the tetrahedron HjO-SiCL-MgAl-FeAl, (b) compatibility diagram showing position of bulk compositions of granitic (L ) and charnockitic (L) melt and variation of water content in these melts. f

(Grt) ^

(CrdH)

v (L Crd) f

(CrdH)

Points invariants de Hensen et Harley, 1990

Fig. 3. P-T grid in the KFMASH system modified after Hensen and Harley, 1990 [2], Solid circles, stable invariant points; open circle, metastable point Invariant points (Bt, Qtz) and (Bt, Spl) after Hensen and Harley, 1990 [2]; (Bt, Qtz) is stable between 8 - 9Kbar, and 950°C [2], and (Bt, Spl) is stable at llilKbar and 1040±15°C f 141 76

PARTIAL MELTING AND LOCAL WATER ACTIVITY, [a(H20J], CONTROL DURING THE FORMATION OF

GRANULITE

FACIES

ASSEMBLAGES

THE

EINASLEIGH

METAMORPHIC

BELT,

NORTHEASTERN AUSTRALIA.

Sance Mundondo Metamorphic

and Structural

Geology Department,

Studies Group

James Cook University,

Townsville,

Q4811,

Australia.

Abstract The Einasleigh Metamorphic Belt exposed in an almost 50km long by 20km wide zone that stretches from the Einasleigh Village in the south to

Mount Surprise in the north within the

Georgetown Inlier, North Queensland, Australia, comprises a package of supracrustal units intensely deformed during middle Proterozoic times. Rocks within the Einasleigh Metamorphic Belt underwent upper amphibolite to granulite facies metamorphism. Biotite + garnet + sillimanite + cordierite + alkali feldspar + plagioclase paragenesis suggest peak metamorphic pressure-temperature (P-T) conditions of 650-750°C at 4-6 kbar (1 bar = 100KPa) and PH20 < Ptor Both psammitic and pelitic gneisses underwent anatexis which may have caused reduction in water pressure.

Field observations, phase relationships and mineral equilibria suggest that granulite facies assemblages at Einasleigh were formed by dehydration-melting reactions in which water from the hydrous silicate phases entered a locally generated melt phase leaving an anhydrous solid phase. Field evidence shows that the melt was locally generated in the absence of externally

derived

vapour phase. The melt phase and the solid residue are effective buffers for a(H20). Granulite facies metapelites and metabsic gneisses contain quartzofeldspathic segregations totally enclosed within granulite facies assemblages. These segregations are the liquid products of the melting reactions surrounded by the solid residue. Melting reactions involved are:

(i) Bio + grt + sil + qtz + plag = Grt + crd + Ksp + liq. (in the metapelites). (ii) Hbl + qtz = Opx + cpx + plag 4- liq. (in the metabasic gneiss)

The shape, size and distribution of the segregations suggest only limited migration and extraction of the melt. The growth of anhydrous poikiloblasts in matrix regions, development of anhydrous haloes around segregations and formation of dehydrated margins on metabsic layers enclosed in migmatitic metapelites all indicate local gradients in water activity. These features also show that individual segregations and bodies of partially melted rock acted as sinks for soluble volatiles. These observation clearly suggest that granulite facies assemblages can be formed locally by internally rather than externally buffered water activity mechanisms.

Formation of anhydrous granulite facies assemblages hinges on the behaviour of the fluid phase in the lower crust. However the role of fluids in lower crustal metamorphism is contentious. Contrasting models invoke influx of C0 2 -rich fluid or non pervasive flow of internally derived compositionally variable fluids. Recent studies of the evolution of the continental crust and the upper mantle have constantly drawn towards increasing recognition of the role of fluids in geological processes e.g. mantle and crustal melting and crustal metamorphism. A significant area of research is concerned with the role of fluids in thermal and geochemical evolution of the lower crust as explored through studies of exposed granulite facies terranes. Considerable controversy centres upon the ubiquitous occurrence of dense C0 2 -rich fluid inclusions in these granulites which have been interpreted as an unbiased sample of the fluid phase attending granulite facies metamorphism and leading to the modelss for the stabilisation of the anhydrous lower crust by pervasive flushing of externally derived C0 2 -rich fluids (e.g. Southern India, Hansen, Newton & Janardhan, 1984;).

Evidence provided by stable isotope and mineral equilibria studies of many granulite terranes (e.g. Adirondacks) demonstrates however both the importance of local fluid buffering mineral assemblages and the occurrence of outcrop scale gradients of volatile species activity and or stable isotope composition, thus indicating a non-pervasive, compositionally variable and transient nature for granulite fluids, (Valley & O'Neil, 1984). Such evidence ultimately questions the widely held belief that fluid inclusions are an acurate sample for granulite facies fluids and could even have wide implications for the study of fluids involved in lower grade metamorphism, ore deposit genesis and crustal geochemical fractionation.

Granulite facies rocks in the Einasleigh metamorphic belt were studied in order to establish the nature of lower crustal metamrphic processes giving rise to these rocks and also to test whether fluid behaviour was internally or externally buffered during granulite facies metamorphism. This paper presents initial results of this investigation. The results provide valuable initial constraints on lower crustal melting processes, water activity control and the evolution of granulite facies assemblages within an ancient cratonic mass.

References

Hansen, E. C., Newton, R. C., & Janardhan, A. S., (1984) Fluid inclusions in rocks from amphibolite facies gneiss to charnokite progression in Southern Karnataka, India: direct evidence concerning the fluids of granulite facies metamorphism., Journal of Metamorphic Geology, 2, 249264.

Newton, R. c „ Smith, J. V., & Windley, B. F., (1980). Carbonic Metamorphism, granulites and crustal growth. Nature, 288, 45-50. Valley, J. W. & O'Neil, J. R., (1984). Fluid heterogeneity during granulite facies metamorphism in the Adirondacks: stable isotope evidence: Contributions to Mineralogy and Petrology, vol. 85, p. 158-173.

THE STRUCTURAL AND METAMORPHIC EVOLUTION OF THE WOOLANGA BORE AREA, NORTHEAST STRANGWAYS RANGE, CENTRAL AUSTRALIA

N.J. Post. Department of Applied Geology, University of New South Wales, PO Box 1, Kensington, 2033.

The Granulite terrain of the Woolanga Bore area, within the Central Tectonic Province of the Arunta Block, central Australia, consists predominantly of mafic granulite and quartzofeldspathic gneiss, with minor calcsilicate rock, metapelitic gneiss and granitic orthogenesis.

Petrological, chemical and structural evidence indicates a protracted geological history, which can be divided into three main stages:

1. Protolith formation 2. High grade metamorphism and deformation, followed by isobaric cooling, and 3. Uplift and final unroofing of the terrain

The protoliths of the Strangways Range comprise a conformably layered bimodal volcanic suite, concordantly interlayered with supracrustal rocks. The protoliths are presumed to have formed in a rifting (not convergent) sub-aqueous environment.

Peak metamorphism (Ml, at approximately 1800 Ma) consisted of two phases (PI & P2) and occurred at 8-9 kbars, 850-950°C. Unaligned idioblastic and polygonal granoblastic textures indicated that no deformation accompanied the peak of the first phase (PI) of the high grade metamorphism. Deformation was responsible for the intense recrystallization which characterises the second high grade phase (P2) and outlasted the granulite facies event. Mineral parageneses indicate that isobaric cooling occurred subsequent to P2, and that early in the period of isobaric cooling, the granulites were partially retrogressed and metasomatised by fluids released from crystallizing partial melts.

Final exhumation of the terrain, during the Alice Springs Orogeny, probably occurred along crustal scale eastwest trending shear zones.

The phlogopite-mineralisations in the Beraketa Belt of southern Madagascar: a spectacular example of channelised fluid flow and fluid-rock interaction Michael M, Raith, Stefan Pierdzig, Stephan Hoernes, Mineralogisch-Petrologisches Institute Universitat Bonn, 5300 Bonn, Germany, and Raymond Rakotondrazafy, Laboratoire de Geologie, Universite d'Antananarivo, Madagascar. The famous phlogopite mineralisations of southern Madagascar (Lacroix, 1941) occur within a 350 km N-S trending zone of isoclinaly folded and intensely sheared granulite-grade supracrustal rocks and orthogneisses (the Pan-African Beraketa shear belt). In order to update the existing data base of the phlogopite deposits and to unravel the processes of fluid-rock interaction which led to their formation, we have undertaken a detailed geological, petrological and geochemical study of the phlogopite mineralisations at Ampandrandava (Pierdzig, 1992; Rakotondrazafy, 1992). The steeply dipping banded rock association at Ampandrandava comprises (1) two pyroxene-plagioclase gneisses (the hosts of the mineralisations) with bands and lenses of scapolite-bearing varieties and orthopyroxenites, (2) light garnet-sillimanite gneisses with bands of garnet-cordierite-sillimanite gneisses, quartzites and leptynitic gneisses, (3) rare metapelitic cordierite-orthopyroxene-garnet-plagioclase gneisses with spinel/sapphirine, (4) rare calc silicate rocks and marbles, (5) late veins and small intrusive bodies of syenite. The phlogopite mineralisations were formed subsequent to high-grade metamorphism and ductile deformation, during Pan-African uplift and cooling (K-Ar phlogopite data range between 491 and 485 m.y.). They are exclusively confined to the basic pyroxene-plagioclase bands; their arrangement, shape and paragenetic zonation indicates formation through channelized fluid flow along a system of late fractures. An extremely coarse-grained aggregate of anhydrite, calcite, giant euhedral phlogopite and apatite forms the central part of the mineralisations Cpoches", pockets). It is surrounded by a monomineralic granoblastic diopsidite zone from which large euhedral diopside crystals grew into the "pegmatitic" aggregate. This points to mineralisation into open fissures. Depending on the size of the pockets (diameters range from a few dm up to 30 m) and the therewith connected amount of infiltrating fluids, the host pyroxene-plagioclase gneisses have been subjected to metasomatic alteration from cm- to m-scale. While the smaller pockets develop only a thin diopsidite seam, at the contact to larger pockets, the surrounding monomineralic diopsidite zone (D) is separated from the host gneiss by subsequent zones with phlogopite+diopside (PHL), diopside+amphibole+ spinel (DA), and amphibole-rich pyroxene-plagioclase gneiss. Geochemical data obtained from sequential samples through several mineralizations and their host gneisses allow to constrain the mass transfer between the metasomatic zones and the pocket, and the concomitant volume changes. Mass transfer calculations following Potdevin and Marquer (1987) reveal a similar behaviour of Ga, Y and Sm in the metasomatic alteration zones D, PHL and DA. If these elements are considered as immobile, a volume increase relative to the host gneiss in the order of 20 to 30% is calculated. A1 behaves similar to Ga, Y and Sm in the outer zones PHL and DA (also indicating a volume factor of 1.32), but was mobile and depleted in the inner metasomatic zone D. The changes in the major element chemistry across the mineralisations reflect the mineralogical changes. Relative to the host gneiss, the diopside-dominated zones DA and D are characterized by a significant import of CaO, MgO and S0 3 , the phlogopite-dominated zone PHL in addition by a significant gain of K 2 0. All the zones (including the poche) were depleted in Na 2 0. The pocket shows extreme gains of CaO, MgO, K 2 0, P 2 0 5 , S0 3 as well as of the incompatible trace elements Rb, Sr, Ba, Th and U. Compared to the host gneiss, mass transfer calculations for the pocket indicate a drastic volume increase by almost five times. It is inferred that the major part of the pocket was created by mineralisation of an opening fissure and only a minor part by replacement of the host gneiss.

Stable isotope data for minerals of the pockets indicate fluid temperatures of about 475 °C and isotopic characteristics of the fluid (8 0=9°/ , d C=-7%o, 5D=-31°/oo) which compare with those of fluids of metamorphic or magmatic sources. The fluid pressure was in the order of 1.5 to 2 kb, as deduced from the isochores for rare C0 -inclusions (Tm=-56 to -58.5 °C; Th=25.5 to 28.5 °C; p= 0.65 gem" ) in the host gneisses and rare aqueous inclusions of high salinity (p= 0.02 gem" ; 23 mol% equ.NaCl) in minerals of the pocket. Based on the new observations and data, a genetic model is discussed and contrasted with previous concepts. 18

References:

Lacroix, A. (1941). Les gisements de phlogopite de Madagascar et les pyrox6nites qui les renferment. Ann. G6ol. du Serbv. des Mines de Madagascar, XI, 7-119. Pierdzig, S. (1992). Geochemie und Petrologie granulitfazieller Gesteine der Ampandrandava-Formation Siidmadagascars und die Entstehung ihrer Phlogopit-Mineralisationen. Unpublished Ph.D. Thesis, Universitat Bonn, Germany. Potdevin, J.-L. and Marquer, D. (1987) M6thodes de quantification des transfers de matifere par les fluides dans les roches m6tamorphiques d6form6es. Geodinamica Acta, 1, 193-206. Rakotondrazafy, R. (1992). Etude thermobarometrique d'une s6rie metamorphique retrograde dans le secteur d'Ampandrandava (Sud Madagascar). Unpublished Ph.D. Thesis, University d'Antananarivo, Madagascar.

Textural constraints on timing relationships between metamorphism and deformation in the west-central Mary Kathleen Fold Belt, Australia Jurgen Reinhardt, Institutfur Mineralogie, Ruhr-Universitat Bochum, D-4630 Bochum, Germany

Most commonly, metamorphic rocks preserve only a small part of the pressure-temperature-deformation history they have experienced. However, the chance to retrieve the P-T-d record increases with the variety of rock compositions present in one terrain. If a prograde succession of metamorphic pangeneses in each rock type can still be recognized to some extent, and effects of retrogression are developed locally, the combined reaction sequences may cover a relatively large part of the P-T path. Microscale textures can then be used to obtain critical information on the timing relationships between successive P-T path segments and corresponding stages of rock deformation. Such timing constraints are crucial for interpreting the P-T history in relation to tectonic and thermal processes that trigger metamorphism.

A detailed examination of aluminous schists and cordierite-orthoamphibole rocks from the Mary Kathleen Fold Belt (central Mount Isa Inlier) revealed that these rocks contain a comprehensive record of the P-T path related to the main contractional episode D 2 (Reinhardt, 1992). This deformation is characterized by non-cylindrical, tight upright folds, with wavelengths on a ten-kilometre scale down to centimetre scale. Associated are pervasive, N-S trending axial planar fabrics, and a vertical mineral alignment in S 2 (Reinhardt, 1992a, 1992b; Holcombe et al. 1992).

The inclusion patterns of porphyroblastic minerals document early to late stages in the development of the S 2 foliation. The oldest preserved microstructures are undeformed to weakly D2-crenulated S J - L J fabrics. Judging from crenulated, S r parallel quartz veins overgrown by andalusite, the bulk D 2 shortening strain at lower amphibolite facies conditions was still below 20%. In contrast, the latest prograde minerals overgrew a pervasive S 2 with no remains of Dj structures. The change in fabric orientation from Dl to D 2 is accompanied by a distinct increase in matrix grain size. These microstructures allow to classify the porphyroblast generations as early- to late-D2, keeping in mind that such timing relationships will commonly change vertically and laterally within a metamorphic rock pile (Reinhardt & Rubenach, 1989). Due to a variation in metamorphic grade from west to east, the foliation development was slightly more advanced in the lower-grade area, for equivalent metamorphic reactions.

Apart from purely structural criteria, the growth sequence of the metamorphic minerals is also constrained by reaction textures (e.g. overgrowth, coronas, partial replacement) which furthermore allow to identify the underlying metamorphic reactions. Using timing constraints and reaction relationships in conjunction with a theoretical reaction grid for the pelite model system KFMASH, the reaction history could be reconstructed (see also Reinhardt, this volume). The early- to late-D2 reaction sequence in the aluminous schists involved early cordierite and four generations of Al 2 Si0 5 polymorphs which formed from different reactions, at different stages 83

of D (early: andalusite I, cordierite I, andalusite II; late: andalusite III, sillimanite I). This sequence cannot be found in a single rock, but requires a certain range of Mg/Fe and Al/(Mg+Fe) ratios in bulk rock composition. In relatively K-Al-poor rock types such as cordierite-orthoamphibole-biotite rocks, early- and late-D generations of cordierite can be distinguished (cordierite I, cordierite II + anthophyllite). The Al^iC^ generations of the retrograde P-T path (sillimanite II, kyanite, andalusite IV) occur in chlorite-quartz-rich rocks that are unfoliated. 2

From the integration of structural field data and microscopic L-S fabrics with reaction textures it is concluded that prograde, low-pressure, high-temperature metamorphism was entirely synchronous with D crustal shortening. The end of D coincided with the thermal peak of metamorphism. Retrogression did not involve any significant ductile deformation at the exposed crustal level. Evidently, the high-temperature geothermal gradient had been established early in (or even before the onset of) D and therefore cannot be considered as a result of the crustal thickening process. It appears that the pervasive thermal weakening of the crust to relatively shallow levels was a precondition for achieving the large amount of ductile shortening (>60%) observed here. 2

References: Holcombe, R J., Pearson, P.J. & Oliver, N.H.S., 1992. Austral. Geol. Survey Org. Bull. 243, 257-287. Reinhardt, J. & Rubenach, MJ., 1989. Tectonophysics 158,141-161. Reinhardt, J., 1992a. Geol. Magazine 129,41-57. Reinhardt, J., 1992b. Austral. Geol. Survey Org. Bull. 243,229-255.

Provenance of the Cambrian Liitzow-Holm Complex, East Antarctica Kaznvnyi Slhiffli-itii, Yoichi Motoyoshi, National Institute of Polar Research, Itabashi-ku, Tokyo 173, Japan, Yoshikuni Hiroi, Department of Earth Sciences, Chiba University, Chiba 260, Japan, David J. Ellis, Department of Geology, Australian National University, Canberra, ACT 2601, Australia and C. M. Fanning, Research School of Earth Sciences, Australian National University, Canberra, ACT 2601, Australia.

In the sector of 30-45°E in East Antarctica, two high-grade metamorphic complexes, the Liitzow-Holm Complex (LHC) in th east and the Yamato-Belgica Complex (YBC) to the west have been distinguished on the basis of protolith, type of metamorphism and igneous activity. The exact boundary between these complexes has not been established because of the continental ice sheet. Ion microprobe (SHRIMP) dating has been carried out on zircon separates from both metasedimentary and meta-igneous rocks of these complexes. U-Pb ages from six widely separated localities throughout the Liitzow-Holm Complex (LHC) show that the timing of regional metamorphism and folding in the LHC were between -520 and -555 Ma as recorded by new growth of metamorphic zircon. Many zircons contain centres which record a wide range of inherited ages from -2900 to -1500 Ma. An -1000 Ma zircon component has been identified in three locations. An -1000 Ma zircon component may indicate that either the -1000 Ma adjoining Rayner Complex to the east was a source for the Liitzow-Holm paragneisses or that a similar crustal prehistory is recorded in parts of the LHC. A heterogeneous zircon population of a metasedimentary gneiss from YBC gives a discordant age of -2470 Ma. Although other analyses from the gneiss do not define unique ages, it is inferred that variable Pb loss at -600 Ma occurred in zircons that originally crystallyzed at -1000 Ma. We have already reported for the first time, the existence of a Pan-African mobile belt within the East Antarctic Shield (Fanning et al., 1991; Shiraishi et al. , 1992). This work provides additional evidence for an improved fit to the once contiguous fragments of Gondwana. The Highland/Southwestern Complex (HSWC) of Sri Lanka has remarkable penological similarities to LHC (Hiroi et al. 1987, 1991; Ogo et al., 1992). This metamorphic correlation is now supported by the identification of -500 Ma age for LHC as the German- Sri Lankan Consortium have also shown that the Sri Lankan HSWC metamorphism and folding was at -550 - 600 Ma. The age relations provide more specific correlations between Sri Lanka and Antarctica. The Wanni Complex, the HSWC and Vijayan Complex of Sri Lanka are the direct continuation of the Rayner Complex ex, Luzow-Holm Complex and the Yamato Mountains respectively of the East Antarctic Shield. Thus the characteristics which enable separation of three belts within each continent correlate with available data in terms of Nd model ages, U-Pb zircon ages, lithologies and style of metamorphism. Gondwanaland did not finally amalgamate until the termination of Pan-African orogenesis at -500 Ma (e.g. Dalziel, 1992). It is noteworthy that the provenance of the LHC developed at the margin of Late Proterozoic Supercontinent including Gondwana and Laurentia cratons. LHC-HSWC orogenesis coincided with largescale convergence between East and West Gondwanaland.

THE STRUCTURAL EVOLUTION OF THE LAKE JULIUS AREA.

Itta Somaia- Department of Earth Sciences, Monash University, Clayton 3168, Australia.

To the immediate west of Lake Julius (80 kilometres north of Mount Isa) lies a pattern of north to east trending folds. The folds are overprinted by an array of faults resulting in a complex puzzle of reoriented folds and fault blocks. The faults, bedding and cleavage form surfaces of the Lake Julius rocks have been mapped in detail at a 1:5000 scale.

The stratigraphy of the Lake Julius area consists of pelites, sandstones, dolomites, quartzite's, and various types of conglomerates. The rock types are divisible into Myally subgroup, Bigie Formation, Quilalar Formation, and the Surprise Creek Formation. Pre-main phase folding (Fj 1 ^) is characterised by an east-west trending synform, of a noncylindrical, open nature. A weak axial planar slaty cleavage is associated with this F j l j fold.

Main phase folds (F2^) have a variety of trends within a north-west to north circumference. The generation of the F 2 ^ folds has been determined in a number of ways. Within the F j ^ fold Si cleavage has been overprinted by a north-south crenulation cleavage associated with an open north-south trending synform. This has resulted in a basin like structure attributed to Di L J -D2 L J interference. Other D2 LJ folds have axial planar slaty cleavages with evidence of earlier intersection lineations.

It is suggested that cleavage nature ie slaty, spaced or crenulation is not necessarily indicative of structural generations in these low grade rocks. Crenulation cleavages are present in fold hinges of Surprise Creek pelites, traced into spaced cleavages in Quilalar sandstones and further into slaty cleavages in Myally rocks. The cleavage type is therefore dependent on the rock type and the amount of strain subjected to the area. A variety of fold types characterise D2 LJ , these range from open, tight, isoclinal to disharmonic. This range in fold types shows how rock competency has effected the natures of the same generation of folds. Fold natures are also effected by post main phase faulting.

The large scale faults (kilometres in length) are dated at post D2 u and generally have an easterly, north-west or north-east trend. The truncation of F2LJ folds are common and occasionly S2 cleavage reorients into these faults. Drag folds (post D2 LJ -syn D3 LJ ) are associated with some of these faults. The fault zones are characterised by becciated quartz and rock gouge. Dips on these faults are thought to be steep due to the steep plunges of the drag folds which lie adjacent and within the fault zone.

Post main phase faulting has been interpreted in terms of strike slip and normal faulting which fit into a 'wrench-type' environment. Transpression and block rotations may explain the variety of orientations found for D2 L J folds and D3 L J faults. This is comparable to the 'popout model', Lister (1986) which has been applied to the Lake Julius area.

Reference Lister G. (1986)- BMR Research Newsletter, 1986. Transpressional strike slip faulting in the Mount Isa Inlier.

Structural history, alteration and gold mineralization in the Lapland Greenstone Belt, Finland. Peter Soijonen-Ward, Geological Survey of Finland, SF-02150, Espoo, Finland. The Lapland Greenstone Belt is the largest contiguous greenstone terrain exposed in the Fennoscandian (Baltic) Shield, extending over 500 km from the Norwegian coast, through Finnish Lapland and into adjacent Russian Karelia. The belt is of particular interest since it records protracted but episodic ensialic rifting and volcanism over a period of more than 500 Ma, commencing around 2.53 Ga and lasting until the Svecofennian Orogeny at 1.89 Ga. Evidence for a sialic substrate comes from deep seismic profiling, indicating a low density basement at 5-8 km depth (Ga41 et al., 1989), which presumably corresponds to the basement gneisses exposed in the late Archean (3.1-2.7 Ga) Karelian Craton at the eastern and southern margin of the belt Evidence of an autochthonous origin for at least the lower part of the succession includes paleoregoliths, abundant intercalations of mature terrigenous clastic sediments, crustal contamination of komatiites and the emplacement of extensive 2.44-2.39 Ga Cr-rich mafic layered intrusions across the basement-greenstone contact However, an allochthonous origin cannot be entirely precluded for the upper part of the sequence, which is characterized by pyroclastic komatiitic and submarine tholeiitic volcanism, with intercalated iron formations. Very little structural evidence for the nature of extension has been found, although if it is assumed that the layered intrusions along the present southern south margin of the greenstone belt represent the disrupted remnants of a formerly contiguous intrusion, their overall geometry is consistent with rotation within a system of SW-dipping listric normal faults, as schematically depicted in Figure la. Further evidence of episodic extension is provided by a variety of dike swarms preserved within the craton south of the Lapland greenstone belt, at 2.2, 2.10, 2.05 and 1.97-1.96 Ga (Vuollo et al, 1992). The latter age coincides with that of the Jormua and Outokumpu ophiolites (Kontinen, 1987), which were emplaced onto the Karelian craton margin during the Svecofennian Orogeny. The evidently modern character of the latter contrasts markedly with the Lapland Greenstone Belt, which is more more reminiscent of classical Archean successions; this is clearly of relevance in assessing putative changes in magmatic and tectonic processes at the Archean-Proterozoic boundary. The youngest volcanics within the Lapland greenstone belt are 1.88 in age and are felsic to intermediate in composition. They occur in restricted areas associated with synorogenic redbed sedimentation and and are possibly consanguineous with coeval granitoids; they clearly postdate the earliest stages of deformation of the greenstone belt and maybe correlative with the Kiruna porphyries in northern Sweden (Ward et al., 1989). This has enabled the timing of deformation to be determined rather precisely, between 1.89 and 1.88 Ga. The presence of lower greenschist facies synorogenic sediments also implies that the main part of the greenstone belt was never deeply buried and possibly indicates that the underlying crust was neither excessively thinned during extension, nor greatly thickened during thrusting. During compressive deformation, the Lapland Greenstone Belt appears to have behaved as a foreland to two thrust systems of opposing polarity. The northern margin was overthrust from the NE by the Lapland Granulite Complex. Related S-SW vergent structures are found along the northern margin of the greenstone belt, with local overturning of strata and asymmetric recumbent to reclined folds; abundant depositional younging criteria nevertheless preclude the existence of major recumbent fold nappes, while greenschist facies metamorphism (excluding contact aureole effects) militates against deep tectonic burial of the terrain as a whole. A maximum age of 1.93 Ga for this thrusting event is imposed by mylonitized and metamorphosed granitoids, while provisional interpretations of Sm-Nd and SHRIMP zircon data (Claou6-Long, Huhma & 88

Soijonen-Ward, in prep.) indicate that the youngest metasedimentary protoliths were scarcely depsoited before granulite metamorphism and rapid exhumation. The timing of deformation at the southern margin is less well constrained but the earliest folding must have predated 1.89 Ga granitoids and synorogenic sedimentation and volcanism. Many of the geometrical features of the Lapland Greenstone Belt can be interpreted in terms of a clockwise torque imposed on at least the upper crust by movement associated with two major N-NE trending dextral shear systems, namely the Tornio Deformation Zone along the Swedish border and the Posio Zone in eastern Finland (Figure lb). This has resulted in spectacular asymmetric fold interference patterns at regional scale, as well the curving and branching of shear zones, producing a mosaic of high and low strain domains and a general eastwards bulging or 'extrusion* of the central part of the greenstone belt (Figure lb). Intense hydrothermal alteration is characteristic of both volcanic and clastic sedimentary lithologies throughout much of the greeenstone belt, with pervasive albitization being particularly prominent and carbonate-scapolite bearing assemblages being widespread. Timing and microstructural observations indicate that these alteration processes generally predate compressive deformation (Eilu & Idman, 1989, Ward et al., 1989) and although a such altered rocks are common associated with gold mineralization, a direct genetic relationship has not been demonstrated. Rather, the altered sequences may be regarded as having been both compositionally and structurally favourable for fluid-rock reaction and gold precipitation during regional metamorphism and deformation. In particular, the common occurrence of sericite and biotite in host-rocks that were originally potassium-poor suggests that such metamorphic mineralizing fluids were not of local or sea-water origin, thus strengthening the argument for gold being epigenetic. Gold mineralization occurs in a wide variety of lithologies and structural settings including (i) retrogressed shear zones within the allochthonous granulites; (ii) carbonate-quartz vein networks in a range of altered mafic to ultramafic host rocks now reprented by massive albitites, or heterogeneous chromian fuchsite schists and marbles. In some cases mineralization occurs in relatively late brittle veins, while elsewhere young veins are barren and mineralized systems have been folded and sheared; (iii) biotite-dominated schists representing syntectonic alteration of talc-chlorite-actinolite parageneses in ultramafic lavas and pyroclastics, with gold occurring in brittle-ductile quartz-baryte-tremolite veins; (iv) syntectonic felsic dikes displaying albite-carbonate alteration; (v) a distinctive sericitic quartzite formation within a predominantly sedimentary sequence at the southeastern margin of the greenstone belt. Several superimposed alteration parageneses have been recognized, with gold being closely associated with biotite and sericite alteration, where rocks are more foliated, or carbonate and silica alteration associated with vein-type mineralization (Pankka & Vanhanen, 1992). Mineralization in this area is broadly associated with a refolded antiform. Therefore, instead of invoking fluid flow along a yet to be identified regional shear zone, a feasible alternative may be structural enhancement of the permeability of particular stratigraphic units during folding, as a result of mechanical contrasts between different lithologies in this multilayered sequence. Selected references: Eilu P & Idman H, 1989. Bulletin of the Geological Society of Finland 60: 115-127. Gail G, et al., 1989. Tectonophysics 162: 1-25. Kontinen A, 1987. Precambrian Research 35: 313-341. Lehtonen M I, et al., 1985. Geological Survey of Finland Report of Investigations 71, 56 pp. Pankka H S & Vanhanen E J, 1992. Precambrian Research 58: 387-400. Vuollo J, et al., 1992. Geological Survey of Finland Bulletin 363, 32 pp. Ward P, et al., 1989. Geological Survey of Finland Special Paper 10: 71-77. 89

E X T E N S I O N A L PHASE

(ca 2 . 5 - 2 . 0 Ga)

- Na and carbonate hydrothermal alteration associated with sea water circulation

Hydrothermal a l b i t e - c a r b o n a t e alteration of clastic quartzite and t h o l e i i t e - k o m a t i i t e sequence

Deep level fluid convection and high heat flow during basin extension

Enhanced fluid flow in listric normal and transfer fault systems

Au in association with porphyry dykes and syntectonic granitoids Au p r e c i p i t a t i o n in shear zone r e l a t e d vein s y s t e m s

Metamorphic inversion beneath allochthonous granulites

R e a c t i v a t i o n of transfer faults as ductile shear zones

Brecciation of altered quartzites during folding enhances flow of Au mineralizing fluids

Inversion of extensional faults as thrusts during Svecofennian orogeny

C O M P R E S S I O N A L PHASE

(ca 1.9 Ga)

- K - e n r i c h m e n t due to m e t a m o r p h i c f l u i d c i r c u l a t i o n , r e s u l t i n g in s e r i c i t e - b i o t i t e a l t e r a t i o n a s s e m b l a g e s .

Episodicity in Metamorphism, Deformation and Magmatisn in Low-P High-T Terrains.

Kurt Stiiwe and Michael Sandiford, Department of Geology and Geophysics, Adelaide University, GPO Box 498, Adelaide, SA5001, Australia

Low-P , high-T metamorphism must be the consequence of advective heat transfer, for example the upward passage of granitic magma. This is because such PT conditions cannot be attained by heat conduction for reasonable lithosphere deformation geometries. Whereas such heating mechanisms are necessarily extremely short-lived, the lifetime of the underlying orogenic processes is likely to be much longer and it is conceivable that, during the evolution of an orogen, repeated transient heating at shallower crustal levels is caused by episodes of segregation and upward passage of melts generated at deep crustal levels. We refer to such repeated transient heating as multiple event superposition. The number and timing of these events would depend on such factors as critical melt segregation volume, Moho-temperature and strain-rate evolution. We investigate some of the controls on multiple event superposition using a simple thermo-mechanical model designed to predict, simultaneously, the strain-rate, isostatic and thermal evolution of convergent orogenesis, subject to constant driving force. A critical melt segregation volume is reflected in the episodic upwards intrusion of granitic melts during a monotonic T-evolution of the Moho, causing repeated metamorphic-deformation events at shallower levels. We show that episodicity of events as the consequence of such processes may be be characterised by wavelengths of the order of tens of Ma and may last up to several 100 Ma (Figure 1). An appealing aspect of this formulation is that, in contrast to models that have attempted to explain event cyclicity through episodic processes acting at orogenic boundaries, it accounts for the repeated occurrence of events through processes inherent to the system itself. In fact, structural, metamorphic and geochronological work in many low-P high-T terrains has shown that multiple event histories frequently occur within time-spans comparable to the lifetime of modern day orogenic belts (100-200 my). Moreover, the greatest effect of the events may be seen focussed on one area. In the light of our model we discuss some possible examples from Central Australia and the Namara belt of southern Africa.

100

150

time (my)

Figure 1.: Strain rate (a) and temperature (b) at mid crustal levels plotted against time for a model lithosphere deforming in a convergent environment. Multiple peaks in the temperature and strain rate evolution arise from our assumptions about the melting bahaviour of the lower crust with a melting interval between T i=700°C and Tnq =1200°C and critical segregation volumes of the order of 30% at geological strain rates and over critical length scales appropriate to pluton sizes of the order of kilometres. The dashed line indicates the strain rate evolution if no melting is assumed. so

Deformation and metamorphism in the southern Halls Creek Mobile Zone, East Kimberley, Western Australia. Kerry Turnock: Dept of Earth Sciences, Monash University, Clayton, Victoria.

The relative timing of deformation and thermal events are important to consider in polydeformational terrains. This study focusses on intensely deformed medium grade rocks towards the.south of the Halls Creek Mobile Zone.

The Halls Creek Mobile Zone is a complex polydeformed terrane located in the East Kimberley, Western Australia. Metamorphic grade within this mobile zone increases from greenschist facies in the south up to granulite facies in the north.

Stratigraphic units of the Halls Creek Group were deposited prior to the

Barramundi Orogeny, dated at 1854±6Ma. The study area is composed of meta-sediments belonging to the mid to upper Biscay Formation and the lower Olympio Formation.

These units were intruded by numerous

Woodward Dolerite sills prior to the Barramundi Orogeny.

The Olympio Formation is located on the east side of the Halls Creek Fault. This Formation is found both above and below the older Biscay Formation. To explain this relationship, in the absence of faulting, it is proposed that this interfingering contact may be due to :

a) simultaneous deposition of the Biscay and Olympio Formations. OR b) tectonic interleaving of the two Formations.

Four phases of deformation have affected the rocks within the study area. Associated with deformation are three phases of regional metamorphism. Prior to regional metamorphism a contact metamorphic event (Mi) took place in the NW corner of the study area as the result of the intrusion of a shallow level igneous complex (predominantly rhyolite/rhyodacite) into carbonate country rock.

Dj is the oldest deformation event in area. It is a weak, locally developed deformation producing a NNE trending Si cleavage. D\ is only preserved in the phyllitic sediments. D2 is a high strain event producing the dominant structures in the study area. It is the result of the regional Barramundi Orogeny. The structural elements formed by D2 include a pervasive NNE trending S2 slaty cleavage (open crenulation cleavage in the presence of Si) and a weak foliation in rocks of the igneous complex in the NW sector of the study area. F2 folds are NNE trending open to isoclinal, upright, shallowly plunging folds. Late D2 to pre D3 macroscopic NNE/NE trending faults are widespread throughout the study area. The sense of displacement along these faults is unknown due to the lack of stratigraphic control either side of the faults.

Associated with D2 is M2, the peak regional metamorphic event.

M2 can be constrained to an upper

temperature limit of 500°C with pressures between 3.5kb to 4.5kb thus indicating upper greenschist to lower amphibolite facies conditions. Comparison of peak metamorphic grade across the Halls Creek Fault (HCF), which bisects the study area, found that there is no difference in the metamorphic grade. Likewise the structures located on either side of the HCF show no significant differences.

The Woodward Dolerite intruded the study area pre-D2- In response to regional deformation and metamorphism the sills have developed different compositional zones. This can be related to the thickness of the sills, with larger sills preserving higher temperature and relict igneous assemblages in their core. Chloritic schists located on dolerite margins are probably an intensely deformed and retrogressed equivalent of the dolerite.

D3 is a semi-pervasive deformation event producing an E-W trending crenulation cleavage, which has developed into a differentiated crenulation cleavage in regions. F3 folds are characteristically kinks trending E-W and have vertical plunges. Faulting associated with D3 is E-W and SE-NW trending conjugate strike-slip faults. M3 is associated with D3. M3 is a regionally extensive retrograde event producing greenschist facies assemblages. The conditions of the M3 are 300°C to 450°C and 2.5kb to 3.5kb.

D4 is the youngest deformational event in the study area. It forms a weak, locally developed crenulation cleavage. Associated with D4 is a minor retrograde M4 event.

Microstructural evolution during deformation and metamorphism the Vortex, Mount Isa Inlier, Queensland, Australia.

David Young: Department of Earth Sciences, Monash University, Clayton, Victoria 3 J 68, Australia.

Microstructural analysis of quartz-rich rocks from the Mount Isa Inlier indicates deformation textures developed during a two-phase tectonic event were greatly modified by static and dynamic recrystallization. High temperatures from syn-tectonic metamorphism, coupled with low strain rates, favoured grain boundary migration over rotation recrystallization processes.

Significant compressional deformation occurred in the Mount Isa Inlier between 1620 and 1520 Ma. Compression was accompanied by low pressure metamorphism, varying from lower greenschist to upper amphibolite facies throughout the Inlier. Between one and three generations of macroscopic structures are present in many areas, indicating deformation was heterogeneous. Debate concerns the question of whether these were distinct events separated by considerable time breaks, or whether they were increments of a progressive shortening event.

The field area under examination, informally referred to as the " Vortex

is located approximately 15

km southwest of Mount Isa. The main lithologies present are quartz-rich rocks (quartzites, feldspathic quartzites, quartzofeldspathic gneisses and quartz/feldspar/mica schists) of the Mount Guide Quartzite and the May Downs Gneiss (lower Haslingden Group). These units are exposed in the core of a major antiformal F1 fold and are flanked by the structurally overlying Sybella Batholith.

Detailed mapping in the study area has defined two phases of deformation, both manifest at all scales. The earliest structures recognized are associated with a large-scale, north/south, tight, inclined antiformal fold (F1), with an S1 axial surface foliation. These features are overprinted by a later fold and foliation episode (F2/S2), resulting in complex fold superposition and modification of the earlier SI fabrics. The intensity of S2 fabrics varies from areas where it is dominant and masks pre-existing features, to those where the S2 imprint is weak and earlier textures are preserved. Peak metamorphic grade of upper amphibolite facies (650-700°C) at 3-4 Kb was reached syn- to post- S1. Timing critera include (1) hornblende in metamorphosed mafic rocks aligned in the SI foliation, ( 2) micas in migmatitic segregations (partial melts) lack an SI fabric (post-Si), while the migmatites themselves are folded by F2 folds (pre-F2), (3) rare sillimanite deformed by S2.

The textures of quartz, potassium feldspar, plagioclase and mica at the micro-scale show the evolution of heat and deformation (Figure 1). S1 fabrics were subject to static recrystallization after their development, and little of their syn-deformational morphology is preserved. Quartz-rich lithologies containing few other phases have undergone exaggerated grain growth, resulting in a very coarse grainsize and almost complete obliteration of the earlier foliation (although a weak to very weak relict foliation is often discernable).

phases such as feldspar and/or mica. In such rocks, a preferred alignment of mica grains (biotite + muscovite) defines the relict SI foliation. Quartz and feldspar show little preferred grainshape and are often relatively equigranular with smooth to gently lobate grain boundaries.

In areas of overprinting by S2, dynamic recovery and recrystallization microstructures are superimposed upon the earlier statically recrystallized textures. Crystal plastic strain (undulose extinction, deformation bands and lamellae) is ubiquitous in quartz grains, indicating deformation outlasted elevated temperature conditions that would have otherwise erased these features (Fig. 1). The SI mica fabric is crenulated, and decussate textures in mica grains suggest deformation took place by intragranular kinking, followed by subgrain rotation and recrystallization. Subgrain recovery structures are also common in quartz, and new grain development by progressive rotation of subgrains is occasionally apparent. Low strain rates may be implied by the relatively low incidence of rotation recrystallization. Complex grainshapes in quartz and feldspar, and irregular and serrated quartz boundaries imply the operation of grain boundary migration driven by intragranular defect energy. Little evidence of lattice strain is apparent in feldspars.

Microstructures developed within the two fabric generations (S1/S2) indicate temperatures were significantly elevated throughout both deformations. This would neccessitate two distinct amphibolite facies metamorphic events if SI and S2 were separate in time and unrelated. It seems more likely that the metamorphism was due to a single thermal pulse, and both structural generations developed during this event.

Figurel. Schematic diagram showing the relationship between temperature and deformation.

temperature

c T

time

The structural, igneous and metamorphic evolution of the Mount Lofty Ranges, South Australia. Sylvia Zakowski, Nicholas H.S. Oliver, and Ian Cartwright, Department of Earth Sciences, Monash University, Clayton 3168, Australia Models have recently been presented for regional thermal weakening of the crust by granite emplacement, with consequent localisation of metamorphism and deformation(Sandiford et al., 1992). Aspects of this model have been tested in this study by detailed structural analysis of fabrics and melt relationships in the south eastern Mt Lofty Ranges, South Australia. The study area consists of Cambrian Kanmantoo Group sediments that were folded, metamorphosed and intruded by various granitoids during the Delamerian Orogeny between 520 and 480 Ma. The rocks have been subjected to at least three deformation events. The first resulted in the development of a spaced cleavage that is now isoclinally folded and only preserved locally. The second deformation is much more pervasive and is evident as a well developed crenulation cleavage, or as a spaced cleavage and a sub horizontal extension lineation. F2 folding is tight to isoclinal and is now present as upright to recumbent folds depending on the degree of reorientation by F3. Shearing is also associated with the D2 deformation event. The original orientation of S2 was probably sub-horizontal. The third deformation has resulted in the local development of an S3 crenulation cleavage as well as open upright folding on both mesoscopic and macroscopic scales. The major anticlinal and synclinal closures in the Ranges are of this generation. The meta-sediments have been metamorphosed to upper amphibolite grade, with peak temperatures of 530° to 680° C, at pressures of 3-5 kbars (Sandiford et al., 1990). Fibrolitic sillimanite is elongated in S2 and is observed pseudomorphing andalusite indicating that peak metamorphic conditions in the high grade rocks were reached pre- to syn- D2. Three generation of melt segregation are recognised. Field observations, together with the petrological data and preliminary geochemical analysis of the migmatites suggests that they originated as partial melts of the quartzofeldspathic schists and gneisses in which they are found. The most abundant migmatites are the stromatic migmatites that are parallel to the dominant schistosity S2. The relationship between the stromatic, nebulitic and ptygmatic migmatites and the structural fabrics indicates that partial melt was present, post Dl, syn D2 and post D2. Therefore the data indicates that the metamorphic peak was reached relatively early in the high grade rocks. In contrast andalusite and staurolite in lower grade rocks overgrow complex crenulations, suggesting the metamorphic peak occurred late. These observations are consistent with the model of Sandiford et al. (1992) in which emplacement of granite triggers high grade metamorphism and partial melting with subsequent heat conduction to cooler regions. The model of Sandiford et al. (1992) predicts that the complexity of deformation should be less in low grade areas compared to the high grade areas because granite emplacement early in the history also 97

localises deformation. However this recent mapping exercise has shown that D2 and D3 deformations are pervasive throughout both the high and low grade regions. Thus, although the thermal peak was reached early in the high grade part of the belt as predicted by Sandiford et al. (1992), the distribution of deformation features is probably not a response to thermal weakening by granites. An alternative may be that there were two separate phases of deformation, the first involving sub horizontal shearing and granite emplacement and the second involving upright folding.

References. Sandiford, M., Oliver, R.L., Mills, K.J., and Allen, R.V. (1990). A cordierite-staurolite-muscovite association, east of Springton, Mt Lofty Ranges; implications for the metamorphic evolution of the Kanmantoo Group, Geol. Soc. Aust. Spec. Publ., 16, 483-495.

Sandiford, M., Foden, J., Zhou, S and Turner, S., (1992). Granite genesis and the mechanics of convergent orogenic belts with application to the southern Adelaide Fold Belt. Second Hutton Symposium, Proc. R. Soc. Edinburgh, 83: 83-93.

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CONTENTS: QUANTITATIVE DATA COLLECTION:Quantitative clastic reservoir geological modelling: problems and perspectives; Alluvial architecture in a sequence stratigraphic framework: a case history from the upper cretaceous of southern Utah, USA; Sedimentary architecture of field analogues for reservoir information (SAFARI); a case study ofthefluvial Escanilla formation, Spanish Pyrenees; Quantitative facies analysis of coal-bearing sequences in the Bowen basin, Australia: applicationsto reservoir description; Quantification of turbidite facies in a reservoir analogous submarinefan channel sandbody, southern-central Pyrenees, Spain; Outcrop studies of shale smears on fault surfaces; Predicting reservoir sandbody orientation from dipmeter date: the use of sedimentary dip profiles from outcrops; Applications of the formation micro-scannerto modelling of Palaeozoic reservoirs in Oman; Permeability patterns in point bar deposits: Tertiary Loranca basin, central Spain; Knowledge base development for the estimation of reservoir rock properties in the interwell area: examples from the Texas Gulf coast; MODELLING METHODS: Sedimentary flow units in hydrocarbon reservoirs: some shortcomings and a case for high-resolution permeability data; The use of 3-D seismic in reservoir geological modelling; The use of length distributions in geological modelling; A theoretical study of fluvial sandstone body dimensions; Stochastic modelling of fluvial sandstone bodies; A 3-D modelling approach for providing a complex reservoir description for reservoir simulations; Index

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In 1992 Included: Effects of fluid production on fluid flow during regional and contact metamorphism, R.B. Hanson Calcite-graphite isotope thermometry: A test for poly metamorphism in marble, Tudor gabbro aureole, Ontario, Canada, S.R. Dunn and J.W. Valley Ionic associations in H 2 0 - C 0 2 fluids at mid-crustal conditions, J. V. Walther Stable isotopic and penological constraints on scapolitization of the Whitestone meta-anorthosite, Grenville Province, Ontario, D.P. Moecher, E.J. Essene and J.W. Valley

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ill You Read In 1993? Fluid infiltration and regional metamorphism of the Waits River Formation, Northeast Vermont, USA, A. Leger and J.M. Ferry

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Partially retrograded eclogites of the Munchberg Massif, Germany: Records of a multistage Variscan uplift history in the Bohemian Massif, P.J. O'Brien

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