Archaean Yilgarn Block Research Articles

The effects of three-dimensional structure on two-dimensional images of crustal seismic sections and on the interpretation of shear zone morphology

SUMMARY Crustal scale seismic images provide information on the geometry of subsurface structure. In this paper we examine shear zones as they provide geometrical constraints on the evolution of the crust and as they provide pathways for the migration of mineral-rich fluids from the lower crust. However, they typically appear in seismic images of the deep crust as laterally continuous bands of discontinuous reflections with individual reflections often having high amplitudes. Geological mapping of exposed shear zones show them to have a complex 3-D structure yet crustal-scale seismic reflection surveys use single or at the most only a few profiles, and therefore only create 2-D images of these structures. The processing and imaging of the multifold common midpoint (CMP) data assumes that the seismic energy comes entirely from within the plane of the section. In this paper, we use full-waveform 3-D synthetic data to consider the effects that 3-D topography on a reflector has on reflection character on a 2-D profile. We base our synthetics on an observed shear zone and test models with both a single layer and anastomizing layers. We show that topography on the reflector out of the plane of the section may cause spurious events both above and below the expected target depth. We derive the basic understanding using a simple isotropic homogeneous model, however, we then demonstrate that this is a robust phenomenon and is endemic on all 2-D sections even if the overburden is not homogeneous. We demonstrate that we obtain similar results with a velocity gradient or, for a more extreme case, with a 2-km-deep basin filled with low-velocity sediment. For crustal scale seismic profiles, in particular, the effect is pervasive as neither stacking nor migration can discriminate against out-of-plane energy and the 2-D stack represents the 3-D seismic response of a broad swath centred on the profile. However, we conclude that using the modelled data it is possible to identify qualitatively where there is significant contamination from out-of-plane topography and show examples from a shear zone in the Archaean Yilgarn Block in Western Australia.

Chromite in Komatiites, II. Modification during Greenschist to Mid-Amphibolite Facies Metamorphism

Chromite compositions in komatiites are influenced by metamorphic chromite is highly susceptible to modification during processes, particularly above 500°C. Metamorphosed chromite is early hydrous alteration and subsequent prograde metasubstantially more iron rich than igneous precursors, as a result of morphism of host rocks. This modification is the subject Mg–Fe exchange with silicates and carbonates. Chromite metaof this paper. morphosed to amphibolite facies is enriched in Zn and Fe, and Metamorphic modification of chromite has been disdepleted in Ni, relative to lower metamorphic grades. Relative cussed extensively in the literature in the context mainly of proportions of the trivalent ions Cr, Al and Fe are not ophiolitic or ‘alpine ultramafic’ complexes (Onyeagocha, greatly modified by metamorphism up to lower amphibolite facies, 1974; Ulmer, 1974; Evans & Frost, 1975; Hoffman & although minor Fe depletion occurs during talc–carbonate alWalker, 1978; Loferski & Lipin, 1983; Kimball, 1990; teration at low temperature. Significant Al is lost from chromite Burkhard, 1993) and in a few studies of komatiitic rocks cores above 550°C, as a result of equilibration with fluids in (Bliss & MacLean, 1975; Donaldson, 1983; Gole & Hill, equilibrium with chlorite. Elevated Zn content in chromite is 1990), and in a detailed study of the Pechenga intrusions restricted to rocks with low (metamorphic) Mg/Fe ratios, and is (Abzalov, 1998). These studies have highlighted two the result of introduction of Zn during low-temperature alteration, important effects. First, chromites become rimmed and with further concentration and homogenization during prograde progressively replaced by chromian magnetite or ‘fermetamorphism. Cobalt and Mn also behave similarly, except where ritchromit’. Second, chromite core compositions become carbonate minerals are predominant in the metamorphic assemblage. progressively modified during prograde metamorphism Chromite at amphibolite facies is typically extensively replaced by as a result of exchange of components with surrounding magnetite. This is the result of incomplete metamorphic reaction silicate minerals (Evans & Frost, 1975; Abzalov, 1998). between chromite and chlorite-bearing silicate assemblages. MagThis paper examines the nature and magnitude of these netite compositions at the inner chromite–magnetite boundary are effects in komatiites from a variety of localities in the indicators of metamorphic grade. Eastern Goldfields Province of the Archaean Yilgarn Block in Western Australia (Fig. 1). Brief locality descriptions are given in Appendix B.

Pb isotope fingerprinting of mesothermal gold deposits from central Victoria, Australia: implications for ore genesis

New Pb isotope data from three major mesothermal lode gold deposits (Ballarat West, Tarnagulla, Maldon) in central Victoria support a model whereby the metals derived from a large reservoir with a long residence time in the crust below the Palaeozoic Lachlan Fold Belt. The Pb isotopic ratios of least radiogenic samples from these deposits are in close agreement with published Pb signatures for turbidite-hosted gold deposits, and for Devonian granites, elsewhere in the Lachlan Fold Belt. Despite their spatial distribution and variations in the geological setting, the Pb signatures point to the extraction and transport of metals from a crustal source area by long-lasting, large-scale hydrothermal systems, resulting in the prominent homogenisation of Pb isotopic ratios. The enduring interaction between large hydrothermal systems and an extensive crustal source reservoir were a vital pre-requisite in the formation of the Victorian gold province. In this regard, the prospectivity of Victoria is analogous to world-class ore provinces elsewhere, such as the Archaean Yilgarn Block in Western Australia.

Alteration and primary geochemical dispersion associated with the Bulletin lode-gold deposit, Wiluna, Western Australia

The Bulletin lode-gold deposit is within the northernmost part of the Norseman–Wiluna greenstone belt in the Archaean Yilgarn Block, Western Australia. It is located within a brittle–ductile shear zone and hosted by tholeiitic metavolcanic rocks. Syn-metamorphic wallrock alteration envelops the gold mineralisation and is pervasive throughout the entire shear zone and extends up to 150 m into the undeformed wallrocks. Alteration is characterised by the sequence of distal chlorite–calcite, intermediate calcite–dolomite, outer proximal sericite and inner proximal dolomite–sericite zones. The thickness of the alteration envelope, and the occurrence of dolomite in the alteration sequence, can be used as a rough guide to the width, extent and grade of gold mineralisation, because a positive correlation exists between these variables. Mass transfer evaluations indicate that chemical changes related to the wallrock alteration are similar in all host rocks: in general, Ag, As, Au, Ba, CO 2, K, Rb, S, Sb, Te and W are enriched, Na and Y are depleted, and Al, Cr, Cu, Fe, Mg, Mn, Nb, Ni, P, Se, V, Zn and Zr are immobile, while Ca, Si and Sr show only minor or negligible relative changes. The degree of mobility of each component increases with proximity to gold mineralisation. The largest potential exploration targets are possibly defined by regional As (>6 ppm) and Sb (>0.6 ppm) anomalies. These anomalies, if real, extend laterally for >150 m from the mineralised shear zone into areas of apparently unaltered rocks. Anomalies defined by Te (>10 ppb), W (>0.6 ppm), carbonation indices, local enrichment of Sb (>2.0 ppm) and As (>28 ppm), and potassic alteration indices also form significant exploration targets extending beyond the HJB shear zone and the Au anomaly (>6 ppb) and, locally, into apparently unaltered rock. Gold, itself, has a restricted dispersion, with an anomaly extending for 1–35 m from ore, and being restricted to within the shear zone itself. Amongst individual geochemical parameters, only As and Sb define significant, consistent and smooth trends (vectors) when laterally approaching the ore. However, the respective dimensions of individual geochemical anomalies can be used as an extensive, though stepwise, vector towards ore.

Factors in the formation of the giant Kalgoorlie gold deposit

Kalgoorlie (Golden Mile plus Mt Charlotte) accounts for more than half the gold won from the 2000 deposits that comprise the widespread gold mineralization of the Archaean Yilgarn Block of Western Australia. It is also one of the largest gold deposits globally (> 1800 t Au production and reserves). An analysis of the various stages of formation of the Kalgoorlie deposits from source, through solution of gold in hydrothermal fluids and their infiltration along channelways, through to gold precipitation at depositional sites reveals several features that make Kalgoorlie favourably disposed as a site for a major gold deposit. However, none alone account for its anomalously large size. It is concluded that the giant Kalgoorlie gold deposit owes its size and unique position in Western Australian gold production history to favourable host-rock chemical composition, favourable host-rock thickness and mechanical properties, and to the favourable geometry of its host units and hosting greenstone belt, at favourable PT conditions within the greenschist facies. Collectively, these factors, together with proximity to the regional-scale Boulder-Lefroy fault, allowed highly focussed fluid flow on both a deposit and district scale. In this regard, it is significant that the structural setting of gold mineralization and the greenstone architecture at the largest Canadian Archaean deposit (Timmins) are remarkably similar to Kalgoorlie. Orebody size-distribution histograms for both the Yilgarn Block and the Abitibi Belt, Canada, are highly skewed, with extremely few giant deposits, but numerous deposits of ≤ 10 tonnes gold existing. This topology can be accounted for in terms of system efficiency. The probability of all six factors in the ore-forming system being optimized (high heat flow, large fluid reservoir, proximity to major structure, efficient fluid focussing, and large and compositionally favourable host lithology) is extremely small. In contrast, the probability of two processes coupling to give low efficiency is very large, resulting in numerous small deposits. Kalgoorlie appears to have formed by the same basic processes that formed the plethora of smaller Yilgarn gold deposits. However, it represents a mineralized domain in which there was a conjunction of a number of favourable parameters, both on a regional and deposit scale, that produced high system efficiency and allowed the effective fluid focussing and gold deposition that formed this giant deposit.

An indirect lead isotope age determination of gold mineralization at the Corinthia mine, Yilgarn Block, Western Australia

The Corinthia lode‐gold deposit in amphibolite‐facies greenstone belt rocks in the Southern Cross Province of the Archaean Yilgarn Block contains a largely undeformed pegmatite dyke emplaced during the last phases of movement along the Fraser's‐Corinthia shear zone. Gold mineralization and shear zone development were synchronous, and a Pb‐Pb isochron age of 2620 ±6 Ma for pegmatite emplacement either indirectly dates mineralization, or places a minimum age constraint on the timing of mineralization. This age is in accord with a broadly synchronous dominant episode of Archaean lode‐gold mineralization throughout the Yilgarn Block.

Archaean Au−Ag mineralisation at Racetrack, near Kalgoorlie, Western Australia: a high crustal-level expression of the Archaean composite lode-gold system

The Racetrack Au−Ag deposit, in the Archaean Yilgarn Block, Western Australia, is hosted by a porphyritic basalt in a low greenschist facies setting and is associated with a brittle strike-slip fault system. Three distinct and successive stages of hydrothermal activity and late quartz-carbonate veining resulted in multiple veining and/or brecciation: Stages I and II are Au-bearing, whereas Stage III and late veins are barren. The ore shows features of both classic epithermal and mesothermal deposits. Alteration assemblages, typified by sericitization, carbonization, silicification and chloritization, are similar to those of mesothermal gold deposits, wheras the quartz vein-textures including comb, rosette, plumose and banded, ore mineralogyof arsenopyrite, pyrite, chalcopyrite, sphalerite, galena, freibergite, tetrahedrite, tennantite, fahlore, electrum and gold, and metal associations (Cu, As, Ag, Sn, Sb, W, Au and Pb) are more characteristics of epithermal deposits. Fluid inclusions related to Stage II are two phase and aqueous with 1–8 (average 4) wt. % NaCl equiv. and CO2 content of <0.85 molal. Pressure-corrected homogenisation temperatures range from 190°C to 260°C. Mineral assemblages indicate that ore fluid pH ranged between 4.2 and 5.3, fO 2 between 10−38.8 and 10−39.6 bars, and mΣs between 10−3.2 and 10−3.6. Calculated chemical and stable isotope compositions require a component of surface water in the ore fluid depositing the mineralisation, but evidence for deep crustal Pb indicates that deeply sourced fluids were also involved. The deposit is interpreted to have formed in a shallow environment via mixing of deeply sourced fluids, from at least as deep as the base of the greenstone belt, with surface waters. It therefore represents the upper crustal end-member of the crustal depth spectrum of Archaean lode-gold mineralisation.

The classification of Western Australian greenstone-hosted gold deposits according to wallrock-alteration mineral assemblages

The epigenetic, greenstone-hosted gold deposits in the Archaean Yilgarn Block of Western Australia can be classified according to their silicate-carbonate gangue, which forms the innermost wallrock-alteration zone of the structurally controlled hydrothermal systems. The proposed classification is descriptive, and emphasizes associations of prominent, macroscopically visible minerals. The gold deposits are divided into two large groups: high-temperature deposits (400°–700°C) formed at pressures of 3–5 kbars in the lower parts of the greenstone belts (10–15 km depth), and medium-temperature deposits (250°–400°C) formed at pressures of 1 to 2 kbars in the upper parts of these belts (3–7 km depth). The high-temperature deposits are usually located in metamorphic terranes of medium grade (amphibolite facies), and are characterized by either microcline-muscovite-andalusite, garnet-pyroxene-biotite or amphibole-biotite-calcite alteration in the ore zones. Deposits showing the latter two alteration styles may also be classified as skarns in the descriptive sense of Einaudi and Burt (1982). The classic medium-temperature (mesothermal) deposits, studies of which form the basis of all current models on Archaean gold metallogenesis, represent only about one half of the total population of deposits in the Yilgarn Block, although they dominate by total gold production, as they include the only giant deposit in Western Australia, i.e., the Golden Mile at Kalgoorlie. The medium-temperature deposits are usually located in metamorphic terranes of low grade (greenschist facies), and are characterized by biotite-ankerite-albite or sericite-ankerite-albite alteration in the ore zones. It is possible that some of these deposits grade into amphibole-biotite-calcite skarns at depth.

The implications to exploration of chalcophile corridors in the Archaean Yilgarn Block, Western Australia, as revealed by laterite geochemistry

The regional distribution of the chalcophile and associated elements As, Sb, Bi, Mo, Ag, Sn and W in laterite over the Archaean Yilgarn Block of Western Australia is dominated by elongated trends within which chalcophile abundances are relatively high compared with general backgrounds. As seen from orientation studies, laterite geochemistry in Western Australia generally provides an imprint of geochemical patterns observed in bedrock, albeit with some degree of dispersion. These chalcophile trends in the lateritic duricrust materials imply the existence of chalcophile geochemical provinces in bedrock and mark out the spatial distribution of such provinces across the Precambrian craton. The chalcophile trends revealed by regional multi-element laterite geochemistry are typically 15–30 km wide and 50–100 km long, some extending to 150 km. There appears to be a broadly coincident spatial relationship between the chalcophile trends and the distribution of a variety of ore types, including gold and base-metal sulphide deposits. The name “chalcophile corridor” is herein proposed because the trends appear to be the geochemical expression of the gold exploration corridors recognized some years ago. Some of the chalcophile corridors appear to spatially link with major faults and regional shear zones. Whether the chalcophile associations in laterite are reflecting hydrothermal processes of rock alteration and mineralization along shear zones, or hydrothermal activity at Archaean volcanic centres or intrusions is presently an open question. Much more research needs to be directed at identifying in bedrock the specific rock types/structural associations that give rise to the anomalous chalcophile elements. The concept of chalcophile corridors appears to have important implications in mineral exploration because of the possibility of delineating the most prospective terrain at the reconnaissance stage of the exploration operations.

A structural and metamorphic traverse across the Albany Mobile Belt, Western Australia

The Albany Mobile Belt is divided into three domains (the Southern, Central and Northern Domains) based on their aeromagnetic response and their distinctive structural and metamorphic characteristics. Amphibolite-facies rocks of the Northern Domain are reworked Archaean Yilgarn Block gneisses and dolerite dykes with a progressive, southwards increase in the intensity of deformation and metamorphism. The Northern Domain-Central Domain contact is marked by an oblique thrust zone with a component of dextral transcurrent movement developed during the second deformation event (D 2). The Central Domain consists of two-pyroxene granulites, together with quartz-magnetite gneiss, which has undergone penetrative, ductile, non-coaxial deformation with a dominant dextral transcurrent component during the first deformation event (D 1), and has subsequently been folded around kilometric-and metric-scale folds overturned to the NNW during D 2. The Central Domain grades transitionally southwards into the Southern Domain, where retrograde amphibolite facies rocks dominate. The Albany-Fraser Province is a Proterozoic mobile belt, formed during two prolonged tectonothermal events, both of which consist of a combination of dextral transcurrence and northwards thrusting.

The distribution of 3.0 and 2.7 Ga volcanic episodes in the Archaean Yilgarn Block, Western Australia

The distribution of 3.0 and 2.7 Ga volcanic episodes in the Archaean Yilgarn Block, Western Australia

The Perseverance Ultramafic Complex, Western Australia: The Product of a Komatiite Lava River

The Perseverance ultramafic complex is a body of olivine-rich komatiitic rocks spatially associated with the Agnew nickel deposit, in the Agnew-Wiluna greenstone belt of the Archaean Yilgarn Block in Western Australia. The complex consists of a central lenticular body, up to 700 m thick, of olivine adcumulates, flanked by laterally extensive sheet-like bodies of olivine orthocumulates and spinifextextured komatiite flows. Rocks progressively further away from the central lens have chemical compositions reflecting higher original proportions of komatiite liquid to cumulus olivine. Parent liquids had MgO contents between 25 and 32% MgO, approximately chondritic Al/Ti ratios and HREE patterns, and moderate depletion in LREE. Olivines within the adcumulate lens show a progressive increase in forsterite content from Fo93 at the bottom to Fo94..5 at the top. Calculated original olivine compositions in ihe flanking rocks are similar to those at the base of the central lens. Original olivine nickel contents show a symmetrical variation from maximum values of 3500 ppm at the top of the central lens, through minimum values of 1000 ppm at the base and margins of the central lens to intermediate values in the distal rocks. The complex as a whole shows evidence for nickel depletion relative to other komatiite suites. These observations are explained in terms of prolonged eruption and flow of komatiitic lava down a major flow channel or lava river. Adcumulates crystallized on the floor and sides of the central channel, which was formed at an early stage by thermal erosion of floor rocks. Episodic overflow of the central channel produced distal 'flood plain' rocks consisting of olivine orthocumulates and layered flows. Lavas became more magnesian and nickel-rich with time, giving rise to the observed spatial variation in primary olivine composition. Nickel depletion of the earliest lavas is attributed to pre-eruption segregation of large volumes of immiscible Fe-Ni-sulfide, which were concentrated to form the underlying Agnew nickel deposit.

High-grade terrains in and around the Yilgarn Block of Western Australia

Summary Two kinds of high-grade terrain occur in the western part of the Archaean Yilgarn Block. One consists of repeatedly deformed and metamorphosed granites (3650 and 3500 Ma), layered basic rocks (3750 Ma), and siliceous metasedimentary rocks, which were metamorphosed to granulite facies at 3300 Ma. The other consists of much less deformed granites and some basic rocks which form deeper, granulite-facies equivalents of the adjacent high-level 3000–2600 Ma granite-greenstone terrain. The Yilgarn Block is bounded by Proterozoic belts of intense deformation and high-grade metamorphism. These belts were active between 2000 and 1600 Ma and comprise both reworked Archaean and early Proterozoic rocks. Two belts were mobile again and metamorphosed to granulite or amphibolite facies between 1300 and 1000 Ma, and one of these belts was also intruded by plutonic rocks, deformed and metamorphosed to granulite facies at about 650 Ma.

Geology and geochronology of the Saddleback Greenstone Belt in the Archaean Yilgarn Block, southwestern Australia

The Saddleback Group is a volcanogenic greenstone sequence developed in the Western Gneiss Terrain of the Archaean Yilgarn Block. It consists of three major units; the Hotham Formation composed mainly of metasediments, the Wells Formation consisting of metamorphosed felsic volcanics and the Marradong Formation which is made up of metabasalts. The sequence has been metamorphosed to greenschist facies and intruded by adamellite. The sequence is poorly exposed, due to an extensive capping of bauxitic laterite which is also host to a major lateritic gold deposit (the Boddington deposit). The Saddleback Greenstone Belt contrasts with the rest of the Western Gneiss Terrain, which is composed mainly of linear, high grade, polyphase metamorphic belts enclosed by complex migmatite and intruded by late‐stage granitoids. Zircon U‐Pb results indicate that felsic volcanism in the Saddleback Greenstone belt extended from ca. 2670 to 2650 Ma ago. This was immediately followed by greenschist facies metamorphism, deformat...

Landsat MSS imagery applied to geological investigation of the Norseman area granitoid–greenstone terrain, southeast Yilgarn Block, Western Australia

AbstractInteractively processed Landsat MSS imagery has been used as an aid to studying the regional geology of approximately 10 800 km2 of terrain at the southeast margin of the Archaean Yilgarn Block in Western Australia. The technique proved successful in extending positions of known lithological contacts and lineaments into poorly exposed, inaccessible areas and in revealing new geological features, especially faults, previously unrecognized at ground level. During this investigation the distribution of granitoids and greenstones was more precisely defined, internal greenstone structures highlighted and three main fault trends were identified: (1) NW–NNW and (2) ENE, both within Archaean shield, and (3) NE–NNE within the transition to adjacent Proterozoic mobile belt. In order for the most information to be extracted from Landsat MSS images it is recommended that, whenever possible, image processing should follow ground-based studies as well as precede them, and that field geologist and Landsat specialist should work at the image processing system together.

Sm‐Nd model ages across the margins of the Archaean Yilgarn Block, Western Australia — III. The western margin

Earlier studies of the nature and extent of Proterozoic activity around the Archaean Yilgarn Block, based on Sm‐Nd model ages, have now been extended by an examination of the western margin. New data from the western Yilgarn Block indicate that rocks of the Chittering Metamorphic Belt (2.8–2.9 Ga) are significantly younger than rocks in the other three metamorphic belts of the Western Gneiss Terrain (literature values ≥3.0 Ga). This suggests possible continental growth by marginal accretion during the Archaean. Data from Yilgarn Block granitoids indicate a complex and extended period of generation, ranging from 2.7 to 3.1 Ga. There is a major time break at the western margin of the Yilgarn Block, marked by the Darling Fault. All rocks sampled W of the fault have Proterozoic model ages. This confirms the existence of a Proterozoic mobile belt along the western edge of the Yilgarn Block and provides further evidence that the present Darling Fault follows a zone of Precambrian crustal discontinuity. The remarkably consistent age of 1.8 Ga for rocks occurring both within and below the Perth Basin is similar to the youngest values determined in the Gascoyne Province and western parts of the Albany‐Fraser Province. This suggests contemporaneous mobile belt activity around the entire western portion of the Yilgarn Block and strongly supports models of Proterozoic continental growth by marginal accretion. Although data are limited, a further westward younging is suggested by available isotopic data from the Leeuwin Block (age ca 0.6 Ga). This suggests a further westward, step‐like decrease in age away from the Archaean Yilgarn Block.

Early Precambrian crustal evolution at Mount Narryer, Western Australia

The oldest known rocks in Australia occur at Mount Narryer and are associated with quartzite that contains detrital zircons far older than any known terrestrial rocks.The region forms part of the Western Gneiss Terrain of the Archaean Yilgarn Block. The main rock unit is a banded biotite adamellite gneiss which gives a Sm-Nd model age of 3630 Ma. It is intruded by a leucocratic granite gneiss which gives a Sm-Nd model age of 3510 Ma and contains abundant inclusions of a meta-anorthosite—gabbro—ultramafic suite. Quartzites and banded quartz—pyroxene—magnetite rocks form concordant layers within both gneisses.Most rocks are strongly deformed, are folded together into large-scale fold interference patterns, and show mineral assemblages in granulite or retrograded granulite facies. One large body of quartzite has internally escaped intense deformation and contains cross-bedding and graded-bedding structures as well as horizons of both polymict and quartz-pebble conglomerates. The quartzites contain detrital zircons which mostly give U-Pb ages of ∼3750–3500 Ma, but a few zircon grains give ages of 4100–4200 Ma. Samples of the adamellite gneiss give a Rb-Sr whole-rock isochron age of 3348±43 Ma which might date a peak of metamorphism.

Chloritoid and staurolite stability: implications for metamorphism in the Archaean Yilgarn Block, Western Australia

Abstract The stability of quartz‐chloritoid‐staurolite‐almandine‐cordierite and aluminium silicates is used to constrain both metamorphic conditions and pressure‐temperature trajectories for two localities within the 2700 Ma Archaean Yilgarn Block in Western Australia. Available experimental data are used to calculate thermodynamic data for a self‐consistent set of equilibria between these minerals. A lower amphibolite facies locality from the margin of a lower strain area contains assemblages including quartz‐chloritoid‐staurolite‐garnet‐biotite with altered cordierite replacing chloritoid, quartz‐staurolite‐andalusite, and quartz‐cordierite‐andalusite‐biotite. This locality was heated to 530–560°C in the andalusite field, at 4.2 kbar. A sample from a mid‐ to upper‐amphibolite facies, highly strained locality contains relict staurolite enclosed by andalusite, in turn replaced by cordierite and muscovite with biotite and sillimanite in the matrix. The assemblage was heated isobarically from conditions near the maximum experienced by the lower grade locality of 560°C at 4.2 kbar to temperatures in excess of the andalusite‐sillimanite transition but within the quartz plus muscovite stability field (600–650°C). The higher grade locality is close to a granitoid dome and sections based on gravity profiles reveal that this locality is underlain by granitoid at shallow depths. The higher grade metamorphism apparently reflects superposition of the thermal aureole on regional metamorphic conditions similar to those in the lower grade areas.

Sm‐Nd model ages across the margins of the Archaean Yilgarn Block, Western Australia—II; southwest transect into the Proterozoic Albany‐Fraser Province

Abstract This paper continues a survey of Sm‐Nd age relationships at the margins of the Yilgarn Block, Australia's largest Archaean craton. Model ages have been determined along an irregular transect extending approximately 120 km from the extreme SW of the Western Gneiss Terrain of the Yilgarn Block through the western extremity of the Albany‐Fraser Province, a major Proterozoic mobile belt. The ages demonstrate the significance of the Manjimup and Pemberton Lineaments as major crustal discontinuities. The sequence of ages across the lineaments strongly supports accretionary models of Precambrian crustal evolution, although some aspects of the age sequence can also be interpreted as mixing trends. Within the Western Gneiss Terrain, the Manjimup Lineament marks a change from older (c. 3.1 Ga) to younger (c. 2.7 Ga) Archaean gneisses. Further south the Pemberton Lineament, marking the northern tectonic boundary of the Albany‐Fraser Province, defines a change from Archaean (c. 2.7 Ga) to Proterozoic (c. 2.1 Ga) mantle differentiation ages. The one exception within the mobile belt is a sample of granulite from Windy Harbour, the age of which (3.1 Ga) implies that the Proterozoic crustal material of the Albany‐Fraser Province is sharply terminated to the west as well as to the north. The ages determined within the Albany‐Fraser Province, together with those previously reported for the Gascoyne Province, point to the existence of a major crust‐forming event surrounding a considerable portion of the Yilgarn Block about 2 Ga ago.

Sm‐Nd model ages across the margins of the Archaean Yilgarn Block, Western Australia; northwest transect into the proterozoic gascoyne province

Abstract A transect of Sm‐Nd model ages across the NW margin of the Archaean Yilgarn Block into the Proterozoic Gascoyne Province was undertaken to investigate the ages and possible origins of the gneiss terrains in this portion of the Proterozoic Capricorn Orogen. The nine sample localities show a decline in model ages from c. 3.5 Ga in the Archaean terrain to c. 2.0 Ga in the heart of the Gascoyne Province. These ages may be interpreted as times of generation of new sialic crustal material, and lend support to a model of crustal accretion against an older stabilised craton. The ages measured are in conflict with the geological evidence that the gneisses within the Proterozoic orogen are reworked Archaean basement. No unique interpretation is possible for the seemingly simple model age progression. It could indicate either the existence of an extended sialic crust‐forming event at c. 2.0–2.4 Ga, or a progressively diminishing degree of mixing of Archaean rocks into Proterozoic crust (<2.0 Ga) across the ...