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.     

Metamorphism in the Olary Block, South Australia: compression with cooling in a Proterozoic fold belt

Abstract The sedimentary and igneous rocks comprising the lower Proterozoic Olary Block, South Australia, were deformed and metamorphosed during the mid-Proterozoic 'Olarian'Orogeny. The area is divided into three zones on the basis of assemblages in metapelitic rocks, higher grade conditions occurring in the south-east. Mineral assemblages developed during peak metamorphism, which accompanied recumbent folding, include andalusite in Zones I and II and sillimanite in Zone III. Upright folding and overprinting of mineral assemblages occurred during further compression, the new mineral assemblages including kyanite in Zone II and kyanite and sillimanite in Zone III. The timing relationships of the aluminosilicate polymorphs, together with the peak metamorphic and overprinting parageneses, imply an anticlockwise P–T path for the 'Olarian'Orogeny, pressure increasing with cooling from the metamorphic peak.     

High abundance of early Archaean grains and the age distribution of detrital zircons in a sillimanite-bearing quartzite from Mt Narryer, Western Australia

SHRIMP U–Pb analyses are reported for a detrital zircon population from a sample of sillimanite-bearing quartzite from the Narryer sedimentary succession in the Narryer Terrane of the northwestern Yilgarn Craton. The detrital zircons define two distinctive age groups, an older group from 4000 Ga to 4280 Ma and a younger group from 3750 to 3250 Ma. The abundance of older group zircons of about 12% far exceeds the abundance of about 2% reported in the first discovery of ancient zircons in a quartzite from the Narryer metasediments, and is equivalent to the abundance of >3900 Ma zircons in metaconglomerate sample W74 from the Jack Hills, confirmed by new measurements reported in this paper. Most analyses of the Narryer and the Jack Hills detrital zircon populations are discordant. The Jack Hills zircon analyses are dominated by strong recent Pb loss whereas the Narryer zircon analyses have had a more complex history and have experienced at least one Pb loss event, possibly associated with the high-grade metamorphism at ca. 2700 Ma, and a further disturbance of the U–Pb systems during relatively recent times. Although the number of analyses is limited and many of the zircon analyses are discordant, the age distributions of the older (>3900 Ma) zircons from the Narryer and Jack Hills samples are different, suggesting a complex provenance for the ancient zircons. The distribution of ages in the younger population of Mt Narryer zircons is similar to that reported for zircons from the surrounding Meeberrie gneiss, supporting previous suggestions that zircons from the gneisses or their precursors were a major contributor to the detrital zircon suite. The younger zircon population from Jack Hills sample (W74), lacks the strong age peak from 3600 to 3750 Ma present in the Narryer zircon population, and conversely the strong zircon age group at ca. 3350–3500 Ma in the Jack Hills population is only weakly represented in the Narryer zircon population. The age distributions for the Narryer and the Jack Hills zircon populations are taken as benchmarks for comparing zircon populations from quartzite occurrences elsewhere in the Yilgarn Craton.     

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.     

Volcanogenic Cu–Pb–Zn bodies in granulites of the central Arunta Block, central Australia

Abstract Small unexploited copper-lead-zinc deposits, characterized by a distinctive wall-rock association of cordierite quartzite, silica-undersaturated rocks, calc-silicate rocks and impure marbles, occur in quartzofeldspathic gneisses and mafic granulites of the Strangways Metamorphic Complex, central Arunta Block, central Australia. Available data support the hypothesis that these are metamorphosed volcanogenic ore bodies. The chemical compositions of the quartzofeldspathic gneisses are comparable with those of less metamorphosed felsic igneous rocks, particularly the felsic igneous rocks emplaced in the North Australian Orogenic Province in the interval 1880–1800 Ma; and the mafic granulites are chemically similar to basalts (olivine-normative tholeiites). The wall-rock suite can be correlated from chemistry and lithological association with the suites of wall rocks found in unmetamorphosed volcanogenic ore deposits. That the protolith of the cordierite quartzites may well have been leached tuff, similar to the illite-chlorite-quartz tuff found in volcanogenic ore deposits, is also shown by retrogression of the granulitefacies assemblage: cordierite-garnet-ortho-pyroxene-biotite-quartz in the cordierite quartzites to cordierite-anthophyllite-bearing assemblages and thence to chlorite-muscovite-quartz assemblages. Lenses of silica-undersaturated rocks with spinel and, less commonly, sapphirine are interpreted as the metamorphosed equivalents of chlorite-rich pods found within leached tuffs in volcanogenic ore deposits. The wall rocks form sheet-like bodies; this suggests that they were deposited in relatively shallow water, thus precluding the formation of massive sulphides.     

Scale-integrated architecture of a world-class gold mineral system: The Archaean eastern Yilgarn Craton,Western Australia

World-class mineral systems, such as those found in the Archaean eastern Yilgarn Craton, are the product of enormous energy and mass-flux systems driven by lithospheric-scale processes. These processes can create big footprints or signatures in the lithosphere, which can be observed at a range of scales and via a variety of methods: including geophysics, isotopes, tectonostratigraphy and geochemistry. We use these datasets to describe both the architecture (structure) of world-class gold systems of the Yilgarn Craton and the signatures of their formation. By applying an understanding of the most critical elements of the process, and their signatures, new areas, especially undercover, may be targeted more predictably than before.     

Grenvillian-aged orogenesis in the Palaeoproterozoic Gascoyne Complex, Western Australia: 1030–950 Ma reworking of the Proterozoic Capricorn Orogen

In situ SHRIMP U–Pb geochronology of monazite and xenotime in pelitic schists from the central Gascoyne Complex, Western Australia, shows that greenschist to amphibolite facies metamorphism occurred between c. 1030 and c. 990 Ma. Monazite from an undeformed rare‐element pegmatite from the same belt gives a ²⁰⁷Pb/²⁰⁶Pb age of c. 950 Ma, suggesting that peak metamorphism and deformation was followed by pegmatite intrusion and coeval granite magmatism. Metamorphism in the central Gascoyne Complex was previously interpreted as Barrovian, largely based on the identification of kyanite in peak metamorphic assemblages, and has been attributed to intense crustal shortening and substantial tectonic thickening during Palaeoproterozoic continent–continent collision. However, the stable Al₂SiO₅ polymorph has been identified in this study as andalusite rather than kyanite, and the prograde assemblages of staurolite–garnet–andalusite–biotite–muscovite–quartz indicate temperatures of 500–550 °C and pressures of 3–4 kbar. These data show that the Palaeoproterozoic Gascoyne Complex underwent an episode of Grenvillian‐aged intracontinental reworking concentrated in a NW–SE striking corridor, during the Edmundian Orogeny. Until now, the Edmundian Orogeny was thought to have involved only reactivation of structures in the Gascoyne Complex, along with deformation and very low‐ to low‐grade metamorphism of Mesoproterozoic cover rocks some time between 1070 and 755 Ma. However, we suggest that it involved regional amphibolite facies metamorphism and deformation, granite magmatism and pegmatite intrusion between c. 1030 and c. 950 Ma. Therefore, the Capricorn Orogen experienced a major phase of tectonic reworking c. 600 Myr later than previously recognized. Our results emphasize the importance of in situ geochronology integrated with petrological studies in order to link the metamorphic history of a terrane with causally related tectonic events.     

Successive overprinting granulite facies metamorphic events in the Anmatjira Range, central Australia

The Anmatjira Range and adjacent Reynolds Range, central Australia, comprise early Proterozoic metasediments and othogneisses that were affected by three, and possibly four, temporally distinct metamorphic events, M_1–4, and deformation events, D_1–4, in the period 1820–1590 Ma. The north-western portion of the range, around Mt Stafford, preserves the effects of ±1820 Ma M₁-D₁, and shows a spectacular lateral transition from muscovite + quartz-bearing schists to interlayered andalusite-bearing migmatites and two-pyroxene granofelses that reflect extremely low-pressure granulite facies conditions, over a distance of less than 10 km. Orthopyroxene + cordierite + garnet + K-feldspar + quartz-bearing gneisses occur at the highest grade, implying peak conditions of ±750°C and 2.5 ± 0.6 kbar. An anticlockwise P–T path for M₁ is inferred from syn- to late-D₁ sillimanite overprinting andalusite, petrogenetic grid considerations and quantitative estimates of metamorphic conditions for inferred overprinting assemblages. The effects of M₁ have been variably overprinted to the south-east by a c. 1760 Ma M₂–D₂ event. Much of the central Anmatjira Range, around Ingellina Gap, comprises orthogneiss, deformed during D₂, and metapelites that have M₁ andalusite and K-feldspar overprinted by M₂ sillimanite and muscovite. The south-eastern portion of the range, around Mt Weldon, comprises metasediments and orthogneisses that were completely recrystallized during M₂–D₂, with metapelitic gneisses characterized by spinel + sillimanite + K-feldspar + quartz-bearing assemblages that suggest peak M₂ conditions of >750°C and 5.5 ± 1 kbar. Overprinting parageneses in metapelitic gneisses imply that D₂ occurred during essentially isobaric cooling. A third granulite facies event, M₃, affected rocks in the Reynolds Range, immediately to the south of the Anmatjira Range, at c. 1730 Ma. A possible fourth event, M₄, with a minimum age of c. 1590 My affected both Ranges, but resulted in only minor overprinting of M_1–3 assemblages. The superimposed effects of M_1–4, mapped for the entire Anmatjira–Reynolds Range area, indicate that only minor or no dislocation of the regional geology occurred during any of the metamorphic and accompanying folding, events. Although the immediate cause of each of the metamorphic events involved advection, the ultimate causes were external to the metasediments and most probably external to the crust.     

Kyanite-paragonite-bearing assemblages, northern Fiordland, New Zealand: rapid cooling of the lower crustal root to a Cretaceous magmatic arc

Fiordland, New Zealand exposes the lower crustal root of an Early Cretaceous magmatic arc that now forms one of Earth's most extensive high‐P granulite facies belts. The Arthur River Complex, a dioritic to gabbroic suite in northern Fiordland, is part of the root of the arc, and records an Early Cretaceous history of emplacement, tectonic burial, and high‐P granulite facies metamorphism that accompanied partial melting of the crust. Late random intergrowths of kyanite, quartz and plagioclase partially pseudomorph minerals in the earlier high‐T assemblages of the Arthur River Complex, indicating high‐P cooling of an over thickened crustal root by c. 200 °C. The kyanite intergrowths are themselves partially pseudomorphed by paragonite, commonly in the presence of phengitic white mica. Biotite–plagioclase intergrowths that partially pseudomorph phengitic white mica and diopside–plagioclase intergrowths that partially pseudomorph jadeitic diopside, combined with published thermochronology results, are consistent with later rapid decompression. A short duration anticlockwise P–T path may be explained by the high‐P juxtaposition of comparatively cool upper crustal rocks following their tectonic burial and under thrusting during the waning stages of Early Cretaceous orogenesis. This was then followed by the decompression giving the rapid exhumation within 20 Myr of peak metamorphism, as suggested by the isotopic data.     

Pro- and retrograde P–T evolution of granulites of the Beit Bridge Complex (Limpopo Belt, South Africa): constraints from quantitative phase diagrams and geotectonic implications

Interpretations based on quantitative phase diagrams in the system CaO–Na₂O–K₂O–TiO₂–MnO–FeO–MgO–Al₂O₃–SiO₂–H₂O indicate that mineral assemblages, zonations and microstructures observed in migmatitic rocks from the Beit Bridge Complex (Messina area, Limpopo Belt) formed along a clockwise P–T path. That path displays a prograde P–T increase from 600 °C/7.0 kbar to 780 °C/9–10 kbar (pressure peak) and 820 °C/8 kbar (thermal peak), followed by a P–T decrease to 600 °C/4 kbar. The data used to construct the P–T path were derived from three samples of migmatitic gneiss from a restricted area, each of which has a distinct bulk composition: (1) a K, Al‐rich garnet–biotite–cordierite–sillimanite–K‐feldspar–plagioclase–quartz–graphite gneiss (2) a K‐poor, Al‐rich garnet–biotite–staurolite–cordierite–kyanite–sillimanite–plagioclase–quartz–rutile gneiss, and (3) a K, Al‐poor, Fe‐rich garnet–orthopyroxene–biotite–chlorite–plagioclase–quartz–rutile–ilmenite gneiss. Preservation of continuous prograde garnet growth zonation demonstrates that the pro‐ and retrograde P–T evolution of the gneisses must have been rapid, occurring during a single orogenic cycle. These petrological findings in combination with existing geochronological and structural data show that granulite facies metamorphism of the Beit Bridge metasedimentary rocks resulted from an orogenic event during the Palaeoproterozoic (c. 2.0 Ga), caused by oblique collision between the Kaapvaal and Zimbabwe Cratons. Abbreviations follow Kretz (1983 ).     

A structural metamorphic study of the Broken Hill Block, NSW, Australia

The prograde pressure–temperature (P–T) path for the complexly polydeformed Proterozoic Broken Hill Block (Australia) has been reconstructed through detailed structural analysis in conjunction with calculation of compositionally specific P–T pseudosections of pelitic rock units within a high‐temperature shear zone that formed early in the tectonic evolution of the terrane. Whilst the overall P–T path for the Broken Hill Block has been interpreted to be anticlockwise, the prograde portion of this path has been unresolved. Our results have constrained part of this prograde path, showing an early heating event (M1) at P–T conditions of at least c. 600 °C and c. 2.8–4.2 kbar, associated with an elevated geothermal gradient (c. 41–61 °C km^?1). This event is interpreted to be the result of rifting at c. 1.69–1.67 Ga, or at c. 1.64–1.61 in the Broken Hill Block. Early rifting was followed by an episode of lithospheric thermal relaxation and burial, during which time sag‐phase sediments of the upper Broken Hill stratigraphy (Paragon Group) were deposited. Following sedimentation, a second tectonothermal event (M2/D2) occurred. This event is associated with peak low‐pressure granulite facies metamorphism (c. 1.6 Ga) and attained conditions of at least 740 °C at c. 5 kbar. A regionally pervasive, high‐temperature fabric (S2) developed during the M2/D2 event, and deformation was accommodated along lithology‐parallel, high‐temperature shear zones. The larger‐scale deformation regime (extensional or shortening) of this event remains unresolved. The M2/D2 event was terminated by intense crustal shortening during the Olarian Orogeny, during which time the first mappable folds within the Broken Hill Block developed.     

The Rayner Complex of East Antarctica: complex isotopic systematics within a Proterozoic mobile belt

Abstract New isotopic (Rb–Sr, U–Pb zircon and Sm–Nd) and petrological data are presented for part of an extensive Proterozoic mobile belt (locally known as the Rayner Complex) in East Antarctica. Much of the belt is the product of Mid-Proterozoic (∼ 1800–2000 Ma) juvenile crustal formation. Melting of this crust at about 1500 Ma ago produced the felsic magmas from which the dominant orthogneisses of this terrain were subsequently derived. Deformation and transitional granulite-amphibolite facies conditions (which peaked at 750 ± 50°C and 7–8 kbar (0.7–0.8 GPa) produced open to tight folding about E–W axes and syn-tectonic granitoids about 960 Ma ago. Subsequent felsic magmatism occurred at about 770 Ma and not, as has been widely advocated, at 500–550 Ma, which appears to have been a time of widespread upper greenschist facies (400–500°C) metamorphism, localized shearing and faulting. Sm-Nd model ages of 1.65–2.18 Ga disprove a previously favoured hypothesis that the Rayner Complex mostly represents reworked Archaean rocks from the neighbouring craton (Napier Complex). Models that involve rehydration of the Napier Complex are no longer required, since the Rayner Complex was its own source of water. Two episodes of Proterozoic crustal growth are identified, the later of which occurred between about 1200 Ma and 1000 Ma, and was relatively minor. Sedimentation took place only shortly before Late Proterozoic orogenesis. The multiphase history of the Rayner Complex has resulted in complex isotopic behaviour. Three temporally discrete episodes of Pb loss from zircon have been identified, the earliest two of which are responses to the c. 960 Ma and 540 Ma tectonothermal events. Fluid leaching was operative during the later event for there is a good correlation between degree of isotopic discordance and secondary mineral growth. Pb loss during the high-grade event was probably governed by the same process or by lattice annealing. Some zircon suites also document recent Pb loss. Most lower concordia intercepts have no direct geological meaning and are explicable as mixed ages produced by incomplete Pb loss during two or more secondary events. Whereas all zircon separates from the orthogneisses produce U–Pb isotopic alignments, zircons from the only analysed paragneiss produce scattered data, in part reflecting a range of provenance. The 960 Ma event was also associated with the growth of a characteristically low U zircon (∼ 300 μg/g) in rocks of inferred high Zr content. There is ubiquitous evidence for the resetting of Rb–Sr total-rock isochrons. Even samples separated by up to 10 km fail to produce igneous crystallization ages. Minor mineralogical changes produced by the 540 Ma upper greenschist-facies metamorphism were sufficient to almost completely reset some Rb–Sr isochrons and to produce open system conditions on outcrop scale, at least in one location.     

Gold deposits of the Bardoc Tectonic Zone: a distinct style of orogenic gold in the Archaean Eastern Goldfields Province,Yilgarn Craton,Western Australia

The Bardoc Tectonic Zone is an ～80 km-long and up to 12 km wide, intensely sheared corridor of Late Archaean supracrustal rocks that is bounded by pre- to syn-tectonic granites in the Eastern Goldfields Province, Yilgarn Craton. This zone has produced over 100 t of gold from a range of deposits, the largest being Paddington (～40 t Au). This shear system is connected along strike to the Boulder – Lefroy Shear Zone, which hosts considerably larger deposits including the giant Golden Mile Camp (>1500 t produced Au). In contrast to the diverse characteristics of gold deposits associated with the Boulder – Lefroy Shear Zone, mineralogical and geochemical data from five representative localities in the Bardoc Tectonic Zone have relatively uniform features. These are: (i) quartz – carbonate veins in competent mafic units with wall-rock alteration characterised by carbonate + quartz + muscovite + chlorite ± biotite + sulf-arsenide + sulfide + oxide + gold assemblages; (ii) arsenopyrite as the dominant sulfur-bearing mineral; (iii) a unique three-stage paragenetic history, commencing with pyrrhotite, and progressing to arsenopyrite and then to pyrite-dominated alteration; (iv) a lack of minerals indicative of oxidising conditions, such as hematite and sulfates; (v) δ³⁴ sulfur compositions of pre- to syn-gold iron sulfides ranging from 1 to 9 ‰; and (vi) a lack of tellurides. These features characterise a coherent group of moderately sized orogenic-gold deposits, and when compared with the larger gold deposits of the Boulder – Lefroy Shear Zone, potentially highlight the petrological and geochemical differences between high-tonnage and smaller deposits in the Eastern Goldfields Province.     

Proterozoic granulite facies metamorphism in the southeastern Reynolds Range, central Australia: geological context, P–T path and overprinting relationships

In the southeastern Reynolds Range, central Australia, a low- P granulite facies metamorphism affected two sedimentary sequences: the Lander Rock Beds and the Reynolds Range Group. In the context of the whole of the Reynolds Range and the adjacent Anmatjira Range, this metamorphism is M3 in a sequence M1–4 that occurred over a period of 250 Ma. In particular, M1 affected the Lander Rock Beds prior to the deposition of the Reynolds Group. M3 has an areally restricted, high-grade area in the southeastern Reynolds Range, affecting both the Reynolds Range Group and the underlying Lander Rock Beds. The effects of M3 are characterized by spinel + quartz-bearing peak metamorphic assemblages in metapelites, which imply peak conditions of ≥750°C and 4.5 ± 1 kbar, and involved isobaric cooling or compression with cooling. It is concluded that one of a series of thermal perturbations caused by thinning of mantle lithosphere contemporaneous with crustal thickening was responsible for M3. In the southeastern Reynolds Range, evidence of both the unconformity between the two rock groups and previous metamorphism/deformation has been completely erased by recrystallization during M3–D3.     

Anti-clockwise P–T paths in the lower crust: an example from a kyanite-bearing regional aureole, George Sound, New Zealand

The George Sound Paragneiss (GSP) represents a rare Permo-Triassic unit in Fiordland that occurs as a km-scale pillar to gabbroic and dioritic gneiss of c . 120 Ma Western Fiordland Orthogneiss (WFO). It is distinguished from Palaeozoic paragneiss common in western Fiordland (Deep Cove Gneiss) by SHRIMP and laser-ablation U–Pb ages as young as c . 190 Ma and ¹⁷⁶Hf/¹⁷⁷Lu >0.2828 for detrital zircon grains. The Mesozoic age of the GSP circumvents common ambiguity in the interpretation of Cretaceous v. Palaeozoic metamorphic assemblages in the Deep Cove Gneiss. A shallowly dipping S₁ foliation is preserved in the GSP distal to the WFO, cut by 100 m scale migmatite contact zones. All units preserve a steeply dipping S₂ foliation. S₁ staurolite and sillimanite inclusions in the cores of metapelitic garnet grains distal to the WFO preserve evidence for prograde conditions of T  <   650 °C and P <  8 kbar. Contact aureole and S₂ assemblages include Mg-rich, Ca-poor cores to garnet grains in metapelitic schist that reflect WFO emplacement at ≈760 °C and ≈6.5 kbar. S₂ kyanite-bearing matrix assemblages and Ca-enriched garnet rims reflect ≈650 °C and ≈11 kbar. Poorly oriented muscovite–biotite intergrowths and rare paragonite reflect post-S₂ high- P retrogression and cooling. Pseudosection modelling in NCKFMASH defines a high- P anti-clockwise P–T history for the GSP involving: (i) mid- P amphibolite facies conditions; preceding (ii) thermal metamorphism adjacent to the WFO; followed by (iii) burial to high- P and (iv) high- P cooling induced by tectonic juxtaposition of cooler country rock.     

Metastable growth of corundum adjacent to quartz in a spinel-bearing quartzite from the Archaean Napier Complex, Antarctica

In a granulite-facies spinel-bearing quartzite, corundum, orthopyroxene and sapphirine (and rarely cordierite and sillimanite) form partial rims separating spinel from quartz. Textures indicate the reactions:
spinel + quartz = orthopyroxene + corundum, and
spinel + quartz = orthopyroxene + sapphirine.
Thus, corundum and sapphirine are produced by reactions involving quartz. The low Al-content of the orthopyroxene (0.5–2.8 wt %) and low values for Mg–Fe distribution coefficient for spinel–sapphirine and spinel–orthopyroxene reflect low-temperature conditions during formation of the reaction products. Absence of zoning in spinel and a constant Mg–Fe distribution coefficient for spinel–sapphirine and spinel–orthopyroxene, over a compositional range, indicate Mg–Fe equilibration. It is suggested that stable reactions such as spinel + quartz = cordierite or spinel + quartz = garnet + sillimanite were over-stepped and that metastable reactions give rise to the anomalous juxtaposition of corundum + quartz.     

Transformation of Archaean Lithospheric Mantle by Refertilization: Evidence from Exposed Peridotites in the Western Gneiss Region, Norway

Orogenic peridotites occur enclosed in Proterozoic gneissesat several localities in the Western Gneiss Region (WGR) ofwestern Norway; garnet peridotites typically occur as discretezones within larger bodies of garnet-free, chromite-bearingdunite and are commonly closely associated with pyroxenitesand eclogites. The dunites of the large Almklovdalen peridotitebody have extremely depleted compositions (Mg-number 92–93·6);the garnet peridotites have lower Mg-number (90·6–91·7)and higher whole-rock Ca and Al contents. Post-depletion metasomatismof both rock types is indicated by variable enrichment in thelight rare earth elements, Th, Ba and Sr. The dunites can bemodelled as residues after very high degrees (>60%) of meltextraction at high pressure (5–7 GPa), inconsistent withthe preservation of lower degrees of melting in the garnet peridotites.The garnet peridotites are, therefore, interpreted as zonesof melt percolation, which resulted in refertilization of thedunites by a silicate melt rich in Fe, Ca, Al and Na, but notTi. Previous Re–Os dating gives Archaean model ages forthe dunites, but mixed Archaean and Proterozoic ages for thegarnet peridotites, suggesting that refertilization occurredin Proterozoic time. At least some Proterozoic lithosphere mayrepresent reworked and transformed Archaean lithospheric mantle. KEY WORDS: Archaean mantle; Proterozoic mantle; Western Gneiss Region, Norway; mantle metasomatism; garnet peridotite     

Crustal architecture of the Capricorn Orogen,Western Australia and associated metallogeny

A 581 km vibroseis-source, deep seismic reflection survey was acquired through the Capricorn Orogen of Western Australia and, for the first time, provides an unprecedented view of the deep crustal architecture of the West Australian Craton. The survey has imaged three principal suture zones, as well as several other lithospheric-scale faults. The suture zones separate four seismically distinct tectonic blocks, which include the Pilbara Craton, the Bandee Seismic Province (a previously unrecognised tectonic block), the Glenburgh Terrane of the Gascoyne Province and the Narryer Terrane of the Yilgarn Craton. In the upper crust, the survey imaged numerous Proterozoic granite batholiths as well as the architecture of the Mesoproterozoic Edmund and Collier basins. These features were formed during the punctuated reworking of the craton by the reactivation of the major crustal structures. The location and setting of gold, base metal and rare earth element deposits across the orogen are closely linked to the major lithospheric-scale structures, highlighting their importance to fluid flow within mineral systems by the transport of fluid and energy direct from the mantle into the upper crust.     

The P–T–deformation path for a mid-Proterozoic, low-pressure terrane: the Reynolds Range, central Australia

Abstract The Proterozoic low-pressure, high-temperature (LPHT) terrane of the Reynolds Range occurs in a 130-km-long, NW-trending belt in the central part of the Arunta Block, central Australia. The Reynolds Range has been affected by two mid-Proterozoic tectonic cycles, DI and DII, associated with two metamorphic events, MI and MII. DI–MI effects are restricted to the older of two sedimentary successions, the Lander Rock beds, which are separated from the younger Reynolds Range Group by an angular unconformity. The dominant structural–metamorphic features formed during DII–MII affected both sedimentary successions and the various granites that intruded them, and reworked most DI–MI effects. The DII deformation history can be subdivided into one prograde, two peak, and one retrograde stage. Average P–T calculations in the southeastern half of the range indicate a peak-metamorphic pressure of 4.1 ± 0.3 kbar. Because the calculated values are derived from the same stratigraphic level corresponding to the base of the Reynolds Range Group, which is exposed throughout the area, it is likely that pressures were similar in the entire range. In fact, however, the peak-metamorphic temperature shows a dramatic increase from greenschist facies (c. 400° C) in the northwest to granulite facies (740 ± 60° C) in the southeast, indicating that MII was associated with anomalously high heat flows. The P–T path is anticlockwise, with isobaric cooling from the metamorphic peak indicated by corona textures. However, the evidence of a prograde increase in pressure is indirect and based on the compressional nature of the structures. Peak-metamorphic mineral assemblages and retrograde mineral assemblages in amphibolite facies shear zones show the same metamorphic zonation, suggesting they formed in response to the same thermal event. If this is true, the implication is that a thermal perturbation external to the crust was maintained for a considerable period of time (110 Ma, based on zircon dating). As it is not clear whether Proterozoic, asthenosphere-active, thermal perturbations operated for this long, the alternative interpretation must be considered, namely that the peak-metamorphic events are separate from the shear zone event associated with reheating of the area.     

Two-stage decompression in orthopyroxene–sillimanite granulites from Forefinger Point, Enderby Land, Antarctica: implications for the evolution of the Archaean Napier Complex

Magnesian metapelites of probable Archaean age from Forefinger Point, SW Enderby Land, East Antarctica, contain very-high-temperature granulite facies mineral assemblages, which include orthopyroxene (8–9.5 wt% Al₂O₃)–sillimanite ± garnet ± quartz ± K-feldspar, that formed at 10 ± 1.5 kbar and 950 ± 50°C. These assemblages are overprinted by symplectite and corona reaction textures involving sapphirine, orthopyroxene (6–7 wt% Al₂O₃), cordierite and sometimes spinel at the expense of porphyroblastic garnet or earlier orthopyroxene–sillimanite. These textures mainly pre-date the development of coarse biotite at the expense of initial mesoperthite, and the subsequent formation of orthopyroxene (4–6 wt% Al₂O₃)–cordierite–plagioclase rinds on late biotite.
The early reaction textures indicate a period of near-isothermal decompression at temperatures above 900°C. Decompression from 10 ± 1.5 kbar to 7–8 kbar was succeeded by biotite formation at significantly lower temperatures (800–850°C) and further decompression to 4.5 ± 1 kbar at 700–800°C.
The later parts of this P–T evolution can be ascribed to the overprinting and reworking of the Forefinger Point granulites by the Late-Proterozoic ( c . 1000 Ma) Rayner Complex metamorphism, but the age and timing of the early high-temperature decompression is not known. It is speculated that this initial decompression is of Archaean age and therefore records thinning of the crust of the Napier Complex following crustal thickening by tectonic or magmatic mechanisms and preceding the generally wellpreserved post-deformational near-isobaric cooling history of this terrain.