Western Gneiss Terrain Research Articles

Argon composition of metamorphic fluids: Implications for 40Ar/39Ar geochronology

The concentration and isotopic composition of argon trapped in fluid inclusions within a mylonitic amphibolite were determined by combining electron microprobe, [sup 40]Ar/[sup 39] Ar laser microprobe, and fluid inclusion analyses on the same samples. In our study, fluid in inclusions formed during ductile deformation was probably derived locally from the Paleozoic, Caledonian sequences, whereas brines, emplaced in cross-cutting fractures, were derived from Proterozoic footwall rocks several kilometers from the mylonite. By utilizing [sup 38]Ar[sub Cl] to correct for [open quotes]excess [sup 40]Ar,[close quotes] we estimate a 370 Ma closure age for the amphibole, which is distinctly younger than the 440-480 Ma plateau ages indicated by step-heating spectra. The [open quotes]excess [sup 40]Ar[close quotes] responsible for the anomalous plateaus probably resulted from diffusive re-equilibration of the amphibole with the fluids present during ductile deformation of the mylonite. The significantly younger cooling age determined by our method is concordant with ages determined from more recent geochronological studies in the region. These ages indicate that the western gneiss terrain, in our study area, records deformation and cooling associated with Siluro-Devonian tectonothermal metamorphism rather than an earlier Caledonian tectonism as proposed in previous [sup 40]Ar/[sup 39]Ar investigators. 46 refs., 8 figs.

The distribution of 3.0 Ga and 2.7 Ga volcanic episodes in the Yilgarn Craton of Western Australia

Abstract The results of a conventional zircon U-Pb investigation of the ages of felsic volcanic rocks in a number of greenstone belts in the Yilgarn Craton of Western Australia indicate that volcanism took place during two distinct episodes at ca. 3.0 and ca. 2.7 Ga. Each episode spans a time period of about 75 Ma and the two ages of volcanism are spatially separated. 3.0 Ga volcanic rocks are found in greenstone belts in the northern part of the Western Gneiss Terrain and in the Murchison Province, whereas 2.7 Ga volcanic rocks are present in the southern part of the Western Gneiss Terrain and in the Southern Cross and Eastern Goldfields Provinces. There is no evidence of inheritance in the zircon U-Pb systems which would support an older crustal origin for these rocks. Present results, together with previous studies of the ages of granites and gneisses, suggest that the volcanic episodes were accompanied by broadly contemporaneous granite plutonism. It is suggested that the evolution of the Yilgarn Craton took place in three main stages. The first was the development of a stable 3.4 Ga granitic nucleus, which incorporated earlier generations of granitoid, a 3.7 Ga layered complex and possibly 4.2 Ga primitive crust. This was followed by the two separate granite-greenstone episodes, at ca. 3.0 and ca. 2.7 Ga., involving the addition of significant amounts of new crust. Each episode was characterised by the accretion of extensive granite-greenstone terranes at the margins of the stable craton, the intrusion of granite plutons into the evolving craton and the reworking of older cratonic rocks to form granites and gneisses with inherited isotopic systems.

Archaean evolution of the Wongan Hills Greenstone Belt, Yilgarn Craton, Western Australia

Ion microprobe and conventional U‐Pb analyses on single zircons from a felsic porphyry from the Wongan Hills Greenstone Belt in the Western Gneiss Terrain of Western Australia define an age of 3010 ± 7 Ma. This is a minimum age for volcanism in the greenstone belt. A sample of gneiss from near the belt contains two populations of zircons with ages of 2997 ± 47 Ma and 2800 ± 9 Ma. The ca 3.0 Ga zircons are interpreted as dating the formation of the granitic parent of the gneiss, and the ca 2.8 Ga zircons are interpreted as dating gneiss formation. This latter event may be an early manifestation of the major 2.65–2.75 Ga granite‐greenstone episode which affected the entire Yilgarn Craton and is expressed at Wongan Hills by the emplacement of post‐tectonic porphyritic granite dated at 2651 ± 4 Ma, and greenschist facies retrogressive metamorphism, dated at 2646 ± 11 Ma by U‐Pb analyses of cogenetic sphene. The present results confirm the significance of ca 3.0 Ga volcanism in the north and western Yilgarn Cr...

The nature of Proterozoic reworking of early Archaean gneisses, Mukalo Creek Area, Southern Gascoyne Province, Western Australia

The Gascoyne Province, a key segment of the early Proterozoic Capricorn Orogen which separates the Archaean Pilbara and Yilgarn Cratons of Western Australia, may be divided into Northern, Central and Southern Zones. The Southern Zone comprises predominantly Archaean granulite facies quartzo-feldspathic gneisses, and passes southwards into the Western Gneiss Terrain of the Yilgarn Block. This area was part of the lower crust of a stabilized continental landmass at the end of the Archaean. Development of the Capricorn Orogen ∼ 2 Ga ago resulted in discontinuous deformation of the northern part of this landmass. Deformation was concentrated in ductile shear zones leaving blocks of relatively undeformed gneiss between them. The shear zones trend between ENE-WSW and ESE-WNW, and dip to the south at 50–60°. Strong down-dip stretching lineations and asymmetric feldspar porphyroclasts indicate that the shear zones are reverse faults. However, metamorphic assemblages in early Proterozoic dolerite dykes which post-date the shear zones show that the area was still in the lower crust following the earliest Proterozoic deformation. Isothermal uplift of the Southern Gascoyne Province/Northern Yilgarn Block, by 10 km or more, occurred 1.7-1.5 Ga ago, and was accompanied by granitoid intrusion. Deformation within E-W trending shear zones continued after uplift, and was accompanied by patchy greenschist-facies retrogression of both static and dynamic style. On the local scale, the effects of early Proterozoic deformation on the early Archaean gneisses are generally confined to shear zones and their immediate surroundings. They comprise: (1) realignment of gneissic trends from N-S to E-W; (2) development of quartz-ribbon mylonites by elongation of quartz grains into lenses and ribbons, and reduction of feldspar grainsize by crystal plastic processes; (3) growth of porphyroblastic and corona garnet in felsic and mafic gneisses. The later Proterozoic deformation occurred at lower temperatures, and resulted in: (1) further realignment of gneissic trends from northerly to easterly; (2) reduction of quartz grainsize by crystal plastic processes and reduction of feldspar grainsize largely by brittle fracture; (3) greenschist-facies retrogression. That the earliest deformation in the Southern Gascoyne Province resulted in crustal shortening, and was followed by significant uplift and granitoid intrusion, indicates that Proterozoic reworking of this area resulted from regional N-S compression. This favours development of the Capricorn Orogen in a collision zone between the Pilbara and Yilgarn Cratons, rather than in an ensialic mobile zone.

Oldest known terrestrial anorthosite at Mount Narryer, Western Australia

Fragments of a deformed and metamorphosed, layered anorthosite-gabbro-ultramafic intrusion called the Manfred Complex occur as inclusions in ∼ 3650 and ∼ 3400 Ma old banded granite gneisses at Mount Narryer in the Western Gneiss Terrain of the Yilgarn Block. The complex is typical of a world-wide group of Archaean anorthosites, characterized by very large primary grain sizes of equidimensional calcic plagioclase. This is the first recorded occurrence of these anorthosites in Australia. Pb-Pb and Sm-Nd whole-rock, and U-Pb zircon geochronology given in the accompanying two papers (Fletcher, I.R., Rosman, K.J.R. and Libby, W.G., 1988. Precambrian Res., 38: 343–354; Kinny, P.D., Williams, I.S., Froude, D.O., Ireland, T.R. and Compston, W., 1988. Precambrian Res., 38: 325–341) shows that the Manfred Complex is about 3730 Ma old. It is therefore the oldest known terrestrial anorthosite complex and one of the oldest known rock units on Earth.

Platinum group elements in mafic-ultramafic rocks of the Western Gneiss Terrain, Western Australia

The New Norcia and the Yornup bodies are situated within the high-grade Western Gneiss Terrain of the Yilgarn Block. The New Norcia body consists of mafic and ultramafic rocks of gabbronoritic, olivine-gabbronoritic and harzburgitic composition respectively, metamorphosed to amphibolite, amphibolitic serpentinite and serpentinite.

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...

Rare earth element patterns in Archean high-grade metasediments and their tectonic significance

Metasediments from two contrasting types of Archean high-grade terrains are interpreted as being derived from distinct tectonic settings. The Kapuskasing Structural Zone, Canada, represents the deep roots of a typical greenstone belt, whereas the Limpopo Province, southern Africa and Western Gneiss Terrain, Australia, mainly consist of shelf sediments deposited on a granitic basement and then buried to the depths required for granulite fades metamorphism. Upper amphibolite to granulite fades paragneisses from the Kapuskasing Structural Zone have REE patterns similar to those of greenstone belt sediments, except where partial melting has occurred, forming restites with Eu enrichment and melts with Eu depletion. Except in this latter instance, metamorphism has not affected the primary REE patterns. REE patterns in Archean upper amphibolite to granulite facies metasediments from the central Limpopo Province and Western Gneiss Terrain show wide differences, ranging from patterns which resemble those in post-Archean terrigenous sediments, to typical Archean sedimentary rock patterns. The diversity in REE patterns for these shallow shelf metasediments is interpreted as resulting from derivation from local provenances. Samples with “post-Archean” patterns, displaying Eu depletion, are interpreted as being derived from K-rich granitic plutons which were portions of small, stable early Archean terrains, precursors of the widespread late Archean-Proterozoic episode of major cratonic development.

Relation between archaean high-grade gneiss and granite-greenstone terrain in western Australia

Two major regions of Archaean crust occur in Western Australia called the Pilbara and Yilgarn Blocks. All the Pilbara and most of the Yilgarn Block consist of granite-greenstone terrain with metamorphic grades in greenschist or low amphibolite facies, but these terrains developed at different times in the two regions: 3500–3000 Ma ago in the Pilbara and 3000–2600 Ma ago in the Yilgarn. High-grade gneisses are only known to occur in a belt along the western margin of the Yilgarn Block in a region called the Western Gneiss Terrain. Direct observations of the relation between this high-grade gneiss terrain and typical granite-greenstone terrain can therefore only be made in this region. Two kinds of relation are seen. In the north, the high-grade gneiss terrain developed between 3800–3300 Ma ago. It is distinctly older than, and contains a different assemblage supracrustal rocks from, the granite-greenstone terrain. The gneiss terrain could have formed a sialic basement to the greenstones but direct evidence of this relation is not preserved. The gneiss terrain is intruded by granites similar to those of the granite-greenstone terrain and the two terrains are deformed and metamorphosed together. Prograde metamorphism of the granite-greenstone terrain increases from mainly greenschist facies in the east, to high amphibolite facies in the west, where it is associated with retrograde metamorphism of the gneiss terrain from granulite to high amphibolite facies. In the south, a portion of the Western Gneiss Terrain also consists of gneisses which pre-date the granite-greenstone terrain. However, most of the gneiss terrain consists of younger granitoid intrusions, similar to those that form a major component of the granite-greenstone terrain, but metamorphosed in granulite facies. As in the north, a regional eastward decrease in metamorphic grade suggests that the exposed crustal section is tilted eastwards. The tectonic significance of the high-grade gneiss and granite-greenstone terrains remains uncertain. Regional relationships on the scale of the Yilgarn Block suggest three possible models: (1) cratonic basinal model, whereby large greenstone basins formed on gneissic basement; (2) continental marginal model, whereby greenstone terrains developed adjacent to pre-existing protocontinent; or (3) collision involving two unrelated segments of crust.

Rb‐Sr, Sm‐Nd and Pb‐Pb geochronology of ancient gneisses from Mt Narryer, Western Australia

New Rb‐Sr, Sm‐Nd and Pb‐Pb data are presented from banded gneisses in the Mt Narryer region of the Western Gneiss Terrain, together with Rb‐Sr and Sm‐Nd data from cross‐cutting granite sheets. The Meeberrie Gneiss, which previously gave a Rb‐Sr isochron age of 3348 + 43 Ma and an initial ratio of 0.7037 + 0.0005, has given a Pb‐Pb age of 3357 ± 70 Ma, whilst an additional suite of the same gneisses collected to the SW of the original samples gives a Rb‐Sr age of c. 3100 Ma with an initial ratio of c. 0.705. Deformed sheets of recrystallized granite which intrude the Meeberrie Gneiss give a Rb‐Sr age of 2579 + 122 Ma with an initial ratio of 0.7143 ± 0.0092. Sm‐Nd analyses on samples of the Meeberrie Gneiss give TCHUR ages of 3710 ±30 Ma and 3620 + 40 Ma, which support the previously determined model age of 3630 ± 40 Ma. An additional TCHUR of 3540 ± 30 Ma for a sheet of Dugel Gneiss within the Meeberrie Gneiss coincides with a previously reported value of 3510 ± 50 Ma for another sample of the Dugel Gneis...

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.

SmNd geochronology of greenstone belts in the Yilgarn Block, Western Australia

SmNd geochronology has been used to date 3 widely spaced metavolcanic (greenstone) belts in the Yilgarn Block. The measured isochron ages are 2.78 ± 0.03 Ga (initial ϵ Nd = 2.5 ± 0.3) for greenstones at Kanowna in the Eastern Goldfields Province, 3.05 ± 0.10 Ga (initial ϵ Nd = 0.9 ± 0.7) for Diemals-Marda in the Southern Cross Province and 2.98 ± 0.12 Ga (initial ϵ Nd = 0.7 ± 1.2) for the Warriedar fold belt in the Murchison Province. These data show that substantial crustal components of age ∼3.0 Ga exist in the Southern Cross and Murchison Provinces, and support published evidence that the Eastern Goldfields Province contains no crustal material ⩾ 2.8 Ga. These ages are all significantly younger than ages commonly observed in the Western Gneiss Terrain and so support the suggestion of a progressive crustal age trend across the Yilgarn Block. The measured age values depend strongly on the inclusion of data for felsic components of the greenstone belts; data for mafic and ultramafic units allow that eruption in the 3 localities could have been contemporaneous at ∼ 2.8 Ga. The age range of metavolcanic rocks within each province is possibly comparable to the maximum possible age difference between study areas (∼ 0.2 Ga), so the documented lithological variations between the greenstone successions of the 3 provinces cannot be attributed to depositional age differences. Leaching experiments on altered Warriedar samples indicate that both alteration and isotopic homogenisation occurred early in the evolution of the system, probably prior to the ∼ 2.6 Ga regional metamorphic event. The initial ϵ Nd values do not conform to previously suggested mantle depletion curves. When considered with published data for greenstone belts they suggest complex mantle heterogeneity, with e Nd diverging over time to both positive and negative values.

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.

Growth of Archaean crust within the western gneiss terrain, Yilgarn Block, Western Australia

Abstract Two groups of Archaean orthogneisses from the Western Gneiss Terrain, approximately 100 km east of Perth, Western Australia, give Nd model ages of 3150 Ma to 3240 Ma, and 2950 Ma to 3050 Ma respectively. A Sm‐Nd whole‐rock isochron of 3210 ± 190 Ma (2σ), obtained by including a mafic granulite with samples having the older group of model ages, is in close agreement with U‐Pb zircon ages of Nieuwland & Compston (1981). This agreement, together with a chondritic initial 143Nd/144Nd, indicates a limited prior crustal residence time (<200 Ma) for the gneiss protoliths. Leucosome and melanosome components of nearby migmatite yield significantly younger Nd model ages of 2860 Ma and 2660 Ma, respectively. Post‐tectonic granites also give younger model ages of 2770 Ma and 2680 Ma, consistent with the addition of new material to the continental crust at this time. When contrasted with Nd model ages of 3630 Ma and 3510 Ma from the northern extension (de Laeter et al., 1981) of the Western Gneiss Terrain, t...