John S. Myers

Tectonic evolution of Proterozoic Australia

Proterozoic Australia has long been interpreted as a single intact continent in which all tectonic and magmatic activity was intracratonic. This paper proposes an alternative hypothesis in which numerous fragments of continental crust were assembled by plate tectonic processes. The assembly was completed between 1300 and 1100 Ma when the crustal fragments were combined as an early component of the Rodinian supercontinent. Rifting and fragmentation of Archaean continents began in the late Archaean and continued into the Proterozoic. Passive margin deposits, such as those of the Hamersley Basin, accumulated on isolated fragments of Archaean crust. These numerous fragments were subsequently assembled into three cratons by ∼ 1830 Ma. A West Australian Craton was established by collision of the Archaean Pilbara and Yilgarn cratons, which were joined along the Capricorn Orogen. Similarly, a South Australian Craton developed by amalgamation of the proto-Gawler and proto-Curnamona cratons along the Kimban Orogen. A North Australian Craton appears to have formed by accretion of numerous crustal fragments, including the Kimberley, Pine Creek, Lucas, and Altjawarra cratons, with sutures marked by the King Leopold, Halls Creek, Tennant Creek and proto-Isan orogens. The southern margin of the North Australian Craton was the site of repeated terrane accretion and orogenic activity between ∼ 1880 Ma and 1400 Ma. This included an orogenic event at ∼ 1880 – 1850 Ma; the Strangways (1780 – 1730 Ma), Argilke (1680 – 1650 Ma), and Chewings (1620 – 1580 Ma) orogenies; and the intracratonic Anmatjira uplift (1500 – 1400 Ma). Intracratonic rifting at ∼ 1750 to 1710 Ma and ∼ 1640 to 1600 Ma produced the McArthur Basin and related minor basins, parts of which were deformed by the Isan Orogeny at ∼ 1600 and ∼ 1530 Ma. Rifting along the line of the Capricorn Orogen led to deposition in the overlying intracratonic Bangemall Basin between 1630 and 1300 Ma. Along the eastern margin of the South Australian Craton, the 1670 to 1600 Ma Olarian Orogeny marks interaction with now obscured continental crust to the east. Tectonic activity between 1300 and 1100 Ma led to the assembly of Proterozoic Australia as an early component of the supercontinent of Rodinia. This first involved the amalgamation of the West Australian and North Australian cratons, followed by collision with the South Australian Craton. The Centralian Superbasin developed over the junction of the North, South, and West Australian cratons between ∼ 830 and 750 Ma. Rifting to the east formed the “Adelaide Geosyncline” at ∼ 830 Ma. This was followed by the breakup of Rodinia, with the rifting apart of Laurentia and Gondwanaland along the eastern margin of Proterozoic Australia at ∼ 750 Ma, and the subsequent formation of the Palaeo-Pacific Ocean. After the breakup of Rodinia, a series of northeast-southwest compressional events followed by periods of relaxation, reflect the assembly of a new supercontinent. Old lines of weakness were reactivated, culminating in the intracratonic King Leopold, Paterson, Petermann Ranges, and Pinjarra orogenies between 620 and 540 Ma. Subsequent reactivation continued into the Phanerozoic, with the widespread eruption of continental flood basalts and the formation of intracratonic basins (540 – 530 Ma).

Precambrian tectonic evolution of part of Gondwana, southwestern Australia

Southwestern Australia has a long dynamic Precambrian history of crustal fragmentation and aggregation by continental collision and accretion. This interpretation contrasts with the widely held belief that most of Australia was part of a Proterozoic supercontinent in which all tectonic, magmatic, and metamorphic activity was intracontinental. The Pilbara and Yilgarn cratons are two quite different fragments of Archean continental crust that collided to form the Capricorn orogen between 2000 and 1600 Ma. The Albany-Fraser orogen formed along the southern margin of the Yilgarn craton during another major continental collision at 1300 to 1100 Ma. Rifting removed the northeastern part of these combined cratons and orogens between 1100 and 700 Ma. Subsequent continental collision along this rifted margin formed the Paterson orogen at 700 to 600 Ma. Therefore, the assembly of this considerable part of Gondwana may not have been completed until about 600 Ma.

Chronology and evolution of the Middle Proterozoic Albany‐Fraser Orogen, Western Australia

Geochemical and Sm‐Nd isotopic data, and 19 ion‐microprobe U‐Pb zircon dates are reported for gneiss and granite from the eastern part of the Albany‐Fraser Orogen. The orogen is dominated by granitic rocks derived from sources containing both Late Archaean and mantle‐derived components. Four major plutonic episodes have been identified at ca 2630 Ma, 1700–1600 Ma, ca 1300 Ma and ca 1160 Ma. Orthogneiss, largely derived from ca 2630 Ma and 1700–1600 Ma granitic precursors, forms a belt along the southeastern margin of the Yilgarn Craton. These rocks, together with gabbro of the Fraser Complex, were intruded by granitic magmas and metamorphosed in the granulite facies at ca 1300 Ma. They were then rapidly uplifted and transported westward along low‐angle thrust faults over the southeastern margin of the Yilgarn Craton. Between ca 1190 and 1130 Ma, granitic magmas were intruded throughout the eastern part of the orogen. These new data are integrated into a review of the geological evolution of the Albany‐Fra...

Early archaean narryer gneiss complex, Yilgarn Craton, Western Australia

Abstract An outline is given of the Narryer Gneiss Complex that forms the northwest part of the Yilgarn Craton. It consists of two major groups of quartzo-feldspathic gneiss: Meeberrie gneiss mainly derived from ∼ 3680 Ma monzogranite, and Dugel gneiss derived from ∼ 3400 Ma syenogranite and monzogranite. The gneisses contain inclusions of an ∼ 3730 Ma layered gabbro-anorthosite intrusion called the Manfred Complex and are tectonically interleaved with metasedimentary rocks, mainly quartzite and banded iron formation. The complex was deformed and metamorphosed in granulite facies about 3350-3300 Ma ago. The gneiss complex is in tectonic contact with a granite-greenstone terrain that forms most of the Yilgarn Craton. These terrains were juxtaposed, intruded by sheets of granite, intensely deformed and metamorphosed about 27002600 Ma ago. The Narryer Gneiss Complex is a substantial addition to known areas of early Archaean crust. In contrast to most previously described terrains of this antiquity, it is largely derived from granite, it is devoid of metavolcanic rocks, and metasedimentary rocks are predominantly siliceous. The latter contain detrital zircons with U-Pb ages of 4275-4000 Ma, far older than any known terrestrial rocks. These features all suggest the former presence of a substantial amount of very old sialic crust (> 3700 Ma) and that remnants of it may still survive in this vast complex.

Preface: Archaean geology of the Eastern Goldfields of Western Australia — regional overview

Abstract The Eastern Goldfields region forms part of the Yilgarn Craton which, after the Superior Province of Canada, is one of the largest fragments of Archaean granite-greenstone terrain exposed at the surface of the Earth. This craton has much in common with the more intensely studied Superior Province, but there are also significant differences between these two cratons. Both cratons were assembled from numerous smaller crustal fragments in a relatively brief period during the late Archaean. The Superior Province was assembled between 2725 and 2695 Ma (Card, 1990), and the Yilgarn Craton was assembled between c. 2670 and 2650 Ma (Myers, 1993). Both cratons contain fragments of older continental crust and volcanic arcs that formed just prior to collision and cratonisation. The Superior Province contains extensive belts of metasedimentary rocks interpreted as accretionary prisms (Card, 1990), whereas metasedimentary rocks form only a minor component of the Yilgarn Craton, and most appear to postdate accretion. Both cratons contain significant concentrations of gold and support thriving gold-mining industries. The most prominent difference between the cratons lies in the nature of the granitoid rocks. The Superior Province is dominated by tonalite and granodiorite whereas the Yilgarn Craton largely consists of granite. This applies to granitoid rocks of all ages in each craton. Granite studies form a major part of this special volume of Precambrian Research.

Early Precambrian crustal evolution at Mount Narryer, Western Australia

Abstract 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.

Isotopic evidence on the age and origin of the Fraser Complex, Western Australia : a sample of Mid-Proterozoic lower crust

SmNd, RbSr and Pb isotopic data have been obtained for a variety of samples from within and around the Fraser Complex, a large (> 400 × 30 km) composite body of metagabbro which is a major component of the Proterozoic Albany-Fraser Orogen. Granites to the north and east of the complex and a granite sheet near its centre have SmNd model ages (tDM) of ∼2000 to ∼ 2300 Ma, similar to values recorded in the western portion of the orogen. SmNd data for Fraser Complex gabbros scatter significantly on an isochron plot, but well-preserved igneous minerals extracted from one sample give 1291 ± 21 Ma with initial ϵNd= -1.3 ± 0.2. RbSr for the biotite-rock pair gives a cooling age of 1268 ± 20 Ma with initial 87Sr/86Sr≈0.706. Slices cut from a mixed gneiss/ amphibolite zone of ductile deformation and retrogression along the western margin of the complex give a ∼ 1300-Ma Pb/Pb date for final tectonic emplacement. Intrusion, granulite-facies metamorphism and upper-crustal tectonic emplacement of the complex were probably all achieved within ≤ 30 Ma. SmNd and RbSr characteristics of the western deformation zone suggest flushing by large volumes of fluid derived from a moderately depleted mantle source (ϵNd ≈ + 2.4; ϵSr, ≈ − 5 at 1300 Ma). The precursors to the Fraser Complex could have been derived from the same (or a similar) source at ∼ 1900 Ma.

Granitoid sheets, thrusting, and Archean crustal thickening in West Greenland

Archean gneisses in part of West Greenland mainly formed from sheets of tonalite and granodiorite emplaced into more mafic volcanic and plutonic rocks. Sheet emplacement was syntectonic and associated with dilation and thrusting, and it led to considerable and irreversible thickening of the continental crust. Large recumbent nappelike folds of these rocks were compressed and refolded on steep axes while still ductile. They were then deformed by steep shear zones as they dried out, stiffened, and became part of a stable craton about 2,850 m.y. ago. Although similar in composition to many Phanerozoic intrusions, these Archean granitoid sheets appear to be unusually extensive. An analog for their generation is the present tectonic setting of the Himalayas and Tibetan plateau.

Origin of granite-greenstone patterns, Yilgarn Block, Western Australia

The Yilgarn Block of Western Australia consists mainly of Archean granite-greenstone terrain with oval areas of granitoid rocks fringed by arcuate greenstone belts. In a typical part of this terrain the outcrop pattern is shown to result from large-scale dome-and-basin fold interference structures rather than diapiric emplacement of steep-sided batholiths. The greenstones and underlying granitoid rocks were initially deformed together in a subhorizontal tectonic regime. As a result, the granitoid rocks recrystallized and a gneissosity developed subparallel to the stratigraphy of the greenstones. Thick subhorizontal monzogranite sheets were later emplaced into the lower part of the greenstone sequence. Subsequent deformation generated two successive sets of folds with vertical axial surfaces at a high angle to each other. These folds formed dome-and-basin interference structures ranging from a few centimetres to more than 50 km in diameter.

Oldest known terrestrial anorthosite at Mount Narryer, Western Australia

Abstract 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.