R.H. Smithies

High pressure amphibolite-granulite facies metamorphism in the Paleoproterozoic Rudall Complex, central Western Australia

Abstract The Rudall Complex forms the basement of the Paleoproterozoic to Neoproterozoic Paterson Orogen, which was deformed during at least three major orogenies. The complex comprises a sequence of metamorphosed clastic and chemical sediments, and basalts, deposited in a marginal basin environment, prior to extensive felsic intrusion. Deformation of the sequence during the first “Yapungku” orogeny between 2000 and 1760 Ma, included extensive thrust stacking from ENE to WSW. The thermal peak of the associated high-pressure metamorphism was late- to post-tectonic with respect to this deformation. Thermobarometry on amphibolites and mafic granulites that contain various combinations of the minerals hornblende, plagioclase, quartz, garnet, othopyroxene and clinopyroxene, indicates that temperatures peaked at around 800°C, while pressures reached 1200 MPa. The late timing of peak temperatures together with the presence, in some amphibolites, of amphibole-plagioclase symplectitic coronas around garnet, indicates a steeply decompressive clockwise P-T-t path. Peak metamorphic pressures indicate that the crust was locally thickened by at least 40 km, during an event that showed similarities to Phanerozoic collisional tectonics. It is suggested that this deformational and metamorphic event records the collision between the Pilbara Craton and a continent to the northeast. The Paleoproterozoic evolution of Australia has previously been attributed to ensialic processes characterised by low-pressure metamorphism and anti-clockwise P-T-t paths. The metamorphic history of the Rudall Complex is not consistent with this view, and the documented high-pressure metamorphism is broadly synchronous with a medium-pressure metamorphic event in central Australia (the Strangways Orogeny), and possibly also in northwestern Australia (the Capricorn Orogeny). Collisional plate-tectonic processes played a significant role in the Paleoproterozoic evolution of northern Australia.

Distinct basement terranes identified from granite geochemistry in late Archaean granite-greenstones, Yilgarn Craton, Western Australia

Abstract Hidden basement terranes have been identified by a geochemical study of granites that intrude deformed greenstones in the Eastern Goldfields of the Yilgarn Craton. The granites are calc-alkaline rocks with a high average silica content (72.5 wt%), lie mainly in the compositional range of granodiorite to monzogranite, and are derived from melting of pre-existing sialic crust. The greenstones comprise several tectono-stratigraphic terranes that were essentially assembled before intrusion of the granites. The distribution of granite types generally shows no spatial relationship to these terranes or to terrane boundary faults. The Emu Fault, which separates the Kurnalpi Terrane to the east, from the Kalgoorlie and Gindalbie terranes to the west, provides an exception. Granites to the east of this fault have lower TiO 2 , FeO 2 and MgO, and generally higher Na 2 O, compared to granites to the west. These contrasting compositions result from differences within the source regions and are interpreted to reflect two distinct basement terranes. The basement terrane beneath the Kurnalpi region had a higher proportion of tonalite than was present beneath the Kalgoorlie and Gindalbie terranes. We suggest that a regional tonalite-trondhjemite-granodiorite basement was accreted prior to complete accumulation of the greenstones and was later recycled, to form the granites. The Emu Fault is inferred to reflect a suture in the underlying basement of the greenstones.