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.