HIGH-PRESSURE AMPHIBOLITE-GRANULITE FACIES METAMORPHISM IN THE PALEOPROTEROZOIC RUDALL COMPLEX, CENTRAL-WESTERN AUSTRALIA

The Rudall Complex forms the basement of the Paleoproterozoic to Neopr oterozoic Paterson Orogen, which was deformed during at least three ma jor orogenies. The complex comprises a sequence of metamorphosed elast ic and chemical sediments, and basalts, deposited in a marginal basin environment, prior to extensive felsic intrusion. Deformation of the s equence 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-tectoni c with respect to this deformation. Thermobarometry on amphibolites an d mafic granulites that contain various combinations of the minerals h ornblende, plagioclase, quartz, garnet, othopyroxene and clinopyroxene , indicates that temperatures peaked at around 800 degrees C, while pr essures reached 1200 MPa. The late timing of peak temperatures togethe r with the presence, in some amphibolites, of amphibole-plagioclase sy mplectitic coronas around garnet, indicates a steeply decompressive cl ockwise 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 tha t this deformational and metamorphic event records the collision betwe en the Pilbara Craton and a continent to the northeast. The Paleoprote rozoic evolution of Australia has previously been attributed to ensial ic processes characterised by low-pressure metamorphism and anti-clock wise P-T-t paths. The metamorphic history of the Rudall Complex is not consistent with this view, and the documented high-pressure metamorph ism is broadly synchronous with a medium-pressure metamorphic event in central Australia (the Strangways Orogeny), and possibly also in nort hwestern Australia (the Capricorn Orogeny). Collisional plate-tectonic processes played a significant role in the Paleoproterozoic evolution of northern Australia. (C) 1997 Elsevier Science B.V.