Intracontinental orogenesis in the heart of Australia: structure, provenance and tectonic significance of the Bentley Supergroup, Western Musgrave Block, Western Australia

Abstract

Volcanic rocks attributed to the ca. 1080 Ma Giles Event were extruded within the western Musgrave Block in the late Mesoproterozoic. SHRIMP U-Pb analysis of two samples of zircon cores from rhyolites approximately 2km SSE of Blackstone Range yielded 238U/206Pb crystallisation ages of 1071±8 Ma and 1073± 7 Ma. These rhyolites, forming the base of the Smoke Hill Volcanics at Mount Jane were unconformably emplaced over the Blackstone Range layered intrusions in a series of separate flows. Soon after being extruded the rhyolites were deformed and developed pervasive NE-or SW dipping foliation within a 2km wide shear zone at Mount Jane. Garnet and epidote replacement of plagioclase phenocrysts indicates at least lower amphibolite-facies metamorphism was attained within the near vertical, south-dipping ultramylonite centre of the shear zone. Shear sense indicators within the shear zone almost exclusively give north side down kinematics in outcrop and in thin section. A syn-tectonic micro-granite intrudes the centre of the shear zone with veins that cross-cut the ultramylonitic foliation and hence post-dates shearing. This micro-granite yields a 238U/206Pb vs 207Pb/206Pb isotope ratio zircon age of 1070±5 Ma. Thin, highly luminescent, structureless rims surround the zircons and yield a similar or slightly younger age than the cores. Therefore, the deformation was most likely synchronous, or soon after, the eruption of the rhyolite as part of the Giles Event. A series of mafic and intermediate volcanic rocks attributed to the Hogarth Formation directly overlie the rhyolite, dip steeply towards the south, and have been cut by a series of later stage gabbros and granites.