Did late Palaeoproterozoic assembly of proto-Australia involve collision between the Pilbara, Yilgarn and Gawler Cratons? Geochronological evidence from the Mount Barren Group in the Albany–Fraser Orogen of Western Australia

Abstract

The Mount Barren Group, which crops out on the southern margin of the Yilgarn Craton, has been considered an allochthonous synorogenic sequence to a Mesoproterozoic (1300 Ma) collision between southwestern Australia, southeastern Australia and eastern Antarctica during assembly of East Gondwana. Reassessment of its stratigraphic geometry confirms the historical perspective that nonconformable and faulted contacts are preserved on autochthonous Archaean orthogneiss of the Yilgarn Craton, emphasising that it is not an allochthonous sequence. SHRIMP UPb dating of early diagenetic xenotime establishes a Late Palaeoproterozoic (1696±7 Ma) depositional age for the Mount Barren Group, thereby negating a Mesoproterozoic synorogenic origin. The Mount Barren Group now correlates in time with strike-slip sequences in the Capricorn Orogen approximately 1000 km to the north, and which were a post-collisional response to ∼1750 Ma sinistral transform collision between the Pilbara and Yilgarn Cratons. SHRIMP UPb detrital-zircon geochronology of the Mount Barren Group, therefore, has an important role in establishing if Pilbara–Yilgarn transform collision were either independent of or a component in the Late Palaeoproterozoic assembly of a proto-Australian continent. Detrital-zircon age-data from four samples of different sandstone beds in the Mount Barren Group define complex age spectra composed of seven significant provenance age-subpopulations. Comparison to age spectra from provinces with a possible geotectonic link to the Yilgarn Craton indicates that either the Pilbara Craton or the Gawler Craton was the source terrain. A barrier to a dispersal-system link to the Pilbara Craton points to the Gawler Craton as the source, a provenance link that was also made for the now-redundant 1300 Ma scenario. The 1696±7 Ma depositional age and zircon provenance analysis support a contentious hypothesis involving coeval collisions between the Pilbara–Yilgarn and Yilgarn–Gawler Cratons during assembly of a proto-Australian continent some 500–400 million years earlier than envisaged by other reconstructions.