Oceanic plateau accretion onto the northwestern margin of the Yilgarn Craton, Western Australia: implications for a mantle plume event at ca. 2.0 Ga

Abstract

The 2.0 Ga Bryah Basin (Group) is one of a series of Palaeoproterozoic basins in the eastern Capricorn Orogen, a major tectonic unit between the Archaean Pilbara and Yilgarn Cratons of Western Australia. The Narracoota Formation forms the bulk of the Bryah Group and consists of ultramafic and mafic schist and metabasaltic hyaloclastites, sheeted dykes and a layered mafic-ultramafic complex, collectively referred to as metabasites, because they are generally metamorphosed to greenschist facies. Chemically these rocks correspond to komatiites, high-Mg basalts and tholeiites. Previous interpretations have purported that these rocks represent rift-related volcanics, ophiolites and mid-ocean ridge settings. In this contribution, I show that the Narracoota Formation metabasites were likely part of an ancient oceanic plateau, created by a 2.0 Ga mantle plume. Geochemical data (eg REE patterns) indicate that the metabasites are identical to modern oceanic plateaux. The presence of orogenic gold lodes further supports the oceanic plateau model. During the 1.96 Ga Glenburgh Orogeny, which brought together an Andean-style magmatic arc (southern Gascoyne Complex) and the northwestern margin of the Yilgarn Craton, the Narracoota oceanic plateau was accreted onto the Yilgarn Craton. The 1.8 Ga Capricorn Orogeny further deformed the Narracoota metabasites, resulting in the present day structural configuration.