Multiple sources for basaltic rocks from Dubbo, eastern Australia: geochemical evidence for plume—lithospheric mantle interaction

Abstract

A wide range of Cenozoic basaltic compositions, including primary or primitive basanites, alkali olivine basalts, olivine tholeftes, and evolved hawaftes and mugearites, is represented in the Dubbo volcanic province, central New South Wales, Australia. Their geological and geochemical characteristics provide the key to unravelling the relative roles of a plume source and the subcontinental lithospheric mantle (SCLM) in the generation of the different types of basaltic melts in this region. Eruptions of the basaltic rocks during Miocene (14-12 Ma) coincide with the locus of the hotspot-related central-volcano magmatism in eastern Australia. The olivine tholeiites have SrNd isotopic compositions similar to those of primitive basalts from adjacent central-volcano provinces and exhibit a smooth incompatible element pattern with Nb and Ta peaks, similar to typical oceanic island basalts. They are most likely generated from a mantle plume which was also the source for the other eastern Australian central volcanoes. In contrast to the olivine tholeiites, basanites and some alkali olivine basalts have SrNd isotope ratios more enriched than the Bulk Silicate Earth reservoir and show fractionated incompatible element patterns with strong primary Rb and K depletions which may be explained either by retention of phlogopite during the partial melting or by selective depletion in the source shortly before the magmatism. Partial melting of a metasomatised SCLM source may have played a significant role in the generation of these alkaline basalts. Some evolved potassic hawaiites could be derived from olivine tholefte parental magmas by high-pressure fractionation with a small amount of crustal contamination. If Neoproterozoic Nd model ages (640–890 Ma) of the Dubbo basaltic rocks are meaningful, these may imply the presence of a Proterozoic lithospheric mantle reservoir beneath certain parts of the Paleozoic Lachlan Fold Belt.