Isotopic and geochemical systematics in Tertiary-Recent basalts from southeastern Australia and implications for the evolution of the sub-continental lithosphere

Abstract

Tertiary-Recent Tasmanian and Newer (Victoria/South Australia) basalts range from quartz tholeiite to olivine melilitite and show systematic increases in their incompatible element abundances with increasing degree of silica undersaturation. These two basalt provinces show similar relative abundances of rare earth elements (REE), differences in the relative concentrations of Rb, Ba, Th, K and Nb, and distinct, restricted isotopic compositions. The Tasmanian basalts have 87Sr 86Sr from 0.7026 to 0.7034, and ϵ Nd from + 7.5 to + 5.8; the Newer basalts have higher 87Sr 86Sr from 0.7038 to 0.7045, and lower ϵ Nd from +4.2 to + 1.7. The range in Sr and Nd isotope compositions can be denned by primary magma compositions for both provinces, using Mg-values, Ni content and the presence of spinel lherzolite nodules. Major and trace element and Sr, Nd and Pb isotope compositions are uniform on a scale of up to 50 km for four separate Newer basanite centers. The chemical and isotopic data are consistent with a model whereby tholeiitic basalts are derived by large degrees of partial melting from a chemically uniform but isotopically variable source, and generation of undersaturated, alkaline basalts by smaller degrees of partial melting of the same source. No isotopic or geochemical evidence was found which would suggest that the more evolved basalts have been contaminated by continental crust. In contrast to tholeiitic and alkalic basalts from Hawaii, there is a continuous spectrum of isotope compositions for the Newer tholeiitic to alkalic basalts. A model is proposed for the generation of these basalts involving mixtures of hotspot mantle plume-derived melt and lithospheric mantle-derived melt, where observed differences between ocean island and continental alkaline basalts are attributed to differences between the sub-oceanic and sub-continental lithospheric mantles. Isotopic differences between tholeiitic and alkalic basalts are interpreted to be due to varying degrees of exchange and mixing between the hotspot plume and lithospheric mantle melt components. The model is consistent with the generation of these basalts from a source which has been recently enriched in the LREE.