Origin of Cretaceous continental tholeiites in southwestern Australia and eastern India: insights from Hf and Os isotopes

Abstract

Rifting between eastern India and western Australia in the Early Cretaceous (∼140–133 Ma) resulted in the formation of the eastern Indian Ocean. Tholeiitic flood basalts erupted on the southwestern margin of Australia (Bunbury basalts at ∼132 and ∼123 Ma) and the eastern margin of India (Rajmahal Traps at ∼118 Ma) shortly after this time, while the Kerguelen Plateau began to form in the nascent ocean basin (∼119 Ma). We studied the Hf and Os isotopic systematics of the continental basalts in order to better understand the relationship between these volcanic events and the Kerguelen plume. The objective of this work was to determine the mantle sources of the continental tholeiites and the nature and quantity of continental contamination. Both the Hf and Os isotopic compositions of these basalts vary with indicators of continental contamination (SiO 2, (Th/Ta) PM, (La/Nb) PM). There is no obvious correlation of γ Os( T) with Sr or Nd isotopic ratios, but rough correlations are present between Os and Hf isotopes and 208Pb/ 204Pb; in contrast, ε Hf( T) shows strong correlations with all isotopic systems and indicators of continental crust assimilation. We demonstrate that the mantle sources of these continental tholeiites cannot originate purely from the depleted asthenosphere. An enriched mantle source is required for both of these basalt suites and the geochemical characteristics of this source are broadly consistent with those inferred to be present in the Kerguelen plume during Early Cretaceous time. The Os isotopic results suggest that at least some continental contamination is present in all samples from both the Bunbury basalts and the Rajmahal Traps. In the Rajmahal Traps, this contaminant appears to be derived from the continental crust and there is little to no evidence for involvement of subcontinental lithospheric mantle (SCLM). The Bunbury basalts have consistently unradiogenic Os isotopic compositions and high ε Hf( T) for a given ε Nd( T), characteristics ascribed to the SCLM. Previous investigations of these basalt suites have concentrated on major and trace elements and Sr, Nd and Pb isotopes; none of these results suggested the involvement of SCLM in the Bunbury basalts. Here, we emphasize that the study of Hf and Os isotopes provide additional, useful tools for deciphering mantle sources and continental contaminants involved in the petrogenesis of continental tholeiites.