Isotopic characteristics of Hannuoba basalts, eastern China: Implications for their petrogenesis and the composition of subcontinental mantle

Abstract

Alkalic and tholeiitic basalts from the Hannuoba region of eastern China have 87 Sr 86 Sr and 143 Nd 144 Nd ranging from the prevalent mantle (PREMA) field to inferred bulk Earth ratios. Ratios of 206 Pb 204 Pb range from 17.10 to 17.94 and all samples have 207 Pb 204 Pb and 208 Pb 204 Pb ratios significantly above the Northern Hemisphere reference line (NHRL). Although the oldest Hannuoba lavas are alkalic basalts, alkalic and tholeiitic lavas are intercalated, and they are isotopically distinct. Relative to the alkalic lavas, tholeiitic lavas have higher 87Sr 86 Sr , lower 143 Nd 144 Nd and less radiogenic Pb isotope ratios. The isotopic ratios and incompatible-element trends defined by all Hannuoba lavas are consistent with mixing of isotopically and compositionally distinct components. We infer that the alkalic basalts were generated by small degrees of melting of an asthenospheric source, similar to that required for some oceanic basalts. However, the tholeiitic basalts contain an enriched mantle component, similar to EMI in the oceanic mantle, which may have resided in the upwelling asthenosphere or subcontinental lithospheric mantle. Mixing between these mantle components may have occurred during the rifting event that caused extension in northeast China ∼ 40 Ma ago. Specifically, we propose that melting within the upwelling asthenospheric created the Hannuoba alkalic lavas whereas the dominantly younger tholeiitic lavas resulted from partial melting of continental lithosphere that was heated by the ascending asthenosphere. Similar models have been proposed for continental basalts erupted in the western U.S.A. and British Tertiary Province.