Geochemistry of Hannuoba basalts, eastern China: Constraints on the origin of continental alkalic and tholeiitic basalt

Abstract

The Cenozoic Hannuoba basaltic province, 200 km northwest of Beijing, China, is composed of intercalated tholeiitic, transitional and alkalic basalt. Although the alkalic basalts contain abundant upper-mantle xenoliths, they range in MgO content from 5.7% to 10.6% MgO. Most of the major- and trace-element abundance variations in the alkalic basalts can be explained by segregation of a clinopyroxene-rich assemblage containing significant amounts of garnet and minor Fe-Tioxides. Incompatible-element abundances in these alkalic basalts are remarkably similar to those of alkalic basalts from oceanic islands. Obviously, there is a worldwide source for alkalic basalts that can be tapped in oceanic and continental settings. Compared with Hannuoba alkalic basalts of similar MgO content, Hannuoba tholeiitic lavas have lower incompatible-element abundances and different incompatible-element abundance ratios (e.g., Ba/La, La/Sm, Sm/Nd, Zr/Nb and Zr/Hf). In contrast to arc basalts and many continental flood basalts, Hannuoba tholeiitic lavas are not anomalously depleted in Nb and Ta. Although radiogenic isotope data (Sr, Nd and Pb) show that the alkalic and tholeiitic lavas are isotopically distinct, variations in incompatible-element abundances and abundance ratios in all lava types define coherent trends suggestive of a mantle mixing process.