Volcanism of the Nanpu Sag in the Bohai Bay Basin, Eastern China: Geochemistry, petrogenesis, and implications for tectonic setting

Abstract

The recently discovered large Nanpu Oilfield is located in the NW part of the Bohai Bay Basin, eastern China. Extensive drilling and geophysical research have revealed widespread occurrence of Eocene to Miocene mafic volcanic rocks. On the basis of stratigraphic constraints, three cycles of volcanism have been recognised. Petrological and geochemical studies of drill core samples indicate that most of these mafic rocks are alkaline basalts. Some basalts experienced fractional crystallization, dominated by clinopyroxene and FeTi oxides. All samples have trace elements patterns similar to oceanic island basalts (OIB). Sr and Nd isotopic analyses show that 87Sr/ 86Sr i values range from 0.703577 to 0.708160 and εNd( t) varies from 0.7 to 6.8. Pb isotope analyses yield Pb ratios ranging from 17.43 to 19.19 for 206Pb/ 204Pb, from 15.43 to 15.67 for 207Pb/ 204Pb and from 37.35 to 38.64 for 208Pb/ 204Pb. The trace element and isotopic characteristics indicate that the Nanpu Sag basalts are similar to oceanic island basalts possibly with some contributions of subcontinental lithospheric mantle. Considering that there is widespread and coeval basaltic volcanism in the Bohai Bay Basin, which has comparable geochemistry, we adopt a model of episodic upwelling and melting of asthenospheric mantle for the origin of these basalts. The old subcontinental lithospheric mantle had interacted with new asthenospheric mantle-derived melts. The replacement of old North China Craton mantle to new and fertile asthenospheric mantle is still ongoing. Early-stage magmas were generated from deeper in the mantle with a higher degree of partial melting than late stage magma. The upward migration of the melting column supported active asthenosphere mantle upwelling or mantle plume induced magmatism under an extensional regime.