Volcanism of Nanpu Sag in the Bohai Bay Basin: Geochemistry, Origin and Tectonic Implications

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 basaltic volcanic rocks of Cenozoic age. On the basis of stratigraphic constraints six cycles of vocanism have been recognised. Petrological and geochemical studies of drill core samples indicate that the basalts belong to the alkaline series. Some basaltic rocks experienced fractional crystallization, dominated by clinopyroxene. All samples examimed are enriched in high field strength elements and have trace elements patterns similar to oceanic island basalts (OIB). High ratios of La/Yb suggest that significant fractionation of rare earth element has occurred. The Sr and Nd isotopic system shows that 87Sr/86Sri range from 0.7036 to 0.7082 and ¦ÅNd(t) vary from -2.336 to 3.564. Measured Pb isotopes range from 17.434 to 18.354 for 206Pb/204Pb, from 15.43 to 15.609 for 207Pb/204Pb and from 37.352 to 38.454 for 208Pb/204Pb, respectively. These geochemical and isotopic characteristics indicate that they are similar to oceanic island basalts and were generated from a within-plate setting. These basalts were formed by low degrees of partial melting from a hybrid mantle of EMI and DMM types, with no significant crustal assimilation. Considering that there is widespread and coeval basaltic volcanism in the Bohai Bay Basin, with comparable geochemistry, we suggest a mantle plume model for the origin of these basalts. We argue that a mantle plume was activated since the Cretaceous, which resulted in uplift and rifting in eastern China. Continuing ascending movement of the mantle plume induced multi-stage mantle upwelling and melting. This process caused crustal uplifting and thinning, resulting in the inception of the Bohai Bay basin, in which a succession of interbedded volcanic and sedimentary rocks accumulated