Trace-element and isotopic geochemistry of volcanic rocks and it's techtonic implications in Weizhou Island and Xieyang Island, Northern Bay

Weizhou Island and Xieyang Island are twin volcanoes in Northern Bay. Based on detailed geological researches, in this paper we report geochemical researches oil trace elements and Sr-Nd-pb isotopes in volcanic rocks and researches oil Re-Os isotopes in mantle xenoliths. In the islands, volcanic rocks of the two stages, early stage and late stage, are all alkaline basalt, including alkaline olivine basalt and picrite. The picrite has porphyritic texture, with glassy matrix and fragments of mantle xenoliths (usually less than 1 cm). This indicates that ascending speed of the magma from the mantle chamber was very quick, and hence the magma evolution was not significant for these volcanoes. Geochemistry of trace elements and Sr-Nd-Pb isotopes in the volcanic rocks in the Weizhou Island and Xeiyang Island show similar characteristics with the surrounding volcanics in Leiqiong, circum Northern Bay and Southern Sea Basin. Sr, Nd isotopes show characteristics of DMM (depleted mantle member), while Pb isotopes show characteristics of Dupal abnormity which means mixing of EMII source. The geochemical characteristics reveals that the basalts in the two islands are not similar with MORB or OIB, and the magma is not derived from a simple mantle source, but from a mixture of two different types of mantles sources. Geochemistry of volcanic rocks indicate that the magma genesis shows no signal of mantle plume, and geochemistry of Re-Os isotopes of the mantle xenoliths also suggest that the magma is from depleted lithospheric mantle, not core-mantle boundary. However we suggest the story of the magma genesis of Northern Bay area is that the extension of South China Sea and disaggregation of the continent induced the asthenosphere upwelling, then mixed and reacted with the young and thin lithospheric mantle, and magma formed.