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南海海盆玄武质岩石的全岩K-Ar年龄为7.9～3.8Ma,为南海扩张停止(15.5Ma)后、晚中新世以来的板内火山作用的产物,可与其周边地区(如雷琼半岛、南海北缘及中南半岛等)同期火山作用的岩石成因和源区性质进行对比研究.在详细的岩相学研究基础上,从中挑选出了新鲜的玄武质岩石,并进行了主量、微量元素及Sr-Nd-Pb同位素分析研究.结果表明,该区玄武岩属于碱性玄武岩浆系列,具有OIB型的稀土及微量元素特点,形成岩浆的部分熔融作用程度较低,岩浆在上升期间或在高位岩浆房中经历了结晶分异作用和堆晶作用.Sr-Nd-Pb同位素地球化学研究表明,南海地幔存在不均一性,岩浆源区性质为EM2与DMM的二端员混合模式,其中EM2为来自地幔柱物质的富集端员,DMM端员为大洋软流圈或岩石圈亏损地幔;Pb同位素特征显示本区存在Dupal异常,结合北冰洋Gakkel脊新发现的Dupal异常,表明此异常并非只限于南半球.从Sr-Nd-Pb同位素变异图解中可见,南海与雷琼半岛、南海北缘以及中南半岛地区可能具有相似的地幔源区特征,与中国华北及东北地区存在一定差异.南海海盆晚中新世以来的玄武岩为海南地幔柱提供了地球化学制约.
The K-Ar ages of the basaltic rocks in the South China Sea Basin are 7.9-3.8 Ma, which are the products of intraplate volcanism since the Late Miocene after the South China Sea expansion ceased (15.5 Ma), which can be related to the surrounding areas , The northern margin of the South China Sea and Indochina Peninsula, etc.) on the basis of detailed petrographical studies, from which the fresh basaltic rocks were selected and the major and minor Element and Sr-Nd-Pb isotope analysis. The results show that the basalt in this area belongs to alkaline basalt series with the features of OIB rare earth and trace elements. The degree of partial melting of magma is low. During the ascending period of magma, Sr-Nd-Pb isotopic geochemical studies show that there is heterogeneity in the mantle of the South China Sea, and the nature of the magma source region is a two-terminal mixed mode of EM2 and DMM, in which EM2 For the enrichment members from the mantle plume, the DMM member is either a marine asthenosphere or a lithospheric depleted mantle. The Pb isotopic characteristics show Dupal anomaly in this area, indicating that this Often, it is not limited to the southern hemisphere. It can be seen from the Sr-Nd-Pb isotope variation that the South China Sea and the Leijon Peninsula, the northern margin of the South China Sea and the Indochina Peninsula may have similar characteristics of the mantle source, which is different from North China and Northeast China. Basalt in the South China Sea Basin since the Late Miocene provided geochemical constraints for the mantle plume in Hainan.