GEOCHEMISTRY OF CENOZOIC MAGMATISM IN THE SOUTH CHINA SEA AND ITS TECTONIC IMPLICATIONS

Upon the review of the published geochonological,petrological and geochemical data of Cenozoic igneous rocks from the SCS,this paper studied the genesis and geochemical characteristics of the rocks,as well as their deep geodynamics and tectonic implications.Data show that there existed a bimodal volcanism in the northern margin of the SCS in earlier times(60~43 Ma or 32 Ma),followed by a relatively silent stage during the Cenozoic seafloor spreading(32~17 or 15.5 Ma),except for the central basin.And a large area of post-spreading volcanism (tholeiitic series for 17~8 Ma,alkali series since 8 Ma)affected the SCS and its adjacent areas(e.g.,Pearl River Mouth Basin,Beibu gulf,Indochina block and the South China Sea basin).The earlier volcanism occurred in an extensional geodynamic setting resulted from the collision between Indian plate and Eurasian plate as a far-field effect,and the post-spreading volcanism may be related to the Hainan mantle plume.The Hainan mantle plume played a subtle but important role in the extensional process of the northern margin and the seafloor spreading of the SCS.Finally,this study proposed that the igneous activities at fossil spreading centers in the SCS are similar to those in the East Pacific Rise and might be a new type of volcanism.The timing and geodynamic setting of the volcanism remain unclear.Why the extinct spreading centers continue to erupt after cessation of spreading is a brand new scientific problem. The further study of the igneous activity along the fossil spreading centers in the SCS is critical to the understanding of the tectonic evolution of the SCS.