LATE CENOZOIC BASALTIC VOLCANISM AROUND THE TAIWAN STRAIT, SE CHINA - PRODUCT OF LITHOSPHERE ASTHENOSPHERE INTERACTION DURING CONTINENTAL EXTENSION

Late Cenozoic intraplate basaltic volcanism in SE China and Taiwan occurred as a result of lithospheric extension related to the opening of the South China Sea. It was gradually terminated by the compression which propagated westwards from the arc-continent collision in Taiwan since approximately 12 Ma ago. The basalts show a spatial variation in chemical composition. Their alkalinity, degree of silica undersaturation and abundance of incompatible elements increase whilst Pb isotopic ratios decrease (Pb-206/Pb-204 from 19.0 to 18.2) progressively away from a NE-trending extension axis in the western Taiwan Strait. Abundant tholeiites were emplaced near the axial zone whereas alkali basalts, basanites and nephelinites took place further away from this axis. Overall, the basalts have Sr-Nd-Pb isotopic systematics similar to those of seamount basalts in the South China Sea which are characterized by a Dupal-type Pb isotopic anomaly. Based on geochemical constraints from the basalts, depth estimates for mantle xenoliths and regional geophysical data, a passive extension model is proposed for the geodynamic evolution of this region. It suggests greatest lithosphere thinning during the Miocene beneath the axial zone. The lithospheric mantle was thermo-mechanically eroded by convective upwelling of the asthenosphere, a process that raised the lithospheric geotherm and resulted in a plum-pudding-type convecting mantle. The spatial chemical and isotopic variation in the basalts can be explained by different degrees of decompression melting of this convecting mantle, corresponding to a change in lithosphere thickness, compounded by various contributions from the continental lithospheric mantle (CLM) -derived plum component. This model, which involves continental extension followed by magma generation via lithosphere-asthenosphere interaction, should be applicable to other areas of Cenozoic rift magmatism around the South China Sea and in the eastern Eurasian continental margin.