SR, ND, AND PB ISOTOPIC COMPOSITIONS OF HAINAN BASALTS (SOUTH CHINA) - IMPLICATIONS FOR A SUBCONTINENTAL LITHOSPHERE DUPAL SOURCE

Sr, Nd, and Pb isotopic ratios were determined for postspreading basalts from Hainan Island, erupted in a quasi-oceanic pull-apart trough at the northern edge of the South China Basin. The basalts comprise quartz tholeiite, olivine tholeiite, alkali basalt, and basanite and show oceanic island basalt (OIB)-type trace element distributions relatively enriched in Nb, Ta, and Th. Despite normal OIB-like Sr and Nd isotopic compositions, Hainan lavas have unexpectedly high Pb-208/Pb-204 and Pb-207/Pb-204 ratios, resembling those of Southern Hemisphere Dupal OIB and some alkalic arc basalts. We propose that the Hainan source comprises a lower lithospheric region of accreted asthenospheric melt (isotopically resembling Central Indian Ridge-type basalt) overprinted by radiogenic melts of subducted sediment. In the absence of a mantle plume, decompression melting of subcontinental lithospheric mantle may result from lithospheric stretching, as a regional response to the Indo-Eurasian collision.