Sm-Nd isotopic constraints on the age of the Bangxi-Chenxing ophiolite in Hainan Island: implications for the tectonic evolution of eastern Paleo-Tethys.

Sm-Nd isotopic data are presented for the metabasites that are present as l enses within the Paleozoic metasediments at the Chenxing and Bangxi regions in central Hainan Island, SE China. A Sm-Nd isochron age of 333+/-112 Ma o btained for the metabasites is interpreted as their crystallization age. Th ese metabasites are characterized by varying degrees of depletion in Th, Nb , Ta and LREEs, and show high positive epsilon(Nd)(t) values of ca. +7, sim ilar to those of mid-ocean ridge basalts (MORB). The combined geochemical a nd Sm-Nd isotopic data suggest that these metabasites were generated by var ying degrees of partial melting from a MORE-like mantle source in an oceani c regime. These rocks are therefore regarded as the mafic member of Bangxi- Chenxing ophiolite. It is proposed that the Bangxi-Chenxing ophiolite can b e correlated with broadly synchronous ophiolites from the Jinsha-Shuanggou- Song Ma. Taken together, these ophiolites probably represent remnants of fr agmented oceanic crust of the eastern part of Paleo-Tethys. The Bangxi-Chen xing ophiolite were obducted onto the continental crust as part of the "Shi lu Melange" in earliest Mesozoic time when southern Hainan (part of the Ind ochina block) collided with northern Hainan (part of South China).