海南岛“邦溪—晨星蛇绿岩片”的时代及其构造意义—Sm—Nd同位素制约

本文报道了邦溪和晨星两个地区变质基性岩的 Sm - Nd同位素定年结果。Sm - Nd等时线年龄 333± 12 Ma代表了变质基性岩的形成年龄。这些变质基性岩高度亏损 Th、Nb、Ta和轻稀土元素 ,εNd(t)≈ 7,与洋中脊玄武岩类似 ,形成于大洋环境。本文将这些洋中脊型变质基性岩称之为“邦溪 -晨星蛇绿岩片”,它们与金沙江 -双沟 - Song Ma蛇绿岩在形成时代上基本一致 ,代表了东古特提斯洋的残片。邦溪 -晨星蛇绿岩片在早三叠世华南 (包括海南岛北部 )和印支块体 (包括海南岛中南部 )块体碰撞拼合时构造侵位在大陆上 ,成为“石碌混杂岩”的一部分

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

SmNd isotopic data are presented for the metabasites that are present as lenses within the Paleozoic metasediments at the Chenxing and Bangxi regions in central Hainan Island, SE China. A SmNd isochron age of 33312 Ma obtained for the metabasites is interpreted as their crystallization age. These metabasites are characterized by varying degrees of depletion in Th, Nb, Ta and LREEs, and show high positive Nd values of ca. 7, similar to those of midocean ridge basalts (MORB). The combined geochemical and SmNd isotopic data suggest that these metabasites were generated by varying degrees of partial melting from a MORBlike mantle source in an oceanic regime. These rocks are therefore regarded as the mafic member of BangxiChenxing ophiolite. It is proposed that the BangxiChenxing ophiolite can be correlated with broadly synchronous ophiolites from the JinshaShuanggouSong Ma. Taken together, these ophiolites probably represent remnants of fragmented oceanic crust of the eastern part of PaleoTethys. The BangxiChenxing ophiolite were obducted onto the continental crust as part of the "Shilu Melange" in earliest Mesozoic time when southern Hainan (part of the Indochina block) collided with northern Hainan (part of South China).