Abstract:

Ophiolites in west of the Namco Lake in Tibet are mainly composed of metamorphic peridotites,gabbro—diabases and basalts．From the analysis of major and trace elements of ophiolite in west of Namco，the charactenstics of the metamorphic peridotite are high content in Mg，and low content in Ti and ΣREE．The diabases and basalts are the transitional IAB／MORB-type basalt，which have the characteristic of depletion in high field elements(Nb，Ta，Zr，Hf)and comparable enrichment in large ion lithosperic elements(Rb，Sr，Ba)，and on the other hand，theREE／chondrite pattern displays a flat-model with LREE
depletion and without Eu anomaly. It is concluded that the geotectonic setting of ophiolite in Namco is formed in back arc basin by the comparison with the ophiolite of typical areas and discrimination diagrams of the tectonlc setting.

Key words: ophiolite, geochemical characteristic, tectonic setting, back arc basin, Namco, Tibet