Geochemical study on metamorphosed mafic rocks in ophiolitic zone in the Yarlung Zangpo Great Bend, eastern Tibet,China.

Based on regional geological mapping at 1∶250000 and thematic studies, we carried out geochemical studies on the metamorphosed mafic rocks, for major, trace elements and Sr-Nd-Pb isotope geochemistry of the Yarlung Zangpo ophiolitic zone in the area of the eastern Himalayan Syntaxis, to further clarify the SSZ-type ophiolite petrogenesis. The metamorphosed mafic rocks can be divided into boninite, IAT and OIB types. The boninite type samples have slightly depleted in LREE or flat-type pattern, with low total REE content (∑REE=7.56×10-6~20.89×10-6). They have enriched Cr, Ni, MgO and Al2O3 contents and originated from fore-arc setting. The IAT samples, as the main type in this zone have Nb-Ta depletion in the spider diagram and flat-type REE pattern with low REE content ( ΣREE=37.84×10-6~55.75×10-6). The OIB samples are LREE-enriched type with higher REE content( ΣREE=102.09×10-6~240.95×10-6). Compared with the IAT and MORB, they show higher La, Nb, P, Ti content, but lower Cr and Ni content. Sr, Nd and Pb isotopic composition suggest that PREMA (or BSE) and EMI provided magma source, suggesting that primitive mantle and lower crust are involved in the original magma forming. We proposed a two-stage hypothesized model for Neotethyan evolution. The Neotethyan evolution involves a plume and oceanic island as the early stage that formed MORB-OIB combination. The late stage formed intra-oceanic arc and resulted in SSZ-type ophiolites and IAT combination.