Geochemical discrimination of tectonic environments of the Yalung Zangpo ophiolite in southern Tibet
西藏雅鲁藏布蛇绿岩形成构造环境的地球化学鉴别

Elemental compositions for the basaltic rocks from Xialu ophiolite,Xigaze,central Yalung Zangpo ophiolite in southern Tibet were studied in this paper.The rocks are low-K tholeiitic basaltic rocks,with low TiO2(0.64% ～ 1.5%),K2O(< 0.69%),high Na2O(2.5% ～ 6.0%) and LOI(2.0% ～ 7.5%),implying a metasomatic processes after the rocks formed in the ocean.Their chondrite-normalized Rare Earth Elements(REEs) and primitive mantle-normalized multi-element patterns are very similar to the normal Mid-Ocean Ridge Basalt(N-MORB),with depletion in light REEs.But they show significant enrichment of Large Ion Lithophile Elements(LILEs;Rb,Ba,U,K) and depletion of High Field Stretch Elements(HFSEs;Nb,Ta,P,Ti,Zr,and Hf),suggesting arc-related components have been added to the source regions.The Xialu ophiolite is SSZ-type(Supra Subduction Zone) oceanic crust,instead of the typical N-MOR type ophiolite.It may have occurred in back-arc or fore-arc tectonic settings in an oceanic subduction-related system.A detailed geochemical discrimination of selected elements from 246 samples from literature and this work,allowed us to identify at least four different types of tectonic settings within the 1500km-long Yalung Zangpo ophiolite.The four-type basalts are(1) continental arc,(2) mantle plume or hot spot,(3) MORB and oceanic arc,and(4) typical oceanic arc.The presence of multiple tectonic settings seen from geochemical signatures of associated basalts within the Yalung Zangpo suture zone reconstructs a long-evolutional history of the Tethyan Ocean.