西藏雅鲁藏布蛇绿岩形成构造环境的地球化学鉴别

Geochemical discrimination of tectonic environments of the Yalung Zangpo ophiolite in southern Tibet

本文对西藏雅鲁藏布蛇绿岩带中部日喀则地区夏鲁蛇绿岩中的基性岩石进行了元素地球化学研究。岩石为低钾亚碱性玄武质岩石,具有低TiO2(0.64%～1.5%)、低K2O(<0.69%)、富Na2O(2.5%～6.0%)、烧失量普遍较高(2.0%～7.5%)的特征,表明样品普遍遭受蚀变。岩石具有与N-MORB类似的稀土元素和总体一致的微量元素特征,但是岩石明显富集大离子亲石元素和亏损高场强元素,显示了洋脊玄武岩向着岛弧岩浆作用的过渡趋势。日喀则蛇绿岩不是典型的洋中脊成因,应为SSZ型蛇绿岩,可能形成于与洋内俯冲有关的SSZ型的弧后或者弧前盆地中。对东西延伸超过1500km的雅鲁藏布蛇绿岩246套基性岩石形成的构造环境进行的地球化学判别,表明至少存在大陆岛弧型、地幔柱-洋内热点型、洋中脊-大洋岛弧型和典型大洋岛弧型等四种类型洋壳,揭示了特提斯洋存在多种构造环境的特征,有利于恢复和还原一个长期演化的、丰富的特提斯洋。

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.