西藏雅鲁藏布江蛇绿岩带中西段萨嘎蛇绿岩及其构造意义

Petro-geochemistry & chronology of Saga ophiolite in mid-western part of Yarlung-Zangbo ophiolite belt and its tectonic significance

基于萨嘎蛇绿岩序列中超基性岩和基性岩地球化学、岩石学、年代学、大地构造与地球动力学的数据资料,对西藏雅鲁藏布江蛇绿岩带中西段萨嘎蛇绿岩进行构造意义分析,认为萨嘎蛇绿岩层序中辉长岩属钙碱性-低钾拉斑玄武岩;蛇绿岩层序中的玄武岩为钙碱性玄武岩和碱性玄武岩,萨嘎蛇绿岩北亚带东侧热嘎、松多处玄武岩为钙碱性-低钾拉斑玄武岩,中西侧萨嘎处玄武岩为碱性-钾玄系列玄武岩。萨嘎蛇绿岩中基性岩具有大洋中脊、低钾拉斑玄武岩和碱性玄武岩特征。热嘎基性岩具有MORB微量元素特征,属大洋中脊玄武岩型;萨拉基性岩具有MORB和大洋岛弧玄武岩特征,属碱性玄武岩和低钾拉斑玄武岩型;松多基性岩具有MORB和大洋岛弧玄武岩特征,属地幔柱玄武岩和碱性玄武岩型。综上所述,位于北亚带萨嘎蛇绿岩多源,普遍贫Cr、Ti、Mn,微量元素具有地幔地球化学特征。在萨嘎蛇绿岩中发现大面积碱性玄武岩,其为后期混染壳源物质后产生,保留了幔源玄武质岩浆的部分地球化学特征,贫硅富碱,形成富碱矿物。萨嘎蛇绿岩达吉岭-萨拉剖面中辉长岩的锆石U-Pb年龄为125.6±2.7 Ma,与整个雅鲁藏布江蛇绿岩中西段基性岩年龄(130～120Ma)一致。推断萨嘎蛇绿岩可能形成于洋内超俯冲带(SSZ),并经历了后期弧前盆地洋中脊扩张改造,属洋内超俯冲带(SSZ)的弧前-弧-弧后环境。

Saga ophiolite from the mid-western part of the Yarlung-Zangbo ophiolite belt was researched in this paper.The data of petrogeochemistry,geochronology,chronology,geotectonics and geodynamics indicate that gabbro in Saga ophiolite was originated from calc-alkaline low-potassium tholeiitic basalt.The basalt in Saga ophiolite is calc-alkaline and alkali.The basalts in Rega and Songduo section line are calc-alkaline low-potassium tholeiitic basalts.The basite in Saga ophiolite has the same geochemical characteristics as MORB,tholeiitic and alkaline basalt.In summary,the Saga ophiolite in the northern sub-belt was originated from various multiple resources.It is generally depleted in Cr,Ti,Mn.The geochemical characteristics of trace elements are similar to mantle.A large area of alkaline basalt was found in the saga ophiolite,retaining geochemical characteristics of mantle with enriching alkali.Zircon U-Pb age of the gabbro in Dajiling-Sala profile is 125.6±2.7 Ma,in accordance with the basaltic age(130 to 120 Ma)in western Yarlung-Zangbo ophiolite.It is inferred that the Saga ophiolite is possible to form in the intra-oceanic supra-subduction zone(SSZ),suffering from expansion in mid-oceanic ridge situated in the forearc basin.