西藏蓬湖西蛇绿岩地球化学及构造背景研究

Geochemistry and Tectonic Setting of Western Pung Co Lake Ophiolite,Xizang(Tibet)

西藏蓬湖西蛇绿岩隶属于班公湖—怒江缝合带的中段,是藏北湖区较好的堆晶岩剖面之一。形成该堆晶岩时矿物分离结晶顺序为铬铁矿橄榄石单斜辉石斜长石,分离结晶作用具有由富镁质、铁质向富铝质、钙质演化趋势。堆晶岩主量元素组成与世界典型蛇绿堆晶岩成分相似;各种堆晶岩石的微量元素和稀土元素配分曲线总体上基本一致,反映其同源性;岩石Nb和Ta亏损,稀土元素含量很低,分配模式呈LREE微弱亏损的平坦型。该蛇绿岩形成于弧后盆地扩张中心,属SSZ型蛇绿岩。

The Western Pung Co Lake ophiolite belongs to the middle segment of the Bangong- Nujiang suture. It is a good ophiolitic accumulate section occurring in Lake Area, northern Xizang（Tibet）. The sequence of fractional crystallization of ophiolitic minerals is chromite →olivine → clinopyroxene -plagioclase. It reflects rocks are abundant in Mg and Fe early while rich in Al and Ca later. The composition of rocks of the Western Pung Co Lake ophiolite is similar to that of the representative ophiolitic accumulative rocks in the world. The distribution of trace and rare earth elements of the rocks are consistent each other, which indicates the same genesis of these rocks. Nb and Ta content of rocks are extremely depletion. REE content is very low. Distribution of REE of rocks are flat type with LREE lightly depletion. Petrology and geochemistry characteristics of the Western Pung Co Lake ophiolite indicate it is Supra-subduction zone type ophiolite and formed in a spreading center of a back are basin.