藏北果芒错蛇绿岩中堆晶岩地球化学特征及其地质意义
Geochemical Characteristics of Cumulates in the Guomangcuo Ophiolites， Northern Tibet and Its Geological Significance

摘要： 西藏果芒错蛇绿岩位于狮泉河—纳木错—波密蛇绿混杂岩带的中段，主要由地幔橄榄岩、铁镁质-超铁镁质堆晶杂岩、枕状熔岩、基性岩墙和硅质岩等单元组成。对果芒错蛇绿混杂岩中超铁镁质堆晶岩、基性岩岩石学及地球化学特征的研究表明，果芒错超铁镁质堆晶杂岩具很好的堆晶韵律，岩石化学特征反映了随着MgO含量的降低其从底到顶的连续分离结晶过程；堆晶杂岩稀土元素总体上具LREE富集、Eu轻微异常特点，微量元素具有富集高场强元素、亏损大离子亲石元素的特征，且相容元素显示良好的变化规律，指示岩石来自受富集组分混染的地幔源区，并在后期经历了俯冲流体作用。果芒错基性岩具有富集型洋中脊玄武岩(EMORB)叠加岛弧型玄武岩(IAB)的地球化学特征，并受到后期俯冲流体的影响。根据果芒错蛇绿岩地球化学属性及其构造环境判别图解，推断该地区蛇绿岩生成于扩张的弧后盆地构造环境。作为狮泉河—纳木错—波密蛇绿岩带的一部分，该区蛇绿岩以班公湖—怒江特提斯洋弧后盆地的形式存在。
Abstract: The Guomangcuo ophiolite in the central ShiquanheNamcoBomi ophiolitic mélange belt is composed of mantle peridotite,ultramaficmafic cumulates,mafic lavas, dikes and siliceous rocks. Petrographic studies of the ultramafic cumulates show that the ultramafic cumulates has a well crystallization order. Trace elements analyses indicate that the ultramafic cumulates are enriched in HFSE,depleted in LILE,and that the compatible elements have a good change rule. The rare earth elements analyses also show a enriched LREE pattern with slight Eu anomaly,displaying that the rocks are formed from a enriched mantle source metasomatized by fluid in the slab subduction.Guomangcuo mafic rocks have both EMORB and IAB affinity and are effected by the subduction fluids.We suggest that guomangcuo ophiolite likely developed in a backarc basin,as a portion of ShiquanheNamcoBomi ophiolitic mélange belt which is the BangonghuNujiang Tethyan ocean backarc basin