青藏高原北羌塘盆地多彩地区蛇绿岩地球化学特征及构造环境

Geochemistry of the Duocai ophiolite in north Qiangtang Basin,Qinghai-Tibet plateau: environments for tectonics

多彩蛇绿岩是通天河蛇绿混杂岩的组成部分,区域上隶属西金乌兰—金沙江缝合带北延部分。多彩蛇绿岩各岩石组分呈构造岩块、岩片产出,出露零星,面积小,但岩石组分较齐全,主要由异剥橄榄岩、单斜辉石岩、辉长岩、玄武岩、辉绿岩及硅质岩等组成,缺乏地幔橄榄岩,蛇绿岩组分均与围岩呈构造接触关系。异剥橄榄岩以富MgO、贫TiO2、贫P2O5为特征,Eu不显异常或显弱负异常,是具有SSZ型蛇绿岩的重要岩石组分;辉长岩具高TiO2、Al2O3;玄武岩与辉绿岩具相似的特性,玄武岩Th/Ta平均为1.78,Ta/Hf>0.15,而Ta/Hf=0.15～0.2,与大洋板内玄武岩相似;硅质岩Al2O3/(Al2O3+Fe2O3)比值为0.9,(La/Ce)N值为1.81,可能具有陆源海特性。由上述特征并结合蛇绿岩形成时代及构造环境判别图解,推测多彩蛇绿岩形成于俯冲带构造环境。

Duocai ophiolite is a component part of mélange ophiolite in Tongtian River area,and regionally belongs to the northward extension part of Xijinwulan-Jinsha River suture zone.Each Duocai ophiolite occurs in the form of tectonite and rock chips exposed as small and scattered outcrops,but has fairly complete kinds of rocks,mainly composed of diallage-peridotite,pyroxenite monoclinic pyroxinite,gabbro,basalt,diabase and siliceous rocks,with no mantle peridotite.Ophiolite components assume tectonic contact with wall rocks.Diallage-peridotite is characterized by rich MgO,poor TiO2 and P2O5,with no or only weak negative Eu anomaly.Diallage-peridotite is an important rock of SSZ type.Gabbro has high TiO2 and Al2O3 content;basalt and diabase have similar characteristics.Th/Ta of basalt has an average value of 1.78,Ta/Hf0.15,and Ta/Hf=0.15-0.2,and characteristics of these elements are similar to those of oceanic intraplate basalts.Al2O3/（Al2O3＋Fe2O3） ratio of siliceous rocks is 0.9,（La/Ce）N is 1.81,showing characteristics of a land-based sea.Based on the above-mentioned characteristics in combination with the age of ophiolite as well as tectonic setting discrimination diagrams of basalt and diabase,the authors hold that Duocai ophiolite was formed in a subduction zone tectonic setting.