东昆仑山木孜塔格蛇绿混杂岩中火山岩的地球化学特征及构造环境

Geochemical characteristics and tectonic setting of volcanic rocks in the Muztag ophiolitic mélange, East Kunlun Mountains, Xinjiang, China

木孜塔格蛇绿混杂岩沿东西走向的木孜塔格-鲸鱼湖断裂断续出露有变质橄榄岩、堆晶岩、火山岩及其上覆的硅质岩,彼此呈构造接触关系。其中,火山岩由玄武岩、玄武安山岩和安山岩构成。野外观察曾认为火山岩属于蛇绿岩的一部分,但火山岩的微量和稀土元素分析表明,其富集LREE、Cs、Ba、Th、Pb贫Nb,Ba/Nb、Ce/Pb、Cs/Rb、Ba/La和Nb/Ta分别为18.59～102.82、1.42～8.13、0.10～0.49、11.76～51.79和6.22～13.58,TiO2含量1.64%～2.09%。这些特征表明该火山岩为形成于较成熟岛弧环境的火山岩而非蛇绿岩成员,也指示木孜塔格地区曾经历了与今西太平洋边缘近似的]化过程,可能形成了较成熟的沟-弧-盆体系。

Metaperidotites, cumulates, volcanic rocks and their overlying cherts in the Muztag ophiolitic mélange are exposed discontinuously along the E-W-trending Muztag-Jingyuhu fault. These rocks show tectonic contact relationships. Of these, volcanic rocks consist of basalt, basaltic andesite and andesite. Field observations show that the volcanic rocks are a part of the ophiolites. However, the trace element and REE analyses of volcanic rocks indicate that they are enriched in LREE, Cs, Ba, Th and Pb and depleted in Nb, with Ba/Nb, Ce/Pb, Cs/R.b, Ba/La and Nb/Ta ratios of 18.59-102.82, 1.42-8.13, 0.10-0.49, 11.76-51.79 and 6.22-13.58 respectively and TiO2=1.64-2.09%. These features suggest that the volcanic rocks are those that formed in a relatively mature island-arc environment rather than a member of the ophiolites and also indicate that the Muztag area underwent an evolutionary process that resembles that on the margins of the Western Pacific Ocean, in which a relatively mature trench-arc-basin regime might form.