东昆仑西段原特提斯洋洋盆闭合时间——来自新疆木孜塔格地区同碰撞花岗岩的证据

    Time constraints on the closure of the Proto-Tethys Ocean in the western part of the East Kunlun orogenic belt: Evidence from the syn-collision granites in the Muztag area, Xinjiang

    • 摘要: 通过对东昆仑造山带西段木孜塔格地区二长花岗岩的锆石U-Pb年龄和地球化学数据研究,探讨其成因及构造环境,为东昆仑造山带的构造演化提供新信息。对木孜塔格地区出露的中粗粒二长花岗岩和中细粒二长花岗岩进行了LA-ICP-MS锆石U-Pb测年,获得其206Pb/238U年龄加权平均值分别为415±4 Ma(MSWD=2.9)和419±6 Ma(MSWD=4.3),表明该花岗岩形成于早泥盆世。全岩地球化学分析结果显示,木孜塔格地区中粗粒和中细粒二长花岗岩均具有高硅、低镁和低钛特征,铝饱和指数大于1.0,主体为钙碱性弱过铝质花岗岩系列。稀土元素配分图和微量元素蛛网图显示,2类花岗岩富集轻稀土元素和Rb等元素,而亏损Nb、Ta、Yb、Y等高场强元素,结合CaO/Na2O和Al2O3/TiO2值及区域地质资料,推测木孜塔格地区花岗岩应形成于同碰撞构造环境,且岩浆源区为贫粘土沉积物的陆壳。综合造山带已有研究成果,提出东昆仑造山带原特提斯洋的闭合在时空上表现出差异性,整体呈现出纵向上北部和中部早于南部,横向上东段早于西段的特征。

       

      Abstract: This paper discusses the rock genesis and tectonic environment of the monzonitic granite from the Muztag area in the western part of the East Kunlun orogenic belt, and provides new information for the tectonic evolution of the East Kunlun orogenic belt, through the study of zircon U-Pb ages and geochemical data.LA-ICP-MS zircon U-Pb dating of the medium-coarse and medium-fine grained monzogranites from the Muztag area yield 206Pb/238U ages of 415±4 Ma(MSWD=2.9) and 419±6 Ma(MSWD=4.3), respectively.These geochronological data show that the intrusive rocks were formed in the Early Devonian.Whole-rock geochemical results suggest that the monzonitic granite samples of the Muztag area are characterized by high silicon, low titanium and magnesium contents, as well as large Aluminum Saturation Index(A/CNK) values.These are indicative of calc-alkaline weakly peraluminous granitic series.Moreover, the monzonitic granite samples are relatively enriched in light rare earth elements(LREE) and Rb element, but depleted in high field-strength elements of Nb, Ta, Yb and Y.In combination with the moderate values of CaO/Na2O, low values of Al2O3/TiO2and regional geological data, it is proposed that monzonitic granite of the Muztag area could be formed in the syn-collision tectonic environment and be derived from the continental crust with characteristics of deficient-clay sediments.By compiling previous studies of the East Kunlun belt, it is suggested that closure time of the Proto-Tethys ocean in the East Kunlun orogenic belt has significant spatio-temporal difference characterized by trending from north and central to south direction vertically and by shifting from east to west in the horizontal direction.

       

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