古特提斯多岛洋洋-陆俯冲:木孜塔格蛇绿岩的矿物学证据

前期研究认识到新疆东昆仑木孜塔格蛇绿岩形成在俯冲带环境.为了进一步研究该俯冲带的类型,对新疆东昆仑木孜塔格蛇绿岩变质橄榄岩的岩石学和矿物学特征进行了分析.薄片观察发现变质橄榄岩的橄榄石以变质残余、变质重结晶和斜方辉石熔融结晶3种结构存在,但探针分析发现它们具有稳定且低Fo值(87.8～89.5);斜方辉石发育变质残余和熔融残余结构,En较低(88～90),Al2O3含量变化大(2.90%～5.13%);尖晶石为他形-半自形结构,其Cr#(=Cr/(Cr Al))集中分布在0.508~0.723和0.100~0.118两个范围内.根据这些来自岩石学和矿物学的证据,并结合该蛇绿岩的构造背景与时代,认识到该蛇绿岩形成在大洋向具有厚陆壳的大陆弧俯冲的俯冲带环境,为新疆东昆仑地区古特提斯多岛洋洋-陆俯冲的结果.

Ocean-Continent Subduction within the Paleotethyan Archiopeligic Ocean from Muztag Ophiolite

The former studies show that the Muztag ophiolite outcropped in the East Kunlun area of Xinjiang Uygur Autonomous Region formed in the supra-subduction zone environment. The petrological and mineralogical characteristics of metamorphic peridotites of this ophiolite are analyzed in order to further study the type of the subducuion zone. Thin-section observations show that the olivines of metamorphic peridotites exist in three textures: metamorphic relict, metamorphic recrystallizition and orthopyroxene-melting crystallizations. Microprobe analyses show that they have stable and low Fo range of 87.8-89.5. The orthopyroxenes show metamorphic relict and melting relict textures, with low En of 88-90 and a wide range of Al2O3 content of 2.90%-5.13%. The spinels develop xenomorphic-semi-automorphic textures, with Cr#(=Cr/(Cr Al)) mainly distributing in two ranges of 0.508-0.723 and 0.100-0.118, respectively. Based on the petrological and mineralogical evidences together with the era and tectonic setting of the Muztag ophiolite, it can be concluded that the ophiolite formed in the supra-subduction zone where the oceanic crust subducted down to the continental arc with thick continental crust, and resulted from ocean-to-continent subduction within the Paleotethyan archiopelagic ocean in the East Kunlun area of Xinjiang Uygur Autonomous Region.