西藏冈底斯带石炭纪—二叠纪岛弧造山作用:火山岩及地球化学证据

    Carboniferous-Permian island arc orogenesis in the Gangdise belt, Tibet: evidence from volcanic rocks and geochemistry

    • 摘要: 系统研究了西藏冈底斯带石炭纪—二叠纪火山岩的时空分布、岩相学、元素及Sr、Nd、Pb 同位素地球化学和构造环境、源区性质,并与喜马拉雅带二叠纪火山岩进行了对比研究。冈底斯带石炭纪—二叠纪火山岩近东西向集中分布在冈底斯构造带中北部地带,空间上从东至西火山活动的强度和规模渐次减小,时间上从早至晚火山活动的强度和规模总体由弱到强。冈底斯带石炭纪—二叠纪火山岩形成于活动大陆边缘的岛弧构造环境,从早到晚岛弧造山作用经历了初始岛弧→早期岛弧→成熟岛弧的发展演变过程,火山岩浆来源于富集型地幔部分熔融作用,原始岩浆在形成和演化的过程中有俯冲洋壳及随带的深海沉积物和再循环进人地慢的地壳物质组分的强烈混染,明显不同于受地壳物质组分强烈混染的喜马拉雅带二叠纪陆缘裂陷型火山岩。综合研究冈底斯带及其邻区近年来的最新调查与研究成果,从北向南拟建了石炭纪—二叠纪冈底斯岛弧→雅鲁藏布江弧后裂谷盆地→喜马拉雅陆缘裂陷盆地的弧盆系时空结构演化模式,探讨了冈瓦纳大陆北缘石炭纪—二叠纪活动大陆边缘的岛弧造山作用与青藏高原古特提斯演化的耦合关系及其动力学机制,讨论了冈底斯带松多乡榴辉岩的形成过程。

       

      Abstract: The authors have not only systemically studied the spatio-temporal distribution, petrography, element and Sr-Nd-Pb isotope geochemistry, tectonic environments and characteristics of source regions of Carboniferous-Permian volcanic rocks in the Gangdise belt, Tibet, but also made a comparative study of them and the Permian volcanic rocks in the Himalaya belt. The Carboniferous-Permian volcanic rocks are mainly distributed in a nearly east-west direction in the north-central segment of the Gangdise belt. In terms of space, the intensity and magnitude of volcanism decreased gradually from east to west, and in terms of time they increased from the Early Carboniferous to Late Permian. The Carboniferous-Permian volcanic rocks of the Gangdise belt formed in an active continental-margin island-arc tectonic environment and the island arc orogenesis went through the evolution process of the primitive island arc, early-stage island arc and mature island arc from the Early Carboniferous to Late Permian. The original magma of the Carboniferous-Permian arc volcanic rocks in the Gangdise belt was derived from partial melting of the enriched mantle (EM) and was strongly contaminated by subducing oceanic crust, attendant deep-sea sediments and crustal substance into the mantle due to recycling during its formation and evolution, which is markedly distinct from the Permian continental-margin rift-type volcanic rocks in the Himalaya belt contaminated strongly by crustal substance. By integrating the latest information and research results of the Gangdise belt and adjacent areas, the authors have constructed an evolutionary model of the Carboniferous-Permian arc-basin system, consisting from north to south of the Gangdise island arc. Yarlung Zangbo back-arc rift basin and Himalayas continental-margin rift basin, and discuss the coupling relation of the Carboniferous-Permian active continental-margin island arc orogenesis along the northern margin of Gondwanaland with the evolution of the Paleo-Tethys of the Qinghai-Tibet Plateau and its dynamic mechanism. They also discuss the formation process of the Sumdo eclogite in the Gangdise belt.

       

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