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    西藏阿索蛇绿混杂岩中硅质岩地球化学特征:对狮泉河−纳木错蛇绿混杂岩带构造属性的启示

    Tectonic attribute of the Shiquanhe-Namco ophiolitic belt: Insights from geochemical characteristics of siliceous rocks in the Asa mélange zone, central Tibet

    • 摘要: 班公湖–怒江缝合带和狮泉河–纳木错蛇绿岩带中与蛇绿岩伴生的中生代硅质岩出露广泛,对于探讨中特提斯洋中生代大洋演化和狮泉河–纳木错蛇绿岩带构造属性都具有重要意义。本文对狮泉河–纳木错蛇绿混杂岩带中段阿索蛇绿混杂岩内的硅质岩岩块的野外地质特征、岩石学特征及全岩主、微量元素地球化学特征开展了详细研究,讨论了硅质岩的岩石成因及沉积环境。阿索蛇绿混杂岩中,硅质岩有紫红色硅质岩和黑色硅质岩两种类型,以岩块形式出露于混杂岩带中。紫红色硅质岩SiO2含量低于纯硅质岩,相对较为富Al,稀土元素北美页岩标准化后表现出微弱的Ce负异常,属于正常沉积成因的硅质岩,成岩过程中有陆源物质加入。黑色硅质岩富集Fe、Mn,北美页岩标准化后表现出强烈的Ce负异常,属于热液成因,其成岩过程中有基性火山碎屑物质加入。两类硅质岩均沉积在靠近大陆边缘的洋盆环境中。班公湖–怒江缝合带和狮泉河–纳木错蛇绿岩中的硅质岩地球化学数据分析显示,狮泉河–纳木错蛇绿岩带中,硅质岩主要沉积于靠近大陆边缘的有限洋盆环境中,而班公湖–怒江缝合带中的硅质岩既有大陆边缘环境下产出的硅质岩,也包含了远洋盆地环境中的硅质岩。因此,两者之间硅质岩沉积环境的差异可能暗示了狮泉河–纳木错蛇绿混杂岩带并非班公湖–怒江缝合带的构造推覆体,而是一条代表了有限洋盆遗迹的原位蛇绿混杂岩带。

       

      Abstract: Mesozoic siliceous outcrops with ophiolite were found in Shiquanhe-Namco Ophiolitic Mélange Zone (SNMZ) and Bangong-Nujiang suture zone (BNSZ), which can reveal the sedimentary environment Bangong-Nujiang Ocean and relationship between BNSZ and SNMZ. In this paper, research on the siliceous rocks from the Asa area, located in the central segment of SNMZ, is reported, including field work, petrology, as well as major- and trace-element analysis. Moreover, the genesis and sedimentary environments of these siliceous rocks have also been analyzed. The Asa siliceous rocks consist of two types: red siliceous and black siliceous, which are exposed as rock blocks in the mélange belt. The SiO2 content in red siliceous rocks is lower than that of pure siliceous rocks, while the red siliceous rocks are relatively enriched in Al. The rare earth element patterns of the red siliceous rocks are similar with flat model with a weak negative Ce anomaly after North American Shale standardization (NASS). The red siliceous rocks belong to the siliceous rocks of normal sedimentary genesis. So, the red siliceous rocks were formed in a continental margin basin environment with the addition of terrigenous materials in the source area. The black siliceous rocks are enriched in Fe and Mn, with a strong negative Ce anomaly after North American Shale standardization (NASS). The black siliceous rocks are of hydrothermal origin, influenced by basic pyroclastic materials and formed by hydrothermal activity near the continental margin. The geochemical characteristics of the siliceous rocks indicate that siliceous rocks in Asa ophiolitic mélange were formed in a limited oceanic basin environment close to the continental margin, which is consistent with siliceous rocks from other ophiolites in SNMZ. Unlike SNMZ, BNSZ not only contains siliceous rocks formed in the continental margin environment but also preserves siliceous rocks deposited in the pelagic environment. Therefore, the difference in the siliceous depositional environments between BNSZ and SNMZ suggests that SNMZ does not represent a tectonic nappe of BNSZ but is an in-situ ophiolitic mélange belt originated from a limited oceanic basin.

       

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