西藏谢通门县春哲地区中二叠统下拉组硅质岩地球化学特征及构造环境意义
Geochemical Characteristics and Tectonic Environmental Significance of Siliceous Rocks from Middle Permian Xiala Formation in Chunzhe Area, Xietongmen County, Tibet
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摘要: 通过详细野外地质调查和岩石化学、稀土元素、微量元素等方面研究，探讨了西藏春哲地区中二叠统下拉组（P2x）硅质岩成因及其形成地质构造环境。研究结果表明，春哲地区硅质岩的SiO2与MgO负相关性明显，主量元素中的SiO2/Al2O3-MnO/TiO2和Fe2O3/FeO-SiO2/Al2O3在双变量图解中均落入热水沉积岩区；微量元素中Ag、As、Sb含量均高出地壳平均值，Ni/Co值为0.11~2.05，Ti/V值为35.90~48.37，均显示明显的热水沉积特征。其Al/（Al+Fe）值及MnO/TiO2值较低，稀土元素的（La/Yb）N值介于1.35~1.97，（La/Ce）N值为1.08~1.15，（Ce）N=0.89~0.95。结合下拉组发育大量的滨浅海相碳酸盐岩等特征，研究表明春哲地区硅质岩形成于大陆边缘浅海构造环境，应为大陆边缘环境下形成的与火山活动有关的热水沉积硅质岩。该套岩石的发现说明冈底斯成矿带西段区域上中二叠世应发育一次火山热液喷流事件。鉴于许多学者在冈底斯成矿带东段古生界来姑组发现了大型喷流沉积型铅锌多金属矿床，该发现指示冈底斯成矿带中西段的古生界地层中存在寻找火山喷流型矿床的前景。Abstract: Based on field geological survey and main, REE and trace elements analyses, the genesis and geological and tectonic environment of the siliceous rocks from middle Permian Xiala Formation (P2x)in Chunzhe area, Tibet have been discussed in this paper.The results show that SiO2 and MgO contents of the siliceous rocks from Chunzhe area have negative correlation.In the double variable diagrams of SiO2/Al2O3-MnO/TiO2 and Fe2O3/FeO-SiO2/Al2O3, they are plotted into the area of hydrothermal sedimentary rocks.The contents of Ag, As and Sb are much higher than the average value of these elements in the crust, and the Ni/Co and Ti/V ratios vary from 0.11 to 2.05 and from 35.90 to 48.37, respectively.The Al/(Al+Fe)ratio and MnO/TiO2 are relatively low, the values of (La/Yb)N, (La/Ce)N and (Ce)N range from 1.35 to 1.97, from 1.08 to 1.15 and from 0.89 to 0.95, respectively.Combined with mass shallow marine carbonate rocks developed within Xiala formation, the conclusions can be drawn that the siliceous rocks in Chunzhe area was formed in a shallow marine tectonic environment of continental margin, which belong to the hydrothermal sedimentary siliceous rocks related to volcanic activity.The discovery of hydrothermal sedimentary siliceous rocks shows that the volcanic hydrothermal exhalative event should happen at the west Gangdese metallogenic belt in middle Permian.Large lead-zinc polymetallic exhalative deposit had been discovered at Paleozoic Laigu formation in east Gangdise metallogenic belt, which indicate a good prospect to find similar mineral resources in the west Gangdise metallogenic belt.
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