ISSN 1009-6248CN 61-1149/P 双月刊

主管单位:中国地质调查局

主办单位:中国地质调查局西安地质调查中心
中国地质学会

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    北羌塘盆地三叠系康南组砂岩地球化学特征及其对物源区和构造背景的制约

    Geochemical Characteristics of the Sandstones from Triassic Kangnan Formation in North Qiangtang Basin (Tibet): Implications for Provenance and Tectonic Setting

    • 摘要: 沉积盆地中碎屑砂岩的地球化学特征主要受物源区的制约,分析碎屑砂岩的化学成分可以揭示盆地砂岩的物源属性和构造背景。笔者对北羌塘盆地三叠系康南组碎屑砂岩进行全岩分析。样品具有高SiO2含量,K2O/Na2O值变化较大,TFe2O3+MgO含量中等。稀土元素配分模式图显示富集轻稀土元素,(La/Yb)N=3.96~9.25,平均为6.32,δEu明显负异常;δCe呈弱负异常。碱土金属元素和化学蚀变指数(CIA)以及成分变异指数(IPV)说明其源区物质较新鲜,成熟度相对较低,化学风化作用较弱-中等。此外,微量元素Th/U、La/Sc、Th/Sc等值以及TiO2-Ni、La/Th-Hf、Co/Th-La/Sc和Th/Sc-La/Sc源岩判别图解反映砂岩的物源具有长英质特征。砂岩的SiO2-K2O/Na2O、La-Th、La/Y-Sc/Cr双变量图解,La-Th-Sc、Th-Sc-Zr/10、Th-Co-Zr/10三变量图解以及Th/U、Rb/Sr等值表明北羌塘三叠系康南组碎屑砂岩的物源区构造背景以大陆岛弧为主,兼有被动大陆边缘构造背景特征。

       

      Abstract: The geochemical composition of sandstones in sedimentary basin has been mainly controlled by the tectonic setting of the provenance. Therefore, it is possible to reveal the tectonic setting of the provenance and the nature of source rocks in terms of the geochemical composition of sandstones. The major and trace element compositions of clastic sandstone rocks in North Qiangtang Basin have been analyzed in this paper. These rocks are characterized by high SiO2 contents and moderate TFe2O3+MgO contents, but the K2O/Na2O ratios vary largely for different samples. Their REE patterns are uniform and similar to the ones of typical post-Archean shales, with LREE enrichment and flat HREE. Their (La/Yb)N ratios vary from 3.96 to 9.25, with an average of 6.32. These rocks have evident Eu depletions and weak Ce depletions. The relationship among alkaline earth elements, Chemical Index of Alteration (CIA) and Index of Compositional Varibility (IPV) indicate that, the source area of these rocks was affected by a moderate weathering history. In addition, the Th/U, La/Sc, Th/Sc and Ba/Sr ratios of Kangnan Formation and the binary diagrams (such as TiO2 vs. Ni, La/Th vs. Hf, Co/Th vs. La/Sc and Th/Sc vs. La/Sc) denote that the origin of these sediments is characterized by superficial and felsic. The tectonic setting discrimination diagrams (such as SiO2vs. K2O/Na2O, La vs. Th, La/Y vs. Sc/Cr and La vs. Th vs. Sc, Th vs. Sc vs. Zr/10, Th vs. Co vs. Zr/10) and the Th/U, Rb/Sr ratios reveal that the tectonic settings of the provenance for Mesozoic sedimentary rocks in Qiangtang Basin belong to a passive continental margin and a continental island arc.

       

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