北羌塘三叠系康鲁组沉积岩地球化学特征及其物源区和构造背景分析

Geochemical Characteristics of Sedimentary Rocks from the Triassic Kanglu Formation,North Qiangtang Basin(Tibet) :Implications for Provenance and Tectonic Setting

沉积盆地中碎屑沉积岩的地球化学特征主要受物源区的制约。对北羌塘盆地三叠系康鲁组碎屑沉积岩进行全岩测试,分析表明,样品的成分变异指数(ICV)和化学蚀变指数(CIA)变化不大,反映其源区物质较新鲜,成熟度相对较低,化学风化作用较弱—中等。岩石Si O2含量普遍较高,TFe2O3+Mg O含量相对较高,但K2O/Na2O比值变化较大。微量元素标准化蛛网图与后太古代页岩和上地壳相似,且REE配分模式图显示富集LREE,δEu明显负异常以及δCe呈弱负异常。此外,Th/U、La/Sc、Th/Sc、Ba/Sr等微量元素比值以及Ti O2-Ni、La/Th-Hf、Co/Th-La/Sc和Th/ScLa/Sc判别图表明沉积物物源具有浅源、长英质的特征。沉积岩的Si O2-K2O/Na2O、La-Th、La/Y-Sc/Cr双变量图解和La-Th-Sc、Th-Sc-Zr/10、Th-Co-Zr/10三变量图解,以及Th/U、Rb/Sr等比值特征表明北羌塘三叠系康鲁组沉积砂岩的物源区构造背景以大陆岛弧为主,兼有大洋岛弧和被动大陆边缘构造背景特征。

The geochemical characteristics of clastic sedimentary rocks in sedimentary basin are mainly con- trolled by the origins of the rocks. In this paper, we report on the whole-rock major and trace element composi- tions of clastic sedimentary rocks in North Qiangtang Basin. The results show that the index of compositional variability and the chemical index of alteration for different samples are similar, revealing a relatively fresh, weakly matured origin with weak-medium chemical weathering. The rocks arc characterized by high SiO2 con- tents, and relatively high TFe203 ＋ MgO contents, but KEO/Na20 ratios vary largely for different samples. The rocks have the similar trace and REE patterns with the post-Arehean Australia shales and upper continental crust, characterized by enrichment in LREE, the ratios of Th/U, La/Sc, Th/Sc, Ba/Sr of evident Eu depletions, and weak Ce depletions. Furthermore, Kanglu Formation and the binary diagrams TiO2 vs. Ni, La/Th vs. Hf, Co/Th vs. La/Sc and Th/Sc vs. La/Sc denote that the origin of the sediments is characterized by superfi-cial and felsic. The tectonic setting discrimination diagrams, such as of SiO2 vs. K20/Na20, 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 ratios of Th/U, Rb/Sr sug- gest that the source material of and partially belongs to passive Kanglu Formation in North Qiangtang mainly belongs to continental arc setting, continental margin and oceanic island arc setting.