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

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.