GEOCHEMISTRY AND SOURCE AREA ENVIRONMENT OF THE MIDDLE JURASSIC SANDSTONE IN SOUTHERN QIANGTANG BASIN

The sandstone of Middle Jurassic Quemocuo Formation is developed in Jiangyumaluo area of southern Qiangtang Basin. The environment of sandstone source area is restored through petrogeochemical analysis. The chemical index of weathering (CIW), chemical index of alteration (CIA) parameters and A-CN-K diagram of sandstones show that the clastic compositions are intensely weathered. The index of compositional variability(ICV) and Th/Sc-Zr/Sc diagram indicate that the sandstone contains mainly the first sedimentary cycle materials, with a little recycled sediment. The Al₂O₃/TiO₂, Th/Sc and Cr/Zr ratios of rock elements and La/Th-Hf discrimination diagram of source rocks suggest that the clastics of sandstone are mainly from the felsic source area of upper crust, mixed with a small number of basic femic rocks. The characteristics of trace elements and SiO₂-K₂O/Na₂O, La-Th-Sc, Co-Th-Zr/10, Sc-Th-Zr/10 diagrams all indicate that the sandstones were formed in the continental margin rifting tectonic setting.