Composition and geochemistry of Permian clastic rocks in the northern margin of the Middle and Upper Yangtze region

The detrital modes and geochemical characteristics of Permian clastic rocks in the northern margin of the Middle and Upper Yangtze region are analyzed.The results show that these rocks are dominantly products of upper continental crust and have low degree of sediment recycling.Most samples were derived from basic and felsic volcanic rocks.However,major elements geochemistry is less useful for interpreting the tectonic setting,because of weathering and post-depositional process,than immobile trace elements such as Cr,Co,Th,Sc,La and Zr.The values of La/Sc,Th/Sc,Th/Co and Th/Cr of the clastic rocks show affinity closer to the upper continental crust and siliceous sandstone than the lower continental crust,oceanic crust and basic sandstone.Based on the composition and geochemical characteristics of major elements,trace elements and REE,it can be summarized that the tectonic settings of Xikou,Zhen’an county was mainly continental island arc-active continental margin which is similar to that of Shangdan Suture Zone,and the similarity of clastic rocks between Permian and Devonian may reflect the feature of paleo-continental margin of the North Qinling.The samples of Huaying Mountain were derived from multiple provenances of intraplate tectonic settings.