Geochemical Characteristics And Tectonic Setting Constraints Of Sedimentary Rocks From Kunlun River Area,Qinghai

Lying in the northern margin of the Tibitan Plateau,the East Kunlun Orogenic Belt(EKOB)is one of the most important metallogenic belts,which experienced two complete orogenic circles.The first orogenic circle was related to the evolution of Proto-Tethys Ocean,while the evolution of the Paleo-Tethys Ocean influtened the other orogenic circle.Therefore,there are two purposes can be realized by the characteristics of petrography and geochemistry of sedimentary rocks in different orogenic stages:(1)realizing the features and verifying the transition time of the different tectonic evolutionary stages;(2)understanding the compound orogeny of the East Kunlun Orogenic Belt.The stratum researched in the article include the Neo-proterozoic Wanbaogou Group(Pt₃W),the Lower Cambrian Shasongwula Formation((?)₁s)and the Middle-Upper Ordovician Nachitai Group(O_2-3N),the Upper Carboniferous-Lower Permian Haoteluowa Formation(C₂P₁h),the Lower Triassic Hongshuichuan Formation(T₁h),the Middle Triassic Xilikete Formation(T₂x)and the Upper Triassic Babaoshan Formation(T₃b).The lithology includes cherts and clastic rocks.It is showed by Al/(Al+Fe+Mn)ratio and?Ce value that the Wanbaogou Group and Shasongwula Formation claret-colored cherts are submarine hydrothermal genesis with little clastic rocks mixed while the Shasongwula Formation offwhite cherts are related to plentiful terrigenous clastic material.At the same time,the characteristics of Mn O/Ti O₂,U/Th,V/Cr,V/(V+Ni),Fe₂O₃^T/Ti O₂,La _N/Ce _N,Al₂O₃/(Al₂O₃+Fe₂O₃^T)ratios showed that the Wanbaogou group claret-colored cherts developed in oxygen-deficient condition near mid-ocean ridge,the Shasongwula Formation claret-colored cherts developted in oxygen-poor condition in pelagic basin,and the offwhite cherts developed in oxidation environment related to continental shelf setting.In addition,the CIA values of the sandstones researched suggest that the source rocks of the Shasongwula Formation sandstones underwent significant chemical weathering,while the other formations were opposite.The geochemical signature suggests that the sandstones in the study area were primarily sourced by felsic rocks with minor intermediate igneous rocks.Moreover,through systematic comparison of the La,Ce,Th,U,?REE contents and the La/Sc,Th/Sc,Sc/Cr,La/Y ratios,it is suggested that the cherts from the Wanbaogou Group developted in a passive continental margin setting related to the rifting between the East Kunlun and Qaidam terranes.The sandstones from the Shasongwula Formation and the Nachitai Group were deposited in an active continental margin setting produced by the subduction of the Proto-Tethyan oceanic crust,whereas the sandstones from the Haoteluowa Formation were deposited in a passive continental margin setting related to the rifting between the East Kunlun and Bayanhar terranes.The detritus of sandstones from the Hongshuichuan Formation were mainly derived from continental margin arc volcanic rocks that were produced by the subduction of the Paleo-Tethyan oceanic crust.Due to the both active and passive continental margin-related geochemical affinity,the sandstones from the Xilikete Formation are considered to be deposited during the initiation of the continental collision.The sandstones from the Babaoshan Formation were deposited in an active continental margin setting caused by the complete continental collision and post-collision.