Sedimentation feature and its tectonic significances of Xingshikou formation in Xiabancheng basin,Yanshan fold-and-thrust belt.

Based on the component of sediment、paleocurrent of Xingshikou formation,and the zircon LA-ICP-MS datas from detrital zircons of sandy fillings、granite and granite gneiss gravel preserved in the middle segment of Xingshikou formation in the Xiabancheng basin,Yanshan fold-and-thrust belt,combined with the research of tectonics and magmatism of provenance,this paper studies dramatic transformation of the depositional environment during Early Mesozoic.The Liujiagou formation and Ermaying formation were fluvial environment in Triassic,its paleocurrent was west to east.However,the sedimentary features of Xingshikou formation indicated the environment was piedmont alluvial fan,and its paleocurrent was north-northwest to south-southeast.The lower and middle segments of Xingshikou formation recorded the process of uplift and denudation of the Mongolian paleo-uplift.Zircons from detrial zircons to gravel of granite and granite gneiss are characterized by three major groups of U-Pb ages:2025～2565Ma、1659～1888Ma and 193～691Ma. Ages of the youngest detrital zircons weighted mean were 197±6Ma.It indicates sedimentation age of the Xingshikou formation was posterior to 197±6Ma.Therefore,Xingshikou formation formed in Early Jurassic.The zircon U-Pb ages of different unit granite in rock complex of the Wangtufang,the northern Xiabancheng basin,author had obtained are 191±1 and 207±1Ma.These ages of the granite are the same as detrital zircons and gravel of granite of Xingshikou formation in error limit.It shows the contemporary magmatic body emplacement with the Wangtufang complex rock and the Mongolian paleo-uplift experienced quick uplift and denudation in the Xingshikou period.Integrated with findings of the predecessor,this paper considers the age of before～180Ma tectonic phase in the Yanshan fold-and-thrust belt was 197～180Ma.