Sedimentary Characteristics,provenance Analysis And Tectonic Setting Of The Yangping Formation,Wudang Group In Northwestern Hubei

The geological age,material source,and tectonic settings of the Yangping Formation,dominated by sedimentary rocks,are still controversial.This study took regional stratigraphic characteristics as the pointcut,and used the method of petrology,petrography and zircon U-Pb chronology.The formation age,source of detrital material and diagenetic tectonic settings of the Yangping Formation were discussed.The main understandings are as follows:The Yangping Formation is at the middle of the Wudang Group.It is subdivided into three members according to lithology and sedimentary features.The first member is mainly thick bedded coarse-medium feldspar-quartz sandstone and graywacke.The second member consists of medium-thin bedded medium-fine feldspar-quartz sandstone and meta-quartz sandstone.The third member contains medium-thin bedded argillaceous siltstone,thin bedded carbonaceous mudstone and few meta-volcanic tuff.Such lithology above presented littoral facies and shallow sea shelf facies.The age spectrum of detrital zircons in the Yangping Formation is concentrated in:1.8?2.0Ga,0.78?1.OGa,as well as 2.4?2.9Ga.The top of the Yangping Formation is mainly concentrated in 0.72?0.75Ga,as a result the depositional age of the Yangping Formation is younger than 720Ma.The geological age of Yangping Formation was the early Middle Nanhunan according to the stratigraphic chart of China(2014).The detrital zircons distribution in Yangping Formaiton is consistent with that in Yangtze Block,indicating the provenance of the Yangping Formation is the Yangtze Block.In this case,the Wudang area is a part of Yangtze Block during Neoproterozoic.The whole rock geochemical data of La/Sc,Sc/Th,Cr/Th,Zr/Sc in the Yangping Formation are all in the felsic rock source area.In the F1-F2 diagram,most of the data are in the area of felsic igneous.The Yangping Formation,characterized by initial deposition,lacking redeposition and rapid accumulaiton,is mainly felsic volcanic rocks of the upper continental crust.The(Fe2O3+MgO)-TiO2 diagram suggests most data drop into and near the passive continental margin.The La/Sc-Ti/Zr diagram and La/Y-Sc/Cr diagram show the distribution is in the passive continental margin as well as in and near active continental margin.Along with petrology and zircon chronological characteristics,it is inferred that the Yangping Formation was deposited under the continental marginal tectonic background,and the detrital material mainly came from acid magmatic rocks of the continent.This study argues that under the background of back-arc basin expansion,the Yangping Formaiton recorded the process of expanding and deepening of the sedimentary basin.The Wudang area was in the active continental margin of the northwest of Rodinia supper continent during Neoproterozoic.