Depositional Age and Tectonic Settings of Neoproterozoic Qianzimen Formation in Southeast Chongqing:U-Pb Dating and Geochemistry Constraint

There are two major glacial events in Neoproterozoic globally, respectively the Sturtian glacial stage and Marinoan glacial stage.Corresponding glacialde posits develop in Xiushan area of Southeast Chongqing, respectively Nantuo formation and Qianzimen formation. ZirconLA-ICP-MS dating of Qianzimen sandstone obtains ²⁰⁶Pb/²³⁸ U age of 736—2 726 Ma, in which the data of 28 measuring points concentrate between 736 and 890 Ma, with anaver age of 834 Ma. Minimum value of 736 Ma defines the sedimentary starting time of Qianzimen formation, which can be compared with Jiangkou glacial stage of southeastern Hunan started in 734±4 Ma, almost unanimously in the same error range.The age of 834 Ma records magmatism related to Sipu-Jinning orogeny movement in provenance.ICV numerical values of all samples being 0.85—1.74(1.23 on average) show that clay minerals are less and Qianzimen formation is less affected by the redeposition. Meanwhile, CIA numerical values of the samples being 43—69(57 on average)indicatethe sedimentation environment of Qianzimen formation is cold, dry and low degree of chemical weathering.SiO₂-K₂O/Na₂O and K₂O/Na₂O-SiO₂/Al₂O₃ discrimination diagrams and REE patternscurve indicate that Qianzimen formation is created in the active continental margin environment.