Dating Of The Key Formations, Sedimentary Geological Characteristics And The Tectonic Significance Of Rift Filling Sequence In Kangdian Rift

Kangdian Rift,developed in the southwest of the Yangtze Block,is an important part of Neoproterozoic rift system in South China and even in the whole world.In order to optimize the chronological stratigraphic correlation framework of the filling sequence of the Kangdian Rift,remodel the structural paleogeographic pattern,and reveal the temporal and spatial development of the rift,Research has carried out a comprehensive study of the Kangdian Rift by using the basic theories and methods of isotopic chronology,stratigraphy,sedimentary geology and petrochemistry through field geological survey,indoor sample testing and comprehensive analysis.It is a comprehensive study of many disciplines and dimensions,such as generation stratigraphy,sedimentary geology,basin structure,etc.The following major progress has been made:1.The LA-ICP-MS zircon U-Pb age of dacite in the Suxiong Formation is 802.0 ± 7.0 Ma at the boundary between the Suxiong and Kaijianqiao formations in Ganluo area,Western Sichuan,which indicates that both Suxiong formation and Chengjiang formations belong to the upper part of Qingbaikou.2.The Kaijianqiao Formation in Dechang area is divided into two parts: the lower part which is dominated by terrigenous clastic rock series and the upper part which is dominated by volcanic rock series.The zircon U-Pb age of LA-ICP-MS in the bottom tuff of the upper member is 778.7 ± 6.2Ma.It is confirmed that the upper member of the Kaijianqiao Formation in Dechang area belongs to the Nanhuan System,while the lower member belongs to the Qingbaikouan System.This zircon U-Pb age provides an important basis for cross regional stratigraphic correlation of rift filling system.3.The study on the sedimentary facies of the key intervals of the Kaijianqiao Formation in Dechang basin and the Chengjiang Formation in Central Yunnan basin shows that the braided river deposition is the main part of the basin filling system.The results of field measurement and rose diagram of large oblique bedding in these braided river sedimentary rock series confirm that the main body of paleocurrent(sediment diffusion direction)points to the East,indicating that the provenance area is in the west of the rift,further demonstrating the cognition that the main body of rift is in the northsouth direction.4.The statistical results of grain composition of sandstone skeleton show that most of the sample points of the Kaijianqiao Formation in Dechang area are located in the stable craton or stable basement area,and a few in the recycle orogenic area,indicating that the source of strata of the Kaijianqiao Formation is mainly from the stable craton or ancient basement,not from the Andes magmatic arc.On this basis,combined with the geochemical characteristics of magmatic rocks of the same period in the area,the plate tectonic environment of the Kangdian Rift will be further discussed.5.Based on the above research results and previous data,this paper demonstrates the structural division plan of the Kangdian Rift: from north to south,it is divided into three extensional basins: Suxiong basin,Dechang basin and Central Yunnan basin,and then discusses and summarizes the temporal and spatial development characteristics and tectonic paleogeographic evolution stage of the Kangdian rift.