Sedimentary Characteristics And Tectonic Significance Of Paleogene Jiachala Formation In Gyangze-Bailang Area,Southern Tibet

The closure of the Neo-Tethys Ocean and the collision between India and Eurasia are one of the most important geological events in the world since the Cretaceous.They not only record the time and process of the extinction of the Tethys Ocean and the collision of plates,but also have important effects on the uplift and erosion of the Qinghai-Tibet Plateau,biological environment and global climate.Therefore,it is an important content in the study of the Qinghai-Tibet Plateau to constrain the closing time of the Neo-Tethys Ocean.The closure of the Neo-Tethys Ocean in all parts of southern Tibet has been recorded and preserved in the form of the highest Marine layer.Among them,the Jiachala Formation is the highest Marine layer in the Gyangze-Bailang area,but the research on this formation is relatively weak and there are big differences.Therefore,on the basis of solid field data,this paper obtained relevant samples and data through field measurements of Jiachala section,Zhexia section and Padu section,and applied multi-disciplinary knowledge of sedimentary petrology,sequence stratigraphy and zircon chronology.The sedimentary environment,provenance and tectonic significance of Jiachala Formation in the study area were analyzed by means of detrital sandstone analysis and UPb age of detrital zircons.Jiachala Formation is composed of a set of gray green medium thick laminated fine sandstone,siltstone and black shale,and there are many turbidities sedimentary structures such as gully mode and channel mould.The results of sandstone composition analysis show that the Jiachala Formation sandstone is mainly lithic quartz sandstone,followed by feldspar quartz sandstone and lithic sandstone.The content of lithic sandstone is high and the lithic sandstone is mainly volcanic rock debris.The mineral particles in the sandstone are of medium sorting,subangular to subround shape,small sphericity,more heterogroups and calcareous cementation.The analysis of sedimentary facies shows that the Jiachala Formation is a subsea fan with the middle fan as the main body,and the high density turbidity current deposits are widely developed.The Dickinson triangle diagram of the detrital components shows that the Jiachala Formation has multiple provenances,belongs to the cyclic orogenic belt in the large tectonic setting,and has the property of subduction zone complex.The detrital zircon are mainly distributed in the ages of 80-125 Ma,450-600 Ma,and 910-1250 Ma.The zircon ages of the Jiachala Formation are compared with those of the adjacent strata.The results show that the provenance of the Jiachala Formation is mainly the re-cycle of the Lhasa terrane and the Gangdise arc,and the erosion of the upper Ngamring Formation and Padana Formation in the Xigatse forearc basin.The provenance was also imported from the Saga-Dênggar region in the NW.In addition,the youngest detrital zircon,at 80 Ma,only restricts the upper limit of the stratigraphic deposition,while the paleontology,such as pollen,dinoflagellates and radiolarians,restricts the stratigraphic age to the Early Paleocene-Early Eocene.In terms of the tectonic environment,through the analysis of the relative ratio of stratigraphic deposition,clastic composition,provenance characteristics and sedimentary age,it is considered that the Jiachala Formation was deposited on the northern margin of India of the Gyangze residual ocean basin,while the Saga-Dênggar area at the same time was located on the peripheral foreland basin and deposited the Sangdanlin Formation and Zheya Formation.