Sedimentary Environment And Provenance Analysis Of Paleogene Clastic Rocks In Jinlugou,Zedang,Southern Tibet

The Xigaze forearc basin is an important unit of the Yarlung Zangbo suture zone,and records the geodynamical evolution history of the upper plate in the subduction zone.However,compared with the North America Great Valley forearc basin,the exposed area and extent of the Xigaze forearc basin are much smaller.The forearc basin only occurs in Xigaze and its western region,while no related strata are reported in the eastern segment of the suture zone.There are two main models to explain the spatial distribution characteristics of the forearc basin:(1)In the eastern part of the suture zone,the Gangdese thrust and the Great Counter thrust are spatially adjacent,implying Xigaze forearc basins was covered by the joint activities of the two faults,resulting in the disappearance of related strata;(2)The occurrence of subduction erosion in the eastern part of the suture zone resulted in most of the material of the upper plate in the forearc region subducted below the island arc along the subduction channel,and thus no forearc basin developed in these areas.Both the above models explain why the forearc basin is not observed in the eastern part of the suture zone.According to previous study which revealed the possible occurrence of forearc sediments in the eastern section of the suture zone,I conducted the stratigraphic measurement and some related indoor analysis of clastic rock at the Jinlu,Zedang area in order to test the above two models.This measured section is southeast dipping,and the bottom of section is in fault contact with ophiolitic mélange.According to the characteristics of lithologic combination,this section is divided into two parts.The lower part is composited of conglomerate and medium-grained sandstone with graded bedding and parallel bedding,while the upper part consists of sandstone and mudstone with parallel bedding and horizontal bedding.In some parts of the measured section,A and B members of Bauma sequence were observed.The above observations indicate clastic rocks were deposited in a deep-sea turbidite sedimentary environment.Clastic components of sandstones are mainly quartz and lithic.Firstly,the lithic fragments are mainly sedimentary lithic fragments,and most of them are radiolarian siliceous rock or sandstone lithic fragments,indicating that these sedimentary lithic fragments should be recycled from strata containing radiolarian chert and sandstone.Secondly,a small amount of fresh and well-preserved volcanic rock lithic fragments was observed,suggesting volcanic rock might be an additional source.More detailed statistical results of clastic compositions show that the Jinlugou sandstones are plotted into the area of recycle orogenic belt in QFL trigonometric graph.The main zircon UPb age peak of the Jinlugou clastic sandstone is 114-120 Ma.These observations,in a combination with the lithologic characteristics of the strata in the Xigaze forearc basin,suggest the Jinlugou clastic rocks were mainly sourced from the Ngamring and Chongdui formations.All the five different youngest detrital zircon U-Pb ages of the Jinlugou sandstones are about 51 Ma,suggestive of the maximum depositional age.Therefore,the Jinlu clastic rock might be deposited in Eocene.The lithologic association,sedimentary environment,provenance characteristics and depositional age of the Jinlugou clastic rocks resemble with those of the Sangdanlin and Enba formations.Therefore,the Jinlugou clastic rocks might be deposited in the foredeep of the Himalayan foreland basin during the India-Asia collision.This collision for the eastern Yarlung Zangbo suture zone probably occurred around 51 Ma.As the major potential provenance,the Xigaze forearc basin might occur in the Zedang area.But its sequences were covered by Gangdese batholith and Tethyan Himalayan strata resulted from the activities of Gangdese thrust and Renbu-Zedang thrust.