昌都地块西缘甲丕拉组地层时代、物源及构造背景:来自孢粉组合、地球化学和碎屑锆石年代学的制约OA

To reveal the provenance and tectonic setting of the western margin of the Qamdo Block in the Late Triassic,the Jiapila Formation was selected as the target for sporo-pollen analysis,petrogeochemistry and detrital zircon dating.This paper presents,for the first time,the discovery of sporopollen fossils found in the Jiapila Formation,eastern Xizang.It is characterized by the combination of dominant molecules gymnosperms Classopollis,Concentrisporites,Chasmatosporite,and pteridophytes Deltoidospora,Cyathidite,which reflects that the sedimentary period was arid and hot tropical-subtropical climate.Based on the previous data and the youngest detrital zircon age of 213 Ma,the age of Jiapila Formation is preliminatively determined to be the Late Triassic Norian.The geochemical characteristics of the rocks show that the provenance of the formation is mainly from the felsic volcanic rocks in the upper crust.The high compositional variation index(ICV values range from 2.02 to 2.95)indicates that the Jiapila Formation was deposited in a relatively active tectonic environment.The multiple structural discrimination diagram shows that the source area is mainly from the active continental margin and continental island arc.The ages of the detrital zircons of the Jiapila Formation are mainly concentrated in four ranges:267-213 Ma,483-434 Ma,1 030-977 Ma,and 1 890-1 730 Ma,which are quite different from that of the Lhasa Block and Zuogong Block in the west,as well as the Yangtze Block and the Yidun Island Arc in the east.The age peak distribution of detrital zircons is consistent with that of the North Qiangtang Block,indicating that the Qamdo Block and the North Qiangtang Block have similar provenance and close affinity.Combined with the regional tectonic evolution data,it is believed that the Jiapila Formation was formed in the back-arc foreland basin resulting from arc-continent collisions.