Age And Tectonic Implications Of Late Cretaceous Redbeds In Asa,Tibet

The early history on the formation of the Qinghai-Tibet Plateau has attracted more and more attention in recent years.Relevant studies show that in late Cretaceous,Lhasa plate had experienced large-scale crustal shortening,crustal thickening and regional uplifting.However,the relative timing,the region impacted,tectonic mechanism and other issues remain controversial.In this theses,the region of the Shiquanhe-YongzhuJiali Suture Zone was selected as the study area,and the depositing age and sedimentary environment of the Late Cretaceous Mamole Formation and the Jingzhushan Formation at Shangale were studied.Detrital zircon dating was applied to samples of redbeds from Zhongcan.Combining with regional geological data,Late Cretaceous tectonics of the Lhasa Plate is discussed in this theses.The characters of fan delta deposition of the strata showed in Jingzhushan Formation at Shangale in Asa and fossils of marine animals Hydra and Rudists discovered in the lower part of the strata,indicate that the strata deposited in the terissal-offshore transitional environment.The youngest single grain age(YSG)of 89±5Ma was obtained by detrital zircon LA-ICP-MS U-Pb dating from a Jingzhushan Formation sandstone sample.A dioritic vein intruding into the northeast of the strata gives a zircon U-Pb age of 88 Ma.Jinzhushan Formation in Shangale depsited at ca.90 Ma.The Mamole Formation deposited in braided river-delta environment.The YSG obtained is 110 ±3Ma,and the age of the dorite instrusive by zircon U-Pb dating is 92±3Ma.Combining with the stratigraphic contact relationship and published data,the deposition time is constrained in 99-92 Ma.The YSG obtained from a detrital zircon sample in the Zhongcang area is 113±4Ma.The detrital zircon ages composition and trace elemental characteristics of detrital zircon show that the provenance of the Jingzhushan Formation has the characteristics of island arc of convergence environment,while the Mamole group shows the relatively more complex provenance tectonic.Both starta' detrital zircon age composition shows a Qiangtang sourced feature.Combined with regional Late Cretaceous stratas' dating results,provenance's characteristics showed and regional tectonics and thermal chronology study,we proposed that Jingzhushan Formation and Mamole Formation are the sedimentary response of the Lhasa-Southen Qiangtang collision,and they jointly record the crustal uplifting process of Lhasa plate in Late Cretaceous.