Sedimentary Records And Significance Of The Late Cretaceous Linzhou Basin In Lhasa,Tibet

As the roof of the world,the Qinghai Tibet Plateau is rich in mineral resources and cultural resources.Since the beginning of the study of the Qinghai Tibet Plateau,it has been a hot spot of geological research in China and even in the world.The Linzhou basin is located in the eastern part of the Gangdise block of the Qinghai Tibet Plateau,adjacent to the Nyainqentanglha Range in the north,the Gangdise block in the South and the upper reaches of the Lhasa River.It is under the jurisdiction of Mozhugongka county and Linzhou County,Lhasa City,Tibet Autonomous Region.The terrain here is high in the East and low in the west,with an average elevation of about 4500 meters.In this paper,the Shexing Formation of Linzhou basin is selected as the main starting point to discuss the provenance stage and basin formation process of the Linzhou basin.The main strata in Linzhou basin include the Linbuzong Formation(J₃K₁l),Chumulong Formation(K₁c),Ttakna Formation(K₁t),Shexing Formation(K₂s)and Linzizong Volcanic Rock Group.The Shexing Formation(K₂s)is mainly distributed in the east of Nyainqentanglha range emerging near Linzhou basin and the Maxiang village and and part in the west of Nyainqentanglha range.Its bottom is in integrated contact with the Takna Formation(K₁t),and its top is in unintended contact with the Linzizong volcanic rock group.The analysis of zircon chronology and geochemistry shows that the youngest detrital zircon U-Pb sample in the bottom of the Shexing Formation in Zongxue Village,Songpan Township,Linzhou Basin is?108Ma,with a weighted average age of 114.5±3.0Ma(MSWD=4.9),which basically represents the lower sedimentary limit of the Shexing Formation near Zongxue Village in Songpan Township,Linzhou County.The age is Early Cretaceous.Combined with previous researches,it shows that the deposition of the Shexing Formation has been deposited until about 80Ma.The ICV values of the samples in the Shexing group collected are 0.74?2.20,with an average value of 1.38 and greater than 1.There are few clay minerals in the sediments of the Shexing Formation in the samples collected from the reaction.Since the Shexing Formation is deposited on the active continental margin near the Yajiang suture zone,the sedimentary rocks of the Shexing Formation are the first weathered and eroded materials in the active tectonic belt.The Al₂O₃/Si O₂ values of samples collected around Linzhou County ranged from 0.06 to 0.2,with an average value of0.15,indicating that the maturity of the sediments of the Shexing Formation is low.Combined with the photographs under the microscope,the particles of sandstone are sorted better and the roundness is average,which implies that the source distance of the Shexing Formation will not be too far.The age span of the Shexing Formation collected in Zongxue Village,Songpan Township,Linzhou Basin is large,showing the characteristics of a single peak,and the age span is between 108Ma and 1945Ma.The overall peak concentration ranges from 108 Ma to 125 Ma,and some zircons are missing between 400 Ma and 500 Ma,500 Ma and 750 Ma,and 1400 Ma and 1500 Ma.The comparison between the detrital zircons of the Shexing Formation and the Xigaze fore arc basin shows a great similarity.From 170Ma to 140Ma,the southern Gangdise block may have experienced a short and intense crustal compression uplift and thickening,which caused the Yeba Formation(J₁y)to undergo strong deformation and metamorphism,and became the base of the Linzhou Basin.The Linzhou area began to show the prototype of the basin.With the continuous northward subduction of the Neo-Tethys Ocean and the continuous reduction of the ocean basin,the basin began to deposit the Chumulong Formation and Takna Formation of shore shallow marine facies.From 138 Ma to 137Ma,with the increasing impact of the Sangri arc activity on the Linzhou basin,the Linzhou basin shows the characteristics of the back-arc foreland basin.Between125Ma and 110Ma,the Neo-Tethys Ocean continues to dive,the Linzhou area uplifted,which began to slowly transition from marine to continental facies,and began to deposit the Shexing Formation of continental red beds.After?78Ma,the Linzhou basin began to deform until the end of deformation before?72ma.With the subduction of the Indian plate and the continuous accretion of the strata,Linzhou basin slowly shows the characteristics of a wedge top basin behind the arc on accretion wedge.