Timing And Process Of The Extinction Of The Bangong-nujiang Ocean

The extinction time and process of Bangong-Nujiang ocean is one of the hot issues in the basic geological research of the Tibetan Plateau.It is not only of great significance to explore the early uplift history of the central Tibet before the India-Asia continental collision,but also related to the accurate understanding of the metallogenic geological background and metallogenic law of large-scale world-class mineral resources in the central Tibet.For the extinction time and process of Bangong-Nujiang ocean,many works have been carried out on ophiolite and ocean island in Bangong-Nujiang suture zone,and late Mesozoic magmatic rocks and sedimentary rocks in blocks on both sides.However,the extinction time and closure process of Bangong-Nujiang ocean are still controversial.One of the key factors to explore the extinction process of Bangong-Nujiang ocean is to determine its closure time.In addition to the direct oceanic lithospheric fragments such as ophiolite and ocean island,the strata in the suture zone,as a faithful record of ancient ocean,is of key significance for restoring the evolution of ancient ocean and paleogeographic pattern.Among them,the identification of the youngest deep-sea sediments,the highest marine strata and the oldest continental molasse is the key to reasonably restrict the closure time of ancient ocean.In this thesis,zircon U-Pb dating of turbidite sandstone and tuff interlayer in 3 study areas in the central and western segment of Bangong-Nujiang suture zone shows that the youngest detrital zircon age[YC1?(2+)] of sandstone in Mabujiacuo area is 114.9 ± 3.9 Ma(n = 3),and the youngest detrital zircon age of 2 sandstone samples in Duochang area are 126.5 ± 4.2Ma(n = 2)and 126.0 ± 3.7 Ma(n = 2),respectively.The weighted average age of volcanic tuff in Kama area is 113.3 ± 2.0 Ma(n = 5),which reveals that there is Aptian flysch deposition(ca.126-113 Ma)in the Bangong-Nujiang suture zone.Turbidite facies analysis shows that Aptian flysch is mainly formed in the middle-lower fan of sub marine fan,and part of it is formed in deep-sea basin plain.In terms of structural deformation,Aptian flysch experienced a strong tectonic emplacement process in subduction period,and mixed structurally with the with ophiolite and other blocks to form the tectonic mélanges.In addition,the provenance analysis results show that it comes from the re-cycle of the early accretionary prism in the southern margin of Southern Qiangtang terrane.On this basis,combined with the soft sedimentary deformation structure developed in turbidite sandstone,this thesis considers that the Aptian flysch was formed in trench environment.Spatially,the east-west extension scale of Aptian flysch is more than 400 km from Mabujiacuo area in Ritu County in the west to Kama area in Dongcuo Town in the East.The above research shows that the youngest deep-sea flysch deposition in the Bangong-Nujiang suture zone has continued to the Aptian.The geochronological study of the Albian in the Rebonco area in the western segment of the Bangong-Nujiang suture zone shows that the youngest detrital zircon age of 2 sandstone samples are 109.4 ± 2.0 Ma(n = 9)and 108.4 ± 1.8 Ma(n = 7)respectively,which is basically consistent with the zircon U-Pb ages of 110.2 ± 1.4 Ma(n = 18)and 109.1 ± 1.4 Ma(n = 17)of two pyroclastic intercalation samples.These chronological data accurately define the sedimentary age of the Albian in the Rebonco area as the Albian.On this basis,combined with the sedimentological characteristics of the upper Shamuluo Formation in the region and the previous chronological data,it is found that the Shamuluo Formation in the western segment of the Bangong-Nujiang suture zone contains two parts with significantly different sedimentary facies and provenance,namely,the shallow shelf-slope facies of the oxfordian-Barremian and the estuarine-coastal facies of the Albian.The regional stratigraphic correlation analysis shows that the Albian estuarine-coastal facies deposits in the upper part of the Albian represent the highest marine strata in the Bangong-Nujiang suture zone.The Qushenla Formation in Nare Village,Gaize County,the middle segment of Bangong-Nujiang suture zone is a typical continental volcanic-sedimentary stratum.The volcanic rock member includes vesicular rhyolite and rhyolitic ignimbrite.The zircon U-Pb weighted average age of rhyolitic ignimbrite is 103.9 ± 1.4 Ma(n = 19).The sedimentary member is mainly alluvial fan conglomerate.The zircon U-Pb ages of quartz diorite porphyritic and granodiorite porphyritic boulders in the conglomerate are106.0 ± 1.6 Ma(n = 20)and 106.1 ± 1.5 Ma(n = 20)respectively.The zircon U-Pb ages of 106.5 ± 3.7 Ma(n = 9)and 121.6 ± 3.9 Ma(n = 10)are measured in 1 conglomerate Matrix sample.These age data accurately constrained the formation of Nare Qushenla Formation in the late Albian(< 106 Ma).Through the analysis of sedimentary facies and provenance,this thesis believes that the alluvial fan conglomerate of the Nare Qushenla Formation is the sedimentary response of strong crustal uplift in the late Albian(< 106 Ma),which is significantly different from the sedimentary records dominated by fluvial facies in the early Albian(ca.110-106 Ma)reported by previous researches.In order to explore the mechanism of the strong crustal uplift at the end of the Early Cretaceous,based on previous studies,this thesis makes an petrological,chronological and geochemical study on the Qushenla Formation volcanic rocks in Gaize area.The results show that the Early Cretaceous ca.105 Ma magmatic rocks in Gaize area are very complex.Spatially,they are basically distributed in a nearly east-west direction along both sides of the suture zone,which can be further divided into two subzones: southern subzone and northern subzone.The rock types of the northern subzone are relatively rich,including bajaite,A-type rhyolite,high-Nb basalt,N-MORB basalt,adakite and so on.The lithology of the southern subzone is relatively simple,and only A-type rhyolite,magnesium rich andesite and adakitic intrusion are found.The complex characteristics of these 105 Ma magmatic rocks reflect a sudden deep tectonic event and record the breakoff of the north branch of the oceanic slab after the closure of the Bangong-Nujiang ocean undergoing divergent double subduction at the end of the Early Cretaceous.Based on the newly discovered and determined youngest deep-sea flysch deposits represented by Aptian trench flysch(ca.126-113 Ma),the highest marine strata represented by Albian(ca.110-102 Ma)estuarine-coastal facies deposits in the upper part of Shamuluo Formation,and the Albian(< 106 Ma)Qushenla Formation continental molasse.In this thesis,the stratigraphic framework of Bangong-Nujiang suture zone is improved,and then combined with the previous research data,the extinction time and process of Bangong-Nujiang ocean are determined: After the divergent double subduction in the early stage,the intra-oceanic island arc collided with the southern margin of Southern Qiangtang terrane along the northern side of the ocean in the Late Jurassic-Early Cretaceous.It not only led to the formation of the Upper Jurassic-Lower Cretaceous Shamuluo Formation and its underlying sedimentary unconformity,but also caused the sedimentary discontinuity and magmatic stagnation of the Southern Qiangtang terrane from Late Jurassic to Early Cretaceous.However,the arc-land collision did not cause the closure of Bangong-Nujiang ocean,and its ocean basin still has a certain scale.Subsequently,the ocean basin experienced a ?scissors?diachronic closure process from east to west,and the region east of Nima County gradually closed before the Aptian period.In the west of Nima County,there is still a certain scale of residual ocean basin,and deep-sea sediments represented by Aptian trench flysch are developed.Entering the Albian,the middle and western segment of Bangong-Nujiang ocean gradually closed,and arc-arc ?soft collision? occurred,resulting in the differential uplift of the suture zone.Therefore,in the western section of Bangong-Nujiang suture zone,the Albian estuarine-coastal facies strata of the Shamuluo Formation and the continental Qushenla Formation are developed.In the late Albian period(ca.106 Ma),the Bangong-Nujiang oceanic slab under the suture zone finally broke off along its northern margin,which controlled the complex magmatism and sedimentation process around Gaize area.After clarifying the extinction process of the Bangong-Nujiang ocean,combined with the paleogeographic pattern of the Late Mesozoic in the central Tibet,this thesis proposes that the early uplift process of the central Tibet is phased,with the characteristics of gradual uplift from north to south in time and space.The mechanism of the early uplift is that the Late Jurassic-Early Cretaceous was mainly controlled by the extinction process of the Bangong-Nujiang ocean,the Late Cretaceous was controlled by the northward subduction of the Neo-Tethys ocean.From Late Jurassic to Early Cretaceous,the intra-oceanic island arc of the Bangong-Nujiang ocean collided with the southern margin of the Southern Qiangtang terrane along the north side of the ocean,resulting in a strong crustal uplift in the south of the Southern Qiangtang terrane.Subsequently,the Albian arc-arc ?soft collision? and the breakoff of the northern branch of the oceanic plate,resulting in the formation of the Bangong-Nujiang suture zone and the rapid uplift of its northern margin.The breakoff also accelerated the subsidence of the Bangong-Nujiang oceanic lithosphere.In the early Late Cretaceous,without the absorption and buffer of the Bangong-Nujiang oceanic lithosphere,the far-field tectonic force formed by the northward subduction of the Neo-Tethys ocean directly actes on the Lhasa terrane,resulting in strong tectonic shortening and the development of many thrust fault belts.