Cretaceous-Paleocene Radiolarian Fauna In Saga-Zhongba,Southwestern Tibet:Implications For The Evolution Of Neo-Tethys

India-Eurasia collision and the closure of the Neo-Tethys are essential geological events in the Qinghai-Tibet Plateau. They are important for understanding the evolution of the Himalayan Orogen and the formation of the Tibetan Plateau. However, the evolutionary history of the Neo-Tethys and the time of collision between India and Asia plates are still controversial.So far, existing estimates to the times of the India-Eurasia collisional ranges include70Ma,65Ma,55Ma,45Ma or≤34Ma. Existing models of the closure of Neo-Tethys include:1)from west to east;2) from east to west;3)from both sides (west and east) to the middle. The one from west to east is generally accepted. Nevertheless, the latest marine deposit(about34Ma)—Pengqu Formation was found in the middle of the Yarlung Zangbo Suture Zone (YZSZ for short).It seems that the collisional process could have happened from both sides (east and west) to the middle.It is direct and effective to constraint the time and process of collision between Indian and Asian plates by sedimentary response especially the morphology and spatial configuration of marine sedimentary of southern Tibet. As a key tectonic boundary in the Tibet Plateau, YZSZ represents the position of collision between India and Eurasia plates.In the suture Paleogene radiolarian-bearing strata are wide-spread and they can provide very important micro paleontological evidence in interpreting the evolution of the Tethys and constraining the time of India-Eurasia collision.All of the studied radiolarian samples were collected in Saga-Zhongba area, which is located in the south of the YZSZ and belongs to Northern Himalayan-Tethyan Zone.Late Cretaceous Archaeospongoprunum tehamaensis Zone and Dictyomitra lamellicostata-Squinabollum fossilis Assemblage and late Paleocene Lamptonium-Calocycloma Assemblage have been recognized in section S31. Archaeospongoprunum tehamaensis Zone from bed10of the section S31includes10species belonging to9genera and is assigned to late Cretaceous Cenomanian; Dictyomitra lamellicostata-Squinabollum fossilis Assemblage from bed11of the section S31includes7species belonging to6genera is assigned to late Cretaceous Turonian; Lamptonium-Calocycloma Assemblage from bed16of the section S31includes4species belonging to3genera is assigned to late Paleocene.Orbula comitata-Lychnocanoma costata Assemblage has been recognized in section S32and includes18species belonging to11genera. It could be correlated to Peritiviator(?) dumitricai subzone of northern Atlantic and its age was assigned to late Paleocene Selandian about60-59Ma.Late Cretaceous Katroma bicornis-Pantanellium lanceola Assemblage, Thanarla lacrimula-Tricapsula costata Assemblage, Cryptamphorella conara-Pseudoeucyrtis tavricus Assemblage, Cavaspongia antelopensis-Acaeniotyle umbilicata Assemblage and late Paleocene Buryella pentadica Zone have been recognized in section S103. Katroma bicornis-Pantanellium lanceola Assemblage in beds27and28includes8species belonging to8genera and3indetermined species and could be correlated with Aptian part of Turbocapsula Zone of the Mediterranean and is assigned to early Cretaceous Aptian. Thanarla lacrimula-Tricapsula costata Assemblage in beds4,6and9includes14species belonging to11genera and could be correlated with Dactyliosphaera ailviae Zone of the Mediterranean and is assigned to late Cretaceous Cenomanian. Cryptamphorella conara-Pseudoeucyrtis tavricus Assemblage in beds37and39includes16species belonging to13genera and could be correlated with Pogonias misslilis subzone and Dactyliosphaera silviae Zone of the Mediterranean and is assigned to Cretaceous Albian-Cenomanian. Cavaspongia antelopensis-Acaeniotyle umbilicata Assemblage in beds41,54and55includes39species belonging to28genera and could be correlated with Alievium superbum Zone of the Mediterranean and is assigned to late Cretaceous Turonian. Buryella pentadica Zone in beds12includes23species belonging to15genera is assigned to late Paleocene about59-56.5Ma.Among the studied Paleocene radiolarian assemblages Buryella pentadica zone of section S103is the latest one, so it is studied in paleobathymetry here.The radiolarian fauna is very diverse and dominated by Nassellarian elements, which suggest a deep-water environment setting. Most of Nassellarian in the fauna have conical shell or bell-like shell with three feet mainly lived in400-3000m bathyal-abyssal area and increase dramatically in1000m and1500m respectively in individuals. Bathropyramis magnifica of the studied fauna is the long pyramidal in shape, mainly inhabits in the water below1200m. Consequently, the occurrence of deep-welling taxa in our materials reveals at least1000m paleobathymetry or bathyal-abyssal setting environment in the study area.The tectonic attributes of radiolarian-based stratum directly constrain the tectonic evolution of the study area. The Sangdanlin section S31and S32comprise asynchronous deposits in ages, such as late Cretaceous Cenomian, Turonian and late Paleocene and the section S103of Zhongba comprise early Cretaceous Aptian, Albian, late Cretaceous Cenomian, Turonian and late Paleocene deposits based on the discovered radiolarians. Moreover, the cherts and clastic rocks are usually divided by faults and disordered in sequence. It could be inferred that the cherts varying in age are remnants of the Tethyan Ocean and were mixed during subduction of the Tethyan crust and perched in sedimentary melange in form of tectonic blocks. The rock associations indicate that there existed the Tethyan Ocean in Saga-Zhongba area along the YZSZ until at least late Paleocene.We collected some basalt blocks in ophiolitic melange to the northwest of the study section S103. The zircon U-Pb ages of the basalt samples were dated by SHRIMP. Seven spot ages of the sample10G139-7are dispersive and the latest one is59.1Ma. The long columnar zircon with weak rings shows the characteristics of magmatic zircon. In the region, the previous work restricted the Yanduo basalt to52.85±1.38Ma and Niuku basalt block in Saga to52.09±0.77Ma by whole-rock K-Ar. Generally, zircon dating results are older than the whole rock K-Ar age determination because potassium mineral has a later closure time than zircon. It is reasonable that the present single zircon SHRIMP U-Pb age is earlier6-7Ma than the previous data. So there may be important magmatism correspondent with the age in this region. The coincidence between ages of magmatic event and radiolarian assemblage reveals that oceanic crust existed until late Paleocene in Zhongba area and Neo-Tethys turned into a remnant oceanic basin, the terminal stage during oceanic crust subduction. It is inferred that initial collision between India and Asia plates had not occurred and closure of Neo-Tethys had not taken place in the region by the late Paleocene.Whereas, previous investigation suggested it was foreland basin in Gamba-Tingri in the late Paleocene. Therefore, the tectonic framework of southern Tibet during late Paleocene could be concluded that it was remnant ocean basin to the west of Saga area and foreland basin to its east. That means in the late Paleocene the initial collision between India and Eurasia plates had happened completely in eastern Gamba-Tingri area while collision had not occurred in western Zhongba area. It is inferred that collision between India and Asia Plates initiated earlier in the east than that in the west. That suggests the Neo-Tethys experienced the closure process from east to west. It agrees with the investigations on pale geography of southern Tibet during Paleogene.Here, it is proposed that the process of the collision between India and Asia can be indived into three stages included in initial collision, the climax of collision and post-collision based on the review of the available documents and every stage continued for a long period.(1)70-56Ma. In the stage, the collision between India and Asia initiate and remnant sea formed. It caused a series of affection as fallowing:the subsidence rate increase rapidly in Zhepure region at70Ma, the basement of Eastern Tethys uplift, sea level dropped significantly, Karst surface be formed and sedimentary facies and depositional model in late Cretaceous changed suddenly ar68Ma, large marginal foreland basin be developed in Tibetan Plateau at65Ma, terrigenous sandstone and giant conglomerate appeared in the bottom of Paleocene of Gamba region. At about70Ma, the sedimentary facies and depositional model changed dramatically in the Late Cretaceous-Paleocene continuous marine seuqence of Zep mountain in Indian passive continental margin. The sudden transformation from marly sandstone to siliciclastic turbidite in mid-Maastrichtian indicate the initial contact between the Indian and Asian plates. While, in the northwest of the Pakistan, the columnar accretionary wedge and trench strata thrusted over the passive continental margin at66-55Ma, which means the disappearance of Oceanic deposition between Indian and Asia plates.These lines of evidence direct that at70-56Ma the hyperplasia edge of the Asian plate loaded above the passive margin of Indian Plate and the initial Indian-Asian occurred along the YZSZ. With the initial collision, Chengdu Basin, Tarim Basin and another large sag basins were formed in the Tibetan Plateau. The convergence between the Indian and Eurasian plates during the Paleocene is not very intense after the initial collision. Therefore, the contact relationship is conformable contact between Paleocene and Cretaceous in Chengdu Basin and Tarim Basin.The channel between Greater India and Tibet has been established because of the the initial collision of the Indian and Asian plates, so that the biota express the characteristic of being mixed and similar between India and Asia. There is typical mammals of Laurasia in Maastrichtian Taki formation of Nagpard in Indian plate. Indian subcontinent Maastrichtian contains the pollen of the Asian and the Paleogene ancient flora and fauna of two continents mixes in the YZSZ. the foraminiferal fauna from both sides of the Yarlung Zangbo suture are quite similar after the Paleocene.In magmatic activity, the age of the inition of the collision was reflected in the age of the syn-collisional volcanic rocks of Linzizong Group in the main collision zone (64.47-40.84Ma) and the age of the muscovite strongly peraluminous granites in southern Gangdese (56-50Ma).(2)55-45Ma. In the stage, the climax of the collision occurred and the Neo-Tethys retreated from Tibet westward along the YZSZ. The collision between the two continents reached its peak and caused a series of strong magmatic activities. The ages of the collisional magmatic rock focused at about50Ma in Gangdese. Considerable collision-related rocks appearced in Qushui granite.In the Qinghai-Tibet Plateau, the comprehensive collision led to Lanzhou-Xining sag basin and Qaidam Basin-Hoh Xil-Qiangtang sag basin be formed in the north margin of the plateau and begin to deposit collision-related terrigenous coarse clastic conglomerate and pebbly coarse sandstone. What’s more, a series of strike-slip pull-apart basins were formed in Yushu-West Sichuan-eastern Tibet,the eastern Tibetan Plateau,The previous paleomagnetic data suggest relative closure rate between Indian and Eurasian Plates decreased roughly at53-48Ma. If the rough decrease in closure rate could be attributed to the collision between plates, the phase of53-48Ma should be thought to be the age of the climax of collision. The available paleomagnetic data of Cretaceous-Paleogene in southern Tibet show the north margin of the India and Lhasa block sutured entirely in55-50Ma and the entire suture could be attributed to the strong collision between the two continents.(3)45-34Ma. In the stage, the Indian-Asian collision complated and the Neo-Tethys remnant sea disappeared in Tibet. Appearance of the earliest molasse, the age of the latest marine sedimentary, muscovite/two-mica granite directing intra-continental convergent and broad and strong tectonic compression deformation events the continent-continent collision end at45-34Ma. In the stage, the Indian-Asian collision complated and the Neo-Tethys remnant sea disappeared in Tibet and they shifted to intra-continental convergent stage.Along the Yarlung Zangbo River valley, there are Eocene thick molasse and Fluvial glutinite, Among of these, Paleocene-Eocene Liuqu conglomerate is earliest outer molasse deposits along the YZSZ, which implies the collision between India and Asia had completed until the P-E transition and southern Tibet had transformed into the stage of uplifting at that time.Latest marine strata of Norhtern Tibet Linzhou Basin, Cuojiangding of Zhongba, Maila, Mount Kailash, Rikangba of Zanda, Gaer and Ritu are carbonate sediments with benthic foraminifera and later than mid-Eocene. However,the nannofossils from the top of marine Pengqu formation in Dingri, southern tibet offer an ageof latest Eocene about34Ma, which suggests the latest sea regression in Tibet may be in the end of the Eocene.In tectonic deformation, the inter-continental convergence cause latest Eocene Ultramafic rock piece obducted above the Late Cretaceous Xigaze Group sandstones and shales and both Xigaze Group and overlying marine Paleocene-Eocene layers strongly folded. The fission track ages of zircons and apatites from the Gangdese belt reflect there exist the rapid cooling events and exhumations reflected by rock uplift during43-36Ma In Himalaya. The existence of regional unconformity and deposition missing in Gangdese belt at about36-34Ma prove Himalayan has uplifted initially and Gangdese continued to uplift at about40Ma. With the completion of Indian-Asian collision Longmenshan fault had tranformed into dextral strike-slip from sinistral strike-slip at40-35Ma and new strike-slip pull-apart basins formed in the eastern edge of the Plateau, such as Nangqian basin.The REE abundance and chondrite-normalized patterns of38.7±0.4Ma trachytic dacite of Lingzizong Group are similar to those of analcime tephriphonolite, trachyte and trachyandesite and it suggests terminal stage of the collision.Above all, the process of the evolution of the Neo-Tethys during Paleogene could be concluded as fallowing:the initial collision between India and Asia started in eastern Himalayan syntaxis during about70-56Ma. Subsequently, the collision spreaded to the west in Scissors-style.During55-45Ma the climax of the collision occurred and the Neo-Tethys retreated from Tibet westward along the YZSZ; at about34Ma the Indian-Asian collision complated and the Neo-Tethys remnant sea disappeared in Tibet and they shifted to intra-continental convergent stage.