Sedimentary Record And Basin Evolution Of The Neo-Tethyan Subduction System In Xigaze,Southern Tibet

Northward subduction of the Neo-Tethyan ocean to the Aisa plate in Cretaceous induced subsequent India-Asia collision and produced the immense Tibetan plateau.Therefore,understanding the Neo-Tethyan subduction is vital for deciphering evolution of Tibetan plateau,and reconstructing the initial India-Asia collision.The Neo-Tethyan subduction system was composed of the Gangdese magmatic arc,the Xigaze forearc basin,the Yarlung-Zangbo Ophiolite and the Xiukang melange,which have been investigated widely and studied systematically since 1980s.However,some scientific issues remain open despite of many achievements.In this study,we investigated the Cretaceous-Tertiary sedimentary strata in the Neo-Tethyan subduction system and related basalt,and presented an integrated study of stratigraphy,sedimentology,sandstone petrology,detrital zircon U-Pb geochoronology and Hf istopes,whole-rock geochemistry to constraint evolution of the subduction system.Between the Yarlung-Zangbo Ophiolite to the south and the Gangdese magmatic arc to the north,the Xigaze forearc basin with well-preserved and superbly exposed succesions provides an excellent opportunity to study the evolution of a forearc basin.The successions include the Chongdui,Sangzugang,Ngmaring and Padana Formations(from bottom to top).Barremian-Late Aptian cherts of the lower Chongdui Formation were deposited in an abyssal basin while the upper member was deposited as distal turbidite on the continental rise.The late Aptian-Early Albian Sangzugang Formation accumulated as a carbonate platform on the southern Lhasa margin.In the Late Albian-Cenomanian,turbiditic sandstones of the Ngmaring Formation were deposited in distal-fan to slope settings while the Padana Formation was developed in the shelf to delta environments during Campanian.Thus,shallowing-upward successions deposited in submarine-fan,shelfal and finally deltaic environment were documented and the transition from the underfilled to the filled stage was recorded in the basin.The lithology,depositional environment,sandstone petrography,detrital-zircon U-Pb ages and Hf isotopes of the upper Chongdui Formation are the same as in the immediately overlying strata of the Ngamring Formation.Thus,the upper Chongdui Formation is equivalent to the lower-middle parts of the Ngamring Formation.From the Chongdui to Padana Formations,sandstone composition evolves from feldspatho-lithic arenite to litho-feldspatho-quartzose and finally litho-quartzose,indicating provenance change from undissected magmatic arc to dissected magmatic arc and recycled orogeny.Detrital zircons from the Chongdui and lower-middle Ngmaring Formations yielded dominantly Mesozoic ages,from unimodal(peak age at 110 Ma)upward to bimodal distribution(peak ages at?110 and?157 Ma).Age patterns are more complex in the upper Ngmaring and Padana Formations,with many pre-Mesozoic ages and more clusters of Mesozoic ages.Besides,?Hf(t)values of Mesozoic zircons change from completely positive to both positive and negative from lower-middle Ngmaring Formation to upper Ngmaring and Padana Formations.Therefore,provenance of forearc-basin sandstones changed from initially Lower Cretaceous Gangdese arc rocks along with its basement and the Sangzugang Formation,next to Cretaceous and Jurassic Gangdese arc rocks,and finally to mixed sources from the Gangdese arc and central Lhasa terrane,documenting the progressive erosional evolution of the Gangdese arc and uplift of the central Lhasa terrane during the Late Cretaceous.Based on our study and literatures published,it's concluded that the forearc basin was first developed in the east,then later to the west while the east-middle parts were with a changing provenance documented above and the west part with a simple one of the Gangdese arc only.The Xiukang melange south of the Yarlung-Zangbo Ophiolite,displaying a typical block-in-matrix structure,contains blocks of chert,limestone,sandstone and basalt,with various ages and scales while matrix is dominant by mudstone and siliceous mudstone,with distal turbidites occasionally.Sampling traverses involving the Yarlung-Zangbo Ophiolite,the Xiukang melange and the Tethyan-Himalaya strata have been investigated from Geding to Saga and outcrops of blocks vary greatly.Four groups of sandstone blocks with different provenance and depositional settings are distinguished by their petrographic,geochronological and isotopic fingerprints.Two distinct groups of volcaniclastic-sandstone blocks were derived from the central Lhasa terrane and Gangdese magmatic arc.One group was deposited in the trench and/or on a trench-slope basin within the Asian active margin during the early Late Cretaceous,and the other group in a syncollisional basin just after the onset of the India-Asia collision in the late Paleocene.Two Groups of quartzarenite,originally sourced from the Indian subcontinent,were deposited before and during the Cretaceous time respectively on the northernmost edge of the Indian passive margin and eventually accreted to the melange in the very first stage of the India-Asia collision.In depositional contact with limestone blocks,basalt blocks with OIB-type geochemical affinities were from seamounts in the Neo-Tethys.The Xiukang melange was thus evolved first in the Cretaceous as a small subduction complex along the erosive Asian margin with blocks of basalt,limestone,chert and turbiditic volcaniclastic-sandstone,then finally developed in the Eocene when sandstone blocks of both Indian and Asian origin were progressively incorporated in the suture zone of the nascent Himalayan Orogen.Exposed in the melange zone south of Saga,the Luogangcuo Formation was faulted by the Zhongba-Gyangze thrust to the Zhongzuo formation in the south.The unit is dominant by grey shales,purple to dark green sandstone,gravelly sandstone and conglomerate with blocks of chert and sandstone.The clastic-supported massive conglomerate with dominantly subrounded-subangular chert pebbles was developed by deep-water debris flow while thin-bedded sandstone was turbiditic.Noteworthy is that blocks of chert and sandstone were slump deposits.The Luogangcuo section was deposited at?92 Ma-81 Ma indicated by detrital zircon ages,with provenance from the Gangdese magmatic arc and the central Lhasa terrane for sandstone and from the Xiukang melange for conglomerates.With the same provenance,the Guozhang section was developed during?88 Ma-81 Ma.The Luogangcuo Formation was developed in the trench basin,representing deposits at the south margin of the Asia plate.Integrated with studies on other sections in the same tectonic position,the Zhongba-Gyangze thrust might be remnant of the vanished Neo-tethyan trench,i.e.,the boundary of India and Asia.In a conclusion,the change of deposit center and provenance was recorded in the Neo-Tethyan subduction system in south Tibet.