Vestige Sedimentary Records Of The Early India-Asia Collision In The Eastern Yarlung Zangbo Suture Zone,southeastern Tibet,china

The research on the India-Asia collision process is of great meaning for people to perceive the plate tectonics,global climate changing,and so on.Although the history of relevant studies is about 50 years,there are still some heated debates on the timing and mode of the initial collision.This may be partly caused by different levels of studies among different parts of the suture zone—high level in the center and west but very low in the east.According to the experience of previous works,sedimentary and stratigraphic attributes of the syn-collisional sedimentary records are vital to constrain the initial timing and mode of the collision.Therefore,in this study we undertook multiple methods,including the sedimentary,stratigraphic,metamorphic,structural,and petrological observations in the field combined with analyses of detrital zircon chronology,morphology,trace element,Hf composition,and bulk-rock Sr-Nd isotopes in the lab,to identify syn-collisional sedimentary records in the eastern Yarlung Zangbo suture zone(YZSZ).Based on the reclassification of previously proposed“Cretaceous Langxian mélange”that exposes in the Jiacha-Jindong area,Shannan District,along the Yarlung Zangbo,we distinguished the syn-collisional Eocene sedimentary Langxian unit and the Upper Triassic Langjiexue Group from the so-called mélange in the eastern YZSZ.And,we further discussed the timing,mode,and diachroneity of the initial India-Asia collision and characters of the syn-collisional foreland basin system,on the basis of the depositional environment,provenance,and the comparison between the Langxian unit and coeval strata in the central and western parts of the Himalaya orogen.Field observations and lab analyses indicate that the previously-called“Cretaceous Langxian mélange”,outcropping east of Luobusha,is made up of two coherent stratigraphic units—the Langjiexue Group at the south and the Langxian unit at the north,instead of a tectonic mélange.The Langxian unit exposes continuously in the region from Jindong,through Langxian,west to Jiacha.It has a total thickness up to ?2500 m and extends up to ?100 km in the east-west along the strike.It can be subdivided into three sub-units.Sub-unit I is mainly composed of marbles interbedded/intercalated with schists/slates,in which some meta-sandstone and meta-conglomerate layers occur.In Jiacha area,Sub-unit I consists of marbles interbedded with slates and meta-sandstones.Protoliths are sandstones with volcanic rock clasts,conglomerates with granite and marble gravels,and minor quartz sandstones.Sub-unit II only exposes at Langxian-Jindong area and is composed of(feldspathic)quartzites and quartz schists.The protolith could be quartz sandstone or chert.Sub-unit III,?300 m thick,is a package of marbles intercalated with schists,exposing at the southernmost of the Langxian unit,between Langxian and Jindong.The protolith of schists could be calcareous siliciclastic rocks with a high proportion of quartz.The Langjiexue Group,however,has a variety of lithological compositions at different places.It is composed of mica schists and quartz schists in Langxian-Jindong area but becomes the alternation of meta-sandstones and slates in Jiacha-Langxian area.No marble/limestone layers have been found in the Langjiexue Group,yet.We have identified four metamorphic mineral zones in the eastern YZSZ:the staurolite zone,garnet zone,biotite zone,and the chlorite zone.The first zone belongs to amphibolite-facies metamorphism,and the last three zones belong to greenschist-facies metamorphism.Because these zones are symmetrically outcropping along the Badun-Jindong fault,we deduce that the metamorphism is caused by the northward thrust of the Badun-Jindong fault.Structurally,the Langxian unit develops only slaty cleavages and schistosities that are broadly parallel with each other and even parallel with primary beddings(S₀).Multi-stage deformations had been taken place in the Langjiexue Group,obviously distinct from the Langxian unit.In Langxian-Jindong area,the Langjiexue Group northwardly thrusts over the Langxian unit,while the Langxian unit,in turn,thrusts over the Oligocene-Miocene Dazhuka Formation or Gangdese batholith in the north.In Jiacha-Langxian area,the Langjiexue Group northwardly thrusts over the Dazhuka Formation in the north,and the Langxian unit is sandwiched into the Langjiexue Group through two bounds of thrust faults in the south.Based on age spectral comparisons of 32 detrital zircon samples from Jiacha to Jindong,the stratigraphic reclassification and boundaries were further verified.20 samples from the Langxian unit incorporates abundant Cretaceous-Paleogene detrital zircon grains.For instance,in Langxian-Jindong area,12 samples have the Cenozoic grains,yielding YC1?(2+)ages between 58.0±1.4 Ma and 53.0±2.8 Ma.Two samples from Jiacha area yielded two YC1?(2+)ages 54±1.5 Ma and 53.7±1.3 Ma,respectively.Significantly,42 spot analyses performed on metamorphic rims have U-Pb ages between 34.1 Ma and 17.2 Ma.Together with other regional metamorphic geochronology,we suggest the Langxian unit deposited during ?56/54-48 Ma and probably represents the sedimentary records of the initial India-Asia collision.Those samples from the Langjiexue Group,however,are overall older than ?200 Ma and have age spectra well matching that of the Langjiexue Group in the Shannan terrane.PAAS-normalized REE+Y patterns of the Langxian marbles are characterized by light REE depletion,La and Gd enrichment,and significantly positive Y anomaly,resembling those of modern seawater and geohistorical marine carbonates.Least altered marbles have high ⁸⁷Sr/⁸⁶Sr ratios,identical to those of coeval sea waters.The geochemistry suggests that protoliths of the Langxian marbles have precipitated in marine environment.1273 concordant detrital zircon ages of 20 samples from the Langxian unit can be subdivided into three groups in terms of provenance.Group I includes samples of meta-sandstones bearing intensely epidotized volcanic clasts,and it has one age population 150-52 Ma with a predominant peak at ?84 Ma and a weak peak at ?54 Ma,manifesting a main source of the Gangdese batholith.Group II has two main age populations,?150-90 Ma and 60-50 Ma,with a distinct peak at ?105Ma.These zircons have?_Hf(t)values ranging from -16 to+13.Most of the zircons were plotted into the continental zircon survey on the U/Yb vs.Nb/Yb diagram,and only minor zircons show MOR or OI characteristic,indicating Group II have shared the identical sources—Asian plate—with coeval strata exposing at the central South Tibet,rather than the intra-oceanic-island arc.Samples of Group II are mica-quartz schists,quartz marbles,and meta-conglomerates with stretched marble and granite clasts.We thus thought that recycled“forearc strata”are the most likely potential source for this group.Group III comes from samples of quartz marble,quartz schist,quartzite,and meta-conglomerate and has three age populations:the Precambrian(peak at ?560 Ma),the Permian-Triassic 290-200 Ma(peak at ?246 Ma),and the main Cretaceous-Paleocene 150-52 Ma(peak at ?108 Ma).The bulk-rock?_Nd(0)values range from -10 to -5,matching that values of Lhasa terrane in the north and Shannan terrane in the south.Furthermore,detrital zircons aged between 290-200 Ma are similar to the Langjiexue Group in age,detrital zircon CL features,and morphological parameters,implying they were recycled from the Langjiexue Group.Detrital zircons younger than 200 Ma have similar Hf isotopic and trace elemental compositions with zircons of Group II.Accordingly,detrital zircons within this age population in Group III and Group II could have been derived from the same source of the Lhasa terrane.So that,the Langxian unit could have received detritus shed from bilateral terranes exposing at both sides of the suture zone,i.e.from the Lhasa terrane in the north and Shannan terrane(the Langjiexue Group)in the south.Integrating the above evidence of stratigraphy,age,environment,and provenance,we herein propose that the Langxian unit is the sedimentary vestige of the initial collision between Indian and Asian plates,which is the residual records in the eastern YZSZ.Further comparisons of the sedimentary records from the forearc basin strata and relicts in mélange in the central YZSZ and the Eocene strata in Tethys Himalaya indicate that the protoliths of the Langxian unit could have deposited in the distal foredeep of an underfilled marine foreland basin.This place likely was the connection between the foreland forebulge(represented by the upper Zongpu Formation)and the proximal foredeep(represented by the Sangdanlin+Zheya Formations and Jiachala Formation).The age determination and basin mode proposal reveal that the initial collision between Indian and Asian plates could have taken place earlier than ?56/54 Ma in the eastern YZSZ,which is almost equal to the initial collision time(?60-56 Ma)in the western(Zanskar)and central parts of the suture zone(Xigaze forearc basin and mélange in suture)and Himalaya orogen(Enba Member).This spatial-temporal relationship allows us to propose that the initial Asia-India collision along the entire YZSZ seems synchronous without a significant diachroneity.Because basins in the eastern YZSZ,Tethys Himalaya,and Lesser Himalaya have started to receive detritus sourced from Lhasa terrane/Asia since the Paleogene,we deduce that the northern margin of Indian plate(or Himalaya microterrane)have attached(or approach to)the southern margin of Asian plate(or Xigaze arc/forearc basin)at that time,no matter whether or not there had developed an intra-oceanic-island arc within the northern Tethys Ocean.However,geophysical approaches stress that the difference of paleolatitude between the northern Indian plate and the southern Asian plate is ?17°during ?60-55 Ma,and the magnitude of convergence between India and Asia since ?60 Ma is distinctly larger than the magnitude of shortening estimated from the upper crust.Meanwhile,the previously proposed reconstruction models,e.g.,the Greater Indian Ocean or North Indian Ocean model and the rifted Xigaze arc/forearc basin model,could not totally match the geological evidence.It is then proposed that the Greater India had developed broader than the present-day Indian plate(north to the north margin of Tethys Himalaya,e.g.,Saga area)but its north part(north of Saga area)had been subducted into the southern margin of the Asian plate without the shortening and accretion of the upper crust.