Provenance And Tectonic Evolution Of MéLange In The Central And Eastern Yarlung-Zangbo Suture Zone,Southern Tibet

The mélange of the Yarlung-Zangbo suture zone?YZSZ?in southern Tibet probably records subduction process of the Neo-Tethyan ocean and the India–Asia collision.The provenance and formation process of the Yarlung Zangbo mélange?YZM?is critical for reconstruct the processes varying from the opening to the subduction of the Neo-Tethyan ocean.Previous studies from the YZM are focused on the tectonic processes and evolution about the Neo-Tethyan ocean subduction and the India-Asia collision.The formation process and its mechanism of the YZM,and the early exhumation history of the Yarlung-Zangbo ophiolite?YZO?remains poorly constrained.Therefore,this study conducted systematical geological investigation of ophiolitic mélange in the Buma,Luqu and Langxian which exposed within the central and eastern YZSZ,respectively.The petrologic,detrital-zircon U–Pb ages and Hf isotopic data,Crspinel geochemical data,and major and trace elements data of different types of sandstone,basalt and chert within the mélange are analyzed and summarized as follows.Clastic rocks in Buma and Luqu mélanges show a typical block-in-matrix fabric.The detrital zircon U-Pb age distributions present as two groups: Group 1 shows a unimodal population of 120⁹0 Ma?peak age at 96 Ma?with positive ?_Hf?t?values.Group 2 shows bimodal population of 120⁸0 Ma?peak age at 104 Ma?and 180¹40 Ma?peak age at 151 Ma?,also with positive ?_Hf?t?values.The above observations indicate that both two groups were sourced from the Gangdese magmatic arc.Furthermore,their detrital zircon age spectra resemble with those of the Lower and Middle Ngamring Formation in the Xigaze forearc basin?XFB?,respectively.Clastic rocks in Nang mélanges also present a typical block-in-matrix fabric,which can be divided into two groups.Group 1 displays bimodal distribution at 120⁹0 Ma?peak age at 102 Ma?and 150¹30 Ma?peak age at 136 Ma?,suggestive of the Gangdese magmatic arc source.Group 2 yields a scattered age spectra?2300²00 Ma?,resembling with that of the Langjiexue Group.Considering the lower plate materials is completely absent in the Luqu mélange,here this thesis proposed that there was an intensely subduction erosion in the Luqu.Such strong subduction erosion might be caused by the subduction of seamounts in the Neo-Tethyan ocean.This inference is confirmed by a nearly simultaneous exhumation event revealed by low temperature thermochronological data in the southern margin of XFB.Both the detrital zircon U-Pb ages and Gr-spinel geochemical compositions indicate sandstones within the mélanges of the central YZSZ may belong to Lower and Middle Ngamring Formation.The unified Ngamring Formation was divided into two parts by the uplift and exhumation of YZO caused by seamount subduction or a regional thrust fault.The timing of the uplift and exhumation of YZO is⁸8/92 Ma.For the Buma mélange,the Permian limestone block can be observed in the south of the mélange,while the Nang mélange obviously has two-direction provenance,which indicated the accretion wedges in their positions.The subduction erosion of the Neo-Tethys Ocean in the late Cretaceous is different in the same subduction zone.The main factor is whether seamounts were involved in the process of subduction or not.