The Magmatism In Zhongba Area, Yarlung Zangbo Suture Zone And Its Tectonic Implications

The formation,evolution and subduction history of the Neo-Tethyan is one of the core problems about the research of Tibet.The accompanied magmatism and mineralization is considered an integral part of the mineral resource of Tibet plateau.We fucused on the rift lava,ophiolite and Gangdese granitoid from Zhongba area,southern Tibet.The>2000-km-long Yarlung Zangbo Suture Zone?YZSZ?in southern Tibet is the southernmost suture across the plateau and includes the remnants of the Mesozoic Neo-Tethyan oceanic lithosphere.We present here our field observations and geochronological,geochemical and Sr-Nd-Pb isotopic data from the Zhongba rift lava.Zircon U–Pb analyses of a doleritic rock has a concordant age of 247Ma.All the rocks show OIB affinity,they were formed at the continental rift or continental-ocean transition zone,indicate the Neo-Tethyan open during the early Triassic.The YZSZ is subdivided by the Zhada-Zhongba microcontinent into northern and southern branches in its western segment.We present here our field observations and geochronological,geochemical and Sr-Nd-Pb isotopic data from mafic rocks of the Zhongba ophiolite,including samples from both branches.Zircon U–Pb analyses of a doleritic rock from the northern branch has a concordant age of 160.5±1.3Ma.All the doleritic samples from the northern branch and the pillow basalt and gabbro samples from the southern branch display similar REE and trace element patterns similar to those of modern OIB rocks.The dolerites from northern branch?Group 1?and the pillow basalts from southern branch?Group 2?have similar geochemistry and Sr-Nd-Pb isotopic data with each other also with OIB-type rocks from other ophiolite massifs in the YZSZ.They might be influenced by the same mantle plume.The gabbros from southern branch?Group 3?also show strong OIB affinity,with enriched Sr-Nd-Pb isotopic character,could be interpreted as from the same mantle plume source which was contaminated by subducted oceanic crust and pelagic sediment.We interpret our results together with the extant isotopic,geochemical and structural data from the entire YZSZ,and infer the tectonic origin of Neo-Tethyan mafic rocks exposed along the YZSZ as a plume-proximal seafloor spreading system.The plume might be the channel for the Ultrahigh pressure minerals in the podiform chromite,which support the deep source genetic model of the podiform chromite.Geochemistry,geochronology,whole rock Sr–Nd–Pb and zircon Hf isotopic data are presented for the Zhongba granites in the western part of southern Gangdese batholiths,Tibet.Zircon U–Pb dating constrains the timing of emplacement as Eocene?38Ma?.Zhongba samples are enriched in LREE,LILE?e.g.,U,Th,Rb and K?,and Pb,and clearly depleted in HFSE?e.g.,Nb,Ta,Ti?,show very radiogenic Sr((⁸⁷Sr/⁸⁶Sr)_i=0.71080 to 0.71639),unradiogenic Nd(?_Nd?t?=-8.7 to-6.5),and fairly radiogenic Pb isotopic signatures((²⁰⁶Pb/²⁰⁴Pb)_t=18.622–18.909;(²⁰⁷Pb/²⁰⁴Pb)_t=15.738–15.755;(²⁰⁸Pb/²⁰⁴Pb)_t=39.304–39.459),negative zircon?_Hf?t?values?-7.7 to-2.6?and ancient crustal model ages.Zhongba granites were most likely formed by a hybrid parental magma with both rising asthenosphere melt followed the slab break-off and the melt of the subducted Indian continental crust materials.Our samples together with the Kailas granites,Zedong granitoids,recorded magmatic activity in the southern Gangdese belt during 42-38Ma,which have distinct generation mode from either syn-collisional Linzizong volcanic successions and Gangdese batholith or post-collisional adakitic rocks in southern Gangdese.They provided the evidence for the significant injection of the Indian crust to Tibetan magmas,which means the Indian continental materials have contributed to crustal thickening beneath the Lhasa terrane by magmatic activities.