The Tectono-magma Events In The Western Margin Of The Eastern Himalayan Syntaxis And Their Geodynamic Implications

The granitoid batholiths and volcanic rocks, distributed in the southern Lhasa terrane (Gangdese belt), resulted from the northward subduction of the Neo-Tethys oceanic slab under the southern Lhasa terrane, and the collision between Indian and Asian continents. A large number of high-quality zircon U-Pb and Hf isotope data for these rocks have been published in recent years. The geochronological framework of the granitoids and volcanic rocks has been established. However, the previous work mainly focused on the central part of the southern Lhasa terrane. How the Gangdese magmatism evolved along the strike of Himalayan-Tibet collisional belt remains poorly known. In addition, the metamorphism of the southern Lhasa terrane is still not clear. In order to better understand the geological evolution of the southern Lhasa terrane, this study focuses on the granitoids and metamorphic rocks of the Nyingchi Complex in the western margin of the eastern Himalayan syntaxis (EHS). We present an integrated study of detiailed field geology, petrography, whole rock geochemistry (including major elements, trace elements and Sr-Nd isotope), zircon U-Pb dating and Hf isotope composition for the granitoids and metamorphic rocks, and further discuss their petrogenesis and geodynamic implications. The main results related to this study are given as follows.1. The zircon U-Pb dating results reveal that the granitoids in the western margin of the EHS formed during at～164Ma,90～80Ma,66～48Ma and26-22Ma, which are consistent with those in the central part of the southern Lhasa terrane.2. The middle Jurassic granitic gneiss (165Ma) has εHf(t) values of+1.4to+3.5, which is lower than coeval granitoids in the central part of southern Lhasa terrane, suggesting that they mainly sourced from partial melting of crustal materials. We attribute the petrogenesis to the northward subduction of the Neo-Tethyan oceanic slab. The late Cretaceous (90-80Ma) granitoids have diversity magma source.The～-83Ma granodioritic gneiss is characterized by positive εHf(t) values of+7.3to+10.7, indicating a derivation primarily from a depleted-mantle or juvenile crustal source. The～81Ma granitic gneiss shows variablei(t) values from-0.9to+6.2, indicating a binary mixing between juvenile and old crustal materials. The～80Ma Wolong pluton displays adakitic characteristics. The Sr-Nd-Hf isotopic compositions indicate that it can be generated by melting of juvenile lower crust, accompanied by small degrees of contamination by older crustal materials. Combined with the coeval HT granulite facies metamorphism, we suggest that the～80Ma adakitic rocks resulted from Neo-Tethyan ocean ridge subduction. The-66Ma granite also shows adakitic characteristics, and has εHf(t) values of-3.8to-1.3, indicating it sourced from partial melting of old crustal materials. The Paleocene (61Ma) granodioritic gneiss hasεHf(t) values of+5.4to+8.0, suggesting that it originated from partial melting of a juvenile crustal material. Both the Comfluence granite (-49Ma) and the granite enclave (-50Ma) show adakitic characteristics, and have (87Sr/86Sr)i of0.706939to0.708162, εNd(t) of-6.7to-4.3and zircon εHf(t) of-11.8to-0.2, suggesting that they mainly derived from partial melting of old crustal materials. The Tianpolong pluton (-53Ma) hasεHf(t) values of+5.3to+7.7, and does not show adakitic characteristics, suggesting it formed under relatively upper crustal level. We attribute the petrogenesis of Eocene granitoids to the break-off of the Neo-Tethyan oceanic slab, which resulted in the upwelling of asthenospheric mantle. The asthenospheric upwelling heated the base of lower crust and resulted in partial melting of the lower crust to generated adakitic magma (lower crust) and non-adakitic magma (relatively upper crust). All the Oligocene-Miocene granitoids (26-22Ma) shows adakitic geochemical characteristics. The Sr-Nd-Hf isotopic compositions suggest that they sourced from partial melting of the Nyingchi Complex. The late Oligocene adakitic rocks resulted from the break-off of the subducted Indian continental crust starting at-25Ma.3. The garnet-bearing amphibolite and impure marble from the granulite fancies unit of Nyingchi Complex have been studied. Petrographic study indicates that the garnet-bearing amphibolite underwent two stages of peak granulite-fancies and retrograde amphibole-facies metamorphism. The peak mineral assemblage is characterized by Grt+high-Ti Amp+Opx+Pl+Qtz+Rt, and the retrograde amphibole-facies assemblage is characterized by low-Ti Amp+Pl+Czo+Qtz+Rt. There are a large number of rutile exsolutions in the garnet, quartz and amphiboles, suggesting that the primary composition of these minerals had high Ti contents and formed under high temperature conditions. Based on the Ti-in-quartz (TitaniQ) thermometer, the peak metamorphic temperatures is803～924℃. The whole rock geochemical characteristics indicate that the protolith of the garnet-bearing amphibolites are sub-alkaline island arc basalt. The zircons U-Pb dating results show that the crystallization age for the protolith of the garnet-bearing amphibolite is89.3±0.6Ma, and the peak metamorphic age is81.1±0.8Ma. Detrital zircons from the marble show core-rim structure in the CL images. The cores yielded206pb/238U ages ranging from86.3t0167Ma, and the metamorphic rims yielded206Pb/238U age of81.4±0.5Ma. The age distribution and Hf isotopic compositions of zircon cores match well with the age spectra of the Jurassic-Cretaceous Gangdese batholiths, suggesting that the protolith of the impure marble deposited in the fore-arc basin of the Gangdese arc. The above results indicate that both the arc magmatic rocks and the fore-arc sedimentary rocks undergone HT granulite facies metamorphism at～81Ma, suggesting a significant heat input into the forearc area. Combined with the presence of coeval adakitic rocks near studied area, we suggest that the～81Ma high-temperature metamorphism resulted from the upwelling of asthenosphere through the slab window opened as a result of ridge subduction.4. The Nyingchi Complex underwent strongly migmatization. Multiphase felsic melts can be recognized based on the intercalating relationships in the field. This study concentrates on four localities where typical migmatite and leucogranite vein crop out. The zircon U-Pb dating results reveal three stages of crustal anatectic events:65～63Ma,50～48Ma and30-25Ma. The inherited zircons and Hf isotope compositions in these felsic veins indicate that they mainly sourced from partial melting of the metasedimentary rocks and granitic gneisses of the Nyingchi Complex, and some derived from partial melting of the old mafic crustal materials. In addition, there are three stages of metamorphic event corresponding to the crustal anatexis. All the metamorphism and crustal anatexis are consistent with the Paleocene-Miocene magmatism in the central part of the southern Lhasa terrane. The65～63Ma,50～48Ma and30-25Ma crustal anatexis and metamorphism in the western margin of the EHS were related to the thermal perturbations caused by the roll-back, break-off of Neo-Tethyan oceanic slab and the break-off of the Indian plate, respectively.5. Detrital zircons from the metasedimentary rocks in lower, middle and upper Nyingchi Complex have been studied in this study. The age distributions of two detrital zircon samples from the lower Nyingchi Complex are dominated by1000-1250Ma and1400～1800Ma. The youngest detrital zircon age is1006±51Ma, which provides a maximum sedimentary age for these metasedimentary rocks. A sample from the volcaniclastic rock overling the metesedimentary rocks yielded an age of507±4Ma, which provides a minimum sedimentary age for these metasedimentary rocks. The age distribution of these detrital zircons is distinctly different from those in the Paleozoic metasedimentary rocks from Tethyan and Great Himalayan sequences, but similar to these in the metasedimentary rocks from Western Australia. This supports the viewpoint that the Lhasa terrane should be placed at the northwestern margin of Australia during the late Precambrian-early Paleozoic. Detrital zircons from the metasedimentary rocks of middle and upper Nyingchi Complex have four major age populations of330～370Ma,450～650Ma,1000～1250Ma and1400-1800Ma. The depositional ages of the protoliths of these metasedimentary rocks are between234and165Ma. The detrital materials mainly derived from the Lhasa terrane. The presence of aboundant330～370Ma detrital zircons indicates a significant magmatism during Devonian and Carboniferous in the Lhasa terrane.6. The whole rock Sr-Nd and zircon Hf isotopic compositions of the granitoids from the western margin of the EHS show that they mainly sourced from partial melting of the old crustal (0.9～1.5Ga) materials, which is different from those in the central part of the southern Lhasa terrane, indicating existence of an Middle-Proterozoic cruatal basement under the southern Lhasa terrane in the western margin of the EHS. The differences between the central and eastern parts of the southern Lhasa terrane probably result from different processes responsible for forming the continental crust during the convergence between India and Asia. Crustal growth mainly occurred in the central part of the southern Lhasa terrane. By contrast, crustal reworking mainly occurred in the eastern part of the southern Lhasa terrane.7. Our results show that the eastern and central parts of the southern Lhasa terrane have a similar tectono-magma evolution history. The northward subduction of the Neo-Tethyan oceanic slab beneath the southern Lhasa terrane resulted in the arc magmatism during Middle Jurassic-Late Cretaceous (165-80Ma). The Neo-Tethyan ocean ridge subduction occurred at-80Ma and resulted in a formation of slab window. The slab window placed the sub-slab asthenospheric mantle against the base of the overlying plate, which resulted in HT metamorphism in the roots of the arc and fore-arc. High heat flow through the slab window further induced partial melting of overlying lower crust to form adakitic magma. Because the ridge is young, hot and thus buoyant, so there is resistance to subduction, leading to flat subduction of the Neo-Tethyan oceanic slab. After a period of shallow-dip subduction, Gangdese magmatism was rejuvenated at-68Ma possibly due to steepening of the subduction angle. At the same time, the asthenospheric mantle filled into the space of previously occupied by oceanic slab, and heated the lower crust, resulting in crustal anatexis and formation of adakitic magma. The break-off of the subducted Neo-Tethyan oceanic slab from the adherent but more buoyant Indian plate occurred at-50Ma. The asthenospheric upwelling heated the base of lower crust and resulted in partial melting of the lower crust to generated adakitic magma (lower crust) and non-adakitic magma (relatively upper crust). The break-off of the subducted Indian continental crust starting at-25Ma, which induced partial melting of the thickened Asian lower crust by thermal advection resulting from asthenospheric mantle upwelling.