Tectonic Evolution Of The Tangjia-Sumdo Ophiolitic Mélange Belt,Tibet

Opening,spreading and subduction of the Paleo-Tethys ocean during the Paleozoic-early Mesozoic were related to the separation and convergence of terranes in the east Cimmerides and west Cathaysides.The Tangjia-Sumdo ophiolitic melange belt in Tibet,as the southernmost relic of the Paleo-Tethys Ocean on the Qinghai-Tibet Plateau,not only plays an important role in understanding the tectonic framework of the Qinghai-Tibet Plateau,but also has an important constraining significance in exploring the ocean-land distribution pattern and its tectonic evolution process of the global Paleo-Tethys domain.However,previous studies on Tangjia-Sumdo ophiolitic melange belt mainly focus on the eclogites and lack of ophiolite and island arc-related magmatism,the tectonic evolution of Sumdo Paleo-Tethys Ocean and the late Paleozoic paleogeography of the Lhasa terrane are still controversial.In order to reconstruct the evolution of the Sumdo Paleo-Tethys Ocean,it is crucial to study on the rift-related magmatism,fragments of the oceanic lithosphere and island arc magmatic rocks in the Tangjia-Sumdo ophiolitic melange belt.The study on rift-related magmatism can be used to invert the opening time and mechanism of the ancient ocean.The fragments of the oceanic lithosphere such as ophiolite and oceanic island are the direct records of the evolution of the ocean,and the island arc magmatic rocks provide important information for the reconstruction of the oceanic subduction process.On the basis of large-scale regional geological survey,we report the results of petrological,petrographic,zircon U-Pb geochronology and Hf isotope,and whole-rock geochemistry and Sr-Nd isotope data of the early Carboniferous to late Triassic magmatism in Tangjia-Sumdo ophiolitic melange belt.On the basis of the studies,combined with the previous results,we reveals the evolution history of Tangjia-Sumdo ophiolitic melange belt and initially reconstruct the spatiotemporal evolution of the Sumdo Paleo-Tethys Ocean,and further discusse the paleogeographic pattern of the Lhasa terrane in the late Paleozoic.The early Carboniferous magmatic rocks in Tangjia-Songdo ophiolitic melange belt include Sumdo meta-gabbro and Tangjia meta-basalt.Zircon U-Pb geochronology of meta-basalt and meta-gabbro yielded crystallization ages of 338.1±6.5 Ma and 345.3 ± 3.4 Ma,respectively.Geochemical characteristics of meta-gabbro are similar to those of oceanic island basalt and have positive whole-rock εNd(t)values(+0.08 to+1.03)and zircon εHf(t)values(+3.4 to+4.9),suggesting that they originated from asthenospheric mantle with minor contributions from lithospheric mantle.The meta-basalt yields positive whole-rock εNd(t)values(+6.47 to+6.60 and zirconεHf(t)values(+14.2 to+15.2)and has E-MORB-like geochemical characteristics;we suggest that the meta-gabbro was sourced from the depleted mantle and interacted with enriched components.The meta-gabbro was formed in the introntinental rift environment,while the meta-basalt was formed in the spreading ridge setting in an initial ocean basin,representing the oldest oceanic crust of the Sumdo Paleo-Tethys Ocean.Based on the geochemical characteristics,the Permian fragments of oceanic crust in the Tangjia-Sumdo ophiolitic melange belt were divided into four groups,depleted and tholeiite mafic rocks(DTMRs),transitional mafic rocks(TMRs),enriched and alkaline mafic rocks(EAMRs)and picritic ultramafic rocks(PUMRs).DTMRs have the oldest crystallization ages in late Early Permian(～272 Ma),followed by PUMRs(～270 Ma)and TMRs(267-254 Ma)in the Middle-Late Permian;EAMRs(252-250 Ma)yield the youngest ages in the Late Permian.Geochemical characteristics of DTMRs,TMRs,EAMRs and PUMRs are similar to those of N-MORB,E-MORB,OIB and depleted-type picritic mafic rocks in other oceanic plateaus,respectively.The geochemical characteristics of the DTMRs suggest that they formed by partial melting of depleted spinel lherzolite in a mid-ocean ridge setting whereas the geochemical characteristics of the PUMRs suggest a high degree of partial melting of depleted lherzolite in a hot mantle plume head at an early stage.This was followed by the TMRs that formed by moderate degrees of partial melting of moderately enriched garnet lherzolite.Subsequent EAMRs could have formed by low degrees of partial melting of a more enriched garnet lherzolite at relatively low temperatures.We propose that the DTMRs represent normal oceanic crust of the Sumdo Paleo-Tethys Ocean and the TMRs,EAMRs and PUMRs are the fragments of the oceanic plateau in the Sumdo Paleo-Tethys Ocean.The middle Permian island arc magmatic rocks in Tangjia-Sumdo ophiolitic melange belt are mainly distributed in Tangjia and Wenmulang areas,including Tangjia high-Mg quartz diorite,Wenmulang trondhjemite and Wenmulang adakitic granite.Zircon U-Pb geochronology of meta-basalt and meta-gabbro yielded crystallization ages of 269～257Ma.We interpreted the high-Mg quartz diorite derived by the interaction of fluids expelled from the subducting slab with the overlying mantle wedge.The trondhjemite may have been derived from partial melting of hydrated basaltic/gabbroic protoliths.The adakitic granite was derived from partial melting of subducted oceanic slab.The trondhjemite and adakitic granite only slightly postdate the oceanic crust basement they intrude,we suggest that middle Permian magmatism occurred in a plume-induced initial intra-oceanic subduction setting in the Sumdo Paleo-Tethys ocean.In addition,we first report the Longyasumdo serpentinite from the Tangjia-Sumdo ophiolitic melange belt.The results show that Longyasongduo serpentinites have high MgO,TFeO and Mg#values but low Al2O3 and TiO2 contents.The chondrite-normalized REE patterns display a U-type.The primitive mantle-normalized spider diagrams exhibit enrichment in U,Ta and depletion in Th,Nb,Zr and Hf.The protolith of Longyasongduo serpentinite is the residual of partially melted spinel lherzolite in the mantle wedge.Metasomatic zircon has been found in Longyasumdo serpentinite,zircon U-Pb dating of Longyasongduo serpentinite yielded ages of 230.3±2.3 Ma,with εHf(t)values of+13.4 to+16.0.Through chronological and geochemical studies,we argue that the mantle wedge was metasomatized by supercritical fluid generated by subduction of Sumdo Paleo-Tethys oceanic crust to form metasomatic zircon during the late Triassic.Based on the discussion and previous results,a spatio-temporal evolution model of the Sumdo Paleo-Tethys Ocean is established.In the late Devonian to early Carboniferous,the initial rifting of the Sumdo Paleo-Tethys Ocean was induced by the subduction of the main Paleo-Tethys Ocean.During the Carboniferous,the Sumdo Paleo-Tethys Ocean transitioned from a continental rift to a mature oceanic setting.This was followed by northward subduction from the Permian to early late Triassic.Meanwhile,mantle plumes may have been active in the ocean,which resulted in the formation of the middle-late Permian oceanic plateau.In the late Triassic to early Jurassic,the Sumdo Paleo-Tethys Ocean closed and followed by tectonic transition from Paleo-to Neo-Tethys Ocean.Based on the regional geological evidence,we infer that the Lhasa terrane experienced two separation events from Gondwana.The northern Lhasa subterrane drifted northward away from the northern margin of Australia Gondwana,forming the Sumdo Paleo-Tethys Ocean during the late Devonian to early Carboniferous.The Sumdo Paleo-Tethys Ocean was a limited ocean basin in the eastern Lhasa terrane rather than an expansive ocean that completely split northern Lhasa subterrane from Australian Gondwana.In the middle Permian,a new stage of extensional magmatism occurred in the northern margin of Australian Gondwana and then gave birth to the Neo-Tethys ocean,which eventually separated the Lhasa terrane from northern margin of the Australian Gondwana.