Tectonic Evolution And Uplift Process Of The Nyainrong Block,Tibet

Nyainrong block with lens-shape is located within the middle of Bangong-Nujiang suture(BNS)zone between Qiangtang block and Lhasa block.It has a extremely complicated tectonic geology,and the only tectonic block with the outcropping of a wide range of metamorphic basement.The geological research of Nyainrong block is very poor,which restricts geologists from understanding the histories of tectonic evolution of BNS and regional geology more deeply.So,Nyainrong block and the north and south of its area is the main research object of the paper,and systematically studies the geochronology and geochemistry of orthogneisses and Mesozoic granites and Cenozoic tectonic by field observation and fission track dating.Then combing the previous researches,this paper discusses that tectonic evolution and uplift process of Nyainrong block.The paper systematically studies the geochronology and geochemistry of granitic gneisses from the basement of Nyainrong block.These data provide constraints on formation age,experienced tectonic events,source characteristics and tectonic setting of their protolith.The LA-MC-ICPMS zircon U-Pb dating is operated for granitic gneisses.All samples(granitic gneiss)showed the distribution of 830-720Ma,580-520Ma,510-470Ma and 175-165Ma.These ages should represent the different tectonic events.Cambrican-Ordovician granitic gneisses are characterized by high SiO2,Na2O,K2O and Al2O3,low Fe and Mg,enrichment in light rare earth elements(LREEs)and large ion lithophile elements(LILEs),depletion in heavy rare earth elements(HREEs)and high field strength elements(HFSEs),with negative Eu anomaly.The Rittmann index(σ)is 1.77 to 2.60,less than 3.3.The aluminum saturation index(A/CNK)values range from 0.88 to 1.26.These features suggest that protolith of granitic gneisses from Amdo basement show characteristics of calc-alkaline and S-type granite,and they could be derived from partial melting of metamorphic greywackes in the upper crust of low maturity.The tectonic setting is syn-collision.U-Pb zircon ages in gneissic rocks revealed Neoproterozoic crystalline crust in the basement of Nyainrong block,and It is suggested that the basement of Nyainrong block experienced the tectonic events of early Neoproterozoic breaking up of Rodinia and late Neoproterozoic Pan-Africa-Brazilian Orogeny,then occurred circum-Gondwana caused by proto-Tethyan oceanic subduction along Gondwana continental margins in Cambrican-Ordovician,and Jurassic metamorphism and magmatism which was related to the closure of Bangong-Nujiang suture zone.Zircon U-Pb dating was conducted for six granites in Nyainrong block by LA-MC-ICPMS.Four granite samples dated at ca.166.2-175.8 Ma and two granite samples dated at ca.101.8-114.4 Ma.Then these dates indicated that the formation age of granites is early-middle Jurassic and early Cretaceous.All granite samples are calc-alkaline and shoshonite series,enriched in Th,U,and light rare earth elements,and depleted in Nb,Ta,Sr,Ti and P,with negative Eu anomaly.The early-middle Jurassic and early Cretaceous granites were formed from the tectonic setting of syn-collision.It is suggested that the subduction and closure of Bangong-Nujiang suture zone led to the formation of Mesozoic granites.By field survey,two thrust nappe structures exposed in the north and south of Nyainrong block.They were composed of many thrust faults with EW-trending,outliers and nappes.The thrust faults dip to north or south.In addition,cleavage,joint,fold,quartz and calcite veins develop in thrust faults.Based on the ages of the layers of hanging wall and footwall of thrust faults and previous dating data,the paper considered that the formation age of thrust faults were middle-late Cretaceous and middle-late Paleogene,and was related to the subduction and closure of Bangong-Nujiang ocean basin and Indo-Eurasian collision,respectively.Fission track dating was applied to analyze the 20 samples from Nyainrong block,and this paper obtained 20 apatite and 15 zircon fission track ages.The results show single population grain ages with a single mean age and associated central ages mainly ranging from 108±7Ma to 35±4Ma.Their mean track lengths are 12.2-13.9μm with a single peak.Zircon fission track age ranges from 78±3Ma to 117±4Ma.The results represent the two tectonic uplift events in the study area,namely the middle and late of Cretaceous and the middle and late of Paleogene periods.According to thermal history modeling results,uplifting rates of two tectonic events is 0.10 mm/a-0.22 mm/a and 0.03 mm/a-0.09 mm/a,respectively.Combined with field condition and study results,it is suggested that the Cretaceous tectonic uplift event was related to the closure of ocean basin and Qiangtang-Lhasa collision,and the Paleogene tectonic uplift event was related to Indo-Eurasian collision.Based on the analysis of gravel fabric characteristics of conglomerate layers around of Nyainrong block of the Late of Cretaceous-Paleogene in the southeast of Qiangtang Basin,Tibet,this paper elaborates character,origin,source,hanging way and tectonic background of the conglomerate,combined with the regional geology.The results of study shows that conglomerate layers are the short-middle distance of the product of alluvial-proluvial role.The gravels have the characteristics of complex composition,different content and good sorting.The paleocurrent of these statistical points possess the characteristic of a single flow direction,and the condition of hydrodynamic force is relatively stable.The analysis results of the compositional features and orientations of the gravel revealed that the gravels are derived from Nyainrong block,Bangong-Nujiang suture zone and surrounding mountains of them,indicated the sediment feature of the water system distributed in the front of mountains,and were formed by alleviation firstly,proluvial role secondly.To combine with regional,it suggested that in the Late of Cretaceous the north of Nyainrong block were uplifting,formed the palaeogeographic pattern which were higher in the northeast and in the lower southwest;in Paleogene,the study area which preserved the previously palaeogeographic pattern were integrally uplifting.So,it concluded that the uplifting area of Tibet occurred in the north of Bangong-Nujiang suture zone in the Late of Cretaceous,and Tibet was uplifting as a whole in Paleogene.According to the analytical results of this paper and previous researches,this paper discussed the tectonic evolution and uplifting process of Nyainrong block.