The Deformation And Metamorphic Process Of The Accretionary Complex And Subduction Of The Intra-oceanic Arc In The Maoershan Area,south Qiangtang,Tibet

The study area is located in the northwestern part of Southern Qiangtang terrane of Tibet and on the northern edge of the Longmucuo-Shuanghu suture zone,which is of great significance to the evolution of the Tethys Ocean.Previous studies in this area mainly focused on the petrology of magmatic rocks and metamorphic rocks.However,precisely research on the deformation and metamorphic process of the accretionary complex and subduction of the intra-oceanic arc are relatively insufficient.Therefore,this thesis takes the accretionary complex in the northwest of Southern Qiangtang as the research object to discuss the evolution history of the Tethys Ocean.Guided by the theory of plate tectonics and accretionary orogeny,this thesis applies the tectonic lithologic mapping and metamorphic petrology as the methods,which include the quantitative calculation of shear zone,isotope dating and phase equilibria modeling,aiming to research the deformation and metamorphism of Maoershan accretionary complex and the characteristic of the intra-oceanic arc.The accretionary complex in the Maoershan area shows the typical "matrix+block" structure,which should be a part of the accretionary complex sourced from the subduction of the Longmucuo-Shuanghu ocean.The youngest peak of detrital zircons from part of the schist is around 340-350Ma,and may be came from the volcanic rocks of the Wangguoshan Formation,which formed by the deep subduction of the intra-oceanic arc in the Paleo-Tethys Ocean,showing that the subduction of the intra-oceanic arc is of great significance for the formation of the accretionary complex in the Southern Qiangtang.The Maoershan ductile shear zone has been studied in detail as the boundary between the accretionary complex and sedimentay strata,with the trend mainly shows northwest direction,the dip angle is 20-60°,and the mica Ar-Ar dating shows 259?260Ma.The quantitative analysis shows that the K value is 1.73?8.58,the v value is-0.18?-0.64,the ss value is 1.13?1.57 and the average vorticity is 0.86,indicating that accretionary complex was exposed by the ductile shearing which moved at middle-late Permian in the Southern Qiangtang;3.Phase equilibrium modeling of the high pressure granulite obtain the peak temperature and pressure condition of 930-970? and 15-16kbar.And four-stage retrograde metamorphism process were recorded.The first stage is the initial process of high-pressure granulite facies to medium-pressure granulite facies,which from 930-970? and 15-16kbar to 860-900? and 11.5-12kbr.The second stage is the decompression stage from medium-pressure granulite facies to low pressure granulite facies,which from 880?and 12kbar to 7kar with the near isothermal decompression.The third stage is the cooling stage with amphibole facies,and the fourth stage is the late low-grade amphibolite facies to greenschist facies.The hornblende Ar-Ar result shows that the granulite returned to the hornblende facies at ? 375Ma.During the early Paleozoic,there might be a local continental collision event on the northern margin of Gondwana,and then Paleo-Tethys Ocean expand slowly to rapidly.The comprehensive analysis showed that the northern margin of the Gondwana was an active continental margin in the early Paleozoic,and the SSZ-type ophiolite in the Proto-Tethys Ocean dived to the depth of 44-47 km to form the high-pressure granulite.And retrograded to the hornblende facies at-375 Ma.There was an ancient island arc(340-350 Ma)in the evolution process of the Paleo-Tethys Ocean,and part of the sediments camed from the island arc subducted to the blueschist facies during the later period.Followed by the closure of the Paleo-Tethys Ocean,the accretionary complex which were formed in the deep level exhumated from the late Paleozoic strata,and the greenschist facies ductile shear zone which moved at-260 Ma was developed with top to the south.