| The research area is located in Zhiduo county,Yushu Tibetan autonomous,Qinhai province,which belongs to the east sector of Tethys-Himalayas tectonic process,and is situated between Songpan-Ganzi continental block and Qiangtang block,and the west sector of Jinsha River Suture zone.The tectonic Mélange About the material composition,properties and the characteristics and stage of the metamorphic deformation,which are controversial.We have a clear understanding through the 1:50000 geological mapping,1:2000 geological section of mélange,and clarify the structure of space distribution in the study area with the study of geometry and kinematics of mélange in this area.And combined with the zircon U-Pb isotopic age,this paper obtains some understandings and processes on the formation of mélange,which are described below.The Zong kequ mélange zone mainly consists of basic lava,intermediate acid volcanics,gabbro-diabase,external limestone blocks and flysch matrix.Basic lave could be classified into two series,one of which is the tholeiitic series that consist of mid-ocean ridge tholeiite and island-arc volcanic rocks.The other is oceanic island basalt whose magma origined from high degree melting asthenosphere.The material composition reflects the characteristics of Jinsha River ocean basin formation and ocean-land plate collision and subduction.Zong Kequ mélange zone metamorphic mineral assemblages includes both characteristic minerals,such as sericite and chlorite,and the pale blue-green amphibole,biotite,but grenatite didn't appear,which above show the characteristic of low greenschist facies and high greenschist facies.Sericite-chlorite metamorphic belt later superposed dynamic metamorphism and thermal metamorphism.By the research of the interior tectonic patterns in the different sector of the mélange profiles,the deformations of Zong Kequ mélange can be divided into four stages:(1)Deep level tectonic deformation in the early subduction process that was characterized by ductile shear zone and rootless folds,whose attitude is 355°?60°,that indicated the left-trending ductile shearing,represents the strong compression in the early subduction stage.(2)Middle-deep level brittle-ductile deformation stage that was characterized by strong foliated sandstone,siltstone,schist,which were mixed with a large number of blocks,and developed folds and fractures,indicated the left-strike-slipping shear,and the attitude was about 152°.(3)Middle-deep level brittle-ductile deformation in the collision stage that was characterized by the south-west thrust fault and ductile shear zone,which indicated the left-trending ductile shearing movement.(4)Shallow-Middle level brittle deformation in the post collision stage developed abundant folds that were plunging vertical folds and inclined folds based,and their kinematic characteristics indicated left-lateral strike slip.Intermediate volcanic rocks and acidic volcanic rocks in Zongkequ melange belt amount of rare earth elements(? REE)between 63.39 and 244.01 percent,Total light rare earth elements(? LREE)is higher than the total heavy rare earth elements(? HREE),(La/Yb)N between 3.05 ~ 7.67,(La/Sm)N between 2.29 ~ 5.01,(Gd/Yb)N between 1.06 ~ 1.41,Belong to the rich light soil type,Light and heavy rare earth element fractionation is obvious,Eu imform negative anomaly from the graph,Eu losses increase gradually from intermediate volcanic rocks to acidic volcanic rocks,it Shows that magma from neutral to acid plagioclase crystallization enhanced gradually,Overall similar to orogenic belt volcanic rock,The single zircon U-Pb age of andesite was 226.6±4.58 Ma,which showed that formed in the end of late Triassic,it Shows that the southward subduction of jinsha river-kekexili ocean and the north qiangtang-changdu plot was started on or before the Permian in Zhiduo county area,and the subduction should be continued in late Triassic.Based on the petrological,geochemical,metamorphic and deformation characteristics of Zong kequ melange zone,the tectonic evolution history could be basically summarized as the following several stages:(1)The stage of crystalline basement formation in the Paleo-mesoproterozoic;(2)the stage of Carboniferous-Permian Jinsha River oceanic basin formation,expansion and closing;(3)the stage of Triassic ocean and land conversion and mélange formation. |
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