Study On Metamorphism-deformation Characteristics Of The Bulunkuole Group In The Western Part Of West Kunlun Orogenic Belt

The West Kunlun orogenic belt,an important part of the Central Orogenic System,is located in the northwest of Tibetan Plateau.The West Kunlun orogene which formed a key tectonic boundary between Pan Asian and Tethyan tectonic domain,experienced multicycle collisional orogeny.The Bunlunkuole group within the orogenic belt subjected to multiphase tectonic deformation and metamorphism recorded multi-stage metamorphism-deformation activities.Study based on a typical section of the Bulunkuole group provided enormous information about petrology,petrography,metamorphism and deformation.The group is composed of several different rocks,such as schist,gneiss,leptynite,mylonite,marble.In this paper,schist and gneiss are selected to examine geochemical compositions.The compositions of major elements,rare elements and trace elements and occurrence indicate that the schist in this group is parametamorphic rock and protoliths of the schist are argillaceous rocks,argillaceous sandstone and siltstone.The primary rock of the plagioclase gneiss is sandstone or greywacke.The original rocks of the hornblende bearing gneiss consist of greywacke and a small amount of pyroclastic sedimentary rocks.According to mineral paragenesis and their interrelationships,metamorphism in the Bulunkuole group are divided into four stages.The first stage(M1)which formed progressive metamorphic zone is regional dynamo thermal flow metamorphism with greenschist facies-amphibolite facies.In the second stage(M2),greenschist facies dynamic metamorphism is dominated and forms mylonitic rocks.Pneumatolytic hydrothermal metamorphism(M3)is characterized by trichoid scattered sillimanite replacing biotite which exists in both sides of strata of pegmatite veins.LAICPMS U-Pb dating of zircons from plagioclase hornblende gneiss yields metamorphism age of Early Yanshan Stage(179±1.4Ma).The forth stage(M4)shows greenschist facies retrogressive metamorphism and low-temperature minerals assemblages without directional arrangement replacing high-temperature minerals assemblages.The Bunlunkuole group in study area underwent four phases of deformation at least.The first stage regional deformation(D1)generates regional schistosity and gneissic schistosity.In the second stage(D2),bedding compressional deformation forms folds.Ductile strike-slip deformation(D3)produces small ductile shear belt and mylonite.The forth stage(D4)underwent overthrusting deformation and generated thrust-nappe structures.Based on the study above,the Bunlunkuole group was subjected to some sophisticated metamorphism-deformation evolution process and their relationships.Regional dynamo thermal flow metamorphism(M1)event was almost simultaneous with regional deformation(D1).Afterwards bedding compressional deformation(D2)took place and mainly lead to distortion.The dynamic metamorphism(M2)followed,and meanwhile ductile strike-slip deformation(D3)occurred.The forth stage overthrusting deformation(D4)occurred successively and just lead to deformation.Then Pneumatolytic hydrothermal metamorphism(M3)and the latest stage retrogressive metamorphism(M4)transformed Bunlunkuole group in sequence,and are mainly about metamorphism.