Deformational Characteristics And ⁴⁰Ar/³⁹Ar Geochronology Of The Ductile Shear Zone In The North Margin Of Yunkai Block,Southeastern Guangxi

The ductile shear zone of northern margin of Yunkai block outcropped between Cathaysia plate and Qinzhou-Hangzhou joint belt in the South China plate southwest.And it is one of the most complex geological structure deformation and metamorphism in Guangxi orogenic belt.Based on the field geologic observation?structural geology,microtectonics analysis,rock fabric science and ⁴⁰Ar/³⁹Ar geochronology of muscovite,we summarize deformational characteristics and geochronology of the ductile shear zone of Shili,Ling Shan-Huang Ling and Xicun-Nabu in the north margin of Yunkai block,and discuss dynamic evolution mechanism.Achievements of study are as follows:?1?Three ductile shear zones are formed the environment of meso-low temperature deformation.The Shili ductile shear zone is a ductile shear zone with the property of sinistral thrusting;The Ling Shan-Huang Ling ductile shear zone is a ductile shear zone with the property of dextral stretch gliding nappe and strike-slip;The Xicun-Nabu ductile shear zone is a ductile shear zone with the property of dextral strike-slip and thrusting nappe structure.?2?From weak deformation to the strong deformation zone,quartzs of the Shili ductile shear zone appear undulatory extinction,banded extinction?irregular rectangular quartz deformation belt?quartz S-C fabric?asymmetry rotated porphyroclast system?polycrystalline quartz ribbons such as microscopic fabric in turn;Feldspars appear the microfracture of transgranular?mechanical twin?boudinage?rotated porphyroclast system;Micas appear mica-fish textures?bookshelf structure?kink band?microscopic delamination.We have acquired ⁴⁰Ar/³⁹Ar plateau age of 155.33±0.91Ma,it has been shown that the ductile shear zone may be occurred in middle-lower crust shortening process of ocean basin subduction-accretion,arc-continental or continental collision orogen in late Jurassic.?3?From weak deformation to the strong deformation zone,quartzs of the Ling Shan-Huang Ling ductile shear zone appear undulatory extinction?deformation lamella?ribbons of quartz?core-mantle structure,subgrain?rotated porphyroclast system?S-C fabric such as microscopic fabric in turn;Feldspars appear the microfracture of transgranular,intergranular failure?mechanical twin?bookshelf structure?boudinage?rotated porphyroclast system;Micas appear mica-fish textures and sheared step?microscopic stratification structure?fold structure.We have acquired ⁴⁰Ar/³⁹Ar plateau age of222.45±1.27 Ma,it has been shown that the ductile shear zone was formed in the post-orogenic stage with stretching and thinning of the crust in late Triassic.?4?From weak deformation to the strong deformation zone,quartzs of the Xicun-Nabu ductile shear zone appear undulating extinction?banded extinction?irregular rectangular quartz deformation belt?fining recrystallization subgrain?rotated porphyroclast system?boudinage such as microscopic fabric in turn;Feldspars appear the microfracture of transgranular?sheared step?mechanical twin?core-mantle structure?S-C fabric?rotated porphyroclast system;Micas appear mica-fish textures?rotated porphyroclast system?bookshelf structure?microscopic delamination.We have acquired ⁴⁰Ar/³⁹Ar plateau age of 232.38±1.70 Ma,it has been shown that the ductile shear zone may be formed in middle-lower crust shortening process of arc-continental or continental collision orogen in middle Triassic.?5?Based on the former investigations,and the paper'research about deformation characteristics of three ductile shear zones in the the north margin of the Yunkai block and⁴⁰Ar/³⁹Ar geochronology study,we believe that the ductile shear zone of Xicun-Nabu and Lingshan-Huangling are sepsrately products of the resurrection of the NE structural belt between Cathaysia and Yangtze plates?the north margin of Yunkai block?.That are caused by the collision stage in middle Triassic and post-orogenic crustal extension in late Triassic of South China and Indian plates.And yet,Shili ductile shear zone could be the products of the romote and oblique stress field that the subduction-induced of Pacific plate and Eurasian plate?the south China plate?in late Jurassic.