Structural Deformation Of Permian Dashizhai And Zhesi Formations In Central Inner Mongolia And Its Implications For The Tectonic Evolution Of Inner Mongolia-Daxinganling Orogenic Belt

Inner Mongolia-Daxinganling Orogenic Belt(IMDOB)located in the eastern segment of the Central Asian Orogenic Belt(CAOB)was developed from Ordovician to Early Permian by the multiple accretions of terranes from different sources,chiefly island arcs and micro-continents.Numerous recent regional studies suggest a long-lived subduction and eventual closure of the Paleo-Asian Ocean from Late Carboniferous to Early Triassic.However,there still exists many fundamental controversies about the compositional assemblages and tectonic environments of early Permian volcanic rocks in Dashizhai Formation.Some workers focused on the mafic and silic components of the Dashizhai formation and treated them as bimodal volcanic rocks formed in a post-collisional extensional setting.Other workers,however,found that intermediate rocks take an important part of Dashizhai Formation in varied localities,opposing against the interpretation of rift environment of the Formation.According to the tectonics evolution of the Paleo-Asian Ocean,Dashizhai Formation was formed in a pre-collision subduction environment.In addition,main and trace element features of Dashizhai Formation indicate that they were formed in Andean arc setting at the early Permian.Furthermore,Dashizhai Formation was interbedded with marine deposits,thick layer crystalline limestone and argillaceous slate,which seems to be from the marine facies volcano lithotype in this study.The residual volcanic primary flowing structure within mylonites implies that mylonization be formed in ductile deformation under conditions of same diagenetic stage at a relative high temperature.The mylonite of Dashizhai Formation was strongly influenced by ductile shear,which kinematic vorticity values(W_k)ranging from 0.77 to 0.97 with an average of 0.91 was estimated by the Polar Mohr diagram,belongs belonging to a thinning shear zone with average rate 9%of the ductile thinning in study area.The quartz C-axis EBSD and the type of recrystallization of the deformed mineral(quartz and modicum feldspar)reveal that its mylonite deforming average temperature is about 300?,and some areas reach up to500?,which can be analogue with metamorphisms of low green schist facies.The mylonite zone was formed in the upper crust settings at the depth of about 5-6 km due to the high rate of geothermal gradient and strain rate.According to the tectonics setting,the thrust transgression ductile shear zone was formed corresponding to subduction stage during the closure of the Paleo-Asian Ocean.In terms of the mylonite shear movement,superimposed folds and independent geochronologic constraints on pluton and strata,three important deformation sequences during Permian in southwestern Erenhot have been clarified in this research.(1)280-277Ma,forming the upper layer shearing was from NNW to SSE in Dashizhai Formation;(2)The middle Permian,forming NW-SE contraction deformations.Macroscopically,the structural styles of P₁ds are mostly neutral folds and produced the NE-SW striking,top-to-the-northwest,thrust faults while P₂z shows a series of SE-vergened asymmetric folds due to the contrast of inter-bedded competence.(3)The late Permian-early Triassic,forming NE-SW contraction deformations.Nearly NE-trending folds formed in early phase have been modified by the NE-SW contraction composing an arc bending south in Dashizhai Formation and contraction produced the W-E striking,south-directed,thrust faults in Zhesi formation,with SW-vergened asymmetric folds on the hanging wall,SWW-vergened asymmetric folds on the footwall wall,Moreover,the changes of regional stress field could be caused by continent rotation-type(i.e.scissor-type)collision between the Southern Mongolian and North China Craton in the Early-Middle Permian.