Early Cretaceous Asymmetric Granitic Domes In China-Mongolia Border Area:Extension Duration,Strain Analysis And Regional Tectonic

Large-scale crustal extension represented by metamorphic core complexes and asymmetric granite domes widely spread in Northeast Asia during the Late Mesozoic.The extensional shearing mechanism has always been one of the important issues in tectonic research.Three asymmetric extensional granitic domes,Hanwula,Nalan and Baoder,are spread along the southeast section of the Sino-Mongolian boundary in the NE direction.This study conducted a detailed field survey of these three domes,ascertained their structural composition and kinematic characteristics,and combined zircon ages to constrain the timing of ductile deformations.In addition,the strain and deformation analysis of mylonites in ductile shear zones are carried out to comprehensively discuss the tectonic evolution process of these domes.The main body of these domes are late Mesozoic granitic intrusions.Ductile shear(mylonitic)zones are developed in the northwest margin of these domes,which is confined by the later high-angle normal faults.According to the structural relationship between the strongly deformed medium-coarse-grained K-feldspar granite(133±1Ma)and the weakly deformed fine-grained granite(128±2Ma)in the ductile shear zone of the Hanwula dome and their zircon U-Pb ages,these granites may be syn-extensional intrusions and the timing of ductile extension is after 133 Ma and lasts to 128 Ma or later,which is the same time with the development of other domes in the NE Asia.Observing the microstructure of mylonite,using mineral dynamic recrystallization pattern and EBSD quartz C-axis fabric experiment to estimate the deformation temperature of these domes.The results show that the high temperature fabric remains in the ductile shear zones.The peak period is mostly mid-low temperature deformation,which may indicate that the early extension is high temperature deformation(650?700?)related to syn-extensional magmatism.The extension peak period is dominated by medium temperature(500?600?)and low temperature deformation(400?500?),reflecting the progressive deformation process of shear zone uplift to shallow level.We use Rf/(?) method to measure the finite strain axial rates of feldspar in mylonites,Hsu diagram shows that the strain type is plane-flattening strain type(average k=0.5).Polar Mohr circle method,the rigid particle network method and the<C>-axis quartz fabric method are used to estimate the the long-term deformation processes of ductile shearing.They gave average kinematic vorticity values of mylonites are 0.74?0.68,which indicated that the ductile shearing of these domes were dominated by general shear with nearly equal pure shear and simple shear components.Kinematic vorticity values of the later stage of ductile deformation obtained by the quartz diagonal strip method are 0.87?0.99 with an average value of 0.93,which indicated that the ductile deformation converted from general-shear-dominated to simple-shear-dominated.However,kinematic vorticity values obtained by the C'(Stretch fold cleavage)method are 0.53?0.74 with an average of 0.68,which indicated the latest incremental strain in progressive deformation process is mainly pure shear.The results show that the ductile shear detachment zones of these domes are mainly formed by general shear.In the early stage,the simple shear component gradually increases.In the later stage,the simple shear component decreases due to the processes of magma upwelling and overall exhumation,the shear deformation is dominated by pure shear.Compared with other extensional domes and metamorphic core complexes in NE Asia at the same period,this shearing pattern may be of general significance.