中蒙边界早白垩世不对称花岗岩穹隆的伸展时限、剪切作用类型和区域构造意义

东北亚大陆于晚中生代时期发生了大规模地壳伸展,发育变质核杂岩和不对称花岗岩穹隆,其伸展剪切机制一直是构造研究的重要内容之一。中蒙边界东南段沿北东向展布了罕乌拉、纳兰和宝德尔等3个不对称花岗岩伸展穹隆,主体均为晚中生代花岗岩侵入体,岩体西北缘发育韧性剪切(糜棱岩)带,并被后期高角度正断层所围限,整体为穹隆状。根据罕乌拉穹隆韧性剪切带内强变形中粗粒钾长花岗岩(133±1 Ma)和弱变形细粒花岗岩(128±2 Ma)的构造关系及其锆石U-Pb年龄,推测该穹隆内岩体可能为同伸展岩体,韧性伸展时间在133 Ma之后并持续至128 Ma或更晚,与同区其他穹隆发育时限相同。笔者用R_f/ф方法测量了3个穹隆剪切带内糜棱岩中长石的有限应变轴率,利用Hsu图解获得其应变类型为平面压扁应变(k=0.5)。用长石极莫尔圆法、刚性颗粒网法和C轴石英组构法估算了韧性剪切的长期变形过程,得到糜棱岩的平均运动学涡度值为0.68～0.74,表明这些穹隆的韧性剪切作用主要是纯剪切和简单剪切分量几乎相等的一般剪切作用。石英斜向条带法测得的韧性变形后期的运动学涡度值为0.87～0.99,平均值为0.93...

Timing of extention, type of shearing and regional tectonic implications of the Early Cretaceous asymmetric granitic domes in China-Mongolia border area

Large-scale crustal extension happened in Northeast Asia continent during the Late Mesozoic,represented by metamorphic core complex and asymmetric granite domes.The extensional shearing mechanism has always been one of the important issues in tectonic research.Three asymmetric extensional granitic domes,including Hanwula,Nalan and Baoder,are spread along the southeast section of the China-Mongolia boundary in the NE direction.The main body of these domes are the Late Mesozoic granitic intrusions.Ductile shear(mylonitic)zones are developed in the northwest margin of these domes,which are confined by the later high-angle normal faults.According to the structural relationships between the strongly deformed medium coarse-grain K-feldspar granite(133±1 Ma)and the weakly deformed fine-grain granite(128±2 Ma)in the ductile shear zone of the Hanwula dome and their zircon U-Pb ages,it is inferred that 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 as the development time of other domes in the NE Asia.We use R_f/фmethod to measure the finite strain axial rates of feldspar in mylonites from the three ductile shears,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 long-term deformation processes of ductile shearing.The methods gave average kinematic vorticity values of 0.68~0.74 of mylonites,which indicated that the ductile shearing of these domes was 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 that 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 increased.In the later stage,the simple shear component decreased 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 kind of shearing pattern may be general significance.