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  

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

作　　者：杜灿 郭磊[1] 王涛[1] 李建波[3] 童英[1] NARANTSETSEG Tserendash ENKH-ORSHIKH Orsoo 张磊[1] DU Can;GUO Lei;WANG Tao;LI Jian-bo;TONG Ying;NARANTSETSEG Tserendash;ENKH-ORSHIKH Orsoo;ZHANG Lei(Institute of Geology,Chinese Academy of Geological Sciences,Beijing 100037,China;School of Geosciences and Resources,China University of Geosciences(Beijing),Beijing 100083,China;School of Geosciences,East China University of Technology,Nanchang 330013,China;Institute of Geology,Mongolian Academy of Sciences,Ulaanbaatar 15160,Mongolia)

机构地区：[1]中国地质科学院地质研究所,北京100037 [2]中国地质大学(北京)地球科学与资源学院,北京100083 [3]东华理工大学地球科学学院,江西南昌330013 [4]蒙古科学院地质研究所,蒙古乌兰巴托15160

出　　处：《岩石矿物学杂志》2022年第1期18-36,共19页Acta Petrologica et Mineralogica

基　　金：国家自然科学基金项目(41972225);国家重点研发计划(2017YFC0601305);中国地质调查局项目(DD20190001,DD20190004)。

摘　　要：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,表明韧性变形由一般剪切为主转变为简单剪切为主。然而,由C’法(伸展褶劈理法)获得的运动学涡度值为0.53~0.74,平均为0.68,表明递进变形中的晚期增量应变主要是纯剪切,可能指示了后期同伸展岩浆持续上涌导致穹隆整体抬升和剥露。估算结果表明,这些穹隆的韧性剪切拆离带由一般剪切作用形成,前期单剪组分逐渐增加,后期因岩浆上涌和整体剥露抬升,简单剪切分量降低,以纯剪切作用为主。对比同时期早白垩世东北亚地区其他伸展穹隆和变质核杂岩,认为此种剪切模式可能具普遍意义。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 cl

关 键 词：花岗岩穹隆 糜棱岩 应变测量 运动学涡度 中蒙边界 

分 类 号：P588.121[天文地球—岩石学] P542[天文地球—地质学]