内蒙赤峰地区若干主干断裂带的构造热年代学

黑里河-宋三家和西拉木仑近东西向断裂的宏观和显微组构特征均指示了右行走滑剪切；小城子-八里罕北北东向断裂显示了平面上的左行剪切和剖面上的正断式剪切。根据断裂糜棱岩中长石残斑与韧性基质在粒径上的显著差异，采用逐级破碎、逐级分选的方法，逐次剔除长石残斑。从细粒韧性基质中分选出钾长石、斜长石和黑云母，进行糜棱岩矿物内部Rb—Sr等时线定年和单颗粒黑云母的激光熔化^40Ar／^39Ar定年。定年结果显示，糜棱岩化促进了手标本尺度上的同位素均一化，造成了原岩同位素时钟的重置。定年结果表明，黑里河-宋三家断裂变形的年龄为232Ma；西拉木仑断裂右行挤压走滑的年龄为165Ma。蒙古古生代岩浆弧与华北地块北缘的碰撞拼贴，以及华北-蒙古联合地块与西伯利亚板块的碰撞拼贴的远程效应。导致了上述2期变形。小城子-八里罕断裂的左行和正断式剪切的年龄介于127～117Ma，与喀喇沁核杂岩的快速隆升时期相同。壳-幔隆升促进了中下地壳的韧性化和拆离，导致了北北东向断裂的左行和正断式剪切。

Thermochronology of three major faults in the Chifeng area, Inner Mongolia of China

On the basis of investigation of the macroscopic and microscopic characteristics of deformation and kinematics of the Heilihe-Songsanjia and the Xilamulun near E-W-trending faults and the Xiaochengzi-Balihan NNE-trending fault in the Chifeng area, Inner Mongolia, this paper performed biotite, K-feldspar, plagioclase, and whole rock internal Rb-Sr isochron dating of mylonitic rocks, and single-grain laser fusion 40Ar/39Ar dating of biotite from these faults. Deformational features of the two E-W-trending faults have shown dextral shearing, whereas the Xiaochengzi-Balihan fault is characterized by sinistral shear on the horizontal plan and normal slip in the cross-section. According to the difference of feldspar porphyroclast and ductile matrix in grain size, the mylonitic rock samples collected from the three faults were broken to successively finer grains from which K-feldspar, plagioclase, and biotite were separated after removal of feldspar porphyroclasts. Mineral internal Rb-Sr isochron dating yielded reliable results. It shows that mylonitization has caused complete isotope resetting of the ductile matrix from the fault mylonitic rocks. Age of deformation along the Heilihe-Songsanjia fault is dated at 232Ma, whereas the shearing and sliding of the Xilamulun fault is dated at 165 Ma. The collisional suturing of the Mongolian Paleozoic magmatic arc with the North China block in the early Mesozoic, and far field effect of the collisional suturing of the combined North China-Mongol block with the Siberia plate from Late Jurassic to Early Cretaceous are responsible for the above two stages of deformation, respectively. Ages of the sinistral shear and normal slip of the Xiaochengzi-Balihan fault are dated at about 125Ma and 117Ma. Both ages are concurrent with the rapid uplift of the Harkin metamorphic core complex. We concluded that the mantle-crust upwelling prompt ductility in the middle-low crust and detachment of cover rocks from the basement. These conditions eventually resulted in the sinistral shear and normal slip along the NNE-trending faults.