内蒙古北山地区石板井韧性剪切带西段变形特征及年龄约束

    Geochronological constraints and deformation of the western section of Shibanjing ductile shear zone in Beishan, Inner Mongolia

    • 摘要: 石板井韧性剪切带为内蒙古北山地区4条重要的蛇绿岩带之一的石板井-小黄山构造带的重要组成部分。通过宏观、微观、显微构造形态分析发现,石板井韧性剪切带西段先后经历了2期北西西向略带逆冲分量的右行走滑型韧性剪切变形,2期韧性变形之间还发生一次明显的静态恢复重结晶作用。根据定向标本中石英、长石的显微变形特征、糜棱物中新生矿物组合及石英C轴组构分析,判断主期韧性变形温度为400~550℃,为高绿片岩相,静态恢复重结晶阶段形成环境同样为高绿片岩相,而晚期韧性变形温度为280~400℃,为低绿片岩相。根据剪切带内及周边经历不同变质变形阶段的侵入岩LA-ICP-MS锆石U-Pb年龄,限定主期韧性变形及静态恢复重结晶阶段形成时限为435.7~405.0 Ma,晚期韧性变形形成时限为405.0~363 Ma。此外,石板井韧性剪切带是在塔里木板块和哈萨克斯坦板块沿牛圈子—蒜井子—白云山一线斜向碰撞背景下,由沿石板井—小黄山展布的古老深大断裂活化形成的。其中,早期变形形成于碰撞造山时期,晚期变形形成于碰撞造山向后造山伸展转换时期。

       

      Abstract: The Shibanjing ductile shear zone is an important component of the Shibanjing-Xiaohuangshan structural belt, one of the four ophiolite belts in Beishan area.Based on the macroscopic and microscopic structural morphology, we found that the western part of the Shibanjing ductile shear zone had undergone two stages of ductile shear deformations of dextral strike slide accompanied with a small portion of NWW-trending thrusting.The main ductile deformation was punctuated by a stage of static recrystallization.The analysis of microstructures of the quartz and feldspar, the newly-developed mineral assemblage in the mylonite, and the quartz C-axis indicates that the temperature of the main ductile deformation remained at 400~500℃ (high greenschist facies).The static recrystallization was also developed in a high greenschist facies environment, while the later-stage ductile deformation was present at 280~400℃ of low greenschist facies.Based on laser ablation (LA-ICP-MS) U-Pb dating of zircons from intrusive rocks undergoing the metamorphism and deformation stages in and around the shear zone, it is suggested that the age of the main ductile deformation and static recrystallization is 435.7~405.0 Ma, and the time limit of the later-stage ductile deformation is 405.0~363 Ma.Under the background of oblique collision between the Tarim plate and the Kazakhstan plate along the Niujuanzi-Suanjingzi-Baiyun shan, the Shibanjing ductile shear zone were formed by the reactivation of pre-existing deep-seated faults along the Shibanjing-Xiaohuangshan, the main ductile deformation occurred in collisional orogeny, while the later-stage ductile deformation occurred in transition period between collisional orogeny and post-orogenic extension.

       

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