Based on the field geological investigation and microstructural observation,we recognized a NEEstriking sinistral thrust ductile shear zone in the Bolejin area of the north of Sonidzuoqi,central Inner Mongolia.The Bolejin ductile shear zone dips northward with angles of 70°-85°.There occur many typical macroscopic structures and microstructures such as mylonite rocks,mylonitic foliation,stretching lineation,S-C fabrics,asymmetrical folds,rotational porphyroclasts,domino structures and biotite-fishes in the ductile zone.In the light of field observation,the Bolejin ductile shear zone,about 10 km in length and 4 km in width,consists of several strong deformational zones and weak deformational domains.The deformational zones are approximately parallel to each other.The macroscopic structures,microstructures of the ductile shear zone,zircon U-Pb chronology,as well as regional geological data suggest that the Bolejin ductile shear zone is a structural feature of the Chaganobo-Arongqi fault.The shear zone was formed at 290-292 Ma in the early Early Permian.The Bolejin ductile shear zone resulted from the S-N collision between the North China and Siberian plates after the final closure of the Paleo-Asian Ocean.Thus,the determination of the Bolejin ductile shear zone has provided constraints on the Late Paleozoic tectonic evolution of the Xing'an Mongolian Orogenic Belt.%通过野外地质调查和室内显微构造分析,在内蒙古中部苏尼特左旗北部勃勒金一带厘定出一条NEE走向,往北倾,倾角70°～ 85°的左行逆冲型韧性剪切带.该韧性剪切带内发育糜棱岩系列、糜棱面理、拉伸线理、S-C组构、不对称褶皱、旋转碎斑、书斜构造和云母鱼构造等宏观和微观构造.野外观察表明,勃勒金韧性剪切带由若干条强变形带和其间的弱变形域组成,平面上呈现平行状展布,出露长约10 km,宽约4 km.在对韧性剪切带宏观、微观构造特征和锆石U-Pb年代学研究的基础上,结合区域资料,认为勃勒金韧性剪切带是区域上查干敖包—阿荣旗深断裂的构造形迹,形成于早二叠世早期(290～292 Ma),是古亚洲洋闭合、西伯利亚板块和华北板块南北向汇聚碰撞的产物.勃勒金韧性剪切带的厘定为兴蒙造山带晚古生代构造演化提供了基础资料.
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