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

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.