1) Department of Earth Sciences, Faculty of Land Resource Engineering, Kunming University of Science and Technology, Kunming, 650093, China2) National Research Center for Geoanalys, Beijing 100037, China
Abstract:The northern margin of North China block experienced multiple tectonic events in the late Paleozoic, with complex and diverse tectonic activities. The Xiajing-changhanying ductile shear zone located in the middle segment of the North China block had a strong tectonic deformation. The study of the deformation characteristics and formation age of the ductile shear zone is of great significance to further explore the tectonic evolution of the northern margin of the North China block, especially the time and mechanism of oceanic subduction crust in the late stage of Paleo-Asian Ocean. In view of this, the kinematic and geometric profiles, microstructural analysis and zircon LA-ICP-MS U-Pb geochronology of the ductile shear zone were carried out in order to define the deformation characteristics and time of the ductile shear zone. The protoliths in the shear zone are a set of dioritic-granitic magmatic rocks, which were formed by strike slip shearing in the later stage, and the synmorphic K-feldspar granitic mylonite veins were precipitated by the tectonic deep melting, and were emplaced by the undeformed hornblende diabase veins in the later stage, thus divided into the early stage of the deformation of granitic diorite - granitic mylonite, long with deformation fine vein potassium granitic mylonite and post deformation undeformed amphibolite diabase three stages. Among them, mylonite foliation is close to E-W or SW-NE trend, inclining to NW-N, with steep dip angle, SW dip direction and gentle dip angle. Macro and micro structures such as tensile lineation, S-C fabric, asymmetric fold, rotary remnant and domino porphyroclastic structure are developed in the zone, indicating that the nature of the shear zone is characterized by left lateral strike slip shear deformation. Zircon LA-ICP-MS U-Pb geochronology of the early, syn and post deformation rocks was carried out. The zircon 206Pb / 238U concordant ages of 286.7 ± 4.1Ma, 280.0 ± 5.5Ma and 278.8 ± 0.57Ma were obtained. It is concluded that the age of left strike slip deformation in Xiajing-changhanying ductile shear zone is ~ 280Ma, that is, the late Early Permian. Based on the data of tectonic magmatic age, dynamic background and evolution process in the southern margin of the Central Asian orogenic belt, it is considered that the Xiajing-changhanying ductile shear zone was formed during the compression of the Xing"an subduction accretionary orogenic belt on the southeastern margin of the Central Asian orogenic belt to the post orogenic extensional tectonic transformation along the Solun Xilamulun river line.