Structural Evolution And Chronological Constraints Of The Kalaqin Area South Of Chifeng

Destruction and lithospheric thinning of the North China Craton (NCC) are hot topics in recent geological researchs. The Kalaqin region to the south of Chifeng, Inner Mongolia is situated in the north of the Yanshan Tectonic Belt (YTB) on the northern margin of the NCC. This area experienced several phases of magmatism from late Paleozoic to Mesozoic and deforamtion events, and is an ideal area for understanding the craton destruction and tectonic evolution of the YTB.Based on detailed field investigation, a sinistral ductile deformation zone was first recognized in this work, which developed in the metamorphic basement south of the Kalaqin Metamorphic core complex (KMCC). The basement deformation zone is characterized by WNW-ESE steep gneissosity, WNW-ESE shallow mineral elongation lineation and many intrafolial or isoclinal folds. Many kinematic indicators show that it is a WNW-ESE steep, ductile sinistral deformation zone. LA-ICP-MS zircon dating for deformaed and undeformed dikes or plutons from the deformation zone constrains the shortenong deformation to the time between 271 and 251 Ma, i.e. Middle-Late Permian. Combined with previous dating results for post-orogenic magmatism in the YTB, it is inferred further that the shortening-related deformation happened in Middle Permian (271～260 Ma). Regional, geological comparsion suggests that the shortening deformation of the Middle Permian in the YTB resulted from the continent-continent collision in the Xing-Meng Orogen as a result of foreland shoetening deformation.The NNE-striking Louzidian-Dachengzi and Jinshan-Meilin ductile shear zones occur along the eastern and western margins of the Kalaqin batholith. The two shear zones dip toward SE and NW respectivelt, but show consistent NE-SE mineral elongation lineation. Exposure structures, microstructures and quartz C-axis fabrics all indicate top-to-the-NE sense of shear for the two shear zones. Both Metamorphic basement rocks and plutons in footwall were also widely involved in this ductile deformation and show penetrative NE-SW foliation and mineral lineation. The footwall show similar fabrics and shear sense to those in the detachment ductile shear zones. Microstructures suggest deformation temperatures of 550℃～600℃ for the two ductile shear zones and a mid-crustal origin for them. Our dating for volcanic rocks demonstrates the presence of the Late Jurassic Tiaojishan Formation in the Xiaoniu basin, suggesting association of the Late Jurassic rift basins with the KMCC. Many LA-ICP-MS zircon dating results for plutons and dykes constrain the KMCC to the time as a result of regional NE-SW extension.During Early Cretaceous (141～100 Ma), extension activity happened again in Kalaqin area and the extensional direction switched to NW-SE, leading to development of two NE-striking major normal faults with opposite dip directions along the eastern and western edges of the core complex. Early Cretaceous half-graben basins developed on the hanging walls of both major faults, as their shared footwall rose in an extensional dome. This doming resulted in exhumation of the Late Jurassic ductile extensional structures to shallow levels. The core complex was overprinted by earlier NE-SW extensional, ductile deformation belts and later brittle normal faults during the extensional doming of Early Cretaceous. The deformation shows characteristics of strain localization. Stress field inversion for many measured fault-slip data from normal faults in the area shows that these Early Cretaceous extensional structures were the result of the regional NW-SE extension. This is consistent with the stretching direction shown by the Early Cretaceous ductile extensional structures.The KMCC experiences two phases of extensional deformation in both the Late Jurassic and the Early Cretaceous, and presents overprints of the two-phase extensional structures. Our new determination for the evolution and formation mechanism of the KMCC confirms extensional deformation in both the Late Jurassic and the Early Cretaceous in the YTB, and implies the northern margin of the NCC might destructed from Late Jurassic. It is also suggested that the Early Cretaceous extension was intense and widespread in the study area, and represents the peak destruction of the eastern NCC.Based on dating of many igneous rocks and determination of the different deformation events, combining with the regional geological data, tectonic evolution history in the Yanshan tectonic belt during Late Paleozoic and Mesozoic could be deduced comprehensively in this study. It shows that the Yanshan tectonic belt was in an active continental margin setting with arc-related magatism during the latest Early Carboniferous and Early Permian due to the subduction of the Paleo-Asian ocean. The closure of Paleo-Asian ocean along the Solonker suture in Middle Permian resulted in foreland shortening deformation zone and appearance of magatic quiescent period correspondently in the Yanshan tectonic belt. The belt was followed by four periods of extension-related magmatic activities in Late Permian-Triassic, Middle Jurassic, Late Jurassic and Early Cretaceous respectively. Each of the magatic events was interrupted by a quiescent period of magmatism related to a compressive event happening in Early Jurassic, Early Late Jurassic and Early Cretaceous respectively. These phenomena suggest that there exists coupling relation between the magmatic activities and tectonic evolution. Each of the intraplate magmatic events happened under the extensional setting whereas every compressive events corresponds to the magmatic quiescent period in the Yanshan tectonic belt.