Origin And Tectonic Setting Of Permian Granitoids Rocks In The Middle Segment Of The North Margin Of The North China Craton

The North China Craton(NCC)is one of the oldest cratons and the Central Asian Orogenic Belt(CAOB)is the largest Phanerozoic orogenic belt in the world.As the boundary between them,the geological evolution history of the north margin of the NCC has always been one of research hotspots.There are a large number of Carboniferous-Permian-Triassic magmatic rocks in the north margin of the NCC,which are zonally distributed in near east-west direction and related to the closure of the Paleo Asian Ocean.Based on the geochronology,analysis of plagioclase zoning,geochemistry and Hf isotopic study of the Permian magmatic rocks in the middle part of the north margin of the NCC,this paper clarifies the magmatic activities,source area and tectonic environment of those rocks to discuss the crust evolution the geological evolution history of study area.This paper reports 12 zircons U-Pb dating results using LA-ICP-MS and SHRIMP: Dadonggou alkali feldspar granite 295 ± 5 Ma,Dananfangzi alkali feldspar granite 285±8 Ma,Yuanbaoshan diorite dyke 284 ± 5 Ma,Yuanbaoshan monzodiorite dyke 282 ± 4 Ma,Lisicun granite 282 ± 2 Ma,Wuhai granite 280 ± 4 Ma,Erdaowa granite 278 ±5 Ma,Gehuasitai granite 276 ± 3 Ma,Shangde granite 269 ± 3 Ma,quartz diorite enclave in Shangde granodiorite 264 ± 4 Ma,Hadenghushu granodiorite 256 ± 2 Ma,and Hadenghushu granite 260 ± 3 Ma.The magmatic activities in the study area can be divided into 4 periods: early Early Permian,late Early Permian,Middle Permian and Late Permian.The early Early Permian granite is A-type granite,enriched in Rb,K,Nb and Th while depleted in Ta,EU,Ti and Sr;the late Early Permian granitoids and dikes are mainly calc alkaline to high-K calc alkaline series,enriched in large ion lithophile elements and depleted in high field strength elements;the Middle Permian granitoids are high-K calc alkaline series,enriched in Rb,Pb and depleted in Ti;the Late Permian granitoids are calc alkaline to high-K calc alkaline series,rich in Rb,Pb,K and Th,while depleted in Nb,Ta and Ti.The magmatic rocks in the northern margin of North China in the Permian showed a tendency to be acidic and rich in potassium and alkaline.The analysis of plagioclase zoning shows that the magma mixing of Permian magma occurred many times,basic magma injected into acid magma.The source areas of Early Permian granite mainly are the ancient crust of North China Craton;the source areas of Middle Permian granitoids are the ancient crust of North China Craton and mantle;the source areas of Late Permian granitoids are mantle and the ancient crust of North China Craton,and the proportion of mantle material is increased.Hf isotope shows that the mantle contribution is more and more significant,and the magma source of Permian granitoids had evolved from the remelting of ancient continental crust to the crust accretion.The early early Early Permian granite is anorogenic granite;the late Early Permian granitoids were formed in subduction environment;the Middle Permian granitoids were formed in continental margin arc to syn-collisional environment;the Late Permian granitoids were formed in post-collisional environment.The north margin of the NCC was in a continental marginal arc environment in the Early Permian,it experienced the transformation from arc environment to syn-collisional environment in early Middle Permian,and the transformation from the syn-collisional environment to the post-collision environment in the late Middle Permian;in the Late Permian,the north margin of the NCC was in a post-collisional environment.The closure of the Paleo Asian Ocean as well as the collision of the north margin of the NCC and the microcontinents in the CAOB was taken place in the late Middle Permian.