The Mesozoic Tectonic Evolution Of Liaodong Area In The Eastern Segment Of The Northern Margin Of The North China Craton

Liaodong area is located in the eastern part of the northern margin of the North China Craton,affected by the Mesozoic subduction of the Paleo-Pacific plate.This area,which has experienced the evolution of the Paleo-Pacific Ocean(PAO)subduction since the Mesozoic,is a ideal region for studying the tectonic-magmatic activities of the Mesozoic in the North China Craton.In this thesis,the early Cretaceous magmatic rocks,fauled basins and regional main faults in Liaodong area are taken as the research objects.Combined with the chronological data of the predecessors and the zircon LA-ICP-MS U-Pb dating results,the tectone magmatic activities of the Mesozoic Yanshan period in the study area are divided into three periods: Late Triassic-Early Jurassic Period(weak stretching)?Middle-Late Jurassic Period(subduction extrusion)and Early Cretaceous Period(regional extension).Through study on the magmatite rock assemblage,geochemistry and Lu-Hf isotope chronology characteristics,discusses the tectonic magmatic source of their properties and the formation of.At the same time,the development and evolution process of the fault basin in the Liaodong area and the spatial distribution and deformation development of the main fault are studied.The research results of the predecessors in the area are comprehensively studied to explore the Mesozoic tectonic evolution history of the eastern part of the northern margin of the North China plate.The mechanism and dynamics of the lithospheric thinning and craton destruction in the North China Craton are also discussed.During the Early Cretaceous,magmatic activity was very active in the eastern part of the northern margin of the North China Plate,and many intrusive rocks were formed in the eastern part of Liaoning Province.Maokuishan monzonitic granite in Dayingzi area,granite porphyry-alkali feldspar-quartz syenite in Guanmenshan area,Fenghuangshan monzonitic granite in Fengcheng area and Taipingshao alkali feldspar granite in Huanren area all belong to type A granite of weakly peraluminous and high-K calc-alkaline series.They are rich in LREEs and LILEs(K,Rb and Th),depleted in HREEs and HFSHs(Nb,Ti).Magma originated from partial melting of the ancient middle-lower crust.Huangdi granodiorites in Dayingzi area and Daxing monzonitic granite in Fengcheng area are quasi-aluminous to weakly prealuminous and high-K calc-alkaline I type granites with distinct fractionation of light and heavy rare earth elements,enrichment of K,Rb and other ion lithophile elements,and loss of high field strength elements(Nb,P,Ti).The magma originated from the lower crust,which was heated and partially melted under high pressure due to the vertical accretion and thickening of mantle floor intrusion.The Chaoyang pluton is norite gabbro.They belong to quasi-aluminous and are riched in LILEs(K,Ba,Sr)and LREEs,depleted in HFSEs(Nb,U,P,Ti).The magma originated from a source or the rising process of crustal material into the upper mantle melting.The discovery and determination of the above-mentioned early Cretaceous magmatic rocks indicate that the North China Craton was in an extensional tectonic setting during the Early Cretaceous.The evolution of Mesozoic faulted basins in eastern Liaoning is closely related to tectonic activities.According to the strata of different ages deposited in the basins,the tectonic evolution of the basins can be divided into Jurassic and Early Cretaceous periods.Among them,Tianshifu-Saima-Aiyang fault basin began to deposit in the early Jurassic,and experienced several crustal uplifts and downfalls in the middle.The strata in the basin are discontinuous and there are depositional discontinuities.Dayingzi-Chaoyang-Huanghuadian fault basin,Liujiahe fault basin and Huanren fault basin were all formed in the early Cretaceous,and Xiaoling Formation volcanic-sedimentary formation developed in the basin.The evolution process of the faulted basins in the Mesozoic belt is obviously controlled by the influence and transformation of tectonic activities.Based on the study of Yanshanian tectonics in eastern Liaoning and the time regularity of Mesozoic magmatic activity,the Yanshanian tectonic-magmatic activity in eastern Liaoning can be divided into three periods.During the Late Triassic to Early Jurassic,the study area was subjected to post-collision tensile stress between the Paleo-Asian Ocean and the North China Plate,and was in a weak extensional tectonic environment.During the Middle Jurassic and Late Jurassic,the Paleo-Pacific plate subducted slowly to Eurasia at low angle.The compressive stress was dominant in the study area,and a typical product of compression-thrust nappe structure was formed.The magmatic rocks of the same period are mainly active continental margin arc granites with thickened crustal background.During the Early Cretaceous,the subduction velocity,direction and angle of the Paleo-Pacific Plate changed,and the retrogression effect occurred,which resulted in the formation of a large number of extensional structures in the study area.During the Early Cretaceous,the regional extensional structures generated in the study area were closely related to the subduction of the Paleo-Pacific Ocean.The disintegration and asthenosphere upwelling occurred almost simultaneously in the study area,and their combined action resulted in the thinning of the lithosphere.In summary,during the Early Triassic to Middle Triassic,the Kaiyuan area and the eastern Jilin area adjacent to the eastern Liaoning area underwent a continuous subduction of the Paleo-Asian Ocean,which eventually closed,and then the tectonic evolution of the land-land collision.However,the above-mentioned process has a weak impact on Eastern Liaoning.During the Late Triassic to Early Jurassic,the eastern segment of the northern margin of the North China Craton was in a post-collisional extensional environment after the closure of the Paleo-Asian Ocean,and the whole region was the initial stage of lithospheric thinning and craton destruction.The Middle Jurassic and Late Jurassic period,the Paleo-Pacific Plate began to subduct to the Eurasian Continental Plate,and the continental crust became thicker.The Liaodong area was in an active continental margin setting.During the Early Cretaceous,the subduction and retreat of the Paleo-Pacific Plate and the direction of subduction changed,the regional compression changed into regional extension,and the North China Craton underwent intense lithospheric thinning and craton destruction.