Petrogenesis And Geological Significance Of Middle Jurassic Intrusive Rocks In Xiajiapu Area,Qingyuan,Northern Liaoning

The Qingyuan area in northern Liaoning is located in the eastern segment of the northern margin of the North China Craton,where extensive Mesozoic magmatic rocks and accretionary complexes are exposed.It is a typical area for studying the evolution and transformation of the Paleo-Asian Ocean tectonic domain and the Paleo-Pacific tectonic domain.In this paper,zircon U-Pb geochronology and geochemical analysis of intrusive rocks exposed in the Xiajiapu area of Qingyuan were carried out,and their formation age,genesis,and tectonic setting were discussed Moreover,combined with regional magmatic activities,it provides new geological evidence for the transformation of the Paleo-Asian tectonic domain and the Paleo-Pacific tectonic domain.The petrographic analysis identifies that the assemblages of the intrusive rocks are gabbronorite,quartz diorite and granodiorite.Zircon U-Pb dating results indicate that gabbronorite was formed in the late Middle Jurassic（163.8±2.4Ma）,quartz diorite（169.9±2.2Ma）and granodiorite were formed in the middle period of Middle Jurassic.The geochemical characteristics demonstrate that gabbronorite and quartz diorite belong to the calc-alkaline series,with low Si O₂ contents,high Al₂O₃ contents and high Mg^#value,weakly enriched light rare earth elements（LREEs）,enriched in large ion lithophile elements（LILEs）such as Rb,Ba,K,Sr,and depleted in high field strength elements（HFSEs）such as Nb,Ta,Eu,showing a weak positive or no anomaly.It is suggested that primary magma of gabbronorite and quartz diorite should originate from the partial melting of lithospheric mantle,which is metasomatized by fluid from the subducted oceanic slab.Granodiorite has a high content of silicon,aluminum,and total alkali,and has the geochemical characteristics of high-K calc-alkaline I-type granite,with high Sr/Yb value,a weak negative anomaly in Eu,enriched in LREEs and LILES（Rb,K,Ba,etc.）,and depleted in HFSEs（Nb,Ta,etc.）,indicating that the granodiorite is a product of partial melting of thickened lower crustal basic igneous rocks metasomatized by fluids in a subduction setting.Based on the regional tectonic evolution history,and the temporal and spatial distribution of regional magmatic activities,it is believed that the formation of the Middle Jurassic intrusions in the Xiajiapu area of Qingyuan,northern Liaoning is related to the subduction of the Paleo-Pacific plate.This paper systematically summarizes the spatiotemporal distribution of magmatic activities,the evolution of crustal thickness,and the evolution characteristics of fault structures during the Triassic to Jurassic period in the eastern segment of the northern margin of the North China Craton.The results indicate that the Triassic magmatic activities in the eastern segment of the northern margin of the North China Craton are generally distributed in an eastwest-ward linear and planar manner.The Jurassic magmatic activities are spread in a northeast direction,with a trend of gradually becoming younger from east to west.Granitic rocks have the evolutionary characteristics from I-type granite and adakite（Early Triassic to early period of Late Triassic）to A-type granite（Middle to late period of Late Triassic）,and then to I-type granite and adakite（Jurassic）.During this period,the crustal thickness of the eastern segment of the northern margin of the North China Craton showed a trend of thickening（Early Middle Triassic）to thinning（Late Triassic）,and then gradually thickening（Jurassic）.The Indosinian tectonic stress field in the Qingyuan area is characterized by north-south compression,while the Yanshanian tectonic stress field is characterized by an NW-SE direction,showing an evolution feature of tension to compression and then to compression strike-slipping.The above evolution of magmatic activities,crustal thickness,and structural characteristics is consistent with the process from continuous orogeny to post-orogenic extension after the closure of the Paleo-Asian Ocean to subduction of the Paleo-Pacific plate,indicating that the subduction of the Paleo-Pacific plate began in the Early Jurassic.