Characteristics,Genesis And Tectonic Significance Of Mesozoic Intrusive Rocks In Liujiang Area,Qinhuangdao

The research object of this paper is the Mesozoic intrusive rocks in the Liujiang area of Qinhuangdao on the northern margin of the North China Craton and the eastern segment of the Yanshan intraplate orogenic belt,and conducted field geological survey.On the basis of summarizing the previous research results,combined with petrography,geochemistry,zircon U-Pb chronology,the formation age was determined,and the evolution and formation of magma were discussed,and the tectonic significance was discussed on this basis.The lithology of the Mesozoic intrusions exposed in the study area are mainly monzonite,quartz monzonite porphyry produced in the form of stock,and diabase porphyry,diorite porphyry and granite porphyry produced in the form of dike.The formation age of the diabase porphyrite in the study area is 159.2±1.3Ma.The source of diabase porphyrite is the asthenospheric mantle,which was contaminated by the lower crust during its formation and experienced the separation and crystallization of phlogopite,plagioclase,ilmenite,and rutile.The source area of diorite porphyrite in the study area is the crust,which was contaminated by asthenospheric mantle material during its formation,and there are separated crystals of pyroxene,phlogopite,ilmenite,plagioclase,and amphibole.The monzonite stock(160.6?158.4Ma),monzonite dike and quartz monzonite(170.7±1.8Ma)in the study area are all adakitic formed by partial melting of thickened crust,mantle-derived materials are added in the form of magma mixture during the formation process.The formation age of the granite porphyry in the study area is 118.8±1.3 Ma.It belongs to the aluminous A-type granite.It is formed by partial melting of shallow crustal calc-alkaline granite under low pressure conditions.After further magma differentiation,forming the Amphibole granite in the Xiangshan Pluton and the Houshihushan Pluton.170?139Ma is the period of continuous subduction of the paleo-Pacific plate to the northern part of the North China Craton;139?130Ma,the paleo-Pacific plate began to retreat from subduction;130?118Ma,the paleo-Pacific plate continued to retreat,resulting in a large-scale asthenosphere upwelling and regional extension.In the northern margin of the North China Craton,there is a thickening of the crust between 170 and 160 Ma,and the thickness can reach more than 50 km.Later,the paleo-Pacific plate began to withdraw,and the asthenosphere upwelled,causing partial melting of the lithospheric mantle and crust.As a result,the crustal thickness of the North China Craton began to thin to 40-50km;around 120 Ma,the crustal thickness was thinned to 30 km.There was partial delamination on the northern margin of the ?159Ma North China Craton.