Chronology Study On The Tectonic Evolution Of The Paleoproterozoic Jiandeng Intrusion Of Zanhuang Uplift

With a history of 3.8 billion years of geological evolution,the North China Craton is one of the oldest continents in the world.It is also a stable landmass with the best preservation,the longest age of formation and evolution,the strongest metamorphism and the largest area in China.The early evolution history of the North China Craton always has some controversy,mainly focused on the tectonic evolution history of 2.5Ga?1.8Ga.This study takes jiandeng plutons from Zanhuang uplift to 2.5Ga and their different stages of granitic intrusions and pegmatites as the research object.Through the study of field geology,chronology,geochemistry and structural geology,analyzes the jiandeng plutons and their inclusions,formation age and geological significance of deformed and undeformed pegmatite dikes,and reveals their genesis and tectonic environment.Then discuss the Paleoproterozoic tectonic evolution and metamorphic deformation history of the North China Craton.The study area is located in Lincheng County,Xingtai City,Hebei Province.The overall lithology of the jiandeng plutons is gneissic potash feldspar granite,granitic inclusions with more complex early deformation and late granitic intrusions can be seen.The entire area has undergone strong ductile shearing structurally,forming a gneissic structure,and local mylonitization has occurred,followed by the development of multiple stages of brittle deformation,forming multiple stages of pegmatite,quartz veins,Shear joints and fault fracture zones.The pegmatite can be divided into two stages.The early pegmatite invaded before the regional metamorphic deformation,and simultaneously managed the strong ductile deformation with the potash feldspar granite of the surrounding rock.The late pegmatite was formed after the ductile deformation of the region,and was filled along the shear joints,without deformation.The map of various intrusive bodies and enclaves in Jiandeng plutons(TAS)shows that they are all granite.On the whole,the aluminum content in the rock mass is relatively low,the aluminum saturation index A/CNK is 1.05?1.11,and the alkalinity index A/NK is1.16?1.27,which is shown in the A/CNK-A/NK diagram and the Si O₂-K₂O diagram,Jiandeng plutons are peraluminous shodonite series.In the normalized trace element distribution curve of the primitive mantle,only the undeformed pegmatite has negative Th anomalies,and the rest are obvious negative anomalies of Ba,Nb,P,Ti,Sr and K.In the rare earth element distribution curve,granite pegmatite and potash feldspar granite have obvious positive Eu anomalies,while the rest are moderately negative Eu anomalies.Through zircon U-Pb chronology research,it is found that:(1)the emplacement age of Jiandeng plutons is 2463-2494Ma;(2)it contains 2518Ma zircon,indicating that its provenance may be 2.5Ga TTG;(3)The metamorphic age is 1906 Ma,indicating that the main regional metamorphic deformation may have occurred during this period;(4)This study also found that the Jiandeng plutons are a complex granite body,in which there are2300-2200 Ma granite and granitic pegmatite..The above research shows that the Jiandeng plutons are granite basement formed by the remelting of 2.7-2.5Ga TTG magma,and then it undergoes structural deformation at?2.5Ga.The granite inclusions contained therein are formed at this stage,and then the potassium feldspar granite,it was emplaced into the rock mass,and then 2.26-2.38Ga granite and deformed pegmatite were formed.Finally,the metamorphic age of the metamorphic edge in the granitic enclave was 1906Ma,indicating that the strongest structural deformation occurred in the region at?1.9Ga event.This study did not find 2.5Ga metamorphic zircon,so its metamorphic significance and the determination of deformation events need further research and discussion.Therefore,we propose that the collision time between the eastern land block of the North China Craton and the Zanhuang Arc occurred at?2.5Ga.After the collision,it also experienced back-arc stretching and rift activities,which caused the Taihang Ocean to open during the 2.4-2.2Ga period,followed by the arc The Guandu Group sedimentary strata formed in the back basin;then,the eastern block subducted below the Zanhuang arc(2.1-1.9Ga);finally,the eastern block and the western block finally merged at?1.9Ga.