Geochronology And Geochemistry Of The Middle Proterozoic Magmatism In The Julin Area Of The Western Yangtze Block And Its Geological Significances

Precambrian is an important period for understanding the formation,evolution and dynamic mechanism of the crust in the early continental period.There are a lot of records of Precambrian magmatic activity in the Yangtze Block and its surrounding areas,which provide a good opportunity for us to study the Precambrian crustal evolution history.The Kangdian area is located in the western margin of the Yangtze Block,where a large number of Proterozoic strata and Precambrian magmatic rocks are exposed.This thesis chooses the Proterozoic granitoid in the Julin area in the western margin of the Yangtze Block as the research object.Detailed field geological survey,petrology,mineralogy,chronology,geochemistry and isotope geochemistry of elements were carried out in order to reveal the space-time distribution of granitoid in the Julin area.Combined with the existing research results in the area,petrogenesis and tectonic environment were also discussed.This study provides new results for understanding the tectonic evolution history of the Yangtze Block during the Precambrian period and for deciphering its relationship within the Rodinia supercontinent.The following research achievements and progress have been made:1.Zircon U-Pb geochronological analysis of Julin area granitoids shows that the formation ages of four representative granites are 1063±9 Ma,1064±6 Ma,1062±6 Ma,and 1063±7 Ma,respectively,which are all Mesoproterozoic.Combined with the previous studies in this area,it shows that the Mesoproterozoic magmatism was widely developed in this area,and the magmatic age was mainly in the late Mesoproterozoic(1168?1007 Ma).2.Major and trace analysis results show that the average content of SiO₂ is 71.4%,of TiO₂ is0.50%,of Ca O is 1.17%,and of(Na₂O+K₂O)is 6.14%.The Julin area granitoids are characterized by high Si and K,low Mg,Ca,P,and Ti.The average A/CNK is 1.37,indicating a high-potassium calc-alkaline peraluminous series.The total amount of rare earth element(?REE)of the sample is between 138×10^-6?364×10^-6.They show enrichment of LREE relative to HREE and have negative Eu anomalies(?Eu=0.24?0.83,the average value is 0.46).They are characterized by depleted of Ba,Nb,Ta,Sr,P,Ti,and enriched of large ion lithophile elements such as Rb,U,K and high field strength elements such as Th,Zr,Hf.The rocks have high 10000×Ga/Al ratios and(Zr+Nb+Ce+Y)contents,which are similar to the geochemical characteristics of A-type granites.3.Isotope analysis of the Julin area granitoids shows that the?_Hf(t)of the two representative granites range from-11.00 to+6.81,and the corresponding two-stage model ages(T_DM2)range from 2.57 to 1.39 Ga.The?_Nd(t)of the granite samples range from-9.90 to-0.86,and the corresponding one-stage model ages(T_DM)range from 3.81 to 1.75 Ga.Comprehensive analysis suggests that the Julin area granitoids were mainly produced by remelting of ancient crustal materials,and few depleted mantle materials and/or new crustal materials were also contained in their source.4.Julin area granitoids formed in a post-collisional or post-orogenic extensional environment,and their formation was likely related to the aggregation of Rodinia supercontinent.