Zircon Dating For The Dikes And Petrogenesis Of The Mafic Enclaves From The Dabie Orogen

NNE-striking dike swarm,which are widespread in the Dabie orogen,are consisted by different rock types with familiar quartz-syenite porphyry,granite porphyry,quartz-diorite and diabase.These dikes cut through the Yanshanian intrusive bodies,indicating the dikes were the latest stage Yanshanian magmatic activity.LA-ICPMS zircon U-Pb dating yield weighted mean ages of 127.1±4.5 Ma,127.5 ±1.9 Ma,127.4±2.5 Ma and 127.9±2.1 Ma for the quartz-syenite porphyry,quartz-diorite and granite porphyry,respectively.The results reveal that the dikes are generated during a short time span and formed in the early Cretaceous,representing the termination of Mesozoic magmatism in this region.Owing to the sinistral strike-slip movement of the Tan-Lu fault zone,the dike swarm intruded along the shear-tensile cracks.The ages of these dikes indicate that the third sinistral strike-slip movement of the TLFZ might take place around 127 Ma.There are a large number of mafic enclaves occurring in Mesozoic quartz monzonite and monzonite in the Dabie orogen.Mafic dioritic enclaves,with a finer mineral size and more melanocratic mineral,have the typical magma structures and similar mineral assemblages to host rocks.K-feldspar porphyroblasts and augen quartz occur in some enclaves,and acicular apatite is common.The zircon U-Pb dating of enclaves and host rocks are about 133~129 Ma and 130 Ma respectively,suggest that the formation age of enclaves and host rocks were coeval in the error range and they were formed in the early Cretaceous.The results of electron microprobe analysis show that the minerals' chemical compositions between enclaves and host rocks are similar,the temperature and pressure conditions are similar too.And there are additions of mantle material in the mafic enclaves.Geochemical data show that enclaves and host rocks have a clear linear relationship in the Hacker diagrams constructed by major elements.Meanwhile,the host and enclaves are all enriched in LILEs and LREEs and depleted in HFSEs and show.show similar REE and trace elements distributed patterns,suggest that they have genetic relation.Through the system analysis of petrograhic,mineralogical,zircon U-Pb dating and geochemical for mafic enclaves and host rocks,it is suggested that mafic enclaves are formed by magma mixing.In the early Cretaceous,the mantle derived magma underplated to the lower crust,and leads to partial melting of crust and mixing of mafic magma and felsic magma.