Zircon U-Pb age, geochemistry and genesis of the Early Cretaceous mafic enclaves and granites in Tiantai area, eastern Zhejiang Province

A large number of Late Mesozoic granitic plutons with dark enclaves are widespread in the coastal area of Zhejiang and Fujian Province. They are considered to be the product of large-scale crust mantle interaction and magma mixing. In this paper, the authors conducted the LA-ICP-MS U-Pb dating and obtained whole-rock geochemical data of the host granites and dark enclaves of Baihe intrusion in Tiantai area, eastern Zhejiang Province, with the purpose of exploring the geochronology, petrogenesis and tectonic implications. The results obtained show that the U-Pb zircon ages of the host granite and dark enclaves are(120.4±1.2) Ma and(120.6±1.1) Ma, respectively. They are products of the Early Cretaceous Yanshanian intrusive activities in eastern Zhejiang in a syncollisional-postcollisional environment. Geochemical analyses show that the host rocks are high-potassium calc-alkaline Itype granite, with enriched SiO₂, total alkalis, and weakly-peraluminous nature; they are enriched in Rb, Th, U and K, and depleted in Sr, Ba, P, Ti, Nb and Ta, with strong negative Eu anomaly. The dark enclaves belong chemically to partial alumina low-potassium tholeiite series, with lower SiO₂, enriched sodium, enriched LREE, relatively depleted HREE, and weak positive Eu anomaly. The Hf isotope composition of zircons from host granite and dark enclaves are all indicative of mixing of crustal and mantle materials. Based on the geochronological and geochemical data, the authors hold that the Early Cretaceous I-type granites and their dark enclaves in eastern Zhejiang were generated by mixing of mantle-derived basic magma and its induced crustal felsic magma in an extension environment which followed the Yanshanian arc-back collision.