Chronology and geochemistry of the early stage of Late Cretaceous andesitic porphyrite in Yinkeng area, Southern Jiangxi Province

The Yinkeng andesitic porphyrite is located in the middle of Yushan metallogenic belt and at the edge of Mesozoic fault basin in Southern Jiangxi Province. To study the emplacement age, petrogenesis and tectonic setting of magmatite, the paper analyzes the petrography, zircon U-Pb chronology and geochemical characteristics. The results show that the Yinkeng andesitic porphyrite belongs to high-K calc-alkaline series, characterized by obvious enrichment of LILEs(K, Ba and Rb), LREEs and active incompatible elements(Th and U), and depleted HSFEs(Nb,Ta,Ti and P), with right-dipping REE distributin curves,(La/Yb)_N of 7.90～8.56 and no obvious Eu anomaly. The Th content of(6.01～6.36)×10^-6, Th/Ta of 10.7～11.7, Zr/Hf of 33.15～35.33 and Rb/Sr of 0.198～0.216, all between crustal and mantle values, indicates the magmatic source rock is mainly from the partial melting of mantle materials and affected by crustal contamination. The LA-ICP-MS zircon U-Pb dating result of(99.89 ± 0.30)Ma show that the magma activity occurred in the early stage of Late Cretaceous, and the geotectonic setting was related to the westward subduction of Pacific Plate, which was the product of back-arc extension of active continental margin. Through the comparative analysis of sedimentary sequence of red-bed basins, it is concluded that the activity age of andesitic porphyrite can roughly represent the formation age of the fault basins.