Zircons in the gabbro enclaves of Fengshudong REE deposit in Eastern Nanling Mountains: Chronology, geochemistry and diagenesis

The eastern Nanling Mountains is the concentrated area of rare earth element(REE)mineral resources in China. By studying the zircon U-Pb age and trace elements of enclaves in ore-bearing granite beds of weathered leaching deposits in Eastern Nanling area, the results show that there are at least two types of gabbro enclaves in REE deposits, namely the Early Jurassic enclave(188±2.8)Ma and Late Triassic enclave(227±3.7)Ma. The zircon CL image and Th/U>0.4 indicate the type is magmatic zircon. The variation characteristics of zircon REEs in the enclave reveal that most zircons are distributed in the transition zone between hydrothermal zircons and magmatic zircons, indicating that the zircons undergo intense hydrothermal reformation in later period and the Late Triassic enclave is most strongly reformed. The Ti temperature of major zircons in the Early Jurassic enclave is 694～1 279 ℃, averagely 816 ℃, nearly half above 800 ℃, while that for the Late Triassic inclusions is 687～925 ℃, averagely 671 ℃, most below 750 ℃, which may originate from the partial melting of various components under the condition of water near saturation. There are multiple groups of zircon age data for the enclaves, showing several magmatic activities occurred in the eastern Nanling during Triassic-Early Jurassic, and the Late Triassic rock mass was melted by Early Jurassic magma, then by Early-Middle Jurassic magma, which reflects the influence of paleo-Pacific Plate on South China Plate is from weak to strong, magma formed by the partial melting of continental crust under the subduction background and the mantle-derived material activity also from weak to strong.