A subduction-related metasomatically enriched mantle origin for the Luoboling and Zhongliao Cretaceous granitoids from South China: implications for magma evolution and Cu-Mo mineralization

摘要

The Luoboling granodiorite porphyry and Zhongliao porphyritic biotite-granodiorite occur within the Zijinshan ore field in Fujian Province, southeast China. LA-ICP-MS zircon U-Pb dating yields ages of 103.1 +/- 1.1Ma (2 sigma, MSWD=3.1) and 95.9 +/- 0.6Ma (2 sigma, MSWD=1.2) for the Luoboling and Zhongliao intrusions, respectively. All rocks show high SiO2, K2O, and light rare earth element levels, variable CaO and Fe2O3T, but low heavy rare earth element and high field strength element (Nb, Ta, Ti) concentrations. They also exhibit uniform initial Sr-87/Sr-86 ratios (0.7064-0.7068) and epsilon(Nd)(t) values (-4.0 to -2.6), falling within the compositional field of Cretaceous basalts and mafic dikes in the Cathaysia Block. Together with their relatively high Hf isotopic ratios (epsilon(Hf)(t)=-5.8 to +0.7), these data suggest that the Luoboling and Zhongliao intrusions were derived from a subduction-enriched mantle source. We infer fractional crystallization processes involving early fractionation of clinopyroxene and olivine, and subsequent fractionation of garnet forming the Luoboling granodiorite porphyry, eventually followed by amphibole- and biotite-dominated crystallization, with minor accessory mineral contribution, producing the (younger) Zhongliao porphyritic biotite-granodiorite. Asthenospheric mantle sources may have also contributed to generating the melts as indicated by initial Pb isotopic compositions and Hf isotopic compositions. The generation of these intrusions was associated with the Pacific subduction in an extensional setting during the Cretaceous.