I-type granitoids in the eastern Yangtze Block: implications for the Early Paleozoic intracontinental orogeny in South China

摘要

The Early Paleozoic intracontinental orogenk belt in the South China Block (SCB) is composed of massive granitoids and high-grade metamorphic rocks. Compared to the widespread distributions of early Paleozoic S-type granites in the eastern SCB, coeval I-type granitoids are rare and consequently receive much less attention. Two spatially associated granodiorite plutons in the northwestern rim of the orogen, namely the Banshanpu pluton and Hongxiaqiao pluton, have been investigated in order to determine how they fit into the geodynamic setting. The Hongxiaqiao pluton shares many lithological similarities with the Banshanpu pluton, except for the presence of abundant mafic microgranular enclaves (MMEs) in the Hongxiaqiao pluton. Zircon U-Pb dating has yielded weighted mean ~(206)Pb/~(238)U ages of 432 ± 3 Ma and 434 ± 3 Ma for the Banshanpu and Hongxiaqiao plutons, respectively, indicating that they were emplaced coevally in the early Silurian. Samples from the two plutons possess similar Nd-Sr isotope compositions (εNd_T = - 8.32 to - 6.88; ~(87)Sr/~(86)Sr_i = 0.7109-0.7169), indicating that they were derived from a similar crustal source. Rocks from the Banshanpu pluton are intermediate- to high-K calc-alkaline and show strongly peraluminous (A/CNK >1.1), adakite-like characteristics (Sr/Y ratios >31; Yb < 0.91 ppm), consistent with an origin of partial melting of amphibolite in the garnet stability field. Samples from the Hongxiaqiao pluton contain lower SiO2 but considerably higher Fe2O3, TiO2, P2O5 and highly incompatible elements (e.g. Rb, Cs, Th and U) than those of the Banshanpu pluton.