Petrogenesis of the Bao'anzhai granite and associated Mo mineralization, western Dabie orogen, east-central China: Constraints from zircon U-Pb and molybdenite Re-Os dating, whole-rock geochemistry, and Sr-Nd-Pb-Hf isotopes

摘要

Recently, many Mo deposits genetically related to emplacement of Early Cretaceous granites have been found in the Dabie-Qinling belt. A typical intrusion that combines magmatism and metallogenesis, the Bao'anzhai granite, yields a zircon ~(238)U-~(206)Pb age of 123.2 ± 1.1 Ma and a molybdenite Re-Os isochron age of 122.5 ± 2.7 Ma. This granite is characterized by high silica and alkali, but low Mg, Fe, and Ca. It is enriched with light rare earth elements (REEs) and large-ion lithophile elements (LILEs, Rb, K, Th, U) but depleted of heavy REEs, high field strength elements (HFSEs, Nb, Ta, Ti, and Y), and Sr. This high-K granite has medium initial ~(87)Sr/~(86)Sr ratios (0.706518-0.707116) and low initial Pb isotopic ratios [(~(206)Pb/~(204)Pb)_i, 16.423-16.699; (~(207)Pb/~(204)Pb)i, 15.285-15.345; (~(208)Pb/~(204)Pb)_i, 37.335-37.633], and is characterized by low ε_(ND)(t) and ε_(Hf)(t) values (—14.92 to —14.22 and —21.67 to —19.19, respectively). These data indicate that this pluton is a high-K calc-alkaline fractionated /-type granitite. It was generated by partial melting of the Yangtze lower crust, which is probably similar to Neoproterozoic TTG-like magmatic rocks at the north Yangtze Block under a non-thickened lower crust environment (<35 km). The ores also have low radiogenic Pb isotopes (~(206)Pb/~(204)Pb, 16.592-17.674; ~(207)Pb/~(204)Pb, 15.300-15.476; ~(208)Pb/~(204)Pb, 37.419-37.911) and low Re content in molybdenite (5.693-10.970 ppm), suggesting a crustal magmatic source for the metallogenic minerals in the Mo deposit.