Petrogenesis of Early Cretaceous post-collisional granitoids at Shapinggou, Dabie Orogen: Implications for crustal architecture and porphyry Mo mineralization

摘要

The Shapinggou molybdenum deposit is located in the Qinling-Tongbai-Hong'an-Dabie Orogen, which accommodates the largest molybdenum ore belt in the world. New laser ablation-inductively coupled plasma-mass spectrometry (LA-1CP-MS) zircon U-Pb dating, geochemical and Sr-Nd-Hf isotope data for the granitoids at Shapinggou are presented in this paper. Two stages of Early Cretaceous magmatism, 138-128 Ma and 116-114 Ma, took place at Shapinggou in a post-collisional setting. These granitoids can be divided into four groups, including the early-stage granites (134-129 Ma, SiO2 = 62.33 to 76.27%) and intermediate rocks (129-128 Ma, SiO2 = 52.41 to 60.72%) and the late-stage quartz syenite (116 Ma, SiO2 = 64.09 to 65.12%) and Mo-bearing granite porphyry (114 Ma, SiO2 = 74.95 to 77.26%). The early-stage granites have high Sr contents, Sr/Y and La_N/Yb_N ratios, but low Y, Yb and MgO contents, showing low-Mg adakitic chemical features. They were possibly generated by partial melting of the thickened mafic lower continental crust. The early-stage intermediate rocks have relatively high MgO contents (2.71 to 4.84%) and Mg# values (46 to 60), but low Sr/Y and La_N/Yb_N ratios, which were likely derived from a hybrid mantle metasomatized by melts from foundered eclogitic lower continental crust and subsequent fractional crystallization. The late-stage quartz syenite shows chemical features of metaluminous A-type granites, including high abundances of alkalis, HFSE (Zr, Nb), and trivalent REE. The late-stage Mo-bearing granite porphyry displays high SiO2, alkalis, Zr, Ce contents, and A/CNK ratios, but low Ba, Sr, P and Ti, showing affinity to highly fractionated 1-type granites. The late-stage quartz syenite and granite porphyry were possibly produced by partial melting of intermediate compositions of middle crust. Our results support the conclusion that there were three-layer continental crusts in the Dabie Orogen prior to Early Cretaceous magmatism and suggest that post-collisional granitoids in the Dabie Orogen were derived from not only lower crust but also middle crust The giant Shapinggou porphyry Mo deposit was most likely formed by effective transport of volatiles and Mo in a converting granitic magma column, rather than by direct melting of Mo-enriched sources. The Mo-bearing rocks at Shapinggou are characterized by a high degree of magma differentiation, close associations with A-type granite, and formation at the end of regional magmatism.