Geochemistry, zircon U-Pb geochronology and Hf-O isotopes of the Late Mesozoic granitoids from the Xiong'ershan area, East Qinling Orogen, China: Implications for petrogenesis and molybdenum metallogeny

The petrogenesis and genetic relationship of molybdenum mineralization in the Late Mesozoic granitoids from the Xiong'ershan area remains controversial. This study presents integrated whole-rock geochemistry, zircon U-Pb geochronology and Hf-O isotopes for Late Mesozoic ore-bearing granite porphyry stock (Leimengou granite porphyry) and ore-barren plutons (Haoping and Jinshanmiao plutons) in the Xiong'ershan area of the East Qinling Orogen Belt (EQOB). The ore-barren granitoids are high-K subalkaline, metaluminous, and show negative correlations between Al2O3, MgO, TiO2, MnO, CaO, FeOT and P2O5 with SiO2. However, the Leimengou granite porphyry lacks distinctive linear correlation between major oxides with SiO2 except Al2O3, K2O and Na2O. These granitoids are enriched in LREE over HREE with slightly negative europium anomalies. Most samples show enrichment of Rb, Ba, Th, U, K and Pb, and depletion of Nb, Ta, P and Ti, mostly overlapping the ranges of the Taihua Group. Zircon U-Pb ages from the Haoping pluton, Jinshanmiao pluton and Leimengou granite porphyry are 130.9 +/- 0.4 Ma, 127.7 +/- 0.7 Ma and 131.5 +/- 0.9 Ma, respectively. The Hf-O isotopic features of zircon from the Leimengou granite porphyry (delta O-18(zircon) = 5.03 parts per thousand - 5.86 parts per thousand, epsilon Hf-(t) = -24.3 to -14.4) are markedly similar with the Haoping pluton (delta O-18(zircon) = 5.17 parts per thousand - 5.61 parts per thousand and epsilon Hf-(t) = -26.6 to -21.4). Zircon grains from the Jinshanmiao pluton have unique delta O-18(zircon) ranging from 6.01 parts per thousand to 8.90 parts per thousand and epsilon Hf-(t) from -15.6 to -12.2. The co-variations of whole-rock geochemistry and Hf-O isotopic features in the samples suggest that Haoping pluton and Leimengou granite porphyry originated from mixture of partial melts of the Taihua Group with mantle-derived material, whereas the Jinshanmiao pluton originated from mixture of Xiong'er Group and mantle-derived material, with minor input of Taihua Group. The zircon grains from the Leimengou granite porphyry have significant higher Ce4+/Ce3+ (mean 547) and delta Eu (mean 0.65) ratios than those in the ore-barren Haoping pluton (mean 207 and 0.54, Ce4+/Ce3+ and delta Eu, respectively) and the Jinshanmiao pluton (mean 235 and 0.62, respectively). The main controlling factors for porphyry-type Mo mineralization are likely the oxygen fugacity (fO(2)) of the magma and input of mantle-derived material in the Xiong'ershan area. The geochemical and isotopic data show that the emplacement of these intrusions in the EQOB occurred in the post-collision settings during 132-127 Ma.