A NEWLY DISCOVERED GABBROIC INTRUSION IN THE BAIMA REGION OF THE EMEISHAN LARGE IGNEOUS PROVINCE: IMPLICATIONS FOR METALLOGENESIS

The Middle Permian Emeishan large igneous province (ELIP) hosts numerous magmatic Ti-V oxide deposits within evolved layered mafic-ultramafic intrusions. The Baima region of the ELIP contains the oxide ore-bearing Baima layered gabbro and the contemporaneous Huangcao, and Woshui A1-type granitoids. The country rock to the west of the Huangcao pluton and Baima layered gabbro is reported to be Neoproterozoic (~886 Ma) two-pyroxene plagioclase gneiss, but this study shows that it is undeformed medium to coarse grained, equigranular biotite-olivine gabbro. Zircon U-Pb LA-ICP-MS/MS dating of the gabbro yielded Middle Permian weighted-mean ages of 257.9 ± 2.5 Ma, 260.2 ± 2.0 Ma, and 262.9 ± 2.2 Ma. The zircon ages are within uncertainty of the spatially associated Huangcao syenite (258.9 ± 0.7 Ma), Baima layered gabbro (261 ± 2.0 Ma), and Woshui syenite (259.6 ± 0.5 Ma) indicating that all of the plutonic bodies were emplaced within a short period of time. The newly discovered gabbro is texturally and mineralogically distinct from the Baima layered gabbro as it is composed primarily of plagioclase (45-55 vol%) and clinopyroxene (25-35 vol%) with a significant amount of interstitial biotite (5-20 vol%) and subordinate olivine and apatite. The rocks are compositionally evolved (SiO₂ = 45.4-50.0 wt%; MgO = 4.3-7.1 wt%; Mg# = 37.9-53.0; Ni = 7-45 ppm), LREE-enriched (La/Yb_N = 4.7-10.2), and do not have pronounced chondrite normalized Eu-anomalies (Eu/Eu* = 0.83-0.97). Zircon Hf isotopes from two samples in different locations have different ε_Hf(t) values with one sample yielding a weighted-mean value of +1.0 ± 0.35 and the other of +8.2 ± 0.47. The ε_Hf(t) values correlate with higher (Th/Nb_PM = 1.5) and lower (Th/Nb_PM = 0.3) Th/Nb_PM ratios suggesting that the intrusion was contaminated by crustal materials (i.e., fluid/melt). The primary melt composition of the uncontaminated rocks is picritic and yielded magma and mantle potential (T_P) temperatures of 1448-1466^oC and of 1567-1586^oC. The high T_P estimate and high Tb/Yb_PM (1.66-1.85) ratios of the uncontaminated rocks suggest that the parental magma was derived from a garnet-bearing sub-lithospheric mantle source. The results of this study demonstrate that the Baima region was one of intense high temperature magmatism which may have been a contributing factor to the metallogenesis of the host magmas.