Petrogenesis and metallogenesis of the Taihe gabbroic intrusion associated with Fe-Ti-oxide ores in the Panxi district, Emeishan Large Igneous Province, southwest China

摘要

The Taihe layered gabbro intrusion in the northernmost part of the Panxi district in southwest China is part of the 260 million year old Emeishan Large Igneous Province. This intrusion hosts a giant Fe-Ti oxide deposit with 810 million tonnes of ore reserves, which makes it one of the largest deposits in the Panxi district. The intrusion covers an areal extent of 13 km~2 and has a vertical stratigraphic thickness of 1400 m. It can be divided into a lower zone (LZ) of coarse-grained gabbro, apatite-bearing gabbro, troctolite and intercalated gabbro and clinopyroxene-bearing troctolite, followed upward by a middle zone (MZ) of gabbro and intercalated clinopyroxenite, plagioclase-bearing clinopyroxenite with major oxide layers, and an upper zone (UZ) of olivine gabbro and layered gabbro including unmineralized leucogabbro and melanogabbro, with some small oxide ore bodies in the lower part. Each of these 'zones' contains oxide minerals and relatively similar lithologies. Ore textures and associated mineral assemblages indicate that the ore bodies formed by crystallization of Fe-Ti-V-rich melt under high oxygen fugacity and a volatile-rich environment during the late-stage of magmatic differentiation. A general systematic variation of major oxides is seen through the intrusion as reflected by a slight overall decrease in MgO and Fe_2O_3 (as total iron) and an increase in SiO_2, Na_2O, Al_2O_3, and CaO upward in the layered sequences. Based on lithology and bulk-rock geochemical features, such as positive Eu anomalies, the Taihe intrusion is inferred to have been derived from a ferropicritic melt and became more evolved in chemistry upward following a tholeiitic differentiation trend with enrichment in Fe, Ti, and V. The Taihe gabbros define a small range of age-corrected _(Nd)(t) (t=260 Ma) from -0.6 to 0.7 and (~(87)Sr/~(86)Sr)_t ratios ranging from 0.7040 to 0.7050. The relatively lower _(Nd)(t) values and higher (~(87)Sr/~(86)Sr)_t ratios compared to those from Lijiang picrite which represents the initial product of Emeishan plume head, combined with the enrichment in light rare earth element (LREE) relative to heavy rare earth element (HREE) as well as negative high field strength element (HFSE; e.g., Nb, Ta, Zr, and Hf) anomalies, suggest that subduction-related material was involved in the source region. We propose that the parental picritic magma was generated from the interaction of the 260 million year Emeishan mantle plume with the lithospheric mantle, and the picritic magma interacted with an eclogitic component in the lithospheric mantle. In our view, the eclogitic component was derived from the earlier Neoproterozoic subduction. The junction of these subduction-modified lithospheric mantle sources and the Emeishan plume was a possible crucial factor leading to the production of large Fe-Ti oxide deposits in the Panxi area.