The panzhihua intrusion, SW China

The ~ 260 Ma Panzhihua intrusion is an elongate layered gabbroic intrusion with a length of ~ 19 km and a thickness of ~ 2 km in the central part of the Emeishan large igneous province, SW China. It is made up from the base to top of microgabbros (Marginal zone, MGZ), gabbros, oxide gabbros and Fe-Ti oxide ores (Lower zone and Middle zone a, LZ and MZa), apatite-bearing leucogabbros (Middle zone b, MZb), and gabbros and oxide gabbros (Upper zone, UZ). Igneous layering is observed in rocks of the MZa, MZb and UZ. Textures and geochemical data indicate that, with the exception of the MGZ, rocks in the other zones represent cumulates. The most plausible parental magma, inferred from rock compositions from the MGZ, is a moderately fractionated ferrobasalt rich in FeO, TiO2 and poor in SiO2 relative to common tholeiitic liquids and such a composition, although rare, is known in the high-Ti basalts of the Emeishan large igneous province. Relatively low (87Sr/86Sr)260 Ma (0.7039–0.7055) and high εNd260 Ma (− 2.1 to + 4.3) indicate a juvenile mantle source and minimal contamination by old continental crust during crystallization. Differentiation of the intrusion was driven by fractionation of a magnetite gabbroic assemblage (clinopyroxene + plagioclase + titanomagnetite ± olivine), from which the Fe-Ti oxide ores formed primarily through gravitational accumulation in the LZ and MZa. Apatite and ilmenite joined the fractionation assemblage in the MZb. The early appearance of liquidus titanomagnetite relative to well-studied layered intrusions might be an effect of the unusual bulk composition of the parental magma, magma-wall rock interaction, the presence of volatiles, or any combination thereof. The differentiation trend was interrupted by at least two magma recharge events and some residual liquid escaped from the magma chamber. The A-type granitoids intimately associated with the intrusion have elemental and Sr-Nd isotopic features consistent with protracted differentiation of the ferrobasaltic parental magma, implying the process was driven towards silica enrichment.