Abundant Fe–Ti oxide inclusions in olivine from the Panzhihua and Hongge layered intrusions, SW China: evidence for early saturation of Fe–Ti oxides in ferrobasaltic magma

Abstract

Abundant Fe–Ti oxide inclusions in cumulus olivine (Fo77–81) from the Panzhihua and Hongge intrusions, Emeishan large igneous province, SW China, document the first evidence for early crystallization of Fe–Ti oxides in ferrobasaltic systems in nature. The intrusions also contain significant stratiform Fe–Ti–V oxide ores. The oxide inclusions are sub-rounded or irregular, range from ∼5 to 50 μm in diameter, and are dominated by either titanomagnetite or ilmenite. The fact that the inclusions are either titanomagnetite- or ilmenite-dominant suggests that they are trapped crystals, instead of immiscible oxide melt, formed during growth of the host olivine. The absence of other silicate phases in the inclusion-bearing olivine is difficult to reconcile with a possible xenocrystic origin of the oxide inclusions. These oxide inclusions are thus interpreted to be cumulus minerals crystallized together and trapped in olivine from the same parental magma. In addition to Fe–Ti oxides, some inclusions contain amphibole + biotite ± fluorapatite that might have formed by reaction of trapped hydrous liquid with the host olivine. Numerical modeling of high-Ti Emeishan basalts using the MELTS program successfully simulates early crystallization of olivine (∼Fo81) and Fe–Ti spinel in the presence of a moderate amount of H2O (∼1.5 wt%) under pressure and fO2 conditions generally pertinent to the Panzhihua and Hongge intrusions. The modal mineralogy of the oxide inclusions is in good agreement with the bulk compositions of the ore, as inferred from whole-rock data, in a given intrusion. This is consistent with the interpretation that the stratiform oxide ores in the intrusions formed by accumulation of Fe–Ti oxide crystals that appeared on the liquidus with olivine and clinopyroxene.