Fractional crystallization and the formation of thick Fe-Ti-V oxide layers in the Baima layered intrusion, SW China

The Baima layered intrusion is located in the central part of the Emeishan Large Igneous Province (ELIP). The N-S striking intrusion is similar to 24 km long and similar to 2 km thick and dips to the west. Based on variations in modal proportions and cumulus mineral assemblages, the intrusion from the base to the top is simply subdivided into a lower zone (12) with most of the economic magnetite layers, and an upper zone (UZ) with apatite-bearing troctolite and gabbro. The rock textures suggest crystallization of the Fe-Ti oxide slightly later than plagioclase (An(67-54)) but relatively earlier than olivine (Fo(74-55)), followed by clinopyroxene and finally apatite. Relatively low olivine forsterite content and abundant ilmenite exsolution lamellae in clinopyroxene indicate that the Baima parental magma is a highly evolved Fe-Ti-rich magma. Via MELTS model, it demonstrates that under a closed oxygen system, extensive silicate mineral fractionation of a picritic magma might lead to Fe and Ti enrichment and oxygen fugacity elevation in the residual magma. When such Fe-Ti-rich magma ascends to the shallower Baima intrusion, the Fe-Ti oxides may become an early liquidus phase. Well-matched olivine and plagioclase microprobe data with the results of MELTS calculation, combined with relatively low CaO content in olivine (0.02-0.08 wt.%) indicate that wall-rock contamination probably plays a weak role on oxygen fugacity elevation and the early crystallization of Fe-Ti oxides. Several reversals in whole-rock chromium and plagioclase anorthite contents illustrate that multiple recharges of such Fe-Ti-rich magma mainly occurred along the lower part of the Baima magma chamber. Frequent Fe-Ti-rich magma replenishment and gravitational sorting and settling are crucial for the development of thick Fe-Ti oxide layers at the base of the Baima layered intrusion. (C) 2012 Elsevier B.V. All rights reserved.