In situ crystallization of Fe-Ti oxides in the Bijigou layered intrusion: Insights from crystal size distributions and compositions of magnetite and ilmenite

[Display omitted] •CSDs for Fe-Ti oxides are used to examine the formation of Fe-Ti oxide ores.•Fe-Ti oxide ores in the Bijigou intrusion are formed by in situ crystallization.•Dissociation of H2O from interstitial liquid can increase oxygen fugacity of melts.•Ion diffusion results in a Cr-increase of magnetite toward the top of each unit. The formation of Fe-Ti oxide ore bodies in layered intrusions is attributed to in situ crystallization of basaltic magmas or accumulation after crystal settling regardless of their thicknesses and occurrences in the intrusion. These two key processes are difficult to be distinguished in terms of chemical compositions of minerals or cumulates. However, the crystal size distributions (CSDs) of Fe-Ti oxides in the rocks can be remarkably distinct in these two processes. In this study, we collected samples from a drill core profile across the major Fe-Ti oxide-rich segment of the Bijigou intrusion, a large and differentiated layered intrusion in Central China, and carried out a detailed study on the CSDs and in situ major and trace elements of magnetite and ilmenite to examine how Fe-Ti oxides are accumulated to form Fe-Ti oxide ore bodies in the intrusion. The major Fe-Ti oxide-rich segment of the intrusion consists of, from the base upward, a Fe-Ti oxide ore layer (∼80 m), a troctolite unit (∼105 m), an oxide gabbro unit (∼153 m) and an apatite-oxide gabbronorite unit (∼165 m). Large grains of magnetite and ilmenite in the Fe-Ti oxide ore layer show logarithmic-linear CSD curves, similar to those yielded by in situ crystallization of Fe-Ti oxides. The modes of Fe-Ti oxides in the oxide gabbro unit and apatite-oxide gabbronorite unit fluctuate along the profile, which cannot be attributed to accumulation after crystal settling. In addition, the magnetite of the Fe-Ti oxide ore layer, oxide gabbro unit and apatite-oxide gabbronorite unit shows a gentle increase trend of Cr concentration from the base upward in each unit, inconsistent with the trend for an accumulation after crystal settling. The magnetite grains enclosed within silicates have similar compositions to those interstitial to silicates, indicating that they all crystallized from similarly interstitial Fe-rich melt. The in situ crystallization of Fe-Ti oxides is likely triggered by increasing oxygen fugacity due to dissociation of H2O in highly evolved interstitial liquid that was expelled from underlying crystal mush. The crystallization of low-Cr magnetite at th