Abstract:
The Bushveld Complex (South Africa) is the largest layered intrusion on Earth and plays a
considerable role in our understanding of magmatic differentiation and ore-forming processes.
In this study, we present new geochemical data for apatite-hosted multiphase inclusions in
gabbroic cumulates from the Bushveld Upper Zone. Inclusions re-homogenized at high temperature
(1060-1100°C) display a range of compositions in each rock sample, from iron rich
(35 wt.% FeOtot; 28 wt.% SiO2) to silica-rich (5 wt.% FeOtot; 65 wt.% SiO2). This trend is
best explained by an immiscible process and trapping of contrasted melts in apatite crystals
during progressive cooling along the binodal of a two-liquid field. The coexistence of both Si rich
and Fe-rich immiscible melts in single apatite grains is used to discuss the ability of
immiscible melts to segregate from each other, and the implications for mineral and bulk
cumulate compositions. We argue that complete separation of immiscible liquids did not
occur, resulting in crystallization of similar phases from both melts but in different
proportions. However, partial segregation in a crystal mush and the production of contrasting
phase proportions from the Fe-rich melt and the Si-rich melt can be responsible for the cyclic
evolution from melanocratic (Fe-Ti-P-rich) to leucocratic (plagioclase-rich) gabbros which is
commonly observed in the Upper Zone of the Bushveld Complex where it occurs at a vertical
scale of 50 to 200 m.
