In the Brits area of the Bushveld Complex, the stratigraphically lowermost magnetitite layer of the Upper Zone is underlain by an anorthosite containing discrete, euhedral magnetite crystals, measuring 0.1–0.3 mm in diameter, which form thin layers (1 mm-1 cm thick) and laminae (commonly one grain in thickness). Discordant relationships between laminae and layers indicate a consistent younging direction upwards towards the contact with the overlying magnetitite. Our interpretation is that these crystals are cumulus and have escaped modification by postcumulus overgrowth. They also record, in places, the morphology of the floor onto which they accumulated. Their preservation is largely attributed to adcumulus growth of plagioclase before, during and after, accumulation, but may also indicate that there was only a limited supply of magnetite to the floor, thus precluding grain contact and prohibiting grain enlargement. The results of a study of compositional variations across individual plagioclase grains suggest that the presence of magnetite within the crystallising mush hindered the development of an ideal plagioclase adcumulate.

The occurrence of spheroidal, ovoid or deeply embayed plagioclase inclusions within cumulus bronzite and olivine grains of both pyroxenites and norites here implies the existence of plagioclase feldspar in melts prior to the separation of ultramafic cumulates from these melts. Variations of Sr-isotope initial ratio are shown to exist within the feldspar populations of such rocks, even on a grain-to-grain scale. Five separate stratigraphic intervals within the Lower and Critical Zones exhibit this texture, and in each case these are intervals wherein normal fractionation trends (decline in both whole-rock and orthopyroxene Mg/(Mg+Fe2+); decline in Cr contents of orthopyroxene) became reversed. The data support a model for evolution of the texture during major episodes of influx of hot, primitive magmatic liquid into the chamber. Progressive mixing of the new liquid with resident liquids (bearing feldspar on the liquidus) effected progressive changes in the composition of the hybrid liquids produced, and resorption and imposition of reversed zoning upon the inclusions.