Abstract

The Lower Critical Zone of the Eastern Bushveld Complex is a layered sequence of bronzitite layers interrupted by layers of chromitite and chromitic bronzitite and by two units in which olivine is an abundant cumulus mineral. Layering involves variations in both number and proportions of cumulus minerals and in mineral compositions.Bulk mineralogical and chemical compositions of successive units of the zone, as determined from modal analyses and electron microprobe analyses, indicate that the pile is the product of fractional crystallization, but the sequence of rock units and the upward changes in the enstatite content of the bronzite indicate a major reversal of normal fractionation, culminating in the appearance of the olivine-bearing units in the middle part of the zone. Of the various possible explanations of the reversal, resorption of olivine sinking from the top of the magma chamber (Macdonald, 1967) and slow decrease in total pressure (Osborn, 1980) appear most attractive, but slow influx and mixing of new parental magma cannot be excluded.The layering of the Lower Critical Zone reflects a system in which the composition of the liquid phase of the magma was on or close to the bronzite-chromite boundary on the liquidus during accumulation of most of the zone but was on or close to the bronzite-olivine boundary, the olivine-chromite boundary, or the bronzite-olivine-chromite triple junction at various stages of the development of the two olivine-bearing units. Slight changes in T, P, or X could therefore result in changes in the phases precipitating. It is suggested that changes in total pressure, causing shifts in phase boundaries on the liquidus, were the prime factor controlling the changes in mineral assemblages which are represented in the layering of the zone. One result was the formation of the remarkably regular and persistent chromitite layers for which the Lower Critical Zone is justly famous.Structural relations, variations in the enstatite content of bronzite, and the bulk mineralogical and chemical compositions of the Lower Critical Zone as a whole and its lowest unit in particular indicate that the zone continues the fractionation represented in the Lower Zone of the Olifants River trough and is not the product of a separate, later magma.

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