The role of intercumulus residua in the formation of pegmatoid associated with the UG2 chromitite, Bushveld Complex

Abstract

The UG2 chromitite in the Critical Zone of the Bushveld Complex is underlain by pyroxenite, the uppermost 50 cm of which is a pegmatoidal pyroxenite. Overlying it is an olivine pyroxenite. Concentrations of K2O, Ba, Zr and P2O5 show no enrichment in pegmatoid relative to over‐ and underlying silicate rocks. Whole rock Mg/(Mg+Fe) ratios and normative plagioclase compositions show no systematic differences between coarsegrained and adjacent finer‐grained lithologies. Potholes are associated with the UG2 but no geographic or structural relationships exist between UG2 and Merensky potholes. Comparison of geochemical profiles through normal and potholed UG2 chromitite show no evidence of fluid channelling as has been documented through Merensky potholes. These observations do not support the hypotheses of preferential trapping of, or reaction with, residual liquid or fluid at the magmatic stage in these pegmatoidal horizons or in the formation of potholes. Injection of hot primitive magma is indicated by the formation of a substantial chromitite layer. Complete melting or disaggregation and dispersion of the uppermost cumulates, especially anorthosites, is a probable consequence of addition of this hotter magma. It may also account for the lateral regional lithological changes in the immediate footwall to the UG2 chromitite. Where pyroxenite forms the immediate floor, reheating will produce an orthopyroxene‐saturated dense layer of liquid that will not mix with the overlying magma. At the base of this liquid layer there will be a zone of pyroxene crystals plus interstitial liquid that will be held above the solidus for longer than in a normal sequence that is not heated from above, and this may permit prolonged growth of crystals resulting in the pegmatoidal texture. Olivine occurs interstitially to orthopyroxene in the rocks overlying the UG2 chromitite. This results from the sinking of orthopyroxene from the elevated residual liquid into the more primitive new magma which crystallizes olivine in situ around the orthopyroxene.