The Bushveld Complex

Abstract

The layered mafic rocks of the Bushveld Complex are derived from three magmatic lineages - a lower part comprising the Lower and Critical Zones which crystallized from high-Mg and - Si parent liquids, an isotopically distinct Lower Main Zone derived from more evolved aluminous tholeiitic liquids, and a succession above the Pyroxenite Marker, which includes the entire Upper Zone, stemming from the mixing of residua of these earlier liquids with a final major injection of tholeiitic liquid. No single process can account for all forms of layering. Despite certain limitations, large-scale magma mixing is the most plausible mechanism to bring the liquid composition into the primary phase volume of chromite to produce chromitite layers. By contrast, the trace-element chemistry of magnetite layers suggests that they were derived from relatively thin liquid layers, and that magma addition and mixing did not occur.