The Bushveld Complex

Abstract

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. The varied mineralogy and thickness of the Marginal Zone indicate that it is not a quenched magma, but represents variable cumulus enrichment into several chemically different magmas, only some of which may be representative of magma producing the layered rocks. The complex was emplaced by repeated injections of magma ranging in volume from small to large. Evidence supporting the concept of multiple injections lies in the presence of distinct breaks in initial Sr-isotopic ratio, textures indicating partial resorption of earlier crystallizing phases, abrupt reversals of fractionation trends defining saw-tooth profiles through successive units, and protracted reversals in mineral compositions through hundreds of metres of section that culminate in primitive olivine-rich cumulates in the Lower and Critical Zones. The small influxes yielded localized partial cyclic units Studies of the Critical Zone in the Western limb, for which more comprehensive data are available than elsewhere, reveal a gradation along ca. 200 km of strike from a more primitive proximal facies in the northwest to a more evolved distal facies in the southeast. This concept helps to explain major problems in the Cr budget of the entire limb, the greater proportion of chromite- and olivine-rich cumulates in the proximal facies, and the more feldspathic character of the distal sequence. In the Eastern limb changes in relative spacing between, and thicknesses of, chromitite layers from north to south suggest a similar process, but the transition is abrupt rather than gradational. No single process can account for all forms of layering. Despite certain limitations, largescale 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 magnetitite layers suggests that they were derived from relatively thin liquid layers, and that magma addition and mixing did not occur. The origin of the incomplete cycles which may range from pyroxene- and/or olivine-rich to feldspar-rich layers of the Lower, Critical, and Main Zones appears to be intimately bound up with the emplacement of fresh magma batches. These mixed with partially crystallized residual liquids to produce cumulates bearing incompletely resorbed plagioclase inclusions, highly variable Sr-isotopic ratios, and non-cotectic proportions of phases. Chemical fingerprinting of pyroxenes in Critical Zone rocks shows that gravitational sorting must have played some part in the generation of pyroxenites and in yielding non-cotectic norites. Prominent layering near the top of the Main Zone in the Eastern limb cannot be explained by injection of fresh magmas, oscillatory nucleation or crystal settling, leaving the action of density currents as the most probable mechanism. Whereas layering is well developed in parts of the Upper Zone, the cyclicity of the Critical Zone is absent. The existence of an extremely thick liquid column in the chamber during the later stages is indicated by the 2500 m-thick, isotopically homogeneous sequence above the Pyroxenite Marker. Layering and reversals in mineral composition within this interval may have resulted from the breakdown of stratified liquid layers or convective overturn, rather than addition of magma.