Cr and Sr: Keys to parental magmas and processes in the Bushveld Complex, South Africa

Large layered intrusions are almost certainly periodically replenished during their protracted cooling and crystallization. The exact composition(s) of the replenishing magma(s) in the case of the Bushveld Complex, South Africa, has been debated, mainly on the basis of major element composition and likely crystallization sequences. The intrusion is dominated by orthopyroxene and plagioclase, and so their Cr and Sr contents, and likely partition coefficient values, can be used to re-investigate the appropriateness of the various proposed parental magmas. One magma type, with about 12% MgO, 1000 ppm Cr and 180 ppm Sr, can explain the genesis of the entire Lower and Critical Zones. A number of other magma compositions proposed to produce the Critical Zone fail to match these trace-element constraints by being too poor in Cr. A fundamentally different magma type was added at the base of the Main Zone, but none of the proposed compositions is consistent with the trace-element requirements. Specifically, the Cr contents are higher than predicted from pyroxene compositions. A further geological constraint is demonstrated from a consideration of the Cr budget at this level. There is an abrupt decrease from about 0.4% to 0.1% Cr₂O₃ in orthopyroxene across this Critical Zone–Main Zone transition. No realistic proportions of mixing between the residual magma at the top of the Critical Zone and any proposed added magma composition can have produced a composition that could have crystallized these low-Cr orthopyroxenes. Instead, it is suggested that the resident magma from the Upper Critical Zone was expelled from the chamber, possibly as sills into the country rocks, during influx of a dense, differentiated magma. Near the level of the Pyroxenite Marker in the Main Zone, there is further addition of a ferrobasaltic magma, with 6% MgO, 111 ppm Cr and 350 ppm Sr, that is consistent with the geochemical requirements.