Mantle-derived parental magmas for the Bushveld Igneous Complex: Reviewed, revisited and reduced

Abstract The immense size of the Bushveld Igneous Complex (BIC) allows for a diversity of different parental magma types. Restricting ourselves to its mantle-derived (ultra-)mafic parents, with a special emphasis on the Rustenburg Layered Suite (RLS), these are first reviewed before presenting field, petrographic and bulk rock geochemical data on 35 new mafic-ultramafic sill samples, from a supra-Machadodorp Mb transect through the uppermost Pretoria Group. As observed by others, these sills can be subdivided into (1) more altered and evolved metadolerites, tentatively linked to low-Ti (LT) lavas of the Dullstroom Formation, (2) generally fresher ‘boninitic’ and melanoritic LT sills that are convincingly recognised as initial B1 magmas for the RLS, and (3) dunitic to harzburgitic sills that are likely basal cumulates to either of the above two groups. Focusing more on bulk rock geochemistry, including published data on parental basic magmas, we caution against the liberal use of parental candidates, concluding that only a very specific B1 composition, with a very strong lithospheric component, is certainly parental to the RLS, while an additional asthenospheric high-Ti (HT) parent still needs to be more confidently linked to it. In fact, we speculate on the possibility for (1) a solitary B1 parent for the entire RLS, (2) its generation from a uniquely TTG melt enriched and orthopyroxenitic sub-cratonic lithospheric mantle, and (3) a scatter of lesser HT magma injections to have mainly avoided the RLS in both space and time.