Garnet and clinopyroxene megacrysts found in the Uintjiesberg and Lekkerfontein kimberlites, southern Africa, have been analysed for their major and trace element compositions. Minerals from both megacrysts suites define coherent geochemical trends that can be attributed to differentiation via crystallisation in a closed system. Clinopyroxene megacrysts from both localities are of the subcalcic, low-Cr variety and have Mg#’s that range from 0.821 to 0.890, whereas garnets have Mg#’s in the range 0.702 to 0.824. Clinopyroxene megacrysts from both localities are strongly light rare earth element (REE) enriched when normalised to chondrites (La/Ybn ∼5), whereas garnets are all strongly LREE depleted (La/Ybn <0.014) with flat HREE patterns; both mineral types show strongly subparallel REE patterns. Primitive mantle normalised trace element patterns are likewise subparallel, with clinopyroxene megacrysts reflecting strong incompatible element enrichment and garnet megacrysts incompatible element depletion. Quantitative modelling of REE patterns in both garnet and clinopyroxene megacrysts provides compelling evidence that an ocean island basalt (OIB)-like parental magma, rather than a primary kimberlite magma, gave rise, over an extended (∼60 to 75%) crystallisation period, to the megacryst suite minerals from both localities. Variations in Sr and high field strength element abundances are consistent with this model.