Petrogenesis of strongly alkaline primitive volcanic rocks at the propagating tip of the western branch of the East African Rift

Strongly silica-undersaturated potassic lavas (kamafugites) and carbonatitic tuffs are characteristic of the Toro-Ankole volcanic field in southwestern Uganda, forming the youngest and most northward volcanics of the western branch of the East African Rift. Lavas contain exceptionally low SiO2 (31.8–42.8 wt.%), high CaO (up to 16.6 wt.%) and K2O (up to 7 wt.%). They exhibit moderately enriched correlated Nd (εNd − 0.1 to − 4.7) and Hf (εHf − 0.1 to − 8.8) isotope signatures, indicating time-integrated enrichment in incompatible elements in the source, attributed to mixing between two metasomatic assemblages, a phlogopite-rich MARID-type and a later carbonate-rich assemblage. The restricted range of 87Sr/86Sr (0.704599–0.705402) is due to Sr being dominated by the carbonate-rich assemblage, which also imparts a Nd and Hf signature similar to convecting upper mantle. Os isotopes (γOs up to 290 and variable Os concentrations of 0.056–1.454 ppb) are curved due to mixing between the carbonate-rich metasome and a second end-member that may be derived from melting peridotite, the MARID assemblage, or a mixture of both. Enrichment of the peridotitic mantle in carbonate and silicate melts at 4–6 GPa occurs also in other areas where geochemically similar ultramafic lamprophyres result. The Ugandan kamafugites thus represent the earliest and deepest-derived magmas in a rift through thick continental lithosphere beneath the continuous Congo–Tanzania craton. The Ugandan rift-related mantle enrichment is older than the earliest known tectonic surface expression of the rift.