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

Abstract

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.