Kamafugites

Abstract

Kamafugites are special volcanic rocks whose mineralogical and compositional peculiarities are mostly related to processes developed in the subcontinental mantle. These rocks are very rare, occurring only in three localities: the western branch of the East African Rift [Toro Ankole (Uganda) and Virunga (Democratic Republic of Congo) volcanic provinces], central Italy (Intra-Apennine Province) and southeastern Brazil (Alto Paranaíba Igneous Province and Goiás Alkaline Province). Kamafugites cannot be used to infer specific geodynamic settings, being the three variants emplaced in geodynamic frameworks ranging from far-from-active intraplate (Brazil) to continental rifting (Uganda) and complex (compressional to extensional; Italy). Their paragenesis, consisting of primary kalsilite, melilite, olivine, leucite, phlogopite, perovskite, nepheline, and other minor phases, reflects the strongly ultrabasic to basic whole-rock compositions and alkali (mainly K₂O) enrichment, with ultrapotassic to potassic affinity. Brazilian and East African Rift kamafugites show considerable overlaps in major oxides (e.g., high TiO₂, Fe₂O_3(tot)) and trace elements distribution (high LILE, low Th-U-Nb), whereas Italian kamafugites strongly differ in their incompatible element pattern (peaks at Th-U-Pb and troughs at Ba-Nb-Ta-Ti). Wide variations are also found in Sr-Nd-Pb-B isotopic ratios. The mineralogical and geochemical characteristics of the kamafugites cannot be generated by four-phase volatile-free peridotite mantle sources but in a phlogopite-bearing clinopyroxenite source with variable amounts of modal carbonate. The Italian kamafugites emerge for peculiar whole-rock and Sr-Nd-Pb isotopic compositions, which clearly distinguish them from the Brazilian and East African Rift variants, and are interpreted as generated from a subduction-modified mantle, with diffuse recycling of crustal materials.