Nephelinites from the Gregory Rift

Nephelinites are strongly SiO2-undersaturated volcanic rocks that are often associated with phonolites and carbonatites. In the Gregory Rift in East Africa several major nephelinitic-phonolitic volcanoes occur, with some of them being associated with carbonatitic rocks (e.g., Oldoinyo Lengai, Kerimas, Mosonik, Shombole, Meru), while others lack carbonatites (e.g, Sadiman, Essimingor, Burko). We characterize the magmatic evolution of the Burko volcano and compare our results with published data from spatially associated nephelinite-phonolite±carbonatite associations in the Gregory Rift and elsewhere. Overall, nephelinites show mineralogical differences, are variably evolved (in terms of XMg, LILE and HFSE), and in some cases peralkaline (Na+K/Al >1) nephelinites do occur. Besides nepheline, clinopyroxene and apatite, garnet, magnetite, perovskite and titanite are magmatic phases in most cases. However, magmatic ne-cpx-grt-ttn assemblages can be distinguished from those with ne-cpx-mag-prv. Other phases, such as wollastonite, melilite, combeite, aenigmatite, sodalite and others are restricted to some occurrences and resemble different geochemical flavors of nephelinites, different crystallization conditions, variable differentiation stages and different levels of peralkalinity. Redox- and silica activity-dependent phase equilibria allow for constraining and comparing the magmatic evolution of the different localities by combining textural with mineral chemical data. In general, high redox conditions above FMQ and peralkalinity seem to favor the formation of carbonatites. However, in several cases that meet these conditions, no carbonatites are exposed and worldwide, carbonatites are often associated with nephelinites that are not peralkaline. We discuss the potential for nephelinites to exsolve carbonate-rich liquids based on a petrological and geochemical comparison of different occurrences.