Abstract
Lavas ocouring in the western Rift of Central Africa are characterized by the presence of leucite both as phenocrysts and in the groundmass. Samples studied from the Bufumbira Bay area of the Birunga volcanic field, in south-west Uganda, form two distinct chemical series diverging from basanite compositions. One differentiates through phonolitictephrites to a trachyte and the other differentiates through leueitites to a phonolite. The low-pressure fractionation controls of both series are discussed in terms of the coexisting phases and ultramafic inclusions observed in the lavas. Equilibrium fusion of a mantle phlogopite-bearing peridotite combined with subsequent fractionation is the favoured petro-genetic process to produce a potash-rich basanite liquid, which is considered a likely parent of the Bufumbira lavas.
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