Over a large area extending from the vicinity of Tadjoura, on the Gulf of Tadjoura, northwards to the Ethiopian border a sequence of thick rhyolitic flows, domes, dykes and pyroclastic rocks, known as the Mablas (Mabla) series, is predominantly of peralkaline composition, but some 30% of the sequence consists of flows of basaltic and intermediate rocks. Gadalia and Varet (1983) estimate that ignimbrites occupy 40% of the area, flows and domes 40% and other pyroclastic rocks 20%. The rhyolitic rocks vary from peraluminous to comenditic and pantelleritic. Modal quartz is absent from the peraluminous rhyolites but they contain sodic plagioclase, augite, zircon and an opaque phase; some rocks contain biotite and tiny grains of fayalite may occur in the groundmass. The peralkaline rhyolites have quartz phenocrysts together with sanidine, hedenbergite and aegirine, arfvedsonite, and accessory aenigmatite, titanomagnetite, apatite and titanite (Gadalia and Varet, 1983; Richard, 1985). Mineralogical and petrochemical data are given by Gadalia and Varet (1983).