Alkaline Rocks and Carbonatites of the World

Setup during HiTech AlkCarb: an online database of alkaline rock and carbonatite occurrences

North Tadjoura


Occurrence number: 
Longitude: 42.95, Latitude: 12.13

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).

Age determinations range from 14.2 to 9.7 Ma (Varet, 1978).
DENIEL, C., VIDAL, P., COULON, C., VELLUTINI, P.-J. and PIGUET, P. 1994. Temporal evolution of mantle sources during continental rifting: the volcanism of Djibouti (Afar). Journal of Geophysical Research, 99, B2: 2853-69.GADALIA, A. and VARET, J. 1983. Les rhyolites Miocènes de l'est de l'Afar. Bulletin de la Société Géologique de France, Serie 7. 25: 139-53.RICHARD, O. 1985. Les formations volcaniques. In Notice explicative. Carte Géologique de la République de Djibouti a 1:100,000. Tadjoura. 9-37. Editions de l'ORSTOM. Institut Français de Recherche Scientifique pour le Developpment en Cooperation. Paris.VARET, J. 1978. Geology of Central and Southern Afar (Ethiopia and Djibouti Republic). Editions du Centre National de la Recherche Scientifique, Paris. (Explanation accompanying Geological map of central and southern Afar. 1:500,000).
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