Alkaline Rocks and Carbonatites of the World

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

Nosy Komba


Occurrence number: 
Longitude: 48.33, Latitude: -13.47

Nosy Komba is an approximately circular island 5 km in diameter lying immediately south of Nosy-Be. It comprises an outer ring of schists of Liassic age that are cut and metamorphosed by a centrally located intrusion of amphibole-bearing gabbro and nepheline syenite and smaller intrusions and dykes of nepheline syenite, phonolite and microsyenite. The nepheline syenites are characteristically heterogeneous varying from leucocratic to mesocratic and in many places are choked with inclusions of essexite or theralite, which are probably hybrids resulting from reaction of gabbro with nepheline syenite. The nepheline syenite is described petrographically in some detail by Lacroix (1922b, p. 614) and consists of microperthite, nepheline, analcime, aegirine and a little zircon, fluorite, biotite and what Lacroix terms rinkite (mosandrite). Various textural and mineralogical types of nepheline syenite and more basic dykes occur (tinguaite, berondrite, mafraite etc.) and variously include arfvedsonite, biotite, lavenite, aenigmatite and nepheline. Analyses of many of these rock types are given by Lacroix (1922b).

The more leucocratic varieties of nepheline syenite have been investigated as a potential raw material for glass manufacture (Allen and Charsley, 1968).
K-Ar on a phonolite, incorrectly described as a flow, gave 10.38±0.73 Ma (Emerick and Duncan, 1982).

ALLEN, J.B. and CHARSLEY, T.J. 1968. Nepheline-syenite and phonolite. Her Majesty’s Stationery Office, London. 169 pp.EMERICK, C.M. and DUNCAN, R.A. 1982. Age progressive volcanism in the Comores Archipelago, western Indian Ocean and implications for Somali plate tectonics. Earth and Planetary Science Letters, 60: 415-28.LACROIX, A. 1922b. Minéralogie de Madagascar. 2. Minéralogie appliquée lithologie. A. Challamel, Paris. 694 pp.

Fig. 3_137 The extensive Ampasindava area and the islands of Nosy-Be and Nosy Komba (after Cartes geologiques, 1:500,000, Feuille 1, Diego-Suarez, and 2, Antalaha. Besairie, 1971a and 1971b).
Scratchpads developed and conceived by (alphabetical): Ed Baker, Katherine Bouton Alice Heaton Dimitris Koureas, Laurence Livermore, Dave Roberts, Simon Rycroft, Ben Scott, Vince Smith