Alkaline Rocks and Carbonatites of the World

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Occurrence number: 
Longitude: 44.95, Latitude: -24.25

In the Mahafaly region there is an extensive dolerite dyke swarm, apparently associated with which are 10 small plugs (Boulanger, 1957a, Fig. 1) all of which consist of ankaratrite (Boulanger, 1957b); four of these are depicted on the 1:500,000 geological map (Besairie, 1970b). The plugs are subcircular, have positive relief and vary between 100 and 500 m in diameter. The ankaratrite (nephelinite) comprises olivine, augite and a little nepheline, an analysis of one being given by Lacroix (1923). Rocks referred to as limburgitic ankaratrite are also described by Lacroix (1923) and Boulanger (1957a) in which the olivine, pyroxene and nepheline are set in glass or analcime, probably the former. Rocks described as basanites of the “Southwestern dyke swarm”, which contain modal nepheline, have been analysed by Dostal et al. (1992). These rocks may derive from the Mahafaly area.

K-Ar on basanites gave 66(1 and 71(3 Ma (Dostal et al., 1992).
BESAIRIE, H. 1970b. Carte Géologique. Feuille 8. Ampanihy. 1:500,000. Service Géologique de Madagasikara, Tananarive. BOULANGER, J. 1957a. Le système volcanique fissural du pays Mahafaly (SW de Madagascar). Bulletin de la Société Géologique de France, Ser 6, 7: 529-37.BOULANGER, J. 1957b. Les dykes du pays Mahafaly. Travaux du Bureau Géologique. Haut Commissariat de Madagascar et Dependances, Service Géologique, Tananarive, 83: 101-12.DOSTAL, J., DUPUY, C., NICOLLET, C. AND CANTAGREL, J.M. 1992. Geochemistry and petrogenesis of Upper Cretaceous basaltic rocks from southern Malagasy. Chemical Geology, 97: 199-218. LACROIX, A. 1923. Minéralogie de Madagascar. 3. Lithologie Appendice - Index Geographique. Société d’Éditions Géographiques, Maritimes et Coloniales, Paris. 450 pp.
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