Alkaline Rocks and Carbonatites of the World

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



Occurrence number: 
Longitude: 46.68, Latitude: -18.95

The Itasy volcanic field covers some 300 km2 and includes numerous domes, scoria cones with summit craters and lava flows and overlies Precambrian granites and gneisses. The volcanism has been divided into six principal phases (Lenoble, 1940) which, from oldest to youngest, comprise: (1) trachyte domes, (2) an older basanite and limburgite series forming domes, cones and flows, (3) a large flow of trachyte (Andranonatoa flow), (4) flows and scoria of ordanchite (hauyne-andesine phonotephrite or trachyandesite), (5) a recent basanite series and (6) a late explosive crater-forming event. The trachytes vary mineralogically from types comprising andesine plus an augitic pyroxene to peralkaline varieties in which the feldspar, of both phenocrysts and matrix, is anorthoclase, the pyroxene aegirine-augite and there may be a little interstitial quartz, while nepheline-bearing trachytes and true phonolites also occur (Lacroix, 1912), in some of which probable nosean is present (Lacroix, 1923). The basanitic rocks are the most voluminous of the field and form scoria, generally as cones, as well as flows. The greater part of these rocks are described as basanitoids by Lacroix (1923) and contain only normative nepheline, together with modal olivine, augite and labradorite and these grade into limburgites, in which both modal nepheline and feldspar are absent. The ordanchite contains phenocrysts of titanaugite, hornblende, hauyne, titaniferous magnetite and apatite in a matrix of oligoclase to andesine, microlites of augite and very tiny crystals of hauyne. Lacroix (1923) describes bombs in which the blue hauyne is very abundant and, with resorbed hornblende crystals, set in a glassy matrix. Chemical analyses of the Itasy rocks will be found in Lacroix (1923) and Bussiere (1957b), while photomicrographs and general views illustrating, particularly, the volcanic topography accompany Lenoble’s (1940) account.

Probably similar to Ankaratra (No. 102-00-013).

BUSSIERE, P. 1957b. Le massif volcanique del’Itasy. Travaux du Bureau Géologique. Haut Commissariat de Madagascar et Dependances, Service Géologique, Tananarive, 83: 159-67.LACROIX, A. 1912. Les volcans du centre de Madagascar: le massif de l’Itasy. Compte Rendus Hebdomadaires des Séances de l'Académie des Sciences, Paris. 154: 313-7. lithologie. A. Challamel, Paris. 694 pp.LACROIX, A. 1923. Minéralogie de Madagascar. 3. Lithologie Appendice - Index Geographique. Société d’Éditions Géographiques, Maritimes et Coloniales, Paris. 450 pp.LENOBLE, A. 1940. Études sur la géologie de Madagascar. III. Le massif volcanique de l’Itasy. Mémoires de l’Académie Malgache, Tananarive. 32: 43-77. NICOLLET, C. 1984. Le volcanisme dans le Sud-Ouest Madagascar. Journal of African Earth Sciences, 2: 383-8.

Fig. 3_141 Itasy (after Lenoble, 1940, map 1:100,000).
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