Itasy

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.