Ankaratra

The Ankaratra volcanic field, which is located immediately southwest of Tananarive, covers about 3,800 km2 and extends for over 100 km in a north-south direction. The greatest elevations reached on Madagascar are represented by volcanoes of the Ankaratra field, several of which have summits higher than 2,500 m, although the gneissose basement is at a height of 1,400 m in the north and 2,000 m in the south of the field. There are numerous explosion craters and breccia and tuff cones and picturesque areas dominated by trachytic and phonolitic domes and spines. The field has undergone heavy tropical weathering and is extensively forested. Five principal rock series have been distinguished for which, from their relationships with associated sediments, a chronology has been erected (Lenoble, 1938; Bussiere, 1957a; Brenon and Bussiere, 1959). These series are (1) Lower Pliocene trachyte, andesite and rhyolite, (2) Middle Pliocene trachyphonolite and phonolite, (3) Upper Pliocene basalt and andesite, (4) Pleistocene ankaratrite and basalt and (5) Quaternary basanite and basanitoid. The ankaratrites, so named by Lacroix (1916), are essentially olivine nephelinites. The early rhyolites are neither extensive nor alkaline but the trachytes are predominantly peralkaline. The rocks constituting many of the domes comprise trachyte containing a riebeckitic amphibole together with some quartz, while aegirine, aenigmatite and biotite are found in some varieties. Trachytic tuffs also occur. Several types of trachyphonolite are described by Lacroix (1923) and they are the rock type comprising several domes. Generally they contain aegirine-augite, which is usually poikilitic towards the feldspar, and a little nepheline or sodalite. Although Bussiere (1957a) and Brenon and Bussiere (1959) list phonolite as a part of the Ankaratra suite, Lacroix (1923) does not describe phonolite from this locality and it is not clear whether the area contains phonolite sensu stricto or only trachyphonolite. Basalt is the most voluminous rock type in the Ankaratra field but Lacroix (1923) and others refer also to andesites. The available analyses (Bussiere, 1957a) indicate a series from basalt, through trachybasalt and basaltic trachyandesite to trachyandesite, according to the nomenclature of Le Maitre et al. (1989). Both basanites, and what Lacroix (1923) calls basanitoides, are found as lavas in the Ankaratra field, particularly in the south between Antsirabe and Betafo, and form some of the more remarkable puys. A similar, but coarser rock, is luscladite which forms a dyke traversing granite. This rock suite comprises labradorite, nepheline, olivine, augite and titaniferous magnetite, with phenocrysts of hornblende in some varieties. The ankaratrites extend over an area of about 35x15 km in the northern part of the field where they form flows. They are compact, black rocks of a very fine grain with some olivine apparent in hand specimen. In section phenocrysts of olivine are seen to be abundant, with rarer phenocrysts of titaniferous augite in a matrix of the same pyroxene, titanomagnetite and a little biotite and perovskite. Nepheline is plentiful and often accompanied by melilite, the latter being present also in some rocks that are free of nepheline; thus they are apparently olivine nephelinites grading to olivine melilitite. A number of varieties are distinguished and described by Lacroix (1923). The most detailed petrographic account of the Ankaratra rocks is that of Lacroix (1923) who also presents numerous rock analyses; there are also two analyses of nephelinite in James (1995).