Le volcanisme à Madagascar

Abstract

The important cretaceous volcanic manifestations on the west and east coasts of Madagascar, as well as the flows in the Androy Range, are formed by calcoalcalinous series of Pacific type, ranging from rhyolites to basalts. Basic veins with diabasic facies frequently cut sedimentary rocks in the west, up to the upper cretaceous, as well as the cristalline basement of the eastern shore. The Pacific type of the emissions shows apparently the great depth reached by the cretaceous breaking tectonic. Strictly tertiary, probably neogene, manifestations, in the south of Alaotra Lake and in Andramidioka Range, (Takarindoha), present with their ankaratrites and basanitoids, an alcalino-calcic series of an evident Atlantic type. In the extreme north of Madagascar, Ambre, Tsaratanana and Ankaizina Ranges, and the Ampasindava province, show a volcanic complex with a wide range of lavas, from rhyolites, trachytes and phonolites to basalts, basanitoids and limburgites. Fissural type emissions, more or less interrupted, range from the end of oligocene up to quaternary time. Petrochemistry shows the existence of two Atlantic series, one alcalino-calcic, the most important, the other alcalinous, shorter, on basic term side. Although the quaternary series is always of the Atlantic type, it differs substantially from the two older other series. Comores Islands belong probably to this extreme North neogenequaternary volcanic series. In the center of Hauts-Plateaux, plio-quaternary emissions of the Ankaratra Range, and quaternary emissions of the Itasy Range, belong also to alcalinous fissural manifestations of the Atlantic type.