Nosy-Be (Nosy Be, Nossi Be)

Nosy-Be is an island of about 17x27 km located northeast of the Ampasindava Peninsula (No. 102-00-004). About half of the island is occupied by sedimentary rocks of Liassic age which are overlain by Quaternary volcanic rocks and intruded by two bodies of peralkaline granite, while a small island near the west coast consists mainly of rhyolite. The main area of volcanic rocks, which form the western part of the island, comprises tuffs and lavas of essentially basaltic composition that form numerous cones while some of the many craters are occupied by lakes. The volcanic rocks are referred to by Lacroix (1922a) as being principally ankaratrite, limburgite and limburgitic basanite, with ‘perhaps’ leucite-bearing rocks. The basanitic and limburgitic rocks are essentially alkali olivine basalts which are commonly rich in olivine nodules. The ankaratrites contain nepheline and an example described by Lacroix (1923) includes exceptionally abundant olivine; they would be classified as basanites in a modern nomenclature. Granite and syenite form an approximately circular body at Lokobe at the southern end of the island (Donnot, 1963), a smaller mass in the north and numerous dykes. According to Besairie (1934a) the granites are alkaline, but a description by Lacroix (1922b) indicates that this is not so. However, a syenite described by Lacroix (1922b, p. 606) is indeed peralkaline containing as it does aegirine. The syenitic rocks are said also to include solvsbergite. The rhyolite forming the island of Sakatia off the west coast of Nosy-Be contains phenocrysts of orthoclase, zircon and biotite and thus is not peralkaline.Age K-Ar on an unclassified ‘flow’ gave 7.49±026 Ma (Emerick and Duncan, 1982).