Cap-Vert

The western end of the Cap-Vert Peninsula, north and west of Dakar, and several small islands, are occupied by Tertiary and Quaternary lavas and tuffs covering an area of about 10x7 km. There are also some dozen smaller occurrences up to 100 km east of Dakar which include pipes, dykes and sills as well as lavas and tuffs. Although many of the extrusive rocks were fed from fissures, at least one large central volcano, the Mamelles volcano west of Dakar, was developed. Layered pyroclastic tuffs, palagonitic lapilli, accetionary lapilli and a high proportion of country rock fragments amongst the pyroclastic ejecta of this volcano are described by Crevola (1974) and interpreted as base-surge deposits. Many of the sills, dykes and pipes are occupied by tuff breccias. The lavas include olivine nephelinite, basanite and alkaline olivine basalt (Dia et al., 1987). The nephelinite contains phenocrysts of olivine (Fo85-7) and salitic pyroxene (Wo52En37Fs11) with groundmass pyroxene (Wo47En38Fs15), nepheline, titanomagnetite with cores of chrome spinel and glass; they are free of feldspar but contain a little normative, but no modal, leucite. The basanites have olivine (Fo77-75) and salite-augite (Wo46En39Fs15) phenocrysts in a matrix of plagioclase (An67-62), titanomagnetite and glass. Analyses of basanites (Dia et al., 1987) indicate similar amounts of normative nepheline to the nephelinites but modal nepheline is apparently not present. The basalts contain phenocrysts of plagioclase (An45-55), olivine (Fo63-56) and augite (Wo45En39Fs16) in a matrix of the same minerals together with titanomagnetite and ilmenite. Analyses of rocks and many of minerals will be found in Dia et al. (1987).