Geochemical difference of basalts between polygenetic and monogenetic volcanoes in the central part of the Cameroon volcanic line.

Abstract

The 1600 km long Cameroon volcanic line is composed of large polygenetic volcanoes and small monogenetic volcanoes. The latter conform monogenetic volcano groups consisting of small lava flows, cinder cones and maars. Chemical analyses on basalts from the central part of the Cameroon volcanic line showed that CaO contents of the basalts of polygenetic volcanoes are higher than those of monogenetic volcanoes, while the reverse relation is found for SiO2 contents. Incompatible trace elements are high in both basalts, but HFS/LIL ratios and La/Ba ratio are higher for polygenetic volcanoes than monogenetic volcanoes. Rb-Ba-La relations, representing the most incompatible elements for uni-, di- and tri-valent cations, indicate that basalts of polygenetic volcanoes are similar to depleted mid-oceanic ridge basalts, whereas, the basalts of monogenetic volcanoes are akin to continental flood basalts or oceanic island basalts. It is suggested that the major sources of the polygenetic volcanoes are the upwelling asthenosphere underneath the African plate, while those of the monogenetic volcanoes are the lowest portion of the subcontinental lithosphere.