The Tombel graben (West Cameroon):: a recent monogenetic volcanic field of the Cameroon Line

The Tombel graben is a recent monogenetic volcanic field located in the continental part of the Cameroon Line. Magmatic activity began with fissure eruptions emitting large lava flows, and continued with explosive episodes generating cinder cones. Tombel lava flows are made up of three major rock types. Ankaramite yields phenocrysts of Fe-Ti oxides, forsterite olivine and diopside-augite clinopyroxene. In addition to this assemblage, basalt and hawaiite (olivine basalt, doleritic basalt and trachybasalt according to their modal compositions) contain phenocrysts of labradorite-andesine plagioclase and scarce kaersutite. The suite of lava flows, ranging from ankaramite to hawaiite, define an incomplete sodic alkaline series. Moreover, basanite, rich in incompatible elements, is locally found as late ejecta. Geochemical data share some oceanic island basalt (OIB) characteristics. It is suggested that the ankaramite-hawaiite series could be due to differing degrees of partial melting of a source at the asthenosphere-lithosphere boundary, followed by low rates of fractional crystallization. (C) 2002 Published by Elsevier Science Ltd.