The Mount Cameroon Volcano, West Africa: an Active Link Between Recent Eruptives and Mantle Signatures of the Deep Past Beneath the Margins of Africa

Mount Cameroon, a 4095 m high stratovolcano is arguably the most active volcano in Equatorial West Africa, with seven eruptions during the last century. It is the only active member of the 1600 km long volcanic belt – “Cameroon Volcanic Line (CVL)” - with a history going back ~ 80 Ma at the initial opening of the Central Atlantic Ocean during the Cretaceous. In order to decipher recent petrogenetic and geotectonic evolution of the CVL at this volcano, Lava samples of unknown ages but older than all 20th century lavas are investigated for their mineral chemistry and whole rock geochemistry and compared with the 20th century lavas that have been the only focus of petrological investigations for the past three decades. The data show all lavas as within plate and lava types (basalt, basanite/trachy-basalt, hawaiite and basaltic trachy-andesite) indicating these historical eruptions involved even more evolved magmas than those of the 20th century. The lavas are nepheline normative and show an alkaline affinity associated with rift-graben structures, fed from fissures and central vents. Most samples are olivine, clinopyroxene and plagioclase feldspar phyric with additional Fe-Ti oxide phenocrysts. Mineral chemistry data show the compositional range of olivine for older lava samples to be very wide (Fo58%-84%) and a broader plagioclase composition extending to intermediate andesine, a composition which has not been reported for any 20th century lavas. Despite these differences, primitive mantle normalized trace element spidergrams show similar pattern for these older lavas as the 20th century lavas, similar to OIB, confirming they all have a common mantle source of HIMU type. Thus, whilst the precise origin of the CVL is still disputed, it is clear that its mantle source area is not a transient plume. This has major implications for understanding mantle processes marginal to evolving passive Atlantic margins.