The Cameroon Volcanic Line revisited: Petrogenesis of continental basalticmagmas from lithospheric and asthenospheric mantle sources

The volcanic activity of Mts Bambouto and Oku (Western Highlands) and of th e Ngaoundere Plateau, in the continental sector of the Cameroon Volcanic Li ne, Equatorial West Africa, ranges in age from Oligocene to Recent. It is c haracterized by basaltic, alkali basaltic mid transitional basaltic series. Mineral chemistry, major and trace element bulk-rock compositions, and geo chemical modelling suggest that the magmatic series evolved mainly at lore, pressure (2-4 kbar) through fractional crystallization of clinopyroxene an d olivine +/- magnetite, at moderately hydrated (H2O = 0.5-1 wt %) and QFM (quartz-fayalite-magnetite) to QFM + 1 fO(2) conditions. Basalts from Ngaou ndere (Miocene to Quaternary) and from the early activity (31-14 Ma) of the Western Highlands have incompatible trace element and Sr-Nd isotopic compo sitions similar to those of oceanic Cameroon Line basalts, pointing to a si milar asthenospheric mantle source. By contrast, the late (15-4 Ma) Western Highlands basanites and alkali basalts have anomalously high concentration s of Sr, Ba and P, and low concentrations of Zr; which are exclusive featur es of continental Cameroon basalts.;The genesis of these latter magmas is c onsistent with derivation from an incompatible element enriched, amphibole- bearing lithosheric mantle source. Western Highlands basalts show a continu ous spectrum from high to low Sr-Ba-P compositions, and may result from var iable amounts of mixing between melts derived from an anhydrous lherzolite source (asthenospheric component) and melts from an amphibole-bearing perid otite source (lithospheric HSr component). New Ar-40/Ar-39 ages for Mts Oku and Bambouto basalts, combined with previous Ar-40/Ar-39 and K/Ar ages of basaltic and silicic volcanics and with volcanics, and with volcanic strati graphy, suggest a NE-SW younging of the Peak magmatic activity in the Weste rn Highlands. This SW younging trend extending from the Oligocene volcanism in northern Cameroon (e.g, Mt Oku) to the still active Mt Cameroon, sugges ts that the African plate is moving above a deep-seated mantle thermal anom aly. However the age and location of the Ngaoundere volcanism does not conf orm to the NE-SW younging trend, implying that the continental sector of th e Cameroon Volcanic Line cannot be easily interpreted as the surface expres sion of a single hotspot system.