A. Marzoli et al., The Cameroon Volcanic Line revisited: Petrogenesis of continental basalticmagmas from lithospheric and asthenospheric mantle sources, J PETROLOGY, 41(1), 2000, pp. 87-109
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.