Journal of Petrology Volume 41 Number 1 Pages 87-109 2000
© Oxford University Press 2000
The Cameroon Volcanic Line Revisited: Petrogenesis of Continental Basaltic Magmas from Lithospheric and Asthenospheric Mantle Sources
1DIPARTIMENTO DI SCIENZE DELLA TERRA, UNIVERSITY OF TRIESTE, VIA E. WEISS 8, 34127 TRIESTE, ITALY
2BERKELEY GEOCHRONOLOGY CENTER, 2455 RIDGE ROAD, BERKELEY, CA 94709, USA
3DEPARTMENT OF GEOLOGY AND GEOPHYSICS, UNIVERSITY OF CALIFORNIA, BERKELEY, CA 94720, USA
4DIPARTIMENTO DI MINERALOGIA E PETROLOGIA, UNIVERSITY OF PADOVA, CORSO GARIBALDI 37, 35137 PADOVA, ITALY
5MINISTÈRE DE RECHERCHE SCIENTIFIQUE TECHNOLOGIQUE, B.P. 4110, YAOUNDÉ, CAMEROON
6LABORATORY FOR EARTHQUAKE CHEMISTRY, UNIVERSITY OF TOKYO, HONGO BUNKYO-KU, TOKYO 113, JAPAN
The volcanic activity of Mts Bambouto and Oku (Western Highlands) and of the Ngaoundere Plateau, in the continental sector of the Cameroon Volcanic Line, Equatorial West Africa, ranges in age from Oligocene to Recent. It is characterized by basanitic, alkali basaltic and transitional basaltic series. Mineral chemistry, major and trace element bulk-rock compositions, and geochemical modelling suggest that the magmatic series evolved mainly at low pressure (2–4 kbar) through fractional crystallization of clinopyroxene and olivine ± magnetite, at moderately hydrated (H2O = 0·5–1 wt %) and QFM (quartz–fayalite–magnetite) to QFM + 1 fO2 conditions. Basalts from Ngaoundere (Miocene to Quaternary) and from the early activity (31–14 Ma) of the Western Highlands have incompatible trace element and Sr–Nd isotopic compositions similar to those of oceanic Cameroon Line basalts, pointing to a similar asthenospheric mantle source. By contrast, the late (15–4 Ma) Western Highlands basanites and alkali basalts have anomalously high concentrations of Sr, Ba and P, and low concentrations of Zr, which are exclusive features of continental Cameroon basalts. The genesis of these latter magmas is consistent with derivation from an incompatible element enriched, amphibole-bearing lithospheric mantle source. Western Highlands basalts show a continuous spectrum from high to low Sr–Ba–P compositions, and may result from variable amounts of mixing between melts derived from an anhydrous lherzolite source (asthenospheric component) and melts from an amphibole-bearing peridotite source (lithospheric HSr component). New 40Ar/39Ar ages for Mts Oku and Bambouto basalts, combined with previous 40Ar/39Ar and K/Ar ages of basaltic and silicic volcanics, and with volcanic stratigraphy, suggest a NE–SW younging of the peak magmatic activity in the Western Highlands. This SW younging trend, extending from the Oligocene volcanism in northern Cameroon (e.g. Mt Oku) to the still active Mt Cameroon, suggests that the African plate is moving above a deep-seated mantle thermal anomaly. However, the age and location of the Ngaoundere volcanism does not conform to the NE–SW younging trend, implying that the continental sector of the Cameroon Volcanic Line cannot be easily interpreted as the surface expression of a single hotspot system.
KEY WORDS: Cameroon Line basalts;40Ar/39Ar geochronology; lithospheric and asthenospheric mantle source; hotspot
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