No abstract

International audienceWe report the mineralogy, geochemistry and geochronology of the mafic rocks from the Bamenda Mountains, part of the Cameroon Volcanic Line (CVL), in order to discuss the origin and evolution of the magmas in this part of the CVL. Mafic rocks in the Bamenda Mountains are basanites, basalts, hawaiites and mugearites with an alkaline affinity. K-Ar ages have been obtained on 10 samples and range from 17.6 Myr to present. Trace element and isotopic compositions (Sr-Nd-Pb) show that some samples among the oldest are slightly contaminated by a crustal component with high La/Nb and 87Sr/86Sr ratios and low Pb isotopic ratios. The mafic rocks strongly resemble OIB in their trace element compositions. Some samples possess a positive Sr and Eu anomaly which cannot be explained by a process of plagioclase accumulation. These anomalies are also observed in some pyroxenites found as xenoliths in the Adamawa volcanic province further north. Furthermore, non-contaminated samples have high Pb isotopic ratios and point towards an HIMU component similar to the St. Helena mantle plume. We propose that the Bamenda mafic magmas with positive Sr and Eu anomalies were formed by hybridization of asthenospheric melts with melts formed by the partial melting of pyroxenites. Samples without these anomalies result from the hybridization of the same asthenopheric melts with melts coming from the metasomatized, amphibole-bearing, lithospheric mantle

Re ´sumeĹes laves felsiques des monts Bamenda, au coeur de la ligne volcanique du Cameroun, sont principalement repre ´sente ´es par des trachytes, et par quelques benmore ´ites et rhyolites alcalines a `hyperalcalines. Les donne ´es ge ´ochronologiques K-Ar de ´finissent deux e ´pisodes de volcanisme, le premier entre 18 et 22 Ma, le deuxie `me entre 12,5 et 13,5 Ma. Les donne ´es ge ´ochimiques montrent que les laves acides sont forme ´es par cristallisation fractionne ´e de magmas basiques et mettent en e ´vidence une contamination significative par les roches crustales pendant la diffe ´renciation. Dans l'ensemble, ces donne ´es sont en accord avec l'absence d'e ´volution dans le temps de la localisation du volcanisme le long de la ligne du Cameroun, et montrent un me ´canisme identique de gene `se des roches acides au cours du temps et pour les diffe ´rentes provinces volcaniques adjacentes (Bamenda, Oku et Bambouto).

Mafic rocks from the Bamenda volcanic province along the Cameroon Volcanic Line have been dated from 17 to 0 Ma. Associated with some trachytes and rhyolites, this volcanism covers a period of more than 25 Ma. The studied rocks are basalts to mugearites. Most of them have been contaminated by continental crust during their transit to the surface. The oldest rocks are the most contaminated. One group of samples shows high Eu, Sr and Ba contents. This characteristic is not due to crustal contamination process, but has a mantle source origin. We argue that these characteristics have been acquired by mixing of melts formed by partial melting of mantle pyroxenites with melts formed in mantle peridotites. Such pyroxenites have been observed as mantle xenoliths in the Adamaoua province, and their chemical and isotopic compositions are consistent with such a model. To cite this article: P.

No abstract