Mt Bambouto Volcano, Cameroon Line: Mantle Source and Differentiation of Within-plate Alkaline Rocks

The Late Cretaceous-Quaternary Cameroon Volcanic Line (CVL) is a 1600km long chain of volcanoes that straddles the continent-ocean boundary and extends from the Gulf of Guinea to the interior of the African continent. The magmatic activity started at 70Ma and has continued until the present. The products of this magmatic activity are distinctive in terms of petrology and isotope geochemistry, the variety of volcanic rocks ranging from ultrabasic, alkaline to sub-alkaline lavas to highly evolved alkaline lavas with isotopic compositions indicating complex combinations of both sub-lithospheric (HIMU, EM, DMM) and lithospheric components (sub-continental lithospheric mantle and crust). We conducted a petrological and geochemical study of a set of volcanic rocks, sampled from the rim and interior of the Miocene Mt Bambouto caldera, one of the 12 main volcanic centres of the CVL. The rocks were analysed for their whole-rock major and trace element contents, 40Ar/39Ar ages and whole-rock Sr-Nd-Pb-Os isotopic compositions. Phonolites and quartz-trachytes of the Mt Bambouto caldera are derived by fractional crystallization of highly alkaline and moderately alkaline parental basic magmas, respectively. Assimilation of the shallow crust has affected both alkaline and subalkaline magmas, suggesting that the petrogenesis of the differentiated rocks cannot be explained by crustal contamination alone. Only minor amounts (usually less than 5%) of assimilation of upper crustal silicic rocks from the local Pan-African basement are required to produce the most differentiated compositions. The rocks with the highest crustal contribution are Q-normative trachytes from peripheral cones, as well as one Ne-trachyte. Mt Bambouto basic-ultrabasic rocks, including basanites and alkali-basalts with high 187Os/188Osi, might have experienced some crustal contamination, but it must have been a limited process. Some Mt Bambouto ultrabasic to basic rocks show large ion lithophile element enrichment, notably of Sr, Ba and P compared with Zr. These samples also have relatively radiogenic Sr and unradiogenic Pb isotopic compositions. Such compositions are similar to those of the high-Sr group identified by previous studies. Most of the basic rocks do not show such characteristics and are identified as a low-Sr group. We interpret the geochemical characteristics of the high-Sr group as resulting from the partial melting of a depleted mantle (DMM-like) peridotite source containing pyroxenite veins that had interacted with carbonatitic fluids. To test this hypothesis, we used a new modelling approach based on Monte Carlo simulation; this method has the advantage of deciphering how different mantle components interacted through time. Our modelling confirms the plausibility of a three-component source. In addition, it suggests that the carbonatitic fluid first mixed with the pyroxenititic component and the resulting melt interacted with a DMM-like mantle. Both high-Sr and low-Sr groups can be produced by such a mixing scenario but with a stronger contribution of the carbonatitic fluid for the high-Sr group. At the time of melting, these source components could have been located in a metasomatized region of the sublithospheric mantle (uppermost section of the asthenosphere) or in the sub-continental lithospheric mantle.