Petrology of Mt Etinde Nephelinite Series

Mt Etinde is a volcano situated on the southwestern flank of the large Mt Cameroon. Its eruptions are dated at 0·65 Ma and took place during the Mt Cameroon eruptive cycle (6 Ma with recorded recent activity). The lava types, unrelated to the Mt Cameroon alkali basalts, are melanephelinites, nephelinites sensu stricto, and numerous and varied nephelinites that contain one or more of the following species: nosean, melilite, perovskite, garnet, aenigmatite, leucite, feldspar and haüynophyres. Clinopyroxene is the dominant mafic phase, with a variable composition between Al–Ti augite and aegirine. Zoning is also present in garnets, with conspicuous Ti enrichment in the border. Aenigmatite includes a fair proportion of Fe3+ Tschermak's component. Melilite is systematically Sr rich; its SrO contents increase continuously with MgO decrease, reaching 16 wt % in some facies. The chemical composition of the lavas is extreme, with unusual concentrations of volatiles (CO2, H2O, SO3), most incompatible elements, such as Ba, Sr and Zr, and the light rare earth elements (LREE). The Mt Etinde lavas define two lineages (MgO poor and MgO rich) that partly overlap. The chemical evolution of these two lineages can be reproduced for major elements using a simple model of crystal fractionation. The major fractionating phase is an aluminous clinopyroxene, in accord with the petrographical observations. The scheme proposed can only be validated if the alkalis are not taken into account, a hypothesis warranted by observations of other nephelinite provinces or ijolite massifs and their fenite aureoles. Nephelinite magmas were obviously generated at great mantle depth, but their exotic composition can only be produced by partial melting of a metasomatic mantle. Comparison with other provinces would point to a source that has undergone carbonatitic métasomatism.