Variation in mantle lithology and composition beneath the Ngao Bilta volcano, Adamawa Massif, Cameroon volcanic line, West-central Africa

Abstract

Mantle peridotites entrained as xenoliths in the lavas of Ngao Bilta in the eastern branch of the continental Cameroon Line were examined to constrain mantle processes and the origin and nature of melts that have modified the upper mantle beneath the Cameroon Line. The xenoliths consist mainly of lherzolite with subordinate harzburgite and dunite. They commonly contain olivine, orthopyroxene, clinopyroxene and spinel although the dunite is spinel-free. Amphibole is an essential constituent in the lherzolites. Mineral chemistry differs between the three types of peridotite: olivines have usual mantle-like Mg# of around 90 in lherzolites, but follow a trend of decreasing Mg# (to 82) and NiO (to 0.06 ​wt.%) that is continuous in the dunites. Lherzolites also contain orthopyroxenes and/or clinopyroxenes with low-Mg#, indicating a reaction that removes Opx and introduces Cpx, olivine, amphibole and spinel. This is attributed to reaction with a silica-undersaturated silicate melt such as nephelinite or basanite, which originated as a low-degree melt from a depleted source as indicated by low Al2O3 and Na2O in Cpx and high Na2O/K2O in amphibole. Thermobarometric estimates place the xenoliths at pressures of 11–15 ​kbar (35–50 ​km) and temperatures of 863–957 ​°C, along a dynamic rift geotherm and shallower than the region where carbonate melts may occur. The melt/rock reactions exhibited by the Ngao Bilta xenoliths are consistent with their peripheral position in the eastern branch of the Cameroon Volcanic Line in an area of thinned crust and lithosphere beneath the Adamawa Uplift.