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