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Wehrlite samples (size: ~4 cm) hosted in basaltic lavas from the SW of Hosséré Dammougalré are located in the western Adamawa Plateau. Porphyritic and allotriomorphic texture characterize respectively host Basalt and wehrlite xenoliths. The phenocrysts of olivine (Fo 68−74 ), and Ti-magnetite are scattered in host basalt. Wehrlite xenoliths (~4 cm size) contain Cr-rich clinopyroxene (diopise-augite), olivine (Fo 76−88 ) and chromiferous spinel. Equilibrium temperatures calculated from Fe/Mg exchange reaction for olivine/spinel vary between 944˚C and 1102˚C. The wehrlite olivine crystals with low Fo (<90) indicate a re-equilibration of Fe-Mg in the host basalt at low temperatures. All the analyzed wehrlite clinopyroxenes have crystallized at high pressures as evidenced by the Al vi and Al iv contents. The studied spinelbearing wehrlite xenoliths represent probably the residual portions of the upper mantle, which are an important source of information about lithospheric composition and thermal evolution beneath the Adamawa Plateau.

Alkaline volcanic activities occurred in the Mt Cameroon at the ocean-continent boundary of the Cameroon Line. It is characterized by a volcanic association of alkali basalts and hawaiites extruded during the late Miocene to Recent times. The major and trace element geochemistry of the Mt Cameroon are consistent with the fractional crystallization of olivine ± clinopyroxene ± plagioclase (± amphibole). Petrographical and mineralogical study reveals the presence of xenocryts (olivine, clinopyroxene and spinel) in Mt Cameroon basalts. Their composition are similar to xenoliths and rocks crystals and they come from cumulates formed in the upper lithospheric mantle. Mt Cameroon magmas were generated near the boundary of garnet and spinel mantle stability domains (60-75 km depth), at the base of the lithospheric mantle that the compositions of the Mt Cameroon magmas are consistent with derivation from a infralithospheric mantle that was metasomatised by carbonatite melts. Basaltic volcanism in the Mt Cameroon occurred probably as a result of minor plume activity coupled with lithospheric extension.