Geochemistry and Petrogenesis of Mafic Doleritic Dykes at Mbaoussi (Adamawa Plateau, Cameroon, Central Africa)

Giant mafic doleritic dyke swarms crosscut the Mbaoussi granitoid basement to the North of Ngaoundéré, in Adamawa plateau (Cameroon, Central Africa). The dyke swarm orientation might correspond to reworked Late Pan-African fault zones. Mbaoussi doleritic dykes display intergranular to ophitic and sub-ophitic textures and are mainly composed of clinopyroxene, plagioclase feldspar and Fe-Ti oxides. ICP-MS and ICP-AES geochemical data show both slight alkaline affinities according to total alkalis-silica contents and continental tholeiitic signature evidenced by Nb-, Ta- and Ti-depletions. Primary magmas suffered fractional crystallization coupled with assimilation of continental materials, to produce a sub-alkali basalt to trachybasalt to basaltic trachyandesite lava series. Low values (3-7) of (Ce/Yb)N suggest fairly high partial melting degree of the source. This source was probably the sub-continental lithospheric mantle, whose composition was close to fertile mantle component, yet Nb-Ta depleted after a former subduction.