Re ´sume Ĺesbasaltes transitionnels e ´oce `nes occupent plus de la moitie ´ouest des affleurements laviques du plateau Bamoun. Les basaltes de ce type restent exceptionnels dans la ligne volcanique du Cameroun (LVC). Ils posse `dent des caracte ´ristiques pe ´trographiques (basaltes sans olivine, BSO ; a ˆge de 51,8 AE 1,2 Ma ; le plus vieux de la LVC) et ge ´ochimiques, diffe ´rentes de celles des basaltes alcalins de ce me ˆme plateau (basaltes a `olivine, BAO ; a ˆge de 46,7 AE 1,1 Ma) et de tous les autres basaltes alcalins, classiques, de la LVC. Inversement, ils apparaissent plus proches des roches analogues des rifts est-africains et des ı ˆles Kerguelen. Ils se caracte ´risent par la rarete ´de l'olivine modale et par la valeur relativement e ´leve ´e du rapport Y/Nb (e ´gal a `1), mais faible de La N /Yb N (10) et de Ce N /Yb N (7). Les valeurs des rapports isotopiques initiaux 87 Sr/ 86 Sr (0,7044) et 143 Nd/ 144 Nd (0,5126) des BSO sont respectivement plus e ´leve ´es et plus faibles que celles relatives aux BAO (0,7034 et 0,5128). Ces re ´sultats mettent en e ´vidence l'existence d'un re ´servoir mantellique enrichi par un po ˆle de type EM lithosphe ´rique a `l'origine des basaltes transitionnels, diffe ´rent du re ´servoir plus proche du po ˆle HIMU, a `l'origine des basaltes alcalins.
Petrographic and geochemical studies have been carried out on the peridotite xenoliths included in the pyroclastite deposits from the Lake Guinnadji and Ngao Djalsoka volcano close to the Dibi locality in the Adamawa plateau, central Cameroon. The peridotite xenoliths exhibit protogranular and porphyroclastic textures, and are composed of olivine, orthopyroxene and minor amount of clinopyroxene, amphibole (?) and spinel, typical of harzburgite type. ICP-MS and ICP-OES analyses of representative samples show that they are in the range of those of the primitive mantle origin (42.4-45.0 wt.% MgO, 65.7-79.0 wt.% normative olivine). Decreasing SiO2, TiO2, Al2O3 and CaO contents vs increasing MgO trends evidence depletion from more fertile mantle by extraction of basaltic melts, leaving a refractory residue of harzburgite composition. High contents of REE and incompatible elements point out secondary enrichment processes which have affected the peridotite xenoliths. For this, metasomatism caused by silicate fluids is invoked. In that way, peridotites of Dibi area witness of the nature and the evolution of lithospheric mantle under the Adamawa plateau.
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