No abstract

Peridotite xenoliths of wehrlite composition, scarcely known in Adamawa plateau, Cameroon, were sampled by Mio-Pliocene basanites from Hosséré Do Guessa volcano. Their origins are discussed and elucidated trough petrography and mineral chemistry. Studied wehrlites exhibit poikilitic or protogranular textures and are composed of four main mantle phases (high Mg-olivine, augite, enstatite and Al-spinel). Petrographic and microprobe (Camebax SX100) chemical data (Fo90.8-91.4 olivine, Wo39.4-42.0 augite, En90.5-91.1 enstatite and Al-spinel) suggest a mantle origin for the Hosséré Do Guessa wehrlites. Hence, these rocks could not be considered cumulate. They have been equilibrated between 1,140 and 1,220°C, at pressures of 1.5–2.0 GPa, at 50–66 km deep, below the crust-mantle boundary. Wehrlites might result in reactions with carbonate/carbonatite melt, accompanying CO2 degassing and metasomatism by fluid phases. They suffered transpressional tectonics, during movement at Tertiary times of Pan-African strike-slip-faults, after solid-state tectonic relaxation.

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.