Lithospheric mantle heterogeneity under Adamawa plateau (Cameroon, Central Africa) evidenced from Gangassao lherzolite xenoliths

Large ultramafic xenoliths (4–8 cm in diameter) of lherzolitic composition have been identified in the Gangassao volcanic area, located in the northeastern part of the Ngaoundéré region. Microscope observations reveal protogranular and porphyroclastic texture. Microprobe analyses of the four major minerals show that olivine crystals are highly magnesian (Fo between 87.5 and 89.1), orthopyroxene and clinopyroxene are respectively enstatite and Cr-augite in composition. Spinel phase is Al-spinel, very poor in TiO2 (less than 0.02 wt. %), with Cr# of 11.0−11.8. Estimated equilibrium temperatures (two-pyroxene equilibrium calculation), pressures and sampling depths of studied lherzolites through empirical formulas vary from 1247 °C to 1079 °C, from 2.00 to 1.13 GPa, corresponding to 66 to 37 km, respectively, i.e. a geothermal gradient close to 30 °C/km. Gangassao xenoliths are fragments of lithospheric mantle which have undergone repeated melt extraction coupled with silicate metasomatism after uplift that occurred at Tertiary time. Filter pressing is the best mechanism favouring the separation of silicate liquid generated by decompression of the sub-continental lithospheric mantle. Lherzolite xenoliths discovered in Gangassao volcano. They equilibrated at 1247°C-1079°C, 2.00-1.13 GPa, 66-37 km depth. Mantle heterogeneity under Adamawa plateau is evidenced.