A phlogopite-amphibole-plagioclase bearing websterite from Guinadji volcano (Adamawa Plateau, Northern Cameroon): evidence of a cumulate origin and crustal event

An unusual phlogopite-amphibole-plagioclase-bearing websterite xenolith has been sampled in an alkali basaltic pyrolastite from Guinadji volcano, SE Ngaoundéré, Adamawa plateau, northern Cameroon, eastern extension of the continental sector of the Cameroon volcanic line. This olivine-free websterite exhibits cumulative texture with diopside (En_{43.88–45.13}Wo_{45.90–46.91}Fs_8.92–9.28; Mg#: 0.83–0.85) and enstatite (En_{75.98–76.65}Wo_1.21–1.76Fs_{21.74–22.45}; Mg#: 0.77–0.79) as cumulus phases while plagioclase, a labradorite (An_58.30–An_61.47), represents the intercumulus phase. Amphibole is an edenite and has a Mg# varying between 0.82 and 0.83 while phlogopite has a Mg# of 0.86. Using clinopyroxene geothermobarometer, the estimated mineral equilibration temperature ranges from 1195°C to 1208°C, at pressure between 4.7 and 5.9 kbar. The metasomatic minerals filling small spaces among these pyroxenes have high potassium (K₂O ∼10 wt% for phlogopite and ∼2 wt% for amphibole), high titanium (TiO₂ ∼4 wt% for phlogopite and ∼2 wt% for amphibole) and high fluorine (F > 1.20 wt%) contents. This websterite would thus be of cumulative origin, formed from tholeiitic magma and metasomatized by a K–F rich crustal-derived fluids/melts with relatively high contents of Ti. The estimated high temperature at low pressure suggests a steep geothermal gradient which could be attributed to the existing plume underneath this region. Upwelling of tholeiitic magma, which represents the parental melt of the cumulate, was emplaced at the crust–mantle boundary, where the cumulate crystallized. Continued plume upwelling subsequently led to the uplift and re-equilibration of this cumulate at depths of approximately 20 km, where it underwent metasomatic modification.