Mineralogical, Petrological, and Geochemical Features of Alkali Basalts from the Dibi Area, Adamawa Plateau (Cameroon Volcanic Line)

Located southeast of the city of Ngaoundéré (Adamawa Plateau) within the continental section of the Cameroon Volcanic Line (CVL), the village of Dibi has been the scene of numerous volcanic manifestations, dominated by three types of dynamism (explosive phreatomagmatic, effusive, and Strombolian eruptions); whose rocks, however, are poorly studied. Here, the lavas emitted during those eruptions are analyzed to constrain their origin. Whole rock geochemistry revealed that these rocks consist of basanites and foidites with Mg# varying from 67.3% to 71.2%. They are porphyritic and mainly composed of olivine [(Fo77.28-87.28 in foidites) and (Fo78.92-87.04 in basanites)], diopside [(Wo51En35Fs14 in foidites) and (Wo50-46.18En34.8-41.29Fs10.41-15.92 in basanites)], plagioclase [(labradorite: Ab37.31-39.67An57.43-60.70Or1.99-2.28) in basanites], and alkali feldspar [(anorthoclase: Ab64.45-65.13An6.81-9.15Or26.87-27.08 in foidites and Ab60.92-64.11An8-8.47Or28.45-32.27 in basanite)]. Accessory minerals include Al-chromite, Ferrian chromite, titanomagnetite, and apatite. The lavas are silica-undersaturated (SiO2: 38.9 - 40.3 wt.% in foidites and SiO2: 42.5 - 43.6 wt.% in basanites). They are within-plate basalts with similar features to Oceanic Island Basalt (OIB) magmas, affinity to High-μ (HIMU), Enriched type I (EM1), and recycled component. Their primary melts derived from low degrees of partial melting (3% - 5%) of a garnet peridotite mantle source, as shown by the Nb/La (1.30 - 1.51), Zr/La (4.91 - 5.85), and Zr/Ce (2.49 - 2.88) ratios that are consistent with an OIB-like asthenospheric mantle source. Equilibration temperatures have been estimated from the clinopyroxene-liquid thermometer and range between 981.6˚C and 1051.6˚C, at pressures of 0.9 - 1.6 kbar.