Baossi–Warack monogenetic volcanoes, Adamawa Plateau, Cameroon: petrography, mineralogy and geochemistry

Abstract

Three monogenetic cones in the Baossi–Warack area, Ngaoundere, Adamawa Plateau forming part of the Cameroon Volcanic Line (CVL) are documented in this study. Basaltic lavas (< 1 km3) scattered around these vents and restricted volcaniclastic deposits were emplaced by Hawaiian and mild strombolian style eruptions. The lavas are porphyritic, mainly composed of olivine (chrysolite) and clinopyroxene (diopside and augite) phenocrysts and plagioclase (andesine) microphenocrysts. Accessory minerals include titano-magnetite and titano-hematite, nepheline, apatite and amphibole xenocrysts. Sanidine occurs in some samples and sodi-potassic albite in others. Some olivines and clinopyroxenes exhibit resorbed margins and thin reaction rims while plagioclase displays oscillatory zoning, and sieved textures as a result of magma mixing. Whole-rock geochemistry data indicates that the lavas are silica-undersaturated, composed of basanites and basalts, showing little compositional variations (SiO2: 39.20 wt.%–48.01 wt.%, MgO: 5.29 wt.%–9.70 wt.%). Trace elements patterns of these lavas suggest they are enriched in LILE including Pb, probably due to crustal contamination. REE patterns suggest cogenetic magmas below Baossi 1 and Baossi 2 volcanoes, and distinct sources below Warack volcano and nearby lavas. The lavas studied show affinity to high-µ (HIMU), enriched type I (EM1) and Oceanic Island Basalt (OIB)-like mantle signatures and thus indicate a heterogeneous mantle source underneath the vents as noted at other monogenetic and polygenetic volcanoes along the CVL. Primary melts derived from low degrees of partial melting (0.5%–2%) and encountered low rates of fractionation, and crustal contamination coupled with magma mixing. These melts evolved independently through structural weaknesses in the basement.