Crystallisation and petrogenesis of Cenozoic alkaline basaltic lavas on the Kapsiki Plateau (Moukoulvi, Far-North Cameroon): Unveiling the mantle's heterogeneity and HIMU signature

Abstract

The Kapsiki Plateau represents the northernmost extension of the Cameroon Volcanic Line (CVL) in West Africa. Unlike other regions of the CVL, this area is characterised by a higher prevalence of felsic and intermediate rocks compared to basaltic ones. Detailed investigations into the petrogenetic evolution of these rocks are currently limited. For this reason, Cenozoic alkaline basaltic lavas from the Kapsiki Plateau (Moukoulvi) were studied to enhance comprehension of their crystallisation process, genetic evolution of the magma, and insights into its petrogenesis and source mineralogy. The examined lavas are comparable to the mafic lavas that span the whole range of the CVL and demonstrate major and trace element characteristics similar to those seen in ocean island basalts (OIB). The major and trace element compositions suggest fractional crystallisation mainly including olivine, clinopyroxene, and Fe–Ti oxide with evidence of crustal contamination. The multi-element diagrams reveal consistency with those reported from other CVL volcanoes, displaying depletion of heavy rare earth elements (HREE) and enrichment of light rare elements (LREE), indicating an enriched source and the existence of garnet. The studied lavas have high large ion lithophile element (LILE) contents (Sr = 812–1065 ppm, Ba = 394–467 ppm) relative to high field strength elements (HFSE) (Sr/Zr = 3.97–4.94; average OIB = ∼1 Ba/La = 9.42–11.72; average OIB = 9.4). They also have Zr/Sm ratios (26.93–29.88) that are similar to the average OIB (Zr/Sm = 28). Moreover, the studied lavas exhibit elevated levels of incompatible trace elements (e.g., Rb, Pr, U, and Th), as well as higher Ta/Yb and Th/Yb ratios compared to normal OIB values, associated with a notable fluctuation in Nb/Ta and Zr/Hf ratios. The correlations between Th, La, U, and SiO₂, coupled with the trend of the samples on Nb/Y vs. Rb/Y diagrams, indicate the influence of crustal contamination on the lavas' composition. The source of these lavas was a heterogeneous source with less than 4 % garnet that underwent partial melting of less than 2 %. The presence of a Pb depletion (Ce/Pb > 30) also implies that these magmas belong to the high μ (HIMU)-OIB type, attributed to lithospheric mantle metasomatism. The formation of these magmas involved partial melting of a mantle source evolving chemically and mineralogically over time with a HIMU composition at very low temperatures. Hydrous minerals, such as phlogopite or amphibole, may indicate modal metasomatism, supported by high Rb/Sr ratios or K₂O high levels. Sr/Zr ratios (3.97–4.94) above OIB values (0.8–1.5) suggest Sr and Ba-rich fluid influence. Elevated Zr/Sm ratios (>28) and variations in Nb/Ta and Zr/Hf ratios point to metasomatic fluids or melts altering the mantle's composition. Therefore, the Moukoulvi lavas, like many CVL alkaline lavas, likely erupted from a metasomatized mantle source that was enriched in incompatible trace elements (Rb, Ba, Ce, Nb, and Zr). This enrichment is reflected in the elevated concentrations of these incompatible trace elements, as well as the relative enrichment in HFSE. The absence of temperature anomalies in the upper mantle beneath the CVL suggests that the magmatism originates from the lithospheric mantle source rather than a mantle plume.