High–μ signature in lavas of Mt. Oku: Implications for lithospheric and asthenospheric contributions to the magmatism of the Cameroon Volcanic Line (West Africa)

Abstract

Abstract This study presents geochemical data for mafic lavas from Mt. Oku to investigate the mantle source for Mt. Oku lavas and regional variations in isotopic ratios along the Cameroon Volcanic Line (CVL). The investigated mafic lavas present trace element and isotope signatures akin to ocean island basalts (OIB) and cover the entire range of the CVL. Major and trace element compositions of the studied lavas indicates that they have undergone fractionation of olivine + clinopyroxene + Fe-Ti oxide ± Cr–spinel with insignificant crustal contamination. The lavas were generated by The highest value of radiogenic 206Pb/204Pb (20.7) so far measured along the CVL is reported in this study. The isotopic features of the lavas suggest that the HIMU signatures are dominantly inherited from the asthenosphere (primary source) and melting of enriched components hosted in the subcontinental lithospheric mantle (SCLM; secondary source). Lavas from Mt. Oku present more comprehensive ranges of Sr–Nd–Pb isotopic composition than those from the continent-ocean-boundary (COB). The isotopic composition of Mt. Oku lavas with 206Pb/204Pb\xa0>\xa019.5 differs from those of the COB but are similar to FOZO. The isotopic composition of Mt. Oku lavas (87Sr/86Sr\xa0=\xa00.7030–0.7036, 143Nd/144Nd\xa0=\xa00.5127–0.5129, 206Pb/204Pb\xa0=\xa017.9–20.7) suggest that the lavas are the result of contributions from the asthenosphere and the SCLM. Using a combination of Sr–Nd–Pb isotope display on a 3D plot and Sm/Yb, we identify four essential components denoted; A, B, C and D involved in the petrogenesis of Mt. Oku lavas and the approximate melting depth. While A and B are from the asthenosphere with HIMU mantle flavour, C and D owe their origin from the SCLM with EM1 and DMM characteristics, respectively. Although depleted in Sr-Nd isotopes, D is radiogenic in 206Pb/204Pb, indicating the influence of metasomatism. Therefore, the petrogenesis of Mt. Oku lavas involves the mixing of at least three mantle end-members: HIMU–DMM–EM1. The “fanning” of samples from”A towards the other components (B, C and D) indicates a HIMU–like end–member dominance in Mt. Oku magmatism. Considering the geophysical studies on mantle convection and plumes, we have developed a schematic model that explains the petrogenesis of the CVL lavas. The continuous spectrum from relatively depleted to HIMU characteristics of CVL lavas is associated with the progressive change in source components (i) SCLM with ± pockets of enriched metasomes to (ii) SCLM + asthenosphere sources and finally to (iii) typically asthenospheric source material ± pockets of enriched metasomes. Lavas derived from stage (i) exhibit geochemical variability involving DMM–like and EM1–like signatures. The magma generated at Stage (iii) is dominated by the HIMU mantle component and accounts for over 85% of CVL lavas.