Petrographic and Geochemical Study of Mantle Spinel Lherzolitic Xenoliths within Alkaline Hosted Lavas of Cenozoic Volcanism in the Head of the Cape Verde Peninsula (Senegal)
Tertiary and quaternary Cenozoic alkaline-hosted lavas of the Senegal Cape Verde Peninsula have incorporated spinel lherzolitic xenoliths. The aim of this study is to characterize the petrography and geochemistry of these spinel lherzolitic xenoliths to better understand their original characteristics and the evolution of the mantle under the Senegal Cape Verde Peninsula. To achieve this objective, field mapping was conducted, and xenoliths were selected and sampled. Thin sections were prepared for petrographical and mineralogical analysis, and whole rocks were used for geochemical studies. The petrographical results show that the spinel lherzolitic xenoliths, ranging in size from 1.5 to 10 cm, are rounded to sub-rounded and sometimes elongated, with a protogranular texture. They are characterized by the absence of recrystallization, intense mineral fracturing, intergranular mineral slippage, and magmatic infiltration acting as a contaminant. The mineralogical characteristics show homogeneity in the mineral phases, comparable to those of xenoliths from oceanic platforms and hotspot basalts, as well as sub-continental fertile peridotites from island arcs. The main mineralogical phases are olivine and pyroxenes (orthopyroxenes and clinopyroxenes), associated with an aluminous spinel phase. The proportion of these mineral phases classifies them as harzburgite, dunites or lherzolites. There is no trace of garnet, plagioclase, amphibole, or phlogopite minerals. The relationship between the xenoliths and the host lavas indicates that these xenoliths are not cumulates and are therefore not cogenetic with the host rocks, suggesting their residual character. Mg# values of the olivines (75.9 to 92.4) and the absence of zonation are characteristic of the lherzolitic xenoliths found in alkaline basalts worldwide. This is confirmed by Mg# values (from 82.8 to 93.7) and the variations of CaO, Al2O3, Cr2O3, NiO, and TiO2 in the clinopyroxenes and orthopyroxenes, as well as the ratios of Mg# (63.00% to 84.2%) and Cr# (18.05% to 41.98%) in the spinels. These characteristics reveal a textural and mineralogical homogeneity of these xenoliths originating from the mantle. The Rare Earth spectra of all xenoliths studied show a progressive decrease from Light Rare Earths (LREE) to intermediate Rare Earths (MREE) to Heavy Rare Earths (HREE). The xenoliths from the Senegal Cape Verde Peninsula originate from a Rare Earth-depleted spinel lherzolite mantle source zone. The low contents of incompatible elements (Ce, La), high
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