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)

Abstract

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, Al 2 O 3 , Cr 2 O 3 , NiO, and TiO 2 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 Ni and Cr contents, and the Mg/(Mg + Fe) ratio in both olivines and spinel lherzolitic xenoliths, coupled with the positive anomaly of Ta and P and a negative anomaly in Ba, Th, K, Pb, U, and Hf, appear compatible with the partial melting process. The variations of highly incompatible elements (U and Rb) indicate these elements are not significantly affected by post-magmatic or secondary alteration processes. The petrographical and mineralogical variations, combined with the geochemical variations of major and trace elements in the whole rocks, are compatible with a low degree of partial melting in an enriched upper mantle, such as the lherzolitic type. The low Rare-Earth element enrichment of these spinel lherzolitic xenoliths could indicate a probable interaction between the upper peridotitic mantle and the host magmas. The presence of these spinel lherzolitic xenoliths is likely linked to the evolution of mantle plumes, as seen in the Cameroon line, Cape Verde archipelago, or elsewhere. The geochemical results also suggest that the crust under the Senegal Cape Verde peninsula may consist of a fertile mantle affected by a low degree of partial melting. This is controlled by Cenozoic tectonics, with deep fractures affecting the head of the Senegal Cape Verde peninsula and facilitating the rapid ascent of host magma.