Revisiting the geodynamic model for the Mako greenstone belt: new evidence for a Paleoproterozoic oceanic plateau in the West African Craton

Abstract. Geochemical and isotopic data from the Paleoproterozoic greenstone belt of Mako (Eastern Senegal) provide evidence for the existence of two tholeiitic and one calc-alkaline series. Tholeiitic series are composed of ultramafic and mafic rocks. Tholeiites 1 are characterized by flat Light Rare Earth Elements (LREE) and Middle Rare Earth Elements (MREE) patterns and by initial isotopic compositions of ⁸⁷Sr/⁸⁶Sr (2.1 Ga) from 0.70083 to 0.70368 and εNd (2.1 Ga) from +1.55 to +5.90 (except sample IL49). Tholeiites 2 are characterized by a more enriched LREE and MREE pattern, and by initial isotopic compositions ⁸⁷Sr/⁸⁶Sr (2.1 Ga) from 0.7009 to 0.70263 and εNd (2.1 Ga) from +2.14 to +7.35. In addition tholeiites 2 show a slightly negative Nb anomaly, while the tholeiites 1 have no Nb anomaly. Calc-alkaline rocks are characterized by more enriched LREE and MREE patterns and more depleted Heavy Rare Earth Elements (HREE), indicating more pronounced general LREE/HREE fractionation; they have a more pronounced negative Nb and Ti anomalies and initial isotopic signatures of ⁸⁷Sr/⁸⁶Sr (2.1 Ga) from 0.7009 to 0.70618 and εNd (2.1 Ga) from +3.12 to +6.40. Comparison of geochemical and isotopic data of the three magmatic series with those of other Paleoproterozoic domains of the West African Craton and juvenile volcanic rocks leads us to propose an evolutive model for the Mako series comprising an initial emplacement of an oceanic plateau, in which the first tholeiitic series was generated, followed by a subduction process that triggers the contamination of the mantle plume source and the generation of the second tholeiitic series at the earliest stages of the arc built; and ending with the maturation of an island arc and the calc-alkaline series emplacement, which has a chemical signature indicating that the source region was likely a metasomatized mantle probably mixed with some plume mantle source.