Geochemistry and Petrogenesis of Basic and Ultrabasic Rocks Elogo Complex in Ivindo Archean Block (Congo Craton): Geodynamic Implications

Abstract

The Elogo complex is a greenstone belt portion located on the Eastern edge of the Archean Congo craton at the junction with the Paleoproterozoic to Neoproterozoic Sembe Ouesso basin. This study was carried out on this complex to determine the context of the placement of basaltic rocks. Metaluminous tholeiitic basalts (basic and ultrabasic), calc-alkaline basalts, andesitic basalts, and peraluminous calc-alkaline dacites represent greenstones. Tholeiitic and calc-alkaline basalts come from deep enriched and depleted mantle sources, including garnet in fusion residues [Al 2 O 3 /TiO 2 > 16 (16.5 to 35.12) and in some samples between 12.45 to 14.48; CaO/Al 2 O 3 < 1 (0.52 to 0.97) and >1 (1.04 to 1.35) in ten samples and (Gb/Yb) PM > 1]. The calc-alkaline dacites come from a shallow depleted mantle source [Al 2 O 3 /TiO 2 > 16; CaO/Al 2 O 3 < 1 and (Gb/Yb) PM > 1]. Tholeiitic and calc-alkaline basalts have a negative Rb, Ba, Ce, and Nb anomaly without negative Ti anomaly, positive Ta, Pb anomalies, and a lack of significant REE [(La/Yb)n = 0.36 to 0.97 and 1 to 2.15; (Ce/Yb)n = 0.27 to 0.96 and 1.04 to 1.72, respectively] fractionation. High Nb/Th (2 to 10) and Nb/U (1.82 to 26) ratios and low La/Ta (5 to 27) ratios are characteristic of divergent margin magmatic sources. Tholeiitic and calc-alkaline basalts correspond to an extensive back-arc basin-type tectonic setting. Calc-alkaline andesitic basalts and dacites show positive Ba, U, Th, K, La, Ce, Pb, and Li anomalies and negative Nb, Ta, and Ti anomalies reflecting crustal contamination and hydrothermal alteration in a compressive tectonic context as a volcanic arc in a subduction regime marking the interruption of the meso-neoarchean Elogo's opening. Elogo's opening and closing are probably as-

Banded Iron Formations in Congo: A Review

“…It primarily consists of oxide-facies BIFs (Moutou, 2021; Gourcerol et al, 2022). They are associated with ultramafic rocks, amphibolites, talc schists and greenschists (Gourcerol et al, 2022;Makamba et al, 2023).…”

“…To date in Congo, only the greenstone belt associated with the Elogo BIFs has been studied to constrain the geodynamic setting (Makamba et al, 2023). On the basis of their geochemical features, it has been suggested that the tholeiitic basalts of the Elogo greenstone belt were formed in a back-arc basin subject to an extensive regime, while the calc-alkaline andesitic basalts and dacites indicate a volcanic arc setting, related to a subduction regime (Makamba et al, 2023). Such a model implies that BIFs were deposited in an extensional back-arc basin, which also recorded hydrothermal vent fluids and was later affected by a compressive orogenic event around 2750 Ma.…”