Petrological Characteristics of the Gneissic Formations in the Kenema-Man Domain: Biankouma-Kouibli Sector (Cote d’Ivoire West African Craton)

The gneissic formations constitute one of the major formations observed within the Archean domain of Côte d’Ivoire. Located in the northwest of Côte d’Ivoire, the gneissic formations of the Biankouma and Kouibli sectors were the subject of this study. In order to determine the geochemical and petrographic characteristics as well as the geotectonic environment of these rocks, petrographic studies associated with geochemical analyses were carried out. The geology of this area includes mainly metamorphic formations such as granulitic gneiss, charnockites, pink granulites, charnockitic gneiss and migmatitic gneiss with biotite. The mineralogy of these formations is dominated by quartz and feldspars associated with either biotite or hypersthene. The geochemical data indicate that these formations are generally granodioritic and tonalitic in composition; they are TTG. They have an essentially calc-alkaline affinity (strongly potassic), however tholeic occurrences are observable. The formations in the Biankouma and Kouibli sectors are weakly metaluminous to peraluminous and poor in alumina (Al2O3 < 15%). Rare earth spectra show an enrichment in light rare earths and a depletion in heavy rare earths. A negative europium anomaly is also observed. This anomaly implies the presence of feldspar, notably plagioclase in the residual liquid. Multi-element diagrams normalized to the early mantle showed enrichment in LILEs and depletion in HFSE. Negative Ta and Nb anomalies were observed in each of the samples. These indicate that the studied formations originate from the partial melting of the crust. Trace element data, including rare earths, indicate that the formations studied are derived from the partial melting of a basic composition rock containing garnet and hornblende. All samples have a composition of arc and collision granites and would be generated in a subduction zone.