Abstract

The presence of both archaic and modern lithological features within the Paleoproterozoic (Birimian) domain of the Man Shield (Southern West African Craton) raises questions once again about the secular change of the Archaean–Proterozoic boundary, which would signify the conclusion of archaic crustal genesis processes. Studying the orthogneisses and paragneisses in south-west Côte d'Ivoire (Sassandra-Cavally [SASCA] domain), particularly those between Bliéron and Grand-Béréby, shows that archaic processes continued in Birimian domains. This is demonstrated by the presence of Tonalite-Trondhjemite-Granodiorite (TTGs). Sm-Nd isotope systematics of two paragneisses associated with the orthogneisses provide model ages of around 2.5 Ga and an εNd value close to zero, respectively. This indicates the presence of Archaean components in the protoliths of the Birimian formations in the studied area. The study area belongs to the SASCA domain, which lies between the Archaean and Birimian domains sensu stricto. Archaean relics dated at 3.2 Ga have been found here. A two-stage geodynamic model for the formation of the studied area in particular and the south west of SASCA domain in general was proposed. Firstly, subduction of an oceanic crust was proposed. Secondly, during this process, the oceanic slab melts to generate an abnormally thickened crust in which TTG magmas occur at the base. The vertical movement of the TTGs and general Northwest to Southeast (NW-SE) shortening generate the vertical structures that characterise the formations in the area.

Key words: Gneisses, Birimian segment, geodynamic model, tonalite-trondhjemite-granodiorite (TTG), West Africa Craton, Côte d’ Ivoire.