No abstract

Magmatic charnockitic rocks and TTG (tonalite-trondhjemite-granodiorite) plutons intruded successively the Archaean greenstone belts of the Ntem Complex, at the northwestern margin of the Congo Craton. Geochemical data, zircon Pb-Pb ages, and Sr-Nd isotope studies, constrain the magmatic features, the genetical timing, and the geodynamic settings of these different suites. Charnockites and TTGs are characterized by high Al2O3 contents, high Na/K ratio, low Th content, LREE enrichment, HREE depletion, negative anomalies in Nb, Ta, and Ti, and positive anomalies in Sr. The Pb-Pb zircon ages indicate that charnockites were emplaced at ca. 2900 Ma and TTGs, which cross-cut the charnockites, at ca. 2830 Ma. TDM Nd mean crustal residence ages of both suites range between 3.10 and 2.93 Ga. The charnockites show slightly positive initial εNd2.9Ga (+0.3 to +1.3), whereas the TTGs have slightly negative values (+0.1 to −1.5). The charnockitic and TTG magmas may have resulted from different partial melting processes of the primitive Archaean basaltic crust and contaminated mantle, possibly in a hot slab subduction convergent regime. They contribute to a two-stage crustal growth of the Archaean craton.

The Paleoproterozoic domain of the Ivory Coast lies in the central part of the West African Craton (WAC) and is mainly constituted by TTG, greenstones, supracrustal rocks and leucogranites. A compilation of metamorphic and radiometric data highlights that: i) metamorphic conditions are rather homogeneous through the domain, without important metamorphic jumps, ii) HP-LT assemblages are absent and iii) important volumes of magmas emplaced during the overall Paleoproterozoic orogeny suggesting the occurrence of longlived rather hot geotherms. Results of the structural analysis, focused on three areas within the Ivory Coast, suggest that the deformation is homogeneous and distributed through the Paleoproterozoic domain. In details, results of this study point out the long-lived character of vertical movements during the Eburnean orogeny with a two folds evolution. The first stage is characterized by the development of "domes and basins" geometries without any boundary tectonic forces and the second stage is marked by coeval diapiric movements and horizontal regional-scale shortening. These features suggest that the crust is affected by vertical movements during the overall orogeny. The Eburnean orogen can then be considered as an example of long-lived Paleoproterozoic "weak-type" orogen.