Pétrologie et géomorphologie dans la région de Kango, Gabon: un grand dyke ultramafique-mafique archéen

The mineral assessment programme of Gabon, carried out during the 1980s, showed a 2870 Ma old igneous mafic-ultramafic complex in the Kango area that was first considered to be made up of five separate intrusions associated with Cr---Ni geochemical anomalies. Following the study of the regional relief, the surface materials and the petrography, a heavily incised zone has been selected where outcrops were abundant enough to reasonably define the structure of the complex. Therefore, where observed, the igneous body looks like a subvertical 1–2 km wide dyke composed of an early unit of cumulus feldspathic bronzitites and norites and a late gabbroic unit showing a chilled border against the Archean gneisses. The strike variation of gneisses foliation shows a right-lateral fault of the N40° fracture responsible for the emplacement of the dyke. The mafic-ultramafic complex presents some alkaline late-magmatic and cataclastic effects only along the N170° fractures that affect the dyke by a left-lateral fault. The very same petrography of the other mafic-ultramafic igneous bodies and their relationship with the fractures of the gneisses basement make it possible to interpret the different intrusions as a great dyke some 100 km long within Gabon and possibly extending into equatorial Guinea.