Zonation of supracrustal relics in the Archaean of Sierra Leone, Liberia, Guinea and Ivory Coast

Abstract

THE West African Craton¹ contains a distinct province of Archaean rocks which outcrop in Sierra Leone, Liberia, Guinea and Ivory Coast. Rb–Sr whole rock isochron ages fall between 2,600 and 2,800 Myr BP (refs 2, 3). This paper presents a comparative stratigraphie interpretation of supracrustal relics in this region, based on new maps of three of the supracrustal belts in Sierra Leone, which I made whilst employed by the Geological Survey of Sierra Leone, and a re-examination of published maps and memoirs in the light of field experience. No previous stratigraphie interpretation has been made of the Sierra Leone belts and no previous comparison has been made of supracrustal belts within the West African Archaean. Supracrustal relics have been mapped over a large area. When comparisons are made they seem to fall into two groups on the basis of their size, stratigraphie thickness, lithologies, metamorphic grade and geographical distribution (Fig. 1). First, in Sierra Leone to the west of this region there are large schist belts (up to 130 km long) with thick successions (up to 6.5 km) metamorphosed to amphibolite grade. Banded iron formation is a relatively minor lithology. Second, in southeastern Sierra Leone, Liberia and western Ivory Coast there are small schist relics (maximum 40 km long) with thinner stratigraphie successions (rarely greater than 1 km) in which banded iron formation is dominant and gabbro anorthosite complexes are minor; these rocks were metamorphosed to granulite grade. Geochronological evidence suggests that the two groups of supracrustal rocks are contemporaneous and it is proposed here that they reflect a primary zonation of the Archaean in southern West Africa.