Upper Proterozoic schist belts in the Nigerian sector of the Pan-African Province of West Africa

Abstract

Schist belts in Nigeria occur in a 400 km wide zone which trends NNE, parallel to the boundary between the Pan-African Province and the West African Craton. A polycyclic migmatite—gneiss—granite terrain, ranging in age from Archaean to Lower Palaeozoic, flanks the schist belt zone to east and west and separates the individual belts within the zone. The schist belts fall into two age groups. The earlier group contains assemblages of mafic igneous rocks, pelitic schists and phyllites, banded iron formation and, locally, coarse grained clastics and carbonate rocks. These belts show complex structural styles and are extensively invaded by granitic plutons belonging to the widely distributed Pan-African magmatic suite. Schists and gneisses within and adjacent to some of these belts have been dated at ∼ 1100 Ma, and this group is thought to belong to the Kibaran Orogeny. Similar Kibaran rock associations can be recognised elsewhere in the Pan-African Province — in the Hoggar and Cameroun. The Kibaran Orogeny in this region is interpreted as a largely ensialic event (although with possible minor ocean basin formation) that led into the more intense and widespread Pan-African Orogeny. The later group of schist belts is characterised by coarse to fine grained clastics, insignificant mafic igneous rocks, a simple, straight structural style and an absence of internal granite plutons. These schist belts are believed to belong to the Pan-African Orogeny. Their sediments may have been deposited in a back-arc basin related to an E-dipping subduction zone at the western margin of the Pan-African Province.