The mafic layered complex of the Kaby´e massif (north Togo and north Benin): Evidence of a Pan-African granulitic continental arc root
Résumé
Between the predominantly Neoarchean–Paleoproterozoic West African Craton (WAC) and the Saharian Metacraton (SMC), the Dahomeyides suture zone represents a valuable witness of continental amalgamation during Pan-African times. In the Kaby´e massif (northern Togo and northern B´enin) mainly granulitic metagabbros, associated with Al-rich kyanite and garnet bearing felsic dykes, are exposed as tectonic lenses within the pre-Pan-African gneisses of the SMC. New geochemical data suggest that the high grade rocks (granulites) in the Kaby´e massif originated in a mature continental arc setting. AFC calculations constrain the amount of contamination of mantle wedge derived magmas by crustal etasediments to about 10%. Reconstruction of initial magmatic stratigraphy has been carried out using CIPW-norms for cumulitic sequences within the massif, indicating a normal igneous polarity from west to east. Published geochemical signatures along the Pan-African suture zone of the Dahomeyides, in Ghana, Togo and Benin, constrain the origin of mafic granulitic and eclogitic complexes. A distinction is made between bodies with mainly MORB signatures originated from the subducted WAC plate passive margin and those with magmatic arc signatures originated from the over-riding plate mantle wedge. This suggests that the closure of the oceanic domain between the WAC and the SMC from 640 to 610 Ma was mainly accommodated by oceanic subduction beneath the active continental margin, before Pan-African collision.