Kimberlites in western Liberia: An overview of the geological setting in a plate tectonic framework

Abstract

The Liberian kimberlite field lies in the southern part of the West African craton. Closely associated kimberlites are present in Sierra Leone, Ivory Coast, Guinea, and Mali and, as a group, are structurally controlled within the Archean basement complex of the Leo uplift. Three major kimberlite dike lineaments are recognized in the northwest sector of Liberia all with prominently developed N‐NE trends. These lineaments are termed the Kumgbo‐Wuese trend, the Border trend, and the Lofa trend. The areas of dike intrusion are covered by the Bopolu geological quadrangle map of Liberia: the Border trend is close to Sierra Leone, the Kumgbo‐Wuese trend is to the east, and the Lofa trend is coincident with the Lofa River. Only one kimberlite pipe has been recognized in Liberia (Mano Godua), but a second may be present, and both lie along the Kumgbo‐Wuese trend. The kimberlite bodies intrude the ∼2700‐m.y.‐old Liberian age province, which is comprised of granitic and granodioritic gneiss complexes. Two prominently developed features dominate the province: a general NE‐SW basement fabric and an en echelon dolerite dike system that parallels the coast, dated at ∼180 m.y. and considered to represent intrusion at the onset of continental fragmentation. Transecting dikes and related faults suggest a relative age for the kimberlites of < 180 m.y., an estimate that corresponds to the 92‐ to 140‐m.y. age for the Sierra Leone field. There is indirect evidence that kimberlites in Guinea and Mali are of a similar age based on the fact these are on strike with kimberlites in Liberia and Sierra Leone, respectively. Trans‐Atlantic correlations between West Africa and northeast South America reveal a cyclic eruption of magma types that were generated in response to plate and continental movements. It is postulated that Liberian kimberlite emplacement was related to postplate motion crustal flexuring that reactivated Precambrian basement sutures. These sutures are parallel to, and correlate with, recognizable paleorift and paleocompressional zones within, and adjacent to, the West African craton. Long sustained periods of tectonic activity can be demonstrated, and it is proposed that, if brittle fractures are continuously reactivated, these will serve as conduits to the upper mantle, tapping kimberlite source regions at depths in excess of 100 km.