Nouvelles données pétrographiques, géochimiques et géochronologiques du massif ‘granitique’ de Bondoukou (Côte d'Ivoire) mise en évidence d'un âge Burkinien, par isochrone Rb/Sr sur roches totales

Abstract

The locality of Bondoukou, NE of Ivory Coast, gave its name to a granite type, opposite to the Baoulé-type granite in Bodin's classification (1951, Bull. Dir. Mines AOF, Dakar, N° 12). This classification has been progressively enlarged to the whole of West Africa. The Bondoukou granite is subcircular in form, intrusive and discordant in the tufaceous schists and had not been dated previously.

Recent age determinations on 15 samples from this massif have been done with the Rb/Sr isochron method on whole rocks. The age obtained is approx. 2170 Ma. Consequently, these new data yield a Burkinian age (ex early Eburnean) which was not expected since a lot of works on Birrimian granites of West Africa assigned a post tectonic character and consequently a later age to the Bondoukou granite, related to the Baoulé-type granites.

Other age determinations with the same Rb/Sr method were done on granitic massifs which were assigned to the Bondoukou type; they yielded ages slightly younger than 2000 Ma in Senegal and Burkina Faso respectively (Bassot, J. P. and Caen-Vachette, M. 1984. In: 12th coll. Inter. Géol. africaine, Bruxelles, et géol. africaine (Edited by Klerkx, J. et Michot, J.),pp. 191–209. Tervuren, Belgique; and Gamsonre, P. E. and Rossy, M. 1974. Esquisse géologique de la région de Ouahi Gouya (N. W. de la Hte Volta). Chronologie des divers types de granitoîdes, p. 181. 2ème Rast, Nancy).

So, these few results point out the complex position of the Bondoukou type granites. Their meaning in the main Eburnean orogeny must be considered.