A 3.5 Ga granite-gneiss basement in Guinea: further evidence for early archean accretion within the West African Craton

A granite-gneiss formation (Guelemata Orthogneiss) was mapped on the northe rn and western slopes of the Mount Nimba hill range in Guinea. The original rocks were high-Al, low-Yb, medium to high-K granites similar to most Arch ean TTG. Analyses of U and Pb isotopes in zircons, using an ion-microprobe (SHRIMP), gave ages of 3542 and 3535 Ma respectively for the granite-gneiss and a neighbouring granulitic metagabbro. These are the oldest ages so far reported from the Archean Kenema-Man domain of West Africa. Partial meltin g of an unfractionated basalt under eclogite facies conditions appears to b e a suitable model for the origin of the granite-gneiss. (C) 2001 Elsevier Science B.V. All rights reserved.