Abstract
A granite–gneiss formation (Guélémata Orthogneiss) was mapped on the northern 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 Archean 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 melting of an unfractionated basalt under eclogite facies conditions appears to be a suitable model for the origin of the granite–gneiss.
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