3.5-GA-OLD TERRANES IN THE WEST-AFRICAN CRATON, MAURITANIA

Evidence for early Archaean relicts in the northern part of the West A frican Craton are provided from ion-microprobe (SHRIMP) U-Pb zircon an d Sm-Nd whole-rock ages in the West Reguibat Rise of Mauritania. The U -Pb ages range between 3515 +/- 15 Ma and 3422 +/- 10 Ma for concordan t zircons from an orthogneiss which has also been affected by a 2.7 Ga old metamorphic event, Nd model ages calculated for adjacent metasedi ments range between 3.9 and 3.5 Ga, suggesting that these supracrustal rocks may be derived from the erosion of a similar early Archaean cru st, Zircon ages and Nd depleted-mantle model ages provide the first ev idence for an early Archaean crustal growth, at least 3.5-3.6 Ga old, in the West African Craton.