ARCHEAN CRUSTAL EVOLUTION OF THE WEST-AFRICAN CRATON - EXAMPLE OF THEAMSAGA AREA (REGUIBAT RISE) - U-PB AND SM-ND EVIDENCE FOR CRUSTAL GROWTH AND RECYCLING

New geochronological data (U-Pb and Sm-Nd) are used to constrain the A rchean crustal growth and evolution of the southwestern part of the Re guibat Rise (Amsaga area) in the West African craton. The results are summarized as follows. (1) U-Pb zircon data indicate an emplacement ag e of 2986+/-8 Ma for the magmatic protolith of a charnockitic pluton, in agreement with a whole-rock Sm-Nd isochron at 3012+/-142 Ma. Positi ve epsilon(Nd(3.0)) values ranging between 0.89 and 1.95 and agreement between Nd model ages (between 3.23 and 3.10 Ga) and U-Pb age indicat e a juvenile origin for the magmatic protolith and imply a crustal gro wth event at ca 3.0 Ga. Seven rims of the charnockitic zircons yield d iscordant Pb-207/Pb-206 ages ranging from 2979+/-11 to 2737+/-11 Ma an d are interpreted as resulting from growth during the granulitic event . (2) Whole-rock Sm-Nd data on migmatitic orthogneisses that form the central part of the area indicate an Archean age for their magmatic pr otolith (T-DM ranging from 3.24 to 2.87 Ga) in agreement with a previo us single zircon evaporation age of 2836+/-16 Ma (Potrel, 1994). (3) T wo late-tectonic, post-granulite plutons (both granite and gabbro) dat ed at ca 2.73 Ga mark the end of the granulitic event. The granite pro vides a U-Pb age of 2726+/-7 Ma and the gabbro a mineral Sm-Nd isochro n age of 2706+/-54 Ma. The T-DM ages ranging from 3.1 to 3.2 Ga indica te an origin by crustal reworking of the granite and negative epsilon( Nd(2.7)) value (-0.7) of the gabbro indicate crustal contamination dur ing emplacement. These results, combined with previous work (Potrel et al., 1996), indicate that the Archean Reguibat Rise formed through a succession of crustal growth events from at least 3.50-3.45 Ga until 2 .73 Ga. A major granulitic event took place at ca 2.73 Ga and was acco mpanied by emplacement of crustally derived magmas. (C) 1998 Elsevier Science B.V.