GEOCHRONOLOGICAL EVIDENCE FOR REWORKING OF ARCHEAN TERRAINS DURING THE EARLY PROTEROZOIC (2.1 GA) IN THE WESTERN COTE-DIVOIRE (MAN RISE WEST-AFRICAN CRATON)

A period of accretion of continental crust is recognized during the Ar chean at ca 3.2 - 3.3 Ga on the basis of whole-rock Nd model ages and Pb ages on detrital zircons from various gneisses of the Man area in C ote d'Ivoire. There is no significant evidence of any older event, suc h as the 3.5/3.6 Ga accretion episode recognized in the Archean of the Reguibat Rise in Mauritania. The earliest magmatic events recorded in the northern domain of the Man area correspond to tonalitic grey gnei sses of a minimum age ca 3.05 Ga (Leonian event). A subsequent major p hase of igneous and metamorphic activity around 2.8 Ga (Liberian event ) is recorded in the entire Archean terrain without any evidence of ne w accretion. This second event began at 2830 +/- 7 Ma, its climax is i ndicated by charnockitic intrusions followed by granodioritic magmas a t ca 2800 +/- 10 (Pb-Pb and U-Pb ages on monazite and zircon). Cooling after the Liberian event occurred ca 2.75 Ga ago (K feldspar-whole ro ck-garnet Sm-Nd ages). Paleoproterozoic reworking is mainly found in t he southern part of the Archean terrain of the Man area: zircon; monaz ite; sphene; and garnet geochronometers all indicate an important ther mal effect during the Birimian event (i.e. 2.1 Ga). The earliest influ ence of this event is recorded in a range of 2250 + 40 / - 70 Ma, with a major event ca 2.1 Ga. Partial to total resetting of U-Pb and Sm-Nd geochronometers indicate that high-temperature conditions were reache d during the Birimian event. This metamorphic overprint and the diapir ic emplacement of coeval Paleoproterozoic juvenile magmas at 2.1 Ga in the southern domain (Toulepleu massif) suggest that the 2.1 Ga overpr int could be related to major underplating processes under the Archean basement. The thermal flow related to the differentiation and the acc retion of these juvenile Birimian magmas may account for the reworking of the surrounding Archean domain which had been possibly thinned dur ing underplating. These results suggest close proximity between the Ar chean continents and an accretion zone associated with juvenile magmat ism 2.1 Ga ago. (C) 1997 Elsevier Science B.V.