Geochronological evidence for reworking of Archean terrains during the Early Proterozoic (2.1 Ga) in the western Cote d'Ivoire (Man Rise West African Craton)

A period of accretion of continental crust is recognized during the Archean 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 Cote d'Ivoire. There is no significant evidence of any older event, such 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 gneisses of a minimum age ca 3.05 Ga (Leonian event). A subsequent major phase of igneous and metamorphic activity around 2.8 Ga (Liberian event) is recorded in the entire Archean terrain without any evidence of new accretion. This second event began at 2830 +/- 7 Ma, its climax is indicated by charnockitic intrusions followed by granodioritic magmas at 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 rock-garnet Sm-Nd ages). Paleoproterozoic reworking is mainly found in the southern part of the Archean terrain of the Man area: zircon; monazite; sphene; and garnet geochronometers all indicate an important thermal effect during the Birimian event (i.e. 2.1 Ga). The earliest influence 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 reached during the Birimian event. This metamorphic overprint and the diapiric emplacement of coeval Paleoproterozoic juvenile magmas at 2.1 Ga in the southern domain (Toulepleu massif) suggest that the 2.1 Ga overprint could be related to major underplating processes under the Archean basement. The thermal flow related to the differentiation and the accretion of these juvenile Birimian magmas may account for the reworking of the surrounding Archean domain which had been possibly thinned during underplating. These results suggest close proximity between the Archean continents and an accretion zone associated with juvenile magmatism 2.1 Ga ago. (C) 1997 Elsevier Science B.V.