PETROGENESIS OF JUVENILE-TYPE BIRIMIAN (PALEOPROTEROZOIC) GRANITOIDS IN CENTRAL COTE-DIVOIRE, WEST-AFRICA - GEOCHEMISTRY AND GEOCHRONOLOGY

The Birimian (Paleoproterozoic) granitoids of Central Cote-d'Ivoire ar e associated either with greenstone belts or with basin sedimentary ro cks. Field data indicate two stages of granitoid emplacement which, ac cording to single-zircon and monazite Pb-207/Pb-206,ge determinations, occurred within a short period of time. The first generation of grani toids (2123-2108 Ma) intruded and metamorphosed the greenstone belts, whereas the second generation (2108-2097 Ma) intruded and metamorphose d the sedimentary pile and subordinate andesitic volcanic rocks of the Bandama basin unconformably overlying the greenstone belts. Geochemic al and isotopic data show that the various granitoids belong to four m agmatic groups: (1) sodic calc-alkaline (NaCG); (2) potassic calc-alka line (KCG); (3) peraluminous (AlG); and (3) alkaline (alkG). All the g ranitoids show juvenile features, with mantle-like low initial Sr isot opic compositions (0.700-0.702) and positive epsilon Nd values (1.5-2. 7). These data indicate that (a) the NaCG may have formed by partial m elting of a garnet-amphibolite crust, (b) the KCG and alkG derived fro m partial melting of crustal, mafic to intermediate metaigneous rocks and (c) the AlG (epsilon Nd = 1.5) represent infracrustal melting of y oung sialic igneous materials during a regional crustal shortening, in a transcurrent tectonic context, Sm-Nd model ages (2.25-2.4 Ga) precl ude the existence of an Archean crust reworked in the Birimian magmas. An underplating model is discussed. (C) 1998 Elsevier Science B.V.