Petrogenesis of juvenile-type Birimian (Paleoproterozoic) granitoids in Central Cote-d'Ivoire, West Africa: geochemistry and geochronology

The Birimian (Paleoproterozoic) granitoids of Central Cote-d'Ivoire are associated either with greenstone belts or with basin sedimentary rocks. Field data indicate two stages of granitoid emplacement which, according to single-zircon and monazite Pb-207/Pb-206,ge determinations, occurred within a short period of time. The first generation of granitoids (2123-2108 Ma) intruded and metamorphosed the greenstone belts, whereas the second generation (2108-2097 Ma) intruded and metamorphosed the sedimentary pile and subordinate andesitic volcanic rocks of the Bandama basin unconformably overlying the greenstone belts. Geochemical and isotopic data show that the various granitoids belong to four magmatic groups: (1) sodic calc-alkaline (NaCG); (2) potassic calc-alkaline (KCG); (3) peraluminous (AlG); and (3) alkaline (alkG). All the granitoids show juvenile features, with mantle-like low initial Sr isotopic 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 melting of a garnet-amphibolite crust, (b) the KCG and alkG derived from partial melting of crustal, mafic to intermediate metaigneous rocks and (c) the AlG (epsilon Nd = 1.5) represent infracrustal melting of young sialic igneous materials during a regional crustal shortening, in a transcurrent tectonic context, Sm-Nd model ages (2.25-2.4 Ga) preclude the existence of an Archean crust reworked in the Birimian magmas. An underplating model is discussed. (C) 1998 Elsevier Science B.V.