STRATIGRAPHY OF THE BIRRIMIAN UNITS IN TH E CENTRAL IVORY-COAST (WEST-AFRICA)

For the first time in this country, the comparative study of three Bir rimian volcano-sedimentary complexes (Lower Proterozoic) of the centra l Ivory Coast (Yaoure, Dianfla, Toumodi), makes its possible to establ ish correlations from lithostructural, geochemical and geodynamic data . In, all areas under study, the Birrimian formations were affected by metamorphism ranging from epizonal (Yaoure, Toumodi) to mesozonal (Di anfla). They lie on an ante-Birrimian, polydeformed, gneissic basement showing a katazonal metamorphism. This basement, dated at 2186 Ma in the Yaoure, is comparable with the Dabakalian terranes of the Fetekro group (in the NE of the Ivory Coast). In the three complexes the Birri mian formations show the same lithostructural subdivisions, similar to those of Senegal - Mali or Ghana : (1) A Lower Birrimian (B1), mainly sedimentary (flysch type with pelitic and graywacke beds, black shale s), affected by polyphased tectonics (D1, D2). (2) An Upper Birrimian (B2), volcano-sedimentary to volcanic, affected by a single folding ph ase (D2), and subdivided in three typical suites : (a) a lower basalti c suite with pillow lavas, tuffs and breccias, showing tholeiitic affi nities (A suite) ; (b) a calc-alkaline suite with rhyolites and dacite s (B suite), well developed around Dianfla and Toumodi, almost absent in the Yaoure; (c) at the top, a volcano-sedimentary suite (C suite) r epresented by polygenic basic conglomerates with a <<Tarkwaian>> facie s in the areas under study. The geochemical and petrographical data on the magmatism show either an oceanic arc context (IAT) near Toumodi a nd Dianfla from B1 to B2, or an oceanic intra-plate context with mantl e plume (OIB) affinities in the Yaoure during B2. The positive epsilon (Nd) values indicate that the Birrimian formations and the late granit oids contain a negligible component of Archean crust, but cannot deter mine the possible contribution of a younger continental crust created during the Burkinian cycle (2.4 - 2.15 Ga). The geodynamic evolution o f the Birrimian terranes, over an E-W line through the Ivory Coast, is described according to collision tectonics models. During the Eburnea n orogen between 2150 and 1950. An E-W compression induces a first str ain stage (D1), about 2110 Ma, on all the complexes. The D2 strain is spread over a period of 2040 to 1950 Ma depending on the different com plexes.