Lithologie, structure et métamorphisme des formations birimiennes dans la région d'Odienné(Coˆte d'Ivoire): roˆle majeur du diapirisme des plutons et des décrochements en bordure du craton de Man

Abstract

New petrostructural data obtained during regional mapping allow the Odiennéregion to be divided into three domains. The western domain comprises a basal series of tholeiitic and komatiitic pillow metabasalts (≥1000 m) overlain by metasediments of oceanic affinity. The overlying younger volcanic complex of andesites, dacites and rhyolites with few interlayered metasediments, all ≥4000 m thick, is probably slightly older than 2.1 Ga, by comparison with adjacent areas. Trajectories of the steep metamorphic cleavage and lineations that formed during the regional tectonometamorphic event are parallel to magmatic foliations and lineations formed in plutons during the magmatic stage. First estimates of the synmetamorphic strain in conglomerates at T around 550°C suggest more than 50% of thinning of the series synchronous with a strong, mostly vertical elongation. The central domain comprises high temperature paragneisses derived from pelites and semipelites. Regional metamorphism culminated in metapelites with anatexis and the crystallisation of kinzigites (T = 700–800°C, P around 5 kbar), which may have formed only close to synmetamorphic mafic intrusives. The transpressive deformation produced a strong subhorizontal north-south elongation and relates to the initiation of the sinistral Sassandra Shear Zone, which comprises also bands of mylonites and ultramylonites formed later under the same stress field at decreasing temperatures. The eastern domain comprises siliciclastic and volcaniclastic metasediments with amphibolite facies metamorphism subjected to the same sinistral transpressive regime, anatectic conditions being reached close to the TieméBatholith. The three domains have been placed close together through the sinistral strike-slip motion along the Sassandra Shear Zone that splits into several branches north of Odienné. By comparison with southern Mali, these deformations, which are part of the Eburnian Orogeny, took place in this area between 2.10 and 2.07 Ga.