Geodynamic setting of the Birimian volcanism in central Ivory Coast (western Africa) and its place in the Palaeoproterozoic evolution of the Man Shield.
Résumé
Palaeoproterozoic volcanic formations having different geochemical features are described in the Katiola-Marabadiassa area (Central Ivory Coast). They consist of (i) metamorphic mafic rocks belonging to the greenstone belts, (ii) rhyodacites intruded into the greenstone belts and interpreted as sub-volcanic apophyses of TTG plutons, and (iii) calc-alkaline andesitic lava interbedded in the lower sedimentary pile of the Bandama Basin. The greenstone belt rocks have a magmatic signature of ocean floor tholeiites. They were generated from moderate partial melting of a fairly depleted spinel lherzolite source. The rhyodacites show the same compositions as the TTG granitoids of the belt. These TTGs generated from melting of garnet-bearing amphibolite, derived from subducted oceanic crust, but also from peridotite melting of the fertilized mantle wedge. The andesites show a common composition of active continental margin potassic calc-alkaline lavas. However, they do not constitute a volcanic belt, but belong to an intracontinental basin settled in a wrench tectonic system. They probably generated from remelting of sub-crustal metasomatised mantle. The tectono-magmatism story of the Katiola-Marabadiassa area comprises four stages. A similar evolution is found in other belt-and-basin areas in Ivory Coast. A review of lithostratigraphic and chronological data of the Birimian terrains leads us to compare the four-stage model of the Central Ivory Coast to the West-African Palaeoproterozoic evolution. The first stage corresponds to ocean crust formation, by mid-ocean ridge accretion and volcanic arc building. The second stage, partly overlapping the previous one, is convergent and lasted from oceanic crust stacking to TTG and associated greenstone belt formation. It led to a protocontinental crust accretion by docking of granitized blocks, from ca 2200 Ma to 2110 Ma. The third stage corresponds to the formation of intracontinental basins by reorganization of the granitoid-greenstone shields and of the remnant oceanic basins, between 2118 Ma and 2093 Ma. It was associated with calc-alkaline magma production. The fourth stage is characterized by transtensional stress, with the closure of the basins by left-lateral wrenching along N-S faults, and by emplacement of new potassic granitoids until 2072 Ma.