THE BIRIMIAN VOLCANISM IN THE NORTHEASTER N IVORY-COAST, EVIDENCE FOR2 DISTINCT VOLCANO-TECTONIC PHASES IN THE GEODYNAMICAL EVOLUTION DURING THE PALAEOPROTEROZOIC

In the northeastern Ivory-Coast, volcanic formations having different geochemical features are located in the Haute-Comoe volcanosedimentary Birimian terrains (Palaeoproterozoic). They consist of (1) tholeiites belonging to greenstone belts and showing an oceanic magmatic signatu re, (2) andesitic calc-alkaline lavas interbedded in the sediments of the Haute-Comoe Basin and related to an active margin-type magmatogene sis, and (3) rhyodacitic intrusions spatially and geochemically linked to granitoid plutons. The magmatic characterization, in terms of geot ectonic contexts leads to the following scheme: (1) formation of the g reenstone belts in a juvenile oceanic context with building of oceanic plateau (2.195 Ga), (2) genesis of TTG granitoid batholites which met amorphose the belts and beget a first continental crust (2.15 Ga), (3) opening of a sedimentary basin in a shear-zone corridor with local pr oduction of calc-alkaline volcanism due to heat transfer along a major lithospheric fault (2.15 - 2.10 Ga), (4) shortening of the basin with leucogranite intrusions in the same transcurrent context (2.09 Ga). T his geodynamical scheme takes account of the distinction between two m ajor volcano-tectonic phases : a tholeiitic phase with the greenstone belt formation and then, a calc-alkaline phase linked to the structura l evolution of the sedimentary basin. This model could be applied to o ther Ivory-Coast Birimian terrains, but it is necessary to distinguish the volcanics and the sediments belonging to the greenstone belts and those of the basins which were emplaced between the TTG batholiths.