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Two generations of Birimian (Paleoproterozoic) volcanic belts in northeastern Côte d'Ivoire (West Africa): consequences for the ‘Birimian controversy’

1996; Elsevier BV; Volume: 80; Issue: 3-4 Linguagem: Inglês

10.1016/s0301-9268(96)00011-3

1872-7433

W. Hirdes, Donald W. Davis, G. Lüdtke, Georges Konan,

Geochemistry and Geologic Mapping

UPb ages have been measured on zircon, monazite and titanite from gneiss, granitoids and rhyolite of the Paleoproterozoic Birimian/Eburnean province in the northern Haute Comoé area, northeastern Côte d'Ivoire, West Africa. Dating reveals that two distinct generations of Birimian volcanic belt terranes (Téhini belt in the east, Ouango Fitini belt in the west), separated in age by a 50 m.y. time span, occur proximal to each other in the Haute Comoé North area, at the Ouango Fitini shear zone. This sinistral NNE-trending high-strain zone seemingly represents a regionally important, several hundred kilometers long lineament on the Man shield dividing the Paleoproterozoic Birimian/Eburnean province, seen up to now as a single entity, into an eastern and a western subprovince. The eastern subprovince, which covers Ghana, eastern Côte d'Ivoire and probably many parts of Burkina Faso, typically displays ∼2185-2150 Ma belt volcanism and coeval belt plutonism. Evidence for 2100 Ma extrusive volcanism is absent. The western subprovince (e.g. central Côte d'Ivoire, western Mali, and probably Guinea) is characterized by younger, ∼2105 Ma old volcanic belts and coeval belt plutons. Supracrustal and intrusive rocks of the 2185-2150 Ma time span are at least locally present as various gneisses which previously had been termed ‘Dabakalian’. A granodioritic biotite gneiss in the western gneiss-granitoid terrane of the Haute Comoé area (=western subprovince), resembling ‘Dabakalian’ gneisses further south, gives a crystallization age of 2152 ± 2 Ma, identical to the age of a belt pluton in the Téhini volcanic belt (2152 ± 3 Ma). This suggests that those greenschist-facies supracrustal rocks originally termed ‘Birimian’ in Ghana by Kitson (1918) are coeval with gneisses in Côte d'Ivoire termed ‘Dabakalian’ by Lemoine (1988). Consequently, the invoked ‘Burkinian’ deformation event, which was assumed to separate Dabakalian basement and Birimian supracrustal rocks, cannot exist. Rather, Birimian rocks of the eastern subprovince may be correlative with protoliths of the higher-metamorphic Dabakalian rocks present in the western subprovince. The term ‘Bandamian’ is suggested to refer to the 2105 Ma old suite of supracrustal rocks of the western subprovince, the term ‘Birimian’ being restricted to the 2185-2150 Ma old volcanic/volcaniclastic rock suite of the eastern subprovince (Ghana). If the Birimian/Eburnean province on the Man shield consists of two different-aged subprovinces, the relative scarcity of economic gold deposits in central Côte d'Ivoire, as compared to Ghana, could not only be a function of underexploration of the area, but also of different geological processes having formed the terranes of the (older) eastern and the (younger) western subprovince.