Petrographic and geochemical study of the Paleoproterozoic formations at Kapogouan, within the Banfora Birimian Belt (Burkina Faso, West African Craton)

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Résumé

We present new petrographic and geochemical data for the Kapogouan area, located in the Banfora Belt in southwestern Burkina Faso. This study is based on an integrated approach combining petrographic and geochemical analysis of the various lithological units.
The study area consists of a series of metavolcanic formations comprising metarhyolites, metadacites, metaandesites, and metabasalt, associated with sedimentary formations. These units are cut by various magmatic intrusions, including granites, granodiorites, monzonites, as well as vein bodies of doleritic and gabbroic composition. All of these formations are affected by a network of regional shear zones and by well-developed penetrating schistosity, reflecting polyphasic compressional tectonics. Regional metamorphism is generally of the green schist facies, with local transitions to the amphibolite facies near the intrusions, suggesting the influence of thermal gradients associated with magmatic episodes. The geochemical signatures of the different lithologies highlight the existence of two distinct magmatic series: a tholeiitic series, comprising meta-andesites, metabasalt, dolerite, and gabbro, and a calc-alkaline series represented by granites, granodiorites, and monzonites. The genesis of the tholeiitic magmas is thought to be linked to a MORB-type mid-ocean ridge setting, whereas the calc-alkaline rocks are associated with a volcanic arc environment. Taken together, these results highlight a complex tectono-magmatic evolution, consistent with a context of accretion and convergence within the Birimian Banfora Belt.

Mots-clés

Geochemistry, Kapogouan, Banfora Belt, Volcanic Arc