U-Pb dating of zircons from the fine-grained gneisses of Léo (Burkina Faso): Evidence of early evolution in the Baoulé-Mossi domain of the southern ridge of the West African Craton

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Résumé

The fine-grained felsic gneisses of the Léo region (southern Burkina Faso) constitute a key group of rocks for understanding the early evolution of the West African Craton. This study combines petrographic, geochemical, and geochronological (U–Pb on zircon) approaches to constrain their origin and geodynamic significance.
Two main facies have been identified: (i) a strongly foliated and banded hornblende ± pyroxene granodioritic gneiss, and (ii) a biotite ± muscovite granitic gneiss, exhibiting remnants of magmatic texture. The petrographic and mineralogical characteristics, notably the dominance of quartz–feldspar–plagioclase assemblages, the absence of aluminosilicates, and their textural features, indicate a magmatic origin for the protoliths.
Geochemical data show granitic to granodioritic compositions, metaluminous to slightly peraluminous, belonging to the Type I calc-alkaline series. The rare earth spectra are enriched in light rare earth elements (LREE) with negative anomalies in Eu, Nb, Ti, and P, suggesting an evolution dominated by fractional crystallization involving plagioclase, amphibole, and accessory minerals.
U–Pb ages obtained on zircon (2254 ± 3 Ma and 2255 ± 4 Ma) indicate magmatic crystallization in the Early Paleoproterozoic. These ages, consistent between the two facies, attest to a synchronous regional magmatic episode.
Geochemical data indicate granitic to granodioritic compositions, ranging from metaluminous to slightly peraluminous, belonging to the Type I calc-alkaline series. The rare earth spectra are enriched in light rare earth elements (LREE) with negative anomalies in Eu, Nb, Ti, and P, suggesting an evolution dominated by fractional crystallization involving plagioclase, amphibole, and accessory minerals.
U–Pb ages obtained from zircon (2254 ± 3 Ma and 2255 ± 4 Ma) indicate magmatic crystallization in the Early Paleoproterozoic. These ages, consistent between the two facies, attest to a synchronous regional magmatic episode.
The data as a whole suggest that these gneisses derive from calc-alkaline magmas originating from a hybrid mantle–crustal source within a magmatic arc setting. They are part of an early episode of crustal accretion around ~2250 Ma, preceding the classic Birimian magmatism, and are comparable to equivalent formations recognized in Ghana, Côte d’Ivoire, and the West-Gourma Belt.

Mots-clés

Fine-grained gneiss – U-Pb dating of zircon – Léo – Burkina Faso – West African Craton