Crustal evolution of the Paleoproterozoic Birimian terranes of the Baoulé-Mossi domain, southern West African Craton: U-Pb and Hf-isotope studies of detrital zircons

© 2015 Elsevier B.V. The Paleoproterozoic Baoulé-Mossi domain of the southern West African Craton is regarded as the result of juvenile crust formation during the 2.3-1.9Ga period. However, little is known about the complex processes that led to the formation of this highly metal endowed lithospheric block. Hence, in order to better constrain its geodynamic evolution, small catchments within modern drainage basins of the Niger and Bani Rivers were sampled for detrital zircons for U-Pb and Hf-isotope analyses, following TerraneChron® methodology. The study area stretches across southern Mali over 40,000 km2 between the Yanfolila (west) and Syama (east) belts, from where more than 1000 zircons representing eight different sub-basins have been analyzed.The results of this study identified 2 main zircon age populations. The older population with ages between 3.6 and 2.7. Ga represents approximately 10% of the total of the analyzed zircons, and is mainly restricted to the western margin of the study area across the Yanfolila Belt sampling sites. The Hf-isotope data from this zircon population yielded model ages between 3.7 and 3.6. Ga and suggest a common evolution. Conversely, the main zircon population is dominated by Paleoproterozoic age zircons ranging between 2.4 and 2.1 Ga. Main peaks within this age group were identified at ca. 2161, 2121 and 2084 Ma, and minor peaks at ca. 2343, 2263 and 2200 Ma. The Hf-isotope data of this age group indicate that the zircons are derived from a mainly juvenile source, but were contaminated with crustal rocks potentially as old as 2.8 Ga. However, a group (approximately 10%) of Paleoproterozoic age zircons, which is interpreted to be the result of stream sediment transport from distal sources or the result of multiple cycles of sedimentation, displays a minor but significant older component, up to 4.0 Ga.This study also highlights the occurrence of >2.7 Ga detrital zircons, which are of an unknown source as no rock unit in the region has been identified to yield similar Archean ages. Moreover, some Paleopro-terozoic zircons show Hf-isotope composition that indicates reworking of an older crustal component.Although zircons of the Baoulé-Mossi domain generally confirm its juvenile origin, they also indicatereworking of an older crust at a much larger scale than previously recognized. The identification ofArchean zircons in the region argues for greater interaction between the Baoulé-Mossi domain and theArchean Kénéma-Man domain.
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