4D evolution of the paleoproterzoic Baoule-Mossi domain of the West African Craton

The geological understanding of the relationship between the Archean Kénéma-Man and Paleoproterozoic Baoulé-Mossi domain of the southern Leo-Man rise in the West African Craton (WAC) is still subject of debate. The 4D geodynamic evolution and resulting lithospheric architecture have puzzled scientists, which have proposed multiple theories to fit the existing datasets. Controversies range over modern plate tectonics as opposed to a tectonic domain dominated by mantle upwellings and deformation of the crust driven by gravity. Another controversy is the potential involvement of Archean crustal material in a region that is considered to be the result of mostly Proterozoic juvenile accretion.

The West African Craton (WAC) is composed of three major crustal blocks. These blocks are known as the Reguibat rise at the northern end, the Kedougou-Kenieba and Kayes Inliers in the westernmost portion of the craton and the Leo-Man rise at the southernmost region. The northern and southern portions are characterized for having Archean exposures in their westernmost portions while in the eastern regions the exposures are of Paleoproterozoic age. The Kedougou-Kenieba and Kayes Inliers, which sit in between the northern and southern portions, are predominately Paleoproterozoic in age.

This study has developed a new insight into the geodynamic evolution and architecture of the Paleoproterozoic Baoulé-Mossi domain of the WAC. To achieve a craton wide understanding of the Baoulé-Mossi domain, a large number of spatially targeted felsic intrusions from Burkina Faso, southern Mali and eastern Guinea were characterized for high precision in-situ U-Pb geochronology, Lu-Hf and O isotopes composition, as well as whole rock major and trace element geochemistry. To complement the coverage of the large study area and increase sample density, samples from small river catchments were also collected. The samples collected with this methodology known as TerraneChron® added more than 1000 zircons to the database of this study, which increased the available number of ages as well as Lu-Hf analyses and thus provided an additional tool to understand the evolution of the Baoulé-Mossi domain.

The treatment of the different datasets allowed the identification of key parameters that were either not previously recognized or that were not considered a driving factor in the evolution of the craton. The subsequent integration of all the identified parameters and key aspects allowed the introduction of a geodynamic and architecture model for the Paleoproterozoic Baoulé-Mossi domain of the West African Craton.

The use of the TerraneChron® method across southern Mali allowed a general overview of the U-Pb and Hf isotope characterization of the region. The methodology shows the presence of zircons with ages between 2400 and 2070 Ma, and Hf isotopes with model ages between 2800 and 2100 Ga. These U-Pb and Hf isotopes model ages are consistent with the felsic intrusions reported in this study and with known volcanic activity of the region. More importantly the TerraneChron® method identified a series of zircons with ages between 3600 and 2100 Ma and Hf isotope signature between 3600 and 2800 Ma. The latest group of zircons, which are sub-rounded to rounded, are considered to be the result of transported material that have been subject to multiple sedimentary cycles.

Geochronological aspects include the identification of diachronous evolution of the Baoulé-Mossi domain. The easternmost portion was subject to a main period of magmatic activity lasting between ca. 2190 and 2130 Ma, that peaked at ca. 2175 and 2135 Ma. This contrasts with the westernmost region where the main magmatic activity only lasted for ca. 30 Ma between ca. 2100 and 2070 Ma, peaking at ca. 2090 Ma. More importantly it was possible to identify the Banfora region in southern Burkina Faso as the boundary between the two diachronous regions. Besides the differences between east and west it is also evident from the presented data that the magmatic activity is sporadic between ca. 2260 and 2100 Ma for the entire study area.

Another significant outcome was the identification of inherited ages ranging between ca. 2200 and 2130 Ma in intrusions with crystallization ages younger than 2130 Ma, which is a strong indication that the melts that originated the younger intrusions interacted with the older intrusions or that the region was active during such period. Inherited zircons were identified yielding ages between 3600 and 2400 Ma across southern Mali for the first time.

Regarding the whole rock geochemistry it was possible to identify differences between age groups that suggest that older intrusions (>2130 Ma), mainly to the east of Banfora) are the result of deep sourced melts, while the younger intrusions, mainly concentrated to the west of the Banfora region, are mostly the result of a progressively shallower source. Furthermore, the identified geochemical signatures (e.g. REE patterns, LILE anomalies) suggest that the Baoulé-Mossi domain formed as the result of accretion in an arc type environment.

The isotopic data presented here reinforces the notion that the Baoulé-Mossi domain is comprised of two regions and that both regions are mainly of juvenile origin with an important crustal contamination. As with the geochronological and geochemical data the Lu-Hf and O isotope characterization indicates that the boundary between the two regions lies along the Banfora region in southern Burkina Faso. The Banfora region presents the less radiogenic Hf isotope signature, while the easternmost region presents the most juvenile Hf isotope characterization. The region west of Banfora presents a juvenile Hf isotope signature, which is slightly less radiogenic than the one identified in the easternmost portion of the study area.

Importantly this study recognizes for the first time the potential interaction of juvenile magmas with crustal components that could be as old as 2800 Ma. Furthermore, the O isotope analyses show that the studied samples west of the Banfora region were contaminated with a crustal component. This crustal component was subject to near surface processes reinforcing the notion of contamination of the juvenile source with a crustal source.

The integration of all the datasets and the key elements previously described suggest that the Paleoproterozoic Baoulé-Mossi domain portion of the craton evolved as a result of an accretionary process that involved an arc type system. The domain can be divided into two major regions and although both regions are of mainly juvenile nature, there is evidence that indicates some reworking or mixing of crustal material were involved and that this crustal component can potentially be as old as 2800 Ma. This renewed knowledge on the area will be key to the understanding of its metal endowment and will help to better constraint how the planet evolved in the Paleoproterozoic.