Chemical and Sr–Nd isotopic compositions and zircon U–Pb ages of the Birimian granitoids from NE Burkina Faso, West African Craton: Implications on the geodynamic setting and crustal evolution

The West African Craton was the site of massive juvenile crustal addition in the early Proterozoic during the Eburnean orogeny (2.1 Ga). In this study, we determined chemical and isotopic compositions as well as zircon U–Pb ages of granitoid samples from NE Burkina Faso. These data were then used to constrain the chemical characteristics, emplacement age and tectonic environment of these rocks in the context of the West African Craton evolution. Birimian (Paleoproterozoic) granitoids of NE Burkina Faso are associated with volcanic and volcano-sedimentary sequences of the greenstone belts. Analyses indicate that the Na-rich Birimian granitoids of tonalite–trondhjemite–granodiorite (TTG) composition were generated by partial melting of a garnet-bearing amphibolite crust. Granite intrusions derived from partial re-melting of the deep basement of felsic calc-alkaline nature with contribution of metasediments. Zircon U–Pb isotopic analyses yielded ages between 2122 ± 15 Ma and 2181 ± 7 Ma for the Birimian Na-rich granitoids and 2151 ± 10 Ma for a biotite granite (IJ10). Sr–Nd isotopic data show very low initial Sr composition (ISr (T) ≤ 0.7007), positive epsilon neodymium values (ɛNd (T) = +0.7 to +2.5) and Sm–Nd model ages (TDM) of 2363–2558 Ma). These data are similar to those of mafic rocks of the Pissila greenstone belt: (ISr (T) = 0.7003–0.7015, ɛNd (T) = +3.2 to +4.1, TDM = 2336–2552 Ma), and further suggest that the Na-rich granitoids are of juvenile nature. The present study together with published data on the Birimian volcanics and meta-sedimentary rocks of the West African Craton preclude contamination of the juvenile Birimian rocks of NE Burkina Faso by reworked Archean materials. The Birimian granitoids of NE Burkina Faso were mostly emplaced during the early stage (2.19–2.15 Ga) of a large scale crustal growth event (2.2–2.0 Ga) where large amount of juvenile materials were added to the crust from the depleted mantle in a tectonic environment comparable to modern volcanic arcs.

► Emplacement of granitoids is well dated at 2.19–2.12 Ga by zircon U–Pb method.
► Chemical and isotopic analyses indicate that these granitoids are largely juvenile.
► New data reinforce that 2.2–2.1 Ga represents a key episode of crustal growth.