Crustal growth and reworking of the Southern West African craton: Implications for regional tectonics

This contribution reviews the crustal growth of the West African Craton and its geodynamic evolution using whole rock geochemistry together with U-Pb geochronological and Lu-Hf isotopic records preserved in igneous zircon grains from felsic igneous rocks. Our study shows that the crustal growth in the southern West African Craton during the late Siderian and most of the Rhyacian (∼2350-2300 Ma and ∼2300-2060 Ma, respectively) was largely driven by the opening and subsequent closure of a large oceanic basin that fragmented the Neoarchean continental lithosphere. This tectonic evolution involved two major orogenic cycles including the Eoeburnean (∼2285-2120 Ma) and Eburnean (∼2120-2095 Ma) orogenies, each marked by distinct pulses of both juvenile magmatism and associated crustal reworking. Geochemical, geochronological, and isotopic data acquired on Eoeburnean felsic igneous rocks are used to investigate the process leading to crustal growth and its evolution during the Rhyacian. In central and western Ghana, central Cote d’Ivoire, and Burkina Faso, melt derivation from sources where hornblende and/or garnet were stable is consistent with crustal thickening and the injection of mantle derived magmas. ... mehrIn contrast, eastern and southeastern portions of the sWAC record reworking of Archean crust, as indicated by negative ƐHf values, whereas the western Kénéma-Man Domain over in Liberia and Sierra Leone exhibit intact Archean continental crust. During the Eburnean orogeny, more extensive crustal melting occurred at both deep and shallow structural levels, with ƐHf values indicating both continued juvenile input and reworking of earlier crust, including ongoing reworking of the northern margin of the Archean Kénéma-Man Domain. Late-orogenic bimodal magmatism (∼2095-2040 Ma) in southern Guinea is interpreted as a response to asthenospheric upwelling, likely triggered by lithospheric delamination or slab break-off following arc-continent collision. These magmatic rocks exhibit mixed isotopic signatures, consistent with contributions from both mantle-derived and reworked crustal sources. We propose that the tectonic evolution of the southern West African Craton was controlled by at least two partly synchronous subduction zone segments. The first was active during the Eoeburnean orogenic cycle and was southeast-dipping beneath the Archean BurGhana block in the eastern part of the craton. The second one was active during the Eburnean orogenic cycle and east-dipping, likely extending northwards into the Reguibat Domain and extended in the southern West African Craton where Precambrian terrains accreted onto the Archean Kénéma-Man Domain. We propose that the convergence and diachronous collision of intra-oceanic arcs with Archean lithospheric blocks (Kénéma-Man, BurGhana, Reguibat) marked the protracted closure of a ∼3,500 km wide oceanic basin.