Lithostratigraphy and composition of 2.1 Ga greenstone belts of the West African craton and their bearing on crustal evolution and the Archean-Proterozoic boundary

We have determined the lithostratigraphy of three, Early Proterozoic (~2.1 Ga), Birimian greenstone belts in the West African craton and measured the concentrations of major and trace elements in samples of volcanic rocks from those belts. The association of tholeiitic basalts and sodic, calc-alkaline, felsic volcanic rocks found in Birimian greenstone belts differs from the association of calc-alkaline basalts and potassic felsic rocks which comprises Proterozoic greenstone belts in North America and Scandinavia but is similar to that present in modern, immature, intraoceanic island arcs and many Archean greenstone belts, which may have formed in ancient island arcs. Taken together, the data suggest that magmas formed in Early Proterozoic, West African island arcs had compositions broadly similar to magmas formed in many Archean island arcs and, hence, the Archean-Proterozoic boundary may not coincide with a worldwide, fundamental change in crust-mantle evolution. -from Authors