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 (approximately 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 Nangodi belt largely consists of tholeiitic basalt flows with mg numbers of 37 to 69 and flat to slightly LREE-depleted chondrite-normalized REE patterns, overlain by sodic (Na2O/K2O = 3.7-29), calc-alkaline, dacitic tuffs and lavas that are HREE-depleted (La/Yb = 4.5-8.9 times chondrites, Yb = 3.9-8.7 times chondrites). The volcanic sequence is capped by calc-alkaline andesitic crystal tuffs with high mg numbers (66-75) and a high La/Yb ratio (10.5 times chondrites) similar to Archean and modem sanukitoids. The Dixcove belt largely consists of tholeiitic hyaloclastites and pillow lavas (mg numbers = 52-61) that grade upward into high-Mg (mg numbers approximately 59), calc-alkaline, andesitic tuffs and flows. The Winneba belt largely consists of massive, tholeiitic, high-Mg and siliceous high-Mg basalt flows with mg numbers of 64-72 and La/Sm ratios of 0.8-1.3 times chondrites, overlain by felsic tuffs and flows including a sodic, calc-alkaline, rhyolite with a La/Yb ratio of 16 times chondrites. 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 modem, immature, intraoceanic island arcs and many Archean greenstone belts, which may have formed in ancient island arcs. Like their Archean counterparts, Birimian tholeiites have La/Yb ratios < 4, La/Ta ratios < 35, and Hf/Ta ratios < 10, and most Birimian felsic rocks have chondrite-normalized Yb enrichment factors of < 8. Like modem, wedge-derived arc tholeiites, Birimian tholeiites have higher Ba/La, Rb/La, Th/La and Sr/Nd ratios, and tend to have lower Ta/La ratios, than the corresponding ratios in NMORB. Birimian dacites have high Ba/La, Rb/La and Th/La ratios relative to NMORB, as do modern-arc dacites and, in addition, have high Sr/Nd and Tb/Yb ratios, relative to NMORB, which are particularly distinctive characteristics of modern-arc dacites derived by direct melting of the subducted slab. Taken together, these 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.