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

Abstract

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 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 ($$Na_{2}O/K_{2}O = 3.7-29$$), 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 modern sanukitoids. The Dixcove belt largely consists of tholeiitic hyaloclastites and pillow lavas (mg numbers = 52-61) that grade upward into high-Mg (mg numbers ~59), calc-alkaline, andesitic tuffs and flows. The Winneba belt largely consists of massive, tholeiitic, h...