Adakitic Magmatism in the Paleoproterozoic Terrane of Ghana, West Africa: Geochemical Evidence from K-rich Granitoids

English Abstract:

Varying views have been held concerning the geodynamic significance of the ca. 2.1 Ga Paleoproterozoic “Birimian” rocks of the Leo-Man shield of the West African craton (WAC). The Birimian terrane, which consists of linear and arcuate volcanic belts with intervening sedimentary basins, has been intruded by several generations of granitoids. Studies into these granitoids present key complements for a full understanding of the evolutionary history and petrogenetic implications of the Birimian terrane of Ghana, and by extension the WAC. We present new research into the unique potassium-rich ‘Bongo-type’ and ‘Banso-type’ granitoids from Ghana, and demonstrate the possibility of K-rich adakitic magmatism in this terrane, thereby shedding more light on the largely debated tectonic setting of the Birimian rocks. The granitoids are magnesian, metaluminous to weakly peraluminous high-K calc-alkaline to shoshonite series rocks, interpreted to have had some interaction with crustal materials or some influence of subducted sediments. The ‘Bongo-type’ and ‘Banso-type’ granitoids show typical subduction-related granitoid signatures with evidence of being emplaced in a volcanic arc geotectonic environment. These late granitoids do not form an independent series as earlier believed, but are linked to other granitoids in Ghana, and by extension several other granitoids in the Birimian terrane of the West African craton. They, however, do not fit the general description of tonalite–trondhjemite–granodiorite (TTG’s) as reported in other parts of the Birimian terrane, and are dissimilar to other post-Archaean granites in terms of chemical composition. Their slightly elevated Sr/Y and (La/Yb)_PM ratios and low Y (< 14 ppm) and Yb (< 1 ppm) contents strongly indicate geochemical signatures similar to adakites. On the basis of their SiO₂ (> 60 wt. %), CaO+Na₂O (< 7 wt.%), MgO (0.4 to 1 wt.%) and Sr (< 1285 pm) contents, together with their REE concentrations, the granitoids may be classified as high-SiO₂ adakitic (HSA) rocks. Implications of adakitic signatures thus link the Birimian rocks to ridge subduction in a volcanic arc setting.