Geochemical and Sr-Nd isotopic records of Paleoproterozoic metavolcanics and mafic intrusive rocks from the West African Craton: Evidence for petrogenesis and tectonic setting

Metavolcanics and mafic intrusive rocks of the Paleoproterozoic Birimian terrane in the southeastern part of the West African Craton, Ghana, were analyzed for major and trace elements and Nd and Sr isotopic data to constrain the geodynamic evolution of the Birimian Supergroup. The metavolcanic rocks consist of metabasalts, meta-andesites, and amphibolites, whereas the mafic intrusions are mainly gabbros, hornblendites, and dolerites. The rocks are tholeiitic in composition and show the classic features of arc magmatism. The metavolcanics display significant enrichments in large ion lithophile elements (LILE) and light rare earth elements, relative to high field strength elements (HFSE) and heavy rare earth elements. The multielement patterns of the rocks also show positive Pb, Ba, Th, and Sr and negative Nb, Ta, and Ce anomalies that are typical characteristics of subduction-related magmas. They also have La/Nb ratios <3 and La/Ta ratios <43 that are similar to other Archean and Birimian greenstone belts in West Africa. The rocks have epsilon(Nd) (2.1Ga) values of -0.96 to +2.60, and Nd model ages of 2.24-2.51Ga (T-DM1) and 2.16-2.45Ga (T-DM2), indicating their juvenile character with possible contributions from pre-Birimian crustal materials in their sources. The epsilon(Nd) values suggest a depleted source and further indicate that they were probably produced in an almost entirely oceanic environment with minor influence from the continental crust. The Nd isotopic results are consistent with the island arc model, which views Paleoproterozoic terranes of the West African Craton in the context of subduction-accretion processes. Accordingly, these processes may have played a role in the formation of the Columbia supercontinent during the Paleoproterozoic (2.1-1.8Ga) orogenic events.