Petrology and geochemistry of TTG and K-rich Paleoproterozoic Birimian granitoids of the West African Craton (Ghana): Petrogenesis and tectonic implications

Abstract

We conducted petrologic, geochemical and isotopic studies on Paleoproterozoic Birimian granitoids that intrude the Bole-Nangodi Belt in northern Ghana, in southeastern West African Craton (WAC), to constrain the geodynamic evolution of the Birimian Supergroup. The rocks are potassic (K)-rich granitoids (KRGs), tonalities, trondhjemites, granodiorites (TTGs), pegmatite and aplite. The K2O contents of the KRGs are up to 5 wt%, with generally high concentrations of Rb, Ba, Sr, Cr and Ni. They are highly fractionated calc-alkaline I-type granitoids, enriched in LILE and LREE relative to HREE and HFSE. Chondrite-normalized REE diagrams indicate fractionated LREE (average LaN/SmN = 5.05) and HREE (GdN/YbN = 4.56) patterns, with slight negative Eu anomalies (average Eu/Eu* = 0.75). This may indicate an evolved magma source with varying degrees of plagioclase fractionation. The KRGs share similar geochemical characteristics with the TTGs into which they intrude. Both rock types are characterized by Ba and Th enrichment and depletion in Nb‐Ta, Zr‐Hf, and Ti, which are typical characteristics of subduction-related magmas. The pegmatite-aplite association is slightly geochemically different and may have formed at a time later than the KRGs. The ԐNd(2.1 Ga) values range from + 0.90 to + 1.24 and −0.86 to + 1.37 respectively for the TTGs and KRGs, with TDM1 ages of 2.34–2.38 Ga and 2.36–2.53 Ga and TDM2 ages of 2.21–2.24 Ga and 2.22–2.39 Ga respectively. These indicate their juvenile character, possibly a depleted mantle source with minor contributions from a pre-Birimian (or Archean?) crustal material in their source material(s). The TTGs and KRGs are volcanic arc granites (VAG) and syn-collisional (Syn-COLG) granites, with formation temperatures and pressures of ~640–750 °C and ~2–6 kbar respectively. Based on the high temperatures, I-type signature, calc-alkaline nature and other geochemical characteristics, we infer that the TTGs, KRGs and pegmatite-aplite association developed under high oxygen fugacity conditions in an orogenic tectonic setting related to subduction. Thus, our geochemical and isotopic results are consistent with an island arc tectonic setting arising from subduction–accretion processes, which are typical for the Paleoproterozoic terranes of the WAC. These processes may have contributed to the amalgamation of the Columbia (Nuna) supercontinent during the Paleoproterozoic (2.1–1.8 Ga) orogeny.