Paleoproterozoic TTG-like metagranites from the Dahomeyide Belt, Ghana: Constraints on the evolution of the Birimian-Eburnean Orogeny

Abstract

Among many Paleoproterozoic Orogens, the Birimian-Eburnean Orogeny is of particular importance due to its vital role in connecting the African and South American blocks during the Columbia supercontinent assembly. Although, several tectonic models are proposed for the Birimian-Eburnean Orogen of West Africa, more and more studies demonstrate that the Birimian-Eburnean Orogen was dominated by arc-related accretion/collision. In this study, we present geochemical characteristics, Lu-Hf-O isotopes compositions and zircon U-Pb ages of metagranites from the Dahomeyide basement of Ghana to constrain crustal thickness during the Birimian-Eburnean Orogeny. The metagranites intrude into the dominant Dahomeyide orthogneisses but are not documented. They are mainly high-K calc-alkaline, weakly peraluminous with A/CNK of 1.0–1.07. They have very high Sr/Y and (La/Yb)N, and are strongly depleted in HREE and HFSE. They exhibit potassic TTGs-like geochemical characteristics and yield zircon U-Pb ages of 2100 ± 6–2118 ± 30 Ma. They show subchondritic Lu-Hf isotope compositions with εHf(t) values of −11.1 to −1.6, and preserve typical I-type granitoids oxygen isotope composition with zircon δ18O of 6.4–6.7‰, quartz δ18O of 9.3–9.7‰, plagioclase δ18O of 7.4–7.8‰, K-feldspar δ18O of 7.0–7.3‰, biotite δ18O of 3.5–4.3‰, and whole-rock δ18O of 8.3–8.5‰. Comparison of data for the metagranites with data previously reported for the host orthogneisses indicate that the metagranites are relatively younger but preserve Lu-Hf-O signatures similar to the orthogneisses. This suggests that both rock suites were derived from melting of common Archaean igneous rocks. However, theoretical modelling of partial melting indicates that the metagranites formed in a thickened lower crust with rutile-eclogites residue in a depth ≥50 km, whereas the host orthogneisses formed in the lower crust with garnet-amphibolite residue in a depth of ≤35 km.This marks gradual crustal thickening as a result of arc-continent collision during the Birimian-Eburnean Orogeny. This supports modern-style plate tectonics controlled by subduction-collision as the main geodynamic mechanism operated during the Birimian-Eburnean Orogen in the background of the Columbia supercontinent assembly.