New insight into the Dahomeyide Belt of southeastern Ghana, West Africa: Evidence of arc-continental collision and Neoarchaean crustal reworking

Abstract

This study presents petrography, geochemistry, geochronology, and Lu-Hf isotope compositions of Paleoproterozoic orthogneisses from the Dahomeyide Belt, southeastern Ghana. Traditionally, it is indicated that these rocks represent part of the Birimian of the West African Craton, remobilized during the Pan-African Orogen. The results obtained from this study indicate that the Dahomeyide basement are mainly amphibole and biotite orthogneisses of medium-K, tonalitic composition, and biotite and muscovite bearing orthogneisses of high-K granodioritic composition. Both suites are mainly magnesian, calc-alkaline, metaluminous to weak peraluminous with A/CNK values of 0.86 – 1.03. They are rich in LREEs, LILEs and elements such as Ba, K, Pb, and Sr. They are moderately rich in V and Zn and show variable (La/Yb)N and Sr/Y ratios with geochemical characteristics akin to the Neoarchaean sanukitoids. However, they are depleted in Ni and Cr relative to the sanukitoids. Zircon U-Pb ages determined on two amphibole-biotite and four biotite-muscovite orthogneisses yield 2145 ± 12–2122 ± 27 Ma and 2105 ± 18–2125 ± 25 Ma respectively. The amphibole-biotite orthogneisses show εHf(~2.1 Ga) values of −9.5 to −3.7, pointing to mean model ages of (TDM1) of 2721 ± 24–2761 ± 14 Ma and T(DM2) of 3053 ± 26–3125 ± 25 Ma. Similarly, the biotite and muscovite orthogneisses present εHf(~2.1 Ga) values of −10.6 to −1.7, corresponding to mean T(DM1) of 2743 ± 14–2733 ± 20 Ma and mean T(DM2) of 2949 ± 26–3111 ± 36 Ma. The data suggest that the Dahomeyide basement rocks are derived mainly from partial melting of Neoarchaean lower crust and sediments with little or no mantle contribution. A collision between Birimian arc and an unknown Archaean crust (possibly, now the Dahomeyide Belt) accounted for the formation of these rocks. This study provides first data on the evidence of reworked Archaean crust in the Pan-African Dahomeyide Belt along the eastern border of the West African Craton. Our data serves as comprehensive evidence supporting older crustal contribution in the geodynamic evolution of the juvenile Eburnean/Birimian crust of the West African Craton.