Provenance of late Ordovician to early Cretaceous sedimentary rocks from southern Ghana, as inferred from Nd isotopes and trace elements

Abstract

Geochemical and Nd-isotopic data are reported for 24 shale and sandstone samples comprising the Upper Ordovician to Lower Cretaceous Sekondian Group, southern Ghana. The data are interpreted in terms of the provenance of these siliclastic sediments and the Paleozoic and Mesozoic geologic development of southern Ghana. The sandstones and shales generally have trace element characteristics typical of material eroded from the upper continental crust. Cr and Ni concentrations are low suggesting their derivation from dominantly felsic sources. There appear to be chemical and Nd-isotopic differences between the lower formations (i.e., Ajua Shale and Elmina Sandstone) and the uppermost formation (i.e., Essikado Sandstone) on one hand, and the upper formations (i.e., Takoradi Sandstone, Takoradi Shale, Effia Nkwanta Beds and Sekondi Sandstone) on the other. The upper formations are characterized by evolved major element compositions, high large-ion lithophile element abundances, large negative Eu-anomalies (0.58–0.73), and low ε Nd values (−10.8 to −4.1), indicating stable craton settings. On the other hand, the Ajua Shale, Elmina Sandstone, and Essikado Sandstone formations have less evolved major element compositions, lower large-ion lithophile abundances, and higher Eu-anomalies (0.69–0.88), and this may reflect an active margin setting of the source (juvenile-type Birimian felsic volcanics and/or granitoids) rather than Phanerozoic active continental settings. The upper formations have T DM model ages of 1.8–2.3 Ga indicating that the Paleoproterozoic Birimian rocks are the ultimate sources, whereas the lower formations and uppermost formation have slightly younger T DM values of 1.6–1.9 Ga suggesting of a component from the Pan-African mobile belt. These differences in model ages may, however, be related to lithology (rather than stratigraphy) where the shales (1.6–2.3 Ga; mean, 1.9 Ga) typically give older ages than the sandstones (1.6–1.9 Ga; mean 1.7 Ga).