Petrology And Geochemistry of Clastic Rocks of the Tamale-Obosum and Oti-Pendjari Groups, Voltaian Supergroup, Ghana: Implications for Provenance

Abstract

The Voltaian Basin of Ghana, one of the largest Neoproterozoic sedimentary basins in West Africa, preserves a thick stratigraphic succession that remains poorly constrained in terms of provenance, depositional history, and tectonic evolution. In particular, the Oti-Pendjari and Tamale-Obosum Groups are minimally studied, limiting their integration into models of West African craton evolution and Pan-African orogenesis. This study addresses these uncertainties by applying an integrated petrographic, geochemical, heavy mineral, and isotopic approach to reconstruct provenance, weathering intensity, and tectonic setting, thereby refining the geodynamic framework of the Voltaian Supergroup. Petrographic data from twelve lithostratigraphic units reveal that the Tamale-Obosum Group consists largely of quartzose sandstones with subordinate litharenites, while the Oti-Pendjari Group is dominated by arkosic and lithic sandstones. Provenance analysis indicates that Tamale Obosum sediments were derived mainly from felsic plutonic rocks with minor metasedimentary inputs, whereas the Oti-Pendjari Group records contributions from volcanic arcs, recycled terranes, and glaciogenic successions. Detrital compositions suggest transitional arc, cratonic interior, and recycled orogenic sources for different formations. Whole-rock geochemistry highlights significant compositional variation. Quartz arenites are SiO₂ rich and chemically mature, reflecting intense weathering. Wackes and litharenites display higher Al₂O₃, Fe₂O₃, and alkali contents, consistent with feldspathic and mafic inputs. Major oxide correlations confirm quartz–clay trade-offs, while depletion and enrichment trends relative to Upper Continental Crust (UCC) distinguish feldspathic versus mafic contributions. Weathering intensity assessed using the Chemical Index of Alteration (CIA), ranges from strong alteration in the Tamale Sandstone and Undivided Obosum formations to weaker weathering in the Densubon Sandstone and Sang Conglomerates. Combined CIA–Index of Compositional Variability (ICV) values reveal that the Tamale Formation was derived from uniform felsic sources under intense weathering, whereas other units reflect mixed sources and moderate alteration. Trace and rare earth element (REE) patterns further refine provenance. Enrichment in light REEs (LREEs) and large ion lithophile elements (LILEs) indicate felsic inputs, whereas localized enrichment in ferromagnesian trace elements (Cr, Ni, Co) suggests mafic or recycled sedimentary contributions. Negative Eu anomalies in most formations reflect feldspar depletion, while occasional positive anomalies in younger units suggest feldspar input or sediment mixing. Tectonic discrimination diagrams differentiate depositional settings: the Tamale and Undivided Obosum formations plot within passive margin fields, while the Densubon and Sang Conglomerates display signatures of active continental margins or island arcs. The Oti-Pendjari Group records a transition from stable continental sedimentation in its older members to arc- and rift-influenced deposition in younger units. Isotopic analyses provide additional constraints. Nd and Sr isotopic compositions confirm continental crustal derivation, with the Tamale-Obosum Group reflecting upper crustal sources linked to Birimian and Kwahu- terranes. The Oti-Pendjari Group, by contrast, records mixed contributions from Proterozoic crust and Pan-African belt rocks, consistent with foreland basin development during the Dahomeyide orogeny (\~590 Ma). Sm–Nd depleted mantle model ages (TDM) support derivation from Proterozoic sources and indicate sediment recycling during Pan African tectonism. Overall, the Oti-Pendjari and Tamale-Obosum Groups represent complementary tectono sedimentary records within the Voltaian Basin. The Tamale-Obosum Group reflects felsic plutonic sources, strong weathering, and deposition in mixed passive and active margin environments, whereas the Oti-Pendjari Group captures volcanic arc erosion, rift tectonism, and foreland basin sedimentation. These results demonstrate that the Voltaian Basin evolved through dynamic interactions between the West African Craton and Pan-African mobile belts, contributing to regional models of crustal recycling, sedimentary basin development, and Neoproterozoic geodynamics.