Search

Contrary to what is currently known, archetypal zircon samples from gneisses and intrusive leucogranites in the Palaeoproterozoic Suhum Basin, SE Ghana, suggest the involvement of Neoarchean crustal material in the formation of the Palaeoproterozoic juvenile crust of the Birimian terranes in Ghana. The zircons dated using U–Pb dating methods and subjected to Lu–Hf isotopic analysis suggest that crustal‐forming events from different contemporaneous magmatic episodes within the Suhum Basin took place over a time interval of 139 Ma from 2224 to 2085 Ma. Whole‐rock Lu–Hf data obtained for the gneissic and leucogranitic rocks gave model ages (TDM2) ranging from 2789 to 2456 Ma with ɛHf(t) values ranging from −1.1 to +5.4. These model ages imply that the magmas that formed these rocks were sourced from the early Palaeoproterozoic juvenile mantle with substantial Neoarchean crustal reworking. [ABSTRACT FROM AUTHOR]

The study presents detailed petrographical, geophysical, structural and geochemical data of the internal nappes zone to establish the deformational history, origin and tectonic setting and constrain the crustal growth and evolution of the active margin of the Dahomeyide belt. Two main lithological units, (i) deformed meta-granitoids (migmatites and gneisses) and (ii) undeformed granitoids, dominate the internal nappes zone. The granitoids are generally I-type, metaluminous to weakly peraluminous, low-K tholeiite to high-K calc-alkaline and of tonalite, granodiorite and granite affinity. The overall trace element patterns of the studied granitoids characterized by the enriched LILE and depleted HFS, with negative peaks of Nb-Ta, Sr, P and Ti, are indications of arc-related magmatism. Structural analysis reveals four deformation phases (D1-D4). D1 represents Northwest-Southeast (NW-SE) Pan African shortening associated with a continent-continent collision, resulting in westward nappe stacking. Progressive NW-SE shortening resulted in D2 and D3 top-to-the-NW dextral and sinistral thrusting events during the Pan-African orogeny. D4 is an extensional event likely associated with the orogenic collapse phase. The gneisses and migmatites, with dominant axial planar foliations, point to their formation in a collisional setting or influence by the Pan-African collisional processes. Continental-arc signatures in these rocks imply continental subduction during their protolith formation. The intrusive granitoid and pegmatite are undeformed, meaning late- to post-orogenic emplacement. These findings suggest that the internal nappes zone archived the subduction-collision and post-collisional phase of the Pan-African orogeny and recorded large-scale migmatization and granitoid emplacement due to partial melting of thickened lower crust between Mid-Cryogenian and late Ediacaran. [ABSTRACT FROM AUTHOR]

The metasedimentary rocks of the Sefwi and Lawra volcanic belts in Ghana respectively were analysed for their major and trace elements and Sr‐Nd isotopic compositions to constrain the provenance, palaeo‐weathering conditions, and tectonic setting of the rocks. Geochemical characteristics of the metasedimentary rocks show low to moderate chemical weathering in their source regions and the rocks are interpreted to have been derived from mixture of mafic and felsic rock components. The LREE show much enrichment in relation to the HREE (LaN/LuN, 4.67‐11.66). The εNd (2.1 Ga) values of −0.12 to +3.72 strongly suggest that the source of the sedimentary rocks was derived from a depleted mantle source and that they were most likely produced in an almost entirely oceanic environment with minor influence from the continental crust. The Nd model ages ranging from +2.05 to 2.41 Ga suggest possible contributions of a pre‐Birimian crustal material (or Archaean?) in the source material of the volcanic rocks. The rock types that contributed as detritus to the Birimian metasedimentary rocks were mainly pyroclastics and basalts, and their REE patterns suggest that the contribution to the phyllites can be modelled after 20% basalt, 13% andesite, and 67% dacite. Similarly, the schists can be modelled as a mixture of 16% basalt, 34% andesite, and 50% dacite. The Nd isotopic modeling implies that the bulk of the sedimentary detritus was supplied by the Palaeoproterozoic volcanic arc with contributions of less than 10% from the pre‐Birimian crust (Archaean?). [ABSTRACT FROM AUTHOR]

Metavolcanics and mafic intrusive rocks of the Paleoproterozoic Birimian terrane in the southeastern part of the West African Craton, Ghana, were analyzed for major and trace elements and Nd and Sr isotopic data to constrain the geodynamic evolution of the Birimian Supergroup. The metavolcanic rocks consist of metabasalts, meta‐andesites, and amphibolites, whereas the mafic intrusions are mainly gabbros, hornblendites, and dolerites. The rocks are tholeiitic in composition and show the classic features of arc magmatism. The metavolcanics display significant enrichments in large ion lithophile elements (LILE) and light rare earth elements, relative to high field strength elements (HFSE) and heavy rare earth elements. The multielement patterns of the rocks also show positive Pb, Ba, Th, and Sr and negative Nb, Ta, and Ce anomalies that are typical characteristics of subduction‐related magmas. They also have La/Nb ratios <3 and La/Ta ratios <43 that are similar to other Archean and Birimian greenstone belts in West Africa. The rocks have εNd (2.1 Ga) values of −0.96 to +2.60, and Nd model ages of 2.24–2.51 Ga (TDM1) and 2.16–2.45 Ga (TDM2), indicating their juvenile character with possible contributions from pre‐Birimian crustal materials in their sources. The εNd values suggest a depleted source and further indicate that they were probably produced in an almost entirely oceanic environment with minor influence from the continental crust. The Nd isotopic results are consistent with the island arc model, which views Paleoproterozoic terranes of the West African Craton in the context of subduction–accretion processes. Accordingly, these processes may have played a role in the formation of the Columbia supercontinent during the Paleoproterozoic (2.1–1.8 Ga) orogenic events. [ABSTRACT FROM AUTHOR]