Geochemical and Sm–Nd isotopic evidence for the composition of the Palaeoproterozoic crust of the West African Craton in Ghana

Abstract

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?).

Neodymium isotope composition of Palaeoproterozoic Birimian shales from the Wa‐Lawra Belt, north‐west Ghana: Constraints on provenance

“…The calculation indicates that the Birimian shales in the Wa‐Lawra Belt received at least 80% of its detritus from juvenile Palaeoproterozoic Terrane. This interpretation is fairly consistent with earlier works carried out on the Birimian sedimentary rocks of the Kumasi Basin (Asiedu et al, 2017) and the Sefwi Belt (Sakyi et al, 2019) that suggest that sediments received at least 90% of its detritus from a juvenile Birimian volcanic arc. Roddaz et al (2007), however, reported that the Birimian sedimentary rocks from the Djibo greenstone belt, Burkina Faso, received 60–100% of its detritus from a juvenile Birimian volcanic arc.…”

“…The interpretations that the Birimian shales were predominantly derived from the juvenile Birimian volcanic rocks and deposited in an arc setting are consistent with the interpretations of earlier provenance studies of Birimian sedimentary rocks carried out by Asiedu et al (2004, 2017, 2019), Roddaz et al (2007) and Sakyi et al (2019).…”

“…Indeed, our understanding of the evolution of the crust largely comes from geochemical and isotopic data of siliciclastic sedimentary rocks (Bhat & Ghosh, 2001). However, the application of such crustal evolution studies to the Palaeoproterozoic rocks of the West African Craton (WAC) is very few despite early attempts by researchers, such as Asiedu et al (2017), Roddaz, Debat, and Nikiéma (2007), and Sakyi et al (2019). For this reason, the Palaeoproterozoic sedimentary rocks of the WAC, which are one of the most extended outcrops of Palaeoproterozoic rocks on earth, remains unrepresented in any computed model for crustal composition (Roddaz et al, 2007).…”

“…Therefore, the Sm–Nd‐model ages provide a good basis for determining the average crustal residence age of siliciclastic sedimentary rocks (McLennan et al, 1993). Nd‐isotopic data has been used to model the relative proportion of Palaeoproterozoic juvenile crust and old recycled upper crust at the time of deposition (e.g., Asiedu et al, 2017; Roddaz et al, 2007; Sakyi et al, 2019; Tran, Ansdell, Bethune, Watters, & Ashton, 2003).…”

“…Previous Nd‐isotopic studies on the Birimian sedimentary rocks of Ghana is not adequate. The few existing studies of the Birimian sedimentary rocks that applied Nd‐isotopes examined were not able to definitely conclude on the existence of pre‐Birimian basements in Ghana (Asiedu et al, 2017; Sakyi et al, 2019; Taylor et al, 1992), probably due to limited samples that were examined and the extent of coverage. All of these studies were carried out on the Birimian sedimentary rocks exposed in the south‐west of Ghana, whereas Birimian sedimentary rocks from other parts of Ghana have received little or no attention with respect to Sm–Nd‐isotopic studies.…”