Elemental geochemistry and Nd isotope constraints on the provenance of the basal siliciclastic succession of the middle Paleoproterozoic Francevillian Group, Gabon
Introduction
The middle Paleoproterozoic Francevillian basin developed in southeastern Gabon and hosting the Francevillian Group, is one of the world’s most studied and least deformed middle Paleoproterozoic sedimentary successions. Compared to sedimentary successions of similar age that have been highly deformed and altered, the Francevillian basin sediments were not metamorphosed and have only been affected by diagenetic and/or low-temperature hydrothermal processes (Bros et al., 1992, Gauthier-Lafaye and Weber, 1989, Gauthier-Lafaye and Weber, 2003, Mathieu et al., 2001, Bankole et al., 2015, Bankole et al., 2016, Bankole et al., 2018). Thus, the basin has been extensively studied and is well known for the oldest high-grade, redox-controlled, and sandstone-hosted uranium ore mineralization (Gauthier-Lafaye and Weber, 1989, Gauthier-Lafaye and Weber, 2003, Bankole et al., 2016), natural fossil nuclear-fission reactors (Gauthier-Lafaye and Weber, 1989), oldest multicellular macrofossils (El Albani et al., 2019, El Albani et al., 2014), and hosts the third largest known manganese-ore reserve in the world (Gauthier-Lafaye and Weber, 2003, Gauthier-Lafaye, 2006).
Although a number of studies have been carried out on the Francevillian Group, little is known about the geochemical characteristics of the sediments and their provenance has not previously been studied in much details. The geochemical composition of siliciclastic rocks is useful in providing information about their provenance, including its age and crustal evolution, surface Earth processes, and tectonic setting of sedimentary basins (McLennan et al., 2003). Thiéblemont et al., 2009, Weber et al., 2016 used geochemical datasets for the Okondja sub-basin to suggest a change from rift-related deposition of the FA Formation to subduction-related tectonic setting with accompanying alkaline, mafic to ultramafic magmatism at the time when the FB Formation was deposited. However, Bankole et al. (2018) inferred from geochemical data that the interlayered thin K-bentonite beds in the FB2b unit in the Franceville sub-basin were derived from calc-alkaline intermediate to felsic magmatism, which was related to a continental arc in a subduction setting developed during plate convergence. Recently, Ossa Ossa et al. (2020) utilized detrital zircon geochronology to suggest that the uranium-bearing sandstones and conglomerates of the FA and FB formations in the Francevillian basin were mainly sourced from the Mesoarchean granitoids of the East Gabonian block of the northwestern Congo craton.
This study presents new whole-rock elemental and Nd isotope data for the two basal sedimentary units of the Francevillian Group, FA and FB formations, in the Francevillian basin. We screened first for post-depositional overprints and elemental mobility to establish preservation of primary geochemical signals before constraining the intensity of chemical weathering, provenance, and tectonic setting for the sedimentary rocks of the FA and lower FB (FB1 Member) formations as well as crustal evolution of their provenance.
Section snippets
Geological background
The middle Paleoproterozoic Francevillian Group is an extensive sequence of unmetamorphosed siliciclastic and volcaniclastic strata that unconformably overlie the Archean basement rocks of the northwestern part of the Archean Congo craton in central to southeastern Gabon (Fig. 1). The basin is bounded by the Mesoarchean (3185–2805 Ma) and Neoarchean (2802–2500 Ma) granitoids of the East Gabonian block on the north (North Gabon massif) and south (Chaillu massif) and by the Neoarchean to early
Samples and analytical techniques
Surface outcrops of the FA Formation and FB1 Member are strongly weathered in the Francevillian basin; therefore, only fresh drill core samples were used for sampling. Samples for this study were collected from eight drill cores from the proximal to distal parts of the Franceville sub-basin (Figs. 2a and 3). Most of the drill cores intersected the transition zone between the upper FA and lower FB formations, except for the BA 2 and GR 15 drill cores where only the FA Formation was encountered.
Major element geochemistry
The major element data for the studied samples are presented in Table A1 (see Appendix). The FA Formation sandstones falls within the arkose, sub-arkose, litharenite, and quartz arenite compositional fields, whereas mudstones from FA Formation and FB1 Member fall within the shale and wacke fields in the Herron's (1988) geochemical classification diagram for sedimentary rocks (Fig. 4). The sandstones generally have higher SiO2/Al2O3 ratios and are therefore quartz-rich compared to the mudstone
Discussion
The chemical and mineralogical composition of siliciclastic sedimentary rocks, especially fine-grained rocks, are mainly controlled by the nature of their source rocks, and they have been widely used in constraining the provenance, sedimentary processes, and tectonic setting of sedimentary basins (Bhatia and Crook, 1986, Cullers, 2000, Cullers, 1994, Large et al., 2018, McLennan et al., 1993, McLennan et al., 1990, Roser and Korsch, 1986). Most of the major elements (e.g., Ca, Fe, Na, K, and
Conclusions
The whole-rock geochemistry and Nd isotope data for the unmetamorphosed, middle Paleoproterozoic Francevillian Group FA Formation and FB1 Member sediments demonstrate that their geochemical signatures have not been considerably disturbed by sedimentary and post-depositional processes and are thus suitable for the provenance study. The moderate paleoweathering indices and position of the whole-rock geochemical data on the A-CN-K ternary diagram suggest that the FA Formation sandstones and most
CRediT authorship contribution statement
Olabode M. Bankole: Conceptualization, Investigation, Methodology, Writing - original draft, Writing - review & editing. Abderrazak El Albani: Conceptualization, Investigation, Methodology, Writing - original draft, Writing - review & editing. Alain Meunier: Writing - original draft, Writing - review & editing. Marc Poujol: Investigation, Methodology, Writing - original draft. Andrey Bekker: Conceptualization, Writing - original draft, Writing - review & editing.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors are grateful to the Gabonese Government, CENAREST, General Direction of Mines and Geology, and Agence Nationale des Parcs Nationaux of Gabon for logistic supports. This work was supported by La Région Nouvelle Aquitaine, France and French Embassy in Libreville, Gabon. The authors would like to thank Prof. P. Mouguiama Daouda for his support. The authors also acknowledge Claude Fontaine, Claude Laforest, Cédric Lebailly, and Laurence Tromas for laboratory and administrative support at
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