Elsevier

Precambrian Research

Volume 348, 15 September 2020, 105874
Precambrian Research

Elemental geochemistry and Nd isotope constraints on the provenance of the basal siliciclastic succession of the middle Paleoproterozoic Francevillian Group, Gabon

https://doi.org/10.1016/j.precamres.2020.105874Get rights and content

Highlights

  • Sediments have undergone moderate weathering and secondary alteration.

  • Sediments mainly derived from felsic source with minor mafic components.

  • Provenance from Mesoarchean East Gabonian block with no juvenile contribution.

  • Sediments sourced from crustal addition and recycling of differentiated granitoids.

Abstract

Whole-rock elemental geochemistry and Nd isotope composition of siliciclastic rocks from the basal part of the unmetamorphosed middle Paleoproterozoic Francevillian Group, Gabon (FA Formation and FB1 Member) were examined to provide insights into their provenance and its crustal evolution. Chemical weathering index diagrams suggest moderate degree of weathering in the provenance and secondary K-addition during illitization. Trace element geochemical proxies including the REEs provide evidence for sediment derivation from felsic-dominated upper continental crust (UCC) similar to Archean trondhjemite-tonalite-granodiorite (TTG) with minor contribution of mafic components. The low 143Nd/144Nd and negative εNd values (−10.83 to −5.76) provide further evidence for old and evolved upper continental crust components in detrital sources. The depleted mantle model ages for the provenance protolith (2.73–3.05 Ga) are consistent with sediments sourced from the Mesoarchean granitoids of East Gabonian block exposed in the northern and southern parts of Gabon. We infer that the evolved, Mesoarchean felsic TTG-like granitoids were the main source for the sediments of the basal Francevillian Group, while minor mafic inputs from the contemporaneous greenstone belt with the Belinga Group cannot be completely discounted. The predominantly old crustal sources for the sediments suggest crustal addition and recycling of differentiated granitoid plutons instead of formation of juvenile crust during deposition in the Francevillian basin.

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

References (83)

  • L. Harnois

    The CIW index: a new chemical index of weathering

    Sediment. Geol.

    (1988)
  • X. Jian et al.

    Geochemistry of Mesozoic and Cenozoic sediments in the northern Qaidam basin, northeastern Tibetan Plateau: Implications for provenance and weathering

    Chem. Geol.

    (2013)
  • R.R. Large et al.

    Role of upper-most crustal composition in the evolution of the Precambrian ocean–atmosphere system

    Earth Planet. Sci. Lett.

    (2018)
  • P. Ledru et al.

    Structural and metamorphic evolution of the Gabon Orogenic Belt: collision tectonics in the lower proterozoic?

    Precambrian Res.

    (1989)
  • A. Maheshwari et al.

    Global nature of the Paleoproterozoic Lomagundi carbon isotope excursion: a review of occurrences in Brazil, India, and Uruguay. Precambrian Res

    Precambrian Isotope Stratigraphy

    (2010)
  • A.P. Martin et al.

    A review of temporal constraints for the Palaeoproterozoic large, positive carbonate carbon isotope excursion (the Lomagundi-Jatuli Event)

    Earth-Sci. Rev.

    (2013)
  • A.P. Martin et al.

    Multiple Palaeoproterozoic carbon burial episodes and excursions

    Earth Planet. Sci. Lett.

    (2015)
  • H. Martin et al.

    An overview of adakite, tonalite–trondhjemite–granodiorite (TTG), and sanukitoid: relationships and some implications for crustal evolution

    Lithos

    (2005)
  • R. Mathieu et al.

    Alteration of monazite and zircon and lead migration as geochemical tracers of fluid paleocirculations around the Oklo-Okélobondo and Bangombé natural nuclear reaction zones (Franceville basin, Gabon)

    Chem. Geol.

    (2001)
  • S.M. McLennan et al.

    Samarium/neodymium elemental and isotopic systematics in sedimentary rocks

    Geochim. Cosmochim. Acta

    (1992)
  • I.M. Mouélé et al.

    2.9–1.9Ga paleoalterations of Archean granitic basement of the Franceville basin (Gabon)

    J. Afr. Earth Sci.

    (2014)
  • H.W. Nesbitt et al.

    Prediction of some weathering trends of plutonic and volcanic rocks based on thermodynamic and kinetic considerations

    Geochim. Cosmochim. Acta

    (1984)
  • L. Ngombi-Pemba et al.

    From detrital heritage to diagenetic transformations, the message of clay minerals contained within shales of the Palaeoproterozoic Francevillian basin (Gabon)

    Precambrian Res.

    (2014)
  • B. Öhlander et al.

    Change of Sm-Nd isotope composition during weathering of till

    Geochim. Cosmochim. Acta

    (2000)
  • F. Ossa Ossa et al.

    Two-step deoxygenation at the end of the Paleoproterozoic Lomagundi Event

    Earth Planet. Sci. Lett.

    (2018)
  • F. Ossa Ossa et al.

    Exceptional preservation of expandable clay minerals in the ca. 2.1 Gablack shales of the Francevillian basin, Gabon and its implication foratmospheric oxygen accumulation

    Chem. Geol.

    (2013)
  • A. Préat et al.

    Paleoproterozoic high δ13C dolomites from the Lastoursville and Franceville basins (SE Gabon): Stratigraphic and synsedimentary subsidence implications

    Precambrian Res.

    (2011)
  • M. Roddaz et al.

    Controls on weathering and provenance in the Amazonian foreland basin: Insights from major and trace element geochemistry of Neogene Amazonian sediments

    Chem. Geol.

    (2006)
  • B.P. Roser et al.

    Provenance signatures of sandstone-mudstone suites determined using discriminant function analysis of major-element data

    Chem. Geol.

    (1988)
  • Y. Sawaki et al.

    Chronological constraints on the Paleoproterozoic Francevillian Group in Gabon. Geosci. Front

    Front. Early Earth History Primordial Life – Part I

    (2017)
  • K. Sugitani et al.

    Geochemistry and sedimentary petrology of Archean clastic sedimentary rocks at Mt. Goldsworthy, Pilbara Craton, Western Australia: evidence for the early evolution of continental crust and hydrothermal alteration

    Precambrian Res.

    (2006)
  • F. Weber et al.

    The 2-Ga Eburnean Orogeny in Gabon and the opening of the Francevillian intracratonic basins: a review

    C. R. Geosci.

    (2016)
  • H. Yang et al.

    Geochemical and Nd isotopic compositions of the metasedimentary rocks in the La Ronge Domain, Trans-Hudson Orogen, Canada: implications for evolution of the domain

    Precambrian Res.

    (1998)
  • J.S. Armstrong-Altrin et al.

    Geochemistry of sandstones from the upper miocene Kudankulam formation, Southern India: implications for provenance, weathering, and tectonic setting

    J. Sediment. Res.

    (2004)
  • J. Aubineau et al.

    Unusual microbial mat-related structural diversity 2.1 billion years ago and implications for the Francevillian biota

    Geobiology

    (2018)
  • Azzibrouck-Azziley, G., 1986. Sédimentologie et géochimie du Francevillien B (Protérozoïque inférieur), in:...
  • Bankole, O.M., 2015. Transformation diagenetiques des depots silicoclastiques FA du bassin de Franceville au Gabon...
  • O.M. Bankole et al.

    Earth’s oldest preserved K-bentonites in the ca. 2.1 Ga Francevillian Basin

    Gabon. Am. J. Sci.

    (2018)
  • O.M. Bankole et al.

    Origin of red beds in the Paleoproterozoic Franceville Basin, Gabon, and implications for sandstone-hosted uranium mineralization

    Am. J. Sci.

    (2016)
  • A. Bekker et al.

    Chemostratigraphy of carbonates from the minas supergroup, quadrilatero ferryifero (Iron Quadrangle), Brazil: a stratigraphic record of early proterozoic atmospheric, biogeochemical and climactic change

    Am. J. Sci.

    (2003)
  • M.R. Bhatia et al.

    Trace element characteristics of graywackes and tectonic setting discrimination of sedimentary basins

    Contrib. Mineral. Petrol.

    (1986)
  • Cited by (14)

    • Geochronological and geochemical constraints for the metavolcanosedimentary succession of the Nyong Complex, northwestern margin of the Congo craton: Implications for depositional age and tectonic setting of associated banded iron formations

      2022, Precambrian Research
      Citation Excerpt :

      The Mesoarchean rocks consist of high-K, calc-alkaline granitoids and tonalites-trondhjemites-granodiorites (TTGs) associated with ca. 2820 Ma charnockites and ca. 2920–2750 Ma greenstone belts characterized by volcanic rocks, BIFs, conglomerates, and mica schists (Thiéblemont et al., 2009). These Archean to early Paleoproterozoic metamorphic terranes are unconformably overlain by the middle Paleoproterozoic Francevillian Group and the late Neoproterozoic West Congolian Group (Bankole et al., 2020; Ossa Ossa et al., 2020). The Congo craton in the northwestern Republic of Congo is composed of the Ivindo Complex, bounded by the North Gabonian Massif in Gabon and the Ntem Complex in Cameroon (Soh Tamehe et al., 2022a).

    • Provenance of metasiliciclastic rocks at the northwestern margin of the East Gabonian Block: Implications for deposition of BIFs and crustal evolution in southwestern Cameroon

      2022, Precambrian Research
      Citation Excerpt :

      The West Gabonian block consists of Neoarchean to early Paleoproterozoic (ca. 2515–2435 Ma) metamorphic terranes, while the Ogooué Complex comprises ca. 2200–2120 Ma gneisses and migmatites (Thiéblemont et al., 2009). The Archean to Paleoproterozoic metamorphic sequences of the Congo craton in Gabon are unconformably overlain by the middle Paleoproterozoic Francevillian Group and the late Neoproterozoic West Congolian Group to the north and south, respectively (Bankole et al., 2020; Ossa Ossa et al., 2020). The Congo craton in the western part of the Republic of Congo is characterized by the Ivindo Complex and Chaillu Massif (Fig. 1b; Meloux et al., 1986).

    • Combined U-Pb isotopic signatures of U mill tailings from France and Gabon: A new potential tracer to assess their fingerprint on the environment

      2022, Journal of Hazardous Materials
      Citation Excerpt :

      The major elements contents (Al, Mg, Fe, Ca, Na, K, Ti, Mn, P) of U mill tailings from France are close to the average value reported for the upper continental crust (UCC) (Rudnick and Gao, 2014; Fig. 2). However, the major elements contents of the U mill tailings from Gabon show a strong depletion of Mg, Ca, Na, and Mn (in a lesser extent for Al, K, Fe, Ti, P) compared to UCC, which is probably due to the low content of these elements in the mineralized quartzite sediment (Gauthier-Lafaye, 1986; Bankole et al., 2020). The Ba content of U mill tailings is similar to UCC.

    • Mineralogy and geochemistry of siliciclastic Miocene Cuddalore Formation, Cauvery Basin, South India: Implications for provenance and paleoclimate

      2021, Journal of Palaeogeography
      Citation Excerpt :

      In the (Ti/Al)‒(Fe/Ti) diagram (after Wang et al., 2012), CS and CM samples fall in near felsic charnockite and granite (Fig. 6f) but plot away from mafic and intermediate rocks, indicating that the source was mainly composed of felsic plutonic rocks with little or no contributions from mafic granulite or basaltic rocks. Distributions of various least-mobile trace elements such as REE, Th, Zr, Sc, Co, Cr are useful in deducing the composition of the provenance (e.g. Taylor and McLennan, 1985; Wronkiewicz and Condie, 1990; McLennan et al., 1993; Cullers, 1994, 2000; Absar et al., 2009; Absar and Sreenivas, 2015; Large et al., 2018; Bankole et al., 2020). The relative concentration and ratios of both the incompatible (e.g. REE, Th, Zr) and the compatible (e.g. Sc, Cr, Co) element groups are useful in discriminating felsic and mafic source rocks (e.g. Bhatia and Crook, 1986; Feng and Kerrich, 1990; Manikyamba and Kerrich, 2006; Mondal et al., 2012; Absar and Sreenivas, 2015; Armstrong-Altrin et al., 2015; Madhavaraju et al., 2020a, 2020b).

    View all citing articles on Scopus
    View full text