Journal of African Earth Sciences 183 (2021) 104327
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Journal of African Earth Sciences 
journal homepage: www.elsevier.com/locate/jafrearsci 
Paleoweathering, Provenance and tectonic setting of gold-bearing 
neoproterozoic rocks from the Atacora Structural Unit, Pan-African 
Dahomeyide belt, northwestern Bénin 
Fatchéssin Bruno Adjo a,c,*, Prosper M. Nude b, Luc Glodji Adissin c, 
Anthony Temidayo Bolarinwa a, Bertrand Anagonou c 
a Department of Geosciences, Pan-African University Life and Earth Sciences Institute, University of Ibadan, Nigeria 
b Department of Earth Science, University of Ghana, Legon, Ghana 
c Department of Earth Sciences, University of Abomey-Calavi, Calavi, Bénin   
A R T I C L E  I N F O   A B S T R A C T   
Keywords: The Atacora Structural Unit (ASU) is part of the Pan-African belt located in Northwestern Bénin Republic. It 
Atacora Structural Unit consists of metasedimentary rocks of greenschist facies and composed of quartzites, mica schists and chlorite- 
Natitingou sericite-quartz schists. A petrochemical study of these rock types was undertaken, geared at inferring their 
Palaeoweathering paleoweathering, provenance and tectonic setting. Texturally, the rocks are, medium to coarse-grained with 
Provenance 
Tectonic setting angular to rounded clasts. Detrital component statistics classified them as sublitharenite/quartzarenite. 
Geochemically, they can be classified as arkose/subarkose and sublitharenite rocks. The average values for PIA, 
CIA, and CIW for the quartzites, mica schists and schists range from 75% to 98%, reflecting an intensive degree of 
chemical weathering. The LREE enrichment (LaN/YbN = 9.56), negative europium anomaly (Eu/Eu* = 0.77), 
and fractionated HREE patterns of chondrite-normalized, are comparable to melts generated from crustal con-
tinental materials. The geochemical studies show that the studied rocks were sourced from ancient quartzose 
sedimentary rocks. Trace element ratios Cr/Th, Zr/Sc, Th/Sc, Th/Co and La/Th and rare earth elements of the 
source components indicate that these mature materials were chiefly derived from felsic rocks, though the 
involvement of basic rocks (basalts) cannot be ruled out. Chondrite-normalized REE patterns indicate that the 
metasedimentary rocks are sourced from pre-existing sedimentary rocks in the ASU, which rocks were mainly 
derived from the Dahomeyide basement and probably from the Amazonia craton. The major and trace elements 
discriminant diagrams suggest these metasediments were deposited in a passive-margin setting, though conti-
nental island arc setting signature was also identified.   
1. Introduction Armstrong-Altrin, 2009; Oghenekome et al., 2016). The chemical 
composition of fine grained sedimentary rocks such as shale is believed 
Geochemical studies are widely used to evaluate the chemical to be the most suitable for the discrimination of the provenance and 
composition of the crustal protolith material and the relative enrichment tectonic context due to the homogeneity of sediments before their 
of the constituents of clastic sedimentary rocks such as major detrital deposition, post-depositional impermeability, and the high concentra-
elements, useful metals and metamorphic phases (Roser, 2000). The tion of trace elements (Condie, 1993; Cox and Lowe, 1995; McLennan 
investigation of bulk rock major, trace and rare earth elements (REEs) et al., 2000). Rare earth elements (REEs) and some trace elements such 
concentrations, can be useful in the discrimination of the tectonic as Hf, Sc, Th, Zr display usually minor concentrations in natural waters 
setting, provenance, and evolution of the continental crust. They can and are conveyed mostly abundantly during the sedimentary process 
also be used to constrain the source components and evolution of clastic from the source rocks to siliciclastic sediments (Condie, 1991; Arm-
metasediments (Bhatia, 1985; Condie, 1991, 1993, 1993; McLennan strong-Altrin, 2009). The moderate distribution of these immobile ele-
et al., 1993; Nesbitt and Young, 1996; Gabo et al., 2009; ments in different types of rocks allow to deduce the probable input of 
* Corresponding author. Pan-African University Life and Earth Sciences Institute, University of Ibadan, Nigeria 
E-mail address: afab8213@gmail.com (F.B. Adjo).  
https://doi.org/10.1016/j.jafrearsci.2021.104327 
Received 20 May 2020; Received in revised form 26 June 2021; Accepted 13 July 2021   
Available online 14 July 2021
1464-343X/© 2021 Elsevier Ltd. All rights reserved.
F.B. Adjo et al.                                                                                                                                                                                   J o  u  r n  a  l  o f   A  f r i c  a n   E  a  r t h   S  c i e  n c  e s 183 (2021) 104327
Fig. 1. Regional geological map, showing main components of Pan-African Dahomeyides belt and the study area, modified after Affaton et al. (1991).  
basic and felsic sources in sedimentary rocks (Wronkiewicz and Condie, provenance features are comparable to that of the lower sequence of 
1990). Volta basin with passive continental margin chemical characteristics. 
Though the geochemical characteristics of classic metasedimentary The Natitingou area located in the ASU (Fig. 1) hosts good exposures 
rocks have been pivotal in decrypting tectonic environment and prov- of metasedimentary rocks. This area is known for its historical artisanal 
enance, geochemical studies on the Atacora Structural Unit (ASU) at the gold mining sites with gold production within the Pan-African Daho-
Bénin side is undocumented. Comparatively, the ASU on the Republic of meyides external zone (Adjo et al., 2019, Adjo et al., 2021). The 
Ghana side (where it is known as the Togo Structural Unit, TSU) and in chemical composition of these rocks has remained poorly documented, 
the Republic of Togo has been well studied and documented. Kalsbeek hence their source and tectonic setting remain unclear. 
et al. (2012), using U–Pb zircon data on quartzite sample of the ASU, This study aims to document the petrochemistry of the ASU rocks in 
indicated that this lithology may be comparable to the sedimentary northwestern Bénin by assessing their source characteristics, prove-
materials of the Lower Volta basin which cover parts of the eastern nance, and tectonic context as well their weathering condition during 
margin of the West African Craton. Geochemical studies by Anani et al. Neoproterozoic time. This is relevant in reconstructing the paleogeog-
(2019), on metasedimentary rocks of the TSU in Ghana suggest that the raphy of the Natitingou area which singularly hosts abundant gold in the 
rocks were mainly from recycled sedimentary sources and the ASU of the Dahomeyides belt. 
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F.B. Adjo et al.                                                                                                                                                                                   J o  u  r n  a  l  o f   A  f r i c  a n   E  a  r t h   S  c i e  n c  e s 183 (2021) 104327
Fig. 2. Geological map of the study area with sample locations.  
2. Geological setting chert, limestones, mafic and felsic volcanics as well serpentinized ul-
tramafic rocks. It is regarded as the westernmost unit of the Daho-
The Pan African orogenic Dahomeyides belt formed as a result of the meyides external nappes and thrusts to west on the Volta Basin 
continental collision between the eastern margin of the West African sediments, which corresponds to the foreland of the belt. The Atacora 
Craton (WAC) and the Benin-Nigerian shield after the closure of a sub- Structural Unit (ASU) is a range of mountains consisting of quartzites 
duction event around 0.6 Ga years ago (Bessoles et al., 1980; Caby, hills and phyllites recrystallized in greenschists to amphibolite facies 
1989; Affaton et al., 1991; Ganade et al., 2016). The belt has a conditions (Affaton et al., 1991; Castaing et al., 1993; Adjo, 2021). The 
well-organized orogenic architecture with its main three litho-structural best exposures of the ASU occur in the Natitingou area which is pre-
zones (Fig. 1) i. e., from West to East, the external zone, the suture zone dominantly made of siliciclastic metasedimentary rocks (Adjo, 2021; 
and the internal zone or the Benino-Nigerian geological province Adjo et al., 2021a). These latter exhibit a polyphase deformation, 
(Affaton et al., 1991; Deynoux et al., 2006; and therein references). reflecting orogenic and collisional effects produced through the 
These zones are separated by thrust contacts and each one constitutes a Pan-African event (Adjo, 2021; Adjo et al., 2021b). 
thrust sheet. The internal nappes mainly composed of granitic gneiss and U–Pb zircon data acquired on two quartzite samples (one from the 
migmatitic rocks; (2) the suture zone (intermediate nappes) made up of ASU and one from the Togo Structural Unit) have detrital zircon sig-
SSW–NNE elongated eclogites, pyroxenites, metanorites and granulite natures compatible with the Volta Basin lower units (Ganade et al., 
bodies parallel to the Pan African structural trend; and (3) the external 2016). Geochronology data on the quartzites of the Togo Structural Unit 
nappes interpreted as the deformed edge of the WAC and its covers rocks (TSU), Kalsbeek et al. (2008); Kalsbeek and Frei (2010) have suggested 
comprising the Buem and Atacora (Togo) Structural Units. The that this unit might be also compared to the Kwahu/Bombouaka Group 
deformed edge of the WAC constitutes the basement Dahomeyan of the Volta Basin. 
external unit (basement complex) and made up of meta granites, 
granodiorites, amphibolites and varied gneisses known in Ghana as Ho 3. Materials and methods 
gneisses, and in Togo as Kara and Palime-Amlame gneisses (Affaton 
et al., 1991; Attoh et al., 1997; Agbossoumondé et al., 2007). Previous For this study, seventeen (17) fresh representative samples composed 
works interpreted these rocks as remobilized rocks derived from partial of mica schists (n=9), quartzites (n=4), and chlorite sericte-quartz 
melting of thickened Archean lower crust during the Eburnean Orogeny schists (n=3) were selected among samples collected during the 
at ~2.1 Ga (Agbossoumondé et al., 2007; Aidoo et al., 2020). Deformed geological mapping within the Natitingou area (Fig. 2). Thin sections of 
alkaline rocks and carbonatite occur at the sole thrust of the suture zone all rock samples were prepared at the Geology Department, Obafemi 
and the basement complex (Attoh and Nude, 2008; Nude et al., 2009). Awolowo University, Ile-Ife, Nigeria and were studied under the petro-
The Buem Structural Unit (BSU) consists of succession of massive ar- graphic microscope to determine their texture, mineral phases and 
koses and a thick pile of shales, siltstones mudstones interbedded with paragenesis, and alteration phases. The seventeen selected samples were 
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F.B. Adjo et al.                                                                                                                                                                                   J o  u  r n  a  l  o f   A  f r i c  a n   E  a  r t h   S  c i e  n c  e s 183 (2021) 104327
Table 1 
Modal composition of the seventeen selected quartzites and mica schist samples from the Natitingou area. (Qm: mono-crystalline quartz; Qp: poly-crystalline quartz; 
Qt: total quartz (Qt = Qp + Qm); P plagioclase grains, K potassium feldspars, F total feldspar grains (F––K + P), L: total lithics; M: mica; G: Garnet; C: Chlorite).  
Lithology Sample Qm Qp Qt P K F L M C G TOTAL 
Quartzite AD4 81.0 10 91.0 0 0 0 1 8 0 0 100 
AD6 90.0 3.0 93.0 2 2.5 4.5 1 1.5 1 0 100 
10 CE 83.0 7.0 90.0 2 3 5 2 2 1 0 100 
12 CE 89.0 3.0 92.0 3 2 5 2 1 0 0 100 
19 CE 85.0 8.0 93.0 1 3 4 0 2 1 0 100 
24 CE 82.5 5.0 87.5 0 0 0 1.5 10 1 0 100 
25 CE 81.0 4.0 85.0 3.5 2 5.5 2 5 2 0 100 
31 CE 83.0 2.0 85.0 3 3 6 1 6 2 0 100 
32 CE 81.5 4.0 85.5 2 4.5 6.5 0 7 1 0 100              
Mica schist AD9 79 2 81 0 2 2 2 13 2 0 100 
11 CE 74.5 0.5 75 1.5 1.5 3 3.5 15.5 3 0 100 
15 CE 77.5 1.3 78.8 1 1 2 0 15 4.2 0 100 
AD25a 67 1 68 2.5 1.5 4 8 17 4 0 100 
27 CE 74 2 76 0 1 1 4 14 5 0 100              
Schist 18 CE 65 0 65 1 1 2 5 8 12 8 100 
AD19a 73 3 76 1 1.5 2.5 0.5 10 11 0 100 
36 CE 62.5 2 64.5 2 2 4 9 7.5 10 5 100  
Fig. 3. Microphotographs of the ASU metasedimentary rocks showing a texturally matured grains (a–d) in quartzites; (e–f) in schistose rocks. [Qtz– quartz; Mus– 
muscovite; Mc– microcline; Pl– plagioclase; Bt– biotite; Chl– chlorite; Ep– epidote; Lt– lithic fragment]. 
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F.B. Adjo et al.                                                                                                                                                                                   J o  u  r n  a  l  o f   A  f r i c  a n   E  a  r t h   S  c i e  n c  e s 183 (2021) 104327
are shown in Table 1. 
4.2. Quartzite 
The analysed quartzites are texturally mature. The framework grains 
are dominated by monocrystalline and polycrystalline quartz, musco-
vite, K-feldspar, plagioclase, lithic fragments, and chlorite, in decreasing 
order of abundance (Fig. 3a–d). The subangular to subrounded quartz 
grains are the dominant components, with monocrystalline quartz (Qm) 
ranging from 81% to 90% and by volume and polycrystalline quartz 
(Qp) ranging from 2 to 10% by volume (Table 1). Most of mono-
crystalline quartz are undulatory (Fig. 3a and c). Polycrystalline quartz 
are composed more than two crystals with little curved intercrystalline 
borders. The feldspar content is low (4–6.5%; Table 1) and composed of 
microcline and oligoclase (Fig. 3b and d). Lithic fragments (mainly 
sedimentary) account for 1%–2% and composed of chert while the 
muscovite content range from 1% to 10% by volume. Minor amounts 
(2%) of chlorite was also observed According to the classification 
scheme of Folk (1980) and based on the modal analysis, the quartzites of 
the ASU can be classified as quartzarenites and sublitharenite (Fig. 4). 
4.3. Mica schist and chlorite-sericite-quartz-schist 
Fig. 4. Petrographic classification of the ASU studied rocks plotted on Q-F-L Textural analysis shows that the framework of the schistose rocks 
diagram after Folk (1980). [Q: total quartz; F: total feldspar; L: lithic fragments; consist of quartz, muscovite, biotite, microcline, plagioclase, chlorite, 
Circle red: quartzites; circle black: schist and micaschist]. (For interpretation of garnet (Fig. 3e and f). Accessories minerals include pyrite, ilmenite, 
the references to colour in this figure legend, the reader is referred to the Web rutile, epidote, sphene and zircon. The percentages of total quartz grains 
version of this article.) (Qt), total feldspar grains (F) and total lithic fragments (L) are 64.5–81% 
(Av. 73%), 1–4% (Av. 2.4%) and 12–17% (Av. 4.2), respectively 
crushed, pulverized to below 200 mesh, packaged and sent to Bureau (Table 1). The percentages of monocrystalline quartz grains (Qm), 
Veritas Commodities Ltd (ACME Laboratories), Vancouver, Canada. polycrystalline quartz grains (Qp), micas, K-feldspar and plagioclase 
0.2g of each sample were mixed with lithium metaborate/tetraborate grains are 62.5–79%, 0–3%, 7.5–17%, 1–2% and 0–2.5%, respectively, 
and fused at 1025 ◦C in a furnace for their decomposition. Inductively- with average percentages of 71.6%, 1.5%, 12.5%, 1.44%, and 1,12%, 
Coupled Plasma Emission Spectrophotometry (ICP-ES) with an analyt- respectively. (Table 1). Monocrystalline quartz are both non-undulose 
ical precision ranging between 0.002 and 0.04 percent was used to and undulose. Some quartz grains and plagioclase contain inclusions 
assess the major oxides compositions and Loss on Ignition (LOI) of the of heavy minerals such as zircons (Fig. 3f). Micas are composed of 
rocks. The weight difference after ignition at 1025 ◦C was used to muscovite and biotite with a dominance of muscovite over biotite. 
calculate the LOI percentage while Inductively-Coupled Plasma-Mass Chlorite probably derived from biotite accounts for 2–12% while garnet 
Spectrometer (ICP-MS) with 0.01–0.5 ppm Detection limits for all trace occupy 0–8% by volume. From the detrital modes, the studied schistose 
elements was used to analyse trace and rare-earth elements in accor- rocks have also the composition of quartzarenites and sublitharenites as 
dance with the Code LF202 analytical package (ACME Laboratories). illustrated in (Fig. 4). 
The reference materials such as STD DS11, STD GS311-1, STD GS910-4, 
STD OREAS262, STD GBM309-15, and STD SO-19 were used for the 4.4. Geochemistry 
analyses. The analytical data obtained were grouped based on their 
chemical similarities. The results of the major and trace elements analysis of the ASU 
Various discriminatory parameters and diagrams of the major and metasedimentary rocks are presented in Table 2. 
trace elements were used to characterize the provenance and tectonic 
environment. The binary plots of Log Na2O/K2O against log SiO2/Al2O3 4.4.1. Major elements 
(Pettijohn, 1975) and the ternary diagram (Q-F-L) of Folk (1980) were The results of the analysis show a large variation in the content of the 
used in classifying the rocks. The ternary plot (Nesbitt and Young, 1982, major elements. 
1984) expressed as [Al2O3-(CaO*+Na2O)–K2O] as well the formulas for The quartzite samples are composed of: SiO2 (88.27–94.03 wt %), 
chemical index of weathering (CIW), chemical index of alteration (CIA), Al2O3 (2.02–6.38 wt %), TiO2 (0.07–0.32 wt%), Fe2O3 (0.85–1.58 wt%), 
and plagioclase index of alteration (PIA) were valuable in the evaluation and MgO (0.10–0.17 wt%). K2O contents range between 0.54 and 1.54 
of the weathering degree. In the formulas, CaO* is the quantity of CaO wt% while sodium oxide (Na2O) contents range from 0.10 to 0.18 wt%. 
amalgamated to the silicate portion of the rock. The contents of CaO, P2O5 and MnO are mostly less than 0.1 wt% 
(Table 2). The graph of Pettijohn (1975) revealed that these quartzites 
4. Results can be characterised as subarkose to sublitharenite (. 5a). 
The schistose rocks have the following compositional ranges: 61.28 
4.1. Petrography to 88.87 wt % of SiO2, 4.28 to 17.55 wt% of Al2O3, 2.68 to 10.5 wt% of 
Fe2O3, 0.13 to 2.55 wt % of MgO, 0.01 to 0.88 wt% of CaO, 0.12 to 0.53 
As stated above, the metasedimentary rocks studied in this paper are wt% of Na2O, 0.68 to 5.27 wt% of K2O, 0.3 to 1.41 wt% of TiO2, 0.02 to 
quartzites, mica schists and chlorite-sericite-quartz schists. Their 0.23 wt% of P2O5, and 0.01 to 0.11 wt % of MnO (Table 3). As presented 
petrographic analysis were done to classify the protolith of rocks and in Fig. 5a, the chlorite-sericite-quartz schist and mica schist show the 
understand the sedimentary histories. Modal compositions of represen- composition of arkose. 
tative medium to coarse-grained textured quartzites and schistose rocks The analysed rocks are rich in quartz as indicated by their high silica 
content with an average of 90.61 wt% in quartzite and 74.77 wt% in the 
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F.B. Adjo et al.                                                                                                                                                                                   J o  u  r n  a  l  o f   A  f r i c  a n   E  a  r t h   S  c i e  n c  e s 183 (2021) 104327
Table 2 
Major (wt %) and trace elements (ppm) composition of quartzites from the Natitingou area.  
Lithology Quartzite 
Sample AD4 AD6 10 CE 12 CE 19 CE 24 CE 25 CE 31 CE 32 CE 
SiO2 (W. %) 88.27 94.03 93.92 92.26 93.14 86.68 92.86 86.98 87.39 
TiO2 0.23 0.07 0.13 0.16 0.09 0.32 0.14 0.27 0.32 
Al2O3 5.79 2.14 2.02 2.95 3.02 6.38 2.53 6.32 5.98 
Fe2O3 1.16 0.97 0.85 1.58 1.73 1.32 1.66 1.29 1.32 
MgO 0.15 0.11 0.14 0.12 0.10 0.16 0.13 0.17 0.16 
CaO 0.03 0.10 0.08 0.09 0.09 0.06 0.11 0.05 0.06 
Na2O 0.15 0.13 0.12 0.13 0.11 0.18 0.10 0.16 0.17 
K2O 1.33 0.54 0.69 0.69 0.70 1.52 0.66 1.54 1.51 
P2O5 0.04 0.05 0.04 0.04 0.04 0.05 0.06 0.05 0.04 
MnO <0.01 <0.01 0.01 <0.01 0.01 0.02 0.01 0.02 0.01 
Cr2O3 0.005 0.003 0.005 0.004 0.004 0.004 0.003 0.005 0.004 
LOI 2.80 1.80 1.90 2.00 2.10 3.20 2.00 3.10 3.00 
Total 99.97 99.99 99.98 99.98 99.99 99.94 99.98 99.96 99.96 
Log (SiO2/Al2O3) 1.18 1.64 1.56 1.50 1.14 1.13 1.16 1.49 1.67 
Log (Na2O/K2O) − 0.95 − 0.62 − 0.82 − 0.72 − 0.98 − 0.93 − 0.95 − 0.80 − 0.76 
PIA 96.75 92.39 92.39 95.59 95.02 96.41 94.46 96.74 96.73 
CIA 79.65 76.11 77.49 78.79 77.85 78.95 78.86 78.89 79.55 
CIW 97.48 94.20 93.83 96.59 96.00 97.24 96.60 97.32 97.42 
Trace elements (ppm) 
Ba 122 37 27 65 26 226 27 118 45 
Ni 20 20 20 20 19 20 20 19 19 
Sc 5.0 2.0 2.0 3.0 6.0 6.0 2.0 2.0 3.0 
Be 2.0 10 7.0 8.0 8.0 3.0 9.0 3.0 3.0 
Co 2.6 1.4 1.5 1.3 1.3 4.8 1.4 2.9 0.7 
Cs 0.9 0.7 0.5 0.8 0.5 1.1 0.6 0.9 20.8 
Ga 6.3 1.6 0.9 2.6 1.0 6.2 0.8 4.9 25.8 
Hf 2.5 1.4 0.8 2.0 0.8 5.9 0.9 1.8 1.3 
Nb 3.4 1.7 1.1 2.2 2.1 4.4 1.0 2.6 31.2 
Rb 37 16.1 12.5 22.1 12.4 43.6 12.7 33.6 41.7 
Sn 2.0 2.0 2.0 2.0 2.0 3.0 1.0 2.0 2.0 
Sr 23 14.6 9.1 13.9 9.1 31 9.4 19.1 18 
Ta 0.3 0.2 0.2 0.3 1.6 0.5 0.1 0.2 5.8 
Th 3.5 1.6 1.0 2.3 0.9 4.9 0.9 2.8 1.3 
U 0.6 0.3 0.1 0.3 0.2 1.1 0.2 0.4 2.1 
V 15 9.0 8.0 9.0 9.0 20 8.0 16 8.0 
W 2.3 1.0 0.7 1.5 0.9 1.8 0.7 18.8 1.7 
Zr 104.8 50.2 30.7 68.4 29 220.1 34.1 70.2 15.5 
Y 11.4 6.3 4.0 7.2 3.9 14.2 4.3 9.3 6.0 
Mo 0.9 1.3 3.3 1.1 2.8 0.8 2.6 2.9 0.3 
Cu 20.8 27.5 23.5 27 21.1 19.1 21.4 19.5 7.3 
Pb 0.7 0.9 1.2 0.9 1.1 2.9 0.9 1.1 2.2 
Zn 5.0 7.0 7.0 7.0 6.0 8.0 8.0 6.0 4.0 
As 1.3 1.9 2 1.7 1.5 1.2 2.1 1.7 0.5 
Au (ppb) 4.4 43.2 29 18.9 13.7 464 38.6 2.9 0.5  
Fig. 5. Geochemical classification of the ASU studied rocks, including (a) Log Na2O/K2O against log SiO2/Al2O3 plot after Pettijohn (1975); (b) K2O (W%) against 
Na2O (W%) plot after Crook (1974). [Circle red: quartzites, circle black: schist and micaschist]. (For interpretation of the references to colour in this figure legend, the 
reader is referred to the Web version of this article.) 
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F.B. Adjo et al.                                                                                                                                                                                   J o  u  r n  a  l  o f   A  f r i c  a n   E  a  r t h   S  c i e  n c  e s 183 (2021) 104327
Table 3 
Major (wt %) and trace elements (ppm) composition of mica schist and schist from the Natitingou area.  
Lithology Micaschist Schist 
Sample AD9 11 CE 15 CE AD25a 27 CE 18 CE AD19a 36 CE 
SiO2 (W. %) 88.87 81.12 81.83 61.28 70.92 69.04 77.92 67.2 
TiO2 0.30 0.53 0.54 0.92 0.91 1.01 0.59 1.14 
Al2O3 4.28 9.4 8.89 17.55 13.35 14.62 12.07 13.77 
Fe2O3 2.68 4.38 4.31 6.96 7.38 7.34 3.31 10.25 
MgO 0.13 0.19 0.18 2.55 0.38 0.59 0.23 0.47 
CaO 0.18 0.01 <0.01 0.88 <0.01 0.07 <0.01 0.18 
Na2O 0.12 0.15 0.14 0.31 0.27 0.53 0.25 0.2 
K2O 0.68 1.97 1.84 5.27 4.23 3.27 3.15 4.06 
P2O5 0.05 0.02 0.02 0.23 0.01 0.04 0.02 0.08 
MnO 0.04 <0.01 0.01 0.10 0.01 0.01 <0.01 <0.01 
Cr2O3 0.005 0.002 0.002 0.013 0.022 0.007 0.002 0.004 
LOI 2.60 2.10 2.20 3.60 2.40 3.30 2.40 2.40 
Total 99.95 99.95 99.94 99.83 99.94 99.87 99.94 99.84 
Log (SiO2/Al2O3) 1.32 0.94 0.96 0.54 0.73 0.67 0.81 0.69 
Log (Na2O/K2O) − 0.75 − 1.12 − 1.12 − 0.79 − 1.10 − 1.23 − 1.19 − 1.31 
PIA 96.69 98.02 98.05 97.42 97.12 95.53 97.27 97.95 
CIA 84.20 81.60 81.78 75.82 74.79 79.36 78.02 76.36 
CIW 97.20 98.43 98.45 98.18 98.02 96.49 97.97 98.55 
Trace elements (ppm) 
Ba 81 249 238 1104 676 200 324 526 
Ni 20 20 20 49 28 37 20 30 
Sc 3.0 11 10 16 19 16 13 24 
Co 6.5 1.9 2.2 17.2 4.5 13.8 2.1 10.3 
Cs 1.5 2.0 2.3 6.3 1.9 3.5 2.5 10.7 
Ga 4.0 9.4 9.7 22.2 13.5 15.7 12.9 20.5 
Hf 4.8 4.5 5.2 6.3 2.9 9.6 5.7 9.6 
Nb 6 5.9 6.6 20.9 4.4 12.6 7.1 10.4 
Rb 30.9 69.4 68.3 168.6 53.9 91.7 81.2 135.2 
Sn 4.0 1.0 1.0 3.0 1.0 3.0 1.0 2.0 
Sr 17.1 20.2 20.1 85.7 39.5 75.3 44.5 108.2 
Ta 0.6 0.4 0.5 1.4 0.3 1.0 0.6 0.7 
Th 5.4 8.3 8.7 20.3 3.8 16.3 7.1 9.4 
U 1.5 1.2 1.3 2.4 0.6 2.3 1.3 2.0 
V 20 16 18 87 81 28 29 63 
W 0.8 3.3 3.6 3.2 2.4 6.4 13.9 1.8 
Zr 179.5 182.2 194 230.4 108.2 366.3 222.7 376.4 
Y 14.4 32 30.4 23.8 12.9 61.3 27.3 63.9 
Mo 2.5 <0.1 <0.1 <0.1 <0.1 0.5 <0.1 <0.1 
Cu 24.5 2 1.8 33.7 2.9 14 3.1 2.4 
Pb 5.2 2.2 0.5 9.2 1.3 0.9 1.3 1.9 
Zn 16 1.0 2.0 71 1.0 4.0 2.0 3.0 
Ni 5.7 0.5 0.4 35.5 0.8 1.7 0.2 2.4 
As 2.1 1.5 1.3 0.7 <0.5 1.5 11.3 <0.5 
Au (ppb) 8.8 1.1 <0.5 49.4 <0.5 15.3 9587 <0.5  
schistose rocks. The plot of Na2O against K2O (Fig. 5b), indicate these the amounts of K2O with Al2O3 (Fig. 6b) indicates that the potassium 
ASU metasedimentary are quartz-rich. The mean content of Fe2O3 + and aluminium absorption is controlled by the mica, the clay minerals 
MgO is 1.90 wt% which suggests that the samples contain traces of mafic (chlorite and illite) and K-feldspars (microcline) in ASU metasedi-
material. The higher Fe2O3 concentration in some samples may be mentary rocks (Khanehbad et al., 2012; Hu et al., 2015). 
related to the presence of magnetite and hematite minerals. The average 
CaO, Al2O3, Na2O, K2O contents of these rocks are respectively 0.12 wt 
%, 7.71 wt %, 0.19 wt % and 2.34 wt %, suggesting an impoverishment 4.5. Trace elements 
of feldspars. However, the rocks have significant amounts of K2O 
(mainly in schistose rocks) which could occur through conveyance or Because of numerous parameters including physical sorting, 
due to the modification feldspars in the source area (Tobia and Aswad, adsorption, weathering, provenance, metamorphism and diagenesis, 
2015). It could also be related to K addition during diagenesis. Similarly, trace elements have a complex behaviour during sedimentary processes 
the studied rocks show appreciable TiO2 content because of the 
(Nesbitt et al., 1980; Wronkiewicz and Condie, 1987; Rollinson, 1993). 
Titanium-rich minerals, for example rutile, ilmenite and sphene. The trace element content of the analysed metasedimentary rock sam-
SiO show strong negative correlation with K O, Al O , Fe O TiO ples show a wide range of concentrations (Tables 2 and 3). 2 2 2 3 2 3, 2 
and MgO, moderate or weak correlation with Na O, CaO and P O The average value of Large Ion Lithophile Elements (LILE) Ba, Rb, K, 2 2 5 
(Fig. 6a). There is no correlation between MnO and SiO . The oxides Sr and Cs, are 240.64 ppm, 54.76 ppm, 16.65 ppm, 41.23 ppm and 3.38 2
such as TiO2, K2O, Fe2O3 and Na2O show strong positive correlations 
ppm, respectively. K2O correlated positively with Ba (r = 0.90), Rb (r =
with Al O (Fig. 6b). According to Fedo et al. (1996), elements such as 0.87), Cs (r = 0.70) and Sr (r = 0.82). This indicates that the higher 2 3 
Al O , TiO , K O, Fe O and Na O indicate high weathering level of the contents of K, Ba, Rb and Sr in the ASU metasedimentary rocks is 2 3 2 2 2 3 2
source area where Calcium is favourably leached while potassium and controlled by K-bearing clay minerals such as muscovite and biotite 
magnesium are incorporated in clay minerals. Conversely, MgO, P O (McLennan et al., 1993; Rollinson, 1993). High field strength elements 2 5 
and CaO show a moderate positive correlation (correlation coefficient R (HFSE values of Th/U range widely (3.6–10; Tables 4 and 5) and greater 
= 0.85, 0.44 and 0.45 respectively) with Al2O3. Moreover, increase in 
than those of the average Neoproterozoic upper continental crust (UCC) 
value of about 3.9 (Condie, 1993); the Th/Sc contents varying from 0.2 
7
F.B. Adjo et al.                                                                                                                                                                                   J o  u  r n  a  l  o f   A  f r i c  a n   E  a  r t h   S  c i e  n c  e s 183 (2021) 104327
Fig. 6. Major oxides relationships diagrams for the Atacora Structural Unit metasedimentary rocks from Natitingou area. (a) Harker variation diagrams, (b) Major 
oxides versus Al2O3 diagrams. [In both diagrams, Circle red: quartzites, circle black: schist and micaschist]. (For interpretation of the references to colour in this 
figure legend, the reader is referred to the Web version of this article.) 
to 1.8 are generally above those of the average UCC value of 0.72 concentrations of schist and micaschist samples are generally higher 
(Condie, 1993). The spider diagrams of the ASU metasedimentary rocks than those of these UCC and PAAS (Fig. 8). This shows that the studied 
compared to UCC (Fig. 7) show parallel signatures with a poor enrich- schistose rocks contain more REE concentrations than those of the 
ment in some elements and a depletion in the same elements in some quartzites; thus confirming the quartz enrichment in the quartzites. 
samples. However, the concentrations of the trace elements are chiefly Also, the chondrite normalized La/Yb ratio varies between 7.18 and 
below those of the UCC particularly in all quartzite samples (Fig. 7a and 20.83, with most rocks samples having contents relatively more abun-
b). dant than those of UCC and PAAS. Furthermore, the Europium anomaly 
The concentrations of base metals, i.e. Ni, Pb, Zn, Cu, and Sn range matches with the depletion in CaO and Na2O, indicating that it 
from 20 to 49 ppm, 0.5–9.2 ppm, 1–71 ppm, 1.8–33.7 ppm and 1–4 ppm, moderately developed in response to plagioclase weathering, where 
respectively (Tables 2 and 3), and are all below those of the averages most of the Europium is incorporated. McLennan and Taylor (1991) and 
upper crust (Fig. 7b). As values are moderate, and vary from 0.5 to 11.3 Awwiller (1994), supposed that the Europium anomaly in sedimentary 
ppm while high values of Au are recorded across the samples (average rocks is commonly believed to be derived from igneous source rocks. 
849.33 ppb). Most of the samples exhibit positive anomaly in Th, K, La, 
Ce, Nd, Ni, Rb, and Zr, and negative anormaly in Ba, U, Nb, Ta, Sr, P, Ti, 5. Discussion 
V, Pb and Cu (Fig. 7a and b). 
5.1. Influence of metamorphism/alteration 
4.5.1. Rare earth elements 
Details on REE contents and some related parameters of the studied The major elements such as Si, Ca, K, and Na and mobility of trace 
rocks are summarized in Tables 4 and 5 The total REE and the ratios of elements such as Cs, Rb, Ba, and Sr of sedimentary rocks can be affected 
LREE to HREE contents are between 32.78 and 501.72 ppm, and by alteration and metamorphism (Bhatia, 1983; Roser and Korsch, 1986; 
4.72–10.96, respectively. The rocks are characterized by enrichment in Bhatia and Crook, 1986; Yang et al., 1998). Consequently, it is essential 
LREE (LaN/SmN, 4.52–2.29), depletion in HREE (GdN/YbN, 3.31–1.17), to examine the scale of metamorphism, alteration and element mobility 
and significant negative Eu anomalies (0.57–0.80). Comparison of the on the analysed rocks samples before making good inferences with the 
chondrite-normalized distribution of ASU rocks to UCC and Post geochemical data obtained in this study. Several observations prove low 
Archean Australian Shale (PAAS) (Taylor and McLennan, 1985, 1995), degree of metamorphism and alteration in the area, and insignificant 
show some similarities (Fig. 8). However, the concentrations of the extent remobilization of some trace elements. In fact, the rocks of the 
quartzites are lower than those of UCC and PAAS (Fig. 8) while the Atacora Structural Unit reflect a greenschist metamorphic facies 
8
F.B. Adjo et al.                                                                                                                                                                                   J o  u  r n  a  l  o f   A  f r i c  a n   E  a  r t h   S  c i e  n c  e s 183 (2021) 104327
Table 4 
REE (ppm), some ratio and main parameters for quartzite samples.  
Lithology Quartzite 
Sample AD4 AD6 10 CE 12 CE 19 CE 24 CE 25 CE 31 CE 32 CE 
La 21.0 12.4 7.5 11.8 7.0 21.5 7.7 16.6 2.3 
Ce 38.5 22.1 13.5 21.1 12.9 41.2 12.9 29.3 3.4 
Pr 4.99 3.06 1.79 2.76 1.7 4.86 1.79 3.84 0.52 
Nd 18.2 11.8 6.6 11.0 6.7 18.2 6.8 14.3 2.0 
Sm 3.07 2.20 1.13 2.03 1.16 3.25 1.21 2.31 0.7 
Eu 0.69 0.42 0.27 0.48 0.27 0.72 0.28 0.55 0.12 
Gd 2.44 1.67 0.97 1.85 1.06 2.83 1.06 2.05 0.88 
Tb 0.36 0.23 0.15 0.27 0.15 0.44 0.15 0.3 0.18 
Dy 2.06 1.32 0.84 1.45 0.77 2.54 0.88 1.71 0.86 
Ho 0.42 0.27 0.16 0.29 0.16 0.53 0.18 0.38 0.18 
Er 1.19 0.65 0.42 0.75 0.4 1.59 0.44 1.03 0.42 
Tm 0.16 0.09 0.06 0.10 0.06 0.23 0.06 0.15 0.07 
Yb 1.12 0.64 0.36 0.70 0.4 1.50 0.42 0.91 0.64 
Lu 0.14 0.09 0.05 0.09 0.05 0.22 0.05 0.15 0.09 
(La/Yb)N 12.64 13.06 14.05 11.36 11.80 9.66 12.36 12.30 12.64 
(Gd/Yb)N 1.76 2.11 2.17 2.13 2.14 1.52 2.04 1.82 1.98 
(La/Sm)N 4.30 3.55 4.17 3.66 3.80 4.16 4.00 4.52 4.30 
La/Sc 4.20 6.20 1.90 3.93 2.50 3.58 3.85 5.53 3.50 
Th/Sc 0.70 0.80 0.33 0.77 0.27 0.82 0.45 0.93 1.3 
La/Co 8.08 8.86 5.20 9.08 5.77 4.48 5.50 5.72 4.12 
Th/Co 1.35 1.14 0.67 1.77 0.69 1.02 0.64 0.97 0.76 
Cr/Th 9.77 12.83 9.77 11.90 8.69 5.58 6.51 3.49 6.01 
Th/Cr 0.10 0.08 0.04 0.08 0.05 0.18 0.04 0.08 0.06 
Th/U 5.83 5.33 10.00 7.67 4.00 4.45 4.50 7.00 6.20 
La/Th 6.00 7.75 7.5 5.13 9.75 4.39 8.56 5.93 1.77 
Zr/Sc 20.96 25.10 10.23 22.80 11.1 36.68 17.05 23.4 15.5 
Eu/Eu* 0.77 0.67 0.79 0.76 0.74 0.73 0.76 0.73 0.77 
ΣLREE 86.45 51.98 30.79 49.17 29.73 89.73 30.68 89.73 66.9 
ΣHREE 7.89 4.96 3.01 5.50 3.05 9.88 3.24 9.88 6.68 
ΣREE 94.34 56.94 33.8 54.67 32.78 99.61 33.92 99.61 73.58 
LREE/HREE 10.96 10.48 10.23 8.94 9.75 9.08 9.47 9.08 10.01  
Table 5 
REE (ppm), some ratio and main parameters for mica schist and schist samples.  
Lithology Mica schist Schist 
Samples AD9 11 CE 15 CE AD25a 27 CE 18 CE AD19a 36 CE 
La 19.7 27.9 29.9 39.0 22.3 60.8 18.3 92.5 
Ce 43.7 54.6 57.5 120.6 21.3 125.4 34.9 167.4 
Pr 4.56 6.43 6.77 10.44 4.98 13.75 4.30 27.59 
Nd 17.8 25.6 26.0 37.2 17.6 51.3 16.2 122.2 
Sm 3.23 5.47 5.57 6.63 3.64 9.46 3.18 25.4 
Eu 0.59 1.42 1.42 1.31 0.91 2.29 0.76 5.56 
Gd 3.06 6.22 5.97 5.51 3.29 9.73 3.71 23.08 
Tb 0.44 0.97 0.97 0.81 0.49 1.66 0.70 3.16 
Dy 2.72 5.97 5.49 4.98 2.82 10.38 4.6 17.16 
Ho 0.52 1.20 1.14 0.99 0.50 2.27 1.01 2.90 
Er 1.65 3.49 3.23 2.91 1.44 6.47 3.12 7.44 
Tm 0.21 0.47 0.40 0.42 0.19 0.86 0.41 0.93 
Yb 1.43 2.91 2.69 2.99 1.21 5.36 2.55 5.62 
Lu 0.20 0.44 0.39 0.46 0.20 0.74 0.36 0.78 
(La/Yb)N 9.29 6.46 7.49 8.79 12.43 7.65 4.84 11.10 
(Gd/Yb)N 1.73 1.72 1.79 1.49 2.19 1.46 1.17 3.31 
(La/Sm)N 3.84 3.21 3.38 3.70 3.85 4.04 3.62 2.29 
La/Sc 6.57 2.54 2.99 2.44 1.17 3.80 1.41 3.85 
Th/Sc 1.80 0.75 0.87 1.27 0.20 1.02 0.55 0.39 
La/Co 3.03 14.68 13.59 2.27 4.96 4.41 8.71 8.98 
Th/Co 0.83 4.37 3.95 1.18 0.84 1.18 3.38 0.91 
Cr/Th 6.33 0.82 0.79 4.38 39.60 2.94 0.96 2.91 
Th/Cr 0.16 1.21 1.27 0.23 1.04 0.34 0.03 0.34 
Th/U 3.60 6.92 6.69 8.46 6.33 7.09 5.46 4.70 
La/Th 3.65 3.36 3.44 1.92 5.87 3.73 2.58 9.84 
Zr/Sc 59.83 16.56 19.40 14.40 5.69 22.89 17.13 15.68 
Eu/Eu* 0.67 0.74 0.75 0.66 0.80 0.73 0.68 0.70 
ΣLREE 89.58 121.4 127.2 215.2 70.73 263 77.64 440.7 
ΣHREE 10.23 21.67 20.28 19.07 10.14 37.47 16.46 61.07 
ΣREE 99.81 143.09 147.4 234.25 80.87 300.47 94.1 501.7 
LREE/HREE 8.76 5.60 6.27 11.28 6.98 7.02 4.72 7.22  
9
F.B. Adjo et al.                                                                                                                                                                                   J o  u  r n  a  l  o f   A  f r i c  a n   E  a  r t h   S  c i e  n c  e s 183 (2021) 104327
Fig. 7. Spider plots for the ASU metasedimentary 
rocks showing (a) trace elements plot of the ASU 
samples normalized against the upper continental 
crust after Taylor and McLennan (1995), (b) 
Multiple-elements diagram of the ASU samples 
normalized against the average crust, after Taylor and 
McLennan (1985). [In both diagrams, Circle red: 
quartzites, circle black: schist and micaschist]. (For 
interpretation of the references to colour in this figure 
legend, the reader is referred to the Web version of 
this article.)   
use the geochemical results obtained from the samples in determining 
the source area weathering characteristics, tectonic setting and prove-
nance (Yang et al., 1998). According to these authors, although it is 
possible that some LILE are susceptible to be remobilized, the wide 
extent remobilization of the HFSE and the REEs is limited.Other factors, 
such as provenance and source area weathering would rather have 
predominantly contributed in defining the dispersals of these elements. 
5.2. Paleoweathering and metasomatism 
Complex clays (e. g. chlorites, smectite) containing Na, Ca, Mn, Fe 
and Mg deteriorate easily during weathering comparatively to musco-
vite and biotite containing K2O which are stable and unaffected by 
weathering (Cox et al., 1995). These major elements relationship allow 
us to deduce the extent of chemical weathering (Nesbitt and Young, 
1982, 1984, 1989, 1996). The diagrams of these authors are helpful for 
deducing the weathering degree of the rocks before being conveyed to 
their depositional environments. As showed on Al2O3–CaO* +
Na2O–K2O (Fig. 10) ternary diagram, the samples plot along the A-K 
trend around muscovite-illite closer to high corindon (Al2O3) contents 
(Kaolinite). This suggests that the sediments have experimented an 
Fig. 8. Chondrite-normalized REE plots for the ASU metasedimentary rocks. advanced chemical weathering and recorded a K-metasomatism action 
[Yellow colour: UCC, blue: PAAS, Circle red: quartzites, circle black: schist and (i.e, plagioclase albitization and/or illitisation of smectite) from the 
micaschist]. (For interpretation of the references to colour in this figure legend, highly aluminous clays minerals (kaolinite to potassic clays) conversion. 
the reader is referred to the Web version of this article.) For interpreting the palaeo-weathering history of sediments/meta-
sediments, Nesbitt and Young (1982) and Harnois (1988) suggested the 
(Affaton et al., 1991; Castaing et al., 1993). The presence epidote and use of the chemical index of alteration (CIA), and of weathering (CIW), 
chlorite indicate alteration processes in these rocks According to Sher- respectively, while Fedo et al. (1995) proposed the plagioclase index of 
vais et al. (2006), rocks affected by low post-refreshing alteration show alteration (PIA). For calculating the three indices, CaO* (quantity of CaO 
K2O/P2O5 > 1. The K2O/P2O5 ratios for the analysed samples (Table 2) assimilated in the silicate portion of the rock only but not in carbonate) 
are greater than 1 ranging between 10.8 and 423. This suggest minor is used within equations. An indirect method (CaO* = CaO −
alteration and almost immobility of the major elements. 10/3*P2O5) recommended by McLennan et al. (1993) for determining 
According to Girty et al. (1996), HFSE and REEs are rarely influenced CaO content has been used in this study. 
by post magmatic alteration or metamorphism. This is corroborated by CIA measures the conversion extent of feldspars to clays minerals. 
the selected REE and HFSE plot against Zr for the ASU metasedimentary CIA values varying from 0 to 50% point out fresh source areas, whilst 
rocks (Fig. 9). The patterns show some levels of scattering and strong values between 50-60% and 60–80% indicate incipient and intermedi-
linear relationships between Zr and selected trace elements. Particu- ate weathering, respectively. Values superior or equal to 95%, express a 
larly, all HFS elements (e.g., Hf, Yb, Nb, Th, and U) illustrate linearly high-degree of weathering at the source area (Nesbitt and Young, 1982). 
consistent inter-relationships with Zr (Fig. 9) indicating a weak mobility Chemical index of alteration values obtained for quartzites and schistose 
or even non-mobility of these elements. The remaining trace elements rocks of ASU vary from 76.11 to 79.65% (average 78.4%) and 
(e.g., Tb, Gd, Ce, and Nd) display a low degree of scattering suggesting 74.49–84.20% (average 78.99%), respectively. This suggests that 
limited mobility. Their relationships show the consistency and chemical moderate tropical weathering prevails in the source area (Feng et al., 
logic of the obtained results. Therefore extensive-scale remobilization, 2003; Ejeh et al., 2015; Lang et al., 2018). The obtained values are 
at least for these trace elements was minimal. Hence, it is reasonable to weakly higher than the PAAS values (70–75%; Taylor and McLennan, 
10
F.B. Adjo et al.                                                                                                                                                                                   J o  u  r n  a  l  o f   A  f r i c  a n   E  a  r t h   S  c i e  n c  e s 183 (2021) 104327
Fig. 9. Multi-plots of Zircon versus selected REE and HFSE for the Atacora Structural Unit. [Red colour: quartzites, black: schist and micaschist]. (For interpretation 
of the references to colour in this figure legend, the reader is referred to the Web version of this article.) 
1985; McLennan et al., 1993). Furthermore, the highest CIA value (CIA sedimentary rocks. 
= 84.2; sample AD9) could primarily be due to the low values of its K2O However, compositional variation and sediment degree reworking 
and Na2O contents or rocks have undergone intensive weathering could be assessed with the Th/Sc versus Zr/Sc plot (McLennan et al., 
because of the tropical climate control in the area at the disintegration 1993). In this bivariate diagram, sediment samples from igneous dif-
time of the source rocks. ferentiation have to accompany the origin-dependent compositional 
PIA values ~50% correspond to the unweathered plagioclase values. variation tendency while those from recycled sedimentary origin will 
Values closer to 100% indicate complete plagioclase conversion into reassemble towards the zircon addition direction. In this study, the 
others aluminous clay minerals (e.g. chlorite, gibbsite, illite and samples plotted along compositional variations trend (Fig. 11a), and not 
kaolinite (Fedo et al., 1995). In this study, the high plagioclase index of recycled materials and sorting associated with zircon enrichment. The 
alteration values (average 96.6%; Tables 2 and 3) of the ASU rocks low concentrations of Zircon in most of the rocks (Tables 2 and 3) are in 
suggest significant chemical weathering degree at source area. Using agreement with this diagram. It can be inferred that the protoliths of 
CIW criterion, most of the analysed quartzite and schistose rocks show ASU rocks are mainly derived from igneous rocks (McLennan et al., 
high (>97%) CIW values (Tables 2 and 3) suggestive of advanced 1993). 
weathering at source area (Condie, 1993). In the studied samples, the Having constrained principally igneous provenance for the studied 
CIA values are lower than CIW and PIA values because of the elimination ASU rocks, we now attempt to characterize whether their source rocks 
of K2O from the index, hence these rocks would have experienced were predominantly mafic or felsic igneous rocks. The dispersion of 
K-metasomatism. transition metals and HFSE rarely undergo the effect of secondary pro-
cesses, diagenesis and metamorphism. For example, Co, Sc, La, and Th 
5.3. Compositional variation and provenance elements are almost immobile during metamorphism and weathering. 
(Taylor and McLennan, 1985; Wronkiewicz and Condie, 1987; 
Using the approach of Roser and Korsch (1988) through major ele- McLennan and Taylor, 1991; Cox et al., 1995). Therefore, the prove-
ments discriminant function diagram, Adjo et al. (2021a) stated that the nance of clastic sedimentary rocks can also be discriminated using HFSE, 
ASU metasedimentary rocks were probably derived from quartzose REEs and some transition elements. While Th and La are more 
sedimentary provenance which are mature polycyclic continental compatible to felsic than mafic igneous rocks, Co and Sc follow the 
11
F.B. Adjo et al.                                                                                                                                                                                   J o  u  r n  a  l  o f   A  f r i c  a n   E  a  r t h   S  c i e  n c  e s 183 (2021) 104327
Fig. 10. Paleo weathering discrimination diagrams, after Nesbitt and Young (1982, 1984, 1989, 1996). [Circle red: quartzites, circle black: schist and micaschist]. 
(For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) 
Fig. 11. Composition variation and provenance discrimination diagrams for the ASU metasediments, (a) after McLennan et al. (1993); (b) after Floyd and Leveridge 
(1987). [Circle red: quartzites, circle black: schist and micaschist]. (For interpretation of the references to colour in this figure legend, the reader is referred to the 
Web version of this article.) 
opposite trend in those rocks. Thus elemental ratios, such as Th/Co, shown in Table 6. From the comparison, the elemental ratios of the ASU 
Th/Sc, La/Co, Zr/Sc and La/Sc are useful in the determination of metasedimentary rocks are almost within or close to the average values 
provenance of sedimentary rocks (Taylor and McLennan, 1985; Cullers of the felsic source. 
et al., 1988; McLennan et al., 1993; Cullers, 2002). Some average Also, the size and behaviour of the Europium anomaly can be used to 
elemental ratios of the ASU metasedimentary rocks have been compared infer the source area of metasedimentary rocks because felsic igneous 
with other sediments sourced from basic and felsic rocks; these are rocks usually hold negative europium anomalies (from REE chondrite- 
12
F.B. Adjo et al.                                                                                                                                                                                   J o  u  r n  a  l  o f   A  f r i c  a n   E  a  r t h   S  c i e  n c  e s 183 (2021) 104327
Table 6 
Range of elemental ratios values in the ASU metasedimentary rocks compared to similar ratios values in fractions derived from felsic and mafic rocks (after Cullers 
et al., 1988; Cullers, 1994, 2000, 2002; Cullers and Podkovyrov, 2000).  
Elements ratios Range of metasediments from ASU a Range of sediments from felsic and mafic sources b 
Quartzites (n = 9) Schists(n = 3) Micaschists (n = 5) Mafic Felsic 
Cr/Th 3.49–12.83 0.96–2.94 0.79–6.33 25–500 4.00–15.0 
Th/Cr 0.04–0.18 0.03–0.34 0.16–1.27 0.018–0.046 0.13–2.70 
Th/Co 0.64–1.77 0.91–3.38 0.83–4.37 0.04–1.40 0.67–19.4 
La/Co 4.12–9.08 4.41–8.98 2.27–14.68 0.14–0.38 1.80–13.8 
Th/Sc 0.33–1.30 0.39–1.02 0.75–1.8 0.05–0.22 0.84–20.5 
La/Sc 1.90–6.20 1.41–3.85 2.44–6.57 0.43–0.86 2.50–16.3 
Eu/Eu* 0.67–0.77 0.66–080 0.67–0.75 0.71–0.95 0.40–0.94  
a This study. 
b Cullers et al. (1988). Cullers (1994.2000.2002). Cullers and Podkovyrov (2000). Armstrong-Altrin (2009). 
normalized plot patterns) while igneous mafic rocks show no or little Th and Hf values (9.84 and 9.6 ppm, respectively). This high Hf con-
europium anomaly (Taylor and McLennan, 1985; Cullers, 2000, 2002). centration can imply high zircon content in the sediment, while high La/ 
The negative europium anomalies ranging from 0.66 to 0.80 in the Th value could be related to low Thorium concentration in the sediment. 
studied rocks (Tables 5 and 6) would suggest felsic materials (Cullers Nevertheless, most of the ASU samples with Hf contents (2–6.3 ppm) 
et al., 1988; Cullers and Podkovyrov, 2000). Thus, it indicates that the and La/Th values (1.92–5.13) are mostly felsic in composition since 
precursors of the studied rocks were extracted from magmatic materials these averages are similar to those of Floyd and Leveridge (1987) who 
(Taylor and McLennan, 1985). By using the approach of Rudnick (1992), demonstrated that, felsic source arcs show constant or low (less than 5) 
the negative patterns of Europium anomalies on the REE diagrams La/Th values and Hf contents range from 3 to 7 ppm. 
(Fig. 8) support felsic source. This is further reinforced by the enrich- From the above discrimination diagrams, trace element ratios, and 
ment in LREE relative to HREE, which according to Wang et al. (2016) other immobile elements ratio diagrams, it is reasonable to propose that 
symbolizes felsic sources for the sediments. Conversely, the Eu/Eu* the protolith of the studied ASU quartzites, mica schists and chlorite- 
range, overlaps those of sediments from both felsic and mafic rock sericite-quartz schists were largely sourced from felsic rocks with in-
sources (Table 6) indicating mixed felsic and mafic sources. All the volvements from mixed mafic and felsic regions beside ancient recycled 
samples showed negative Europium anomalies, an enrichment in LREE components. The felsic material could also have evolved from the frac-
and fractionated HREE patterns (Fig. 8). These characteristics, notably tionation of a mafic melt. This interpretation of combined provenance is 
the negative Europium anomalies point to a differentiated photoliths also supported by detrital zircon which yielded three different ages 
(McLennan et al., 1993; Asiedu et al., 2000). It is known that siliciclastic (993–597 Ma (Neoproterozoic), 1598–1006 Ma (Mesoproterozoic) and 
rocks with Cr/Th values of between 2.5 and 17.5 as well as Eu/Eu* (2225–1704 Ma (Paleoproterozoic) in the schists and quartzites of the 
values varying from 0.48 to 0.78, are frequently from felsic sources ASU (Ganade et al., 2016). 
(Cullers, 1994). Thus, all Eu/Eu* as well average Cr/Th values (Table 6) From the above deduction, it appears that the quartzite and schist 
for the studied rocks fall within the felsic source domain. rocks of the ASU mainly derived from felsic sources (probably granit-
On the Hf against La/Th plot (Fig. 11b) which can also discriminate oids) with mafic source rocks (basalt) contributions. 
the nature of source rocks and different arc constituents (Floyd and 
Leveridge, 1987), the studied rocks are deduced to have originated from 5.4. Source area location 
felsic sources with some involvement from mixed felsic and mafic 
provenance. In fact, mainly the schistose rocks and a quartzite (24 CE) Having deduced a principally granitic materials with involvements 
samples have high Hf values (4.5–9.6 ppm) and low La/Th contents from mafic regions derivation for the precursors of the studied ASU 
(1.92–4.39) compared to the other quartzite samples; this suggests that metasediments, it seems useful to discriminate these source regions. The 
most of sediments originated from felsic rock sources with some impli- metasediments of ASU are interpreted as metamorphic and tectonic 
cations from the recycling of ancient sediment. Three quartzite samples lateral equivalents of the sediments of Bombouaka Group of Volta Basin. 
have low Hf concentrations (1.4–2.5 ppm) and slightly high La/Th ratios (Affaton, 1990; Affaton et al., 1991). Thus, the Bombouaka Group 
(5.13–9.75) indicating a combination of mafic and felsic source areas, sedimentary rocks and the ASU metasedimentary rocks might have a 
and probably implicating subduction of oceanic crust on continental common source area or at least a felsic rocks source. Ganade et al. 
margin (Floyd et al., 1989). (2016) support this inference using compatible detrital zircon signatures 
One of the sample (36 CE, schist) plotting outside these fields may between the ASU and Volta Basin rocks. Also, (a) given that the Volta 
probably be a recycled sediment (Fig. 11b). It contains slightly high La/ basin thrusts to the East of the WAC while the ASU is thrusted by the 
Table 7 
Comparison of some trace elements ratios of the ASU metasedimentary rocks to those of the Boumbouaka Group and Birimian metasedimentary rocks.  
Elements ratios Range of metasediments from ASU a Range of Neoproterozoic Bombouaka sandstones b Range of Paleoproterozoic 
Birimian sedimentary rocks c 
Quartzites Schists Micaschists 
Th/Cr 0.04–0.18 0.03–0.34 0.16–1.27 0.05–0.44 0.026–0.061 
Th/Co 0.64–1.77 0.91–3.38 0.83–4.37 0.52–8.26 0.16–0.35 
La/Co 4.12–9.08 4.41–8.98 2.27–14.68 1.46–28.1 0.33–2.33 
La/Th 1.77–9.75 2.58–9.84 1.92–5.87 1.93–14.4 2.08–8.87 
Th/Sc 0.33–1.30 0.39–1.02 0.75–1.80 0.73–6.24 0.09–0.47 
La/Sc 1.90–6.20 1.41–3.85 2.44–6.57 2.07–28.1 0.38–2.71 
Eu/Eu* 0.67–0.77 0.66–080 0.57–0.75 0.50–0.71 0.71–0.97  
a This study. 
b Anani et al., 2017. 
c Asiedu et al., 2004 & 2017. 
13
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Fig. 12. Plot of the ASU metasediments data on various diagrams using major oxides and trace elements contents such as (a) CaO–Na2O–K2O ternary diagram after 
Toulkeridis et al. (1999); (b) Th–Co–Zr/10 ternary diagram; (c) La–Th-Sc ternary diagram and (d) Th-Sc-Zr/10 ternary diagram after Bhatia and Crook (1986). 
[Circle red: quartzites; circle black: schist and micaschist]. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web 
version of this article.) 
basement nappes, and (b) that the studied metasedimentary rocks yiel- can have a same source area and igneous provenance. This under-
ded Pan-African age (662–579 Ma; Attoh et al., 1997; Ganade et al., standing is enhanced by similar detrital zircon ages yielded in both the 
2016), the only older rocks displayed in West Africa around the basin sedimentary rocks of Bombouaka Group and the ASU metasedimentary 
and ASU and able to provide detritus to the Bombouaka Group/ASU are rocks with 993–2225 Ma (ASU; Ganade et al., 2016) and 1100–2200 Ma 
Paleoproterozoic rocks (2000–2200 Ma; Abouchami et al., 1990; Leube (Bombouaka Group; Kalsbeek et al., 2008) which overlap that of the 
et al., 1990; Taylor et al., 1992; Davis et al., 1994), and migmatitic and flanking Birimian Paleoproterozoic basement (2000–2200 Ma). 
granitic gneisses and associated rocks of the basement nappes (basement Contrary to the geochemical similarities noticed between the 
rocks). Bombouaka Group sedimentary rocks and those analysed, the 
To investigate these hypothesis a comparison between some sensitive geochemical characteristics of the ASU rocks are expressively dissimilar 
trace elements ratios of the studied ASU rocks and those of the Boum- to those of the Birimian metasedimentary rocks reported by Asiedu et al. 
bouaka Group (Neoproterozoic) and Birimian metasedimentary rocks (2004 & 2017). For example, the Eu/Eu* of these Birimian metasedi-
were performed (Table 7) since the high concentrations of least frac- mentary rocks are larger than those of ASU metasedimentary rocks. 
tionated REEs, Sc and Th in the sediments are linked to that of their Likewise, the Th/Sc, La/Sc, La/Co, Th/Co and Th/Cr values of the 
source (McLennan et al., 1980). The geochemical proprieties of the studied ASU rocks are considerably higher than those of the Birimian 
studied ASU rocks are analogous to geochemical particularities of the metasedimentary rocks documented by Asiedu and his collaborators in 
Bombouaka sandstones documented by Anani et al. (2017). Their Th/Cr, 2004 and 2017. Consequently, the Birimian rocks, susceptible to be 
La/Co, La/Sc, Th/Co, Th/Sc ratios and europium anomalies values are exposed for erosion and fed the Volta basin (indirectly the ASU) with 
similar (Table 7). Further, both show Na2O and CaO depletion, and sediments, are improbable to be the main source of the ASU metasedi-
K2O-rich concentrations higher than Upper Continental Crustal values. mentary rocks. Hence, the proximal granitoids from the basement 
These various geochemical comparisons suggest that the ASU meta- nappes are the most likely candidates. Furthermore, the rocks of the 
sedimentary rocks and the sedimentary rocks of the Bombouaka Group Bombouaka Group and those of Togo Structural Unit, all lateral 
14
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Table 8 
Sensitive parameters discriminator of tectonic setting (Bhatia and Crook, 1986).  
Tectonic setting La Ce ΣREE La/Yb (La/Yb)N ΣLREE/ΣHREE Eu/Eu* 
CIAa 27 ± 4.5 59 ± 8.2 146 ± 20 11 ± 3.6 7.5 ± 2.5 7.7 ± 1.7 0.79 ± 0.13 
OIAa 8 ± 1.7 19 ± 3.7 58 ± 10 4.2 ± 1.3 4.2 ± 1.3 3.8 ± 0.9 1.01 ± 0.11 
PMa 39 85 210 15.9 10.8 8.5 0.56 
ACMa 37 78 186 12.5 8.5 9.1 0.6 
Quartzitesb 04 CE 21 38.5 94.34 18.75 12.64 10.96 0.77 
06 CE 12.4 22.1 56.94 19.38 13.06 10.48 0.67 
10 CE 7.8 13.5 33.8 19.5 14.05 10.23 0.79 
12 CE 11.8 21.1 54.67 16.86 11.36 8.94 0.76 
19 CE 7.5 12.4 32.78 20.83 11.80 9.75 0.74 
24 CE 21.5 41.2 99.61 14.33 9.66 9.08 0.73 
25 CE 7.7 12.9 33.92 18.33 12.36 9.47 0.76 
31 CE 16.6 29.3 99.61 18.24 12.30 9.08 0.73 
32 CE 7.0 3.4 73.58 10.94 12.64 10.01 0.77 
Micaschistsb AD9 19.7 43.7 99.81 13.78 9.29 8.76 0.57 
11 CE 27.9 54.6 143.09 9.59 6.46 5.60 0.74 
15 CE 29.9 57.5 147.44 11.12 7.49 6.27 0.75 
AD25a 39 120.6 234.25 13.04 8.79 11.28 0.73 
27 CE 22.3 21.3 80.87 18.43 12.43 6.98 0.68 
Schistsb 18 CE 60.8 125.4 300.47 11.34 7.65 7.02 0.66 
AD19a 18.3 34.9 94.1 7.18 4.84 4.72 0.80 
36 CE 92.5 167.4 501.72 16.46 11.10 7.22 0.70  
a Sandstones from different tectonic setting (Bhatia, 1985). 
b This study. 
equivalent of ASU rocks, revealed zircon ages of 1000–1300 Ma (Kals- Furthermore, the most sensitive discriminators of the tectonic 
beek et al., 2008). This age tranche are characteristic of rocks sourced context (Bhatia and Crook, 1986), Sc, Hf, Th, Zr, and ΣREE concentra-
from outside the WAC, likely from the Amazonian Craton (Kalsbeek tion, Eu/Eu* and Zr/Hf ratio, with means of 10.67 ppm, 7.63 ppm, 5.03 
et al., 2008). Considering the parallel ages of deposition and the lateral ppm, 191.93 ppm, 158.94 ppm, 0.73 and 37.9, respectively, are similar 
coherence between the ASU, TSU and sequences of Bombouaka, the to those recorded in the sedimentary rocks of continental island arc 
protolith of the ASU metasedimentary rocks might also source from the (Table 8). Inversely, other indicators, such as Zn, Nd, La, Ce, V, Co Nb, 
Amazonian Craton. The rocks of Bombouaka Group and those of the ASU Th/U, Rb/Sr and La/Y with averages of 10.5 ppm, 31.09 ppm, 31.43 
yielded detrital zircon ages analogous with the adjoining WAC which is ppm, 62.36 ppm, 32.92 ppm, 5.72 ppm, 7.13 ppm, 6.04, 1.83 and 1.39, 
similar to that of the Martinópole in North-east Brazil situated within the respectively are close to those documented in sedimentary rocks of 
Amazonian Craton, referred as a perfect equivalence of the ASU Ganade passive margin environment (Tables 4, 5 and 8; Bhatia and Crook, 
et al. (2016). 1986). In addition, Zr/Th (28.93), La/Yb (14.18) and La/Yb normalized 
In sum, considering the above geochemical analysis results, previous (9.56) approximate the two different tectonic settings. In sum, the above 
data of Zircon ages and various correlations between the units, we different discrimination parameters and diagrams indicate features of 
propose that the ASU metasedimentary rocks resulted from sedimentary both PM and CIA tectonic setting for the ASU metasedimentary rocks, 
rocks which might be originated from the migmatitic and granitic but with a dominance of passive margin materials in the source. Even-
gneissic rocks of the basement nappes and/or rocks from the Amazonian tually, the rocks that plotted in CIA field, are built on either ‘thin con-
Craton. tinental margin or on well-developed continental crust’ with a source of 
a “dissected magmatic arc-recycled orogen” (Bhatia, 1983; Bhatia and 
Crook, 1986). However, the sample which plotted in OIA is probably due 
5.5. Tectonic setting to a contaminant introduced during the preparation of samples. 
Previous works (Affaton et al., 1980; Trompette, 1994; Ganade et al., 
According to Verma and Armstrong-Altrin (2016), siliciclastic sedi- 2016) in the Volta Basin and Atacora Structural Unit have interpreted 
ments can be classified considering the tectonic environment of their the tectonic context of the regions as passive margin deposits similarly to 
source rocks since the composition of the sediemnts is predominated by those of Amedjoe et al. (2018) that concluded passive margin settings for 
composition of the provenance area which is furthermore controlled by Volta Basin sediments with painting of continental island arc settings 
the tectonic context. This contribution attempts to constrain the tectonic materials. Though the geochemistry of sediments does not constrain age 
context of the analysed ASU metasedimentary rocks using the approach of CIA sediments, it seems that, mature CIA basement residues from 
of Bhatia and Crook (1986) and Toulkeridis et al. (1999). cropping rocks at the Passive Margin location may have played on the 
On the calc-alkaline plot (CaO–Na2O–K2O) proposed by Toulkeridis sediments chemistry (Amedjoe et al., 2018). Consequently, the tectonic 
et al. (1999), all the studied samples shown characteristic of being context metasedimentary rocks of the Atacora Structural Unit point out 
within the passive margin (PM) field (Fig. 12a). Moreover, based on characteristics of both Continental Island arc and passive margin with 
some characteristics, including constancy and uniformity of some ele- predominant of passive margin materials. 
ments (La, Sc, Co, Th and Zr) of sediments during the post depositional 
process, Bhatia and Crook (1986) suggested several types of diagrams to 6. Conclusion 
discriminate tectonic regime. In Fig. 12b showing the ternary model of 
Th–Co–Zr/10, the analysed rocks predominantly plot within the PM Petrography and geochemical studies were performed to constrain 
field, but two samples plotted in the CIA field. On the Th–La-Sc diagram the paleoweathering, tectonic setting and provenance of quartzites, 
the rocks plotted largely within the continental margin field (Fig. 12c), mica schists and chlorite-sericite-quartz schist succession of the Atacora 
while six samples are situated close to it but in the CIA field. As showed structural unit within the Natitingou area. From the obtained data, the 
on the ternary diagram of Th-Sc-Zr/10 (Fig. 12d), almost all of the ASU following four key conclusions emerge: 
rocks plotted within or inside the vicinity of the CIA area, with two of Firstly, petrography characteristics show that the studied rocks are 
them within the PM field and one plot in OIA. 
15
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quartz-rich with a moderate proportion of K-feldspar, which is also craton paleoproterozoic margin reactivated during the pan-african collision. 
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