# Structural architecture and metamorphism of the Mayombe Chain and Niari Basin (West Congo Belt) in Congo Brazzaville

Thomas Fullgraf, Yannick Callec, Hugues Bauer, Florent Boudzoumou, Nicolas Charles, Eric Gloaguen, Benoit Issautier, Sage Tsoumou Kebi, Benjamin Le Bayon, Joël Le Metour, et al.

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## To cite this version:

Thomas Fullgraf, Yannick Callec, Hugues Bauer, Florent Boudzoumou, Nicolas Charles, et al. Structural architecture and metamorphism of the Mayombe Chain and Niari Basin (West Congo Belt) in Congo Brazzaville. Journal of African Earth Sciences, 2024, pp.105369. 10.1016/j.jafrearsci.2024.105369. hal-04694669

HAL Id: hal-04694669

https://brgm.hal.science/hal-04694669v1

Submitted on 26 Sep 2024

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# Structural architecture and metamorphism of the Mayombe Chain and Niari Basin (West Congo Belt) in Congo Brazzaville

Thomas Fullgraf<sup>a, *</sup>, Yannick Callec<sup>a</sup>, Hugues Bauer<sup>a</sup>, Florent Boudzoumou<sup>b</sup>, Nicolas Charles<sup>a</sup>, Eric Gloaguen<sup>a</sup>, Benoit Issautier<sup>a</sup>, Sage Tsoumou Kebi<sup>c</sup>, Benjamin Le Bayon<sup>a</sup>, Joël Le Metour<sup>a</sup>, Fabien Paquet<sup>a</sup>, Sergey Sergeev<sup>d</sup>, Klaus Wemmer<sup>e</sup>

## ARTICLE INFO

Handling Editor: DR Damien Delvaux

**Keywords:**

West Congo Belt
Eburnean
Pan-African
Structure
Metamorphism
Geochronology

## ABSTRACT

Geological mapping of the Mayombe Chain and Niari Basin of Congo Brazzaville allows for the first time defining the structural architecture and metamorphism of the West Congo Belt. Four different tectono-metamorphic domains, separated by crustal-scale shear zones, are now distinguished (Niari Basin (NB), Eastern (EMC), Central (CMC) and Western (WMC) Mayombe Chain).

The NB is marked by only weak regional deformation under middle to upper diagenetic conditions. It is delimited in the west from the EMC by the Mount Belo Shear Zone forming the terminal thrust system of the West Congo orogen.

The tectonic style in the EMC is characterized by discrete, widely-spaced low-angle thrusts, reverse faults and strike-slip faults resulting in the formation of duplex and/or positive flower structures. Off these high-strain zones, the rocks are folded into gentle syn- and anticlines. Penetrative schistosity starts in shales in the western part. The metamorphism increases from eastern anchizational conditions to lower greenschist facies in the west. The EMC is juxtaposed along the Moukondo thrust/back-thrust system with the CMC.

The CMC is typified by open to closed upright to NE-verging folds, S₁ schistosity with moderate to steep SW dips, onset of regional crenulation cleavage (S₂), frequent reverse thrusts and numerous faults. Metamorphic conditions remain in the greenschist facies. The Loukenéné-Mandji Thrust marks the CMC-WMC contact and coincides with a jump in metamorphic grade marked by biotite-in.

The WMC consists of Palaeoproterozoic basement stacked with Neoproterozoic rocks. Autochthonous Palaeoproterozoic gneiss and schist record Late Eburnean sedimentation, magmatism and metamorphism between 2110 and 1970 Ma, which are compared with the Eburnean history in Gabon and the Transamazonian orogeny in Brazil. The allochthonous Bikossi Group was thrust during the Pan-African event from the west over Tonian metavolcaniclastic and plutonic rocks before further folding and stacking of both units. The intensity of Pan-African deformation increases from open to closed folds with spaced cleavage in the southeast of the WMC to thrust-dominated tectonics in the northwest, where the Palaeo- and Neoproterozoic rocks are transposed into parallelism with the pronounced schistosity.

Geochronology of illite and muscovite documents two Pan-African events at 590-570 Ma (M1) and at 520-500 Ma (M2) that are related to the main collisional and late thermal events in the Araçuai-Ribeira Belt in Brazil. Metamorphic isogrades shifted from M1 to M2 for more than 30 km to the west. Detrital mica and metamorphic illite of the Mpioka Group record M1 and M2, respectively constraining sedimentary deposition between 570 and 520 Ma, which implies the interpretation of the group as molasse of the West Congo Belt.

<footnote><sup>a</sup> Bureau de Recherches Géologiques et Minières (BRGM), 3 ave C. Guillemin, PO 36009, 45060, Orléans, Cedex, 2, France</footnote>

<footnote><sup>b</sup> Université of Mariam Ngouabi, Brazzaville, Congo</footnote>

<footnote><sup>c</sup> Université of Rennes, France</footnote>

<footnote><sup>d</sup> A.P. Karpinsky Russian Geological Research Institute, St. Petersburg, Russia</footnote>

<footnote><sup>e</sup> University of Göttingen - Geoscience Centre, Goldschmidstr. 3, 37077, Göttingen, Germany</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail addresses: t.fullgraf@brgm.fr (T. Fullgraf), y.callec@brgm.fr (Y. Callec), h.bauer@brgm.fr (H. Bauer), florentboudzoumou@yahoo.fr (F. Boudzoumou), ncharles@brgm.fr (N. Charles), e.gloaguen@brgm.fr (E. Gloaguen), b.issautier@brgm.fr (B. Issautier), sagekebi@yahoo.fr (S.T. Kebi), b.lebayon@brgm.fr (B. Le Bayon), j.lemetour@yahoo.fr (J. Le Metour), f.paquet@brgm.fr (F. Paquet), Sergey_Sergeev@vsegei.ru (S. Sergeev), kwemmer@gwdg.de (K. Wemmer).</footnote>

<footnote>https://doi.org/10.1016/j.jafrearsci.2024.105369</footnote>

<footnote>Received 14 March 2024; Received in revised form 3 August 2024; Accepted 5 August 2024</footnote>

<footnote>Available online 20 August 2024</footnote>

<footnote>1464-343X/© 2024 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).</footnote>
# 1. Introduction

The geology of the Republic of Congo has to date never attracted much attention from the international science community even though it tracks earth's history back to the Archaean era and is complementary to the geology of the coeval Ribeira/Araçuai belts and the São Francisco craton on the Brazilian side. This is due to a number of geographical and political factors such as difficult access to remote areas which are often swampy or covered by tropical forest (with generally poor outcrops exposing strongly weathered rocks), the lack of roads and developed infrastructures as hospitals, railways and airports across the country, a poorly-equipped geological department at the University of Mariam Ngouabi in Brazzaville and also some political instabilities during the past years in the south of the country (Pool area and along the Cabinda border). The results of detailed geological studies in the Coastal Basin and offshore by oil companies and for phosphate remain confidential. Mineral exploration in the interior of the country focused on the feasibility of various Archaean BIF deposits for iron mining (Mayoko, Zanaga, and Mount Avima) and on the copper or gold potential of the Neoproterozoic Niari basin and Mayombe Chain. However, aspects of regional geology were generally neglected and geological mapping just a by-product of exploration. Few geological studies from 1980 onwards give a first insight into the structure, age and composition of the Mayombe Chain but needed update by modern analytical techniques

and also re-interpretation. Therefore, a national geological mapping program was initiated by the Ministry of Mines in 2011 to improve the knowledge on the country's geology and mining potential, which in turn shall attract investors of exploration and mining industry.

The first part of the geological mapping program was carried out by BRGM geologists in southwest Congo from 2012-13. It resulted in a set of five 1:200,000 scale and one 1:100,000 scale map(s) that cover the western and central part of the country (Fig. 1).

The maps are accompanied by comprehensive sheet explanations (in French) documenting the stratigraphy, petrography, structures, geochemistry, geochronology, mineral potential and geodynamic setting of investigated rocks (Bauer et al., 2015a, b; Callec et al., 2015a, b; Charles et al., 2015; Fullgraf et al. 2015; Le Bayon et al., 2015a, b). However, to reach a wider readership, the main results will be given in a series of publications dealing with specific geological domains instead of covering the complete geological record of a selected area (i.e. of a map sheet). Here, we present the structural architecture and metamorphism of the Mayombe Chain and Niari Basin and compare it with the evolution of the corresponding terranes in Gabon, Democratic Republic of Congo and Brazil. The revised Neoproterozoic stratigraphy and basin evolution will be presented in a second publication and is therefore only briefly summarised here. The focus of the paper is on the field description of the rocks to demonstrate that geological mapping remains a fundamental task in the geosciences as a primary source for knowledge.

<figure><img src="image_4.png" /><figcaption>**Fig. 1.** The main geological domains of Congo-Brazzaville and the mapping program of the National Plan for the Development of the Mining Sector. The position of the country in Africa is shown in the upper-left overview map.</figcaption></figure>
## 1.1. Research history

Geological mapping of the Western Congo Belt started in the framework of mineral exploration from Gabon through Congo-Brazzaville to the Democratic Republic of Congo (Babet, 1932; Lepersonne, 1951; Nickles and et Rourcq, 1952; Tkatchenko, 1954; Cosson, 1955; Gérard, 1958; Nicolini, 1959; Bigotte, 1959; Dadet, 1963, 1964; Scolari, 1965; Scolari and Van Daalhoff, 1965). The general stratigraphic terminology has hardly evolved since Gérard (1958) due to the marked lithological contrasts that exist between the formations in the entire area. Several marker formations, such as the glaciogenic deposits of the Lower and Upper Diamictite, can be correlated across the Western Congo Belt, and along the perimeter of the central Congo Basin (Delpomdor and Préat, 2015). The latest stratigraphic correlations combine lithologic and chemostratigraphic variations and resulted in comparative lithostratigraphic diagrams for all Neoproterozoic terrains of Central Africa (Tack et al., 2001; Frimmel et al., 2006, 2011; Cailteux et al., 2007; Delpomdor and Préat, 2013, 2015; Affaton et al., 2016). However, they remain in many details poorly constrained because of only few geochronological data and lack of detailed mapping. The lithostratigraphic nomenclatures differ significantly from one sector to another even though the main boundaries between the units remain unchanged (Table 1).

The first geological map and stratigraphy of the Mayombe Chain in its confines of the Republic of Congo was presented by Cosson (1955). The author distinguished the lower Mayombe System comprising the Loémé, Bikossi and Loukoula Series, the central Mount Bamba System with the lower Mvouti and upper Mossouva Series and the West Congo System with the Lower Tillite, Louila/Bouenza, Upper Tillite, Schisto-calcaire and Schisto-gréseux series (Table 1).

The boundaries between the three main systems were classified as major tectono-stratigraphic unconformities. An additional major unconformity was located by Devigne (1959) between the Bikossi and Loukoula Series, who combined the Bikossi and Loémé Series into the so-called Pre-Mayombien System. A completely different schema was presented by Dadet (1965, 1966, 1969), who merged all systems into a single major orogenic cycle called the West Congolese Belt. This model was further elaborated by Hossie (1980) who, based on structural analysis, divided the Mayombe Chain into an internal domain, characterised by polyphase metamorphism (Loémé or Guena), an intermediate domain in the amphibolite facies (Bikossi and Loukoula) and an external domain characterised by greenschist facies conditions (Mvouti and Mossouva).

Based on gravimetric reconnaissance surveys (Byamungu et al., 1987) it was demonstrated that i.) Guena basement rocks were thrust to the east and ii.) Bikossi and Ncessé series are allochthonous. The study of the Les Saras granodiorite by Boni (1990) showed that the intrusion has a calc-alkaline composition, is of Eburnean age (2000 ± 80 Ma) and was overprinted by two stages of deformation under P/T conditions of 475–550 °C and about 5 kb related to the Pan-African orogeny. However, the age of emplacement was at odds with the younger (Neoproterozoic age) of the country rock (Ncessé Series) and two possible solutions were proposed: i) the interpretation of the Les Saras Granodiorite as thrust unit, or ii) the Eburnean age of the Ncessé Series, but this was considered unlikely given the differences in composition and tectonic fabrics compared to the western Guena Block. Multi-isotope studies by Djama (1988) and Djama et al. (1992) gave an Eburnean emplacement age (2014 ± 56 Ma) for the Guena gneiss and a Late Mesoproterozoic age (1050 ± 25 Ma) for the Mfoubou granite, while Sm-Nd T<sub>DM</sub> model ages between 2.6 and 3.0 Ga support an Archaean protolith for both intrusions.

The general stratigraphy of the overlying Neoproterozoic to Palaeozoic Mvouti to Schisto-gréseux series has been much less controversial and remained almost unchanged over the years (Table 1). Most of the discussions concerned the definition of the stratigraphic boundaries and the nature of the contacts.

## 2. Methods

### 2.1. Geological mapping

Geological mapping and sampling were carried out in 2012–2013 by five teams in two campaigns, each lasting about six weeks. Due to intense tropical weathering, outcrops are limited mostly to rivers and areas of rugged topography as well as the newly constructed National Road from Pointe Noire to Dolisie, which provides an excellent profile of 60 km length across the Mayombe range. The area was accessed by 4x4 vehicle and on-foot using flying camps. Planning of field traverses took into account archival data and remote sensing analysis. Fig. 2 shows the location of 5250 field observation points indicating the accuracy of the geological map at different locations.

### 2.2. Laboratory analysis

#### 2.2.1. U-Pb zircon SHRIMP, VSEGEI, St Petersburg, Russia

SHRIMP-II ion microprobe at the Centre for Isotopic Research (VSEGEI, St. Petersburg, Russia). Zircon grains were hand selected and mounted in epoxy resin, together with chips of the TEMORA (Middledale Gabbroic Diorite, New South Wales, Australia, age = 417 Ma, Black and Kamo, 2003) and 91500 (Geostandard zircon, age = 1065 Ma, Wiedenbeck et al., 1995) reference zircons. The grains were cut approximately in half and polished. Each analysis consisted of five scans through the mass range; the spot diameter was about 18 µm and the primary beam intensity about 4 nA. The data were reduced in a manner similar as described by Williams et al. (1999) and references therein, using the SQUID Excel Macro of Ludwig (2000). The Pb/U ratios were normalised relative to a value of 0.0668 for the ²⁰⁶Pb/²³⁸U ratio of the TEMORA zircon, corresponding to an age of 416.75 Ma (Black and Kamo, 2003). The uncertainties reported for the individual analyses (ratios and ages) in Table 2 are at the 1sigma level, whereas uncertainties in calculated Concordia ages are reported at the 2sigma level. Th/U ratios of all zircon grains are always higher than 0.4, suggesting their magmatic origin.

#### 2.2.2. K-Ar mass spectrometry, illite crystallinity, GZG, Göttingen, Germany

Slate, siltstones, shales and sericite schist of 2–3 kg were sampled to separate the fine mineral fractions <2 µm and <0.2 µm to determine both their illite crystallinity (IC) and K-Ar age. Illite crystallinity analysis and K-Ar geochronology was performed at the Geowissenschaftliches Zentrum of University Göttingen (GZG), Germany. The rocks were crushed in a shatter mill (vibration mill) for about 20 s and divided into the size fractions >63 µm and <63 µm. The <63 µm fraction was used to recover the <2 µm clay fraction by settling in Atterberg cylinders. The <0.2 µm fraction was separated from the <2 µm fraction using an ultracentrifuge. Both fractions were analysed by XRD to determine their mineralogical composition and the IC. The IC is defined as the peak width at half the height of the 10 Å peak and was calculated using a computer program developed at the GZG. Digital measurement of the IC was carried out by step scan (301 points, 7–10° (2θ), scan step 0.010° (2θ), integration time 4 s, receiving slit 0.1 mm, automatic divergence slit). IC values are given in (Δ°2θ). For samples from which only small amounts of the fine fractions were available for XRD analysis, an internally developed “minipreparation” was used. The resulting systematic large shifts of the peaks could not be compensated for by the software, so the analysis was performed manually. Iron oxides are probably much more frequent than shown in the table as is chlorite whose peaks match those of kaolinite. The diagenesis/anchizone and anchizone/epizone boundaries are 0.600°–0.400° and 0.240° (Δ°2θ), respectively. These limits were verified with the interlaboratory standardisation program of Warr and Rice (1994). The reported IC values are nearly identical to the crystallinity index standard (CIS) and were determined using the machine parameters recommended by Kisch
<caption>Table 1</caption>

Previous stratigraphic systems of the West Congo belt. Mt = Mount, inf. = inferior, sup. = upper (Dévigne, 1959; Thiéblemont et al., 2009; Cosson (1955); Scolari and Van Daalhoff, 1965; Hossié, 1980; Cahen (1978); Stanton et al., 1963; Vellutini et al., 1983).

<table><thead><tr><th rowspan="3"></th><th colspan="3">GABON</th><th colspan="6">CONGO BRAZZAVILLE</th><th colspan="3">DRC</th><th>ANGOLA</th></tr><tr><th colspan="2">Devigne, 1959</th><th>Thieblemeont et al. 2009</th><th colspan="2">Cosson, 1955</th><th colspan="2">Vincent, 1961</th><th colspan="2">Scolari et al., 1965</th><th colspan="2">Dadet, 1965, 1966, 1969</th><th colspan="2">Hossie, 1980</th><th colspan="2">Vellutini et al., 1983</th><th colspan="2">Cahen, 1978</th><th colspan="2">Stanton et al., 1963</th></tr><tr><th>Système du Congo Occidental</th><th>Système du Congo Occidental</th><th>Système du Congo Occidental</th><th>Système du Congo Occidental</th><th>Système du Congo Occidental</th><th>Système du Congo Occidental</th><th>Système du Congo Occidental</th><th>Système du Congo Occidental</th><th>Système du Congo Occidental</th><th>Système du Congo Occidental</th><th>Système du Congo Occidental</th><th>Système du Congo Occidental</th><th>Système du Congo Occidental</th><th>Système du Congo Occidental</th><th>Système du Congo Occidental</th><th>Système du Congo Occidental</th><th>Système du Congo Occidental</th><th>Système du Congo Occidental</th><th>Système du Congo Occidental</th></tr></thead><tbody><tr><td rowspan="6">Système du Pre-Mayombiéen</td><td rowspan="2">Série de la Douigni</td><td>Série de la Bamba</td><td>Série de la Mbeia</td><td rowspan="2">Série de la Mossouva</td><td rowspan="2">Série de la Mvouti</td><td rowspan="2">Série de la Loukoula</td><td rowspan="2">Série de la Loukoula Sup.</td><td rowspan="2">Série de la Loukoula Inf.</td><td rowspan="2">Série de la Loukoula</td><td rowspan="2">Série de la Loukoula Inf.</td><td rowspan="2">Série de la Loukoula</td><td rowspan="2">Série de la Loukoula</td><td rowspan="2">Série de la Loukoula</td><td rowspan="2">Série de la Loukoula</td><td rowspan="2">Série de la Loukoula</td><td rowspan="2">Série de la Loukoula</td><td rowspan="2">Série de la Loukoula</td><td rowspan="2">Série de la Loukoula</td><td rowspan="2">Série de la Loukoula</td><td rowspan="2">Série de la Loukoula</td></tr><tr><td>Série de la Sounda</td><td>Série de la Banza</td><td>Groupe de Loukoula</td><td>Groupe métamorphique de Loukoula</td><td>o Magmatisme Mayombien</td><td>o Ceinture plutonique éburnéen</td><td>o Bassin syn-orogénique</td><td>o Bassin anté-orogénique</td><td>o Domaine gneisso-migmatique</td><td>Série de la Loéme</td><td>Série de la Loéme</td><td>Série de la Loéme</td><td>Série de la Loéme</td><td>Série de la Loéme</td><td>Série de la Loéme</td><td>Série de la Loéme</td><td>Série de la Loéme</td><td>Série de la Loéme</td></tr><tr><td rowspan="2">Série de la Kouboula</td><td>Série de la Loukoula</td><td>Série de la Bikossi Sup.</td><td>Série de la Bikossi Inf.</td><td>Série de la Bikossi Inf.</td><td>Série de la Bikossi Inf.</td><td>Série de la Bikossi Inf.</td><td>Série de la Bikossi Inf.</td><td>Série de la Bikossi Inf.</td><td>Série de la Bikossi Inf.</td><td>Série de la Bikossi Inf.</td><td>Série de la Bikossi Inf.</td><td>Série de la Bikossi Inf.</td><td>Série de la Bikossi Inf.</td><td>Série de la Bikossi Inf.</td><td>Série de la Bikossi Inf.</td><td>Série de la Bikossi Inf.</td><td>Série de la Bikossi Inf.</td><td>Série de la Bikossi Inf.</td><td>Série de la Bikossi Inf.</td></tr><tr><td>Série de la Loéme</td><td>Série de la Loéme</td><td>Série de la Loéme</td><td>Série de la Loéme</td><td>Série de la Loéme</td><td>Série de la Loéme</td><td>Série de la Loéme</td><td>Série de la Loéme</td><td>Série de la Loéme</td><td>Série de la Loéme</td><td>Série de la Loéme</td><td>Série de la Loéme</td><td>Série de la Loéme</td><td>Série de la Loéme</td><td>Série de la Loéme</td><td>Série de la Loéme</td><td>Série de la Loéme</td><td>Série de la Loéme</td><td>Série de la Loéme</td></tr></tbody></table>
<figure><img src="image_5.png" /><figcaption>Fig. 2. The position of 5250 field observation points recorded in the study area, the main infrastructure, and the main geological units of the study area.</figcaption></figure>

(1991). Argon isotopic composition was measured in a Pyrex glass extraction and purification line connected to a VG 1200 C noble gas mass spectrometer operating in static mode. The amount of radiogenic ⁴⁰Ar was determined by the isotope dilution method using a highly enriched ³⁸Ar spike from Schumacher, Bern (Schumacher, 1975). The spike was calibrated against the biotite standard HD-B1 (Fuhrmann et al., 1987). The age calculations are based on the constants recommended by the IUGS, cited in Steiger and Jäger (1977).

### 2.2.2.1. Illite crystallinity (KI) and K-Ar geochronology.
Since the early studies by Weaver (1960) and Kübler (1968), it is known that the degree of illite crystallinity (expressed as Kübler Index, KI) depends mainly on temperature conditions, but there is also an additional relationship to deformation. According to Warr (2018) the following assignment to mineral facies is appropriate: diagenetic zone to zeolite facies; anchizone to the prehnite–pumpellyite facies and the pumpellyite–actinolite facies; epizone to the greenschist facies (Fig. 3). The IC technique is considered a semi-quantitative, empirical or statistical technique that gives best results in the range from low diagenesis to very low-grade metamorphism (anchizone), but can also be used for very low-grade metamorphism (epizone). Problems with sample preparation and interpretation and standardisation of the IC values have been discussed since the introduction of this method (i.e., Kisch, 1991; Warr and Rice, 1994; Warr, 2014, 2018). The geochronology of Pan-African metamorphic events is an interesting by-product of this method and provides clues to possible links with the adjacent circum-Atlantic Pan-African orogens of the Brasilia, Araçuai and Kaoko-Ribeira belts.

## 3. Revised stratigraphy

The revised stratigraphy for the Mayombe Chain and Niari Basin is presented in detail by Fullgraf et al., (2015) (Table 2) while the simplified revised geological map and cross section are shown in Fig. 4. Only the main points important for understanding of the tectono-metamorphic history are summarised here.

### 3.1. Palaeoproterozoic era

All rocks of Palaeoproterozoic (Eburnean) age are now combined into the Loémé Supergroup; the unit is further subdivided into the autochthonous Loukoula Group intruded by the calc-alkaline Bilinga Suite, and the allochthonous Bikossi Group. The maximum age of sedimentation and high-grade metamorphism of the migmatitic paragneiss is constrained by U-Pb SHRIMP analyses of detrital zircons to 2111-2064 Ma (Table 3, Fullgraf et al., 2015). Late Eburnean U-Pb zircon SHRIMP ages between 2028 and 2060 Ma have been determined on orthogneisses of tonalitic to granitic composition from the Bilala, Bilinga and Les Saras quarries which, together with similar geochemistry and Neo-Archaean NdT<sub>DM</sub> model ages, constrains their common origin in the late stage of the Eburnean orogeny (Djama, 1988; Djama et al., 1992; Fullgraf et al., 2015; Affaton et al., 2016; Bouenitela, 2019). These intrusions are now combined into the Bilinga Suite. Deposition of the Bikossi Group post-dates Eburnean plutonism and pre-dates Tonian magmatism. U-Pb SHRIMP and LA-ICP-MS analysis of detrital zircons from basal quartzite constrains maximum deposition ages of 2043 ± 13 Ma (Fullgraf et al., 2015) and 1971 ± 40 Ma, respectively (Affaton et al., 2016) being indicative for the erosion of Eburnean middle crust (Table 3) and the interpretation as a molasse.
<caption>**Table 2**</caption>

Revised stratigraphy (down to subgroup level) together with the main tectono-metamorphic events and geodynamic settings for the West Congo belt integrating previous models with new regional mapping, geochemistry and geochronology.

<table><thead><tr><th>Era</th><th>Supergroup</th><th>Group</th><th>Subgroup</th><th>Composition</th><th>Intrusion</th><th>age [Ma]</th><th>Geodynamic setting</th></tr></thead><tbody><tr><td rowspan="10">Neoproterozoic</td><td rowspan="10">West Congo</td><td colspan="7"><strong>Pan-African II (mainly thermal, minor deformation)</strong></td></tr><tr><td>Mpioka</td><td>Breccia, sandstone, shale</td><td></td><td></td><td></td><td>580–500</td><td>Molasse</td></tr><tr><td colspan="7"><strong>Pan-African I (tectono-metamorphic)</strong></td></tr><tr><td>Schisto-calcaires</td><td>Sc3<br>Sc2<br>Sc1</td><td>Limestone, slate<br>Limestone, slate<br>Limestone, slate</td><td></td><td></td><td></td><td>630–580</td><td>Passive margin</td></tr><tr><td>Niari</td><td></td><td>Pebble shale, sandstone, shale</td><td></td><td></td><td></td><td>630</td><td></td></tr><tr><td>Mayombe</td><td>Louila/<br>Bouenza<br>Lower<br>Diamictite<br>Mossouva</td><td>Shale, slate, sandstone±carbonate<br>rocks<br>Pebble schist (diamictite), ±<br>sandstone, ± dolerite<br>Quartzitic sandstone, quartz-sericite<br>schist, ± graphite schist</td><td></td><td></td><td></td><td>630–725<br>725–750<br>750–870</td><td></td></tr><tr><td rowspan="3">Sounda</td><td>Mvouti</td><td>Various schist (graphite),<br>metaarkose (graphite-bearing) ±<br>dolerite</td><td></td><td></td><td></td><td>&gt;870</td><td>Neoproterozoic rift<br>(1000-870 Ma)</td></tr><tr><td>Kakamoéka</td><td>Quartzite, bimodal metavolcanics,<br>various schist (partly graphite-<br>bearing)</td><td></td><td>Mfoubou, Mt Kanda</td><td></td><td>930–910</td><td></td></tr><tr><td>Nemba<br>Complex</td><td>Metabasalt and gabbro, green schist,<br>epiclastics</td><td></td><td></td><td></td><td>915</td><td></td></tr><tr><td colspan="7"><strong>Major hiatus and unconformity</strong></td></tr><tr><td>Palaeoproterozoic</td><td>Loéme</td><td>Bikossi</td><td>Quartzite, conglomerate, ± mafic<br>dyke/sill swarm</td><td></td><td></td><td>&lt;2040<br>(deposition)</td><td>Nappe of unknown age<br>(Pan-African?)</td></tr><tr><td colspan="7"><strong>E3 (amphibolite facies affecting the Bilinga Suite)</strong></td></tr><tr><td colspan="7"><strong>E2 (gneissic event, migmatite)</strong></td></tr><tr><td></td><td>Loukoula</td><td>Paragneiss, schist±quartzite,<br>amphibolite</td><td></td><td>Bilinga Suite (Les<br>Saras, Bilinga,<br>Bilala)</td><td></td><td>&lt;2100</td><td>unknown</td></tr></tbody></table>

<figure><img src="image_6.png" /><figcaption>**Fig. 3.** The relationship between metamorphic grade, mineral facies, illite crystallinity (KI) and coal rank (Warr, 2018) and between metamorphic grade, the presence of hydrocarbons and sheet silicates as well as the estimated temperatures (modified from Hunziker, 1986).</figcaption></figure>

## 3.2. Neoproterozoic era

The Eburnean orogeny was terminated at around 1970 ± 30 Ma with the emplacement of post-tectonic granites (Thiéblemont et al., 2009; Feybesse et al., 1998) and is followed in the Congo by a major hiatus of about 1000 my for which no rock record is available. Neoproterozoic deposition began around 1 Ga ago and was probably triggered by the break-up of the supercontinent Rodinia (Li et al., 2008). This initiated a new orogenic cycle that lasted about 500 Ma and whose rocks are now grouped into the West Congo Supergroup subdivided into four groups: i) Sounda, ii) Mayombe, iii) Niari, and iv) Schisto-calcaire.

The Neoproterozoic rock record starts with the Sounda Group, which was deposited in a continental rift setting between 1000 and 870 Ma (Vicat and Pouclet, 2000; Tack et al., 2001). It comprises mafic igneous and epiclastic rocks of the Nemba Complex, dated at 915 ± 8 Ma

(Fullgraf et al., 2015), quartzite, bimodal volcanic rocks and schists of the Kakamoéka Subgroup, dated at 903 ± 6 Ma and 915 ± 17 Ma, and meta-arkosic rocks and schists of the Mvouti Subgroup, some of which contain graphite. The rocks are intruded by coeval granites of the Mount Kanda and Mfoubo granites.

The overlying sediments of the Mayombe Group were deposited in a passive margin setting between 870 and 635 Ma. The ages are constrained by the youngest age of underlying rift volcanism and overlying glaciogenic sediments associated with the Marinoan glaciation. Basal quartzitic sandstone, quartz-sericite schist and minor graphite schist of the Mossouva Subgroup are overlain by pebbly schist (diamictite) with minor sandstone intercalations of the Lower Tillite Subgroup, which is correlated at global scale with the Sturtian glaciation. This is followed by a series of shales, sandstones with minor carbonate rocks subdivided into the eastern Bouenza and western Louila subgroups. Deposition is
<figure><img src="image_7.png" /><figcaption>**Fig. 4.** Simplified revised geological map of the Mayombe Chain and Niari Basin (see also electronic appendix 2). Stratigraphic units below the Subgroup level have been merged. The stratigraphy of these units is given in the legend in Table 2.</figcaption></figure>

constrained to 635–720 Ma by the underlying and overlying glacial sediments of the Cryogenian period.

The Niari Group overlies the Mayombe Group along a regional unconformity and is associated with the Marinoan glaciation, implying the age of about 635 Ma. It consists of siliceous shales, sandstones and shales deposited as mass flows before glaciers.

The return to warm climatic conditions is recorded in the Schisto-calcaire Group, which consists of limestone and shale and whose sedimentary structures reflect lagoonal, reef and forereef environments. The group has been subdivided into three subgroups reflecting major transgression-regression cycles. The minimum depositional age is limited by the onset of the Pan-African orogeny at about 590 Ma.
<caption>**Table 3**</caption>

Results of U-Pb SHRIMP analysis of detrital zircons from samples of the Bikossi and Loukoula groups. The numbers marked in red indicate high common lead values or very inconsistent data.

<table><thead><tr><th rowspan="2">detrital zircons</th><th rowspan="2">Spot</th><th colspan="4">concentrations [ppm]</th><th colspan="4">ages [Ma]</th><th colspan="8">ratios</th></tr><tr><th>206Pb<sub>c</sub></th><th>U</th><th>Th</th><th>206Pb*</th><th>206Pb /238U</th><th>±1s err</th><th>207Pb /206Pb</th><th>±1s err</th><th>% Disc</th><th>232Th /238U</th><th>238U /206Pb*</th><th>±%</th><th>207Pb*/206Pb*</th><th>±%</th><th>207Pb*/235U</th><th>±%</th><th>206Pb*/238U</th><th>±%</th><th>err corr</th></tr></thead><tbody><tr><td rowspan="16">Bikossi (quartzite)</td><td rowspan="16">DOL2153</td><td>23.1</td><td>0.10</td><td>405</td><td>124</td><td>131.3</td><td>2061</td><td>22</td><td>2043</td><td>13</td><td>-0.8</td><td>0.317</td><td>2.654</td><td>1.3</td><td>0.1260</td><td>0.7</td><td>6.544</td><td>1.5</td><td>0.3766</td><td>1.3</td><td>0.87</td></tr><tr><td>8.1</td><td>0.26</td><td>491</td><td>338</td><td>144.2</td><td>1891</td><td>19</td><td>2044</td><td>11</td><td>8.1</td><td>0.712</td><td>2.931</td><td>1.1</td><td>0.1261</td><td>0.6</td><td>5.928</td><td>1.3</td><td>0.3409</td><td>1.1</td><td>0.88</td></tr><tr><td>3.1</td><td>0.03</td><td>315</td><td>135</td><td>101.1</td><td>2046</td><td>21</td><td>2047</td><td>11</td><td>0.1</td><td>0.442</td><td>2.677</td><td>1.2</td><td>0.1263</td><td>0.6</td><td>6.504</td><td>1.3</td><td>0.3735</td><td>1.2</td><td>0.89</td></tr><tr><td>1.1</td><td>0.07</td><td>447</td><td>125</td><td>145.2</td><td>2067</td><td>20</td><td>2049</td><td>9</td><td>-0.9</td><td>0.290</td><td>2.646</td><td>1.1</td><td>0.1264</td><td>0.5</td><td>6.588</td><td>1.2</td><td>0.3779</td><td>1.1</td><td>0.90</td></tr><tr><td>29.1</td><td>0.10</td><td>345</td><td>119</td><td>107.5</td><td>1994</td><td>21</td><td>2049</td><td>14</td><td>2.7</td><td>0.358</td><td>2.757</td><td>1.2</td><td>0.1264</td><td>0.8</td><td>6.321</td><td>1.5</td><td>0.3626</td><td>1.2</td><td>0.83</td></tr><tr><td>10.1</td><td>0.36</td><td>1128</td><td>287</td><td>258.1</td><td>1516</td><td>15</td><td>2050</td><td>9</td><td>35.2</td><td>0.263</td><td>3.768</td><td>1.1</td><td>0.1265</td><td>0.5</td><td>4.626</td><td>1.2</td><td>0.2651</td><td>1.1</td><td>0.90</td></tr><tr><td>5.1</td><td>0.06</td><td>332</td><td>78</td><td>108.3</td><td>2075</td><td>21</td><td>2064</td><td>11</td><td>-0.5</td><td>0.244</td><td>2.634</td><td>1.2</td><td>0.1275</td><td>0.6</td><td>6.675</td><td>1.3</td><td>0.3797</td><td>1.2</td><td>0.89</td></tr><tr><td>6.1</td><td>0.04</td><td>569</td><td>167</td><td>174.9</td><td>1971</td><td>19</td><td>2068</td><td>8</td><td>4.9</td><td>0.304</td><td>2.796</td><td>1.1</td><td>0.1278</td><td>0.5</td><td>6.301</td><td>1.2</td><td>0.3576</td><td>1.1</td><td>0.92</td></tr><tr><td>9.1</td><td>0.12</td><td>982</td><td>142</td><td>319.4</td><td>2067</td><td>20</td><td>2100</td><td>9</td><td>1.6</td><td>0.149</td><td>2.645</td><td>1.2</td><td>0.1302</td><td>0.5</td><td>6.785</td><td>1.3</td><td>0.3780</td><td>1.2</td><td>0.92</td></tr><tr><td>24.1</td><td>0.06</td><td>729</td><td>347</td><td>220.5</td><td>1943</td><td>19</td><td>2110</td><td>11</td><td>8.6</td><td>0.492</td><td>2.842</td><td>1.1</td><td>0.1309</td><td>0.6</td><td>6.351</td><td>1.3</td><td>0.3518</td><td>1.1</td><td>0.88</td></tr><tr><td>17.1</td><td>0.09</td><td>227</td><td>34</td><td>78.1</td><td>2164</td><td>22</td><td>2132</td><td>15</td><td>-1.5</td><td>0.155</td><td>2.506</td><td>1.2</td><td>0.1325</td><td>0.8</td><td>7.290</td><td>1.5</td><td>0.3990</td><td>1.2</td><td>0.82</td></tr><tr><td>22.1</td><td>0.03</td><td>320</td><td>181</td><td>111.4</td><td>2190</td><td>24</td><td>2163</td><td>12</td><td>-1.3</td><td>0.582</td><td>2.471</td><td>1.3</td><td>0.1349</td><td>0.7</td><td>7.526</td><td>1.5</td><td>0.4046</td><td>1.3</td><td>0.87</td></tr><tr><td>21.1</td><td>0.09</td><td>92</td><td>1</td><td>32.8</td><td>2242</td><td>27</td><td>2223</td><td>22</td><td>-0.8</td><td>0.006</td><td>2.404</td><td>1.4</td><td>0.1397</td><td>1.3</td><td>8.009</td><td>1.9</td><td>0.4159</td><td>1.4</td><td>0.74</td></tr><tr><td>15.1</td><td>0.35</td><td>147</td><td>73</td><td>59.1</td><td>2468</td><td>26</td><td>2438</td><td>17</td><td>-1.2</td><td>0.512</td><td>2.142</td><td>1.3</td><td>0.1583</td><td>1.0</td><td>10.181</td><td>1.6</td><td>0.4664</td><td>1.3</td><td>0.77</td></tr><tr><td>25.1</td><td>0.11</td><td>328</td><td>176</td><td>115.9</td><td>2217</td><td>22</td><td>2486</td><td>11</td><td>12.1</td><td>0.554</td><td>2.435</td><td>1.2</td><td>0.1629</td><td>0.7</td><td>9.223</td><td>1.4</td><td>0.4105</td><td>1.2</td><td>0.87</td></tr><tr><td>12.1</td><td>0.31</td><td>68</td><td>49</td><td>28.3</td><td>2544</td><td>33</td><td>2492</td><td>21</td><td>-2.1</td><td>0.751</td><td>2.064</td><td>1.6</td><td>0.1634</td><td>1.2</td><td>10.906</td><td>2.0</td><td>0.4839</td><td>1.6</td><td>0.78</td></tr><tr><td>26.1</td><td>0.18</td><td>235</td><td>118</td><td>94.3</td><td>2465</td><td>25</td><td>2514</td><td>13</td><td>2.0</td><td>0.517</td><td>2.145</td><td>1.2</td><td>0.1657</td><td>0.7</td><td>10.641</td><td>1.4</td><td>0.4658</td><td>1.2</td><td>0.85</td></tr><tr><td>30.1</td><td>0.18</td><td>81</td><td>57</td><td>34.2</td><td>2577</td><td>31</td><td>2523</td><td>21</td><td>-2.1</td><td>0.733</td><td>2.033</td><td>1.4</td><td>0.1665</td><td>1.2</td><td>11.283</td><td>1.9</td><td>0.4915</td><td>1.4</td><td>0.76</td></tr><tr><td>13.1</td><td>0.14</td><td>143</td><td>83</td><td>59.3</td><td>2541</td><td>26</td><td>2539</td><td>12</td><td>-0.1</td><td>0.602</td><td>2.068</td><td>1.3</td><td>0.1681</td><td>0.7</td><td>11.200</td><td>1.5</td><td>0.4832</td><td>1.3</td><td>0.86</td></tr><tr><td>19.1</td><td>0.11</td><td>229</td><td>136</td><td>96.9</td><td>2574</td><td>26</td><td>2555</td><td>12</td><td>-0.7</td><td>0.615</td><td>2.037</td><td>1.2</td><td>0.1698</td><td>0.7</td><td>11.488</td><td>1.4</td><td>0.4908</td><td>1.2</td><td>0.86</td></tr><tr><td>16.1</td><td>0.05</td><td>429</td><td>245</td><td>175.5</td><td>2510</td><td>24</td><td>2561</td><td>9</td><td>2.1</td><td>0.590</td><td>2.101</td><td>1.1</td><td>0.1704</td><td>0.5</td><td>11.181</td><td>1.3</td><td>0.4760</td><td>1.1</td><td>0.91</td></tr><tr><td>14.1</td><td>0.04</td><td>626</td><td>467</td><td>244.7</td><td>2417</td><td>23</td><td>2569</td><td>8</td><td>6.3</td><td>0.771</td><td>2.198</td><td>1.1</td><td>0.1711</td><td>0.5</td><td>10.735</td><td>1.2</td><td>0.4550</td><td>1.1</td><td>0.93</td></tr><tr><td>2.1</td><td>0.03</td><td>178</td><td>65</td><td>75.5</td><td>2590</td><td>25</td><td>2573</td><td>10</td><td>-0.7</td><td>0.377</td><td>2.022</td><td>1.2</td><td>0.1716</td><td>0.6</td><td>11.702</td><td>1.3</td><td>0.4946</td><td>1.2</td><td>0.90</td></tr><tr><td>20.1</td><td>0.45</td><td>106</td><td>70</td><td>44.7</td><td>2560</td><td>29</td><td>2717</td><td>19</td><td>6.1</td><td>0.678</td><td>2.047</td><td>1.4</td><td>0.1872</td><td>1.1</td><td>12.582</td><td>1.8</td><td>0.4876</td><td>1.4</td><td>0.77</td></tr><tr><td>28.1</td><td>0.09</td><td>127</td><td>75</td><td>40.2</td><td>2015</td><td>23</td><td>2737</td><td>16</td><td>35.8</td><td>0.607</td><td>2.724</td><td>1.3</td><td>0.1894</td><td>1.0</td><td>9.582</td><td>1.7</td><td>0.3669</td><td>1.3</td><td>0.80</td></tr><tr><td>7.1</td><td>0.14</td><td>206</td><td>142</td><td>92.4</td><td>2706</td><td>29</td><td>2743</td><td>10</td><td>4.2</td><td>0.712</td><td>1.916</td><td>1.3</td><td>0.1901</td><td>0.6</td><td>13.673</td><td>1.4</td><td>0.5217</td><td>1.3</td><td>0.91</td></tr><tr><td>4.1</td><td>0.45</td><td>324</td><td>164</td><td>98.0</td><td>1935</td><td>20</td><td>2754</td><td>12</td><td>42.4</td><td>0.522</td><td>2.852</td><td>1.2</td><td>0.1914</td><td>0.7</td><td>9.236</td><td>1.4</td><td>0.3500</td><td>1.2</td><td>0.85</td></tr><tr><td>18.1</td><td>0.06</td><td>428</td><td>120</td><td>191.9</td><td>2708</td><td>25</td><td>2804</td><td>8</td><td>3.6</td><td>0.289</td><td>1.915</td><td>1.2</td><td>0.1973</td><td>0.5</td><td>14.205</td><td>1.3</td><td>0.5220</td><td>1.2</td><td>0.92</td></tr><tr><td>11.1</td><td>0.06</td><td>421</td><td>156</td><td>203.8</td><td>2879</td><td>29</td><td>2936</td><td>7</td><td>2.0</td><td>0.382</td><td>1.776</td><td>1.2</td><td>0.2140</td><td>0.4</td><td>16.614</td><td>1.3</td><td>0.5630</td><td>1.2</td><td>0.95</td></tr><tr><td>27.1</td><td>0.04</td><td>545</td><td>200</td><td>278.3</td><td>3006</td><td>28</td><td>3077</td><td>6</td><td>2.4</td><td>0.380</td><td>1.683</td><td>1.2</td><td>0.2337</td><td>0.4</td><td>19.141</td><td>1.2</td><td>0.5942</td><td>1.2</td><td>0.94</td></tr><tr><td rowspan="16">Loukoula (paragneiss)</td><td rowspan="16">DOL9134-1</td><td>14.1</td><td>3.23</td><td>2770</td><td>1367</td><td>553</td><td>1299</td><td>12</td><td>1893</td><td>170</td><td>46</td><td>0.510</td><td>4.447</td><td>1.0</td><td>0.1160</td><td>9.2</td><td>3.570</td><td>9.3</td><td>0.2233</td><td>1.0</td><td>0.11</td></tr><tr><td>3.1</td><td>0.14</td><td>510</td><td>167</td><td>144</td><td>1825</td><td>13</td><td>2069</td><td>17</td><td>13</td><td>0.337</td><td>3.055</td><td>0.8</td><td>0.1279</td><td>1.0</td><td>5.771</td><td>1.3</td><td>0.3272</td><td>0.8</td><td>0.65</td></tr><tr><td>4.1</td><td>0.09</td><td>330</td><td>385</td><td>111</td><td>2121</td><td>17</td><td>2111</td><td>14</td><td>1.3</td><td>1.204</td><td>2.565</td><td>1.0</td><td>0.1310</td><td>0.8</td><td>7.038</td><td>1.2</td><td>0.3897</td><td>1.0</td><td>0.76</td></tr><tr><td>15.1</td><td>0.09</td><td>175</td><td>159</td><td>58.8</td><td>2123</td><td>19</td><td>2128</td><td>20</td><td>0.2</td><td>0.938</td><td>2.564</td><td>1.0</td><td>0.1322</td><td>1.2</td><td>7.110</td><td>1.6</td><td>0.3900</td><td>1.0</td><td>0.66</td></tr><tr><td>18.1</td><td>0.13</td><td>482</td><td>234</td><td>166</td><td>2175</td><td>15</td><td>2144</td><td>13</td><td>-1.4</td><td>0.502</td><td>2.492</td><td>0.8</td><td>0.1335</td><td>0.7</td><td>7.383</td><td>1.1</td><td>0.4012</td><td>0.8</td><td>0.74</td></tr><tr><td>2.1</td><td>0.12</td><td>265</td><td>132</td><td>91.1</td><td>2167</td><td>16</td><td>2153</td><td>15</td><td>-0.7</td><td>0.515</td><td>2.502</td><td>0.9</td><td>0.1341</td><td>0.9</td><td>7.391</td><td>1.2</td><td>0.3996</td><td>0.9</td><td>0.69</td></tr><tr><td>14.2</td><td>0.04</td><td>1385</td><td>410</td><td>331</td><td>1581</td><td>11</td><td>2207</td><td>10</td><td>40</td><td>0.306</td><td>3.597</td><td>0.8</td><td>0.1384</td><td>0.6</td><td>5.30</td><td>0.9</td><td>0.2779</td><td>0.8</td><td>0.80</td></tr><tr><td>1.1</td><td>0.64</td><td>2865</td><td>105</td><td>550</td><td>1291</td><td>9</td><td>2344</td><td>16</td><td>82</td><td>0.038</td><td>4.502</td><td>0.8</td><td>0.1498</td><td>0.9</td><td>4.58</td><td>1.2</td><td>0.2217</td><td>0.8</td><td>0.65</td></tr><tr><td>9.1</td><td>0.29</td><td>594</td><td>337</td><td>219</td><td>2296</td><td>16</td><td>2398</td><td>13</td><td>4.4</td><td>0.586</td><td>2.335</td><td>0.8</td><td>0.1546</td><td>0.8</td><td>9.12</td><td>1.1</td><td>0.4278</td><td>0.8</td><td>0.72</td></tr><tr><td>19.1</td><td>0.11</td><td>579</td><td>297</td><td>221</td><td>2364</td><td>18</td><td>2418</td><td>12</td><td>2.3</td><td>0.530</td><td>2.257</td><td>0.9</td><td>0.1564</td><td>0.7</td><td>9.56</td><td>1.2</td><td>0.4430</td><td>0.9</td><td>0.79</td></tr><tr><td>11.1</td><td>0.76</td><td>305</td><td>121</td><td>122</td><td>2455</td><td>18</td><td>2426</td><td>18</td><td>-1.1</td><td>0.411</td><td>2.153</td><td>0.9</td><td>0.1573</td><td>1.1</td><td>10.05</td><td>1.4</td><td>0.4634</td><td>0.9</td><td>0.65</td></tr><tr><td>17.1</td><td>0.09</td><td>130</td><td>116</td><td>51.9</td><td>2457</td><td>22</td><td>2461</td><td>19</td><td>0.2</td><td>0.925</td><td>2.155</td><td>1.1</td><td>0.1605</td><td>1.1</td><td>10.27</td><td>1.6</td><td>0.4639</td><td>1.1</td><td>0.69</td></tr><tr><td>7.1</td><td>0.17</td><td>339</td><td>197</td><td>137</td><td>2486</td><td>18</td><td>2473</td><td>12</td><td>-0.5</td><td>0.600</td><td>2.124</td><td>0.9</td><td>0.1616</td><td>0.7</td><td>10.49</td><td>1.1</td><td>0.4705</td><td>0.9</td><td>0.76</td></tr><tr><td>16.1</td><td>0.13</td><td>275</td><td>238</td><td>99.3</td><td>2249</td><td>18</td><td>2515</td><td>19</td><td>-0.9</td><td>0.894</td><td>2.392</td><td>1.0</td><td>0.1657</td><td>1.1</td><td>9.54</td><td>1.5</td><td>0.4175</td><td>1.0</td><td>0.65</td></tr><tr><td>6.1</td><td>0.07</td><td>441</td><td>368</td><td>185</td><td>2556</td><td>18</td><td>2534</td><td>11</td><td>-0.9</td><td>0.863</td><td>2.054</td><td>0.8</td><td>0.1676</td><td>0.7</td><td>11.25</td><td>1.1</td><td>0.4866</td><td>0.8</td><td>0.78</td></tr><tr><td>8.1</td><td>0.03</td><td>383</td><td>97</td><td>184</td><td>2862</td><td>20</td><td>2836</td><td>9</td><td>-0.9</td><td>0.261</td><td>1.789</td><td>0.9</td><td>0.2012</td><td>0.6</td><td>15.50</td><td>1.0</td><td>0.5589</td><td>0.9</td><td>0.83</td></tr><tr><td>10.1</td><td>0.70</td><td>238</td><td>121</td><td>116</td><td>2861</td><td>22</td><td>2871</td><td>14</td><td>0.4</td><td>0.527</td><td>1.785</td><td>0.9</td><td>0.2056</td><td>0.9</td><td>15.84</td><td>1.3</td><td>0.5587</td><td>0.9</td><td>0.74</td></tr><tr><td>5.1</td><td>0.47</td><td>72</td><td>87</td><td>36.6</td><td>2971</td><td>35</td><td>2979</td><td>21</td><td>0.3</td><td>1.247</td><td>1.705</td><td>1.5</td><td>0.2198</td><td>1.3</td><td>17.74</td><td>1.9</td><td>0.5854</td><td>1.5</td><td>0.75</td></tr><tr><td>13.1</td><td>0.13</td><td>262</td><td>123</td><td>142</td><td>3141</td><td>22</td><td>3192</td><td>10</td><td>1.6</td><td>0.486</td><td>1.591</td><td>0.9</td><td>0.2512</td><td>0.7</td><td>21.75</td><td>1.1</td><td>0.6280</td><td>0.9</td><td>0.81</td></tr></tbody></table>

Errors are 1-sigma; Pbc and Pb* indicate the common and radiogenic portions, respectively.

Error in Standard calibration was 0,43%(not included in above errors but required when comparing data from different mounts).

Common Pb was corrected using measured 204 Pb; red figures indicate high discordance of data or high common lead.

### 3.3. Late Neoproterozoic to Cambrian era

The Mpioka Group is the youngest lithostratigraphic unit of the West-Congo Supergroup (Tack et al., 2001; Frimmel et al., 2006) and records the molasse stage of the Pan-African orogeny (Scolari, 1965; Scolari and Van Daalhoff, 1965; Frimmel et al., 2006). Analysis of metamorphic and detrital white mica constrains the depositional age of the unit between 575 ± 8.7 Ma and 499 ± 19 Ma after the main orogenic event at 580 Ma (this study) confirming the interpretation as molasse of the Mayombe Chain.

## 4. Structural architecture and metamorphism

The location of the sample sites and key outcrops, referred to below, are shown in Fig. 5 while their geographic coordinates are given in Appendix 1. The selection criteria for the key outcrops were i) informativeness for understanding the regional geology ii) good exposure and low weathering (i.e. quarries and outcrops along the National

Road), and iii) generally easy access.

### 4.1. Palaeoproterozoic (Eburnean) structures and metamorphism

The identification of Palaeoproterozoic structures and kinematics is complicated by the strong Pan-African overprint and relatively few instructive outcrops in the WCB, which are mostly limited to quarries. No systematic study was carried out at regional scale, however, detailed structural observation in several outcrops demonstrate tectonic style and metamorphism of the Eburnean orogeny. The classification of Eburnean metamorphic events E2 and E3 refers to the Eburnean history in Gabon, E1 is not recorded in the study area.

Outcrop DOL9134 in the Loukenéné River exposes layered paragneiss of the Loémé Supergroup that probably records the oldest tectono-metamorphic fabrics. Leucogneiss alternating with quartz-feldspar mica schist corresponds to former alternating beds of sandstone and semipelite. Migmaticitic (stromatic) textures in the metapelitic rocks record regional E2 high-grade conditions, whereas the leucogneiss
<figure><img src="image_8.png" /><figcaption>Fig. 5. U-Pb zircon sample positions and key outcrops referred to in the text; geographic coordinates are given in Appendix 1.</figcaption></figure>

remained unaffected by anataxis (Fig. 6a–d). The absence of crosscutting leucosome suggests that melts in the metapelites were not mobile. The migmatitic gneisses are overprinted by isoclinal folding of decimetre size with axial planes parallel to the main gneissic layering. They are intruded at high angle by a thin granite dyke of unknown age that in turn is affected by a weak foliation associated with gentle folds that slightly rotated the feldspars. Late brittle faulting represents the youngest tectonic feature. M1 upper amphibolite facies conditions are indicated by the paragenesis quartz-plagioclase-biotite-muscovite-sillimanite in the semipelitic rocks; partial replacement of biotite by chlorite documents retrograde low-grade conditons.

DOL2724, 200 m west of Nkougni station, illustrates further aspects of polyphase Palaeoproterozoic deformation and metamorphism (Fig. 6e and f). The gneissic layering in mafic and felsic orthogneiss is again folded into tight to isoclinal folds. Subsequent shearing and boudinage is recorded in a E to NE inclined low-angle thrust; it is in turn overprinted by open folding. Further structures within the shear zone are small-scale sheath folds with subhorizontal NNE-directed fold axes parallel to the direction of movement. A second generation of shear zones displays asymmetric E to SE inclined SC-fabrics suggesting W to NW directed transport. 200 m further southeast, near the Nkouni station, these fabrics become dominant and completely transpose the older structures. Late stage structures comprise asymmetric open to closed folds with ESE-ward inclined axial planes lacking any schistosity.

In the Bilala quarry (DOL2141, 9039) the southern wall (Fig. 6g) exposes a layered sequence of gneissic tonalite alternating with granite, amphibolite and migmatitic paragneiss. The Mfoubou Granite (c. 1 Ga) nearby shows little tectonic overprinting by the Pan-African orogeny, suggesting a Palaeoproterozoic age for most of the tectono-metamorphic fabrics in the rocks of the quarry. The intrusion of the tonalite post-dates an early M1 event, which led to anatexis of the country rock and

associated with a first phase of isoclinal folding D1. The predominant M2 event overprints all rocks, thereby transposing the older D1 fabrics.

Moderately south to SE-dipping gneissic layering (D2) is associated with shallow E to SW-plunging stretching lineations. Kinematic indicators (delta- and sigma clasts) suggest W to SW directed transports. M2 amphibolite facies metamorphism is indicated by the mineral paragenesis garnet-amphibole-biotite-plagioclase (oligoclase). Two generations of syn- and post-kinematic garnet growth show that M2 metamorphism outlasted D2 deformation. Subsequent deformation occurred under retrograde metamorphism. The orientation of several locally developed small-scale structures suggests their pre-Mayombic and therefore Late Eburnean age. This includes (i) NE-dipping crenulation cleavage associated with subhorizontal NW-trending crenulation lineation; (ii) Low-angle reverse thrust with shallow E to moderate NW dip; and (iii) two sets of open folds with shallow SE and moderate NE-dipping axial planes. The final stage of ductile deformation is recorded by NE striking steep crenulation cleavage of Pan-African age. Younger brittle deformation comprises SSE-inclined low-to moderate-angle faults.

In the Les Saras quarry (DOL9036), layered granodiorite displays over 150 m distance considerable gradient in tectonic overprint. Rocks in the interior of the pluton are hardly affected by deformation and igneous structures and enclaves of various origin are largely preserved (Fig. 6h). Towards the contact with the hosting paragneiss, progressively increasing shearing transformed the rocks into protomylonite to ultra-mylonite (Fig. 6i). Intercalated amphibolite is transposed and thin layers transformed into chlorite-biotite-epidote schist. Where developed, the foliation is associated with a pronounced down-dip stretching lineation. Shear indicators indicate both reverse and normal movements. Considerable variation in dip direction from S to W is attributed to subsequent folding.
<figure><img src="image_9.png" /><figcaption>**Fig. 6.** Key outcrops in the WMC illustrating Eburnean deformation and metamorphism. A) Outcrop DOL9134 in the Loukenéné River exposes layered paragneiss; B) phlebitic and stromatic structures are limited to semipeltic layers whereas metapsammitic rocks are unaffected by anatexis; C) close up shows layer-pallel small-scale tight to isoclinal folding in semipelitic rocks; D) the emplacement of a granite dyke (of unknown age) post-dates regional metamorphism; E) Outcrop DOL2724 exposes highly deformed migmatitic paragneiss; here refolded isoclinal folds overprinted by late crenulation cleavage (ccf); F) small-scale sheath fold; G) Bilala quarry (DOL9039) exposing metatonalite; h) Les Saras granite gneiss with mafic enclaves i) alternating protomylonitic and mylonitic orthogneiss.</figcaption></figure>
At DOL2474, about 3 km west of the Les Saras quarry, a diamond drill core of granodioritic orthogneiss illustrates again highly variable deformation over ca 80 m length. Mylonitic quartz-feldspar-biotite gneiss and augen gneiss often grade into quartz-feldspar-mica schist. In less deformed parts, very coarse K-feldspar porphyroclasts are set in a matrix of quartz-feldspar-mica schist. Intercalated thin layers of amphibole schist and sericite-chlorite schist record polyphase deformation including isoclinal folding with axial planes parallel to the main schistosity. Similar U-Pb zircon crystallisation ages in samples from Les Saras quarry and this drill core demonstrate coeval evolution of the igneous precursors and constrain the maximum age of E3 regional metamorphism and deformation at ca 2040 Ma ((Fullgraf et al., 2015, Table 2).

### 4.1.1. Geochronology

Geochronology of detrital zircons from migmatitic paragneiss (DOL9134) of the Loukoula Group yielded ages between 3192 and 2069 Ma with peaks at P2 [2420–2535 Ma] and P3 [2100–2150 Ma], demonstrating erosion of Palaeoproterozoic and Archaean source areas (Table 3, Fig. 7). The classification as P2 and P3 (P = plutonism) refers to the history of Palaeoproterozoic magmatism in Gabon (Thiéblemont et al., 2009). P2 zircons appear rounder than P3 zircons suggesting their origin from distal sources. The youngest $^{207}$Pb/$^{206}$Pb age of 2069 ± 17 Ma is 13% discordant and therefore only provides the minimum age of this grain. The youngest concordant $^{207}$Pb/$^{206}$Pb age of 2111 ± 14 Ma provides the maximum age of sediment deposition and high-grade metamorphic event that led to the anatexis of the metapelites.

Isoclinally folded gneissic sill granite (DOL2724-2) yielded a concordia age of 2045 ± 7 Ma which is interpreted as time of magmatic emplacement (Fullgraf et al., 2015); it constrains the maximum age of the main gneiss-forming event, isoclinal folding and low-angle shearing under medium-to high-grade conditions at this locality. The ages of several xenocrysts from this and the other sample (DOL2724-1) vary between 2200 and 2620 Ma documenting Palaeoproterozoic to Neoarchaean source or host rocks.

Magmatic zircons from four samples of the Bilinga, Bilala and Les Saras intrusions yielded ages between 2028 ± 12 and 2048 ± 12 Ma, indicating (i) their origin in the late phase of the Eburnean orogeny (ii) the minimum age of the M1 event they post-date (iii) the maximum age of the M2 event that converted the intrusions into orthogneiss, and (iv) the minimum age of the M1 anatectic event that affected the country rock but was not recorded in the intrusions (Fullgraf et al., 2015). Similar results were obtained by Affaton et al. (2016) dating the intrusions at 2049 ± 29 and 2060 ± 18 Ma.

## 4.2. Pan-African structures and metamorphism

The Mayombe Chain forms the central part of the West Congo Belt, which extends southwards through Kabinda into the Democratic Republic of Congo (DRC) and Angola and northwards into Gabon. On continental scale, it forms the external part of the 580-500 Ma Pan-African Ribeira-Araçuai orogen in Brazil (Brückner et al., 2000; Alkmim et al., 2001, 2006; Pedrosa-Soares et al., 1992, 1998, 1999, 2001, 2008). On the African side, it possibly continues into the 620-590 Ma Kaoko belt of southern Angola and northern Namibia (Goscombe et al., 2005). Previous studies in the West Congo Belt show that the orogen records two principal tectonic events (Hossié and Caby, 1979; Hossié, 1980; Boudzoumou, 1986; Vicat and Vellutini, 1988; Bonie, 1990; Maurin et al., 1991, Maurin, 1993). An older event D1 is characterised by i) NW-SE trending schistosity $S_1$ (synkinematic biotite, white mica and chlorite), ii) SW plunging mineral lineation, and iii) NE-verging folds and NE-directed tectonic transport. D2 crenulation cleavage $S_2$ dips in D1 directions but is steeper. This is attributed to a second stage of tangential tectonics resulting in duplex structures (Hossié and Caby, 1979). The intensity of D2 deformation increases from Congo Brazzaville where large open folds and faulting are characteristic

to Gabon, where E-directed shearing becomes predominant transposing the older D1 structures.

Early E-W fold axes were recognised in DRC that towards the north rotate progressively into the typical NW-SE trend of the central and northern Mayombe (Cahen, 1977, 1978; Franssen and Andre, 1988). Late-stage D3 deformation in the Loémé Supergroup is characterised by steeply NW-inclined crenulation cleavage and refolding of D1/D2 structures (Boudzoumou, 1986).

### 4.2.1. Field observation

The geological map and cross section together with the new tectono-metamorphic map of the Dolisie map sheet illustrate the new subdivision of the West Congo Belt into the Western, Central and Eastern domains of the Mayombe Chain and the Niari Basin (Figs. 4 and 8). On an orogenic scale, all domains represent the eastern external zone of the Araçuai/Ribeira orogen in Brazil. The distinction between a chain and a basin takes the historical terms into account, but the boundary of the Mayombe Chain has been shifted about 30 km eastwards to Mount Bélo and now includes all rocks that are affected by folding, faulting and thrusting. In the new framework, the Niari Basin is structurally defined by almost undeformed sedimentary rocks in subhorizontal position. The contacts between all domains are first-order structures forming a regional reverse thrust (Western/Central Mayombe; Loukenéné-Mandji thrust), a thrust-back thrust (Central/Eastern Mayombe; Moukondo back-thrust) and a terminal thrust system (Eastern Mayombe/Niari Basin; Mount Bélo thrust). The contact between the Central and Western Mayombe Belt further marks a jump in metamorphic grade and stacking of Palaeo- and Neoproterozoic rocks in the WMB. The main features of each domain and their contacts are described below.

#### 4.2.1.1. Niari Basin (NB). 
The Niari Basin comprises, from base to top, i) the Louila/Bouenza subgroups deposited under periglacial deltaic to shallow marine conditions, ii) the Niari Group of glaciogenic origin, iii) the Schisto-calcaire Group, recording lagoon and marine platform environments, and iv) the silici-clastic Mpioka Group, representing the eastern molasse of the Western Congo Belt.

Structurally the domain is characterised by the absence of foliation and folding, subhorizontal bedding and only rare subvertical normal and strike-slip faults, sometimes forming conjugate shear systems. In the northern part, normal faults are the main structural element. In the southern Boko-Songho and Mfouati areas additional strike-slip faults and horsetail graben structures are developed locally, which are associated with economic Pb-Zn-Cu mineralization (Charles et al., 2015).

The metamorphic grade increases in the Niari Basin from diagenesis in the eastern part to anchizonal conditions at Mount Bélo (this study).

#### 4.2.1.1.1. Eastern Mayombe Chain (EMC). 
The EMC comprises the same stratigraphic units as the Niari Basin. The metamorphic grade increases from anchizonal conditions in the east to greenschist facies in the west (see below). Structurally, the EMC is characterised by strain partitioning into discrete shear zones. Most of the deformation is concentrated in low-angle thrusts, reverse and strike slip faults resulting in duplex and/or positive flower structures. Normal faulting of unknown age further complicates the geology in this domain.

Two of such high-strain zones have been studied at a road aggregate quarry (DOL0333) and some 20 km further to the west at the N1 (DOL2093; Fig. 9). Outcrop DOL0333 is located within the Mount Bélo thrust at the EMC-NB boundary. Carbonate rocks of the Schisto-calcaire Group are folded into disharmonic NE-verging folds several tens of metres in size with SW-inclined axial planes of shallow to moderate dip. The folds are offset by several shallow-dipping reverse faults and, in the upper part of the quarry, complexly deformed red and dark shales and carbonates are in thrust and fold contact with the underlying rocks. The main thrust probably lies in front of the quarry but is not exposed.

The sedimentary rocks in the surrounding hills are in a subhorizontal position and appear to be almost undeformed, illustrating strain
<figure><img src="image_10.png" /><figcaption>**Fig. 7.** Detrital zircons of paragneiss (DOL9134-1) arranged by age. The data indicate two major magmatic episodes P2 [2420–2535 Ma] and P3 [2100–2150 Ma] in the source area of the former sediments. Magmatic periods [2717–2804 Ma] and [2043–2068 Ma,] which are recorded in sample DOL2150 of the Bikossi Group, are only weakly represented.</figcaption></figure>
<figure><img src="image_11.png" /><figcaption>**Fig. 8.** The new tectono-metamorphic map of the West Congo belt (example Dolisie sheet) shows the main geological units and illustrates the structural architecture. The Loukenéné-Mandji Thrust marks the biotite-in-isograd, which coincides with the boundary of the western Mayombe domain.</figcaption></figure>

<figure><img src="image_12.png" /><figcaption>**Fig. 9.** The aggregate quarry 55 km east of Dolisie (DOL0333) exposes the Mt Bélo shear zone, characterised by folded and thrust rocks of the Schisto-calcaire Group. The sedimentary rocks in the surrounding hills show subhorizontal bedding and appear to be almost undeformed, suggesting strain partitioning into discrete shear zones.</figcaption></figure>

partitioning. Small-scale low-angle thrusting has been observed at Mount Bélo already 3 km in front of this major thrust, while the geological map illustrates that the Mount Bélo shear zone comprises an

anastomosing system of discrete SE-NW striking thrusts.

A second outcrop (DOL2093) exposes a sequence of shales and interbedded sandstones of the Louila Group, folded in disharmonic, NE-
verging gently inclined metre-scale folds and associated with a shallow SSW dipping schistosity $S_1$ (Fig. 10). Syn-kinematic vein quartz is crystallised in the hinges of the folds. Approximately 75 m to the east, two blocks of nearly undeformed sandstone and shale are offset from each other by vertical NE-SW striking faults and to the west they are juxtaposed with highly deformed shale characterised by the same dis-harmonic folds and shallow dipping fold axial planes described before. Striations on the fault planes plunge at low-angle to the SW and indicate sinistral movement. The juxtaposition of strongly folded and almost undeformed shales clearly documents strain partitioning, whereas shallowly dipping fold axial planes parallel to the local $S_1$ argue for underlying low-angle thrust systems.

The geological map and profile (Fig. 4) show between Mountt Belo and Dolisie (i.e. outsidethese high-strain zones) gentle syn- and anticlines of about 5–8 km wavelength with NW-SE trending subhorizontal fold axes but no schistosity associated. Further west, regional increasing deformation is indicated by the transition to upright open symmetric folds that define the geometry of the geological map. The second characteristic structural element are positive and negative NW-SE striking flower structures bounded by high-angle reverse and normal faults. The largest of these between Dolisie and the Moukondo toll station is about 5 km wide can be followed more than 30 km along strike.

Penetrative schistosity $S_1$ at moderate to high angles to the bedding first occurs near Dolisie in shales of the Louila Group and continues westwards from there. Its eastern limit remains unknown due to poor outcrop of the generally deeply weathered shales.

The Moukondo thrust/back-thrust marks the boundary between the Eastern and Central Mayombe Chain (CMC); it is a complex structure several 100 m wide, characterised by a tectonic breccia of various Mossouva and Louila subgroup rocks forming isolated tectonic clasts up to several metres size in a matrix of Niari Group rocks (Fig. 11). The eastern boundary of the structure is marked by a folded sequence of Mossouva quartzite and quartz-sericite schist thrust over the tectonic mélange and overlain further east by steeply dipping shales of the Louila Subgroup. To the west, the fault is bounded by the steeply overturned limb of folded Mossouva rocks. The final stage of deformation transformed the Moukondo thrust into the present back-thrust, characterised by NE-dipping foliation and SW-verging folds.

#### 4.2.1.2. Central Mayombe Chain (CMC).
The domain is about 25–30 km wide and bound by the Moukondo back-thrust to the east and the Loukenéné-Mandji Thrust to the west (Fig. 8). The lithological units of the CMC are: i) marine distal and proximal silici-clastic slope facies of the Mvouti Subgroup, ii) silici-clastic platform facies of the Mossouva Subgroup, iii) distal marine basin sediments with a glaciogenic component of the Lower Tillite Subgroup, and iv) shales and sandstones of the Louila Subgroup.

The main structural elements are: (i) open to closed, upright to NE-verging syn- and anticlines, up to 10 km wide, with shallowly plunging axes to the NW and SE, (ii) subvertical to moderately SW dipping schistosity $S_1$, (iii) the onset of regional crenulation cleavage, or schistosity $S_2$, (iv) moderate-to high-angle reverse thrusts and, (v) several generations of subvertical normal faults some of which are infilled by major tectonic breccias.

The typical fold style of the CMC is illustrated at outcrops DOL2104 and DOL2106, which also demonstrate the change from symmetrical upright folds in the east to NE-vergent asymmetric folds in the west (Fig. 12). Numerous small-scale tectonic structures associated with folding can be observed on both outcrops, mainly due to the competence contrast between soft sericite schist alternating with rigid quartz sandstone. These include layer-parallel gliding marked by polished surfaces, slickensides and slicolites, development of symmetric and asymmetric boudinage, asymmetric parasitic minor S, M and Z folds, S-C fabrics, conjugate shear zones and local faulting in the fold hinges. All small-scale structures are consistent with a vertical to steeply dipping strain ellipsoid. On a regional scale, the folds are classified as D2 structures. They are affected by reverse thrusting during progressive deformation and are locally refolded around E-W axes during a late stage of deformation. One such D3 fold of about 6 km size has been identified by remote sensing in the central part of the Dolisie sheet (Fig. 8). The $S_1$ schistosity generally forms a moderate angle with the bedding but may also be absent in sandstones, or show only slight obliquity in zones of high shear and on the steep limbs of the folds. Subsequent deformation is shown by regional crenulation cleavage of greatly varying orientation.

The most important fold structure of more than 100 km length and about 20–30 km width has been denominated “Dimonika syncline”; the fold closure of the structure is located south of the study area in Cabinda (Fig. 8). The Dimonika syncline is juxtaposed in the NE by the Mvouti thrust with Mossouva quartzite, which, however, is preserved in the core of the syncline, too. Reverse thrusting sheared off the lower part of the Mvouti Subgroup and resulted in uplift of Sounda Group rocks, which are locally outcropping. In addition, the structure may have played an important role as conduit for gold mineralizing fluids of the Dimonika gold field (Fig. 8). A further change in fold style in the SW part of the CMC is illustrated along the National Road N1 from DOL2248, with shallow west-dipping arkosic rocks, to DOL2289 two kilometres further west (Figs. 4 and 8), where the beds suddenly change to an overturned, steeply SSW-dipping position. This reveals asymmetric, NNE-vergent, km-scale folds refolding the SW limb of the Dimonika Syncline. It is consistent with polyphase homoaxial D2 deformation characterising the geometry in the western part of the CMC. The regional increase in deformation is further documented by the $S_1$ schistosity, which changes

<figure><img src="image_13.png" /><figcaption>**Fig. 10.** Sketch of road outcrops DOL2093 about 15 km east of Dolisie.</figcaption></figure>
<figure><img src="image_14.png" /><figcaption>Fig. 11. Sketch of the Moukondo back-thrust system (DOL2252).</figcaption></figure>

<figure><img src="image_15.png" /><figcaption>Fig. 12. Typical disharmonic upright to slightly E-verging folds of decca-metre scale in the central Mayombe, here in quartz sandstone and quartz-sericite schist of the Mossouva Subgroup.</figcaption></figure>

from steep to vertical dips in the east to a moderate SW dip in the west.

Finally, at road outcrop DOL2166 near the Loukenéné-Mandji thrust, graphitic schist of the Mvouti Subgroup exposes isoclinal D1 folds that are refolded into asymmetric, disharmonic, NE-verging, meter scale D2 folds (Fig. 13). Their axial planes are parallel to the penetrative, moderately SW dipping, S₂ schistosity, which predominates in this place. Steeply W-dipping crenulation cleavage associated with shallow to moderate N-plunging crenulation lineations represent the last

increment of deformation. The D1 folds are likely to be local and related to the Loukenéné-Mandji Thrust at the CMC-WMC contact. Tectonic stacking of Loukoula gneiss and Mvouti schist was observed in this zone over ca. 200 m cross section. However, tectonic slices of basement rocks have been found at DOL2948 in graphitic schist up to 2 km NE of the main contact, indicating the much wider width and regional significance of the structure.

Numerous faults of several generations occur in the CMC the most
<figure><img src="image_16.png" /><figcaption>**Fig. 13.** Outcrop DOL2166 near the Loukenéné-Mandji Thrust shows strong deformation of graphitic schists of the Mvouti Subgroup with (locally?) developed isoclinal D1 folds.</figcaption></figure>

important of which are shown on the geological map (Fig. 4). An early generation developed under ductile conditions. The most spectacular of these is the so-called Portes du Mayombe fault (DOL2248), which consists of a WNW- striking vertical layer of dark, silicified mylonitic quartzite about 30 m high and 20 m wide with abundant vein quartz on both sides. Highly deformed rocks in vicinity of the fault display S-C fabrics suggesting NE-directed compressional movements with vein quartz crystallising in the C-planes. The pronounced local crenulation cleavage is SW-inclined and associated with shallow NW-plunging lineations. Vertical D1 structures change to moderate south dips 50 m away from the fault and are overprinted by S-C fabrics with normal down-dip movements. Late lateral movements are recorded by subhorizontal striae plunging variously to the E and SW.

4.2.1.3. *Western Mayombe Chain (WMC).* The WMC comprises Palaeoproterozoic gneiss and schist of the Loémé Supergroup that are tectonically stacked with Tonian rift-related rocks of the Sounda Group and their coeval granites. The domain was transported NE over the CMC along the Loukenéné-Mandji Thrust. The significance of the structure is emphasized by a metamorphic jump to the biotite-in grade (Fig. 8). Garnet-bearing phyllite also has been observed at several locations, however, it is unclear whether the rocks belong to the Loémé Supergroup or the Nemba Subgroup.

The intensity of Pan-African deformation increases from early open

folds with a subvertical, widely spaced schistosity in the SE, as documented in the Mountt Mbaou Syncline (Fig. 8), to thrust-dominated tectonics in the NW stacking Palaeo-with Neoproterozoic rocks and associated with penetrative S₀-₁ fabrics (Fig. 14).

The gradient of deformation varies considerably laterally and across the domain: zones of higher shear show progressive rotation of linear structures into the transport direction and curvilinear fold hinges. At outcrop scale, this is demonstrated at DOL9117 within the Loukenéné-Mandji Thrust where fold axes rotate from a shallow NW plunge, perpendicular to the transport to a down-dip SW plunge parallel to the transport. On the geological map, curvilinear traces of fold axial planes either result from differential finite strain or D3 refolding (Figs. 4 and 8).

The Palaeoproterozoic Bikossi Group is another important element of the WMC. Structurally, the unit is characterised by polyphase deformation, including refolded S₀-₁ fabrics and several generations of thrusts and faults. Contacts with adjacent Precambrian units remain poorly documented due to the lack of instructive outcrops. However, quartzite and quartz-muscovite schist of the group appear to structurally overlie both Neoproterozoic and Palaeoproterozoic rocks. A structural unconformity has already been indicated on BRGM archive maps and has been confirmed by the recent fieldwork, suggesting the interpretation of the Bikossi Group as a Neoproterozoic nappe. The geological map also shows that the Bikossi Group beds are refolded with the underlying rocks suggesting nappe emplacement prior to the main stage of D2
<figure><img src="image_17.png" /><figcaption>Fig. 14. Thin sections of metasediment from the Mount Mbaou Syncline (left) and metaignimbrite from the Kakamoéka area (right) demonstrating the change in deformation style and intensity along strike. Crossed polarized light (Xn).</figcaption></figure>

deformation.

The structural base of the Bikossi Group is exposed at DOL2153 along the N1 where the rocks overlie weathered greenschist of the Nemba Complex, which is in thrust contact with underlying orthogneiss. The 3–4 m wide, moderately SW dipping contact between the greenschist and the Bikossi Group is highly weathered but appears to be transitional. Rocks in this zone are highly sheared and form a tectonic mélange of up to metre-sized quartzite boudins and sigma clasts within a matrix of greenschist. A pronounced down-dip stretching lineation in the shear plane indicates reverse NE movements. Late stage deformation is shown by the steeply SW inclined crenulation cleavage. Petrography of the greenschist reveals faint compositional layering at low angle to S₁. An isolated graphite clast suggests that Sounda Group sediments were involved in the thrust. Few unoriented chlorite flakes illustrate post-kinematic retrograde metamorphism or hydrothermal overprint.

DOL2145 at the N1 documents over about 200 m polyphase deformation of the Bikossi Group (Fig. 15). The structural data and the interpreted sequence of deformation events are shown in Fig. 16. The penetrative schistosity S₀.₁ transposing all pre-existing sedimentary and tectonic fabrics, dips generally at low angle to the NW; it is associated with isoclinal folds of decametre-scale and an oblique stretching lineation, implying a dextral component in the E to ENE-directed movements. D2 comprises two generations of steeply S-dipping crenulation cleavages (S₂). The directions of the associated low-angle crenulation lineations scatter widely in the NW quadrant, corresponding to a stage of N-S to NE-SW shortening. The continuation of NE directed low-angle thrusting

(D3) is illustrated by a SW-dipping shear zone of metre-scale where rocks in the hanging wall are refolded into small-scale closed folds with subvertical NW striking axial planes. It is followed by late SE directed thrusting (D4), resulting in ramp/flat geometries. The structures were folded into gentle D4 folds with subvertical NW striking axial planes without associated cleavage, representing the last stage of ductile deformation. The final stage of semi-ductile to brittle deformation during Cretaceous rifting is recorded by subvertical NW and NNE-striking normal faults (D5).

Metamorphic conditions of the Bikossi Group are poorly constrained by field observation, but appear to reach amphibolite facies. Petrography of the meta-arenite shows the metamorphic texture of recrystallised quartz, varying proportions of muscovite and accessory chlorite and tourmaline (Vicat and Vellutini, 1988). The intercalated mica-schist described by the authors consists of chlorite, muscovite and biotite as main minerals, variable amounts of calcite and accessory syn-kinematic D1 garnet altered to retrograde chlorite and epidote. D2 deformation at a low angle to D1 is recorded by new growth of muscovite, chlorite and sometimes biotite around recrystallised polygonal arcs. Metamorphic petrology of garnet-bearing graphite-mica schist by Bouenitela (2019) revealed peak metamorphic conditions of 510–568 °C and 8 kbar.

### 4.2.2. Stereogram presentation

Stereogram plots of planar and linear structural data illustrate systematic change in the orientation of the small-scale structures in the West-Congo Belt (Figs. 17 and 18).

<figure><img src="image_18.png" /><figcaption>Fig. 15. Cross section at key outcrop DOL2145 illustrating polyphase deformation of the Bikossi group rocks.</figcaption></figure>
<figure><img src="image_19.png" /><figcaption>**Fig. 16.** Stereogram presentation of structural data from DOL2145 and sequence of deformation phases D1-D5.</figcaption></figure>

#### 4.2.2.1. Eastern Mayombe.
841 poles to the S₀ planes define the maximum density at 060/87 corresponding to a subhorizontal SW dipping plane (240/03). The remainder of the S₀ poles are scattered with little variation around a subvertical AC-plane (i.e. the perpendicular plane to the fold axis) whose pole coincides with the maximum of mostly subhorizontal fold axes (141/03). The data distribution defines slightly asymmetric NE-verging folds with long shallow SW-dipping limbs (many data) and short, steeper NE-dipping limbs (less data). This geometry is confirmed by locally developed S₁ fabrics (116) where the pole maximum corresponds to a moderately SW-dipping plane (219/66). S₂ fabrics are almost absent in this domain.

Shear planes (40) vary widely in orientation but define two main groups: the maximum of poles (036/60) defines a shallow SW-dipping shear zone (216/30). This group probably combines low-angle thrusts at the base of duplex structures as demonstrated in several localities. The second group are subvertical NW and SE-dipping shear zones associated with ductile normal and reverse faults. Consequently, the poles of the fault planes (87) define maxima in the same directions (Fig. 17). A number of fault planes dip again at low angles to the SW and probably correspond to reverse shear zones. Moderate to steep SW plunges of associated striae document predominantly normal movements. Subhorizontal fold axes and intersection lineations plunge in NW and SE directions with the maximum at 141/03. Few steep to vertical fold axes and stretching lineations are probably part of steeply dipping high-strain shear zones.

#### 4.2.2.2. Central Mayombe.
The AC-plane through 429 S₀ bedding poles defines a subhorizontal NW-plunging fold axis (131/02). The S₀ pole maximum (041/24) corresponds to steep SW limbs (221/66) of upright to slightly NE-verging folds. Few subhorizontal planes where measured in the hinge of the closed folds. Similar distribution patterns are shown in the S₀.₁/S₁ and S₂ stereograms (Fig. 17), which are marked by maxima at 042/39 (S₀.₁/S₁) and 049/24 (S₂), arguing for homoaxial, polyphase deformation in this domain.

The maximum density of 81 poles to thrust planes at 040/63 defines a low-angle SW-inclined plane (220/27) and, combined with the maximum of stretching lineations (230/36), indicates reverse NE-

directed transports. The local density maximum of a group of subvertical shear planes at 227/88 can be associated with small-scale bedding-parallel slip along steep fold limbs. The orientation of 32 fault planes varies widely, but the majority of poles cluster around position 019/24, corresponding to a moderate to steep SSW dip (199/66). It is uncertain whether these faults are related to late Pan-African uplift or Cretaceous rifting.

The maximum of 45 fold axes at 317/06 is close to the pole of the AC-plane (Fig. 18). However, a number of fold axes plunge moderately to the SW (238/40), close to the maximum of 27 intersection lineations (249/30), suggesting their rotation into the transport direction by simple shear. The maximum density of 39 crenulation lineations at 316/00 documents continuing NE-directed transport during the later stages of Pan-African deformation.

#### 4.2.2.3. Western Mayombe.
Stereograms of all structures illustrate more complex deformation patterns as compared to the other domains. The maximum of 180 S₀ bedding poles at 032/51 corresponds to a moderate SW-dipping average plane (212/49). However, the poles no longer define an AC-plane but scatter in a small circle with a radius of about 25° around the maximum (= contour 2.00). This pattern is repeated in the S₁/S₀.₁ stereogram, where the poles of S₁ (669) and S₀.₁ (147) planes are concentrated in the NE (and less in the SW) and define a similar maximum (042/51) corresponding to a moderate SW dip (222/49). Even S₂ crenulation cleavages show a large orientation variation: Most of the data plot in the NE quadrant where they define two maxima at 048/15 and 072/14, corresponding to steep SW (228/75) and WSW dips (252/76). In addition, shallower dips in this group indicate subsequent folding around subhorizontal NW-plunging axes (301/05). Local maxima of a second group in the NW and SE quadrants correspond to subvertical NE-SW planes (030–210) parallel to the axial planes of these folds and possibly represent S₃ fabrics, similar to those observed by Boudzoumou (1986). A third cluster at 338/07 defines a steep SSE plane (158/83).

Shear planes (99) vary widely in orientation, with their poles clustering mainly in the NE quadrant. The poorly defined maximum at 032/51 corresponds to a moderate SW dip (212/49), parallel to the maxima
<figure><img src="image_20.png" /><figcaption>Fig. 17. Stereogram presentations of planar structures of the eastern, central and western domains of the Mayombe Chain.</figcaption></figure>
<figure><img src="image_21.png" /><figcaption>Fig. 18. Stereogram presentations of linear structures from the eastern, central and western domains of the Mayombe Chain.</figcaption></figure>

of S₀ and S₁/S₀-₁ planes.

Fault planes (29) show mainly steep to vertical orientations, but the small dat set only poorly defines the dominant NE (045) and SE (156) directions. The remaining data correspond to moderate and low-angle dips without any preferred orientation. Subhorizontal to moderate plunging fault striae record mainly lateral to slightly oblique movements.

Fold axes (63), intersection lineations (54) and crenulation lineations (168) all show the same distribution pattern: their orientations scatter in the NW, SW and SE quadrants corresponding to a SW-dipping plane (240/32). Local maxima are, in decreasing order, at 305/12, 250/33 and 158/10. In contrast, stretching and mineral lineations are concentrated in a large circular area with the centre at 228/45 and a radius of 25°. A second smaller local maximum is located at 058/37.

### 4.2.3. *Pan-African metamorphism*

To date, no systematic metamorphic study has been carried out in the area between Central Mayombe and the Niari Basin. Illite crystallinity data were presented by Mickala et al. (2014) and suggest diagenetic to epizonal facies conditions in the NB. However, without additional geochronology, it is impossible to differentiate between detrital origin and new mica growth. The metamorphic paragenesis chlorite-sericite-epidote indicates overall greenschist facies conditions in the CMC, decreasing eastwards to possibly diagenetic conditions in the NB.

Precise geographical knowledge of the palaeotemperatures in this basin is one of the key parameters for determining the potential of Neoproterozoic hydrocarbon systems, the existence of which has already been proven by graphite schist of the Mvouti and Mossouva subgroups. These sediments could also occur in the NB and would be much less affected by metamorphism here.
The sample positions are shown in Fig. 19 and the geographical co-
ordinates are given in appendix 1. The analytical results are presented in
Tables 4 and 5, which show the mineral composition of the samples, the
illite crystallinity [KI; Kübler Index] determined by XRD under gly-
colised and dry air conditions, and the K-Ar ages of the <0.2 µm and <2
µm fractions.

4.2.3.1. Mineral composition. The fine-grained nature of most samples precludes determination of their mineral composition by microscopic observation alone. However, XRD analysis provides an additional tool for identifying and classifying phyllosilicates based on their crystallographic proportions. The onset of low-grade metamorphism is indicated from about 3 km west of Dolisie where penetrative S1 schistosity overprints Louila Subgroup shales (DOL2251). The schistosity at high angle to laminar bedding is outlined by metamorphic sericite, indicating temperatures above 250 °C, which is within the range of estimated closure temperatures for the K-Ar illite/muscovite system for clay size minerals. The K-Ar ages of most samples west of Dolisie therefore date the cooling of the rocks below this temperature. Exceptions are samples DOL2278 from a fault gouge of presumably Cretaceous age and DOL2155 from a mylonitic muscovite-bearing pegmatite (see below).

XRD analysis of the fine powders shows that most samples contain high to very high proportions of illite, allowing the systematic application of the Kübler Index [KI] method. Kaolinite and goethite in many samples are interpreted as tropical alteration products, documenting the onset of lateritisation in the fine-grained shales and slates whereas the common rutile is of magmatic/metamorphic origin but very resistant to weathering. Chlorite was identified in several samples of the EMC (DOL0674, DOL0676) and throughout the NB (DOL0091, SB0031, SB0040), but this has no influence on the paleotemperatures as it also crystallises under hydrothermal conditions. The peaks of this mineral coincide with those of kaolinite and iron oxides, and it is probably more common than indicated in the table. Potassium feldspar was identified in the fine fraction of only one sample (DOL2177). The absence of this mineral is crucial for the interpretation of the analysed K-Ar isotope data

because, if present, the resulting mixing ages would be of little geolog-
ical significance.

Mixing of metamorphic and detrital mica cannot be ruled out by XRD analysis, but can be evaluated by comparing the ages of the 0.2 and 2µ fractions. If both fractions give more or less the same age (Δ < 25 my), it is assumed that any detrital illite component has been reset during metamorphism. In cases where Δ > 25 my, the <0.2 µm fraction provides the age of metamorphism, while the <2 µm fraction probably still contains detrital components. However, from the high anchizone to the epizone, the <2 µm fraction provides the reliable age of metamorphic uplift and cooling below the c. 250° isotherm.

4.2.3.2. Illite crystallinity [KI] and K-Ar ages

4.2.3.2.1. *Niari Basin.* KI values of the NB samples show consistent thermal trends in the <0.2 and < 2 µm fractions but with a significant shift between the two fractions (Fig. 20). Diagenetic conditions are only indicated in one sample (SB0039) from the Bouenza Subgroup argillite at the NE start of the profile. KI values of 0.489 (<0.2 µm) and 0.426 (<2 µm) correspond to upper and uppermost diagenetic conditions with temperatures below 200 °C. The determined ages of 574.5 and 562.8 Ma are high compared to the average of 481 Ma (0.2 µm) and 507 Ma (2 µm) in the epizone (see below). Pre-Pan African ages of 792 Ma (<2 µm) and 699 Ma (<0.2 µm) were determined in sample SIB0040 with the very low illite content (indicated by the XRD method) preventing determination of the KI. These features indicate that metamorphism was too low to completely reset the detrital component, implying diagenesis.

Middle anchizone conditions are recorded by the 0.2 µm fractions of
samples SB0044, SB0031 and DOL0091 from the Schisto-calcaire Group.
A systematic increase of the palaeotemperatures to the SW is docu-
mented by decreasing KI of 0.33, 0.31 and 0.30, respectively. The cor-
responding ages of 503 Ma (SB0044), 522 Ma (SB0031) and 484 Ma
(DOL0091) reveal new illite growth in the late Pan-African. Much lower
KI values of 0.25, 0.27 and 0.22 in the 2 µm size fraction of the same
samples and significantly increased ages of 577 Ma and 583 Ma (Δ =
60–75 Ma) in the northeastern samples (SB0044, SB0031) suggest

<figure><img src="image_22.png" /><figcaption>Fig. 19. Position of samples for the analysis of the illite crystallinity and K-Ar geochronology.</figcaption></figure>
<caption>**Table 4**</caption>

Key parameters of the analysed samples and their composition determined by XRD (ab = albite, cc = calcite, chl = chlorite, dol = dolomite, gth = goethite, ill = illite, kao = kaolinite, kf = K-feldspar, ms = muscovite, qz = quartz, rt = rutile, ser = sericite, smc = smectite).

<table><thead><tr><th rowspan="2">Sample</th><th rowspan="2">Stratigraphy</th><th rowspan="2">Rock type</th><th>Size</th><th colspan="5">Minerals identified by XRD</th></tr><tr><th>[µm]</th><th>chl</th><th>ms/ill</th><th>kao</th><th>qz</th><th>others</th></tr></thead><tbody><tr><td>DOL0084</td><td>Louila</td><td>shale</td><td>&lt;2</td><td>+++</td><td>++</td><td></td><td>+</td><td></td></tr><tr><td></td><td></td><td></td><td>&lt;0,2</td><td>++</td><td>+++</td><td>+</td><td></td><td>++ chl/smc mixed layer</td></tr><tr><td>DOL0091</td><td>Schisto-calcaires (ScIIc)</td><td>limestone</td><td>&lt;2</td><td>++</td><td>++</td><td></td><td>+</td><td>++ cc + dol +++ unknown</td></tr><tr><td></td><td></td><td></td><td>&lt;0,2</td><td>++</td><td>++</td><td></td><td>+</td><td>++ cc + dol ++ unknown</td></tr><tr><td>DOL0674</td><td>Mvouti (Mv1)</td><td>qz-ser-chl schist</td><td>&lt;2</td><td>+++</td><td>+++</td><td></td><td></td><td></td></tr><tr><td></td><td></td><td></td><td>&lt;0,2</td><td>+++</td><td>+++</td><td></td><td></td><td></td></tr><tr><td>DOL0676</td><td>Mvouti (Mv3)</td><td>qz-ser schist</td><td>&lt;2</td><td>++</td><td>++</td><td>++</td><td>+</td><td>++ gth + rt</td></tr><tr><td></td><td></td><td></td><td>&lt;0,2</td><td>++</td><td>++</td><td>++</td><td></td><td>++ gth + rt</td></tr><tr><td>DOL0679</td><td>Louila</td><td>shale</td><td>&lt;2</td><td>+++</td><td>++</td><td></td><td></td><td></td></tr><tr><td></td><td></td><td></td><td>&lt;0,2</td><td>+++</td><td>++</td><td></td><td></td><td>++ gth</td></tr><tr><td>DOL2146</td><td>Bikossi</td><td>qz-ms schist</td><td>&lt;2</td><td>+++</td><td>++</td><td>++</td><td>+</td><td></td></tr><tr><td></td><td></td><td></td><td>&lt;0,2</td><td>+++</td><td>++</td><td>++</td><td>+</td><td></td></tr><tr><td>DOL2155</td><td>Loukoula</td><td>pegmatite</td><td>&lt;2</td><td>+++</td><td>+</td><td></td><td>+</td><td>peak at 15.7° unidentified</td></tr><tr><td></td><td></td><td></td><td>&lt;0,2</td><td>+++</td><td>+</td><td></td><td>+</td><td>peak at 15.7° unidentified</td></tr><tr><td>DOL2157</td><td>Bikossi</td><td>qz-ms schist</td><td>&lt;2</td><td>+++</td><td>+</td><td></td><td>+</td><td>++ gth</td></tr><tr><td></td><td></td><td></td><td>&lt;0,2</td><td>+++</td><td>+</td><td></td><td>+</td><td>++ gth</td></tr><tr><td>DOL2175</td><td>Mossouva (Mv1)</td><td>qz-ser schist</td><td>&lt;2</td><td>+++</td><td>+</td><td></td><td>+</td><td>o rt</td></tr><tr><td></td><td></td><td></td><td>&lt;0,2</td><td>+++</td><td>+</td><td></td><td>+</td><td>++ rt + gth</td></tr><tr><td>DOL2176</td><td>Mossouva (Mv1)</td><td>qz-ser schist</td><td>&lt;2</td><td>+++</td><td>++</td><td></td><td>+</td><td>++ gr o rt</td></tr><tr><td></td><td></td><td></td><td>&lt;0,2</td><td>+++</td><td>++</td><td></td><td>+</td><td>++ gr o rt</td></tr><tr><td>DOL2177</td><td>Mossouva (Mv1)</td><td>qz-ser schist</td><td>&lt;2</td><td>+++</td><td>++</td><td>++</td><td>+</td><td>o kf</td></tr><tr><td></td><td></td><td></td><td>&lt;0,2</td><td>+++</td><td>++</td><td>o</td><td>o kf</td><td></td></tr><tr><td>DOL2179</td><td>Mvouti (Mv3)</td><td>green schist</td><td>&lt;2</td><td>+++</td><td>++</td><td></td><td>+</td><td>++ rt o gth</td></tr><tr><td></td><td></td><td></td><td>&lt;0,2</td><td>+++</td><td>++</td><td></td><td>o</td><td>++ rt o gth</td></tr><tr><td>DOL2248</td><td>Mvouti (Mv2)</td><td>ser schist</td><td>&lt;2</td><td>+++</td><td>+</td><td></td><td>+</td><td>o rt</td></tr><tr><td></td><td></td><td></td><td>&lt;0,2</td><td>+++</td><td>+</td><td></td><td>o</td><td>++ gth o rt</td></tr><tr><td>DOL2251</td><td>Louila</td><td>shale</td><td>&lt;2</td><td>+++</td><td>++</td><td></td><td>+</td><td>++ gth o rt</td></tr><tr><td></td><td></td><td></td><td>&lt;0,2</td><td>+++</td><td>++</td><td></td><td>+</td><td>++ gth o rt</td></tr><tr><td>DOL2253</td><td>Louila</td><td>shale</td><td>&lt;2</td><td>+++</td><td>+++</td><td></td><td>+</td><td>o rt</td></tr><tr><td></td><td></td><td></td><td>&lt;0,2</td><td>+++</td><td>++</td><td></td><td>o</td><td>o gth o rt</td></tr><tr><td>DOL2259</td><td>Mossouva (Mv1)</td><td>ser schist</td><td>&lt;2</td><td>+++</td><td>++</td><td></td><td>o</td><td>++ rt</td></tr><tr><td></td><td></td><td></td><td>&lt;0,2</td><td>+++</td><td>++</td><td></td><td>o</td><td>++ rt</td></tr><tr><td>DOL2278</td><td>Cretaceous (?); Mvouti</td><td>fault gouge</td><td>&lt;2</td><td>+++</td><td>++</td><td></td><td>+</td><td>++ rt</td></tr><tr><td></td><td></td><td></td><td>&lt;0,2</td><td>+++</td><td>++</td><td></td><td>+</td><td>++ rt</td></tr><tr><td>SB0031</td><td>Schisto-calcaires (ScIb)</td><td>micaceous argillite</td><td>&lt;2</td><td>++</td><td>++</td><td></td><td>+</td><td>++ cc + ab ++ unknown</td></tr><tr><td></td><td></td><td></td><td>&lt;0,2</td><td>++</td><td>++</td><td></td><td>+</td><td>++ cc ++ unknown</td></tr><tr><td>SB0039</td><td>Bouenzia</td><td>micaceous argillite</td><td>&lt;2</td><td>+++</td><td>++</td><td>++</td><td>+</td><td></td></tr><tr><td></td><td></td><td></td><td>&lt;0,2</td><td>+++</td><td>++</td><td>++</td><td>o</td><td></td></tr><tr><td>SB0040</td><td>Bouenzia</td><td>micaceous argillite</td><td>&lt;2</td><td>+</td><td>+</td><td></td><td>o</td><td>++ cc +++ unknown</td></tr><tr><td></td><td></td><td></td><td>&lt;0,2</td><td>++</td><td>+</td><td></td><td>o</td><td>++ cc +++ unknown</td></tr><tr><td>SB0044</td><td>Schisto-calcaires (ScIb)</td><td>calcareous pelite</td><td>&lt;2</td><td>+++</td><td>+++</td><td></td><td>+</td><td>++ cc (not in x-ray)</td></tr><tr><td></td><td></td><td></td><td>&lt;0,2</td><td>++</td><td>+++</td><td></td><td>+</td><td>++ cc +++ unknown</td></tr></tbody></table>

+++ predominant, ++ major, + minor, o probable.

The identification of minerals was carried out on oriented compounds as they are used for illite-crystallinity measurements. Therefore, 00l-peaks are overemphasized and hkl-peaks are reduced. In some fractions calcite is listed, but is not presented in the XRD pattern, because it was removed before the x-ray analysis. In few cases the main rutile peak may conceal a very small amount of potassic feldspar (ks). The unknown mineral, peak at 7.7° in samples DOL0091, SB0031, SB0040 and SB0044, remained unidentified, because no reference pattern fits, even in a regular powder diffraction compound.

ab = albite.

cc = calcite.

chl = chlorite.

dol = dolomite.

gth = goethite.

ill = illite.

kao = kaolinite.

kf = K-feldspar.

ms = muscovite.

qz = quartz.

rt = rutile.

ser = sericite.

smc = smectite.

incomplete isotope resetting of an older component or no resetting at all.

Two samples from the Mpioka Group, rich in detrital mica, were collected near the contact with the Eastern Mayombe Domain. They yield similar KI values of 0.32/0.35 (DOL0018) and 0.27/0.28 (DOL2061) for size fractions <0.2 and < 2 µm, indicating upper and middle anchizone conditions. The corresponding ages of 521/548 Ma (DOL0018) and 503/522 Ma (DOL2061) show that illite in these

fractions crystallised or was reset during the late Pan-African. In contrast, sample DOL0091 from the underlying Schisto-calcaire Group nearby has a similar KI in the <0.2 µm fraction (0.30), while the KI of the <2 µm fraction is much lower (0.22) indicating epizonal conditions. Additional coarser size fractions of these were analysed to obtain information on their provenance. Three size fractions (2–6/6–10/10–20 µm) yielded KI values between 0.17 and 0.23 (DOL0018) and 0.17–0.22
<caption>Table 5</caption>

Presentation of the XRD analysis under dry air and glycolised conditions. The classification of illite crystallinity (using the Kübler Index, KI) into diagenesis, anchizone and epizone was done by using the standardisation program of Warr and Rice (1994).

<table><thead><tr><th rowspan="3">sample</th><th colspan="5">K-Ar isotope analysis</th><th colspan="4">Illite crystallinity [KI]</th><th colspan="3">Analytical parameters</th></tr><tr><th>Size</th><th>K2O</th><th>40 Ar *</th><th>40 Ar *</th><th>age</th><th colspan="2">2 sigma error</th><th colspan="2">Dry air</th><th colspan="2">Glycol</th><th rowspan="2">Potassium</th></tr><tr><th>[µm]</th><th>[wt. %]</th><th>[nl/g] STP</th><th>[%]</th><th>[Ma]</th><th>[Ma]</th><th>[%]</th><th>Φ Δ*2Θ</th><th>Metam. degree</th><th>Φ Δ*2Θ</th><th>Metam. degree</th><th>40K : 0.011670%</th><th>40K : 0.011670%</th></tr></thead><tbody><tr><td rowspan="5">DOL0018</td><td>10-20</td><td>nd</td><td></td><td></td><td></td><td></td><td></td><td>0.172</td><td>epi</td><td>0.173</td><td>epi</td><td rowspan="5">tot Ar : 39,9477</td></tr><tr><td>6-10</td><td>4.203</td><td>91.15</td><td>98.59</td><td>571.6</td><td>5.9</td><td>1.0</td><td>0.188</td><td>epi</td><td>0.178</td><td>epi</td></tr><tr><td>2-6</td><td>4.473</td><td>94.98</td><td>98.55</td><td>561.3</td><td>8.7</td><td>1.5</td><td>0.228</td><td>epi</td><td>0.215</td><td>epi</td></tr><tr><td>&lt;2</td><td>4.752</td><td>98.08</td><td>98.87</td><td>547.7</td><td>8.6</td><td>1.6</td><td>0.319</td><td>MA</td><td>0.321</td><td>MA</td></tr><tr><td>&lt;0.2</td><td>4.723</td><td>91.98</td><td>98.61</td><td>520.9</td><td>9.0</td><td>1.7</td><td>0.353</td><td>MA</td><td>0.322</td><td>MA</td></tr><tr><td rowspan="2">DOL0084</td><td>&lt;2</td><td>4.396</td><td>75.26</td><td>99.02</td><td>465.3</td><td>6.9</td><td>1.5</td><td>0.208</td><td>epi</td><td>0.209</td><td>epi</td><td rowspan="2">tot Ar : 39,9477</td></tr><tr><td>&lt;0.2</td><td>2.469</td><td>44.62</td><td>98.87</td><td>488.0</td><td>8.7</td><td>1.8</td><td>0.218</td><td>epi</td><td>0.247</td><td>epi</td></tr><tr><td rowspan="2">DOL0091</td><td>&lt;2</td><td>7.240</td><td>128.75</td><td>98.70</td><td>481.1</td><td>6.4</td><td>1.3</td><td>0.237</td><td>UA/EPI</td><td>0.223</td><td>epi</td><td rowspan="2">tot Ar : 39,9477</td></tr><tr><td>&lt;0.2</td><td>6.389</td><td>114.64</td><td>97.99</td><td>484.9</td><td>7.0</td><td>1.4</td><td>0.264</td><td>UMA</td><td>0.297</td><td>UA</td></tr><tr><td rowspan="2">DOL0674</td><td>&lt;2</td><td>3.373</td><td>119.57</td><td>98.97</td><td>858.4</td><td>17.2</td><td>2.0</td><td>0.147</td><td>epi</td><td>0.146</td><td>epi</td><td rowspan="2">tot Ar : 39,9477</td></tr><tr><td>&lt;0.2</td><td>3.187</td><td>54.19</td><td>91.42</td><td>462.6</td><td>5.1</td><td>1.1</td><td>0.151</td><td>epi</td><td>0.153</td><td>epi</td></tr><tr><td rowspan="2">DOL0676</td><td>&lt;2</td><td>1.985</td><td>41.32</td><td>99.08</td><td>551.8</td><td>12.0</td><td>2.2</td><td>0.158</td><td>epi</td><td>0.158</td><td>epi</td><td rowspan="2">tot Ar : 39,9477</td></tr><tr><td>&lt;0.2</td><td>1.639</td><td>29.69</td><td>99.53</td><td>489.0</td><td>7.2</td><td>1.5</td><td>0.171</td><td>epi</td><td>0.189</td><td>epi</td></tr><tr><td rowspan="2">DOL0679</td><td>&lt;2</td><td>7.700</td><td>139.19</td><td>99.69</td><td>484.2</td><td>6.5</td><td>1.3</td><td>0.224</td><td>epi</td><td>0.206</td><td>epi</td><td rowspan="2">tot Ar : 39,9477</td></tr><tr><td>&lt;0.2</td><td>6.041</td><td>112.76</td><td>98.90</td><td>502.0</td><td>5.1</td><td>1.0</td><td>0.183</td><td>epi</td><td>0.234</td><td>UA/EPI</td></tr><tr><td rowspan="5">DOL2061</td><td>10-20</td><td>nd</td><td></td><td></td><td></td><td></td><td></td><td>0.166</td><td>epi</td><td>0.139</td><td>epi</td><td rowspan="5">tot Ar : 39,9477</td></tr><tr><td>6-10</td><td>3.364</td><td>70.47</td><td>99.04</td><td>554.8</td><td>13.0</td><td>2.3</td><td>0.184</td><td>epi</td><td>0.174</td><td>epi</td></tr><tr><td>2-6</td><td>3.689</td><td>77.44</td><td>98.98</td><td>555.8</td><td>9.2</td><td>1.7</td><td>0.224</td><td>epi</td><td>0.216</td><td>epi</td></tr><tr><td>&lt;2</td><td>3.818</td><td>74.60</td><td>98.89</td><td>522.4</td><td>10.2</td><td>1.9</td><td>0.262</td><td>UA</td><td>0.259</td><td>UA</td></tr><tr><td>&lt;0.2</td><td>3.875</td><td>72.42</td><td>98.82</td><td>502.5</td><td>5.5</td><td>1.1</td><td>0.254</td><td>UA</td><td>0.268</td><td>UA</td></tr><tr><td rowspan="2">DOL2146</td><td>&lt;2</td><td>6.100</td><td>90.77</td><td>94.36</td><td>410.9</td><td>5.4</td><td>1.3</td><td>0.186</td><td>epi</td><td>0.196</td><td>epi</td><td rowspan="2">tot Ar : 39,9477</td></tr><tr><td>&lt;0.2</td><td>5.381</td><td>83.44</td><td>93.70</td><td>426.2</td><td>8.6</td><td>2.0</td><td>0.194</td><td>epi</td><td>0.197</td><td>epi</td></tr><tr><td rowspan="2">DOL2155</td><td>&lt;2</td><td>7.261</td><td>117.74</td><td>97.07</td><td>443.5</td><td>5.6</td><td>1.3</td><td>nd</td><td></td><td>nd</td><td></td><td rowspan="2">tot Ar : 39,9477</td></tr><tr><td>&lt;0.2</td><td>7.058</td><td>115.39</td><td>96.78</td><td>446.8</td><td>5.0</td><td>1.1</td><td>nd</td><td></td><td>nd</td><td></td></tr><tr><td rowspan="2">DOL2157</td><td>&lt;2</td><td>5.336</td><td>85.79</td><td>95.31</td><td>440.2</td><td>4.7</td><td>1.1</td><td>0.151</td><td>epi</td><td>0.154</td><td>epi</td><td rowspan="2">tot Ar : 39,9477</td></tr><tr><td>&lt;0.2</td><td>5.307</td><td>83.74</td><td>94.33</td><td>432.9</td><td>4.9</td><td>1.1</td><td>0.173</td><td>epi</td><td>0.170</td><td>epi</td></tr><tr><td rowspan="2">DOL2175</td><td>&lt;2</td><td>7.159</td><td>140.78</td><td>99.55</td><td>525.3</td><td>6.3</td><td>1.2</td><td>0.182</td><td>epi</td><td>0.202</td><td>epi</td><td rowspan="2">tot Ar : 39,9477</td></tr><tr><td>&lt;0.2</td><td>5.837</td><td>105.37</td><td>98.95</td><td>487.5</td><td>8.7</td><td>1.8</td><td>0.179</td><td>epi</td><td>0.179</td><td>epi</td></tr><tr><td rowspan="2">DOL2176</td><td>&lt;2</td><td>6.797</td><td>125.38</td><td>99.09</td><td>496.8</td><td>5.6</td><td>1.1</td><td>0.194</td><td>epi</td><td>0.187</td><td>epi</td><td rowspan="2">tot Ar : 39,9477</td></tr><tr><td>&lt;0.2</td><td>5.732</td><td>105.88</td><td>99.25</td><td>497.4</td><td>5.6</td><td>1.1</td><td>0.186</td><td>epi</td><td>0.186</td><td>epi</td></tr><tr><td rowspan="2">DOL2177</td><td>&lt;2</td><td>5.525</td><td>101.19</td><td>99.05</td><td>493.7</td><td>5.7</td><td>1.1</td><td>0.178</td><td>epi</td><td>0.174</td><td>epi</td><td rowspan="2">tot Ar : 39,9477</td></tr><tr><td>&lt;0.2</td><td>3.492</td><td>63.17</td><td>96.37</td><td>488.4</td><td>7.6</td><td>1.6</td><td>0.152</td><td>epi</td><td>0.151</td><td>epi</td></tr><tr><td rowspan="2">DOL2179</td><td>&lt;2</td><td>2.147</td><td>38.85</td><td>98.28</td><td>488.6</td><td>8.0</td><td>1.6</td><td>0.169</td><td>epi</td><td>0.158</td><td>epi</td><td rowspan="2">tot Ar : 39,9477</td></tr><tr><td>&lt;0.2</td><td>1.765</td><td>30.38</td><td>98.35</td><td>467.5</td><td>7.0</td><td>1.5</td><td>0.152</td><td>epi</td><td>0.160</td><td>epi</td></tr><tr><td rowspan="2">DOL2248</td><td>&lt;2</td><td>6.276</td><td>115.74</td><td>96.73</td><td>496.7</td><td>6.2</td><td>1.2</td><td>0.188</td><td>epi</td><td>0.189</td><td>epi</td><td rowspan="2">tot Ar : 39,9477</td></tr><tr><td>&lt;0.2</td><td>5.660</td><td>122.11</td><td>95.15</td><td>569.0</td><td>7.7</td><td>1.3</td><td>0.175</td><td>epi</td><td>0.170</td><td>epi</td></tr><tr><td rowspan="2">DOL2251</td><td>&lt;2</td><td>4.790</td><td>92.93</td><td>98.99</td><td>519.5</td><td>8.3</td><td>1.6</td><td>0.193</td><td>epi</td><td>0.202</td><td>epi</td><td rowspan="2">tot Ar : 39,9477</td></tr><tr><td>&lt;0.2</td><td>3.758</td><td>72.38</td><td>99.00</td><td>515.9</td><td>7.1</td><td>1.4</td><td>0.206</td><td>epi</td><td>0.201</td><td>epi</td></tr><tr><td rowspan="2">DOL2253</td><td>&lt;2</td><td>3.380</td><td>68.67</td><td>99.40</td><td>539.8</td><td>9.7</td><td>1.8</td><td>0.194</td><td>epi</td><td>0.197</td><td>epi</td><td rowspan="2">tot Ar : 39,9477</td></tr><tr><td>&lt;0.2</td><td>2.794</td><td>52.81</td><td>99.64</td><td>507.5</td><td>7.9</td><td>1.6</td><td>0.261</td><td>UA</td><td>0.250</td><td>UA/EPI</td></tr><tr><td rowspan="2">DOL2259</td><td>&lt;2</td><td>5.822</td><td>113.38</td><td>99.55</td><td>520.9</td><td>6.7</td><td>1.3</td><td>0.194</td><td>epi</td><td>0.183</td><td>epi</td><td rowspan="2">tot Ar : 39,9477</td></tr><tr><td>&lt;0.2</td><td>3.850</td><td>72.26</td><td>96.61</td><td>504.4</td><td>5.7</td><td>1.1</td><td>0.181</td><td>epi</td><td>0.181</td><td>epi</td></tr><tr><td rowspan="2">DOL2278</td><td>&lt;2</td><td>1.277</td><td>21.84</td><td>99.03</td><td>465.0</td><td>7.0</td><td>1.5</td><td>0.173</td><td>epi</td><td>0.174</td><td>epi</td><td rowspan="2">tot Ar : 39,9477</td></tr><tr><td>&lt;0.2</td><td>0.695</td><td>8.32</td><td>68.06</td><td>337.6</td><td>13.3</td><td>3.9</td><td>0.162</td><td>epi</td><td>0.159</td><td>epi</td></tr><tr><td rowspan="2">SB0031</td><td>&lt;2</td><td>4.150</td><td>92.18</td><td>98.55</td><td>583.4</td><td>9.6</td><td>1.6</td><td>0.269</td><td>UA</td><td>0.265</td><td>UA</td><td rowspan="2">tot Ar : 39,9477</td></tr><tr><td>&lt;0.2</td><td>5.599</td><td>109.40</td><td>99.01</td><td>522.4</td><td>6.5</td><td>1.3</td><td>0.286</td><td>UA</td><td>0.306</td><td>MA</td></tr><tr><td rowspan="2">SB0039</td><td>&lt;2</td><td>4.185</td><td>91.30</td><td>98.98</td><td>574.5</td><td>12.9</td><td>2.2</td><td>0.432</td><td>UD/LA</td><td>0.426</td><td>UD/LA</td><td rowspan="2">tot Ar : 39,9477</td></tr><tr><td>&lt;0.2</td><td>5.354</td><td>114.05</td><td>98.85</td><td>562.8</td><td>6.3</td><td>1.1</td><td>0.494</td><td>UMD</td><td>0.489</td><td>UMD</td></tr><tr><td rowspan="2">SB0040</td><td>&lt;2</td><td>3.954</td><td>126.72</td><td>98.71</td><td>791.6</td><td>11.4</td><td>1.4</td><td colspan="3">no peak</td><td colspan="2">no peak</td></tr><tr><td>&lt;0.2</td><td>4.040</td><td>111.14</td><td>97.80</td><td>698.5</td><td>9.6</td><td>1.4</td><td colspan="3">no peak</td><td colspan="2">no peak</td></tr><tr><td rowspan="2">SB0044</td><td>&lt;2</td><td>4.340</td><td>95.31</td><td>97.80</td><td>577.7</td><td>6.0</td><td>1.0</td><td>0.266</td><td>UA</td><td>0.250</td><td>UA/EPI</td><td rowspan="2">tot Ar : 39,9477</td></tr><tr><td>&lt;0.2</td><td>6.323</td><td>118.34</td><td>98.88</td><td>503.2</td><td>5.8</td><td>1.2</td><td>0.285</td><td>UA</td><td>0.327</td><td>MA</td></tr><tr><td rowspan="2">DOL2155<br>pegmat.</td><td>&lt;2</td><td>10.791</td><td>1047.92</td><td>99.40</td><td>1771.0</td><td>18.5</td><td>1.0</td><td colspan="3">no peak</td><td colspan="2">no peak</td></tr><tr><td>&lt;0.2</td><td></td><td></td><td></td><td></td><td></td><td></td><td>0.161</td><td>epi</td><td>0.166</td><td>epi</td><td rowspan="2">tot Ar : 39,9477</td></tr><tr><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>0.213</td><td>epi</td><td>0.211</td><td>epi</td><td></td></tr></tbody></table>
<figure><img src="image_23.png" /><figcaption>**Fig. 20.** Tectono-metamorphic domains of the study area and sample positions together with the results of illite crystallinity analysis (under glycolised conditions) and K-Ar geochronology. A) < 0.2µ size fraction; b) < 2µ size fraction.</figcaption></figure>

(DOL2061), indicating epizonal or higher metamorphic conditions. The corresponding K-Ar ages of 561/571 Ma (DOL0018) and 555/556 Ma (DOL2061) are significantly increased compared to those of the <0.2 and < 2 µm fractions, but similar to those in the samples further north-east (Fig. 20).

#### 4.2.3.2.2. Eastern Mayombe.
KI values between 0.20 and 0.25 with no systematic shift were obtained in the <0.2 µm size fraction of samples DOL0679, DOL2251, DOL0084 and DOL2253, which constrains thermal conditions of the uppermost anchizone to epizone. The corresponding <2 µm size fraction of these samples yielded significant lower KI values, resulting in a shift of the anchizone/epizone for more than 30 km to the east, coinciding with the Mount Bélo Shear Zone (Fig. 20). The K-Ar isotope ages of both fractions vary between 488 and 516 Ma and are interpreted as late Pan-African cooling ages.

#### 4.2.3.2.3. Central Mayombe.
Epizonal conditions are recorded from the Moukondo back-thrust onwards in both the CMB and WMB in both size fractions of almost all samples (Table 5). The KI values vary between 0.15 and 0.21, with no systematic trend evident.

The <0.2 µm ages of Mossouva Group samples decrease almost continuously from 516 Ma at Moukondo to 468 Ma at Mvouti (Fig. 20).

The same applies to a lesser extent to the < 2 µm fraction. Again, the age difference between the two fractions is less than 25 my in most samples, indicating a complete isotope reset during the late Pan-African. The systematic westward rejuvenation could be caused either by longer cooling times of the increasingly higher metamorphic rocks or by increasing thermal overprint due to the Cretaceous rifting.

The samples of the Mvouti Subgroup show considerable age differences in the <0.2 and < 2 µm fractions from 63 to 395 Ma (Fig. 20). The <0.2 µm fraction can be both younger and older than the <2 µm fraction, arguing against a complete reset of the K-Ar mica system during the Pan-African orogeny. Alternatively, small amounts of detrital potassium feldspar of the often arkosic rocks could lead to mixing ages, even though the mineral was not detected by XRD.

Sample DOL2278 from a fault gouge yielded the youngest age of all samples for the <0.2 µm fraction (338 ± 13 Ma). Field observation reveals reactivation of an older Pan-African reverse fault in the hinge of a regional fold by normal faulting. A major fault breccia nearby may be related to Cretaceous rifting.

#### 4.2.3.2.4. Western Mayombe.
Epizonal or higher metamorphic conditions are found in all samples of the Western Mayombe Domain in both
size fractions (Table 5, Fig. 20). The analysed KI values vary between 0.15 and 0.21 without a systematic trend being discernible. Three fine-grained muscovite concentrates were analysed from samples of the Bikossi Group. The rocks are characterised by pervasive $S_{0.1}$ schistosity and record lower amphibolite facies conditions (Bouenitela, 2019). Muscovite geochronology defines the youngest age group of 447-411 Ma. Age differences of always <15 my between both size fractions document complete reset of the K-Ar isotope system.

One K-Ar age of 1771 ± 19 Ma was determined on coarse-grained muscovite from a Palaeoproterozoic (Eburnean) pegmatite overprinted by Pan-African thrusting and mylonitisation (DOL2155). The data show that metamorphic temperatures were not high enough to reset the K-Ar clock. However, the fine fractions of this sample yielded nearly identical post-Pan-African ages of 444 Ma (<2 µm) and 447 Ma (<0.2 µm), indicating the complete reset of the fine-grained fraction in the Upper Ordovician.

# 5. Discussion

## 5.1. Eburnean orogeny

The geochronological data of our study show that the Loémé Supergroup consists of Palaeoproterozoic rocks that evolved from about 2100 Ma during the middle and final stages of the Eburnean orogeny. Geological mapping currently allows the distinction of three main units.

i) The Loukoula Group comprises isoclinally folded migmatitic paragneisses of semipelitic and psammitic origin with minor intercalated layers of amphibolite derived from basalts. The maximum depositional age is constrained by detrital zircon geochronology at about 2111 ± 14 Ma (Table 3). This implies correlation of the main phase of metamorphism and deformation with E2 in Gabon. Detrital ages from 2200 to 3192 Ma document erosion of Mesoarchaean to Palaeoproterozoic rocks in the source area.

ii) The Loukoula Group was intruded by the calc-alkaline Bilinga Suite comprising granite, granodiorite and tonalite. The age of magmatism is dated at 2065 ± 13 Ma and 2028 ± 12 Ma, and constrains the minimum E2 age in the hosting Loukoula paragneiss (Fullgraf et al., 2015; Affaton et al., 2016; Bouenitela, 2019). The Bilinga Suite is in turn overprinted by late-Eburnean (E3) amphibolite facies metamorphism as indicated by the syn-tectonic biotite-garnet-feldspar paragenesis. Structures in the Bilinga and Bilala plutons record early NNE or SSW and subsequent westerly transports. Post-kinematic garnet growth shows either that E3 metamorphism has outlasted deformation or is associated with Pan-African metamorphism. The Les Saras granite orthogneiss, 20 km further east, records both E- and W-directed transports. Subsequent ENE-directed reverse thrusting, parallel to the main foliation took place in discrete shear zones near the contact with the country rocks and may be of Pan-African age.

iii) The Bikossi Group comprises mainly muscovite-bearing quartzite and quartz-muscovite (biotite) schist with minor intercalated metaconglomerate and mafic sills or transposed dykes. The maximum depositional age is constrained by detrital zircon geochronology at ca 1971 ± 41 Ma while the minimum age predates Tonian rift magmatism. The unit is thrust over, stacked and folded with Neoproterozoic and Palaeoproterozoic rocks suggesting its structural interpretation as Pan-African nappe. Metamorphic petrology of intercalated garnet-biotite-graphite schist yielded P/T conditions of 510–568 °C and 8 kbar (Bouenitela, 2019). The rather unusual combination of graphite schist and quartzite resembles the Francevillean A and B in the foreland of the Eburnean orogeny of which the unit could be a

metamorphic equivalent but also could result from stacking of Bikossi quartzite with Sounda Group rocks.

Comparison with the Palaeoproterozoic history in Gabon (Thiéblemont et al., 2009; Fig. 21 shows in our study area no evidence for the pre-Eburnean (2500-2400 Ma) orogeny (M1, P1, S1) nor the first high-grade metamorphic event E1 (2200-2120 Ma) recorded in the Ogooué and Lambaréné-Waka domains. Instead, detrital zircon ages between 2400 and 2500 Ma in samples of the Loukoula and Bikossi groups demonstrate erosion of pre-Eburnean Palaeoproterozoic rocks in the hinterland of the sediments.

The age of the (main) Eburnean E2 event is bracketed in Gabon (Table 6) between 2122 ± 43 Ma (Feybesse et al., 1998) and 2074 ± 8 Ma (Thiéblemont et al., 2009). It is interpreted as accretion of Archaean blocks and Palaeoproterozoic sediments along the active margin of the West Gabon Block followed by thrusting of all units over the East Gabon Block. Crustal thickening and a prograde clockwise P-T path in the internal zone of the Eburnean orogeny reached peak conditions of 9 kbar and 810 °C (at ca. 2100 Ma) whereas a counterclockwise P-T path marks the evolution of the external zone. P2 plutonism from 2080 to 2040 Ma accompanied and outlasted E2 deformation and metamorphism.

In the study area, E2 metamorphism reached in Loukoula paragneiss upper amphibolite facies resulting in the anatexis of semipelitic rocks. The intrusion of the non-migmatitic Bilinga Suite constrains the minimum age of E2 at 2065 ± 13 Ma. By excluding metamorphic ages older than 2111 Ma, the remaining data from Gabon yield a mean of 2091 ± 18 Ma (Table 6) further narrowing the minimum depositional age of the group to between 2110 and 2090 Ma. This suggests stratigraphic correlation with the fore-arc related Ndjole Group (2118 ± 10 Ma to 2027 ± 6 Ma), or the basal Francevillian Group FA (>2075 Ma), that in Gabon was deposited in eastern foreland basins of the Eburnean orogen (Feybesse et al., 1998; Prian et al., 2009a-b; Thiéblemont et al., 2009; Weber et al., 2016). Sedimentation of the Loukoula Group between E1 and E2 suggests short-lived synorogenic basins that soon after opening underwent thrust tectonics and metamorphism.

The late Eburnean event E3 [2040–1970 Ma] has been associated in Gabon with an advanced stage of uplift and retrograde metamorphism, completing the return of the overthickened crust to isostatic equilibrium (Feybesse et al., 1998). The period also marks the transition from tangential to transcurrent tectonics associated with P3 granite magmatism. In our study area, E3 can be related to a similar scenario: NNE-SSW movements parallel to the orogen are locally preserved and in line with transcurrent tectonic style. E3 gneissic layering and crenulation cleavage vary considerably in orientation but verge in several places westwards, towards the internal part of the Eburnean orogen. Kinematic indicators document in different localities widely varying transports into NE, NW and SW directions. These features are possibly expressions of orogenic uplift leading to the formation of gneiss domes typically characterised by such structural complexity (Whitney et al., 2004). Decompression melting during exhumation can also explain the post-E2 magmatism of the Bilinga Suite similar to the setting described by Yang et al. (2020). NdT<sub>DM</sub> model ages of 2.6–3.0 Ga (Djama, 1988; Djama et al., 2006) document considerable contribution of Archaean crust to magma genesis. The P3 phase of magmatism in Gabon is not recorded in the study area or cannot be separated from P2 magmatism.

Our reconstruction of Eburnean orogeny differs from that proposed in Gabon (Thiéblemont et al., 2009; Weber et al., 2016). The main phase of Eburnean plutonism P2 (2080–2040 Ma) also post-dates E2, but is considered in Gabon as active continental arc magmatism. E3 subsumes the final stage of ocean closure, continental collision and regional metamorphism, as well as late orogenic uplift. The main argument against this model is that P2 magmatism clearly post-dates the main phase E2 of deformation and regional metamorphism in the Loukoula Group. The model in Gabon also does not explain why P2 plutonism only occurred towards the end of the 2200-2040 Ma long subduction phase shortly before or during continental collision.
<figure><img src="image_24.png" /><figcaption>**Fig. 21.** Comparison of the geological history of the Palaeoproterozoic in the eastern Bahia Belt (Brazil), in Gabon, and in the study area (Congo-Mayombe) and their plate tectonic interpretation.</figcaption></figure>

The correlation of the Eburnean event with the Transamazonian orogeny in Brazil has been widely discussed (Teixeira and Hereida Figueiredo, 1991; Ledru et al., 1994; Feybesse et al., 1998; Alkmim and Marshak, 1998; Zhao et al., 2002; Barbosa and Sabaté, 2004; Lerouge et al., 2006; Heilbron et al., 2008). The Minas Basin in Brazil records western passive margin sequences between 2600 and 2400 Ma.

Eastward-directed subduction of oceanic crust at the eastern active margin produced successively younger continental and oceanic magmatic arcs (Teixeira et al., 2015) with multiple phases of accretion, metamorphism and magmatism from about 2350 to 2100 Ma. Continent collision with the Congo Craton from 2125 to 2100 Ma includes all, overall crustal thickening, regional metamorphism, thrust tectonics and
<caption>**Table 6**</caption>

Summary of Palaeoproterozoic metamorphic ages determined in Gabon. Only ages younger than 2122 Ma were used to calculate the mean age and 2-sigma errors of granulite facies metamorphism.

<table><thead><tr><th>Domain</th><th>Rocktype</th><th>sample</th><th>Lat</th><th>Lon</th><th>Method</th><th>Mineral</th><th>Age [Ma]</th><th>Interpretation</th><th>Reference</th></tr></thead><tbody><tr><td rowspan="2">West-Congo Belt</td><td>tillite</td><td>FOU1234</td><td>10.987</td><td>-1.6199</td><td>microprobe</td><td>monazite</td><td>2074±8</td><td>inherited (metamorphic age)</td><td rowspan="2">Thiéblemont et al., 2009</td></tr><tr><td>tillite</td><td>FOU1233b</td><td>10.9862</td><td>-1.62062</td><td>microprobe</td><td>monazite</td><td>2076±11</td><td>inherited (metamorphic age)</td></tr><tr><td rowspan="7">Lambarene/Wake</td><td>Fe-quartzite</td><td>FOU1065</td><td>10.8491</td><td>-1.27377</td><td>SHRIMP</td><td>zircon</td><td>2064±13</td><td>granulite facies metamorphism</td><td></td></tr><tr><td>Fe-quartzite</td><td>FOU1065</td><td>10.8491</td><td>-1.27377</td><td>SHRIMP</td><td>zircon</td><td>2094±19</td><td>granulite facies metamorphism</td><td></td></tr><tr><td>granodiorite</td><td>LAM1266</td><td>10.6556</td><td>-0.75445</td><td>SHRIMP</td><td>zircon</td><td>2093±13</td><td>granulite facies metamorphism</td><td></td></tr><tr><td>gneiss</td><td>LAM1232A</td><td>10.1895</td><td>-0.582917</td><td>microprobe</td><td>monazite</td><td>2147±19</td><td>metamorphism</td><td></td></tr><tr><td>migmatitic orthogneiss</td><td>LAMB028A</td><td>10.2172</td><td>-0.6309</td><td>SHRIMP</td><td>zircon</td><td>2445±27</td><td>migmatitic event</td><td></td></tr><tr><td>metagabbro</td><td>FOU0122A</td><td>10.5564</td><td>-1.06303</td><td>SHRIMP</td><td>zircon</td><td>2461±8</td><td>HT metamorphism</td><td></td></tr><tr><td>metagabbro</td><td>FOU0122A</td><td>10.5564</td><td>-1.06303</td><td>Sm/Nd</td><td>wr-ga</td><td>2469±35</td><td>HT metamorphism</td><td></td></tr><tr><td rowspan="6">Ogooué</td><td>Abamié migmatite</td><td></td><td></td><td></td><td>microprobe</td><td>monazite</td><td>2088±32</td><td>metamorphism (?)</td><td>Mayaga-Mikolo, 1996</td></tr><tr><td>garnet amphibolite</td><td></td><td></td><td></td><td>Sm/Nd</td><td>wr-ga</td><td>2120±38</td><td>HP metamorphism</td><td>Feybesse et al. (1998)</td></tr><tr><td>Abamié migmatite</td><td></td><td></td><td></td><td>microprobe</td><td>monazite</td><td>2122±43</td><td>metamorphism (?)</td><td rowspan="3">Mayaga-Mikolo, 1996</td></tr><tr><td>Abamié migmatite</td><td></td><td></td><td></td><td>microprobe</td><td>monazite</td><td>2208±38</td><td>metamorphism (?)</td></tr><tr><td>Abamié migmatitic orthogneiss</td><td></td><td></td><td></td><td>microprobe</td><td>monazite</td><td>2494±49</td><td>emplacement or metamorphism</td></tr></tbody></table>

<figure><img src="image_25.png" /><figcaption>**Fig. 22.** The orientation of lineations varies systematically across the Mayombe Chain illustrating the change in strain ellipsoid and deformation style from coaxial (pure shear) in the eastern and central domains to non-coaxial (with increasing component of simple shear) in the western domain.</figcaption></figure>
development of synorogenic intramontane and external molasse basins. Orogenic collapse of the over-thickened crust resulted in crustal extension with the formation of metamorphic core complexes dated at 2095 Ma. They are marked by peak P/T conditions of 7 kbar and 850 °C, leading to local anatexis of the rocks.

This model is similar to the one of our study, but diachronous and indicates post-organic uplift and extension already during the E2 phase in Congo (i.e. 2095 ± 13 Ma). Furthermore, metamorphism and deformation in metamorphic core complexes is typically of rather local extent (Whitney et al., 2013), whereas the data from Gabon and Congo clearly document the regional scale of E2.

## 5.2. Pan-African deformation

Geological mapping suggests subdivision of the study area into four tectonic domains separated from each other by first order tectonic structures. An unfolded eastern domain, the Niari Basin (NB), is bounded by the Mount Belo Shear Zone with folded Mayombe rocks, which are subdivided into the eastern EMC), central (CMC) and western Mayombe (EMC) chain. Structural data and field observation (Figs. 16 and 17) documents increasing deformation from the EMC to the WMC.

The orientation of planar structures changes from predominant subhorizontal $S_0$, sub-vertical $S_1$ in the EMC through moderate to steep SW-inclined $S_0$, $S_1$ and $S_{0-1}$ in the CMC to moderate SW-dipping planes in the WMC. $S_{0-1}$ and $S_2$ structures are absent in the EMC, rare in the CMC and abundant in the WMC. In the CMC, $S_2$ and shear planes are SW inclined, while they have a complex arrangement in the WMC. All this reflects the increasing stress acting on the rocks and progressive change from pure to simple shear regimes. Further differences in the tectonic history of the WMC are the onset of Pan-African nappe tectonics and the stacking of Neoproterozoic rocks with the Palaeoproterozoic basement.

Fig. 22 illustrates the variation in the orientation of lineations and associated finite strain ellipsoids. In the EMC and most of the CMC, the structures show little variation around the general NW trend with the $σ1$ axis of strain ellipsoids perpendicular to the main transport direction. Few SW-plunging mineral and stretching lineations were noted in discrete shear zones and faults. Together with the predominant symmetric fold style, this documents co-axial deformation under mainly pure shear conditions. The drastic change across the Loukenéné-Mandji Thrust is illustrated by numerous SW plunging lineations and strain ellipsoids with variable orientations, indicating the onset of NE-directed thrust tectonics under increasingly simple shear conditions.

The fold style changes in the WMC laterally from open to closed folds with east-trending axes and steep $S_1$ schistosity at high angle to the bedding in the south (i.e. Mount Mbaou Syncline) to tight NE-verging folds with bedding-parallel $S_{0-1}$ schistosities in the NE. Fold axes, intersection and crenulation lineations plunge within a shallow SW-inclined plane to the NW, SW and SE, reflecting variable deformation of the rocks. Subhorizontal NW axes and crenulations perpendicular to the main NE transport document low-strain, co-axial deformation and pure shear. SW-plunging lineations (fold axes, crenulation, stretching and mineral lineations) are attributed to high-strain, and non-coaxial deformation.

Fig. 22 shows that many SW plunging stretching lineations occur in Palaeoproterozoic rocks near the Loukenéné-Mandji Thrust. Their origin within the Eburnean orogeny cannot be excluded, for which in Gabon similar eastward-directed transports were described (Feybesse et al., 1998). However, the same directions have been measured in Neoproterozoic rocks near the thrust and, together with the large variation in the orientation of Eburnean stretching lineations outside the thrust zone, argues against such interpretation.

## 5.3. Pan-African metamorphism

The K-Ar mica ages vary widely from 699 to 338 Ma (<0.2 µm fraction) and 858-411 Ma (<2 µm fraction). KI values of 0.15-0.49

(<0.2 µm fraction) and 0.15-0.43 (<2 µm fraction) document upper diagenetic to epizonal or higher metamorphic conditions. The profile from the Niari Basin to the WMC reveals systematic trends in the illite crystallinity (KI) and K-Ar ages documenting superposition of several geological events (Fig. 23).

KI values in the easternmost part of the NB correspond to upper diagenesis conditions that in location SB0040 were too low to reset the K-Ar clock in the detrital fine fractions. The ages of 700 ± 10 Ma and 791 ± 11 Ma are interpreted as mixing of Pan-African illite with an older detrital component (Fig. 23). Sample SB0039 nearby yielded ages of 562 ± 6 and 574 ± 13 Ma and the highest KI values typifying upper diagenesis conditions. In the central part of the basin, anchizational conditions were determined in two samples (Schisto-calcaire Group) with systematically higher KI values in the <0.2 µm fractions. The corresponding K-Ar ages shift from 577 ± 6 and 583 ± 10 Ma (<2 µm) to 503 ± 6 and 522 ± 7 Ma (<0.2 µm). The combined data suggest two Pan-African events, one between 590 and 570 Ma (M1) and onther one between 520 and 500 Ma (M2). In the Mpioka Group, near the EMC-NB boundary, detrital mica (6-20 µm) yielded M1 ages whereas the finer-grained illite fractions record mostly M2 ages. This argues for the interpretation of the group as molasse of the West Congo Belt and its deposition and burial between 570 and 520 Ma between a first phase of orogenic uplift and late thermal overprint.

In the EMC, KI values indicate epizone (greenschist) conditions in the <2 µm fraction of all samples and anchizone conditions in the <0.2 µm fraction of 4/5 samples. The K-Ar ages of both fractions vary mainly in the interval 525-475 Ma. These features are interpreted to record M1 greenschist facies metamorphism and recrystallization of the fine fraction (<0.2 µm) to anchizone conditions during M2. The same K-Ar ages in both fractions document complete resetting of the argon clock during M2 and temperatures above 250 °C. The model implies that the epizone-anchizone contact shifted during M2 for more than 30 km towards the west coinciding with the EMC-CMC boundary (Fig. 20).

In the CMC, KI values of both fractions document greenschist facies or higher metamorphic conditions and most K-Ar ages scatter in the M2 interval of 525-475 Ma. However, samples of the Mvouti Subgroup yielded several older ages between 858 and 550 Ma. Detrital K-feldspar is a major component in the sediments of this subgroup and may have disturbed the geochronological analysis event though it was not detected by XRD.

In the WMC, the KI values of all samples indicate greenschist facies or higher metamorphic conditions, while much younger post-Pan-African K-Ar ages scatter in the interval of 450-400 Ma. They may evidence long-lasting differential uplift of the West Congo belt in the final stage of the Pan-African orogeny or partial isotope resetting during Cretaceous rifting.

The average K-Ar age of M2 was calculated by selecting only those samples where the difference between the <2 µm and <0.2 µm fraction is less than 25 Ma (i.e. from Mvouti to Mount Bélo). Samples from the WMC were excluded as they obviously form a younger age group of their own. The same applies to samples east of Mount Bélo, which either document M1 or mixing with detrital components. The remaining eleven samples (DOL2179-DOL2061, excluding DOL2253) yield a mean age of 495 ± 17.7 Ma using the algorithm of Wendt and Carl (1991), which is interpreted as the time of M2 cooling below the 250 °C isotherm.

Metamorphic petrology and geochronology by Bouenitela (2019) shows that the Palaeoproterozoic Loémé and Loukoula groups were affected by at least two phases of metamorphism, reaching peak conditions of 550-600 °C and 4-6 kbar, and associated with the Eburnean and Pan-African orogenies. The structurally overlying Bikossi Group records a single metamorphic event characterised by peak conditions of 510-568 °C and 8 kbar implying the interpretation as nappe. ⁴⁰Ar-³⁹Ar analyses of biotite, muscovite and amphibole indicate three phases of Pan-African metamorphism and deformation at 620-610 Ma, 545-540 Ma and 510-496 Ma. The oldest and youngest phases can be correlated with M1 and M2 of our study, whereas the memory of the intermediate
<figure><img src="image_26.png" /><figcaption>**Fig. 23.** Presentation of the K-Ar data and KI of the samples in relation to their relative position on the geo-traverse. The KI values decrease steadily from Sibiti to Moukoundou, indicating the increase in palaeotemperatures. The solid squares enclose K-Ar ages of the individual samples. The regression curves (3rd order) through the K-Ar ages of both size fractions are shown as dark and light green dashed lines, documenting the general decrease in age from ENE to WSW with a mean age of 575 Ma and 500 Ma in the ENE and eastern parts of the profile, respectively. The sample localities are indicated in the bottom line, while the sampled stratigraphic units are listed below the sample numbers.</figcaption></figure>

phase is not preserved in the K-Ar system of the illite fine fractions.

## 5.4. Pan-African orogeny

Review of circum-Atlantic Pan-African terranes (Almeida et al., 2000; Alkmim et al., 2006; Pedrosa-Soares et al., 2008, Pedrosa-Soares and Alkmim, 2011; Valeriano et al., 2011; Machado et al., 1996; Schmitt

et al., 2004; Teixeira et al., 1987 and references therein, Fig. 24) reveals the polyphase and diachronous evolution from Neoproterozoic to Palaeozoic times and allows the interpretation of the West Congo Belt in a broader regional context. The most important stages of this history are outlined here again:

A first phase of global diachronous Neoproterozoic rifting is commonly associated with the break-up of the Mesoproterozoic
<figure><img src="image_27.png" /><figcaption>**Fig. 24.** The main crustal architecture and Pan-African tectono-metamorphic events affecting the circum-Atlantic terranes of the South American and African continents.</figcaption></figure>

supercontinent Rodinia (Li et al., 2008). In central to SW Africa, it is recorded by rift volcanics and sediments of the West Congo and Damara belts, dated at ca. 1000-860 Ma (Zadinian, Mayumbian groups, Congo; Tack et al., 2001) and 750-700 Ma (Naauwpoort volcanics, Namibia; Miller and Burger, 1983a, b).

Diachronous ocean spreading and island arc accretion are documented in various terranes around the São Francisco and Congo cratons. The earliest record of arc magmatism (and possibly metamorphism associated with arc accretion) has been dated between 890-630 Ma and 790-585 Ma in outboard terranes of the Brasilia and Kaoko belts, respectively (Laux et al., 2005; Goscombe and Gray, 2007).

However, a continental bridge connected the São Francisco and Congo cratons during the Neoproterozoic in the north. Limited ocean opening from 630 to 575 Ma is evidenced by the Ediacaran Rio Doce magmatic arc and ophiolite complex (Tedeschi et al., 2016).

Diachronous continent-continent collision comprises three main events. (1) Initial collision between the Paranapanema and the São Francisco paleocontinents records peak metamorphism and regional

anatexis between 640 and 630 Ma (Valeriano et al., 2004, 2008) but this could already be related to decompression melting during orogenic collapse. The event did not affect the African side, which at that time was separated from the São-Francisco craton by an inland sea basin, or large gulf, partly floored by oceanic crust. (2) Closure of the confined basin between 580 and 540 Ma resulted in the Kaoko-Ribeira-Araçuai-Western Congo orogen (Brückner et al., 2000; Goscombe et al., 2005). High T regional metamorphism and widely varying transport directions (Alkmim et al., 2006, Fig. 24) can be related to oblique collision between the Congo and São Francisco cratons, which in turn may have been triggered by the collision of the Amazonia and São Francisco cratons. This orogeny represents the principal Pan-African event in the Western Congo Belt. (3) In the south, the main collision of the Kalahari and Congo cratons took place from 550 to 530 Ma. The subsequent phase of orogenic collapse and decompression melting between 530 and 500 Ma (Miller, 1983) may also have affected the West Congo Belt. Emplacement of numerous late-orogenic granite intrusions in the central Ribeira Belt between 513 and 481 Ma (Valeriano et al.,
2011) coincides with K-Ar metamorphic white mica ages in the Araçuai and West Congo belts (525-480 Ma; Teixeira et al., 1989; Machado et al., 1983; this study). The Cambro-Ordovician granite belt probably marks the zone where preferential heating and melting of lower continental crust occurred, caused by slab break-off, or extensional collapse of the hot, overthickened continental crust (Valeriano et al., 2016).

The K-Ar age data of this study record the main (M1, 580-540 Ma) Pan-African tectono-metamorphic event of the Araçuai Belt (i.e. the closure of the confined orogeny) in sediments of the Bouenza and Schisto-calcaire groups. M1 is also documented in detrital mica of the Mpioka Group, which thus represents the molasse basin of the West Congo Belt. M1 metamorphism is characterised by anchizonal metamorphic conditions in the central and western part of the Niari Basin, increasing west of the Mount Bélo shear zone to the epizone (greenschist facies).

The finer-grained <0.2 µm fraction was reset during M2 and therefore records the metamorphic conditions and age of this event with a shift of the contact between the epi- and anchizones 30 km to the west. The age of 495 ± 17.7 Ma is consistent with late magmatism and K-Ar metamorphic ages in the Araçuai Belt. The West Congo Belt therefore forms the eastern external zone of the Araçuai Belt. The onset of nappe tectonics is documented in the western Mayombe by the Palaeoproterozoic Bikossi Sediments that have been thrust over Neoproterozoic rift volcanics.

## 5.5. Hydrocarbon potential of the Niari Basin

The present study demonstrates the only limited hydrocarbon potential of the Niari Basin. The first metamorphic event (580-570 Ma) already reached upper anchizonal conditions in the central part of the basin. Diagenetic to lower anchizone conditions have been determined in the easternmost part filled by sandstones and shales of the Bouenza Subgroup without any potential source rocks. Neoproterozoic rift sediments may occur below the Bouenza Subgroup whose stratigraphic equivalent would be the graphitic sediments of the Sounda and Mayombe groups in the central and western Mayombe. The study of the basin geometry of both the Niari and the Komba basins in its northeastern continuation, by geophysical methods will be interesting to establish the presence of an underlying pre-Bouenzian rift. Numerous Meso-to Neoproterozoic dolerite dykes in the Archaean basement to the east demonstrate that the entire study area underwent crustal extension during this time.

## 6. Conclusions

Geological mapping of the Mayombe Chain and Niari Basin in Congo Brazzaville, supported by geochronology and illite crystallinity studies allows defining the main tectono-metamorphic domains of the Western Congo Belt. It is now subdivided from east to west into four tectono-metamorphic domains, separated by crustal-scale structures: the Niari Basin (NB), Eastern (EMC), Central (CMC) and Western (WMC) Mayombe Chain.

The NB is structurally marked by only weak deformation such as tilting and faulting and, towards the west, by gentle folding. Very low-grade metamorphism varies from middle to upper diagenetic conditions. It is delimited from the EMC by the Mount Bélo Shear Zone, which represents the terminal thrust system of the West Congo orogen.

The tectonic style in the EMC is characterized by discrete low-angle thrusts, reverse faults and strike-slip faults resulting in the formation of duplex and/or positive flower structures. Off these high-strain zones, the rocks are folded into gentle syn- and anticlines, 5–8 km wide with subhorizontal NW-SE fold axes. Penetrative schistosity at high angle to the bedding starts in shales in the western part of the EMC. The metamorphism increases from eastern anchizonal conditions to lower greenschist facies in the west. The EMC is juxtaposed along the Moukondo thrust/back-thrust system with the CMC.

The CMC is typified by open to closed upright to NE-verging folds, up to 10 km wide, with shallow NW and SE plunging axes, S₁ schistosity with moderate to steep SW dips, onset of regional crenulation cleavage (S₂), frequent reverse thrusts and numerous faults. Metamorphic conditions remain in the greenschist facies. The Loukenéné-Mandji Thrust marks the CMC-WMC contact and coincides with a jump in metamorphic grade marked by biotite-in.

The WMC consists of Palaeoproterozoic basement stacked with Neoproterozoic rocks. Autochthonous gneisses and schist record Late Eburnean sedimentation, magmatism and metamorphism between 2110 and 1970 Ma. Deposition of the Loukoula Group is constrained by detrital zircon and metamorphic monazite geochronology between 2111 ± 14 Ma and 2091 ± 18 Ma. The allochthonous Bikossi Group was thrust during the Pan-African orogeny from the west over Tonian meta-volcaniclastic and plutonic rocks before further folding and stacking of both units. The intensity of Pan-African deformation increases from open to closed folds and spaced cleavage in the southeast of the WMC to thrust-dominated tectonics in the northwest, where the Palaeo- and Neoproterozoic rocks are tectonically stacked and associated with pronounced SW-dipping schistosities.

The Mayombe Chain and Niari Basin represent the eastern external zone of the Araçuai-Ribeira Belt in Brazil, providing complementary information for the reconstruction of the Transamazonian-Eburnean and Pan-African Western Congo-Araçuai orogens.

The first Eburnean high-grade metamorphism at 2091 ± 18 Ma, recorded by migmatitic paragneiss, corresponds to E2 in Gabon and can probably be correlated with the main collision phase (<2125-2100) in the Transamazonian orogen. Emplacement of the Bilinga Suite between 2060 and 2020 Ma corresponds with P2 (and partly P3) plutonism in Gabon and took place within the extensional phase of the Transamazonian orogeny. The magmatism is explained by decompression melting during uplift of overthickened crust in the late Eburnean. Subsequent amphibolite facies metamorphism in the Bilinga Suite corresponds to E3 in Gabon and may be associated with a late transpressive phase.

The rifting stage of the Pan-African orogenic cycle started in the early Tonian with deposition of clastic and bimodal volcanic rocks of the Sounda Group into N-S trending troughs. Subsequent silici-clastic sediments of the Mayombe Group were deposited between 870 and 635 Ma in a passive margin setting. The group is overlain by glaciogenic sediments of the Niari Group associated with the global 635 Ma Marinoan event. The return to warm climatic conditions is documented by the Schisto-calcaire Group, which evolved in lagoonal, reef and forereef environments. The minimum deposition age of the group is limited by the onset of the Pan-African orogeny at about 590 Ma.

In the context of Panafrican collision, the Mayombe Chain and Niari Basin represent the eastern foreland and external zone of the Western Congo/Araçuai Orogen. The tectono-metamorphic conditions change from pure shear dominated deformation and very-low-grade metamorphism in the Niari Basin to an important simple shear component and lower amphibolite conditions in the Western Mayombe Belt. The main transports are NE-directed.

Geochronology of detrital and metamorphic mica documents two discrete Pan-African events at 590-570 Ma (M1) and at 520-500 Ma (M2), which can be associated with the main collision stage and late thermal overprint in the Araçuai-Ribeira Belt.

In the Mpioka Group, detrital mica yielded M1 ages whereas the finer-grained illite fractions record mostly M2 ages. This classifies the group as molasse of the West Congo Belt deposited between 570 and 520 Ma after a first phase of orogenic uplift and before late thermal overprint. The group marks in the Western Congo Belt the end of the Pan-African orogenic cycle.

### CRediT authorship contribution statement

Thomas Fullgraf: Writing – original draft. Yannick Callec:
Supervision, Project administration, Investigation. Hugues Bauer:
Investigation. Florent Boudzoumou: Investigation. Nicolas Charles:
Writing - review & editing, Investigation. Eric Gloaguen: Investigation.
Benoit Issautier: Investigation. Sage Tsoumou Kebi: Investigation.
Benjamin Le Bayon: Investigation. Joël Le Metour: Supervision,
Investigation. Fabien Paquet: Investigation. Sergey Sergeev: Investi-
gation. Klaus Wemmer: Investigation.

Declaration of Generative AI and AI-assisted technologies in the
writing process

During the preparation of this work the authors used DeepL software
in order to improve readability and language of the manuscript. After
using this tool, the authors reviewed and edited the content as needed
and take full responsibility for the content of the publication.

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.

Data availability

Data will be made available on request.

Acknowledgements

The fieldwork and laboratory studies of this study were carried out
under a commercial contract between partners BRGM, TEP Congo and
the Congolese government. The fieldwork would not have been possible
without the support of many local guides, drivers, and local authorities.
TF would like to thank Damien Delvoe and an anonymous referee for
their helpful suggestions improving the manuscript.

Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.
org/10.1016/j.jafrearsci.2024.105369.

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