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Home All issues Volume 196 (2025) BSGF - Earth Sci. Bull., 196 (2025) 11 Full HTML
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BSGF - Earth Sci. Bull.
Volume 196, 2025
Article Number 11
Number of page(s) 21
DOI https://doi.org/10.1051/bsgf/2025014
Published online 13 June 2025

© T.D. Suzanne et al., Published by EDP Sciences 2025

Licence Creative CommonsThis is an Open Access article distributed under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Highlights

  • Djourdé-Sinassi magmatic-migmatitic complex has recorded reworking of a juvenile mafic crust into migmatites and plutonic rocks that formed the root of the Central African Orogenic Belt.

  • This root has then been vertically extruded under a pure-shear dominated constrictional regime with vertical stretching in the presence of melt.

  • The shear zones which is located at the Djourdé-Sinassi magmatic-migmatitic complex has developed in the presence of melt but then under solid-state during progressive retrogression from amphibolite to greenschist facies.

1 Introduction

The Central African Orogenic Belt (CAfOB) also known as Central African Fold Belt (Fig. 1), lies between the West African Craton, Congo Craton and Saharan Metacraton (Bessoles and Trompette, 1980; Nzenti et al., 1988; Castaing et al., 1994; Abdelsalam et al., 2002; Toteu et al., 2004; Van Schmus et al., 2008; Liégeois et al., 2013; Jackson and Ramsay, 1980; Kröner and Stern, 2004). It formed during the Pan-African orogeny as a result of the assembly of Gondwana Supercontinent during late Precambrian time (Trompette, 1997; Trompette, 2000). The Sinassi − Mayo Kebbi region marks the transition between the Adamawa-Yadé Domain with Paleoproterozoic and Archean inheritances, and the Western Cameroon Domain (Toteu et al., 2004) or North-Western Cameroon Domain (Bouyo Houketchang et al., 2009) dominated by juvenile Neoproterozoic rocks. It has been interpreted as a major batholith owing to the predominance of plutonic rocks with a calc-alkaline signature dated from ca. 750 to 580 Ma and to their locally intrusive contact into the low to medium grade greenstone belts (Penaye et al., 2006; Pouclet et al., 2006; Toteu et al., 2006; Isseini et al., 2012; Bouyo Houketchang et al., 2015, 2016). However, plutonic rocks of the Sinassi − Mayo Kebbi batholith are associated with migmatitic gneisses hosting a series of mafic to ultramafic rocks that were not fully considered in previous works. What is the timing of emplacement of the mafic to ultramafic magmas relative to the emplacement of intermediate to felsic plutons? Does partial melting occur as a consequence of magma intrusion or does it respond to crustal thickening? What is the significance of the structural record of these plutonic and metamorphic rocks? The goal of this paper is to address these questions by documenting the petro-structural and microstructural record of the metamorphic and plutonic rocks in order to characterize PT conditions and kinematic of deformation, and discuss the relative timing of magmatism, deformation and metamorphism within the Sinassi batholith that we name, in the following, the Djourdé-Sinassi magmatic-migmatitic complex.

thumbnail Fig. 1

(a) Geological map of the Pan-African belt north of the Congo craton (after Toteu et al., 2022) showing the Sinassi location with yellow star and the main lithotectonic domains. CCSZ: Central Cameroonian Shear Zone; TBSZ: Tibati-Banyo Shear Zone; NBSZ: Ngoro-Belabo Shear Zone; CC: Congo Craton; AYD: Adamawa-Yade Domain; YD: Yaoundé Domain; WCD: Western Cameroon Domain. (b) Cameroon Domain Geological sketch map of Northern Cameroon and Southwestern Chad (modified after Toteu et al., 2001; Pinna et al., 1994; Toteu et al., 2004; Bouyo Houketchang et al., 2015, 2016). TBF: Tchollire-Banyo Fault; NP: Neoproterozoic; PP: Paleoproterozoic. Inset: location of the study area within the network of the Pan-African belts of West and Central Africa. Solid Square: Central African Fold Belt (CAfOB); polygon: study area (Fig. 2).

2 Geological background

2.1 Central african orogenic belt

In Cameroon, the CAfOB had been originally subdivided into (1) the West Cameroon Domain, (2) the Adamawa-Yadé Domain and (3) the South Cameroon Domain (Ngnotué et al., 2000; Toteu et al., 2001, 2004). More recent geological mapping supported by new geochronological and geophysical data allowed to refine the geotectonic domains (into (1) the Garoua Domain (GD); (2) the Bamenda-Tignère Domain (BTD); (3) the Nyong Bayomen Domain (NBD); and (4) the Extended Congo Craton Domain (ECCD) subdivided into the Adamawa-Yadé, Yaoundé, Ntem and Moloundou Sembé-Ouesso subdomains (Fig. 1a; Toteu et al., 2022).

The GD, including the Djourdé-Sinassi magmatic-migmatitic complex, extending to SW Chad into the Mayo Kebbi Domain (Penaye et al., 2006; Pouclet et al., 2006; Isseini, 2011) (Fig. 1b), is limited eastwards by the Tcholliré-Banyo Shear Zone (TBSZ) and southwards by the Poli Thrust System (PTS). The GD includes the Poli-Bibémi Greenstone Belt made-up of low to high grade schists and gneisses with a protolith age dated at ∼830 Ma (Toteu et al., 1987, 2006) thrusting southward onto the BTD, and the Rey Bouba Greenstone Belt with a protolith age of ∼670 Ma (Bouyo Houketchang et al., 2015) consisting of low to medium grade schists located along the TBSZ. The Rey Bouba Greenstone Belt, composed of metapelite and metagrauwacke with a protolith that was deposited approximately between ∼645 and −630 Ma, underwent low-grade metamorphism around ∼600 Ma. This belt has been interpreted as a basin supplied by a continental arc (Toteu et al., 1987, 2006; Pinna et al., 1994; Bouyo Houketchang et al., 2015; Mbanga et al., 2023). The BTD, extending to the SW of the GD, is in contrast, characterized by Paleo- to Mesoproterozoic basement including orthogneisses, micaschists, quartzites and high grade pelitic paragneisses and metabasites dated from ∼2180 to <1400 Ma. This period marks the emplacement of a magmatic protolith occurring with granulitic metamorphism and the source of sediments respectively (Toteu et al., 2001; Penaye et al., 2004; Bouyo Houketchang et al., 2009, 2013; Delor et al., 2021). Both domains comprise pre- to post-tectonic calc-alkaline granitoid plutons dated from ∼686 to −563 Ma (e.g. Toteu et al., 2001, 2006; Penaye et al., 2006; Kwékam et al., 2010, 2020; Bouyo Houketchang et al., 2016; Tchameni et al., 2016; Nomo Negue et al., 2017), and post-tectonic alkaline granites and syenites forming small plutons dated between ∼600–550 Ma (Dawaï et al., 2013, 2022; Noudiédié and Tcheumenak, 2023; Bello et al., 2024). These plutons have been interpreted as part of a magmatic arc formed in a subduction context. At the regional scale, they are aligned parallel to the NNE-SSW to NE-SW trending regional schistosity and include numerous mafic to ultramafic enclaves (e.g. Njanko et al., 2010; Fozing et al., 2015, 2021; Bouyo Houketchang et al., 2016; Njiki Chatué et al., 2020).

The structural record of the GD and BTD encompasses a low-grade amphibolite facies flat-lying foliation associated with isoclinal folds and a NW-SE trending stretching lineation attributed to a D1 deformation phase of crustal thickening (Ngako et al., 1992; Nzenti et al., 1992). This foliation is transposed into sinistral strike slip shear zones, attributed to a D2 deformation phase, marked by isoclinal folds with a vertical axial planar schistosity bearing an NNE-SSW to NE-SW trending mineral lineation developed under high grade amphibolite to granulite facies prograde metamorphism (Nzenti et al., 2007; Bouyo Houketchang et al., 2013) synchronous of intensive migmatization (Ngako, 1986). The D2 is attributed to a regime of transpression with sinistral shear zones such as TBSZ. Subvertical dextral shear zones trending WSW-ENE, which are parallel to the Central Cameroon Shear Zone, are associated with the formation of mylonitic foliation under medium to low-grade amphibolite retrograde metamorphism. These zones also localize the emplacement of the syntectonic leucocratic granite veins, interpreted as related to D3 deformation (Bouyo Houketchang et al., 2013; Dawaï, 2014; Fozing et al., 2015; Saha-Fouotsa et al., 2019. A last deformation phase D4 has been invoked based on transposition of previous structures into regional-scale N-S and E-W trending ductile sinistral shear zones that controlled the emplacement of felsic veins (Fozing et al., 2015; 2021; Mbanga et al., 2023).

2.2 Tcholliré-banyo shear zone

The TBSZ is a NE-SW trending crust-scale shear zone initially interpreted as a dextral shear zone (Dumont, 1986) but revised as a sinistral shear zone (Penaye et al., 1989; Ngako et al., 2008; Nomo Negue et al., 2017; Saha-Fouotsa et al., 2018). The TBSZ corresponds to the transposition of a shallowly dipping S2 foliation/schistosity bearing a subhorizontal mineral stretching lineation, into a subvertical NE-SW striking mylonitic foliation (Ngako et al., 2008; Nomo Negue et al., 2017; Saha Fouotsa et al., 2018) and is considered to be the major boundary between the GD − BTD (former Western Cameroon Domain) and the Adamawa-Yadé Domain (Fig. 1, Toteu et al., 2001, 2004; Ganwa et al., 2008, 2016; Bouyo Houketchang et al., 2009; Nomo Negue et al., 2017; Tchakounté et al., 2017; Saha-Fouotsa et al., 2019; Delor et al., 2021). Deformation along the TBSZ was potentially active from ∼650 to −580 Ma on the basis of U/Pb zircon dating of syn-tectonic plutons (Penaye et al., 1989; Toteu et al., 2001; Saha-Fouotsa et al., 2019) which contain also zircon grains as old as ∼719 Ma (Nomo Negue et al., 2017), which might be inherited from the source of the magma.

2.3 Geology of the djourdé-sinassi area

The first geological surveys in the study area were carried out by Koch (1959) and Schwoerer (1965). Subsequent works by Pinna et al. (1994), Penaye et al. (2006) and (Bouyo Houketchang et al. 2013, 2015, 2016) enabled to identify the main geological formations in the Djourdé-Sinassi area (Fig. 2):

  • the Sinassi batholith, composed of granitoids described as a tonalite-trondhjemite-granodiorite suite associated with diorite and granite that contains enclaves of amphibolite/ultramafic rocks, is cross-cut by dykes of rhyolite and syenite and locally contain sulfur and gold (Penaye et al., 2006; Bouyo Houketchang et al., 2016);

  • the Rey Bouba Neoproterozoic greenstone belt (RBGB; Pinna et al., 1994; Bouyo Houketchang et al., 2015; Mbanga et al., 2023), which marks the eastern edge of the Sinassi batholith and is intruded by the post-collisional Vaïmba granite;

  • the Palaeozoic Mangbaï formations (Bea, 1987; Bea et al., 1990) and the Cretaceous Benoué sandstones, deposited in basins controlled by E-W and ESE-WNW trending faults, respectively and representing the post- Pan-African cover.

The granitoids of the Sinassi batholith exhibit a moderately to highly potassic calc-alkaline affinity with I-type metaluminous signature and moderate to weak negative Nb-Ta, Ti and Eu anomalies interpreted to reflect emplacement in a continental magmatic arc setting (Bouyo Houketchang et al., 2016). These granitoids have recorded P-T emplacement conditions of 4.04–5.82 kbar/698–728 °C and 4.23–5.76 kbar/667–720 °C using geothermobarometric calculations hornblende-plagioclase thermometry and aluminum-in hornblende barometry, in the Djourdé and Sinassi areas, respectively (Bouyo Houketchang et al., 2016). U/Pb geochronology on zircon grains from these magmatic rocks points to (i) sources with ages at ∼746 Ma, ∼722 Ma, ∼713 Ma and ∼692 Ma; (ii) emplacement ages at ∼686–670 Ma (orthogneiss group), ∼666–661 Ma (synchronous emplacement of the Djourdé and Sinassi granodiorites groups) and ∼644 Ma (emplacement age of quartz-eyed diorites in the Sinassi group); and (iii) post-magmatic alteration or metamorphism at ∼600 Ma (Bouyo Houketchang et al., 2016).

Bouyo Houketchang et al. (2016) reported the presence of mafic and ultramafic rocks exposed as enclaves in the Sinassi batholith and that display foliation, lineation, folds, shear zones and fractures. The present work focusses on new petrographic data of mafic-ultramafic rocks and migmatitic gneisses, as well as the structural and microstructural characteristics of the Sinassi rocks, in order to update the available geological map from Bouyo Houketchang et al. (2016) (Fig. 2), and to unravel the kinematic evolution and the tectonic significance of the so-called Djourdé-Sinassi magmatic-migmatitic complex in the deformation history of CAfOB in Cameroon.

thumbnail Fig. 2

A hillshade map with outcrop station highlights geological structures such as folds and faults by emphasizing variations in elevation. We use it by making geological contours visible.

3 Methodology

3.1 Field data collection and processing

Several field campaigns enabled to map about 932 stations ((Fig. 2) in the study area. Structures were measured in the field and the projection of the poles of foliations, lineations and fold axes (directions and dips/plunge) was carried out using Stereonet v.11 software (Cardozo and Allmendinger, 2013), in the lower hemisphere of the Schmidt diagram (Ragan, 1973).

3.2 Sample collection and processing for microstructural study

Fresh rock samples were collected in the field for microstructural analysis. The thin sections were made at the Geosciences Environnement Toulouse (GET) laboratory (Observatoire Midi-Pyrénées, France) and at the SARM Nancy (France) laboratory. A total of 55 thin sections were obtained from the selected rock samples, distributed as follows: migmatitic gneisses (02), amphibolites (17), pyroxenites (05), metahornblendites (07), metagabbros (03), hornblendites (03), diorites (07), tonalites (03), granodiorites (03) and mylonitic (grano)diorites (05) were observed using a polarizing optical microscope.

4 New geological map and lithological units

A refined geological map (Fig. 3) and tentative cross section (Fig. 3) are presented for the Djourdé-Sinassi complex based on new field observations, which highlights its subdivision into an eastern domain dominated by migmatitic rocks and a western domain made of plutonic rocks. The eastern domain of the Djourdé-Sinassi complex, exposes hectometer to kilometer thick mafic-ultramafic rafts that are parallel to the syn-migmatitic foliation of the host migmatitic gneiss and are more or less continuous over tens of kilometers at Sinassi, Tokormaye, Warkla, Koindéri, Vaimba 2, Bamboro, Mayo Sinassi, Mayo Warkla, New Warkla-Koindéri quarter district and eastwards of Laoubou. The plutonic rocks of the western domain encompass diorite, tonalite, granodiorite and granite in which mafic-ultramafic rocks are present as boudinaged or folded enclaves.

thumbnail Fig. 3

New geological map of the study area (modified after Bouyo Houketchang et al., 2016) and tentative cross-sections NNW-SSE show the interpreted structures and possible lithological contacts respectively.

4.1 Mafic-ultramafic rocks

Mafic-ultramafic rocks comprise interlayered pyroxenite, metahornblendite, metagabbro and amphibolite that are exposed as more or less continuous and boudinaged rafts within the host migmatitic gneiss and are crosscut by a network of felsic veins. Boudins are angular to rounded, display a S0/n foliation, and are wrapped into the Smgm syn-migmatitic foliation (Fig. 4a, b and c) and the mylonitic foliation as well (Fig. 4d). The syn-migmatitic foliation is also affected by isoclinal folds with folded leucosomes (Fig. 4e). Asymmetrically folded and sigmoid-shape mafic boudins provide kinematic criteria (Fig. 4f and g). These features indicate that deformation occurred in the presence of melt that migrated into dilatation sites. Locally, all these structures are crosscut and transposed with a relic gneissic foliation that is subparallel (Fig. 4h).

Pyroxenite is made of subrhombic clino- and orthopyroxenes with a symplectitic or poikiloblastic texture highlighted by hornblende inclusions (Fig. 5a). Quartz forms ocelli inclusions trapped within plagioclase or hornblende blasts. Pyroxenite locally alternates with metahornblendite and metagabbro. Metahornblendite displays a nematogranoblastic to porphyronematoblastic texture indicating the retromorphosis of clinopyroxene into hornblende and the overgrowth of sphene grains on opaque minerals (Fig. 5b). Metahornblendite locally contains leucite or quartz (Fig. 5c). Metagabbro shows a granonematoblastic texture with subrectangular or subrhombic green hornblende, clinopyroxene and zoned plagioclase blasts associated with scarce orthoclase and microcline blasts (Fig. 5d). Quartz generally appears as small grains at the edge or as ocelli inclusions within hornblende blasts. The preferred orientation of hornblende, clinopyroxene and plagioclase define a foliation that is locally mylonitic (Fig. 5e). Symplectite growths are noticed along mylonitic shear bands. Metagabbro is locally rimmed by amphibolite (Fig. 5f), which attests for the superposition of metamorphism and deformation on a former magmatic protolith. Amphibolites are mainly made of plagioclase blasts, and porphyroblastic hornblende (Fig. 5g) with a composition that falls within the ranges of edenite, magnesio-hornblende, magnesio-pargasite, ferro-hornblende, pargasite and actinolite. At Bamboro, a migmatitic amphibolite is characterized by alternation of mesosome and leucosome with subglobular garnet poikiloblasts that contain quartz, plagioclase and biotite inclusions, preferentially in the mesosome (Fig. 5h). Overall accessory mineral phases in these mafic-ultramafic rocks include apatite, zircon and opaque minerals; secondary phases encompass epidote, calcite, chlorite, prehnite, pumpelleyite and sericite.

The primary mineral assemblage in amphibolites is represented by Hbl ± Grt + Bt +Pl. Relics of early assemblage (Cpx + Opx + Pl) from the low-pressure granulite facies (8-10 kbar/800-850 °C; Bucher and Frey, 2002; Bucher and Grapes, 2011) are found in pyroxenite and pyroxene metahornblendite (Fig. 5a and b). These relic assemblages show destabilization phases around the pyroxenes, indicating a retrograde metamorphism (Fig. 5a and b). In metagabbro, the metamorphic peak is defined by the mineral assemblage Cpx + Hbl + Pl, while the assemblage Grt + Hbl + Bt + Pl is developed in the garnet-bearing migmatitic amphibolite, characteristic of regional metamorphism in the high-grade amphibolite facies at high pressure (7–9 kbar/650–700 °C; Bucher and Grapes, 2011). Leucosome veins of tonalitic/dioritic/granodioritic composition in the migmatitic amphibolite point to partial melting, which requires a temperature of ∼750 °C in anhydrous condition (Rapp et al., 1991; Wyllie and Wolf, 1993; Wolf and Wyllie, 1994) or 650 °C in hydrous condition (Green et al., 2016; Palin et al., 2016). The presence of ubiquitous green hornblende porphyroblasts in these mafic-ultramafic rocks (Fig. 5), suggests pervasive amphibolite facies conditions reached at T ˃ 500–550 °C (Pons, 2001; Bucher and Grapes, 2011). Retrogression of pyroxene into actinolite and actinolitic hornblende is symptomatic of greenschist facies conditions at about 3–5 kbar/350–500 °C (Pons, 2001; Bucher and Grapes, 2011).

thumbnail Fig. 4

Outcrop of mafic-ultramafic rocks. (a) Sigmoid-shaped pyroxenite boudins in migmatitic pyroxenite. (b) Sigmoid-shaped amphibolite boudins in migmatitic amphibolite. (c) Ovoid-shaped metahornblendite enclave rimmed by amphibolite bands concordant to the Smgm foliation. (d) Amphibolite enclaves hosted in mylonitic diorite. (e) Fn+2 isoclinal folded granitic vein in metahornblendite. (f) Fn+3 disharmonic folds of amphibolite showing sinistral motion within mylonitic granodiorite. (g) δ-type marker showing a dextral kinematic movement. (h) S0/n compositional banding developed in the banded garnet amphibolite. The illustrations were observed on subhorizontal planes.
thumbnail Fig. 5

Microphotographs of mafic-ultramafic rocks. (a) Destabilization of pyroxene into amphibole + quartz + opaque. (b) Overgrown sphene grains around opaque minerals. (c) Leucite in metahornblendite. (d) Microcline blasts in metagabbro. (e) Granonematoblastic texture of metagabbro showing concordant fabric to the mylonitic foliation in the adjacent shear band. (f) Outcrop photographs showing contact between metagabbro, amphibolite and metahornblendite. (g) Porphyroblastic green hornblende in biotite amphibolite. (h) Poikiloblastic garnet showing inclusion of plagioclase-quartz-biotite in garnet amphibolite.

4.2 Migmatitic gneisses

Migmatitic gneisses derive mainly from mafic rocks (Fig. 6a). They grade from metatexite to diatexite and display diffuse contacts with granodioritic and granitic plutonic rocks (Fig. 6b). Migmatitic gneisses exhibit a syn-migmatitic foliation characterized by alternation of centimeters to decimeter thick mesosome layers made of hornblende and biotite, and leucosome layers made of plagioclase and quartz. (Fig. 6c and d). The mesosome and leucosome display a coarse grained texture. In the mesosome, the preferred orientation of hornblende and biotite defines a planar fabric. Interstitial plagioclase and quartz display only limited evidences for solid state deformation (Fig. 6e). This suggests that deformation occurred in the presence of a melt fraction that segregated to form a film in between residual crystals (Fig. 6f).

The syn-migmatitic foliation is transposed into a gneissic foliation defined by the preferred orientation of biotite/amphibole blasts in the mesosome and by quartz ribbons and feldspar blasts in the leucosome (Fig. 6g). The syn-migmatitic foliation displays a steeply-plunging Len+1 mineral stretching lineation defined by the alignment and preferred stretching of biotite/amphibole blasts in the melanosomes, or quartz ribbons and feldspar blasts in the leucosomes (Fig. 6h). The syn-migmatitic foliation is affected by isoclinal folds (Fig. 7a) and leucosome concordant veins are boudinaged (Fig. 7b). In the migmatitic gneiss, amphibole is magnesio-hastingsite and plagioclase is andesine. Accessory mineral phases are represented by sphene, apatite, zircon and opaque minerals, whilst secondary phases include epidote, chlorite, damourite and sericite.

The main mineral paragenesis in the migmatitic gneiss is Hbl + Bt + Pl + Qtz ± Kfs, which is diagnostic of amphibolite facies regional metamorphism (Fig. 6f). The absence of garnet and pyroxene in the mesosome indicates a pressure < 7–8 kbar (Wyllie and Wolf, 1993; Bucher and Frey, 1994). Widespread quartzofeldspathic leucosome veins associated with biotite-hornblende mesosome (Fig. 6e and f) point to in situ partial melting potentially in presence of water possibly followed by progressive dehydration-melting of biotite and hornblende (Le Breton and Thompson, 1988; Rapp et al., 1991; White et al., 2005; Weinberg and Hasalová, 2015). Rare potassium feldspar is observed in the mesosome or leucosome layers (Fig. 7c), thus evidencing in situ melting of a plagioclase-rich protolith at a temperature above 650 °C (Green et al., 2016). Nevertheless, hornblende destabilizes into Bt + Qtz + Op ± Ep (Fig. 7d) and Qtz + Ep ± Op ± Chl (Fig. 7c). The persistency of epidote in these secondary mineral assemblages suggests a retrogression of hornblende under transition conditions from low-grade amphibolite facies to green schist facies (4–6 kbar/500–600 °C; Bucher and Grapes, 2011). Biotite stability suggests a temperature around ∼600 °C (Bucher and Grapes, 2011). Hornblende and biotite are retrogressed in epidote and chlorite.

thumbnail Fig. 6

Outcrop photographs of migmatitic gneisses (a, b, d, g and h), (b): exposure showing migmatitic gneiss with a shallow dipping foliation delineated by concordant granitic veins, (c): sample of migmatitic gneiss showing the melanosome, leucosome and mesosome layers, (d) Smgm foliation affected by asymmetrical folds Fn+1, (e): quartzofeldspathic leucosome layers including biotite flakes, (f): Interstitial plagioclase between residual hornbelnde crystals, (h): Migmatites with a foliation transposed into the shear zone with a down-dip stretching lineation marking the transition from the Sinassi to the Djourdé domains.
thumbnail Fig. 7

Field structures and microphotographs in migmatitic gneiss: (a) sheath-like or isoclinal folds Fn+2 showing elliptical sections, (b) (d) Bn+1 asymmetrical boudins, (c): Mesosome with hornblende, retrogressed into quartz, epidote, and opaques, and interstitial microcline and quartz, (d): Hornblende retrogressed into biotite+quartz+opaque.

4.3 Plutonic rocks

Plutonic rocks comprise diorite, tonalite, granodiorite and granite, and display a magmatic planar fabric Sm. In the western domain of the studied area, the predominant plutonic rock is a heterogeneous granodiorite characterized by a coarse-grained dominantly magmatic texture. It is made of plagioclase, green hornblende, potassium feldspar, quartz and biotite flakes. The granodiorite contains angular enclaves of diorite made of plagioclase, quartz, potassium feldspar and green hornblende crystals (Fig. 8e). The granodiorite is cross-cut by shallow-dipping granitic sills (Fig. 8d) and by felsic veins with sharp contacts (Fig. 8c). At the regional scale, the granodiorite is in gradational contact with circular plutonic bodies of tonalite and granite. Tonalite is made of plagioclase, quartz and green hornblende phenocrysts, and biotite lamellas as well. Granite is made of quartz, potassium feldspar, plagioclase and green hornblende phenocrysts, and biotite lamellas as well. Accessory mineral phases include sphene, zircon, apatite and opaque crystals. In the eastern domain of the study area, granite dykes either concordant to the S0/n foliation in amphibolite wall rock (Fig. 8a) or discordant to the Smgm and S0/n foliations of their metamorphic host rocks (Fig. 8b).

Amphibolite and metagabbro are present as sub-rounded enclaves with straight boundaries in diorite, granodiorite and granite and are interpreted as equivalent of the mafic-ultramafic rafts of the eastern domain of the Djourdé-Sinassi complex (Fig. 8f). In contrast, the granitoids also contain trains of hornblendite enclaves dispersed into the granitic host and displaying lobate/diffuse magmatic contacts (Fig. 9a). Locally plagioclase crystals are incorporated into the hornblendite (Fig. 9b). The network of felsic veins that invades the hornblendite ranges from millimeter to centimeter in thickness, and are predominantly composed of plagioclase, quartz, hornblende and opaque crystals (Fig. 9c and d). These relationships point to magma mingling and mixing. Hornblendite outcrops are also cross-cut by bands of pyroxene metahornblendite that show straight boundaries (Fig. 9e).

In granitoid a Sm magmatic fabric is marked by the preferred orientation of feldspar phenocrysts, amphibole needles or biotite flakes but also of schlieren and mafic-ultramafic enclaves distributed within the rock volume. Sm magmatic fabric is cross-cut by Cn+1 syn-magmatic shear planes defined by the reorientation of biotite schlieren and feldspar phenocrysts obliquely to the main direction of the magmatic fabric (Fig. 10a). Textures range from medium- to coarse-grained, with development of magmatic, submagmatic to solid-state microstructures. Euhedral feldspar and amphibole crystals, and biotite flakes are devoid of intracrystalline deformation (Fig. 10b). Plagioclase forms synneusis aggregates with local zoning and with the development of magmatic myrmekite buds on the edges of feldspars (Fig. 10c), associated with microfractures affecting feldspar crystals and sealed by plagioclase or quartz late melts. Quartz mostly displays a homogeneous extinction but is locally marked by a chessboard texture and lobate grain boundaries indicative of high-temperature dynamic recrystallization (Fig. 10d). High-temperature solid state deformation is also attested by bended feldspar twins and kink bands in plagioclase. Feldspar crystals are intensely transformed into sericite or damourite whilst biotite flakes are locally recrystallized into chlorite and hornblende into epidote, which marks retrogression and hydrothermal alteration (Fig. 10e). Sm magmatic fabric is transposed into mylonite fabric along localized sinistral or dextral shear zones (Fig. 10f). This mylonite fabric is associated with asymmetric quartz ribbons and feldspar porphyroclasts. (Fig. 10g and h).

thumbnail Fig. 8

Contact between the mafic-ultramafic rocks and the granitoids in the Sinassi area. (a): dyke of granite into mafic rocks, (b): Migmatitic amphibolite with steep dipping synmigmatitic foliation crosscut by granodioritic veins. (c): Granodiorite with shallow-dipping granitic vein. (d): Granodiorite with shallow-dipping granitic vein. (e): enclave of diorite with diffuse boundary in the host granodiorite, (f): Granodiorite with enclave that has preserved the contact between a metagabbro and an amphibolite.
thumbnail Fig. 9

Relationships of hornblendites and metahornblendites with felsic rocks (a, b, c and e). (a) lobate or diffuse magmatic contacts (b): magmatic contact between hornblendite and granodiorite, (c): magmatic contact between hornblendite and host diorite, (d): Felsic vein in diffuse contact with host hornblendite. (e): interlayered pyroxene metahornblendite and hornblendite.
thumbnail Fig. 10

Field structures and microstructures in the granitoids. (a): Sm magmatic fabric reoriented by Cn+1 synmagmatic sinistral shear zone, (b): magmatic texture with euhedral feldspar crystals, (c): myrmekitic buds along plagioclase rim, (d): chessboard pattern of quartz crystal, (e): plagioclase affected by kink band and damouritization, (f): mylonite fabric defined by quartz ribbons, (g): anticlockwise bookshelf structure, (h): asymmetric porphyroclasts of feldspars describing the sinistral motion.

5 Regional structural trends

The Djourdé-Sinassi magmatic-migmatitic complex displays a complex polyphased structure. The S0/n foliation preserved in the mafic-ultramafic rafts, as illustrated at Mayo Sinassi and Mayo Tokormaye stations, is E-W striking and is steeply dipping, parallel to the preferred orientation of the rafts (Fig. 11). The strike of this S0/n foliation is subparallel to the foliation in the adjacent Rey Bouba greenstone belt, except that the latter is shallowly dipping (Fig. 2). The mafic-ultramafic rafts are wrapped into the Smgm syn-migmatitic foliation that is ENE-WSW striking and is steeply dipping (Fig. 11). This Smgm syn-migmatitic foliation is parallel to the S2 foliation trajectories in the Rey Bouba greenstone belt and to the trend of the TBSZ. All structures are transposed into NE-SW striking steeply dipping sinistral and E-W dextral shear zones bearing a subvertical Ln+1 mineral and stretching lineation (Fig. 12).

In the plutonic domain of the Djourdé-Sinassi complex, the structure is mostly characterized by a gently to steeply dipping (21°–90°) magmatic planar fabric Sm in tonalite, granodiorite, granite and diorite with variable strikes (Fig. 11). This magmatic fabric is locally transposed into the sinistral and dextral shear zones identified in the migmatitic domain. Indeed, this Sm magmatic fabric is transposed into NE-SW striking steeply dipping Sn+2 foliation bearing a steeply SW plunging Ln+2 mineral and stretching lineation with a mean of N033°E/74°SW (Fig. 12). This transposition is marked by asymmetric folds with axes with a mean orientation N067°E/52°SW (Fig. 12) and by sigmoid-shape boudins consistent with a sinistral sense of shear.

The contact between the migmatitic domain and the plutonic domain of the Djourdé-Sinassi complex, is marked, in the southern domain of the studied area, by a NE-SW trending subvertical sinistral mylonitic shear zone characterized by sheath folds with a steeply dipping axis, subparallel to the stretching lineation. This mylonitic shear zone is parallel to the sinistral TBSZ (Fig. 6d, g, h). In the northern domain of the studied area, the contact between the migmatitic domain and the plutonic domain is marked by a E-W trending subvertical dextral mylonitic shear zone. No crosscutting relationships have been identified between these shear zones.

thumbnail Fig. 11

Litho-structural map showing field foliation and planar fluidality with lower hemisphere equal area projection strereograms (contour intervals = 1σ) of the metamorphic, migmatitic, mylonitic and planar fluidality rocks.
thumbnail Fig. 12

Litho-structural map showing mineral stretching lineation and fold axes with lower hemisphere equal area projection strereograms (contour intervals = 1σ) of the metamorphic, migmatitic, mylonitic and magmatic rocks.

6 Discussion

6.1 Significance of the relationships between mafic-ultramafic rocks, migmatites and granitoid plutons

Based on mineralogical, geochemical and geochronological data, Bouyo Houketchang et al. (2016) proposed that the Sinassi batholith is the Cameroonian extension of the Mayo Kebbi batholith in south-eastern Chad (e.g. Penaye et al., 2006; Pouclet et al., 2006; Isseini et al., 2012), both recording an identical tectonic-magmatic evolution during Neoproterozoic times. Considering these previous studies, recent petrographic observations, including migmatitic gneisses and mafic-ultramafic rocks that locally form migmatites with various meso-scale field relationships reflecting different structural levels, indicate a need to reinterpret the regional significance of the Djourdé-Sinassi magmatic-migmatitic complex.

Migmatitic gneisses of the Djourdé-Sinassi complex, especially in the eastern domain of the studied area, correspond to metatexites that represent former partially-molten rock, in agreement with criteria based on the continuity of the syn-migmatitic foliation (Sawyer, 1994; Vanderhaeghe, 2001, 2009). Pyroxenite, metahornblendite and metagabbro are surrounded by amphibolite, which attest for deformation and metamorphism of the mafic-ultramafic rafts (see Figs. 4c, 5f). The diffuse contact of these rafts with the mesosome of the host migmatites suggests that these rocks represent, at least in part, the protoliths of the migmatites. Partial melting of a mafic rock leaves a residue rich in plagioclase and pyroxene and produces a tonalitic/dioritic/granodioritic melt (Bonzi et al., 2021). Expected peritectic phases (e.g. pyroxene, garnet) from amphibole-consuming dehydration melting reactions are absent in tonalite/diorite/granodiorite, but are present in pyroxenites and migmatitic amphibolite. The network of texturally continuous felsic veins concordant to discordant relative to the syn-migmatitic foliation of migmatitic amphibolite and metatexite migmatite (see Fig. 8a) is interpreted to represent channels for migration of granitic melt (see Fig. 8b). Mafic-ultramafic rocks contain patches, felsic veins that might reflect evidences of in situ partial melting or infiltration of the nearby generated melt into residual solids (see Figs. 4a, b, g and 5f). Overall, the distribution of leucosome veins within foliation, boudins necks, fold axial planes and shear planes, suggests that deformation played a key role in the segregation and migration of magma (Brown and Rushmer, 1997; Vanderhaeghe, 2001; Toé et al., 2013).

Heterogeneous granodiorite/diorite/tonalite, exposed in the western domain of the studied area, characterized by their richness in mafic-ultramafic enclaves (see Fig. 9a), are consistent with diatexite migmatite representing a deeper structural level compared to the metatexite migmatite. This is consistent with the estimated PT conditions using geothermobarometric calculations (hornblende-plagioclase thermometry and aluminum-in hornblende barometry) pointing to a depth of ∼16–18 km documented by magmatic petrology (Bouyo Houketchang et al., 2016). Nevertheless, this proposition of an origin of plutonic rocks by in situ partial melting of mafic rocks does not preclude an intrusion of mantle-derived magmas. Indeed, the diffuse and cuspate contacts between some hornblendite and diorite to granodioritic magmatic rocks points to mingling associated with some mixing (Chappell, 1996). Furthermore, the dismembered aspect of some hornblendite enclaves (Fig. 5b) and round- to ovoid-shaped hornblendite enclaves, suggests that they formed by injection of ultramafic melt in a host granodioritic/tonalitic magma and/or partially molten rock (Dorais et al., 1990; Blundy and Sparks, 1992; Bonin, 2004). Some hornblendite enclaves have included feldspar crystals originating from their dioritic/granodioritic host (Fig. 5a, b). Their presence reveals that they have been mechanically included in the enclave when it was not completely crystallized (Didier, 1984; Solgadi et al., 2007; Nédélec and Bouchez, 2011). Similar field relationships between mafic-ultramafic enclaves and their host rocks have been also described in the Mbe − Sassa-Mbersi granitoids (Saha-Fouotsa, 2018), the Numba pluton (Njiki Chatué, 2021), the Pitoa pluton (Happi Djofna, 2024) and in a worldwide review (e.g. Weinberg et al., 2021).

6.2 Tectonic significance of the fabric of plutonic rocks and migmatites of the Djourdé–Sinassi complex: constriction with vertical stretching in the core of the CAfOB

The structural and petrological features presented above indicate that deformation of the Djourdé-Sinassi magmatic-migmatitic complex was dominated by a vertical direction of stretching coeval with NE-SW trending sinistral shearing and WNW-ESE dextral shearing in the horizontal plane, perpendicular to the stretching lineation.The resulting finite strain ellipsoid is in the constriction field. This deformation started in the presence of melt and continued during progressive retrogression. At the lowest structural level, the magmatic fabric of the tonalite and granodiorite delineates kilometer scale domes. Local constriction might be localized in triple points between domes such as described in the east Pilbara craton (Western Australia) where the formation of domes and constriction have been attributed to partial convective overturn (Collins et al., 1998). At higher structural, constriction is rather evidenced by mutual crosscutting relationships between dextral and sinistral shear zones. The overall subvertical foliation and strike-slip shear zones with a down-dip lineation reflects a vertically elongated finite deformation ellipsoid within the constriction field with strain accommodation alongside subvertical strike-slip shear zones marked by vertical flattening and stretching (Gapais et al., 2008; Cagnard et al., 2007; Fueten and Robin, 1989). This is consistent with the vertical orogen-parallel flow mode of Chardon et al. (2009) but with a vertical direction of stretching. The steep plunge of lineation points to a pure-shear dominated transpression (Fossen and Tikoff, 1993) and the parallelism between lineation and axes of isoclinal folds are diagnostic of a shear-component at a low-angle to fold axes, as illustrated by the model of Tikoff and Peterson (1998). This deformation regime was coeval to injection of mafic magmas and was then recorded during progressive retrogression from granulite to amphibolite facies and even greenschist facies, which suggests progressive exhumation of these rocks. From a regional overview, this subvertical constrictional strain has never been described wherever in the GD and BTD. However, similar observation of steeply-plunging streaks and lineation were made in the neighboring Mbé − Sassa-Mbersi region by Saha-Fouotsa (2018) and Saha-Fouotsa et al., (2019).

7 Conclusion and perspectives

  • The Djourdé-Sinassi magmatic-migmatitic complex comprises an eastern domain made of a heterogeneous assemblage of migmatitic gneisses including rafts and enclaves of mafic-ultramafic rocks (pyroxenites, metahornblendites metagabbros and amphibolites), and a western domain made of diorite, tonalite and granodiorite plutons mingled with ultramafic magmas (hornblendites) and containing numerous mafic enclaves.

  • The polyphase structure of the Djourdé-Sinassi complex is characterized by a S0/n foliation preserved in the mafic-ultramafic rocks that is wrapped into the syn-migmatitic foliation of the host migmatites. Deformation, partial melting and melt segregation-migration were coeval as attested by a texturally continuous network of leucosome veins concordant to discordant relative to the syn-migmatitic foliation. The syn-migmatitic Smgm foliation is boudinaged, affected by isoclinal folds and transposed into subvertical NE-SW and WSW-ENE striking shear zones bearing a subvertical mineral and stretching lineation and associated with sinistral and dextral kinematic criteria, respectively. At the lowest structural level exposed in the western domain of the studied area, plutonic rocks are characterized by a gently to steeply dipping planar magmatic fabric locally transposed into sinistral and dextral shear zones parallel to the ones identified in the eastern migmatitic domain.

  • Mineral parageneses, textures and microstructures highlight a metamorphism under granulite facies (peak: ≥ 800 °C/8–10 kbar) associated to partial melting, melt segregation and migration to form migmatites and plutons, followed by retrogression in the greenschist facies conditions (≤ 500 °C/3–5 kbar) along shear zones.

  • The Djourdé-Sinassi magmatic-migmatitic complex represents the exhumed mid- to lower structural level of the orogenic root of the Central African Orogenic Belt deformed in a non-plane strain 3D regime with a vertical stretching direction associated with non-coaxial deformation in the horizontal plane, resulting in a finite strain ellipsoid in the constriction field. Persistence of this regime from high-grade metamorphism in the presence of melt to retrogression under greenschist facies is consistent with progressive vertical extrusion and exhumation of these rocks. This peculiar tectonic pattern might be related to the position of the Djourdé-Sinassi magmatic-migmatitic complex, surrounded by greenstone belts and confined between the West African Craton, the Congo Craton and the Sahara Metacraton.

Acknowledgments

This manuscript is part of the first author’s PhD thesis. Dr. Ngassam Mbianya Ghislain is highly acknowledged for constructive discussions during field works. We would like to thank Mr. Tchikri Koi for invaluable support during field works. Mr. Ouing Michelle is acknowledge for his hospitality and the facilities allowed during field trips. We also want to thank Dr. Kamguia Woguia Brice and Mr. Tchouandom Eric who significantly contributed to improve the language quality of first version of the present paper. Fabienne De Parseval at GET and the SARM team at the CRPG Nancy are thanked for the very nice thin sections. This project was financially supported by the CNRS LithoCOAC project and IRN FALCoL as well as the French Embassy in collaboration with the Agence Universitaire de la Francophonie (AUF), which covered analytical costs and the travel and daily expenses of the first author in Toulouse, France.

Conflicts of interest

The authors declare that they have no competing interests or personal relationships that could have influenced the data presented in this manuscript.

Data availability statement

The data used to support the findings of this study are available from the corresponding author upon request.

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Cite this article as: Djuimou ST, Houketchang MB, Vanderhaeghe O, Megnemo LA, Mbounou LRA, Gregoire M, Dongmo AK. 2025. The Djourdé-Sinassi magmatic-migmatitic complex, Northern Cameroon: a record of vertical extrusion of the Pan-African partially molten orogenic root, BSGF - Earth Sciences Bulletin 196: 11. https://doi.org/10.1051/bsgf/2025014

All Figures

thumbnail Fig. 1

(a) Geological map of the Pan-African belt north of the Congo craton (after Toteu et al., 2022) showing the Sinassi location with yellow star and the main lithotectonic domains. CCSZ: Central Cameroonian Shear Zone; TBSZ: Tibati-Banyo Shear Zone; NBSZ: Ngoro-Belabo Shear Zone; CC: Congo Craton; AYD: Adamawa-Yade Domain; YD: Yaoundé Domain; WCD: Western Cameroon Domain. (b) Cameroon Domain Geological sketch map of Northern Cameroon and Southwestern Chad (modified after Toteu et al., 2001; Pinna et al., 1994; Toteu et al., 2004; Bouyo Houketchang et al., 2015, 2016). TBF: Tchollire-Banyo Fault; NP: Neoproterozoic; PP: Paleoproterozoic. Inset: location of the study area within the network of the Pan-African belts of West and Central Africa. Solid Square: Central African Fold Belt (CAfOB); polygon: study area (Fig. 2).
In the text
thumbnail Fig. 2

A hillshade map with outcrop station highlights geological structures such as folds and faults by emphasizing variations in elevation. We use it by making geological contours visible.
In the text
thumbnail Fig. 3

New geological map of the study area (modified after Bouyo Houketchang et al., 2016) and tentative cross-sections NNW-SSE show the interpreted structures and possible lithological contacts respectively.
In the text
thumbnail Fig. 4

Outcrop of mafic-ultramafic rocks. (a) Sigmoid-shaped pyroxenite boudins in migmatitic pyroxenite. (b) Sigmoid-shaped amphibolite boudins in migmatitic amphibolite. (c) Ovoid-shaped metahornblendite enclave rimmed by amphibolite bands concordant to the Smgm foliation. (d) Amphibolite enclaves hosted in mylonitic diorite. (e) Fn+2 isoclinal folded granitic vein in metahornblendite. (f) Fn+3 disharmonic folds of amphibolite showing sinistral motion within mylonitic granodiorite. (g) δ-type marker showing a dextral kinematic movement. (h) S0/n compositional banding developed in the banded garnet amphibolite. The illustrations were observed on subhorizontal planes.
In the text
thumbnail Fig. 5

Microphotographs of mafic-ultramafic rocks. (a) Destabilization of pyroxene into amphibole + quartz + opaque. (b) Overgrown sphene grains around opaque minerals. (c) Leucite in metahornblendite. (d) Microcline blasts in metagabbro. (e) Granonematoblastic texture of metagabbro showing concordant fabric to the mylonitic foliation in the adjacent shear band. (f) Outcrop photographs showing contact between metagabbro, amphibolite and metahornblendite. (g) Porphyroblastic green hornblende in biotite amphibolite. (h) Poikiloblastic garnet showing inclusion of plagioclase-quartz-biotite in garnet amphibolite.
In the text
thumbnail Fig. 6

Outcrop photographs of migmatitic gneisses (a, b, d, g and h), (b): exposure showing migmatitic gneiss with a shallow dipping foliation delineated by concordant granitic veins, (c): sample of migmatitic gneiss showing the melanosome, leucosome and mesosome layers, (d) Smgm foliation affected by asymmetrical folds Fn+1, (e): quartzofeldspathic leucosome layers including biotite flakes, (f): Interstitial plagioclase between residual hornbelnde crystals, (h): Migmatites with a foliation transposed into the shear zone with a down-dip stretching lineation marking the transition from the Sinassi to the Djourdé domains.
In the text
thumbnail Fig. 7

Field structures and microphotographs in migmatitic gneiss: (a) sheath-like or isoclinal folds Fn+2 showing elliptical sections, (b) (d) Bn+1 asymmetrical boudins, (c): Mesosome with hornblende, retrogressed into quartz, epidote, and opaques, and interstitial microcline and quartz, (d): Hornblende retrogressed into biotite+quartz+opaque.
In the text
thumbnail Fig. 8

Contact between the mafic-ultramafic rocks and the granitoids in the Sinassi area. (a): dyke of granite into mafic rocks, (b): Migmatitic amphibolite with steep dipping synmigmatitic foliation crosscut by granodioritic veins. (c): Granodiorite with shallow-dipping granitic vein. (d): Granodiorite with shallow-dipping granitic vein. (e): enclave of diorite with diffuse boundary in the host granodiorite, (f): Granodiorite with enclave that has preserved the contact between a metagabbro and an amphibolite.
In the text
thumbnail Fig. 9

Relationships of hornblendites and metahornblendites with felsic rocks (a, b, c and e). (a) lobate or diffuse magmatic contacts (b): magmatic contact between hornblendite and granodiorite, (c): magmatic contact between hornblendite and host diorite, (d): Felsic vein in diffuse contact with host hornblendite. (e): interlayered pyroxene metahornblendite and hornblendite.
In the text
thumbnail Fig. 10

Field structures and microstructures in the granitoids. (a): Sm magmatic fabric reoriented by Cn+1 synmagmatic sinistral shear zone, (b): magmatic texture with euhedral feldspar crystals, (c): myrmekitic buds along plagioclase rim, (d): chessboard pattern of quartz crystal, (e): plagioclase affected by kink band and damouritization, (f): mylonite fabric defined by quartz ribbons, (g): anticlockwise bookshelf structure, (h): asymmetric porphyroclasts of feldspars describing the sinistral motion.

In the text
thumbnail Fig. 11

Litho-structural map showing field foliation and planar fluidality with lower hemisphere equal area projection strereograms (contour intervals = 1σ) of the metamorphic, migmatitic, mylonitic and planar fluidality rocks.
In the text
thumbnail Fig. 12

Litho-structural map showing mineral stretching lineation and fold axes with lower hemisphere equal area projection strereograms (contour intervals = 1σ) of the metamorphic, migmatitic, mylonitic and magmatic rocks.
In the text

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