Structural Characterization of the Pan-African Banyo Area (Western Cameroon Domain): Constraints from Field Observations, Structures and AMS

Abstract

The Banyo area, located in the southern prolongation of the Mayo Nolti shear zone trend, belongs to the western Cameroon domain of the Neoproterozoic Central African Belt (NCAB). It is made of granitic rocks that intrude metamorphic banded rocks. Both are sometimes mylonitized. The pluton is dominantly of paramagnetic behavior, as shown by the hysteresis loops and the Fe-bearing silicates crystals are the susceptibility carriers. AMS ellipsoids are dominantly of oblate shape, pointing to the importance of flattening during pluton emplacement. The anisotropy degree of magnetic susceptibility values (≤1.20) characterize the magmatic fabric flow. The microstructural study of the granite reveals magmatic, sub-magmatic, solid-state and mylonitic deformations. Field and AMS fabrics show evidence of polyphase deformation (D₁–D₃). The D₁ phase is of flattening mechanism (flat-laying foliation). The D₂ phase points to sinistral ductile simple shear accommodating moderate to steep dipping and N-S- to NW-SE-oriented foliations in plutonic and country rocks and conjugated E-W mylonitic foliation in country rocks bearing sub-horizontal- to moderate-plunge mineral stretching lineation. The D₃ phase is of dextral ductile simple shear. σ- and δ-type kinematic markers in the pluton indicate sinistral top-to-south sense of shear movement, indicating a non-coaxial component of the tectonics. The magnetic fabrics of the pluton are parallel to those of the D₂ deformation phase of the study area. The transpressive D₂ and D₃ events correlate with the D₂ and D₃ phases of the Pan-African tectonic dated at 613–585 Ma and 585–540 Ma, respectively. The pluton, then, emplaced during regional sinistral D₂ deformation under transpressive regime. The emplacement of the NE Banyo granite took place as rock strips sheared in sinistral sense of shear movement.

1. Introduction

The relationship between granitic magma production and orogeny is regularly associated with their emplacement events. This raises, according to [1], granitic plutons to the position of tectonic markers recorder and leads to the conclusion that massive and continuous granitic magma production and emplacement are closely linked to convergent oblique orogens ([2,3]). The regional tectonics and the dynamics of magmas can then be deciphered by structural analysis of granitic plutons and surrounding country rocks, as demonstrated by several studies (e.g., [4,5]). In most orogenic belts, the time relationships between plutons and regional tectonics are usually expressed as syn-, late-, or post-tectonic [4]. But, their tectonic history is not usually well deciphered. This is the case of the Neoproterozoic Central African Belt (NCAB) in western Gondwana. In old and recent studies ([6,7,8,9]), the lack of detailed structural studies has generated controversial tectonic models for the reconstruction of western Gondwana, despite (i) the petrologic and isotopic data and (ii) a definition of the regional geochrono-stratigraphy in the Pan-African domains. The NCAB involves (1) three major landmasses with three-plate collision belts (Figure 1a) including (a) the São-Francisco–Congo craton, (b) the Eastern Saharan Block and (c) the West African craton and (2) terranes cross-cut by differently oriented transpressive shear zones from different deformation events [5,10,11,12]. Some syn-tectonic granitic plutons within the belt in Cameroon, NE Brazil and Nigeria are recorded along transpressive zones of shear movement [13,14,15,16,17,18,19,20,21,22].

According to [24], shear zones within the NCAB in Cameroon affect the Pan-African basement made of meta-sedimentary and meta-igneous rocks of medium- to high-grade metamorphism in which syn- to post-tectonic granitoids are emplaced. During the past two decades, several research works in the NCAB in Cameroon [25,26,27,28,29,30,31,32,33,34,35,36] point to the kinematics and tectonics of the plutons and its basement rocks. However, their structural evolution is not yet well understood. Indeed, for a better understanding of the regional kinematics, the previous studies outline the lack of structural analysis and the emplacement conditions of rocks within deformed areas and also that some areas remain uninvestigated. Moreover, within the NCAB in Cameroon, the time–relationship interaction between the functioning of the N-S-oriented shear zones, the N70°E-oriented Central Cameroon shear zone (CCSZ) and its N48°E branch (Foumban–Bankim shear zone; [28]) remains a subject of controversy. The Banyo area, located to the north of the junction between the two last shear zones [22,28] and on the southward prolongation of the N-S-oriented Mayo Nolti shear zone (MNSZ; Figure 1b), is chosen for the characterization of the deformation in this area in order to better understand the interactions between the two shear zones. Except for the reconnaissance geological maps by [37,38], the area has never been subjected to any detailed research work in structural analysis, and its tectonics remain misunderstood. In this paper, we integrate detailed petrographic, structural, magnetic and microstructural data to provide the tectonic evolution and emplacement context of the Banyo granitoids and discuss the correlation with the tectonics of the NCAB in Cameroon.

2. Geological Setting

The NCAB in Cameroon (Figure 1b) is also known as the Pan-African North Equatorial Fold Belt [39], Pan-African mobile zone [40] or Pan-African Belt of Central Africa (PBCA; [41,42]). This Neoproterozoic belt is considered to be the extension of the Neoproterozoic Brazilian belt in NE Brazil ([13,43,44,45]), while in the end-Precambrian geological reconstruction of central–west Africa and NE Brazil, the Central Cameroon shear zone appears to be the prolongation of one of the major Brazilian shear zones of the Borborema Province (Pernambuco or Patos principal shear zones and secondary branches; [16,46,47]). It is bordered to the south by the Congo craton; to the North, it is limited by the Sahara metacraton (Figure 1a; [48]), while to the west, it is connected to the Trans-Saharan orogenic belt. The NCAB in Cameroon is the result, according to [5,10,49], of the collision between the Congo craton, the West-African craton and the Saharan metacraton. It is marked by three deformation events: (i) the D₁ deformation event marked by crustal thickening and dated at 630–620 Ma [41,50], (ii) the D₂ deformation event marked by the left lateral wrench movement along the N-S- to NNE-SSW-oriented shear zones (Godé Gormaya, Mayo Nolti, Rocher du Loup; Figure 1b) and dated at 613–585 Ma [11,51] and (iii) the D₃ deformation event marked by the right lateral wrench movement along the CCSZ and dated at ca. 585–540 Ma [52]. Refs. [11,12] developed a model involving two subduction zones including four main steps: (i) breaking up and basin development (from Tonian to at least 620 Ma) on the northern edge of the Congo craton; (ii) pre-tectonic plutonism within the belt (ca. 800 Ma); (iii) collisional events (at ca. 620 Ma) in granulite facies metamorphism reached at ca. 600 Ma; and (iv) nappe tectonics, involving the thrusting of the Yaoundé–Yangana units onto the Congo craton, accretion of the Poli–Leré arc to the Adamawa–Yadé domain, and emplacement of syn-tectonic magmatism (600–580 Ma) and post-tectonic granitoids (ca. 550 Ma).

Table 1. Comparison between Pan-African deformation events of two tectonic evolution models by [5,11,12].

	[5] Deformation Events (Three Major Tectonic Events)	[11,12] Deformation Events (Four Major Tectonic Events)
D1	Pre-orogenic environment marked by back-arc basin at about 1000–640 Ma with sedimentary and magmatic events (volcanism and volcano sediments in the Poli group and sediments in the Yaoundé groups). Crustal thickening and early thrusting/nappe 1 (640–630 Ma) with main tectonic and metamorphic events (collision between the ESB prong and the SFCC; burial and metamorphism of the Yaoundé group in HP-HT granulite facies; early stages of the delamination of the lithosphere).	(1) Pre-collisional state marked by pre-tectonic calc-alkaline granitoids’ emplacement at about 660–670 Ma.
D2	Crustal shortening and thickening upright to recline folds/nappe 2 (around 600 Ma) with: ○ horizontal shortening; ○ thrusting and exhumation of Yaoundé granulites and ultramafic fragments; ○ retrogression of Yaoundé HP-HT granulites and widespread migmatization; ○ increasing delamination of the lithospheric mantle and uprise of the hot asthenosphere.	(2) Syn-collisional stage inducing crustal thickening and the delamination of the subcrustal lithospheric mantle. It comprises D1 and D2 deformations, MP-to-HP metamorphism with granulitic facies rocks, migmatization and the emplacement of syn-tectonic calc-alkaline and S-type granitoids (640–610 Ma).
D3	Conjugate wrench movements (pointing to the superimposition of E-W-oriented Balché, Vallées des Roniers and Demsa cross-cutting the N-S-oriented Gordé, Gormaya and Mayo Nolti shear zones that suggest successive compressional and extensional evolution) at the prong front (585–580 Ma) with: ○ Wrench movements following the penetration of the prong; ○ Advanced delamination of the lithospheric mantle due to indentation; ○ Widespread melting and granitization of the crust causing the dismembering of the ESB; ○ Pan-African dextral shear along the major CCSZ.	(3) Post-collisional stage associated with D3 deformation (nappe and wrench) concomitant with the exhumation of granulites; Development of D4 shear zones and emplacement of late-tectonic calc-alkaline to sub-alkaline granitoids (600–570 Ma). The evolution ends with the development of molassic basins and emplacement of high-level alkaline granitoids (dated at 545 Ma) in an extensional context.

presents a comparison between the different deformation phases of the two tectonic evolution models.

The NCAB began at around 670–660 Ma by the emplacement of calc-alkaline granitoids, followed by crustal thickening between 640 and 610 Ma and high-grade metamorphism and calc-alkaline magmatism. Finally, it was overprinted, between 600 and 545 Ma, by post-collision nappe formation, subalkaline to alkaline magmatism and molasses basin sedimentation [11].

Based on petrological and isotopic data and according to [10,11], the NCAB in Cameroon used to be divided into three geotectonic domains: (i) The southern Cameroon domain or Yaoundé domain [53] that lies between Congo craton to the south and the Sanaga shear zone (SSZ; Figure 1b) to the north. Ref. [12] fixed the actual northern limit of the domain at the Ngoro–Belabo shear zone, situated north of Bafia locality (see Figure 15 in [12]). (ii) The Central Cameroon domain, also known as Adamawa–Yadé domain [24], cross-cut by the CCSZ, is located between the SSZ to the south and the Buffle Noir Mayo Baléo (BNMB) shear zone, also known as the Tcholliré–Banyo shear zone [42], to the north. (iii) The northern or western Cameroon domain that is separated from the Central Cameroon domain by the BNMB shear zone in its eastern part (Figure 1b).

(1) The Yaoundé domain comprises Pan-African metasedimentary units (e.g., Ntui-Bétamba unit, Yaoundé unit [53]) whose detrital material was derived from juvenile Palaeoprotorozoic and Neoproterozoic rock sources deposited at the northern edge of the Archean Congo craton in a passive margin environment. This domain is analogous to the Neoproterozoic Sergipano (Borborema province; NE Brazil, [54]) and Gbayas nappes (Central African Republic; [55]). The rocks of this domain were thrust, towards the south, onto the Congo craton [56]. Deformations and coeval granulites (of medium to high pressure) are dated at about 624 Ma ([55,57]). Geophysical data demonstrate, according to [11], the continuation of the Congo craton under this southern domain and [58] dated Archean rock in the vicinities of Bafia (30 km north; Figure 1b).

(2) The Central Cameroon domain is especially rich in syn- to late-tectonic granitoids of a transitional composition and crustal origin, displaying mainly high-K calc-alkaline affinities and dated at around 640–590 Ma [24,27,45,59,60,61,62]. They were emplaced along differently oriented shear zones in the Pan-African basement made of low- to medium-grade metavolcaniclastic and metasedimentary rocks that contains, according to [8,45,57,60], Paleoproterozoic metasedimentary and orthogneissic remnants with assimilated Archaean crust similar to the Ntem Complex. The shear zones include the following: (i) the N70°E-oriented CCSZ or Adamawa shear zone [17], (ii) the N48°E-oriented Foumban-Bankim shear zone, the junction section between the CCSZ and FFSZ [22,28] and the N30°E-oriented Fotouni–Fondjomekwet shear zone (FFSZ; [30]), Buffle Noir Mayo Baléo (BNMB; Figure 1b) and other anastomosed shear zones, which constitute the CCSZ system (Figure 1b).

(3) The western Cameroon domain, to which belongs the study area, is made of varied rock types, namely (i) Neoproterozoic medium- to high-grade volcanic sedimentary schists and gneisses (dated at about 700 Ma with possible 2.1 Ga inheritances) from the Poli series; (ii) calc-alkaline granitoids from about 660 to 580 Ma [52] composed of diorite, granodiorite and granite; (iii) anorogenic alkaline granitoids; and (iv) volcanic (tholeiitic to alkaline affinities) and sedimentary basin sequences of low-grade metamorphism [8]. According to [45,63], at least two episodes of Pan-African plutonism mark the domain: (i) 640–620 Ma pre- to syn-tectonic calc-alkaline granitoids and (ii) S-type-affinity late-tectonic granitoids emplaced at around 580 Ma. This domain is also marked by differently oriented shear zones: (i) the N-S-oriented Mayo Nolti and Godé Gormaya shear zones [5,10] and (ii) the E-W-oriented Demsa and Vallée des Roniers shear zones [5,10] along which some plutons are emplaced. The superposition of these N-S mylonitic zones and their association with the E-W steep plunging (Gormaya mylonites) and N-S sub-horizontal (Godé mylonites) lineations suggest the successive compressional and extensional evolution of the area [5].

3. Materials and Methods

Various lithological units within the study area are described, and oriented representative samples and cores for structural analysis and AMS measurements were collected from 51 different observation sites (29 sites from the granitic pluton and 22 sites from the metamorphic country rocks; Figure 2b). In the field, some structural elements (foliations, mineral stretching lineation, folds, shear planes) and kinematic markers (σ- and δ-type porphyroclasts) were observed and measured (orientation: strike direction, dip or plunge) when possible, using a clinometer attached to a magnetic/sun compass. Structural elements were also measured by carrying out their geometric analyses that consisted of determination and measurements of (i) the strikes and dips of planar structures (foliations, shear planes, faults) and (ii) plunge directions and plunges of linear structures (fold axes, mineral stretching lineation).

Petrographic and microstructural analyses, using a binocular microscope (plane-polarized light) hosted at the Soil Analysis and Environmental Chemistry Research Unit (SAERU) of the Department of Earth Sciences, University of Dschang (Cameroon), were performed on 25 oriented, thin sections, cut perpendicular to the foliation planes (XY) and parallel to the mineral stretching lineation (X; i.e., the XZ reference frame) as indicated by [62,63]. Statistical analyses of structural elements (planar and linear structures) were performed using Stereonet software v.8 [64,65]. Measurements of strikes, dips and/or plunges in structural elements were plotted in a lower-hemisphere projection diagram (contour intervals = 2%). In the overall view of the mean orientation of structural elements, such as foliations, mineral stretching lineations and fold axes, they were marked by their best-fit pole or best-fit line in the projection diagram using the conventional techniques of [66]. Detailed studies of structural elements in addition to mesoscopic to microscopic criteria of non-coaxial strain (e.g., fold asymmetry, boudins, shear bands, porphyroclast tails and pressure shadows) will allow us to propose a kinematic analysis and reconstruct the tectonic history of the study area.

Oriented rock core samples were collected using a portable motor fitted with a diamond-tipped water-cooled drill. Per sampling site, two or three oriented rock cores were collected, each sample measuring approximately 2.5 cm in diameter and 6 to 7 cm in length. For each core sample, field parameters were measured using a magnetic compass associated with a clinometer. In the laboratory, the samples were sliced into two or three individual cylinders of a 2.2 cm height for AMS measurements. In the study area, 36 sampling sites (21 sites from the granitic pluton and 15 sites from the metamorphic country rocks) allowed us to collect oriented sample cores. The sample cores were cut into standard specimens and a total of 180 specimens were obtained for AMS measurements from which were calculated the AMS parameters.

AMS and hysteresis measurements were conducted in the geophysical laboratory of Centre de Recherche et d’Enseignement de Géosciences de l’Environnement (CEREGE) UMR6635 at Aix-Marseille University-CNRS Europole de l’Arbois (Aix-en-Provence, France).

AMS data were obtained with the MFK1 Kappabridge. The three principal axes, K₁, K₂ and K₃, represent the AMS axes and the AMS ellipsoid with K₁ ≥ K₂ ≥ K₃. K₁ is the magnetic lineation, the rotation axis or zone axis of the preferred orientation of Fe-bearing silicate minerals. K₃ is the pole of the (K₁K₂) plane of the preferred orientation of Fe-bearing silicate minerals, the magnetic foliation plane. AMS parameters, including the mean magnetic susceptibility (Km), the anisotropy degree of magnetic susceptibility (P_j) and the T_j shape parameter, were calculated on the basis of the K₁, K₂ and K₃ magnitudes [67]. They were calculated from the following equations: Km = (K₁ + K₂ + K₃)/3; P_j = exp [{2[(η₁ − η_m)² + (η₂ − η_m)² + (η₃ − η_m)²]}^1/2]; and T_j = (2η₂ − η₁ − η₃)/(η₁ − η₃). Here, η₁ = ln K₁; η₂ = ln K₂; η₃ = ln K₃; and η_m = (η₁·η₂·η₃)^1/3. The T_j value varies between −1 and +1; T_j ˂ 0 corresponds to a prolate shape ellipsoid, while T_j > 0 indicates an oblate shape.

Two representative samples from the plutons were used for hysteresis analyses. Measurements were performed using the following instruments: CS2 apparatus (for thermomagnetic measurements) coupled with a KLY-3S Kappabridge (Agico, Czech Republic) and Micromag VSM (for hysteresis loops up to 1T). Due to the fact that VSM sample volume is not easy to determine, the mass of the sample was used as a normalized value and we computed the volume susceptibility using the arbitrary density of 2.5 × 10³ kg/m³.

4. Results

4.1. Petrography

On the reconnaissance geological map [37], the Banyo region is made of (i) amphibole–biotite and amphibole anatexitic gneiss, (ii) amphibole–biotite gneiss and (iii) amphibolites that host concordant granites (i.e., K-feldspar megacrystals and biotite–amphibole granites) and syenites. However, the geological map by [38] in the Banyo region (Figure 2a) evokes a NE-SW-stretched porphyritic granitic pluton that intrudes a TTG migmatitic orthogneiss. The present work reveals that the study area is made of high–grade metamorphic banded rocks intruded by a plutonic unit (Figure 2b). The metamorphic banded rocks comprise banded hornblende–biotite gneiss while the plutonic unit is composed of diorite, coarse-grained granite (CGG) and fine-grained granite (FGG). Both units are sometimes mylonitized, giving, respectively, mylonitic banded garnet–biotite gneiss in the northern and northeastern parts of the study area and sensu stricto mylonite of granite in the northwestern part of the pluton. Rocks crop out as flagstones, boulders, dykes and veins in the valleys (sometimes in the river beds), on the flanks and on the summits of hills referred to as Hoséré in the local language.

The metamorphic rocks are mostly composed of banded hornblende–biotite gneiss that outcrops in form of flagstones at the borders of the granitic pluton, in the Mayo Soum–Soum and Mayo Foorou tributary river beds, and sometimes as centimetric to metric enclaves within the granitic pluton. Outcrops show an alternation of thick (20–30 cm) mafic and felsic bands (Figure 3a). These bands show internal millimetric banding with quartz- and feldspar-rich bands alternating with hornblende- and biotite-rich bands. The rock, grayish to dark-gray in color and medium- to coarse-grained, is made of hornblende, biotite, plagioclase and K-feldspar in the dark bands, while the light bands are made of quartz ribbons and K-feldspar (Figure 3b). It shows grano-nematoblastic, porphyroblastic or lepidoblastic texture with preferred orientation of crystals. The mylonitic facies is identified as banded biotite–garnet gneiss (Figure 3c) that outcrops along ductile shear zones, presenting the same organization as the banded gneiss with the specific presence of garnet porphyroclasts in the dark bands (Figure 3d,e) and mylonitic texture. The garnet crystals, molded by biotite flakes and quartz polycrystalline ribbons, are usually transformed into plagioclase, biotite and oxide (Figure 3f). Accessory minerals include epidote, sphene, oxides and apatite.

The plutonic unit includes diorite, coarse-grained granite and fine-grained granite. The coarse-grained granite develops mylonitic texture, along the N-S-oriented mylonitic band, on the northwestern edge of the pluton (Figure 2b).

Diorite outcrops either as flagstone (i) in the Mayo Wouroum river bed and (ii) on the Hoséré Wouroum flanks in the southern part of the pluton or as xenoliths in the coarse-grained granite. The rock is dark in color and contains mainly hornblende, plagioclase, clinopyroxene and K-feldspar (Figure 3g,h). Clinopyroxene is sometimes transformed into hornblende and biotite flakes. Accessory minerals include oxides, epidote and sphene.

Coarse-grained granite is the most outcropping rock type in the study area (Hosérés Djiddéré and Wouroum). The rock is massive and either pink or gray to dark-gray (Figure 3i,k) in color. It shows bimodal grain size marked by a major phase assemblage of phenocrysts of alkali feldspar, biotite and hornblende wrapped by a fine-grained matrix phase made of quartz, K-feldspar, zoned plagioclase, hornblende, biotite and myrmekite (Figure 3j,l). Primary biotite flakes, sometimes included in K-feldspar phenocrysts, are progressively transformed into chlorite. Zoned, euhedral to sub–euhedral crystals of K-feldspar phenocrysts molded by quartz ribbons and biotite flakes sometimes contain zoned inclusions of euhedral feldspar crystals. Quartz crystals show evidence of dynamic recrystallization with (i) polycrystalline ribbons that mold some feldspar phenocrysts, (ii) irregular and lobed grain boundaries, (iii) dislocation glide and (iv) intracrystalline deformation. Apatite, epidote, sphene, magnetite and other oxides are accessory minerals. The mylonitized facies corresponds to the deformation of the pluton under greenschist facies conditions. It is made of quartz, sometimes in the form of stretched polycrystalline ribbons, plagioclase, and a variable amount of biotite flakes.

Fine-grained granite outcrops in form of dykes and veins in the coarse-grained granite and in the metamorphic rock unit. The rock, of millimetric grain size, is whitish and composed of quartz, orthose, microcline, plagioclase and biotite. Accessory minerals are sphene, apatite and oxide.

4.2. Structural Data

The rocks preserve structural elements in metamorphic (banded hornblende–biotite gneiss and banded biotite–garnet gneiss) and plutonic (diorite, CGG and FGG) units. Structural elements include foliation, mineral stretching lineation, strain–slip schistosity, folds, boudins and kinematic markers. The field relationships between structural elements, added to macroscopic and microscopic structures, indicate that the study area developed polyphase deformation marked by three phases, namely D₁, D₂ and D₃.

4.2.1. D₁ Deformation

The D₁ deformation is recorded in banded hornblende–biotite gneiss from Ngounaouté and Mayo Soum-Soum. It is a flat layering S₁ foliation marked by the alternation of thick (pluri-decimetric) felsic (quartz- and feldspar-rich) bands with thick mafic (hornblende- and biotite-rich) bands superposed with the alternation of thin millimetric to centimetric dark bands and light bands (Figure 4a). In the lower-hemisphere projection diagrams, it is essentially NE-SW-oriented fabric with low to moderate dips (27–44°) in the western and eastern parts of the study area. The best-fit poles of the foliation (Figure 5a) vary between 130°/60° (i.e., 60°→130°; N40°E/30°NW; site MA60) and 163°/63° (i.e., 63°→163°; N73°E/27°NW; site MA33) with a maximum at 132°/62° (i.e., 62°→132°; N42°E/28°NW; Figure 5b).

4.2.2. D₂ Deformation

The D₂ deformation phase, recorded in metamorphic and plutonic units, is evidenced by the following structures: S₂ foliation (metamorphic and magmatic), strain–slip schistosity, asymmetric F₂ folds, shear bands, L₂ mineral stretching lineation and σ- and δ-type kinematic markers.

The S₂ foliation in banded hornblende–biotite gneiss is marked by the alternation of dark and light bands (Figure 4b), whereas in granites, it is marked by the preferred orientation of feldspar megacrysts (Figure 4c) which represents magmatic foliation. The S_2m mylonitic foliation is observed in the mylonitized facies of both rock types (Figure 4d).

In the banded hornblende–biotite gneiss, the S₂ foliation shows NW-SE to N-S directions with low to moderate dips. Lower-hemisphere projection diagrams show best-fit poles (Figure 6a) varying between 100°/48° (i.e., 48°→100°; N10°E/42°WNW for site MA73b) and 47°/63° (i.e., 63°→47°; N137°E/27°SW; site MA70) with a maximum at 81°/66° (i.e., 66°→81°; N171°E/24°WSW; Figure 6b). In the mylonitic facies (banded garnet–biotite gneiss) of the country rocks (sites MA58, MA73, MA74), the S₁ fabric was completely transposed and rotated slowly to develop E-W mylonitic fabric (S_2m; Figure 4h,j,l,m). It shows high-strain deformation rocks made of banded garnet–biotite gneiss. The fabric shows low to moderate dip angles towards the south. The best-fit poles of the mylonitic foliation (Figure 6a) vary between 358°/72° (i.e., 72°→358°; N88°E/18°S; site MA73) and 03°/44° (i.e., 44°→03°; N93°E/46°S; site MA58) with a maximum at 02°/45° (i.e., 45°→02°; N92°E/45°S; Figure 6b).

In the coarse-grained granite, the S₂ fabric, also known as the magmatic fabric, is NW-SE- to NE-SW-oriented with low to moderate dips. It shows best-fit poles (Figure 7a) that vary between 129°/46° (i.e., 46°→129°; N39° E/44° NW; site MA40) and 62°/48° (i.e., 48°→62°; N152° E/42° SW; site MA41) with a maximum at 92°/66° (i.e., 66°→92°; N02° E/24° W; Figure 7b). In detail, in Hoséré Wouroum in the south, the best-fit poles of the magmatic foliation vary from 304°/60° (i.e., 60°→304°; N34° E/30° SE; site MA62) to 81°/46° (i.e., 46°→81°; N171° E/44° WSW; site MA72) with a maximum at 86°/73° (73°→86°; N176° E/17° W), and in Hoséré Djiddéré, they vary between 97°/61° (i.e., 61°→97°; N07° E/29° W; site MA18) and 302°/45° (i.e., 45°→302°; N32° E/45° SE; site MA64) with a maximum at 92°/68° (i.e., 68°→92°; N02° E/22° SW). Fusiform asymmetric enclaves of banded hornblende–biotite gneiss and diorites, on the metric long axis, oriented sub-parallel to the magmatic and metamorphic fabrics, are observed at the border and the center of the pluton. The mylonitic fabric within the coarse-grained granite (Figure 4d) shows fabric that strikes N-S with moderate dips towards the west. The best-fit poles are close to homogenous (Figure 7a) at 99°/59° (i.e., 59°→99°; N09° E/31° E; site MA23) and 99°/53° (i.e., 53°→99°; N09° E/37° E; site MA22).

Shear planes are recorded in banded hornblende–biotite gneiss and in high-strain mylonitic zones. At the outcrop scale, the foliation (i) curves smoothly by rotation across shear planes (from the outside to the center) to become parallel to the shear plane or (ii) is cross-cut by sinistral rhythmic shear planes with oriented directions of N-S and N13°E (Figure 4e). This is also observed in the contact zone between the coarse-grained granite and diorite. It is also important to note the S-C fabrics recorded in localized deformed zones in the CGG (Figure 4f). This fabric is marked by two sets of planar surfaces: the preferred orientation of K-feldspar porphyroclasts displaying the “S” fabric, oblique to the narrow “C” shear planes represented by small-scale shear surfaces that disrupt the “S” fabric.

L₂ mineral stretching lineation is marked by stretched quartz ribbons, biotite flakes, amphibole needles recorded in banded hornblende–biotite gneiss and banded garnet–biotite gneiss (in the high-strain mylonitic zone). It is NW-SE- to NNW-SSE-oriented with low to moderate plunges. The best-fit lines (Figure 8a) vary between 318°/11° (i.e., 11°→318°; N138° E/12° NW; site MA58) and 154°/12° (i.e., 12°→154°; N154° E/12° SSE; site MA73) with a maximum at 146°/01° (i.e., 01°→146°; N146° E/01° SE; Figure 8b). The mylonitic mineral stretching lineation (Lm₂; Figure 8a) is oriented at 237°/57° (i.e., 57°→57°; N57° E/57° NE; site MA58).

Asymmetric F₂ folds, characterized by thinned long and short flanks with thickened hinges and “S” shapes with vergence towards NW (Figure 4h,i), are recorded in banded hornblende–biotite gneiss and in the high-strain mylonitic zone. The directions of the fold axes vary between N09°E and N145°E in the banded hornblende–biotite gneiss and between N43°E and N59°E in the high-strain mylonitic zone. They show low to moderate plunges. The best-fit axes (Figure 9a) vary between 09°/11° (i.e., 11°→09°; N09° E/11° NNE; site MA50) and 145°/27° (i.e., 27°→145°; N145° E/27° SE; site MA27) in the banded hornblende–biotite gneiss and between 43°/17° (i.e., 17°→43°; N43° E/17° NE; sites MA59) and 59°/39° (i.e., 39°→59°; N59° E/39° NE; site MA58) in the high-strain mylonitic zone with maxima at 177°/17° (i.e., 17°→177°; N177° E/17° SW) and 50°/30° (i.e., 30°→50°; N50° E/30° NE), respectively (Figure 9b).

β₂ boudins are either domino boudins marked by the shear of pegmatitic and granitic veins within diorite in the vicinity of the CGG (Figure 4g) or shear band boudins according to [68,69]’s boudin classifications. In the diorite, pegmatitic and granitic veins boudinage defines trains of asymmetric domino boudins of pegmatite, diorite and fine-grained granite parallel to the magmatic foliation of the CGG in a sinistral shear movement (Figure 4g).

σ- and δ-type kinematic markers are observed in the high-strain mylonitic zones (in the pluton and the country rocks). They are characterized by asymmetric K-feldspar, garnet porphyroclasts (Figure 4d,k–m) and biotite fish. These markers develop rolling structures with (i) pressure shadows marked by recrystallized quartz crystals or ribbons and biotite flakes that mold the porphyroclast edges (Figure 4m) and (ii) bended stair-stepping tails (for δ-type markers; Figure 4k) showing sinistral top-to-NNE and -south senses of shear movement.

4.2.3. D₃ Deformation Phase

The D₃ deformation phase, recorded in banded hornblende–biotite gneiss and in the pluton, is evidenced by the following structures: C₃ shear planes, S₃ foliation, asymmetric β₃ boudins and asymmetric F₃ folds.

C₃ shear planes are ductile bands of high-strain rock recorded in banded hornblende–biotite gneiss and banded garnet–biotite gneiss. They are marked by either (i) narrow shear corridors about 50 cm wide, in the NE-SW direction, that cross-cut the N-S-oriented S₂ foliation in dextral sense of shear movement (Figure 4o) or (ii) discrete planes that show a NW-SE direction (N150° E) where the metamorphic foliation of the country rock curves progressively to become parallel to the shear plane direction (Figure 4h).

Asymmetric β₃ boudins are shear band boudins, recorded in the high-strain mylonitic zone. They have rhomb- to lens-shape, σ-type or pinch-and-swell quartzo-felspathic boudins showing antithetic sinistral top-to-SE sense of shear movement (Figure 4j). They are result of the stretching and folding (“Z” shape fold) of the quartzo-feldspathic vein that cross-cuts the mylonitic foliation within banded garnet–biotite gneiss.

Asymmetric F₃ folds are recorded in banded hornblende–biotite gneiss (Figure 4n,o) and in the high-strain mylonitic zone. They are asymmetric “Z” shape folds showing thinned limbs and thickened hinges pointing to dextral sense of shear movement with vergence towards NE and east (Figure 4j,o).

4.3. Microstructures in Granite

According to [70,71,72,73,74,75,76], mesoscopic and microscopic rock fabric studies allow the determination different states of pluton deformation. Applied to the granitic pluton, oriented thin sections reveal deformation from a magmatic state to a solid state from high to low temperatures. The magmatic state of the granites is marked (1) at the mesoscopic scale by (i) the preferred orientation of the K-feldspar euhedral to sub-euhedral crystals and (ii) elongated fusiform-shape enclaves known as magmatic fabric that defines the magmatic flow and (2) at the microscopic scale by (i) the presence of sub-euhedral, coarse crystals that tend to be polygonal, exhibiting triple joins, (ii) quartz crystals showing homogenous (or weak undulose) extinction (Figure 10a), (iii) feldspar crystal zonation (Figure 10b) and (iv) small, euhedral, zoned, early-formed feldspar crystals hosted by late-stage anhedral feldspar crystals (Figure 10c). The sub-magmatic state deformation is marked by (i) the presence of myrmekite budding crystals on the border of K-feldspar crystals (Figure 10d) and (ii) feldspar porphyroclast micro-fractures filled up with quartz and/or feldspar crystals (Figure 10e) indicating, in this stage of deformation, the presence of at least a small quantity of silicate melts at the time of the fracture of the feldspar porphyroclasts. The solid-state deformation microstructures from high to low temperatures are marked by (i) cleavage and twin deformations (bending and kinking) in biotite and plagioclase crystals (Figure 10d), (ii) quartz crystals dynamically recrystallized with grains that show irregular and lobed boundaries, illustrating the grain boundary migration recrystallization (Figure 10c,e,f), (iii) quartz grain chessboard patterns, (iv) elongated quartz ribbons showing dynamic recrystallization with elongated sub-grain rotation (Figure 10d), (v) the presence of small recrystallized quartz grains and (vi) pervasive mylonitic deformation including biotite and hornblende fishes (Figure 10g) and microcracks in feldspar crystals, sometimes showing synthetic movement in the dextral sense (Figure 10h).

4.4. AMS Data

AMS data per site are presented in

Table 2. AMS parameters: the mean magnetic susceptibility (Km = (K₁ + K₂ + K₃)/3), the anisotropy degree of magnetic susceptibility (P_j = exp [{2[(η₁ − η_m)² + (η₂ − η_m)² + (η₃ − η_m)²]}^1/2]) and the T_j-shape parameter (T_j = (2η₂ − η₁ − η₃)/(η₁ − η₃)) with η₁ = ln K₁, η₂ = ln K₂, η₃ = ln K₃ and η_m = (η₁·η₂·η₃)^1/3 according to [67].

Sites	Geographic Coordinates		Mean AMS Parameters			Mean Eigenvectors
						K1		K2		K3
	Long. E (°)	Lat. N (°)	Km (×10−3 SI)	Pj	Tj	Dec	Inc	Dec	Inc	Dec	Inc
Country rock
MA1	11.840	6.775	0.42	1.08	−0.07	26	12	116	26	268	75
MA2	11.839	6.778	0.58	1.09	0.54	8	3	304	60	97	25
MA3	11.854	6.793	9.83	1.16	0.10	240	31	318	1	72	45
MA4	11.860	6.785	1.04	1.14	0.30	184	23	93	6	1	68
MA5	11.852	6.774	0.65	1.12	0.75	225	63	62	26	329	7
MA6	11.869	6.788	0.35	1.09	0.05	212	10	306	23	104	65
MA9	11.852	6.805	10.59	1.30	−0.09	45	8	313	5	199	81
MA11	11.833	6.805	5.82	1.26	−0.08	137	25	232	7	345	61
MA12	11.836	6.803	4.06	1.20	−0.13	213	4	122	23	314	66
MA16	11.869	6.811	0.66	1.13	0.37	25	4	294	11	131	77
MA26	11.845	6.767	0.18	1.14	0.51	207	34	299	3	34	58
MA27	11.864	6.773	0.19	1.07	0.04	193	25	291	18	53	59
MA33	11.901	6.772	0.57	1.11	0.47	61	80	264	10	174	4
MA56	11.861	6.806	0.86	1.24	0.15	8	23	274	13	162	60
MA58	11.884	6.806	0.57	1.12	0.52	199	2	287	65	119	22
Pluton
MA13	11.869	6.809	0.08	1.03	0.20	339	16	139	72	248	6
MA18	11.873	6.808	0.20	1.09	0.04	120	9	220	42	18	46
MA19	11.867	6.836	0.19	1.06	−0.49	140	3	46	61	230	31
MA20	11.868	6.822	0.18	1.06	−0.29	97	65	212	11	306	22
MA22	11.870	6.799	0.15	1.06	−0.24	259	1	235	67	86	45
MA23	11.858	6.799	0.14	1.07	−0.16	115	11	347	67	208	19
MA24	11.854	6.769	0.18	1.06	0.13	278	7	82	83	188	3
MA28	11.863	6.756	0.19	1.07	0.04	309	38	109	46	210	11
MA31	11.886	6.797	0.16	1.04	0.65	20	40	115	7	213	50
MA32	11.909	6.770	0.16	1.04	0.80	349	1	98	10	274	83
MA34	11.891	6.806	0.28	1.20	−0.28	317	16	169	74	49	8
MA39	11.857	6.763	0.17	1.03	0.14	330	35	87	42	207	37
MA45	11.867	6.757	0.16	1.04	0.56	301	9	44	57	205	32
MA46	11.869	6.753	0.15	1.04	0.63	288	1	19	43	199	47
MA57	11.865	6.800	0.13	1.06	−0.25	111	23	213	7	231	74
MA64	11.885	6.076	0.14	1.06	−0.04	72	17	307	62	169	26
MA47	11.861	6.750	1.29	1.03	0.18	214	9	4	67	118	4
MA51	11.856	6.756	0.48	1.03	0.25	281	32	168	2	51	17
MA52	11.857	6.757	4.89	1.08	0.02	176	1	260	14	92	76
MA40	11.868	6.751	0.40	1.02	0.14	148	27	251	13	3	57
MA41	11.863	6.745	0.53	1.01	0.41	149	11	55	17	277	69

. The magnetic susceptibility bulk magnitude (Km in 10⁻³ SI) in the study area (Figure 11) varies between 0.08 (sample MA13 from the pluton in the coarse-grained granite) and 10.6 (sample MA9 from the country rock). Km values range between 0.18 and 10.6 in the country rocks and between 0.08 and 4.8 in the pluton (in the diorite). In the country rock, 77% of the sites show a Km value > 0.5 while 87% of the sites from the pluton show a Km value ≤ 0.5. The three sites presenting a Km value > 0.5 over twenty-one AMS sites belong to the unmappable dioritic facies that outcrops in the central south of the pluton. Rocks showing Km values ≤ 0.5 range in the paramagnetic group, likely dominated by iron-bearing silicate signal such as biotite and hornblende, while those with Km values > 0.5 range in the ferromagnetic group, usually dominated by magnetite as magnetic susceptibility carrier [77]. There is no direct evidence for the attribution of the ferromagnetic component of susceptibility to magnetite, but the constant observation in similar plutons in the Neoproterozoic Central African Belt indicates that, when present, this component is carried by magnetite. Figure 11 shows the central southern part of the pluton as zone of high Km values (with the ferromagnetic zone where outcrops are unmappable (due to the outcrop width) dioritic facies) while the northern and eastern parts are of low to very low Km values. The paramagnetic behavior of granitic rocks from NE Banyo is confirmed by the hysteresis loops of the representative samples that show typical curves of paramagnetic behavior (Figure 12). The anisotropy degree of magnetic susceptibility (P_j) values vary between 1.01 (site MA41 from the pluton in diorite) and 1.30 (site MA6 from the country rock). It ranges between 1.07 and 1.30 in the country rock and between 1.01 and 1.20 in the pluton (site MA34 in the coarse-grained granite; Figure 13). Rocks showing P_j ≤ 1.20 display the fabric due to magmatic flow as magnetic fabric [78] and are known as magnetite-free rocks [79,80]. In the pluton, there is no P_j value > 1.20, while in the country rock, P_j values > 1.20 are located in the northern, northwestern and western parts of the pluton, without specific organization. The shape parameter (T_j) ranges between −0.49 (sample MA19) and 0.80 (sample MA32) in the pluton in the coarse-grained granite within the study area (Figure 14a). It ranges between −0.13 and 0.75 in the country rock and between −0.49 and 0.80 in the pluton. In the granitic pluton, the constricted AMS ellipsoids are essentially located in the northern part of the pluton where they seem to be influenced by the N-S (Mayo Nolti) and WNW-ESE shear zones. The T_j value isoline within the granitic pluton (Figure 14a) shows a zonation of the pluton with the northern part as a zone of T_j < 0 (with the constricted AMS ellipsoid) while the southern part is dominated by T_j > 0 (with the flattened AMS ellipsoid). The isoline −0.05 works well with the N-S Mayo Nolti shear zone. Furthermore, the T_j vs. P_j diagram (Figure 14b) indicates that most of the sites are plotted in the field of oblate shape of the deformation ellipsoid with 67% for the pluton and 73% for the country rock. Thus, flattening and constriction played important roles during the emplacement and deformation of the pluton.

In the whole pluton, the magnetic foliation displays moderate to high dips (30–90°) for 81% of the sites and low dips (0–30°) for 19% of the sites, for directions varying mostly NW-SE (52% of the sites) and dips towards SW and NE (Figure 15a). The NW-SE-oriented fabric is observed all over the pluton. The mean foliation fabric shows the best-fit pole at 214°/36° (i.e., 36°→214°; N124° E/54° NE). In the country rocks, the magnetic foliation displays moderate to high dips for 54% of the sites with directions varying N-S, E-W and NE-SW. Its best-fit pole is at 69°/76° (i.e., 76°→69°; N159° E/14° SW; Figure 15a).

The magnetic lineation in the whole pluton plunges mostly lowly (76% of the sites) with dominant NW-SE directions (62% of the sites). The other directions are N-S and NE-SW (Figure 15b). The best-fit line is at 309°/3° (i.e., 3°→309°; N129° E/3° NW). In the country rocks, the magnetic lineation also displays low plunges (73% of the sites) for directions varying N-S to NNE-SSW. Its best-fit line is at 203°/11° (i.e., 11°→203°; N43°E/11° SW; Figure 15b).

5. Discussion

5.1. Relationship Between Magnetic Fabric of NE Banyo Granite and Regional Deformation

Section 4.2 described three deformation phases recorded in the study area: D₁, D₂ and D₃. The ductile D₂ deformation phase is recorded both in the country rocks and the pluton and displays the following geometric parameters: (i) S₂ magmatic foliation with mostly N-S to NW-SE strike and low to moderate dips with a best-fit pole at 92°/66° (Figure 7); (ii) S₂ metamorphic foliation with NW-SE to N-S strike and low to moderate dips with a best-fit pole at 81°/66° (Figure 6); (iii) NW-SE-oriented L₂ mineral stretching lineation with low to moderate plunges in the country rocks with a best-fit line at 146°/01° (Figure 8); and (iv) NNE-SSW- to NW-SE-oriented F₂ fold axis with low to moderate plunges with a best-fit axis at 177°/17° (Figure 9). The magnetic fabric in the NE Banyo granitic pluton displays magnetic foliation with N-S to NW-SE strike and moderate to high dips and N-S and mostly NW-SE magnetic lineation with low plunges. The magnetic fabrics of the country rocks show NW-SE-strike foliation with low- to moderate-dips and N-S- to NNE-SSW-directed magnetic lineation with low plunges. The field orientations of the structures of the D₂ deformation phase are roughly sub-parallel to the magnetic fabrics recorded in the country rock and/or the pluton. The L₂ mineral stretching lineation, whenever observed, strikes NW-SE with low to moderate plunges, parallel to the fold axes and the magnetic lineation. The above observations can allow us to infer that the magnetic fabric preserved in the NE Banyo granitic pluton is mainly related to the D₂ deformation phase that is recorded in the country rocks.

5.2. Syn-Tectonic Emplacement of NE Banyo Pluton

Section 5.1 demonstrates that the magnetic fabric of the NE Banyo pluton was acquired during D₂ deformation phase. Fabric features indicating syn-magmatic, sub-magmatic and/or solid-state deformations need to be recognized in the pluton before stating its syn-tectonic emplacement [75]. In the NE Banyo granitic pluton, the preferred orientation of megacrystals of feldspar (Figure 3i–l and Figure 4c) and the elongation of the fusiform shape of country rocks and dioritic enclaves parallel to the S₂ magmatic foliation within the coarse-grained granite at the mesoscopic scale, added to the feldspar crystals’ zonation (Figure 3l) at the microscopic scale, indicate magmatic state imprints. Such structures, associated with sub-magmatic and high- to moderate-temperature solid-state microstructures (see Section 4.4), strongly support that the pluton records a continuum of deformation from the magmatic state with the crystal mush state [21,81,82] to its entire crystallization. Therefore, it can be interpreted as syn-kinematic. Kinematic markers recorded in the pluton and in the country rocks indicate the sinistral sense of shear movement during the emplacement of the study pluton. In the western domain of the NCAB in Cameroon, the N-S fabric with a sinistral sense of shear movement, characterizing the D₂ deformation, has also been recorded and described in many syn-tectonic elongated granitic plutons. These include the Fomepéa [21], Misajé [34], Tcholléré [24], Mbakop [25] and Numba [38] plutons, dated between 613 and 580 Ma.

5.3. Banyo Tectonics Integrated to the Regional Tectonic Evolution of the NCAB

In the NE Banyo area, the country rocks preserve flat-laying S₁ foliation, considered to belong to the D₁ deformation, striking ENE-WSW to NE-SW with low to moderate dips, consistent with the NNW-SSE to NW-SE regional stress shortening direction with a flattening mechanism. This deformation phase lacks mineral stretching lineations and folded structures for a better comprehension of its material transportation kinematics. However, (i) previous work carried out by [83] defined the shortening direction, from NNW towards SSE direction, of material transport in the Mbé–Sassa–Bersi area (located in NNE Banyo, along the BNMB shear zone), and (ii) field observations of Ngounaouté outcrops in the study area and (iii) the rock structure (alternate amphibolite and gneiss) are quite similar to those identified by [84] and [85], respectively, on the southwestern and the southeastern borders of the Poli series in the Mbé group and to those which [85] considered probably from early Pan-African crustal thrusting. In Poli, situated 150 km northwestward of the study area, refs. [84], characterizing the Proterozoic compression and cratonization of north Cameroon, and [86], focusing on Eburnean and Archean provinces in eastern Nigeria (at the Poli series latitude), attributed the first deformation phase to the early Pan-African tectonic emplacement of nappes. According to these authors, the ENE-WSW to NE-SW plunging of the fold axes inducing the NNW-SSE or NW-SE dipping of fold axial planes suggests a verging of the tectonic nappe towards SSE or SE.

The D₂ deformation is observed in the country rocks and in the granitic pluton. It is characterized by (i) S₂ metamorphic and magmatic foliations with moderate dips; (ii) L₂ mineral stretching lineation with sub-horizontal to moderate plunges; (iii) “S”-type F₂ folds with vergence towards NW or W; and (iv) N-S “C” shear planes, S/C structures and asymmetric boudins in sinistral sense of shear movement. The organization of these structures consists of the ESE-WNW to E-W shortening direction that developed sinistral N-S-oriented direction of the foliation by simultaneous combination of shortening and simple shear motion mechanisms. The developed asymmetric “S”-shape folds point to a ductile deformation mechanism that develops NNE-SSW- to NW-SE-oriented fold axes that plunge gently.

In the north of the study area (Poli serie), the country rocks developed localize mylonitic zones where the S₁ fabric is overprinted by the S₂ E-W fabric with moderate dips associated with moderate plunging and stretching lineation. Previous studies (in the Poli series; [61,84,85]) evoke this S₂ E-W fabric as a conjugate wrench movement cross-cutting the N-S-oriented shear zones, suggesting successive compressional and extensional evolution. These localized high-strain ductile zones in the country rocks which record kinematic markers showing a sinistral sense of shear movement (Figure 4h,j–m), such as the one in the pluton, likely developed during the emplacement of the pluton under transpressive regime where simple shear was dominant [87,88,89,90]. Here, the mineral stretching lineation L₂ is quite parallel to F₂ fold axes; this suggests that folding is the result of shearing. Foliation and folding indicate a significant implication of shortening in the WNW-ESE to NW-SE directions. There is, then, a spatial association of shortening and shear during the second deformation phase, as identified by [83] in the Mbé–Sassa–Bersi area, along the BNMB shear zone. Although there is no available age in the Banyo area, according to the tectonics of this area, the correlations with the tectonics of the NCAB can be addressed. Indeed, ref. [5] identified three deformation events in the NCAB with the D₂ deformation event, dated between 613 and 585 Ma [11,51], considered to have been developed in the context of a sinistral sense of shear movement and marked by synthetic shear zones such as the MNSZ (which partly cross-cuts the study area in the northern part), the GGSZ in the north of the study area and the major BNMB shear zone that borders the western domain of the NCAB in Cameroon. Many other authors [18,22,31,33,35,91] have characterized the D₂ regional tectonics in the western Cameroon domain as N-S striking in the sinistral sense of shear movement. Thus, it could be envisaged that the D₂ regional tectonics of the NCAB was recorded in the Banyo area where the N-S MNSZ was responsible of its development. The tectonics of the high-strain zone localized in the country rocks could be due to the emplacement of granite and also be linked to the activation of the major BNMB shear zone during its functioning, like other E-W sinistral shear zones described in the western domain (see [41,92]).

The D₃ deformation is characterized by the following: (i) the presence of S₃ metamorphic foliation which strikes roughly E-W; (ii) the folding of granitic veins, which previously showed asymmetric boudins with sinistral movement, in a “Z”-shape (Figure 4j) with vergence towards ENE; and (iii) C₃ shear planes in the E-W direction (Figure 4o). This D₃ deformation phase, occurring under dextral sense of shear movement, suggests simple shear as dominant mechanism of deformation. In the NCAB in Cameroon, the D₃ phase, striking ENE-WSW to E-W, is in dextral sense of shear or right lateral movement [5,10,33]. It is dated at ca. 585–540 Ma [52].

Based on the structural analysis of the Banyo area, it seems like this D₃ phase can be correlated to the one developed and characterized by previous authors in the NCAB [5,10,41,53]. Thus, the previous description maintains, despite the fact that the age of the rocks of the study area is not available, that the Banyo area was deformed during the tectonics of the NCAB in Cameroon. The pluton emplaced between 613 and 580 Ma, the age of the D₂ tectonics of the NCAB, during a syn-D₂ sinistral sense of shear movement under a transpressive regime where simple shear was dominant.

5.4. NE Banyo Granitic Pluton Emplacement Mechanism

The N-S-oriented granitic pluton located to the NE of Banyo displays magnetic fabrics that strike NW-SE to N-S. These magnetic fabrics are sub-parallel to the D₂ deformation of the area, also recorded in the country rocks. Microstructural analysis of the pluton allowed us to identify magmatic, sub-magmatic, solid-state and mylonitic deformation markers in the rocks. The magnetic carrier and the P_j values (˂1.20) indicate that the magnetic fabric in the pluton is due to the magmatic fabric or flow fabric. Furthermore, the T_j vs. P_j diagram indicates the dominant oblate shape of the AMS ellipsoid. The emplacement strain of the pluton is then mostly due to the flattening component. Thus, the emplacement mechanism of the NE Banyo granite that should be explained must take into account all above descriptions that integrate field fabrics, magnetic fabrics and microstructures. Moreover, going through the pluton, we observe mostly the NW-SE direction of the magnetic fabrics with moderate to steep dips magnetic foliation and low-plunge magnetic lineation parallel to the D₂ fold axis that shows a best-fit line at 117°/17°.

The solid-state and mylonitic markers are well organized within the pluton and along the shear zone with sinistral N-S strike in the northwestern part of the pluton, respectively, indicating that the deformation was not stopped after the complete crystallization of the granitic melt. Taking into consideration the (i) field foliation with low to moderate dips in the country rocks and in the pluton, associated with low-plunge, stretching mineral lineation, and (ii) magnetic foliation with mostly moderate to steep dips in the pluton, associated with very-low-plunge magnetic lineation, we conclude, in accordance with [93], that it is demonstrated that the flattening and simple shear components of the deformation played important roles during the emplacement of the granitic pluton.

The organization of the magnetic foliation and lineation trends (Figure 16) in the whole pluton suggests a nearly NW-SE shearing in the rock strips in the sinistral sense of movement. In the southwestern part of the pluton, the concentration of steep-dip (73–86°) magnetic foliation allows us to suggest that the feeding conduit was not so far from there. The NW-SE-oriented direction of the magnetic lineation and foliation trends that regularly show moderate to steep dips and mostly low to very low plunges is evocative of the slip zone of the rock strips, while the NE-SW-oriented direction of the fabrics indicates the flow direction of the melt. This shows that, when deforming, the magma flowed parallel to the direction of the sub-horizontal magnetic lineation. As the direction of the magnetic foliation is parallel to that of the sub-horizontal magnetic lineation, we can conclude that parallelism between the fold axes and the direction of the stretching mineral lineation points to a low angle of active shear component deformation [94]. We then imagine that the direction of the mean shear stress component that induced the deformation of the pluton is thought to have been in the NE-SW to WNW-ENE direction (Figure 16).

6. Conclusions

The present study highlights the use of field data with the aim of characterizing the tectonic evolution of the studied area and linking it to the regional tectonics of the NCAB in Cameroon and the tectonic emplacement of the granitic pluton. The main conclusions are the following:

a—The NE Banyo area is made of metamorphic rocks intruded by N-S granitic pluton. Fine-grained biotite granite is observed as dykes/veins.

b—The area records three deformation phases. The D₁ deformation phase consists of the NW-SE shortening direction that developed sub-horizontal S₁ flat-laying foliation by a flattening mechanism. The D₂ deformation phase with a sinistral, ductile sense of shear movement developed N-S to NW-SE fabrics. The D₃ deformation phase with a dextral, ductile sense of shear movement suggests simple shear as the dominant mechanism of deformation.

c—The NE Banyo granitic pluton emplaced during the syn- to tardi-D₂ sinistral deformation of the regional tectonics of the NCAB in Cameroon under a transpressive regime. This deformation was coeval to the functioning of the N-S Mayo Nolti shear zone.