Rocks within deformation areas are generally slightly, moderately, or highly strained and may exhibit very nice deformation microstructures. Deformed rocks present matrix with reduced grain size surrounding elongated faulted or sheared grains or grain aggregates. Many questions are steel a debate nowadays. Among them, we have the question of the identification of the presence or the absence of melt during deformation in rocks. Those questions are important for the problem of the melt extraction, the interpretation of plutonic magma ascent, and the mechanisms of emplacement. To have a good understanding of those questions, it is important to use macroscopic, mesoscopic, and microscopic structural criteria.

Coming to the present study, field relationships, mesoscopic and microscopic observations, and the structural data testify that Mgbanji granitoids were emplaced during the brittle-ductile deformation events occurring under the influence of an active strike-slip shear zone.

Conjugated mechanism may have acted simultaneously during the studied granitoid emplacement. They are cataclasis (microcracks, microfaults), diffusive mass transfer by solution (interpenetrating grain, pressure shadow), intracrystalline plasticity (deformation twin, odulatory extension, kink band, subgrain, grain shape fabric and ribbon, new grain), and solid-state diffuse mass transfer transformation (reaction rim, corona, relic mineral). All these mechanisms can be regrouped into magmatic, nonmagmatic (solid state), or submagmatic (few melt) deformation and can help to qualify the shear zone as magmatic, submagmatic, or nonmagmatic.

It is important to mention that nonmagmatic microstructures are those formed during deformation without any melt present. Such a deformation has previously been called subsolidus or solid-state deformation. Submagmatic deformation is characterized by the presence of few melt during deformation, while the magmatic deformation refers to the total presence of melt.

On Mgbanji granitoids, magmatic microstructures are only seen on granite. They are coarse-grained texture containing euhedral feldspar, amphibole, biotite, and sphene minerals (Figure 4(a)) and the presence of magmatic myrmekitic textures [21, 22]. In contrast to granites, granite mylonites (protomylonites, mylonites, and ultramylonites) present nonmagmatic to submagmatic deformation microstructures.

The nonmagmatic structures on Mgbanji granitoids are represented by plagioclase deformation lamellae, pressure shadows filled by epidote around megacrysts, and ribbon grains formed by intracrystalline plasticity in quartz, embayed, or overgrown grain boundaries [23, 24]. Granite mylonites contain new recrystallized quartz grains with higher frequencies of contacts between like grains (Figure 7(b)). This feature has been also revealed in [25] and is justified by the fact that new grains preferentially nucleate on the same phase.

Photomicrographs presenting microstructures ((a), (b), (c), (d), and (f) = cross-polarized light; (e) = plane polarized light). (a) Megacryst of feldspar (orthoclase) from granite mylonites presenting microcracks filled with biotite and quartz. The biotite and quartz are the same phases observed in the groundmass. Also note the core-mantle structure on plagioclase. (b) Typical feature of dynamic recrystallization in quartz involving grain boundary migration. Note the sutured margins on quartz ribbons and also domains of small strain free polygonal grains. Also note the K-feldspar presenting a leave like shape with unfilled microcracks (c) perthite exsolution and stream quartz molding K-feldspar porphyroblasts. Note the K-feldspar microcryst presenting microcrack filled with pasty quartz. (d) Tectonically induced myrmekites characterized by their grow in volumes perpendicular to the shortening direction. Also note exsolution lamellae on plagioclase. (e) Reaction rims of altered minerals around K-feldspar megacryst. This texture testifies to the transformation by diffusive mass transfer (DMT). Also note the biotite embayment and (f) calcite microveins filled with epidote and quartz.

The submagmatic structures on studied granite mylonites are highlighted by the tectonically induced myrmekites (Figure 7(d)) formed in volumes perpendicular to the shortening direction [21, 22].

The Mgbanji granite mylonites present elongated ribbons of quartz grains showing undulatory extinction. Quartz presents sutured margins on ribbons and some areas of small strain free polygonal grains. The new quartz or feldspar grains are in places concentrated in a mantle that partly surrounds the older mega feldspar grains, giving the core-and-mantle structure to the mineral (Figure 7(a)). These features are characteristic of syntectonic emplacement of the granite mylonites. Reaction rims of altered mineral around feldspar grains are present on granite mylonite. This texture may surely testify to DMT (Figure 7(e)).

The high frequency of undulose extinction in quartz within granites mylonites suggests that intracrystalline plasticity occurs during submagmatic deformation (Figure 7(b)). This assumption is mentioned here because quartz subgrain formation or recrystallization has been taken as an indicator of submagmatic deformation in rock where an overall igneous texture is preserved without any nonmagmatic deformation [26]. Plagioclase and calcite microcracks present biotite, quartz, and sphene or epidote trapped within the fillings (Figure 7(a),(c), and (f)). In some places, microcracks are intragranular and filled with pasty quartz suggesting that the plagioclase crystal was in contact with melt (Figure 7(c)). The microcracks filling is petrographycally continuous with groundmass phase of the rock.

Other submagmatic microstructures are the deformation twins and bent twins (kinking) in granite mylonites plagioclases. Such deformation features are known to be formed by intracrystalline plasticity in submagmatic phase (Figures 6(c) and 7(d)).

Paradoxically, some authors demonstrated that the cataclasis of the crystal is not only occurring exclusively during the non-magmatic deformation but can also occur during sub-magmatic events [22, 27, 28]. Also, it has been proved that S-C fabrics may form either in submagmatic or nonmagmatic deformation [29].

On the field scale, the melt was localized in pressure shadows at the ends of boudins, and also faults and joints on Mgbanji granitoids were filled [30]. This observation coupled to the microscopic observation of some kinematic indicators (“σ” and “δ” porphyroclasts) indicate that the rock was syntectonically emplaced [31]. The presence of quartz replacing plagioclase in some granite mylonite samples suggests the possible incipient metasomatism by fluids derived from evolved crustal rocks (Figure 4(b)) [32].

Considering all abovementioned features, the granitoids of the Mgbanji area seem to be emplaced during plastic- to solid-state deformation stage.

It has been also demonstrated that intracrystalline plasticity and solid-state DMT are strongly controlled by temperature but relatively insensitive to pressure. That is why cataclasis is generally attributed to the upper crust (because of the low pressures at that level). Following that rule, crystal plasticity is supposed to occur in the lower crust or in the lithospheric mantle due to the high temperatures in deep earth areas.

The brittle-plastic transition (cataclasis—plasticity) results to intermediate deformation regime of semi-brittle behavior characterized by microfractures interaction with intracrystalline plasticity [33, 34]. Two transitions in deformation mechanism, namely the brittle-semi-brittle transition and semi-brittle-plastic transition are registered with increasing depth respectively in the upper crust and in the middle crust levels. But, even though it has been proved that high stresses may also cause cataclasis at greater depths or higher temperatures, the mechanism we suggest for the Mgbanji magma emplacement operates on three different crustal levels, namely lower, middle, and upper crustal levels (Figure 8).

A schematic model for the emplacement of the Mgbanji granitoids.

In the lower crustal level, the granitic magma may have originated from the in situ crustal melting [35]. That level favored the formation of coarse-grained granite in the study area.

The middle level is characterized by the presence of mylonites and protomylonites, presenting submagmatic and mylonitic structures, typical of syntectonic magmatism. The presence of subvertical lineation on mylonites and protomylonites may indicate that the middle level zone reacted as feeder zones acting as melt collectors (Figure 8) [36].

The presence of both submagmatic and solid-state deformation features demonstrates that the strike-slip event may have been the active phenomenon during and after the total crystallization of the magma.

The magmatic to mylonitic fabric of the Mgbanji granitoids are in general parallel to the solid-state deformation features encountered in the country rocks, demonstrating that the Mgbanji granites seems coeval with the transcurrent tectonics in the Cameroon Central Shear Zone [20].

The upper crustal level or subsurface display spots of fine-grained granites (with homogeneous appearance in the field and few or no solid-state deformation features) surrounded progressively by ultramylonites, mylonites, and protomylonites (Figure 8). The fine-grained granite may be the outcome of tectonically driven solid-liquid partitioning, operating as filter-press, in the middle crust. At upper crustal levels, real magmatic fabrics are not common because of the higher temperature contrasts with the surrounding rocks, facilitating rapid magma cooling.

Magma intrusion may have been forced to flow toward local dilatational, low pressure sites, such as the boudin necks [37].

Field and microscopic observations of kinematic indicator point to the emplacement of the Mgbanji granitoids during the D3 sinistral deformation within an active strike-slip shear zone.