The host rock consists mainly of quartz sandstone and metabasalt. The quartz microdiorite is leucocratic, micrograined, and composed mainly of amphibole, biotite, plagioclase, and quartz (Figure 4(a)). The metabasalts are greenish, microlithic porphyry, containing plagioclase phenocrysts (2 mm - 4 mm) in an almost entirely chloritized matrix of amphibole and biotite (Figure 4(b)). The sandstones (Figure 4(c)) are composed mainly of quartz with a blunt edge, evidence of a slightly significant transport (Figure 4(d)), and represent a minor component.

Figure 4. The main facies of the surrounding Deou granite (Amp: amphibole; Bio: biotite; Pl: plagioclase; Qz: quartz; Qz-Mcb: quartz-micas blancs; Op: opaque.)

4.2.1. Petrographic Characteristics of the Deou Pluton

The granite outcrops in the form of hills and is rarely flush with the ground. The rock is leucocratic with a pink heterogranular background (0.5 mm - 4 mm), but microscopic examination distinguishes a non-granophyric facies (Figure 5(a)-(c)) and a granophyric facies (Figure 5(b), Figure 5(d)), to which a rosette facies is added (Figure 5(e)). However, apart from the proportion of minerals, the mineralogical assemblage is the same for all facies: amphibole-biotite-potassium and quartz feldspars.

Figure 5. Main facies of the Deou granite seen under microscopy; same legend as the previous figure; (a) Non-granophyric coarse facies; (b) Granophyric coarse facies; (c) Non-granophyric fine facies; (d) Granophyric fine facies; (e) Rosette facies.

4.2.2. Geochemical Characteristics of the Deou Pluton

The Deou granite is classified as an alkaline feldspar granite in the Q-A-P [27] normative diagram (Figure 6). The alkaline nature of the feldspars is confirmed by geochemical data (Figure 7).

Figure 6. Diagram Q-A-P [27] showing representative facies of the Deou granite; MG: Monzogranite; GD: Granodiorites; TO: Tonalites; MDQ: Monzodiorite quartzique; DQ: Diorite quartzique; D: Diorite; MD: Monzodiorite; Sfa: Syenite à feldspath alcalin; Sqfa: Syenite quartzique à feldspath alcalin; SQ: Syenite quartzique; MQ: Monzonite quartzique; Gfa: granite à feldspath alcalin; SG: Syéno-granite; QZ: Quartzolite.

Figure 7. Composition of plagioclases in the coarse non-granophyric facies of Deou (De 72 = solid squares; De 98 = empty squares) and fine-grained granite (solid triangles = De 60) in the Or-Ab-An diagram. An = anorthite; By = bytownite; La = labrador; And = andesine; Ol = oligoclase; Ab = albite; Anor = anorthose; Sa = sanidine; Or = orthose.

It is peraluminous (A/CNK between 1.29 and 1.39) (Figure 8), siliceous (74 < SiO₂ < 77), sodi-potassic (7 < Na₂O + K₂O < 9) with moderate CaO (0.51 - 1.13), Fe2O3 (1.7 - 3.67), and MgO (0.02 - 0.16) contents (Table 1). Zr, Nb, Y, Ga, Ce, and Ta contents are high, with Zr + Nb + Ce + Y > 400 ppm, typical of alkaline granites, while the Eu anomaly (Eu/Eu* < 0.7) remains negative, reflecting the depletion of residual plagioclase or alkaline feldspar. In the diagrams [28], it unambiguously occupies the domain of type A granites (Figure 9).

Figure 8. Chemical discrimination in the A/CNK diagram [Al₂O₃/(CaO + Na₂O + K₂O)] as a function of SiO₂ in the Deou granite.

Table 1. Analytical and normative data on the total rocks of the Deou granite.

Figure 9. Discrimination diagram between I, S, M-type and A-type, in which the Deou granite is plotted [28].

Microscopically, almost all the microstructures of the Deou pluton are acquired in the magmatic state, marked by ± lobate quartz, free from any trace of recrystallisation, and automorphic feldspars (Figure 10(a)~(b)). This once again confirms the late-tectonic nature of the Deou pluton. Only a few sites show a low-temperature solid-state microstructure, marked by quartz banding with sub-grain recrystallisation at the edges of the large crystals, giving rise to a “core and mantle” type structure (Figure 10(c)), which is the expression of weak deformation that sometimes occurs at the end of crystallisation in any plutonic massif.

Figure 10. Microstructures of the Deou alkaline granite; same legend as the previous figure.

4.4.1. Scalar Data for Magnetic Susceptibility and Thermomagnetism

At the scale of the pluton, susceptibility has a random map distribution (Figure 11), as already observed by other authors on granitic plutons in Burkina Faso [17]-[22]. This is due to the presence of magnetite, as can be seen from the various thermomagnetic curves (Figure 12), where there is a sudden drop in temperature at 580˚C, characteristic of this mineral.

Figure 11. Map of the magnetic susceptibility (km in µSI) of the Deou alkaline granite and its surrounding area.

Figure 12. Thermomagnetic curves for several sampling sites in the Deou alkaline granite.

4.4.2. Directional Magnetic Susceptibility Data and Structural Analysis

1.Magnetic Foliation

The general shape of the foliation is more or less concentric, although some sites show foliations that are completely secant or even almost orthogonal to this general layout (Figure 13). The foliation of the host rock is completely secant to that of the pluton. The internal foliations of the pluton, which are discordant with the others, probably correspond to the planar fabrications of the late-magmatic veins. It is at these sites that the textures are granophyric.

Figure 13. Map of foliations and stereograms of density contours of the Deou alkaline granite.

Figure 14. Lineation map and stereogram of the density contours of the Deou alkaline granite.

2. Magnetic Lineation

For lineation, the dip is considered shallow for values ≤ 30˚, moderate for values between 30˚ and 60˚, and steep for dips ≥ 60˚. The azimuths of the lineations roughly follow the directions of the foliations, with low to medium plunges, except for a few sites (12%) where the lineations are sub-vertical (plunge ≥ 60˚) (Figure 14). These zones of high plunge are interpreted as pluton-feeding zones [17][29][30] while the zones of medium plunge reflect the finite extension of the magma at the time of crystallization [17][29][30]. In our case, this direction would be WNW-ESE to W-E, which is one of the directions followed by certain late fractures in the Deou zone.

The sub-circular Deou pluton outcrops in a host rock composed of metavolcanic and metasedimentary rocks and TTG-type granitoids. It is unconformably overlain by Neoproterozoic quartz sandstones to the north. Petrogeochemical analyses (Figure 5) show that this is indeed an alkaline granite. Furthermore, the plagioclases in the various facies consist entirely of albite (Figure 6). Whose granophyric texture argues in favour of this and also indicates emplacement at a relatively high level in the crust [31]-[33]. The formation of this texture is interpreted as the result of eutectic crystallisation of a water-rich silicate magma that underwent rapid cooling [34][35].

The pluton feed zones are somewhat scattered at this scale, but the most extensive is in the north. The low plunge values, indicating the direction of magma flow, give a WNW-ESE to W-E direction.

Studies conducted on the Dori granitoids in north-eastern Burkina [22] and at Bouabou in south-eastern Burkina [36] show that the orientation of their foliation is concentric, indicating diapiric emplacement. However, their elliptical shape, the concordance of their foliation with their host rock, and the presence of solid-state deformation microstructures in their vicinity suggest that this diapirism is driven by regional tectonics.

In the Déou area, the structure (foliation and lineation) inferred from measurements of magnetic susceptibility anisotropy is predominantly magmatic and is therefore linked to the mechanisms that prevailed during the emplacement of the Deou granite pluton. As the foliation is concentric, this is characteristic of diapiric bodies formed in anorogenic or orogenic settings [7][37]. In the specific case of the Deou pluton, the context would appear to be anorogenic since the few structures measured in the host rock are discordant with that of the pluton (Figure 13). Furthermore, the fact that it belongs to the A-type granite group indicates that the Deou granite is either anorogenic or was emplaced in a context of extension [38]. This post-orogenic character is also confirmed by the position of the Deou granite in the geotectonic classification diagram [39]. In this diagram, the Deou granite is positioned in the compositional range of post-orogenic to anorogenic granites (Figure 15).

Note: A: anorogenic alkaline granite; PA: post-orogenic alkaline granite.

Figure 15. Geochemical discrimination diagram of the Deou alkaline granite [39].

Diagrams [40][41] (Figure 16 and Figure 17) show that all the samples are in the field of intra-plate granites. This confirms their position at the end of the orogeny.

Figure 16. Geotectonic characterization diagram [40].

Figure 17.Geotectonic characterization diagram [41].

Based on these data, we can propose the following model for the emplacement of the Deou granite (Figure 18).

Note: 1: Lithospheric mantle; 2: Volcanic metalaves; 3: Deou granite.

Figure 18. Depositional model of the Deou alkaline granite.