The West African Craton is essentially made up of the Reguibat Ridge to the north and the Man/Leo Ridge to the south [1] [2] . The Man Ridge comprises an Archean (3.00 - 2.50 Ga) or Kenema-Man domain corresponding to the core of the ridge and a Palaeoproterozoic or Baoulé-Mossi domain, representing the juvenile part [3] - [6] ( Figure 1 ). The Baoulé-Mossi domain is composed of granites, suites of TTG (Tonalite-Trondjhemite-Granodiorite) and belts of Birimian greenstone [7] [8] .

The Birimian greenstone belts comprise elongated sequences of metavolcanic and metavolcanosedimentary rocks that are Birimian trenches. These are essentially metabasalts, metaandesites and volcanic sediments metamorphosed in the Eburnian [9] - [12] . The volcanic activity that gave rise to these rocks occurred around 2.40 Ga and lasted locally until 2.10 Ga [13] - [15] .

Numerous authors have demonstrated that lithostratigraphic succession first favours the emplacement of suites of TTGs commonly known as first-generation granitoids [16] - [18] . These TTGs result from intense plutonic activity before magmatism progressively evolves towards more differentiated terms (potassic granites and alkaline syenites) or second-generation granitoids [19] - [21] . After this magmatic accretion of juvenile crust, the Eburnian orogenic cycle led to the tectonic assembly of the Archean and the various Palaeoproterozoic domains between 2.15 and 2.08 Ga [14] [22] - [25] .

This orogenic cycle coincides with the establishment of vast granitoid domains. The most undifferentiated terms or first generation granitoids or TTGs are the oldest granitoids and are known as rocks that are most often structured by field observation. However, second-generation granitoids are generally poorly structured and appear to be isotropic when observed in the field. By examining microstructures, it is possible to characterise the rheological state of the material during or after its emplacement, and the Magnetic Susceptibility Anisotropy (MSA) approach will reveal structures that would not have been apparent during field observation.

The aim of this study is to contribute, through petrographic, microstructural and structural studies, to a better understanding of the geological processes that led to the emplacement of the granitoids in the central region.

Geological reconnaissance work has been carried out in the central part of Burkina Faso, and an extensive mapping campaign covered the area in 2003 [17] [20] .

Like the other regions of the Baoulé-Mossi domain, and on the basis of the work already carried out, the study area comprises greenstone belts within which metavolcanic rocks (metabasalts, metaandesites, etc.) and, to a lesser extent, metaplutonic rocks (metagabbros, metadiorites, etc.) and metasedimentary rocks (schists, metapelites, etc.) are found. This schistose series also contains intercalations of intermediate to acid volcanics, including a rhyolitic tuff dated at 2238 ± 5 Ma, corresponding to the oldest age obtained to date in Burkina Faso and indirectly confirming that amphibolitised basalts are the oldest rocks in Burkina Faso [20] .

All these formations are affected by greenschist to amphibolite facies metamorphism, with hydrothermal circulation superimposed in places.

Within the Goren arc, these formations develop a schistosity (S1) that is the result of regional NW-SE shortening. This S1 is interpreted as being the result of the first deformation phase of the Eburnian orogeny and corresponds broadly to phase D1 [17] [20] .

The greenstone belt formations corresponding to the Goren belt are cut by two generations of granitoids. Granitoids with a geochemical signature close to that of the Archean TTGs (Tonalites, Trondhjemites and Granodiorites) [17] [21] [26] [27] or Andean adakites [2] constitute the first generation of granitoids that cut the greenstone belts, especially in the Ouagadougou area.

The emplacement of these granitoids produced a halo of contact metamorphism in the surrounding formations [28] [29] . The first-generation granitoids and greenstone belts are intersected by a second generation of granitoids. The TTG granitoids are generally more or less clearly banded at field scale, whereas the second-generation granitoids are generally poorly structured. In terms of mineralogical composition, TTG granitoids generally contain both biotite and amphibole as ferromagnesian minerals, whereas second-generation granitoids generally have biotite as the only ferromagnesian mineral.

In this central part of Burkina Faso, which belongs to the Ouagadougou sheet [17] , it is these granitoids that are the focus of this article ( Figure 2 ).

For this study, the methodology consisted of documentation, fieldwork, laboratory work and interpretation of satellite images and data.

The literature review provided an overview of previous and ongoing work in the study area.

For the field work, sampling was carried out using the ASM approach, where it is common practice to sample regularly and evenly across the pluton [26] [29] - [31] .

Laboratory work consisted of making thin sections for detailed petrographic descriptions and for examining microstructures.

The N-S to NE-SE alignment due to the E-W to NW-SE regional shortening of the Birimian trenches is known in the West African Palaeoproterozoic terrains of the Léo Dorsal [4] [13] [19] [32] - [34] and elsewhere in Guinea [35] , Morocco [36] [37] , Senegal [38] [39] , Ghana [4] [5] , Niger [40] and Burkina Faso [16] [21] [41] [42] . These same forms of green belt are found in the study area and correspond to the oldest formations in this central part of Burkina. They define the Goren arc [43] and border the northern and north-western parts of the study area [20] [44] .

The shaping of these Birimian furrows is the result of a regional E-W to NW-SE shortening corresponding to deformation phase D1. It is ductile and has developed S1 schistosity when visible in the field or not, as in the case of our study area [45] .

This phase of deformation was partially or totally delayed, as was the case in our study area, by the second phase of deformation, D2, which is ductile and brittle and played in a sinister direction. This sinister play of D2 is consistent with the plays of the major shear zones that exist within in the Palaeoproterozoic domain, such as the Tiébélé-Dori-Markoye fault [42] [43] [46] , the Sassandra fault in Côte d’Ivoire [13] and the Senegal-Malian fault [38] .

In the case of our study area, this D2 phase affected first-generation granitoids whose deformation markers are visible in the field, such as fractures, cracks and foliations. This played a fundamental role in the redistribution of magmatic-state microstructures to high-temperature or low-temperature solid-state microstructures, sometimes with incipient orthogneissification. By deduction, these microstructures were acquired after the massif had cooled completely. This reflects the rheological state of the material, which could be characterised by examining the microstructures [47] . This same approach has been used to characterise granitoids in NE Benin [42] [48] and Burkina Faso [29] .

Subsequently, the D3, which is dextral, took over from the D2 in places. It is brittle and of low intensity in the sense that, despite its effects, the Ziniaré granite pluton remains undeformed with microstructures of a dominant magmatic state within the study area. The transition from D3 is nevertheless marked by an alignment of low-temperature solid-state microstructures.

In the northern part of the study area, four deformation phases have been identified, of which D1-x, described by Tangaen, is ante-Birimian and phases D1, D2 and D3 are Birimian [42] . In addition, in the southern part of the study area, near the Kiaka gold deposit, four deformation phases have been identified, of which D4 is very localised and the other phases are similar to those in our study area [46] .

The three phases of deformation identified and proposed in this central part of Burkina Faso are consistent with the evolution of deformation in other regions studied by certain authors, with which a fourth phase, whether local or not, is associated [6] [35] [40] [44] .

The methodological approach adopted has enabled us to distinguish two major lithological groups in the study area: volcano-sedimentary rocks and granitoids. The former are highly altered and are interpreted as volcano-sedimentary schists, the relics of which are the elongated structures of the birimian furrows. The latter are inherited from the D1 deformation phase, characterised by ductile behaviour. The granitoids can be divided into two generations. The first is represented by TTG-type granitoids (Tonalite-Trondhjemite-Granodiorite), while the second includes later intrusions such as PGZiniaré. The microstructural study revealed the presence of four types of microstructure, ranging from magmatic state structures to incipient orthogneissification structures, via submagmatic states and structures acquired in the solid state, either at high or low temperature.

Microstructural analysis shows that the PGZ is dominated by magmatic-state microstructures, suggesting complete crystallisation of the magma without major subsequent structural alteration. In contrast, the Barkoundba granitic pluton is dominated by incipient orthogneissification microstructures, indicating significant deformation associated with the D2 phase. This ductile to brittle phase contributed to the gradual transformation of the magmatic texture towards a gneissic texture.

Interpretation of the structures, based on a relative chronology, has enabled three major successive deformation phases to be reconstructed: an initial ductile D1 phase, a marked sinister D2 phase, and a late dextral D3 phase, of weaker intensity, but locally identifiable by the alignment of low-temperature solid-state microstructures.

This work is a contribution to the characterisation of granitoids from the Ziniaré region, which is located in central Burkina Faso.

We would like to thank the head of the Geosciences and Environment Laboratory (LaGE) for making the slides and observing them under the microscope, and the Burkina Faso Bureau of Mines and Geology for drilling the samples.