1) Amphibole granite

The amphibole granite forms a chaotic-looking outcrop, elongated hills ENE-WSW in the Department of Mayo-Rey in Touboro. The blocks are decimetric to decametric in size (Figure 4(a)). At the level of the Vina river, the amphibole granite outcrops as a slab and contains enclaves of amphibolite. The rocks have a pinkish color.

Microscopic observations show granular porphyritic texture (Figure 4(b)) and consist of amphibole, orthoclase, microcline, plagioclase, quartz, sphene, apatite and opaque minerals.

Amphibole is a green hornblende that forms elongated sections of 1 to 4 mm long and 0.5 to 1 mm wide. It represents 10% of the rocks. The elongated sections display single Carlsbad twinning. It contains inclusions of opaque minerals and apatite. Orthoclase (about 20%) forms elongated crystals of up to 5 mm long and 2 mm wide. It is regularly perthitic with exsolutions in bands. Orthoclase crystals have myrmekite buds at their edges. Orthoclase is associated with plagioclase. Microcline (about 15%) forms elongated crystals 7.5 mm long and 2.5 mm wide. It displays polysynthetic twinning of albite and pericline which gives it a shimmering grid. It is associated with plagioclase, quartz, orthoclase and opaque minerals. Plagioclase makes up 25% of the volume of the rock. It forms small crystals compared to orthoclase (on average 3 mm long and 1 mm wide). Small crystals of plagioclase are also found included in the orthoclase. Plagioclase is often zoned with 3 to 4 concentric zones. This zoning is materialized by a differential distribution of submicroscopic inclusions in the mineral. Plagioclase shows myrmekite buds at its edges. Quartz (about 20%) forms polycrystalline mineral with rolling extinction. It contains trails of submicroscopic fluid inclusions perpendicular to the elongation of the crystals. The sphene forms subautomorphic crystals of 0.2 mm on average. It is arranged in a chain about 3 mm long at the level of the rock. Accessory minerals (approximately 10%) are represented by sphene, apatite and opaque minerals.

2) Biotite and muscovite granite

The biotite and muscovite granite form hills respectively in the villages of Mbidere to the NW and Touga1 SE of the study area. These hills present a chaotic aspect, with decimetric to decametric blocks. Some of Mbidere blocks contain in places enclaves of amphibole and biotite gneiss. The biotite and muscovite granite also outcrops as a slab in the savannah and also contains enclaves of migmatitic gneiss at Koré (Figure 4(c)). It sometimes outcrops in stream beds and at the foot of certain hills. It has a dark gray weathering patina. Microscopic observation shows a granular porphyroid texture and is mylonitized in given areas. The rock is composed of 6-8mm orthoclase phenocrysts, in a grainy matrix made of biotite, muscovite, plagioclase, microcline, quartz, zircon, apatite and opaque minerals. Biotite (7%) forms dark brown flakes of 1.5 mm x 0.2 mm to 2 mm x 0.4 mm. It often turns into chlorite with cleavages thickened by the concentration of opaque minerals. Its borders are corroded by alkaline feldspars, plagioclase, quartz and muscovite. It is sometimes associated with muscovite (Figure 4(d)). Muscovite (8%) forms elongated flakes 2.5 mm long and 1.3 mm wide on average, associated with other rock minerals. There are two types of muscovites (primary muscovite (Ms1) and secondary muscovite (Ms 2)). Secondary muscovite results from the phenomenon of feldspars alteration. Orthoclase (25%) forms subautomorphic crystals with a cloudy appearance due to submicroscopic inclusions. We also notice a phenomenon of feldspar alteration to muscovite at the fracturing planes of the mineral. The size of the crystals in the grainy matrix of the rock varies from 0.7 mm to 1.5 mm. It has cracked sections in which small quartz crystals crystallize. Orthoclase crystals have myrmekite buds at their edges. It is poorly represented compared to plagioclases. Plagioclase (about 15%) forms subautomorphic to xenomorphic crystals 4.5 mm long on average. Plagioclases are often affected by microfractures which cause twins of the mineral to shift a little. These polysynthetic twins are sometimes blurry in places and end as a bevel in the mineral. The microcline represents 20% of the volume of the rock. It shows a very fine and almost subrectangular shimmering grid. It is sometimes surrounded by small crystals of quartz and plagioclase. Quartz (about 15%) forms automorphic to subautomorphic crystals of variable size from 0.3 to 2 mm. It is also found in small crystals around feldspars. Accessory minerals (approximately 10%) are zircon, apatite, and opaque minerals.

The syenite forms elongated hills WSW-ENE from Bingo village to Baibokum. It outcrops in the form of blocks on the hills (Figure 4(e)). The syenite shows large blocks scattered throughout the landscape from Mbaissaye to Libome (Yoli) passing through Baïbokoum. It sometimes outcrops as a slab around the village of Dagbao. This rock has a gray color with a dark gray weathering patina. The rock has a granular porphyroid texture. It is composed of pyroxene, amphibole,

biotite, orthoclase, more or less quartz, sphene, zircon, apatite and opaque minerals. Pyroxene (approximately 7%) consists of orthopyroxene and clinopyroxene. Orthopyroxene is characterized by its first order polarization tint and the presence of a very fine additional cleavage (010). Clinopyroxene is recognized by its vivid second order polarization color. Both types of pyroxenes have cleavages thickened by concentrations of opaque minerals (Figure 4(f)). Amphibole (about 10%) is a green hornblende varying in size from 1.3 to 3.5 mm. It grows on the edges and along the pyroxene cleavages. Amphibole is always associated with pyroxenes and biotite. Biotite (about 8%) forms brown to dark brown flakes measuring 1.3 mm × 0.5 mm on average. Biotite occurs under two habits. The first consists of flakes associated with quartz and feldspar in the grainy matrix of the rock. These flakes have cleavages thickened by concentrations of opaque minerals. This biotite often contains zircon inclusions. The second habit is represented by light brown biotite associated with amphibole. These biotites are arranged along the cleavages of the amphibole, which are planes of weakness allowing the circulation of fluids. Orthoclase (about 35%) forms elongated slats of 4 mm × 1.5 mm on average. All the crystals are perthitic with strips of exsolutions perpendicular to the mineral’s elongation.

Plagioclase (about 25%) forms crystals smaller than that of orthoclase and occurs only in the grainy bottom of the rock. It presents in places fine and irregular twins. It is sometimes affected by micro cracks which shift the twins of the mineral. Quartz forms subautomorphic crystals associated with other minerals in the rock matrix. Quartz (about 3%) forms small crystals in the interstices between the orthoclase slats. Some quartz patches show a slight rolling extinction. Sphene forms crown around opaque minerals. It is generally associated with green hornblende and biotite. Opaque minerals form patches associated with amphibole. They are also found in inclusion in the latter. Zircon is generally included in biotite. The accessory minerals (about 12%) in this rock are: apatite, sphene and zircon.

The monzonite forms small hills located to the SE of the study area. It outcrops as a slab in the savannah and sometimes at the foot of certain hills (Figure 4(g)). This rock is dark gray in color with a black weathering patina. The rock has a granular porphyroid texture (Figure 4(h)). It is composed of amphibole, biotite, quartz, orthoclase, plagioclase and opaque minerals.

Amphibole (10%) is a green hornblende of 1 to 3 mm in length. It associates with biotite to materialize the schistosity of the rock. The amphibole changes to give biotite. Biotite (10%) forms small light brown flakes of dimension 0.4 mm × 0.2 mm on average. It associates with the amphibole to materialize the schistosity of the rock. Biotite is well crystallized in shadows under pressure from alkali feldspar.

Alkaline feldspar is primarily orthoclase (30% - 35%) which forms subautomorphic crystals. It is often associated with amphibole, plagioclase and opaque minerals. Plagioclase (25% - 30%) forms subautomorphic to xenomorphic crystals 4.5 mm long on average. The mineral of this plagioclase is often shifted by the microfractures which affect cause a small shift of the twins of the rock. These polysynthetic twins are sometimes blurry in places and end as a bevel in the mineral. Opaque minerals (15%) occupy along the cleavages of amphiboles.

Granitoids have variable major element contents: 55.92% to 74.75% SiO₂, 0.09% to 1.16% for TiO₂, 13.36% to 17.9% Al₂O₃, 1.36% to 7.47% Fe₂O₃, 0.01 to 0.09 MnO, 0.98 to 5.2 for CaO, 2.81 to 4.43 of Na₂O, 3.96 to 6.93 for K₂O, 0.03 to 0.64 of P₂O₅. The sum of the alkalis (Na₂O + K₂O) is between 7.73% and 11.52% and the Na₂O/K₂O ratio varies from 0.43% to 1.0%. The diagram of Cox et al. [20], adapted to plutonic rocks by Wilson [21] allows discriminating the granitoids of the Baïbokoum-Touboro-Ngaoundaye region (Figure 5(a)) for granites, syenites and monzonite (syeno-diorite). In the K₂O versus SiO₂ diagram of [22] (Figure 5(b)), the majority of samples fall into the shoshonitic series. Two (02) biotite and muscovite granites are observed in the field of the hyperpotassium calc-alkaline series. Harker diagrams [23] are used to see the evolution of major rock elements in the Baïbokoum-Touboro-Ngaoundaye region (Figure 6). There is a decrease in the content of TiO₂, MgO, CaO, Fe₂O₃, MnO and P₂O₅ with increasing SiO₂ content. This illustrates a more or less clear negative correlation between these oxides and SiO₂. The Na₂O, Al₂O₃ versus SiO₂ diagrams show a dispersion of the representative points of the samples analyzed.

The rare earth spectra normalized to chondrites according to the values of [24] (Figure 7(a)), underline an enrichment in light rare earths element (LREE) compared to heavy rare earths element (HREE). Rare earth spectra of amphibole granite show high fractionation (La/Yb) N = 14.27 to 25.38 and heavy rare earth profile segments almost flat [(Gd/Lu) N = 1.47 to 1.70]. The negative Eu anomaly is very small, Eu/Eu* = 0.80 - 0.91. The earth spectra of the two-mica granite show a slightly variable light rare earth fractionation ((La/Sm) N = 3.39 to 6.56) unlike the heavy earths which show a fractionation to the other. [(Gd/Lu) N = 0.93 - 8.68]. We note the presence of a negative anomaly in europium (Eu/Eu* = 0.26 - 0.52). The syenite samples have parallel profiles, with a high fractionation [(La/Yb) N = 33.89 to 34.85] and an absence of an anomaly in Eu (Eu/Eu* = 0.89

to 0.98). The monzonite (syeno-diorite) shows strong fractionation [(La/Yb) N = 9.23] and the almost horizontal heavy rare earth profile [(Gd/Lu) N = 2.37]. We also note an absence of an anomaly in Eu (Eu/Eu* = 1.00). The phased array spectra of rocks from the Baïbokoum-Touboro-Ngaoundaye region normalized to the primitive mantle (Figure 7(b)) show an enrichment in lithophile elements (LILE) compared to elements with high ionic potential (HSFS). We note for all these rocks the presence of a negative anomaly in Nb-Ta and Ti. The biotite and muscovite granite and amphibole biotite have a negative Ba and Sr anomaly. The monzonite presents positive Ba anomaly. Syenite and monzonite (syeno-diorite) show positive Zr and Hf anomaly.