FUENTE: OEFA Hidrocarburos

The GGGT is intruded by the Mrs Pink granite, and hornblende granodiorite-tonalite, and these are crosscut by gabbro-pyroxenite, aplite-granite, and dolerite dykes. The Mrs Pink granite intrudes the Lilengo gneiss complex. The Mrs Pink granite is intruded by a hornblende bearing granodiorite- tonalite pluton (Tshibubudze et al., 2009). The age of the Lilengo gneiss complex and the Mrs Pink granite was established from geochronological study (Chapter 7) to confirm that the Mrs Pink is younger than the Lilengo gneiss complex.

6.4.1. Mrs Pink granite

The Mrs Pink granite, as the name implies, is dominantly pink in colour reflecting potassic alteration in the chilled margin and throughout the pluton (Figure 6.4a). The Mrs Pink granite is medium to coarse crystalline and composed of millimetre-sized crystals of potassium feldspar (microcline), quartz, plagioclase, biotite and aggregates of hornblende in a quartz-feldspar rich groundmass. The pluton is deuterically altered, and silicified in places, and biotite is chloritized. The Mrs Pink granite has granophyric and myrmekitic intergrowths of quartz and potash feldspar, or plagioclase. It hosts ovoid mafic xenoliths (Figure 6.4b). It is foliated and lineated adjacent to the MSZ (Tshibubudze et al., 2009) and the Sabce Shear Zone (Section 6.5). The aureole of Mrs Pink granite in the Lilengo gneiss complex is dominated by quartz-potash feldspar alteration as disseminated vein sets.

The Mrs Pink granite is crosscut by granodiorite-tonalite, gabbro and aplite-granite dykes, indicating that it is older than these rocks in the GGGT (Figure 6.4c, d). It is also crosscut by northwest and north to north-northeast trending discrete shear-mylonite zones indicating that it is older.

Figure 6.4: (a) The name of Mrs Pink is associated with the pink colour of the granite (UTM 30P, 0819922, 1616778). (b) The Mrs Pink granite outcrop with rounded mafic xenoliths, where Mrs Pink and the xenoliths are crosscut by a weak foliation (UTM 30P, 0785692, 1610204), (c) The contact between Mrs Pink granite and the granodiorite-tonalite (UTM 30P, 0781517, 1617563). The granodiorite-tonalite is phenocrystic in plagioclase and amphibole, while Mrs Pink granite is recrystallised at the contact. (d) A granodiorite-tonalite dyke crosscutting the Mrs Pink granite (UTM 30 P, 0799963, 1585180) indicating that the granodiorite-tonalite is younger than the Mrs Pink granite.

6.4.2. Hornblende-bearing granodiorite-tonalite

The hornblende-bearing granodiorite-tonalite is composed of hornblende phenocrysts in a ground mass of plagioclase and quartz, with variable proportions of potash feldspar. In some places it is tonalitic and is composed of plagioclase, hornblende, and minor quartz, with equigranular crystals. It hosts mafic xenoliths whose long axes predominantly trend northwest (Figure 6.5a, b). The granodiorite-tonalite also host xenoliths of the Lilengo gneiss complex (Figure 6.5c), the Mrs Pink granite (Figure 6.5d), amphibolite (Figure 6.5e) and layered sedimentary rocks (Figure 6.5f). This indicates that the emplacement of the granodiorite-tonalite postdates these rocks. The granodiorite- tonalite is chloritised and kaolinised in places.

Moreover, the granodiorite-tonalite crosscuts east-west trending proto-mylonite and mylonite zones, as discussed in Section 6.5. It is sequentially crosscut by;

(3) North-northeast trending shear-faults (Figure 6.6). (4) Pegmatite veins (Figure 6.6).

(5) Aplite-granite dykes (Figure 6.5g; 6.6).

These relationships are presented in a schematic diagram in Figure 6.6.

6.4.3. Pegmatite veins

The pegmatite veins in the GGGT are composed of 1-2 cm sized quartz and plagioclase crystals, and potash feldspar. Crystal growth is generally syntaxial. Some veins are zoned with coarse crystals central to the vein and fine to medium sized crystals at the margins of the veins. Two phases of pegmatite veining were established; pegmatite veins that are deformed (folded and schistosed) and crosscut by northwest trending shears and faults, and pegmatite veins that are not deformed and composed of quartz and plagioclase crystals.

6.4.4. Gabbro-pyroxenite dykes

The gabbro and pyroxenite dykes in the GGGT are phenocrystic in pyroxene with a ground mass of clino-pyroxene, plagioclase, and hornblende. The phenocrysts of pyroxene are 2 cm in size, giving the pyroxenite a spotted texture (Figure 6.7).

Several pyroxenite dykes are oriented parallel to the north-northeast trending shear zones and these are sometimes dominated by hornblende, biotite, pyroxene, tremolite and plagioclase (thus hornblendite), with well-defined mineral lineation (defined by hornblende). Gabbro dykes are composed of hornblende, plagioclase, quartz, biotite and potash feldspar.

6.4.5. Aplite-granite dykes

Aplite dykes in the GGGT consist of fine equigranular crystals of potash feldspar (microcline) and quartz, and are gradational in composition with granite dykes. The granite dykes are composed of potash feldspar (microcline), quartz, and plagioclase, with accessory muscovite, and biotite. The size of the aplite-granite dykes ranges from 10 cm to ~1m wide (Figure 6.5a, g, 6.6). They are similar in composition to aplite dykes mapped in the Goren and Sabba region by Hein et al. (2004) and Simoko (2011) and across northeast Burkina Faso.

6.4.6. Dolerite dykes

Northwest trending dolerite dykes crosscut all units in the GGGT. They are composed of pyroxene, amphibole, and plagioclase. They are readily identified using the LANDSAT, ASTER and RTP magnetic data of the region. They are abundant to the north of the study area and are dated at 250 ± 13 Ma (K-Ar whole rock method) (Hottin and Ouedraogo, 1992).

Figure 6.5: (a) The granodiorite-tonalite hosts elongated and stretched mafic and angular xenoliths and have potassic alteration in places (UTM 30P, 0803958, 1585126). The granodiorite-tonalite is also intruded by aplite-granite dykes (b). The mafic xenoliths are dominantly stretched and elongated along the orientation of the foliation (UTM 30P, 0797401, 1564022). (c) The Lilengo gneiss xenoliths within the granodiorite-tonalite, indicating that the Lilengo gneiss is older than the granodiorite-tonalite (UTM 30P, 0800724, 1585124). (d) Mrs Pink granite xenoliths in the granodiorite-tonalite (UTM 30P, 0799704, 1608584). The contact is crosscut and displaced by northeast trending faults with a dextral oblique-slip displacement. (e) Amphibolite xenoliths within the granodiorite-tonalite (UTM 30P,

Figure 6.5 (continued): (f) Metasedimentary xenoliths with compositional layering within the granodiorite-tonalite indicating that the metasedimentary sequences are older than the granodiorite- tonalite (UTM 30P, 0809052, 1580334). (g) Fine grained potash feldspar-rich aplite-granite dykes that crosscut both the granodiorite-tonalite and the gneiss xenolith (UTM 30P, 0803958, 1585126).

Figure 6.6: The map of an outcrop showing different phases of magmatic intrusions that dominate the GGGT (UTM 30P, 0801190, 1585594). The granodiorite-tonalite with mafic xenoliths is in contact with the granodiorite. The relationship indicates that granodiorite intruded the granodiorite-tonalite. The granodiorite is crosscut by north-northeast trending, steeply dipping to the southeast shear zones that have a dextral displacement. The granodiorite- tonalite is crosscut by pegmatite veins that do not crosscut the granodiorite. Both the granodiorite with mafic xenoliths and the granodiorite and the pegmatite

Figure 6.7: A photograph of a phenocrystic pyroxenite that have crystals that are greater than 2 cm long in a pyroxene, hornblende, biotite and plagioclase groundmass, giving the pyroxenite a spotted texture (UTM 30P 0800213, 1562317).