Petrochemistry and petrogenesis of the Precambrian Basement Complex rocks around Akungba-Akoko, southwestern Nigeria

The petrology of Akungba-Akoko area comprises mainly Migmatite–Gneiss Complex rocks intruded by Pan-African granitoids (Figure 2). The Migmatite–Gneiss Complex rocks occurring in the area are migmatite, granite gneiss and biotite gneiss. Granite gneiss is the predominant rock type covering more than 85% of the area. The granite gneiss is intruded by biotite granite, which trends NE–SW from the central to the northeastern end of the area. In the north-central part of the area, a core of charnockite and an enclave (large xenolith) of biotite gneiss occur within the granite gneiss. This lithologic association suggests that the charnockite is intrusive into the granite gneiss; hence, it is younger than the latter and the biotite gneiss is probably older than the granite gneiss. Field photographs and modal compositions of the various lithologies are shown in Figure 3 and Table 1, respectively.

Figure 2

Figure 3

The granite gneiss of Akungba-Akoko area (Figure 3a) is weakly to moderately foliated, light grey and medium to coarse grained with a megacrystic (blastoporphyritic to porphyroblastic) fabric. It is composed of alternating bands of light- and dark-coloured minerals, most of which are folded. The light-coloured bands are quartz and feldspar rich, while the dark-coloured bands are rich in biotite, hornblende and other ferromagnesian minerals. Quartz and feldspar are the porphyroblasts in the gneiss, and they often have an augen (eye-like) shape. The granite gneiss, in some locations, contains garnets and quartzitic inclusions. The rock mainly trends WNW-ESE to ENE-WSW with moderate to steep dips to the south. Pegmatites, vein quartz, quartz lenses, basic dykes and sills are abundant in the granite gneiss, sometimes giving the gneiss a migmatitic appearance.

Biotite gneiss occurs in the northern part of the area forming mountains/ridges, which extends northwards to Ikare-Akoko area and beyond. [5, 13] referred to this rock as grey gneiss. The rock is composed essentially of quartz, feldspar and biotite (Table 1) and is dark grey, medium grained and strongly foliated with thin alternating bands of light- and dark-coloured minerals (Figure 3b). A ridge of migmatite composed of granite gneiss, biotite gneiss, granite, aplite, pegmatite and vein quartz extends from the southwestern part of Akungba to Supare-Akoko area in the ENE–WSW direction. The rock is highly deformed with the component rocks being intricately mixed and folded. Ptygmatic folds, faults and joints trending in different directions are abundant in this rock.

The biotite granite intruding granite gneiss in Akungba-Akoko is very rich in biotite and hornblende giving it a very dark colour. The granite is medium to coarse grained; rich in quartz, feldspar, biotite and hornblende (Table 1) and contains massive melanocratic segregations of biotite, hornblende and iron (Figure 3c). The coarse-grained, dark-coloured charnockite (Figure 3d) with a shining appearance occurring in the granite gneiss is pyroxene-bearing (Table 1) and rich in quartz, feldspar, biotite and muscovite. The charnockite occurs as boulders, some of which are weathered. Minor rocks occurring in the area include pegmatite, vein quartz, aplite, and basic dykes and sills are often found as intrusives within the gneisses and migmatite. The QAP diagrams of [14, 15] revealed that the granite gneiss, biotite gneiss and biotite granite are granitic in composition (Figure 4a) and the charnockite is a true charnockite (Figure 4b).

Figure 4

Petrochemical data of the basement rocks around Akungba-Akoko (Tables 2 and 3) revealed that the rocks have moderate to high SiO₂ contents: granite gneiss (73.86–74.58 wt.%), biotite gneiss (67.80–73.62 wt.%), biotite granite (59.83–60.06 wt.%) and charnockite (57.50–59.70 wt.%). Based on silica contents, the gneisses are classified as acidic rocks, while the biotite granite and charnockite are classified as intermediate rocks. Al₂O₃ in the rocks range from 12.60 to 17.90 wt.%. The moderate to high silica and alumina contents of these rocks are correlatable with their high quartz and feldspar contents (Table 1). Fe₂O₃ and MgO are relatively higher in the charnockite (Fe₂O₃: 6.68–6.73 wt.%; MgO: 2.41–3.11 wt.%), biotite granite (Fe₂O₃: 6.66–6.68 wt.%; MgO: 2.92–3.02 wt.%) and biotite gneiss (Fe₂O₃: 3.58–5.33 wt.%; MgO: 0.76–1.00 wt.%) compared to the granite gneiss (Fe₂O₃: 3.26–3.83 wt.%; MgO: 0.21–0.32 wt.%). This is as a result of the higher amount of mafic minerals (biotite + hornblende + pyroxene + opaques) in the charnockite, biotite granite and biotite gneiss relative to the granite gneiss. CaO is relatively higher than Na₂O and K₂O in almost all the rocks. All the rocks are also characterized by low to moderate Ba and low Cr, Nb and V contents pointing to a felsic to intermediate composition. The average K/Rb ratios of the rocks (granite gneiss: 228.59, biotite gneiss: 231.70, biotite granite: 178.35 and charnockite: 193.22) are less than that of crustal rocks (283) [16], therefore suggesting crustal sources of the rocks.

Geochemical discrimination of the rocks in the area using the Na₂O + K₂O versus SiO₂ plot [17] revealed that they are sub-alkaline rocks (Figure 5a). On the K₂O versus SiO₂ plot [18], the biotite and granite gneisses are essentially medium-K calc-alkaline rocks, while the biotite granite and charnockite are high-K calc-alkaline rocks (Figure 5b). All the rock samples except one charnockite sample are plotted within the peraluminous field of the Al₂O₃/(Na₂O + K₂O) versus Al₂O₃/(CaO + Na₂O + K₂O) molecular plot of [19] (Figure 5c). The Al₂O₃/(CaO + Na₂O + K₂O) versus SiO₂ plot [20] further distinguished the granite gneiss as S-type peraluminous granitoid and the biotite gneiss, biotite granite and charnockite as I-type peraluminous granitoids (Figure 5d). The biotite and granite gneisses are ferroan, while the biotite granite and charnockite are essentially magnesian (Figure 5e). The magnesian nature of the biotite granite and charnockite is due to low FeO_total/(FeO_total + MgO) and SiO₂ relative to the gneisses. All the rocks are calcic as shown by the K₂O + Na₂O - CaO versus SiO₂ plot ([21]) (Figure 5f).

Figure 5

The granite gneiss, biotite gneiss, biotite granite and charnockite of the Akungba-Akoko area plot within the igneous field on the TiO₂ versus SiO₂ discrimination diagram [22] (Figure 6a). This suggests that the granite gneiss and biotite gneiss of the area are orthogneisses and are formed by metamorphism of igneous protoliths, and the biotite granite and charnockite are of igneous origin. The moderate to high silica contents, K/Rb values (<283), sub-alkalinity and I-type peraluminous nature of the biotite granite, charnockite and biotite gneiss indicate that they are I-type granitoids. This suggests that the biotite granite, the charnockite and the igneous protoliths of the biotite gneiss are formed from crustal igneous-sourced melt(s) [21, 23, 24]. The igneous protoliths of the granite gneiss were probably formed from shallow crustal or sedimentary-sourced melt(s) as indicated by its S-type peraluminous character, occasional appearance of garnets and other Al-silicate (especially muscovite) in its mineralogy as well as presence of quartzite inclusions in the rock at some locations.

Figure 6

Tectonic discrimination of Akungba-Akoko rocks revealed that they are all volcanic arc granitoids (Figure 6b) suggestive of rocks formed during a phase of magmatic activity related to collision and subduction [20, 25].