Retrogression of Orthopyroxene-bearing Gneiss of Iboropa Akoko, Southwestern Nigeria

Orthopyroxene-bearing gneiss is usually referred to as charnockitic gneiss or granulite, most especially if the orthopyroxene is hypersthene. In this article, the use of the term charnockitic gneiss will be adhered to. Charnockite are hypersthene-bearing rocks [1]. There has been an extensive study on charnockites and charnockitic rocks in India [2,3,4,5]. Charnockites are orthopyroxene-bearing anhydrous granitoids [1, 6], and they are of magmatic or metamorphic origin [7]. Charnockites are restricted to high-grade belt and those that are of metamorphic origin take their source either from igneous or from sedimentary protoliths during high-grade metamorphism under anhydrous conditions [8]. In some parts of south India and Sri Lanka, ‘patchy’ charnockites have been termed as ‘arrested growth’, ‘in situ’ charnockites or charnockitisation of amphibolite facies [9,10,11,13,14,15]. For the patchy charnockites of east Gondwana, metamorphic transformation from amphibolite facies gneiss occurred by two mechanisms: CO₂ ingress from deep level and a drop in the pressure of fluid [11, 16,17,18]. The process of charnockitisation is propagated by the influx of the CO₂–H₂O fluid, which results in the migration of most basic elements from the initial rock towards the transition zones [19]. Touret and Huizenga [20] studied charnockite microstructures from magmatic to metamorphic and realised that microstructures alone cannot always provide solution to the igneous or metamorphic origin of charnockites due to high temperature of recrystallisation. In their study, further evidence for the presence of brine fluids includes the high-temperature fluid–mineral reactions at inter-grain boundaries (K-feldspar microveins and myrmekites) observed in both igneous and metamorphic charnockites. Yang et al. [21] gave the first report of Paleoproterozoic incipient charnockite from the North China Cratons. They documented centimetre- to decimetre-scale anhydrous zones of incipient charnockite within tonalite–trondhjemite–granodiorite (TTG) rocks that are adjacent to an intrusive charnockite. From their study, they concluded that the incipient charnockite formed at ultrahigh-temperature conditions of 890°C–970°C.

The study area for this research is part of the Neoproterozoic basement of Nigeria (Figure 1). Rocks of the Basement Complex of Nigeria are made up of predominantly migmatitic and granitic gneisses; quartzites; slightly migmatised to unmigmatised meta-sedimentary schists and meta-igneous rocks; charnockitic, gabbroic and dioritic rocks and members of the Older Granite suite mainly granites, granodiorites and syenites [22]. The migmatite gneisses of South-Western Nigeria are usually composed of three components, all of which sometimes may be present in a single outcrop [22]. These components are (a) early gneiss, (b) mafic–ultramafic bands and (c) granitic or felsic components. Iboropa Akoko is about 10 km from Ikare (Figure 2), and Ikare area is dominated by migmatite gneiss–quartzite complex, making up 90% of the rock units [23]. The charnockitic gneiss of Iboropa is dark grey in colour (Figure 3), and it is poorly foliated. Other associated rocks in this area are grey gneiss, granitic gneiss, pelitic gneiss and pegmatite. Almost all granulite facies rocks described so far from the Nigerian Basement Complex are of charnockitic affinity. Rocks of the granulite facies are found in all crystalline basement of the world [24]. Hubbard [25] suggested that these granulite facies rocks represent relicts of an earlier and more widespread granulite facies metamorphism of unspecified age, predating the Pan-African Orogeny.

Figure 1

Figure 2

Figure 3

However, Rahaman and Ocan [23] suggested that the charnockitic rocks were original igneous rocks that retained their anhydrous affinity during the Pan-African Orogeny. Most of the other rocks of the basement complex described so far range in metamorphic grades from greenschist to upper amphibolite facies. Metamorphic rocks that have attained higher grades of metamorphism are probably the cordierite–sillimanite gneisses at Bena village in northern Nigeria, which Sacchi [27] ascribed to the granulite facies. On the published 1:250,000 G.S.N. Sheets 61, Akure, some localities within the migmatite gneiss–quarzite complex were indicated by Dempster [28] as containing granulite facies mineral paragenesis. Petrographic evidence presented strongly suggests that the granulite facies mineral paragenesis in Ikare area is a result of prograde metamorphism [29]. In this respect, rocks of the granulite facies mineralogy in Ikare area are different from others of similar mineralogy described so far [23], as original igneous rocks, because of composition, especially low water content, retained their original (igneous) granulite facies mineralogy. Data used for this work are from the PhD thesis of the first author.

Detailed petrography was carried out using Petrographic Microscopes, both at the Department of Geology, Obafemi Awolowo University and Department of Geology, Rhodes University, South Africa. Major elements and trace elements were determined at the Central Analytical Facility (CAF), Stellenbosch University, South Africa, using X-ray fluorescence (XRF) and laser ablation ICP-MS, respectively. The mineral chemistry of the minerals was done using electron microprobe analysis (EPMA) at the Department of Geology, Rhodes University South Africa by a JEOL JXA 8230 Superprobe, using 4 WD spectrometers. Equipment operating conditions employed for the EPMA were 15 kV acceleration voltage, 20 nA probe current, beam size of ~1 μm and counting time 10 s on peak and 5 s on each lower and upper background, respectively. Natural standards were used for measuring the characteristic X-rays, and the ZAF matrix correction method was employed for quantification.

The orthopyroxene found in the charnockitic gneiss of Iboropa is hypersthene, that is Fe-Mg-rich (Table 1) and it is associated with amphibole (hornblende) and plagioclase. Pyroxene analysed is a solid solution between enstatite and ferrosilite (En₄₅Fs₅₄Wo₁), and because of the high Fe content it can be referred to as ferrohypersthene. Orthopyroxene is an essential mineral for all rocks in the charnockite suite.

The percentage of CaO in the analysed pyroxene is very low (0.5–0.6%), a value which is less than 1.0 and is consistent with the fact that Ca atoms per formula does not exceed 1.0, which occurs only in the M2 site [30]. The aluminium oxide content of the pyroxene is also low. Orthopyroxene can be found to be metamorphic because the weight percentage of MgO + FeO_T is greater than 44.00% [31]. Orthopyroxene present could have formed during increase in grade of metamorphism from either hornblende or biotite. In Northern Nigeria, charnockite has been derived from the transformation of biotite by the addition of ferrous iron-rich juvenile solution [32]. Orthopyroxene could be formed by any of the following reactions: (1) Hornblende+Quartz=>Clinopyroxene+Orthopyroxene +Plagioclase+K-feldspar+H2O \matrix{{{\rm{Hornblende}} + {\rm{Quartz}} = > } \hfill \cr {{\rm{Clinopyroxene}} + {\rm{Orthopyroxene}} \,+ } \hfill \cr {{\rm{Plagioclase}} + {\rm{K}} {\rm -} {\rm{feldspar}} + {{\rm{H}}_2}{\rm{O}}} \hfill \cr } (2) Biotite+Quartz=>Orthopyroxene+K-feldspar+H2O \matrix{{{\rm{Biotite}} + {\rm{Quartz}} = > } \hfill \cr {{\rm{Orthopyroxene}} + {\rm{K}} {\rm-} {\rm{feldspar}} + {{\rm{H}}_2}{\rm{O}}} \hfill \cr }

The earlier reactions are dehydration reactions resulting in a decrease in the activity of water and an increase in alkali activity [19]. Mineral assemblage show altered form of biotite (first-generation biotite) in close association with K-feldspar (Figure 8a). This process is a high-grade dehydration of rocks of the amphibolite facies to granulite facies and it can involve partial melting and in some cases fluid solid-state dehydration [33,34,35]. Orthopyroxene mantled by amphibole is an indication of retrogressive metamorphic event (Figure 5). The retrogression of the charnockite could be due to various dehydration processes [36]. Retrogression involving the breakdown of orthopyroxene is quite common in granulite complexes [37, 38]. Orthopyroxene replaced by amphibole has been described in the granulite facies of gneiss in the granite Falls-Montevideo [39]. The hypersthene observed has numerous microfractures that may serve as a pathway for the movement of fluids (Figure 4a). The development of biotite along cleavage planes (Figure 4a) is also an indication of retrogression [40]. Amphibole could have been formed by a process in which orthopyroxene reacts with plagioclase (anorthite-rich) and quartz in the presence of water, and this can be represented by the following equation: (3) Orthopyroxene+Plagioclase 1 +Quartz+H2O=>Amphibole+Plagioclase 2 \matrix{{{\rm{Orthopyroxene}} + {\rm{Plagioclase}}\,1 +\, } \hfill \cr {{\rm{Quartz}} + {{\rm{H}}_2}{\rm{O}} = > {\rm{Amphibole}} + {\rm{Plagioclase}}\,2} \hfill \cr }

In this process, amphibole replaces orthopyroxene in a rehydration reaction. Biotite (second-generation biotite) that is in close contact with orthopyroxene (Figure 4a) could have been a product of a reaction in which K-feldspar is consumed by a rehydration process, and thus it can be represented by the following equation: (4) Orthopyroxene+K-feldspar+H2O=>Biotite {\rm{Orthopyroxene}} + {\rm{K}} {\rm{-}} {\rm{feldspar}} + {{\rm{H}}_2}{\rm{O}} = > {\rm{Biotite}}

The biotite is annite-rich, an iron end member of biotite as indicated by the mineral chemistry (Table 2). Such retrogressive reactions have been found to result from fluid (CO₂ and H2O) activity and oxygen fugacity in the later stages of dehydration zone [41]. Based on the BSE image and elemental maps (Figures 5b and 6), there is no evidence of zoning in the pyroxene crystals. Hornblende and biotite in charnockites have been found to be due to retrogressive metamorphism of pyroxene granulite facies rocks [42, 43], and retrograde form of metamorphism has been found to obliterate the mineralogy of the granulite facies [24]. Plagioclase occurring as inclusions in pyroxene and biotite is an indication that it was formed very early enough as primary minerals. Opaque minerals that are concentrated along the cleavage planes of amphiboles crystals are evidences of metamorphic alteration. The Al₂O₃ content of the amphibole is fairly high, and chornockitic rocks usually have Al₂O₃ higher than that of the normal igneous rocks. Hornblende has less magnesium than the coexisting orthopyroxene and a moderate composition of TiO₂ (2.04%). Hornblende has high TiO₂ (>2) which some authors have linked to the increase in the grade of metamorphism, and this link between temperature and Ti content of amphibole has been reported [44, 45]. There is a high value of TiO₂ (4.84–5.23%) in biotite, an indication that it is uniformly rich in Ti, and a higher value of X_Fe to X_Mg. Biotite could have reacted with quartz to give the Fe-Ti ilmenite. There has been suggestion that increasing Ti content leads to increase in the stability field of biotite [46]. The crystals of biotite have MgO of nearly the same range of composition as that of amphiboles but far less than that of orthopyroxene. Plagioclase is the dominant feldspar observed and it is albite-enriched (Ab_55.9An_41.9Or_2.2–Ab_63.2An_34.1Or_2.8) that is andesine and is indicative of amphibolite zone. The K-feldspar has perthitic texture (Figure 8b). Perthite is an intergrowth of one feldspar within another, albite in orthoclase [47], and it is common in high-grade metamorphic rocks [48] due to the high temperature involved. The perthite observed has straight lamellae (Figure 7b), an indication of granulite facies. Based on the association of orthopyroxene and plagioclase, the granulite can be said to be of intermediate pressure [24], and one of the feature of this facies is the depletion of Th and U. The values of Cs have also been found to be depleted in high-grade metamorphosed rocks [24]. There is a depletion in the values of K, Rb, Th and U, and pyroxene gneiss with values of this nature has been suggested as a residue of partial melting in which there has been the removal of these elements from the original rocks [49]. The elements Th, U, Rb and Cs are usually present in the lattices of micas of the amphibolite facies but there is breaking down of micas to K-feldspar in the granulite facies, and K-feldspar do not concentrate Rb and Cs [24]. The Sr value in this rock is high, which might compound the low value of Rb, thus giving rise to low Rb/Sr ratio. REE patterns show LREE enrichment, and HREE-depleted patterns with negative Eu anomaly (Table 5), suggesting that this charnockitic gneiss of Iboropa could have been formed by partial melting and crystallisation. The enrichment of LREE and negative Eu anomaly are similar to the charnockitic rocks of Ado-Ekiti area of Nigeria [50]. The negative Eu anomaly reflects an intracrustal form of differentiation of the original igneous parent rock [51].

The detailed petrographic observation and the mineral assemblage suggest a retrograde metamorphic reaction. There are two generations each for biotite and plagioclase feldspar. The first-generation biotite is altered and it is concentrated in and around plagioclase as well as K-feldpar, while the second-generation biotite surrounds orthopyroxene. The high-grade minerals such as hypersthene and plagioclase gave rise to hydrous minerals amphiboles and biotite. Fluids responsible for the hydration reaction could have been transported along the numerous microfractures within the hypersthene grains. The presence of straight lamellae in the perthite is an evidence of granulite facies. The charnockitic gneiss could have been resulted from a partial melting process.