Mineral Chemistry of Pyroxene Gneiss in Obudu, SE Nigeria, and Its Petrological Significance

The Benin–Nigeria Shield exposed in Nigeria comprises rocks that are commonly grouped into the Migmatite Gneiss Complex, Schist Belts, and Pan-African granitoids [1,2,3]. The pyroxene gneiss which forms part of the basement cover in southeast Nigeria belongs to the Migmatite Gneiss Complex and has a reported age of 2062.4 ± 0.4 Ma from single zircon evaporation method [4]. These rocks outcrop in places such as the Okorotong, Bebi, Bogene, and Abado areas of Obudu (Figure 1). Lithologically, the rocks are mesocratic and coarse-grained with minor leucocratic bands appearing in some places. Ekwueme and Kröner [4] suggested the pyroxene gneiss may have originated from a metagreywacke or granodiorite protolith; Ugwuonah et al. [5], however, termed the rock a meta quartz diorite. The rocks have experienced high-grade metamorphism up to granulite facies [4,5,6]. This work aims to study the mineral chemistry obtained from an electron microprobe analyzer (EMPA) in order to distinguish the different mineral phases and thus infer their petrological significance.

Figure 1:

The petrography of samples collected from the pyroxene gneiss was ascertained using a petrographic microscope at the Department of Geology, University of Calabar. Electron microprobe analyses were subsequently performed on two polished samples using a CAMECA SX FIVE FE at Ruhr Universität, Bochum, Germany. Analyses were performed using LTAP, TAP, LPET, PET, and LLIF crystals on five wavelength dispersive spectrometers at 15KV accelerating voltage, 15nA beam current, and 5μm probe diameter. Ten elements (Si, Al, Fe, Ca, Na, K, Ti, Cr, Mn, Mg) were quantitatively analyzed using natural and synthetic minerals as standards. The concentrations were calculated using the PAP matrix correction factors.

Rocks of intermediate to mafic composition, such as andesite, diorite, basalt, and gabbro, when metamorphosed at high metamorphic grade, are usually marked by the appearance of certain minerals. The appearance of orthopyroxene in a rock with typical metabasite composition defines the onset of granulite facies metamorphism. The typical assemblage of such rocks at granulite facies include orthopyroxene + clinopyroxene + plagioclase ± hornblende ± garnet ± biotite ± quartz ± rutile ± ilmenite [10]. At higher pressures above 7 kbar, the distinctive assemblage is plagioclase + augite + garnet, while at lower pressures between 5–7 kbar, the typical assemblage is plagioclase + augite + hypersthene [11].

During the transition from the amphibolite to the granulite facies, hydrous phases such as amphibole decrease in modal amount with increasing temperature, by the reaction: Hornblende + quartz = orthopyroxene + clinopyroxene + plagioclase + H2O(1)[10] \matrix{{{\rm{Hornblende }} + {\rm{ quartz }} = {\rm{ orthopyroxene}}} \hfill \cr {\;\; +\; {\rm{ clinopyroxene }} + {\rm{ plagioclase + }}\;{{\rm{H}}_2}{\rm{O}}\left( 1 \right)\left[ {10} \right]} \hfill \cr }

As observed in the pyroxene gneiss, hornblende is completely absent from sample IA5 and is only about 1% vol in sample IA3. Also, the orthopyroxene and plagioclase occupy more than 60% volume of the rock. This suggests that pyroxene gneiss had attained granulite facies metamorphism.

At the beginning of granulite facies metamorphism, during anatexis, orthoclase is formed by the reaction: Biotite + quartz = orthopyroxene + orthoclase + H2O(2)[11] \matrix{{{\rm{Biotite }} + {\rm{ quartz }} = {\rm{ orthopyroxene}}} \hfill \cr {\;\; +\; {\rm{ orthoclase }} +\; {{\rm{H}}_{\rm{2}}}{\rm{O }}\left( 2 \right)\left[ {11} \right]} \hfill \cr }

The orthoclase and orthopyroxene produced from the dehydration reaction of biotite will enter into the melt phase and be preserved in the rock as leucocratic quartzofeldspathic bands called leucosomes. This explains the presence of orthoclase only in the leucocratic bands of IA5, where it is associated with orthopyroxene and minor biotite. Anhydrous mafic granulites form at temperatures above 850 °C, but in the presence of water-rich fluids, mafic rocks can undergo partial melting and granulites will form at temperatures below 750 °C [11]. Geothermometric calculations based on hornblende and plagioclase geothermometer by Holland and Blundy [12] gave temperatures of 690–740 °C at 5.2 kbar [5]. Although these low values fall within the temperature and pressure values for migmatitic pyroxene granulites, it has been interpreted as post-peak reequilibration. The exsolution of ilmenite to ilmenite + titanohematite (Figure 2d) occurs due to slow cooling or retrogressive metamorphism [13] and is a further indication of retrogressive metamorphism.

The pyroxene gneiss has the mineralogic assemblage of plagioclase + clinopyroxene + orthopyroxene characteristic of metabasites that attained granulite facies metamorphism at pressures below 7 kbar. The presence of migmatitic structures attests to the presence of H₂O-rich fluids during metamorphism and ensuing lower peak metamorphic temperature. Mineral chemistry data shows that the rock is Mg-rich, as X_Mg values vary from 0.54–0.74 across the Fe-Mg minerals.