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

Introduction

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:

Materials and methods

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.

Results and discussion

Petrography

These rocks are weakly foliated, and, in some locations, metamorphic segregation is observed as irregular banding. One of the thin sections (IA5) was cut through a leucosome band. In terms of texture, the rocks are coarse-grained and exhibit deformation structures such as fractures, pinch and swell structures, and augens. Mineralogically, the rock contains the assemblage plagioclase + orthopyroxene + clinopyroxene + biotite + quartz ± k-feldspar ± amphibole. Accessory phases include ilmenite, magnetite, apatite, pyrite, and zircons. A slight alignment of minerals is observed under the microscope. Plagioclase is the dominant mineral in the matrix (40%–50% vol) and contains inclusions of quartz and apatite. K-feldspar is only present in the leucosome, where it is associated with plagioclase, quartz, and orthopyroxene. Hematite occurs as exsolved laths in ilmenite (Figure 2).

Figure 2:

Mineral chemistry

Feldspar

The sodic plagioclase andesine (An₃₀ to An₃₇) with less than 4% orthoclase content occurs in the pyroxene gneiss. In the leucocratic band where antiperthite occurs, the exsolved laths are orthoclase-rich (Or _{93 – 95}, Ab_{4 – 6}, An_{0.1 – 0.4}) and occur within sodic-rich plagioclase of similar composition (An₃₅) as the matrix plagioclase.

K-feldspar, where it occurred, had a similar composition to the exsolved laths in the antiperthite (Or ₉₁, Ab₉, An_0.3). (Table 1, Figure 3).

Figure 3:

Table 1:

Representative chemical composition (wt%) and atom per formula unit of plagioclase based on 8 oxygen. (sample IA3/5).

Sample	IA3					IA5
Wt%	1	2	3	4	5	1	2	3	4	5
SiO2	60.55	60.58	60.57	60.48	60.44	60.13	60.07	60.20	59.91	60.95
Al2O3	25.33	25.02	25.12	25.43	25.55	25.75	25.18	25.17	25.49	24.90
FeO*	0.09	0.09	0.11	0.10	0.11	0.02	0.05	0.05	0.10	0.07
CaO	6.31	6.48	6.67	6.36	6.55	6.82	6.75	6.72	6.91	6.24
Na2O	7.40	7.67	7.47	7.49	7.65	7.45	7.32	7.43	7.41	7.66
K2O	0.68	0.50	0.40	0.55	0.43	0.35	0.53	0.62	0.39	0.46
Total	100.36	100.34	100.35	100.41	100.73	100.52	99.90	100.19	100.20	100.29
apfu
Si	2.692	2.690	2.694	2.686	2.673	2.668	2.684	2.681	2.667	2.709
Al	1.327	1.309	1.317	1.331	1.332	1.347	1.326	1.321	1.337	1.304
Fe3+	0.000	0.000	0.000	0.000	0.001	0.000	0.000	0.000	0.000	0.000
Fe2+	0.003	0.003	0.004	0.004	0.003	0.001	0.002	0.002	0.004	0.003
Ca	0.301	0.308	0.318	0.303	0.310	0.324	0.323	0.320	0.330	0.297
Na	0.638	0.661	0.644	0.645	0.656	0.641	0.634	0.641	0.640	0.660
K	0.038	0.028	0.023	0.031	0.024	0.020	0.030	0.035	0.022	0.026
Total	5.000	5.000	5.000	5.000	5.000	5.000	5.000	5.000	5.000	5.000
End Member Fractions (mol%)
An	30.77	30.91	32.26	30.93	31.33	32.92	32.72	32.15	33.26	30.21
Ab	65.30	66.27	65.43	65.91	66.21	65.09	64.23	64.33	64.51	67.12
Or	3.93	2.82	2.31	3.16	2.46	1.99	3.06	3.52	2.23	2.67

Orthopyroxene

The orthopyroxene is hypersthene, as seen in Figure 4 and Table 2. X_Mg = 0.56–0.62.

Figure 4:

Table 2:

Representative chemical composition (wt%) and atom per formula unit (apfu) of orthopyroxene based on 6 oxygen. (sample IA3/5).

Sample	IA3					IA5
Wt%	1	2	3	4	5	1	2	3	4	5
SiO2	51.68	51.86	52.20	52.05	52.20	52.94	52.82	52.87	52.98	52.98
TiO2	0.10	0.09	0.05	0.11	0.09	0.08	0.10	0.09	0.08	0.10
Al2O3	0.87	0.85	0.92	0.77	0.80	0.49	0.58	0.53	0.57	0.64
Cr2O3	0.02	0.01	0.00	0.03	0.00	0.05	0.10	0.05	0.05	0.08
FeO*	25.46	25.42	24.74	25.07	26.72	24.49	22.65	23.40	23.30	23.15
MnO	0.66	0.64	0.61	0.64	0.76	0.69	0.61	0.63	0.61	0.59
MgO	19.69	19.83	20.38	20.03	18.94	20.89	21.07	21.49	21.23	21.35
CaO	0.82	0.64	0.51	0.80	0.46	0.57	1.69	0.73	0.85	1.04
Na2O	0.00	0.02	0.01	0.03	0.01	0.00	0.05	0.00	0.04	0.04
K2O	0.00	0.01	0.00	0.00	0.01	0.03	0.01	0.00	0.01	0.00
Total	99.32	99.37	99.43	99.54	99.99	100.23	99.67	99.80	99.72	99.99
apfu
Si	1.971	1.975	1.979	1.976	1.990	1.989	1.985	1.985	1.992	1.985
Ti	0.003	0.003	0.002	0.003	0.003	0.002	0.003	0.002	0.002	0.003
Al	0.039	0.038	0.041	0.035	0.036	0.022	0.026	0.024	0.025	0.028
Cr	0.001	0.000	0.000	0.001	0.000	0.001	0.003	0.001	0.001	0.003
Fe3+	0.013	0.007	0.000	0.008	0.000	0.000	0.000	0.000	0.000	0.000
Fe2+	0.799	0.802	0.785	0.788	0.852	0.770	0.712	0.735	0.733	0.725
Mn	0.021	0.021	0.020	0.020	0.025	0.022	0.019	0.020	0.020	0.019
Mg	1.119	1.126	1.152	1.134	1.076	1.170	1.181	1.203	1.190	1.192
Ca	0.033	0.026	0.021	0.033	0.019	0.023	0.068	0.029	0.034	0.042
Na	0.000	0.001	0.001	0.002	0.001	0.000	0.004	0.000	0.003	0.003
Total	4.000	4.000	4.000	4.000	4.000	4.000	4.000	4.000	4.000	4.000
XMg	0.58	0.58	0.59	0.59	0.56	0.60	0.62	0.62	0.62	0.62
End Member Fractions (%)
Wo	1.70	1.32	1.07	1.67	0.97	1.18	3.47	1.50	1.76	2.13
En	56.98	57.40	58.85	57.77	55.28	59.61	60.22	61.15	60.80	60.85
Fs	41.32	41.28	40.08	40.57	43.75	39.22	36.31	37.36	37.44	37.02

Clinopyroxene

The clinopyroxene is also magnesium-rich (X_Mg = 0.70–0.74). In terms of wollastonite, enstatite, and ferrosilite, the range is Wo_{42 – 45}, En_{39 – 42}, Fs_{14 – 17}. It plots in the field of augite (Figure 4, Table 3).

Table 3:

Representative chemical composition (wt%) and atom per formula unit (apfu) of calcic amphibole and clinopyroxene based on 23 oxygen and 6 oxygen, respectively. (sample IA3/5)

Amphibole	Clinopyroxene
Sample	IA3					IA3		IA5
Wt%	1	2	3	4	5	1	2	1	2
SiO2	42.71	42.70	42.86	41.79	43.34	52.67	52.74	53.24	52.89
TiO2	2.59	2.39	2.06	2.49	2.48	0.16	0.13	0.07	0.16
Al2O3	11.11	10.57	10.77	11.40	10.51	1.49	1.39	1.44	1.50
Cr2O3	0.13	0.05	0.06	0.10	0.08	0.00	0.02	0.08	0.08
FeO*	14.09	15.61	15.77	15.84	15.30	9.41	10.37	9.66	9.17
MnO	0.11	0.15	0.15	0.16	0.15	0.31	0.30	0.31	0.32
MgO	11.58	11.30	11.01	10.55	11.46	13.66	13.42	14.27	14.30
CaO	11.42	11.18	11.51	11.48	11.29	21.48	20.74	20.17	19.95
Na2O	1.35	1.53	1.01	1.22	1.35	0.41	0.48	0.36	0.41
K2O	1.61	1.37	1.43	1.55	1.33	0.00	0.00	0.00	0.00
Total	96.70	96.86	96.63	96.58	97.29	99.59	99.58	99.60	98.76
apfu
Si	6.382	6.379	6.414	6.296	6.427	1.972	1.980	1.991	1.991
Ti	0.291	0.269	0.231	0.282	0.277	0.004	0.004	0.002	0.004
Al	1.956	1.861	1.900	2.024	1.836	0.066	0.062	0.063	0.066
Cr	0.016	0.005	0.007	0.011	0.010	0.000	0.000	0.002	0.002
fe3	0.326	0.555	0.548	0.448	0.518	0.012	0.006	0.000	0.000
Fe2+	1.435	1.394	1.426	1.548	1.379	0.283	0.319	0.302	0.289
Mn	0.013	0.019	0.019	0.020	0.019	0.010	0.009	0.010	0.010
Mg	2.581	2.517	2.456	2.371	2.534	0.762	0.751	0.796	0.802
Ca	1.829	1.790	1.845	1.854	1.794	0.861	0.834	0.808	0.804
Na	0.391	0.442	0.293	0.356	0.390	0.030	0.035	0.026	0.030
K	0.306	0.262	0.273	0.297	0.251
Total	15.527	15.493	15.411	15.506	15.434	4.000	4.000	4.000	4.000
XMg	0.59	0.56	0.55	0.54	0.57	0.72	0.70	0.72	0.74
End Member Fractions (mol%)
Wo						44.90	43.66	42.40	42.44
En						39.74	39.31	41.75	42.33
Fs						15.36	17.03	15.85	15.22

Calcic–Amphibole

The calcic amphibole occupies about 1% volume in the studied samples and can be classified as tschermakite according to the classification scheme of Leake et al. [9]. X_Mg = 0.56–0.59 (Figure 5).

Figure 5:

Biotite

The biotite in the samples is Mg-rich (X_Mg = 0.58–0.69) and has Ti content of 0.5–0.6 p.f.u. for both samples. Matrix X_Mg values are lower than values for biotite adjacent to orthopyroxene (Table 4).

Table 4:

Representative chemical composition (wt%) and atom per formula unit (apfu) of biotite based on 22 oxygen (sample IA3/5).

Sample	IA3				IA5
Remarks	Core adjacent Opx	Rim adjacent Opx	Matrix 1	Matrix 2	Inclusion	Rim adjacent Opx	Core adjacent Opx	Matrix 1	Matrix 2
SiO2	36.65	36.42	35.82	35.79	37.68	38.06	37.54	37.34	37.29
TiO2	5.05	4.80	5.18	5.07	5.54	5.27	5.15	5.80	5.88
Al2O3	14.87	13.94	13.99	14.03	14.18	14.39	14.36	14.23	13.87
Cr2O3	0.04	0.04	0.05	0.07	0.45	0.44	0.40	0.44	0.41
FeO*	14.65	16.62	16.40	16.81	12.74	12.19	13.12	13.69	13.67
MnO	0.01	0.09	0.11	0.06	0.03	0.05	0.04	0.07	0.04
MgO	13.81	13.23	12.27	12.66	14.80	15.45	15.12	14.26	14.04
CaO	0.00	0.00	0.02	0.00	0.07	0.00	0.01	0.07	0.00
Na2O	0.03	0.03	0.03	0.04	0.11	0.05	0.05	0.11	0.04
K2O	9.37	9.34	9.42	9.34	9.50	9.64	9.67	9.41	9.63
Total	94.49	94.51	93.28	93.86	95.09	95.53	95.47	95.41	94.87
apfu
Si	5.519	5.547	5.535	5.504	5.591	5.601	5.563	5.550	5.580
Ti	0.572	0.550	0.602	0.586	0.618	0.584	0.574	0.648	0.661
Al	2.640	2.504	2.548	2.543	2.480	2.497	2.509	2.493	2.447
Cr	0.005	0.005	0.006	0.009	0.053	0.052	0.047	0.052	0.049
Fe2+	1.845	2.117	2.119	2.162	1.581	1.500	1.626	1.702	1.711
Mn	0.002	0.011	0.014	0.008	0.003	0.006	0.005	0.008	0.005
Mg	3.099	3.003	2.825	2.901	3.273	3.388	3.338	3.159	3.132
Ca	0.000	0.000	0.003	0.000	0.011	0.000	0.001	0.011	0.000
Na	0.009	0.007	0.009	0.013	0.031	0.013	0.015	0.031	0.012
K	1.800	1.816	1.857	1.832	1.799	1.810	1.828	1.783	1.839
Total	15.491	15.560	15.519	15.557	15.439	15.452	15.507	15.437	15.436
XMg	0.63	0.59	0.57	0.57	0.67	0.69	0.67	0.65	0.65

Other minerals

The TiO₂ content of the ilmenite in this rock varies from 41.9 wt% to 49.9 wt%, while the FeO content of the ilmenite varies from 46.99 wt% to 52.96 wt%. Hematite microprobe data yields 89.64 wt% to 90.36 wt% FeO with about 1.79 wt% TiO₂ occurring in the exsolved lamella. In contrast, magnetite had a FeO content of 92.60 wt%.

Petrological significance

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:

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:

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.

Conclusions

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.