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

Ifeoma Agbi
Ifeoma Agbi
 e 
Emmanuel M. Iroka
Emmanuel M. Iroka
   | 29 ago 2023
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Pubblicato online: 29 ago 2023
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Ricevuto: 23 gen 2023
Accettato: 14 feb 2023
DOI: https://doi.org/10.2478/rmzmag-2023-0001
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© 2023 Ifeoma Agbi et al., published by Sciendo
This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License.

Figure 1:

Geologic map of Obudu Plateau.
Geologic map of Obudu Plateau.

Figure 2:

Photomicrograph of pyroxene gneiss showing (a) mineral association of orthopyroxene + plagioclase+ biotite + quartz in sample IA5 (PPL); (b) Antiperthite texture in plagioclase of IA5 and apatite inclusion (XPL); (c) Mineral assemblage in sample IA3 showing clinopyroxene occurring alongside orthopyroxene (PPL); (d) BSE image of exsolved laths of hematite in ilmenite in sample IA3.
Photomicrograph of pyroxene gneiss showing (a) mineral association of orthopyroxene + plagioclase+ biotite + quartz in sample IA5 (PPL); (b) Antiperthite texture in plagioclase of IA5 and apatite inclusion (XPL); (c) Mineral assemblage in sample IA3 showing clinopyroxene occurring alongside orthopyroxene (PPL); (d) BSE image of exsolved laths of hematite in ilmenite in sample IA3.

Figure 3:

An-Ab-Or plot for the pyroxene gneiss of Obudu showing andesine as the main plagioclase.
An-Ab-Or plot for the pyroxene gneiss of Obudu showing andesine as the main plagioclase.

Figure 4:

Pyroxene triangular diagram showing the occurrence of hypersthene and augite in the pyroxene gneiss.
Pyroxene triangular diagram showing the occurrence of hypersthene and augite in the pyroxene gneiss.

Figure 5:

Si Vs Mg/(Mg +Fe2+) diagram for amphiboles from sample IA3 shown as tschermakite.
Si Vs Mg/(Mg +Fe2+) diagram for amphiboles from sample IA3 shown as tschermakite.

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

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

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

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
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