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

E.J. Oziegbe; V.O. Olarewaju; O.O. Ocan; G. Costin

doi:10.2478/rmzmag-2020-0009

Figures & Tables

Sketch geological map of the Hoggar-Aïr-Nigeria Province showing the Neoproterozoic Trans-Saharan belt, resulting from terrane amalgamation between the cratons of West Africa and Congo and the East Saharan block [26].

Field photograph of charnockitic gneiss with quartzo-feldspathic material at Iboropa Akoko.

Photomicrographs showing (a) porphyroblast of pyroxene (Px) having numerous fractures. Pale biotite (Bt) along cleavage planes (PPL). (b) Plagioclase (Pl) occurring as inclusions in pyroxene (XPL). PPL, plane polarised light; XPL, crossed polarised light.

(a) Photomicrograph showing pyroxene (Px) rimmed by amphibole (Am). The opaque mineral (Opq) (PPL). (b) Backscattered electron image of minerals shown in (a). PPL, plane polarised light; XPL, crossed polarised light.

The compositional maps of minerals shown in Figure 5, showing elemental levels of Si, Mg, Na, Ca, Fe, Al, K, Mn and Ti.

Photomicrographs showing: (a) amphiboles (Am) with inclusions of pyroxene (Px) (PPL). (b) Amphiboles with inclusions of pyroxene, surrounded by plagioclase (Pl) (XPL). (c) Biotite having inclusions of plagioclase feldspar (XPL). (d) Biotite having inclusions of plagioclase feldspar (PPL). PPL, plane polarised light; XPL, crossed polarised light.

Photomicrographs showing: (a) pyroxene in close contact with areas occupied by transparent minerals: plagioclase and K-feldspar. Altered biotite (brownish mineral) surrounded by transparent mineral (PPL). (b) Pyroxene (Px), perthitic K-feldspar (Kfs) and plagioclase feldspar (Pl). The straight lamellae in the perthite (XPL). (c) Biotite (Bt) and apatite. The smaller needle-shaped apatite crystals (PPL). (d) K-feldspar (Kfs), quartz (Qtz), apatite (Ap) and plagioclase (Pl) (XPL). PPL, plane polarised light; XPL, crossed polarised light.

Ternary diagram showing feldspar solid solution.

The chemical composition of pyroxene and amphibole

Pyroxene			Amphibole
Sample	1c	1r	Crystal	1	2	3	4	5	6
SiO₂	49.680	49.830	SiO₂	42.453	41.804	43.680	42.904	43.139	42.786
TiO₂	0.037	0.114	TiO₂	2.038	1.450	1.505	1.669	1.826	1.993
Al₂O₃	0.618	0.682	Al₂O₃	10.389	10.342	9.819	10.039	10.174	10.186
FeO	32.470	31.910	FeO	18.095	17.894	17.156	18.178	18.975	18.580
Cr₂O₃	0.000	0.010	Cr₂O₃	0.000	0.000	0.043	0.132	0.053	0.101
MnO	0.585	0.712	MnO	0.058	0.016	0.111	0.132	0.049	0.054
MgO	15.200	15.070	MgO	9.163	9.361	9.943	9.474	9.399	9.622
CaO	0.618	0.495	CaO	11.305	11.394	11.575	11.465	11.431	11.455
Na₂O	0.014	0.055	Na₂O	1.533	1.338	1.108	1.386	1.406	1.443
K₂O	0.057	0.030	K₂O	1.409	1.430	1.215	1.411	1.347	1.349
Total	99.290	98.900	Cl	0.071	0.085	0.063	0.077	0.043	0.140
TSi	1.958	1.970	Total	96.510	95.110	96.180	96.740	97.79	97.610
TAl	0.029	0.030	O_Cl	0.020	0.020	0.010	0.020	0.010	0.030
TFe³⁺	0.014	0.000	TSi	6.553	6.517	6.681	6.580	6.556	6.512
M1Al	0.000	0.002	TAl	1.447	1.483	1.319	1.420	1.444	1.488
M1Ti	0.001	0.003	Sum_T	8	8	8	8	8	8
M1Fe³⁺	0.044	0.026	CAl	0.442	0.416	0.449	0.393	0.377	0.337
M1Fe²⁺	0.062	0.080	CFe⁺³	0.000	0.038	0	0	0	0
M1Cr	0.000	0.000	CTi	0.237	0.170	0.173	0.193	0.209	0.228
M1Mg	0.893	0.888	CMg	2.109	2.176	2.267	2.166	2.129	2.183
M2Mg	0.000	0.000	CFe⁺²	2.213	2.201	2.106	2.232	2.279	2.240
M2Fe²	0.950	0.949	Sum_C	5	5	5	5	5	5
M2Mn	0.020	0.024	BFe⁺²	0.123	0.095	0.089	0.099	0.132	0.125
M2Ca	0.026	0.021	BMn	0.008	0.002	0.014	0.017	0.006	0.007
M2Na	0.001	0.004	BCa	1.870	1.903	1.897	1.884	1.861	1.868
M2K	0.003	0.002	Sum_B	2	2	2	2	2	2
Sum (cat)	3.997	3.998	ANa	0.459	0.404	0.329	0.412	0.414	0.426
Ca	1.337	1.069	AK	0.277	0.284	0.237	0.276	0.261	0.262
Mg	45.770	45.260	Sum_A	0.736	0.689	0.566	0.688	0.675	0.688
Fe2_Mn	52.890	53.670	Sum_cat	15.736	15.689	15.566	15.688	15.675	15.688
JD1	0.000	0.108	CCl	0.019	0.022	0.016	0.020	0.011	0.036
AE1	0.202	0.186	Sum_oxy	23.102	23.000	23.023	23.031	23.016	23.002
CFTS1	2.056	1.168
CTTS1	0.056	0.174
CATS1	0.000	0.000
WO1	0.000	0.000
EN1	45.780	45.560
FS1	51.902	52.800
Q	1.931	1.939
J	0.002	0.008
WO	1.299	1.055
EN	44.460	44.670
FS	54.240	54.280
WEF	99.890	99.570
JD	0.000	0.032
AE	0.100	0.396

The composition of ilmenite in a charnockitic gneiss

Sample	1
SiO₂	0.046
TiO₂	50.787
Al₂O₃	0.02
FeO	46.372
Cr₂O₃	0.075
MnO	0.477
MgO	0.267
CaO	0
Na₂O	0.061
Total	98.11
Si	0.001
Al	0.001
Ti	0.986
Fe²⁺	1.001
Cr	0.002
Mn	0.01
Mg	0.01
Ca	0
Na	0.003
Cations	2.014

Chemical composition of biotite crystals

Crystal	1	2	3	4	5	6	7	8
SiO₂	35.20	35.10	35.19	34.71	35.52	35.34	35.95	35.42
TiO₂	5.23	5.15	4.97	4.84	4.84	4.97	5.09	5.00
Al₂O₃	13.63	13.79	14.14	13.75	13.92	13.35	13.82	13.83
Cr₂O₃	0.04	0.06	0.00	0.03	0.02	0.05	0.03	0.00
FeO	21.02	20.48	20.75	20.96	19.97	20.59	20.41	20.72
MnO	0.00	0.04	0.03	0.03	0.00	0.01	0.00	0.02
MgO	9.88	9.95	10.15	9.93	10.00	10.07	9.99	9.64
CaO	0.01	0.07	0.00	0.04	0.02	0.00	0.00	0.04
Na₂O	0.13	0.11	0.09	0.14	0.09	0.08	0.14	0.11
K₂O	9.74	9.71	9.57	9.62	9.56	8.54	9.63	9.74
Cl	0.15	0.11	0.12	0.15	0.09	0.09	0.10	0.10
H₂O	1.80	1.81	1.81	1.78	1.81	1.80	1.83	1.81
Total	95.03	94.57	95.01	94.2	94.03	93.09	95.16	94.62
Si	5.739	5.735	5.717	5.714	5.803	5.823	5.812	5.782
AlIV	2.261	2.265	2.283	2.286	2.197	2.177	2.188	2.218
AlVI	0.356	0.388	0.423	0.380	0.481	0.414	0.443	0.441
Ti	0.641	0.633	0.607	0.599	0.595	0.616	0.619	0.614
Fe²⁺	2.866	2.798	2.819	2.885	2.729	2.837	2.759	2.829
Cr	0.005	0.008	0.000	0.004	0.003	0.007	0.004	0.000
Mn	0.000	0.006	0.004	0.004	0.000	0.001	0.000	0.003
Mg	2.401	2.423	2.458	2.437	2.436	2.474	2.408	2.346
Ca	0.002	0.012	0.000	0.007	0.004	0.000	0.000	0.007
Na	0.041	0.035	0.028	0.045	0.029	0.026	0.044	0.035
K	2.026	2.024	1.984	2.020	1.993	1.795	1.986	2.028
Cations	16.338	16.327	16.323	16.381	16.270	16.170	16.263	16.303
CCl	0.083	0.061	0.066	0.084	0.050	0.050	0.055	0.055
OH	1.959	1.970	1.967	1.958	1.975	1.975	1.973	1.972
O	24	24	24	24	24	24	24	24
Fe/(Fe + Mg)	0.5400	0.54	0.530	0.540	0.530	0.530	0.530	0.550
Mg/(Fe + Mg)	0.460	0.460	0.470	0.460	0.470	0.470	0.470	0.450

The chemical composition of plagioclase crystals

Crystal	1	2	3	4	5	6	7	8	9	10	11	12	13	14
SiO₂	58.96	59.05	59.39	59.47	58.84	58.19	58.45	59.22	59.23	58.7	59.76	60.19	58.14	58.57
TiO₂	0.02	0	0	0	0.04	0	0.37	0.01	0	0	0.04	0	0.03	0
Al₂O₃	25.53	25.59	25.55	25.03	25.72	25.77	25.8	25.65	25.34	25.26	24.06	24.78	25.24	25.49
FeO	0.12	0.07	0.05	0.15	0.02	0.07	0.24	0.07	0.11	0.17	0	0.08	0.01	0.01
MnO	0.01	0.01	0.05	0.02	0.01	0.04	0	0.03	0	0.04	0.02	0	0	0
MgO	0	0.01	0	0	0.01	0	0	0	0.02	0	0	0.01	0	0
BaO	0.01	0.04	0.08	0.05	0.07	0	0.07	0.03	0.09	0	0.06	0.01	0.08	0.03
CaO	8.05	8.04	7.98	7.82	8.13	8.58	8.49	8.21	7.78	7.88	7.18	7.3	8.34	8.15
Na₂O	6.87	6.88	5.99	7.04	6.75	6.7	6.26	6.62	6.93	7.01	7.35	7.35	6.07	6.53
K₂O	0.38	0.37	0.39	0.39	0.34	0.37	0.36	0.34	0.37	0.36	0.49	0.41	0.37	0.39
Total	99.95	100.06	99.48	99.97	99.93	99.72	100.04	100.18	99.87	99.42	98.96	100.13	98.28	99.17
Si	2.639	2.64	2.659	2.661	2.633	2.616	2.617	2.642	2.651	2.642	2.697	2.683	2.642	2.639
Al	1.346	1.347	1.347	1.319	1.356	1.364	1.36	1.347	1.336	1.339	1.279	1.301	1.351	1.353
Fe3⁺	0	0	0	0	0	0	0	0	0	0	0	0	0	0
Ti	0.001	0	0	0	0.001	0	0.012	0	0	0	0.001	0	0.001	0
Fe2⁺	0.004	0.003	0.002	0.006	0.001	0.003	0.009	0.003	0.004	0.006	0	0.003	0	0
Mn	0	0	0.002	0.001	0	0.002	0	0.001	0	0.002	0.001	0	0	0
Mg	0	0.001	0	0	0.001	0	0	0	0.001	0	0	0.001	0	0
Ba	0	0.001	0.001	0.001	0.001	0	0.001	0.001	0.002	0	0.001	0	0.001	0.001
Ca	0.386	0.385	0.383	0.375	0.39	0.413	0.407	0.392	0.373	0.38	0.347	0.349	0.406	0.393
Na	0.596	0.596	0.52	0.611	0.586	0.584	0.543	0.573	0.602	0.612	0.643	0.635	0.535	0.571
K	0.022	0.021	0.022	0.022	0.019	0.021	0.021	0.019	0.021	0.021	0.028	0.023	0.021	0.022
Cations	4.994	4.995	4.937	4.997	4.989	5.003	4.971	4.979	4.992	5.002	4.998	4.995	4.958	4.98
X	3.986	3.987	4.006	3.98	3.99	3.98	3.989	3.989	3.987	3.981	3.977	3.984	3.994	3.992
Z	1.008	1.007	0.93	1.016	0.998	1.023	0.981	0.989	1.003	1.021	1.02	1.011	0.963	0.987
Ab	59.4	59.5	56.2	60.6	58.9	57.4	55.9	58.2	60.4	60.4	63.2	63.1	55.6	57.9
An	38.4	38.4	41.4	37.2	39.2	40.6	41.9	39.8	37.4	37.5	34.1	34.7	42.2	39.9
Or	2.2	2.1	2.4	2.2	1.9	2.1	2.2	1.9	2.1	2.1	2.8	2.3	2.2	2.2

Bulk rock composition of charnockitic gneiss; major elements (%), trace elements (ppm), REE (ppm)

Major element	1	Trace element	1	REE	1
Wt%		ppm		ppm
SiO₂	50.93	V	188.28	La	57.71
Al₂O₃	15.51	Cr	95.31	Ce	119.2
CaO	7.79	Co	107.09	Pr	14.13
Fe₂O₃	12.3	Ni	52.36	Nd	54.64
MgO	5.14	Cu	41.17	Sm	9.05
K₂O	1.01	Zn	124.1	Eu	2.27
MnO	0.15	Rb	18.36	Gd	7.325
Na₂O	3.33	Sr	583.8	Tb	0.856
TiO₂	2.15	Zr	294.09	Dy	4.365
P₂O₅	0.65	Nb	20.61	Ho	0.8
Cr₂O₃	0.02	Mo	1.54	Er	2.02
LOI	0.34	Cs	0.31	Tm	0.251
Total	99.32	Ba	479.77	Yb	1.63
		Hf	6.49	Lu	0.22
		Ta	1.08	Total	274.4
		Pb	8.51	La_N/Yb_N	25.39
		Th	2.89	Gd_N/Yb_N	3.72
		U	0.45	Eu_N/Eu_N^*	0.852
				Ce/Ce^*	1.153

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

Figures & Tables

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

The chemical composition of pyroxene and amphibole

The composition of ilmenite in a charnockitic gneiss

Chemical composition of biotite crystals

The chemical composition of plagioclase crystals

Bulk rock composition of charnockitic gneiss; major elements (%), trace elements (ppm), REE (ppm)

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