Research Article ISSN 2278–0092 International Journal of Advances in Earth Sciences, Volume 3, Issue 1, 2014, 24-31 © Copyright 2014, All rights reserved Research Publishing Group www.rpublishing.org
Geochemical Characteristics of Amphiboles from Sara-Fier Complex, North Central Nigeria. Ajigo, I. O. Department of Applied Geology, Federal University of Technology P.M.B. 704, Akure, Ondo State, Nigeria. Tel: +2348032107038; e-mail:
[email protected]. Received May 17, 2014; accepted July 31, 2014
Abstract The study area (Sara-Fier complex) is located within the Plateau Younger Granite province. It forms part of the Jurassic Younger Granite province of north central Nigeria. Crystallo-chemical studies on the mafic minerals in the gabbros and granites of the complex were carried out to know the chemical characteristics of the Complex. Nine polished samples of rock slides were analyzed using the electron microprobe at the Faculty of Earth and Planetary Sciences, McGill University, Montreal, Canada.Amphiboles of the studied rocks show high Al and Fe content with low Mg’ =Mg/Mg+Fe; belong to hastingsitic ferro-edenite in the Mg’-Si classification diagram of amphiboles. Oxygen fugacity is inversely related to Fe/(Fe + Mg) ratio. The relatively high Fe/(Fe + Mg) ratio in the amphiboles (0.604 to 1.00) suggests low to intermediate oxygen fugacity. This same is true for Aliv, (Aliv between 0.9701.417). Based on the chemical studies carried on the rocks of Sara-fier complex, it is evident that majority of the rocks may have originated from the same or similar parent magma whose compositional variation in time and space is very minimal. From the crystal formulae of amphiboles, it was observed that with the exception of the riebeckite granite, the T-site, which should be occupied by Si, needed other element (Al) in addition before being filled due to insufficient Si. The presence of Al in the T-site helps in achieving better articulation. In the riebeckite granite the T-site has enough Si to fill it and the B-site is entirely occupied by Na. Both T, C and B sites are filled in all the selected samples. Keywords: Amphibole, Sara and Fier, Crystal, Magma, Younger Granite, Plateau.
1.
Introduction
Sara and Fier, which are the type locations for the complex, are two localities within Bauchi and Plateau states respectively. It cuts across Pankshin sheet 190 and Maijuju sheet 169 and lies between latitudes 9 014’N and 9038’N, and longitudes 9017’E and 9030’E. Sara-Fier complex forms part of the eastern margin of the plateau Younger Granite province, standing apart from the main group of plateau Younger Granites which lies to the west and northwest, (Fig. 1). It is composed almost entirely of intrusive rocks with reportedly preserved remnants of volcanic in some places. Members of the amphibole group are common constituents of major igneous rocks, ranging from ultrabasic to acid and alkaline types. They occur characteristically in the plutonic rocks and in general are relatively unimportant minerals of the volcanic rocks. The amphibole structure allows great flexibility of ionic replacement, and the minerals of the group exhibit an extremely wide range of chemical composition, Deer et al (1979). Considering the chemistry of the amphiboles it has been generally accepted that the X position (M4) may be occupied by Ca, Na and K atoms, and, the Y position (M 1 M2 M3) by the smaller cations Mg, Fe+2,Fe+3 and Al. Although structural investigations indicate that the occupation of these structural positions by the larger and smaller ions respectively is not always strictly fulfilled, the conception nevertheless is valuable as a basis for discussing amphibole chemistry. In many amphiboles Si is replaced by Al (this substitution being limited to approximately 2Al atoms per formula unit), and the whole or a proportion of the A sites, vacant in the tremolite structure, are commonly occupied by Na and K atoms, Leake et al, (1998). Although the substitution of Mg↔Fe is of prime importance in consideration of the optical and physical properties of amphiboles, and their parageneses, this is a comparatively simple substitution which involves no charge unbalance, and which can be described by a simple nomenclature using the prefixes magnesio or ferro -. Four other important substitutions may, however, occur in the amphiboles, i.e. Al ↔ Si, (Mg, Fe) ↔ Al, Na ↔ Ca, and the introduction of Na (K) into the A site. More than one substitution of this type must occur in a given amphibole in order to maintain charge balance, and the range of amphibole compositions can be expressed in terms of various end-members, Leake et al, (1998).
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Ajigo / International Journal of Advances in Earth Sciences, Volume 3, Issue 1, 2014, 24-31
Fig. 1 Geological Map of Nigeria showing the Younger Granite Province and the study area. (modified after Buchanam et al, 1971).
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2.
Materials and Methods
Samples were collected from fresh outcrops, using geological hammer and prepared as standard-size 30 x 45 mm rectangular sections; Rectangular sections of rock were prepared as 30-micron-thick sections without cover slips. Chips or grains were mounted in epoxy disks, and then polished half way through to expose a cross-section of the material. Probe-polishing of the selected sample slides was conducted at the Saskatchewan Research Council, Saskatoon, Canada. Electron microprobe analysis for major elements (crystal chemistry), characteristics on nine selected samples was conducted at the electron microprobe laboratory, Faculty of Earth and Planetary Sciences, McGill University, Montreal Canada. Determination was done using an electron probe micro-analyzer with wavelength dispersive X-ray spectrometry (JXA JEOL- 8900L). Operating conditions were 15kV accelerating Voltage, 20nA beam current, and 10um beam size, using the oxide ZAF (atomic number, absorption, and fluorescence) matrix correction program. The structural formulae of the amphiboles were calculated using the stoichiometric limit method (Deer et al, 1992), and on the basis of 23 (O,OH,F,Cl). Also, the proportion of ferric iron in the electron microprobe analysis was estimated after ensuring that the following criteria were satisfied: Si ≤ 8, ∑Ca ≤ 15, ∑K ≤ 16; ∑ Al ≥ 8, ∑ Mn ≥ 13 and ∑ Na ≥ 15. Ferric iron is estimated to be equal to (23∑Ox) x 2, where ∑Ox is the sum of the oxygen in the normalized formula (∑Ox = ∑R 4+ x2 + ∑R3+ x1.5 + ∑R2+ + ∑R1+ x 0.5, where ∑R= the sums of cations with the same valence). The number of moles of FeO equal FeT –Fe3+, where FeT = total Fe in the normalized formular, (Leake, 1998). 3.
Results
FeO is from 29.40wt% (at Gonzi) to 31.19wt% (at Gilling) for the hornblende biotite granite; 34.15wt% to 34.38wt% in the riebeckite granite; It is 19.94wt% and 33.19wt% in the gabbro and hornblende fayalite granite respectively. The relatively high Fe, Ca and Mg in the Fier River Gabbro and Gate Hornblende-fayalite-granite are probably due to their respective mafic and intermediate (petrogenetic) affinities, (Table 1.). In Alvi vs Mg’ (Mg’ = Mg/(Mg+FeT) plot, amphiboles of riebeckite granite and fier river gabbro are richer in Al vi compared to the Gilling, Gonzi and Kumbul hornblende biotite granites. Conversely fier-river gabbro and Gonzi hornblende biotite granite display a relatively higher Mg/(Mg+FeT) than riebeckite granite, gilling and Kumbul hornblende granite as well as Gate hornblende-fayalite granite, (Figure 4). 4.
Discussions
In the general classification diagram of leake, (1998), the amphiboles from the studied rocks are hornblendes, plotting in the ferroedenite field. Amphiboles of Gilling, Gonzi, Kumbul and Gate hornblende granites form a cluster close to the boundary between lower ferroedenite and hastingsite. They could be referred to as hastingsitic ferroedenite. Gonzi hornblende-biotite-granite shows a relatively high Mg content of 0.847; this may be as a result of its activity and/or availability and insufficient Fe in the Y position. Fier river gabbro on the other hand, plot separately towards the edenite-ferroedenite boundary line, displaying a higher Mg’-Si ratio (Fig. 2). Riebeckite granite plot at zero Mg’ and low Si content; this is probably because of its alkaline nature. The composition of the amphiboles can hardly be defined by the bulk chemistry of the host rock because their composition is found to have been controlled by the composition of the fluid phase (Offler 1984). This phase contained ionic concentrations which are dependent on mineral fluid interactions occurring in the different parts of the rocks at any time (Offter 1984). This follows that the type of ion that is released into the fluid is defined by the composition of the igneous minerals undergoing alteration, their susceptibility to breakdown, ph (oxidation/reduction potential) of the medium and temperature. Fluctuation in Si, Ti, Al, Fe2+ and Ca in time and in different part of the melt could have led to the formation of chemically inhomogeneous amphiboles (Table 1); high Fe2+ and Fe3+ results in the crystallization of Fe-rich amphiboles in the studied rocks (Offler 1984). It has been shown in experimental studies on amphiboles that Ti and Fe2+ decreases and the Mg and Fe3+ increases with increasing oxygen fugacity at constant pressure and temperature (spear, 1981). The chemistry of the amphiboles from the studied rocks indicates that oxygen fugacity had been a factor during at least, their early stage of crystallization (Offler 1984). At this time, extraction of oxygen from the fluid would result in the nucleation of phases more enriched in the cations Fe 3+ Mg, Si and Mn which are favored by higher oxygen fugacity (Spear, 1981). However, with further crystallization, variation in the activity of the cations may have been the main factor in defining composition. This is evidenced in the slight variation of composition of adjacent crystals e.g. in the hornblende-biotite granite, MgO is between 2.323wt% at Gilling and 3.656wt% at Gonzi; in the riebeckite granite, it is between 0.00wt% and 0.014wt% while in the fier river gabbro and Gate hornblende fayalite granite it is 10.45wt% and 0.956wt% respectively.
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Table 1. Microprobe Analyses of representative samples from Sara-Fier Complex. NAME
Gilling [T1(1)]
Gonzi [T8(6)]
Kumbul [T23(15)]
SiO2
Gate Hb-FaGranite P6(19)
Fier River Gabbro
40.85
42.29
40.56
49.08
49.47
49.55
40.03
40.46
46.49
TiO2
1.21
1.33
1.99
0.98
1.01
0.82
1.37
1.37
1.43
Al2O3
7.59
6.99
8.07
1.08
1.12
1.08
7.82
7.63
6.37
Cr2O3
0.00
0.00
0.00
0.00
0.01
0.00
0.00
0.00
0.00
FeO
31.19
29.40
30.36
34.23
34.38
34.15
33.19
32.88
19.94
MnO
0.65
0.92
0.69
0.57
0.54
0.45
0.60
0.54
0.26
MgO
2.32
3.66
2.43
0.00
0.01
0.00
0.96
1.18
10.45
CaO
10.75
10.23
10.10
0.69
0.44
0.40
9.96
10.00
10.84
K2O
1.22
0.96
1.21
1.19
1.19
1.21
1.30
1.13
0.65
Na2O
1.76
1.91
2.21
7.88
7.89
7.99
2.17
2.12
1.24
F
0.13
0.38
0.48
1.13
1.02
1.04
0.06
0.11
0.00
Cl
0.37
0.34
0.35
0.01
0.02
0.02
0.50
0.29
0.08
Total
97.88
98.16
98.18
96.35
96.68
96.26
97.82
97.58
97.73
Si Ti Aliv Alvi ∑ Al Cr Fe+2 Fe+3 Mn ∑Mn Mg Ca ∑ Ca Na ∑ Na K ∑K F Cl
6.54 0.15 1.31 0.12 8.12 0.00 2.94 1.24 0.09 12.38 0.56 1.85 14.78 0.55 15.33 0.25 15.58 0.07 0.10
6.57 0.16 1.17 0.11 8.00 0.00 1.91 1.91 0.12 11.94 0.85 1.70 14.49 0.58 15.06 0.19 15.25 0.20 0.09
Riebeckite granite [G23(8)]
6.29 0.23 1.35 0.12 8.00 0.00 1.16 1.78 0.09 11.03 0.56 1.68 13.27 0.66 13.93 0.24 14.17 0.25 0.10
7.90 0.12 0.00 0.21 8.22 0.00 3.13 1.47 0.08 12.90 0.00 0.12 13.02 2.32 15.34 0.25 15.59 0.62 0.00
7.98 0.12 0.00 0.21 8.32 0.00 3.45 1.19 0.07 13.03 0.00 0.08 13.11 2.47 15.58 0.25 15.83 0.53 0.01
8.00 0.10 0.00 0.21 8.31 0.00 3.63 0.99 0.06 12.98 0.00 0.07 13.05 2.50 15.56 0.25 15.80 0.54 0.01
6.41 0.16 1.29 0.14 8.00 0.00 2.12 2.24 0.07 12.43 0.28 1.70 14.41 0.65 15.06 0.23 15.29 0.03 0.14
6.37 0.16 1.34 0.13 8.00 0.00 2.23 2.18 0.08 12.50 0.23 1.70 14.42 0.67 15.09 0.27 15.36 0.06 0.08
6.75 0.16 0.93 0.16 8.00 0.00 0.06 2.36 0.03 10.46 2.26 1.69 14.41 0.35 14.76 0.12 14.88 0.00 0.02
0.33
0.00
0.00
0.00
0.12
0.09
0.97
*
Mg’
0.16 0.31 *Mg’ =Mg/(Mg + FeT)
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Table 2. Structural formulae based on chemical and stoichiometric limits of Amphiboles from Sara-Fier Complex, (After Leake, 1998).
Hb-bt-Granite
Riebeckite Granite G23(8)
Gate Hb-bt-Granite P6 (9)
Fier-RiverGabbro
Si
Gilling 6.542
Gonzi 6.567
Kumbul 6.292
7.896
7.982
8.000
6.412
6.369
6.754
Al
1.432
1.279
1.475
0.104
0.018
0.000
1.425
1.467
1.091
∑T
8.000
8.000
8.000
8.000
8.000
8.000
8.000
8.000
8.000
Al
0.000
0.000
0.000
0.101
0.194
0.206
0.000
0.000
0.000
0.145
0.155
0.232
0.119
0.123
0.100
0.164
0.164
0.156
1.236
1.912
1.783
1.474
1.187
0.986
2.238
2.183
2.363
0.555
0.847
0.561
0.000
0.003
0.001
0.280
0.226
2.263
2.941
1.906
1.156
3.132
3.453
3.629
2.120
2.234
0.060
Mn
0.088
0.121
0.091
0.078
0.074
0.061
0.072
0.081
0.033
Ca
0.035
0.059
1.177
0.096
0.000
0.017
0.126
0.112
0.125
∑C
5.000
5.000
5.000
5.000
5.000
5.000
5.000
5.000
5.000
Ca
1.810
1.643
0.501
0.023
0.076
0.053
1.567
1.586
1.562
Na
0.190
0.357
0.664
1.977
1.924
1.947
0.433
0.414
0.350
∑B
2.000
2.000
1.165
2.000
2.000
2.000
2.000
2.000
2.000
Na
0.355
0.218
0.000
0.343
0.545
0.556
0.218
0.255
0.000
K
0.249
0.190
0.240
0.245
0.246
0.249
0.228
0.265
0.120
∑A
0.604
0.408
0.240
0.588
0.791
0.805
0.446
0.520
0.120
Total
15.604
15.408
14.573
15.588
15.791
15.805
15.446
15.520
15.120
Ti Fe
3+
Mg Fe
2+
The observed correlation between Fe-Mg and pressure has some implications for the interpretation of magmatic oxygen fugacity in the sense that, if two magmas of similar bulk composition crystallize under the same temperature and oxygen fugacity conditions but at different depths, then amphiboles in the shallower intrusion will have higher (Mg/Mg+FeT) than amphiboles in the deeper one (Ague, 1989; Fig. ). Thus, the difference in amphibole Mg/(Mg+FeT) between the intrusive of the studied rocks do not reflect much of magmatic oxygen fugacity differences, but is instead a consequence of consolidation at different pressure. Mg/(Mg+FeT) also decreases with increase in amphibole Alvi and Al (Fig. 4). Correlations of Mg/(Mg+FeT) with other compositional variables, such as amphibole Al content, appear to be weak (Ague, 1989). The distribution of Fe2+ and Mg on the M1, M2 and M3 octahedral sites of amphibole was specified, the partitioning of Fe 2+ and Mg between amphibole and biotite was calculated as a function of temperature and pressure (Ague, 1989). In natural calcic amphiboles, there is a tendency for Fe2+ to substitute preferentially into M1 and M3, whereas Mg may be ordered on M2 (Hawthorne, 1981). The amphiboles of the studied rocks have low proportion of Al vi, which could have led to higher Mg/(Mg+FeT) and thus very little or no exclusion of Mg from M2 by Alvi substitution (Hammarstrom and Zen, 1986). However, from limited amount of A-site occupancy data available, an overall pattern of Fe2+ and Mg ordering is not apparent. Indeed, it has been found that M1, M2 and M3 have quite similar Fe2+ contents (Hawthorne, 1981). However, because amphibole Al iv, Alvi and A-site content may be related by studied rocks coupled substitutions, it is unclear whether the dominant cystallochemical control, on amphibole Mg/(mg+feT) is Al content or the presence of A-site cations. It should be pointed out that if the alkalis in amphiboles contribute to the observed Mg/(Mg+feT) variations, then the correlation is probably an indirect one because oxygen adjacent to the octahedral sites are not directly coordinated
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with A-site cations; it is sufficient to conclude that Fe-Mg is a strong function of amphibole compositions, such that decreases in amphibole Al, Na and K lead to increase Mg/(Mg+feT). The Al content of amphibole in granitic magmas increases linearly with increase in pressure of crystallization. This fact is important implication for the geochemical systematic of Fe and Mg in coexisting amphibole-biotite pairs because of the observed dependence of Fe-Mg partitioning upon amphibole total Al content (Ague, 1989). Fe-Mg distribution and Al content of amphibole are related, such that decreasing Mg/(Mg+FeT) is correlated with increasing amphibole total Al (Hawthorne, 1983). In all the analyses (Table 1.), the 23 (O+OH+CL+F) formula normalization procedure assigned Al between 0.3% to 19% to fill the balance of tetrahedral sites because of insufficient Si; Ca is assigned partly to C-sites and then B-sites, while Na is assigned between B-sites and A-site. The analysed samples therefore have their A-sites vacant, which were partially filled with Na and/or K. It is also observed from results of the studied rocks that, increasing Fe correlates with decrease in Mg and Ti.
LEGEND Graph 1 Riebeckite granite Gilling Hornblende-biotite-granite Gonzi Hornblende-biotite-granite Kumbul Hornblende-biotite-granite Gate hornblende-fayalite-granite Fier river gabbro 1
Pargasite Al>Fe 0.8
Magnesiosadanagaite
mg'=(mg/mg+Fe)
Edenite Magnesio hastingsite Al < Fe
0.6
0.4
Ferropagasite (Al>Fe)
Ferro Edenite
Sadanagaite 0.2
Hastingsite (Al
