Classification of minerals from post-collisional granite

CHAPTER 5. RESULTS

5.2. Mineral classification

5.2.2. Classification of minerals from post-collisional granite

a. Biotite

The classification diagrams for biotites (Figure 5.5) indicating all biotites from post-collisional granites are primary magmatic biotite (Figure 5.5c) which close to Fe-biotite to siderophyllite end-member (Figure 5.5a, b). They progressively increase in the Fe# ratio from the south to the north of the 560 Ma plutons and from north to south of 550 Ma plutons, and from the east to the west of 570 Ma plutons.

Specifically, the biotite of the granite from the eastern part of 570 Ma pluton (570 Ma-E, 14ZA01), the southern part of 560 Ma intrusion (560 Ma-S, 14ZA10) and 550 Ma Ngoura (NA01, -03, -06) are biotite II. Meanwhile, the western part of 570 Ma intrusion (570 Ma-W, 14ZA23), the northern part of 560 Ma intrusion (560 Ma-N, 14ZA18, -19B, -20B, -21B), and 550 Ma Moyto intrusion are biotite I (Figure 5.5, Figure 6.7).

Figure 5.5. Classification of biotite from post-collisional granites in Guéra massif

and Lake Fitri region. (a) The Fe^t/(Fe^t+Mg²⁺) versus total Al content diagram (ASEP diagram) after Rieder et

al. (1998), and the tectonic discrimination line from Shabani et al.

(2003), Fe^t= Fe²⁺+ Fe³⁺; (b) (Al^iv+Fe³⁺+Ti)-Mg-(Fe²⁺+Mn) diagram

after Foster (1960); (c) (FeO+MnO)-10*TiO2-MgO diagram after Nachit et al.

(2005) * denote data from Shellnutt et al.

(2018).

71 b. Amphibole

Similar to collisional granites, the post-collisional granites contain calcium amphibole ranging from ferro-pargasite to ferro-edenite with extremely low Mg#

(0.03-0.29), which is generated by primary magma (Figure 5.6).

Figure 5.6. Classification of amphibole from post-collisional granites in Guéra massif. (a) Amphibole classification for calcic group recommended by the IMA in the modified version

of Hammarstrom and Zen (1986); (b) Amphibole classification follows the IMA recommendation (Leake, 1978) for the calcic group; (c) The plot of

amphiboles in this study in terms of cations (Ca+Na+K) versus Si (apfu) diagram (after Giret

et al., 1980).

72 5.3. Mineral composition

Biotite is the greatest common mafic mineral observed in the Guéra granites. Their chemical compositions are listed in Appendix 3 and the concentrations of major elements are plotted in Figure 5.7 and Figure 5.9. They display a trend of increasing Fe^t and Mn content but decreasing in Mg content through time. The Fe³⁺/(Fe³⁺+Fe²⁺) ratio of biotites are distinct among them, the collisional granite shows a wide range from 0 to 0.99, compared to a narrow range of 0-0.39 and 0-0.14 apfu for 570 Ma and 560 Ma post-collisional granite, respectively. The biotites from the collisional granites have lower Ti content (0.13-0.47 apfu) than the biotites from the post-collisional rocks (0.18-0.63, 0.1-0.5 apfu for 570 Ma- and 560 Ma-granite, respectively). The biotites biotites are fresh and satisfy the requirements to estimate the crystallization temperature of biotite (Stone et al., 2000; Liu et al., 2010).

In this study, amphiboles were mostly found in post-collisional granites (≤570 Ma) but rarely observed in collisional granite (590 Ma). The chemical compositions are shown in Appendix 2, and the major elements of amphibole are summarized in Figure

5.8 and Figure 5.10. All amphiboles show a narrow compositional range with the low magnesium number (Mg# = 0.03-0.35). Overall, the chemical composition of amphibole shows a systematic decreased in Si, Mg, Mn, and total Al content and increased in Ti and total Fe content from oldest to youngest rocks. Moreover, the amphibole from the collisional granite is slightly higher in Mg# (0.2-0.33, Figure 5.4a) in comparison with post-collisional granites (0.03-0.29, Figure 5.5a). This characteristic suggests that the collisional granite crystallized from magma of higher silica activity and magnesium content, but less aluminum activity than post-collisional granites.

74 5.3.1. Post-collisional granite

a. Biotite 14ZA10

Biotite of sample 14ZA10 is characterized by Fe# = 0.83-0.86, SiO2 = 31.96-36.17 wt%, TiO2 = 1.68-3.7 wt%, Al2O3 = 15.21-17.58 wt%; silica (Si = 5.28-5.69 apfu), total aluminum (Al^t = 2.87-3.29 apfu), calcium ( 0.36 apfu), magnesium (Mg = 0.59-0.76 apfu) titanium (Ti = 0.2-0.44 apfu), total iron (Fe^t = 3.52-4.13 apfu), manganese (Mn = 0.06-0.09 apfu) and alkali content (Na+K = 1.09-1.94 apfu) (Figure 5.7).

14ZA18

The main composition of biotite of sample 14ZA18 consists of SiO2 = 32.74-34-37 wt%, TiO2 = 2.62-3.42 wt%, Al2O3 = 12.69-13.78 wt%; Si = 5.50-5.67 apfu, Al^t = 2.47-2.70 apfu, Ca  0.004 apfu, Mg = 0.3-0.39 apfu, Ti = 0.33-0.43 apfu, Fe^t = 4.56-5.11 apfu, Mn = 0.03-0.06 apfu, and Na+K = 11.61-1.89 apfu (Figure 5.7), with Fe#

ranging from 0.92-0.94. Na+K = 1.72-1.89 apfu (Figure 5.7), with Fe# ranging from 0.91-0.95.

14ZA20B

Biotite of sample 14ZA20B consist of main components as SiO2 = 30.42-35.73 wt%, TiO2 = 1.78-3.5 wt%, Al2O3 = 12.35-15.41 wt%; Si = 5.17-5.78 apfu, Al^t =

75 Fe³⁺/(Fe³⁺+Fe²⁺) (0-0.14), Fe# (0.73-0.78) ratio and major components range in atom per formula unit from 5.39 to 5.71 of silica, 0.18 to 0.46 of titanium, 3.31 to 4.11 of total iron, 0.04 to 0.09 of manganese, 1.54 to 1.93 of alkali, 2.56 to 2.90 of total aluminum, 0-0.27 of calcium, 1.02 to 1.31 of magnesium content (Figure 5.7); and range in weight percent from 32.6 to 36.16 of SiO2, from 1.52 to 3.92 of TiO2, from 13.42 to 15.35 of Al2O3.

14ZA23

Compared to sample 14ZA01 in the 570 Ma group, the chemical compositions of biotite of sample 14ZA23 generally has a wide range of SiO2 (30.29-41.66 wt%), TiO2

(2.34-4.89 wt%), Al2O3 (10.94-15.56 wt%), silica (5.31-6.36 apfu), titanium (0.3-0.63 apfu), manganese (0-0.08 apfu) and total aluminum (1.97-3.17 apfu, and higher in Fe#

ratio (0.92-0.97), total iron (3.77-5.33 apfu) but lower than magnesium (0.17-0.43 apfu) content (Figure 5.7).

Figure 5.7. The major element composition of biotite in the post-collisional granite during magma evolving. apfu: atom formula unit.

77 b. amphibole

14ZA18

Amphibole of sample 14ZA18 is characterized by an average compositional range of Mg# = 0.05-0.14, SiO2 = 36.25-40.72 wt% and Al2O3 = 8.16-9.62 wt%, Fe^t = 3.9-4.39 apfu, Ti = 0.1-0.28 apfu, Na+K = 0.75-0.94 apfu, Ca = 1.75-1.97 apfu, Si = 6.21-6.64 apfu, Mg = 0.2-0.59 apfu, Mn = 0.04-0.09 apfu, Al^t = 1.57-1.86 apfu, Al^iv = 1.36-1.69 apfu (Figure 5.8).

14ZA19B

The spread of amphibole composition average values in atom per formula unit of sample 14ZA19B ranges from 3.96 to 4.39 of total iron, 0.09 to 0.28 of titanium, 0.48-0.92 of alkali, 1.47 to 1.90 of calcium, 6.28 to 6.89 of silica, 0.25 to 0.51 of

78 14ZA21B

In sample 14ZA21B, most of the major element of amphibole has a relatively wider and higher compositional range as compared to others, such as Fe^t = 4.23-4.58, Ti = 0.05-0.23, Na+K = 0.42-1.17, Ca = 1.774-2.36, Si = 6.35-7.18, Mg = 0.1-1.17, Mn = 0.07-0.13, Alt = 1.41-3.50, Al^iv = 1.34-2.12 apfu (Figure 5.8), but lower in Mg#

= 0.03-0.08, SiO2 = 38.03-41.02 wt% and Al2O3 = 7.18-8.83 wt%.

14ZA01

The major component of amphibole of sample 14ZA01 is characterized by Mg # ratio from 0.23 to 0.24, SiO2 from 40.67-41.21 wt%, Al2O3 from 8.25 to 9.13 wt%, total iron from 3.61 to 3.69, titanium from 0.11 to 0.24, alkali from 0.73 to 0.91, calcium from 1.76 to 1.84, silica from 6.48 to 7.18, magnesium from 0.89 to 0.98, manganese from 0.09 to 0.13, total aluminum from 1.55 to 1.70 and tetrahedral aluminum 1.46 to 1.53 apfu (Figure 5.8).

14ZA23

Amphibole of sample 14ZA23 consists of major elements that have a compositional average range following as Mg# = 0.05-0.06, SiO2 = 39.16-53.69 wt%, Al2O3 = 6.69-8.69 wt%, Fe^t = 3.00-4.29, Ti = 0.09-0.12, Na+K = 0.42-0.70, Ca = 1.24-1.78, Si = 6.48-8.17, Mg = 0.14-0.22, Mn = 0.09-0.12, Al^t = 1.20-1.77, Al^iv = 0-1.52 apfu (Figure 5.8).

Figure 5.8. The major element composition of amphibole in the post-collisional during magma evolving.

80 5.3.2. Collisional granite

a. Biotite 14ZA16A

The major chemical composition of biotite of sample 14ZA16A is characterized by high SiO2 (33.66-35.64 wt%), Al2O3 (13.23-14.59 wt%), TiO2 (1.58-3.84 wt%), silica (5.26-6.08 apfu), total iron (3.81-4.76 apfu), relatively low Fe# (0.7-0.76) magnesium (0.15-0.8 apfu) content, and the same range of titanium, manganese, alkali, total aluminum and calcium (0.17-0.42, 0.03-0.12, 1.57-1.88, 2.56-3.16, 0-0.02 apfu, respectively) (Figure 5.9).

14ZA12D

Similar to the sample 14ZA16A, biotite of sample 14ZA12D has relatively higher Fe# ratio (0.92-0.98), SiO2 (31.14-39.78 wt%), Al2O3 (13.94-16.21 wt%), silica (5.47-5.69 apfu), total iron (3.55-4.03 apfu), lower magnesium (1.25-1.59 apfu) content, TiO2 (1.32-3.41 wt%) and extremely poor calcium (≤0.02 apfu) as compared to others (Figure 5.9). Besides, the concentration of total iron and magnesium of sample 14ZA16A and -12D indicates a transitional trend from Mg-biotite to Fe-biotite in the biotite classification diagram (Figure 5.3a, b).

14ZA02

Biotite of sample 14ZA02 is chemically distinct by the following average compositional range: Si = 4.85-6.24 apfu, Ti = 0.19-0.46 apfu, Fe^t= 1.6-3.21 apfu, Mn

= 0.02-0.05 apfu, Na+K = 0.46-1.87 apfu, Al^t = 2.66-3.4 apfu, Ca = 0-0.19 apfu, Mg = 1.63-3.31 apfu (Figure 5.9); Fe# = 0.46-0.49, SiO2 = 28.48-42.63 wt%, Al2O3 = 14.98-17.54 wt%, TiO2 = 1.49-3.98 wt%.

81 14ZA06

Biotite of sample 14ZA06 consists of Fe# = 0.41-0.51, SiO2 = 34.79-36.56 wt%, Al2O3 = 15.39-16.43 wt%, TiO2 = 1.36-3.27 wt%, Si = 5.44-5.63 apfu, Ti = 0.16-0.38 apfu, Fe^t= 2.69-2.99 apfu, Mn = 0.02-0.05 apfu, Na+K = 1.35-1.93 apfu, Al^t = 2.84-3.02 apfu, Ca ≤0.01 apfu, Mg = 1.78-2.24 apfu (Figure 5.9).

14ZA25C

Biotite of sample 14ZA25C has similar average compositional range to sample 14ZA02 and -06, following as Fe# = 0.49-0.57, Si = 5.37-5.62 apfu, Ti = 0.13-0.47 apfu, Fe^t = 2.41-2.81 apfu, Mn = 0.01-0.04 apfu, Na+K = 1.61-1.90 apfu, Al^t = 2.56-3.12 apfu, Ca ≤0.08 apfu, Mg = 1.89-2.54 apfu. Noticeably, the sample 14ZA25C, -02 and -06 contain the lowest range of total iron and the highest range of magnesium content (Figure 5.9) that are characteristic for the Mg-biotite bearing granites, consistent with ferroan whole-rock compostions (Figure 5.2a).

Figure 5.9. The major element composition of biotite in the collisional granite during magma evolving.

83 b. Amphibole

14ZA16A

Amphibole of sample 14ZA16A contains significant components that have compositions of: SiO2 = 34.11-39.7 wt%, Al2O3 = 10.44-11.78 wt%, Fe^t = 3.6-4.01 apfu, Ti = 0.01-0.12 apfu, Na+K = 0.03-0.70 apfu, Ca = 1.53-1.90 apfu, Si = 6.52-7.77 apfu, Mg = 0.74-1.36 apfu, Mn = 0.1-0.16 apfu, Al^t = 0.12-1.61 apfu, and A^liv = 0.12-1.40 apfu (Figure 5.10), with Mg# = 0.2-0.26.

14ZA12D

Amphibole of sample 14ZA12D has relatively higher Mg# = 0.2-0.33, SiO2 = 41.26-50.12 wt%, in total iron (3.57-4.24 apfu), alkali (0.76-0.97 apfu), calcium (1.72-1.94 apfu), total aluminum ((1.72-1.94-2.27 apfu) and tetrahedral aluminum (1.66-2.27 apfu), but lower in Al2O3 = 0.67-8.69 wt%, and same range of titanium (0.1-0.19 apfu), silica (5.57-6.34), magnesium (0.74-0.99 apfu) and manganese (0.05-0.09 apfu) in comparison with 14ZA16A (Figure 5.10).

Figure 5.10. The major elements composition of amphibole in the collisional granite during magma evolving.

CHAPTER 6. DISCUSSION

6.1. Temperature estimates

The presence of Fe-Ti oxides in granitic rocks is the most convenient mineral pair to calculate a magmatic temperature estimate (Buddington and Lindsley, 1964;

Ghiorso and Sack, 1991). However, the sensitivity of Ti and Ti/Fe²⁺ in biotite to temperature, it can also be used to obtain reliable temperature estimation in igneous and metamorphic rocks (Luhr et al., 1984; Moradi et al., 2017). Granitic rocks from Guéra massif are dominated by hornblende, biotite, plagioclase, potassium feldspar and quartz, and a lesser amount of apatite, oxides minerals (hematite, ilmenite, magnetite), titanite, and sulfides. This feature satisfies the requirement of temperature estimated based on compositions of Fe-Ti oxides. Thus, this study used the equation of Luhr et al. (1984) to calculate the biotite crystallizes temperatures based on the coupled exchange of Ti and Fe²⁺ in biotite, where temperatures are known from Fe-Ti oxides in his study. The biotites from the Guéra massif and Lake Fitri region show average temperatures are given in Table 6.1, which are resulted from equation (1) and the detail results have given in Appendix 4. Overall, the biotite from the collisional granites crystallized at higher temperatures (639.2 ± 73^oC) than the Guéra and Lake Fitri post-collisional granites which yielded similar temperature (619.6 ± 44^oC, 615.8

± 46^oC and 612.9 ± 45^oC for 570 Ma-, 560 Ma-, 550 Ma-granite, respectively) range.

Table 6.1. The range of crystallization temperature of biotites from Guéra massiff and Lake Fitri region estimates based upon biotite composition.

Pluton Sample Temperature (^oC)

87 6.2. Pressure estimates

The estimate of crystallization pressure of a pluton is necessary for understanding its evolution as well as the reconstruction of petrologic mechanisms of its formation.

This estimation is commonly constrained by an appropriate mineral assemblage that mainly occurs in granitic rocks. Recently, pressure can be estimated based on both biotite (Uchida et al., 2007) and amphibole composition (Hammarstrom and Zen, 1986; Hollister et al., 1987; Johnson and Rutherford, 1989; Schmidt, 1992). This study has utilized both biotite and amphibole compositions to estimate pressure.

Amphibole is one of the most common mafic minerals in calk-alkaline rocks and is that stable at water magmatic content above ca. 2% and within the pressure range of 1.5-12 kbars. The chemistry of amphibole is susceptible to intensive parameters such as temperature, pressure, oxidation state, and water fugacity (Spear, 1981; Wones, 1981; Helz, 1982). The amphibole composition broadly reflects the parameters of the parental melt but also varies with such features such as bulk composition. Thus, many geothermobarometers are calibrated using the composition of calcic amphiboles (Hammarstrom and Zen, 1986; Hollister et al., 1987; Johnson and Rutherford, 1989;

Blundy and Holland, 1990; Thomas and Ernst, 1990; Schmidt, 1992; Holland and Blundy, 1994; Anderson and Smith, 1995; Anderson, 1996; Ridolfi et al., 2010; Mutch et al, 2016). To estimate the solidification pressure of a calc-alkaline granitoid body by using the concentration of Al in hornblendes, it is generally assumed that the hornblende barometer and the host rocks are the same in the equilibrium pressure, and also similar to the pressure of emplacement of the pluton. The computed pressure is controlled by ion substitutions in hornblende such as the substitution between sodium

and calcium, or among magnesium, ferrous iron, and manganese, etc., oxygen fugacity, volatiles, and magma composition. Some doubt there are adequate constraints for an accurate geobarometer estimate using amphibole given all of the parameters that could potentially affect the result (Hollister et al., 1987; Ghent et al., 1991). Also, the computed pressure may display the depth at which the hornblende crystallizes but may not represent the pressure at which the granite nucleates because its integration could occur after hornblende crystallization (Ghent et al., 1991).

The granitic rocks from Guéra massif have similar mineral assemblages as granitic rocks in the study of Hammarstrom and Zen (1986) which permits the application of hornblende geobarometer of Hammarstrom and Zen (1986). To estimate the emplacement pressure of Guéra granitic rocks based on the composition of amphibole (hornblende) by the empirical equation (2).

P = -3.92 + 5.03Al^T ± 3 kbar (2).

Where Al^T designates the total number of Al atoms in amphibole based on 22 oxygen. The calibration is based on the relation between Al^Tin hornblende and pressure for calc-alkalic granitic rocks that are made up of an igneous mineral assemblage consisting of hornblende, plagioclase, biotite, K-feldspar, quartz, sphene, magnetite or ilmenite, ± epidote.

All obtained pressures of the studied amphiboles show consistency with the calibration reference line of hornblende geobarometer from Hammastrom and Zen (1986) (Figure 6.4a). The results show that the early crystallization phase of granitic rocks from Guéra massif might have occurred at 4.65 ± 0.6, 4.3 ± 1.3 and 4.47 ± 0.2 kbars for 590 Ma, 570 Ma, and 560 Ma granite, respectively. In particular, the

pressure peak of plutons follows as ~6 kbars (N = 26), ~3 kbars (N = 9), ~5 kbars (N

= 3), ~4 kbars (N = 84) for 590 Ma-N, 590 Ma-S, 570 Ma-W and 560 Ma-N pluton (Figure 6.1). The average values of pressure obtained are given for each pluton corresponding to rock-type in Table 6.2. The detailed results are shown in Appendix 2 and illustrated in Figure 6.1.

Table 6.2. The range of amphibole pressure of Guéra granies obtained based on the amphibole composition.

Pluton Sample Pressure (kbars)

Max Min Average

590 Ma-N 14ZA16A 7.49 5.83 6.17 ± 0.6 (2)

590 Ma-S 14ZA12D 4.16 0.21 2.40 ± 0.7 (2)

570 Ma-W 14ZA01 4.63 3.85 4.20 ± 1.5 (2)

570 Ma-E 14ZA23 5.00 5.00 5.00 ± 3.0 (2)

560 Ma-N 14ZA18 5.43 3.97 4.78 ± 0.4 (2)

560 Ma-N 14ZA19B 5.31 1.68 4.62 ± 0.4 (2)

560 Ma-N 14ZA20B 13.66 0.82 4.60 ± 1.7 (2)

560 Ma-N 14ZA21B 4.61 3.17 4.14 ± 0.7 (2)

Figure 6.1. The average pressure inferred from amphibole composition including propagation error and its relative probability in respect of plutons in Guéra massif.

Unfortunately, amphibole is not present in all of the studied samples. Thus, there is a limited application to the granitic rocks from the Guéra massif granites. Uchida et al. (2007) demonstrated that the total Al content of amphibole and total Al content of biotite increase together with increasing pressure, in other words, the solidification pressure of granitoid constrains the total concentration of Al in biotite. Therefore, Al-in-biotite can be utilized instead of the Al-in-hornblende for geobarometry. They suggested an empirical equation to calculate pressures by using biotite composition

for granitic rocks containing a mineral assemblage of plagioclase + biotite + muscovite + hornblende + K-feldspar + magnetite + ilmenite:

P (kbars) = 3.33Al^T – 6.53 (±0.33) (3)

where Al^T designates the total number of Al atoms in biotite on the basis of 22 oxygen.

The similarity in the mineralogical composition of Guéra granites and the mineral assemblage employed for equation (3), and the temporal and compositional similarities between Guéra granites and Lake Fitri granites (Shellnutt et al., 2018) permit the application of the Al-in-biotite geobarometry in this study. The calculated average pressures of crystallization based upon biotite composition from granitic rocks of the Guéra massif and Lake Fitri region by using equation (3) are shown in Figure 6.2. The average values for individual pluton are summarized in Table 6.3 with the following pressure peaks: 595 Ma = 2.2 kbar (N = 19), 590 Ma-N = 1.35 kbar (N

= 20), 590 Ma-S = 2.8 and 2.2 kbar (N = 6 and 4, respectively), 570 Ma-W = 1.8 kbar (N = 9), 570 Ma-E = 1.6 kbar (N = 14), 560 Ma-N = 1.3 kbar (N = 23), 560 Ma-S = 2.8 kbar (N = 18), 595 Ma-Ngoura = 4.0 kbar (N = 14), 550 Ma-Moyto 1 = 4.8 kbar (N = 10), 550 Ma-Moyto 2 = 3.4 and 3.6 kbar (N = 7 and 7, respectively) (Figure 6.2).

The detail results are presented in Appendix 4. As a result, the biotite crystallization phase of collisional granite occurred at an average pressure of 1.87 ± 0.06 kbars.

Meanwhile, the biotite of post-collisional granites has variable pressure (1.80 ± 0 .06, 2.10 ± 0.09, 4.12 ± 0.12 kbars for 570 Ma, 560 Ma, 550 Ma granite, respectively).

Moreover, the estimated solidification pressures of biotite from studied rocks show a positive correlation with the total aluminum content of biotite, which is consistent

with the calibration reference curve of Uchida et al. (2009) (Figure 6.4b) indicating practicability of biotite geobarometer in this study.

Table 6.3. The range of biotite pressure computed for granites from Guéra massiff and Lake Fitri region.

Pluton Sample Biotite Pressure (kbars)

Max Min Average

Figure 6.2. The average pressure inferred from biotite composition, including propagation error and its relative probability in respect of plutons in Guéra massif (this study) and Lake Fitri region (data

from Shellnutt et al., 2018).

The concentration of TiO2 and Al^vi closely link to the depth emplacement of the pluton. In particular, the granites having low TiO2 and high Al^vi content are typical for an abyssal depth of formation. Meanwhile, granites that crystallize at deeper depth are characterized by high TiO2 and Al^vi content (Machev et al., 2004; Bora and Kumar, 2015). In this study, the TiO2 and Al^vi content in biotite of collisional granite are TiO2

= 1.12-4.05 wt%, Al^vi= 0.05-0.9 apfu. The values of post-collisional granites follow as TiO2 = 1.52-4.89wt%, Al^vi = 0.07-0.55 apfu, for 570 Ma granite; TiO2 = 0.72-4.03wt%, Al^vi = 0.1-0.82 apfu, for 560 Ma-granite; TiO2 = 1.32-3.97wt%, Al^vi = 0.46-1.15 apfu, for 550 Ma-granite. The Lake Fitri post-collisional granite has relatively high Al^vi contents and low TiO2 content (Figure 6.3d), indicating the deepest depth of crystallization among studied granites. The relatively high TiO2 and Al^vi contents in Guéra granites (Figure 6.3a, b, c) suggest that they crystallized at a mid-crustal level.

Figure 6.3. The plot of titanium oxide and octahedral aluminum content in biotite showing level crystallization of depth. * denotes data from Shellnutt et al. (2018).

Figure 6.4. (a) Comparison of hornblende geobarometers, with calibration curves obtained by a sphalerite geobarometer (after Uchida et al., 2007); (b) Biotite geobarometer in this study within a

calibration curve of biotite geobarometer for granitic rocks from Uchida et al. (2007).

96 6.3. Oxygen fugacity estimates

Oxygen fugacity (O2) is an essential parameter constraining magmatic activities (Kilinc et al., 1983; Kress and Carmichael, 1991; Ottonello et al., 2001; Botcharnikov et al., 2005). It is considered as a useful tool for determining the redox condition of melts to understand petrogenesis, due to its effect on aspects of magma sequences such as the composition of crystallizing minerals, the differentiation of magma (Carmichael, 1991), geophysical properties of magma, e.g., melt structure and viscosity (Jayasuriya et al., 2004).

The empirical equation (4) to calculate the oxygen fugacity suggested by Eguster and Wones (1962), is built based upon the compositions of biotite based on the relation of O2 with P and T for various oxygen buffers and the relationships between oxygen buffers and Fe²⁺- Fe³⁺- Mg²⁺ in compositions of biotite.

Where T is the temperature in K, P is the pressure in bar, and A, B, and C are corresponding coefficients for different oxygen fugacity buffers. T is calculated by equation (1). P is calculated by equation (3).

This is one of the most common methods used to apply in oxygen fugacity estimation of granitic rocks from different environments (e.g., Lalonde and Bernard, 1993; Yavuz 2003a, b; Semiz et al., 2012; Zhang et al., 2013; Shen and Pan, 2015;

Zheng et al., 2015, Li, et al., 2017).

The calculation of O2 depends on the value of coefficients A, B, and C that represent different oxygen fugacity buffers (Eugster and Wones, 1962). The coefficients are chosen based on the proportions of Fe²⁺, Fe^3+, and Mg²⁺ in biotite,

which are calculated from EPMA data using the charge balance method (Nenova, 1997). Wones and Eugster (1965) established the Fe²⁺-Fe³⁺-Mg²⁺ternary diagram using the contents of biotite (Figure 6.5), where the QFM, NNO, and HM buffers are defined based on experimental data. In the ideal case, the A, B, and C values are able to be directly obtained from Eugster and Wones (1962) for a given buffer, if the data plot directly on one of the buffers. In fact, there are many cases indicating the biotite data falls on two buffers, that lead to complications in the calculation. The corresponding value of the coefficient will be assigned for the buffer that is closest to the data point (Yavuz, 2003a, b; Ayati et al., 2013, Li et al., 2017), which may produce significant errors in cases of data that plot relatively far from the buffer. In this study, all samples plot along with the QFM and NNO buffers (Figure 6.5), therefore, the value of A, B, and C coefficients are interpolated between these buffers. In which, the QFM buffer is given for samples of 550 Ma-Moyto 1, 560 Ma-N pluton, 570 Ma-E pluton, and 590 S pluton; meanwhile, the NNO buffer is designated for 550 Ma-Ngoura and Moyto 2, 560 Ma-S pluton, 570 Ma-W pluton, and 595 Ma pluton. The average values of logO2 and their corresponding values to ∆QFM are summarized in

在文檔中 An assessment of the magmatic conditions of Late Neoproterozoic collisional and post-collisional granites from the Guéra Massif, South-Central Chad (頁 85-0)