Temperature estimates - An assessment of the magmatic conditions of Late Neoproterozoic collisi

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 Table 6.4. The detail results are listed in Appendix 4.

Table 6.4. The range of oxygen fugacity for granites from Guéra massif and Lake Fitri region estimated based on biotite composition.

Pluton Sample

Figure 6.5. Composition of biotite from granitic rocks of Guéra massif and Lake Fitri region in the Fe²⁺-Fe³⁺-Mg²⁺after Wones and Eugster (1965) along with the three common oxygen fugacity (fO2) buffer: hematite-magnetite (HM), Nickel-Nickel oxide (NNO) and quartz-fayalite-magnetite (QFM). *

denotes data from Shellnutt et al. (2018).

The diagram indicating the relationship between oxygen fugacity and temperature of this study (constructed by Wones, 1981) (Figure 6.6) shows the change from oxidizing to reducing environment during magma evolution. In particular, the collisional granites crystallized at or near the NNO and QFM buffer, under oxidizing conditions rather than reduced conditions. The Guéra post-collisional granites display a transition from QFM to WM (wüstite-magnetite) buffer, according to the change of redox condition from oxidizing to reducing; while the Lake Fitri post-collisional granites fall between QFM and NNO under oxidizing condition.

Lalonde and Bernard (1993) mentioned that the concentration of FeO, TiO2, and MgO closely relates to the color of biotite as observed in thin section. Reddish biotite is enriched in both total Fe and Fe2+, probably including Ti present as Ti3+ which are typical for reduced peraluminous granites; however, green or greenish-brown biotite formed in oxidizing conditions, e.g., arc-related setting, showing the trend of

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enhancement in Mg and Fe3+ content. Despite the variety in color, all biotites in this study are predominately brown, red-brown, and greenish to green color with respect to Guéra post-collisional granites and collisional-granite. Interestingly, this distinctive feature of biotite color is consistent with the trending concentration of total Fe, Fe2+, Ti and Mg in investigated rocks (see section 5.3) and their behavior of oxygen fugacity on the logO2-T diagram (Figure 6.6). Hence, the Guéra post-collisional rocks are likely reduced peraluminous granites, whereas, the collisional rock is likely oxidized granites.

Figure 6.6. The correlation between estimation values of oxygen fugacity in log unit and temperature of the granitic rocks from Guéra massif ad Lake Fitri region (after Wones, 1981). The sample plotted areas represent for oxygen fugacities trend of crystallizing magmas. HM: hematite-magnetite buffer;

TMQ: titanite-magnetite-quartz buffer; Hd Il: hedenbergite-ilmenite buffer; QFM: quartz-fayalite-magnetite; GC: graphite-CO2 equilibrium. In which, HM is referred to the highest level of oxidizing condition, the direction from QFM toward HM is an oxidizing (Ox) trend of magma crystallization by

the addition of oxygen or hydrogen loss during crustal emplacement; while the opposite direction is reducing (Rd) trend of magma by the addition of hydrogen or carbon. * denotes data from Shellnutt et

al. (2018).

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6.4. The change in magmatic conditions during magma course

The magmatic conditions estimated for the Neoproterozoic granites from the Guéra massif (Figure 6.7) show a systematic variation from high to low in temperature, pressure, and oxidation state (Figure 6.8).

The biotite crystallization temperatures of granites from Guéra massif and Lake Fitri region drop from ~652 to ~612^oC corresponding from the oldest to youngest rocks (Figure 6.8a), for each group, the low temperatures associated with the low Ti (Figure 6.10a) and silica (Figure 6.9) content. This variation is consistent with the transition in solidification conditions of magma from collision to extension regimes (Figure 6.12). Also, the low-temperature range (~611-652^oC) of these sample suites possibly indicates a late magmatic stage (Shand, 1944). Moreover, many experimental studies have proposed that granitic rocks will crystallize when the solidification temperature of granitic bulk compositions have cooled down to around 650-700^oC during metamorphism or hydrothermal alteration (Ackerson et al., 2018). Indeed, the yield of the average biotite temperature from 611 to 652^oC of this study lends support to this assertion.

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Figure 6.7. The simplified location map showing the subdivision of biotites using the Fe-number, and the magmatic condition estimated for each intrusion. (a) modified from Pham (2018), (b, c) modified from Shellnutt et al. (2018). * denotes data from Shellnutt et al. (2018).

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Figure 6.8. (a, b, c) the magmatic conditions of granitic rocks from the Guéra massif and Lake Fitri deduced from biotite composition.

(d, e) The plot of biotite emplacement pressure and oxidation state of studied rock with biotite crystallization temperature, and

(f) the plot of oxidation state and biotite emplacement pressure indicate a clear trend among Guéra and Lake Fitri granites, (g) the

amphibole pressure of Guéra granites generally drops during rock formation. * denotes data from Shellnutt et al. (2018).

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Figure 6.9. The positive correlation between biotite temperature crystallization and silica content. * denotes data from Shellnutt et al. (2018).

Figure 6.10. The concentration of Ti reflects the variation of the crystallization temperature of biotite and oxygen fugacity of magma from Guéra massif and Lake Fitri region. * denotes data from

Shellnutt et al. (2018).

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The oxygen fugacity of all investigated granites generally shows a trend toward slightly reducing condition from NNO to QFM buffer for collisional granites, QFM to WM buffer for Guéra collisional granites, and NNO to QFM for Lake Fitri post-collisional granites (Figure 6.6). In general, biotites have low O2 associated with low Ti content (Figure 6.9b). The concentration of ferrous iron usually varies among different sample suites that differ in redox condition, and the oxidation state of magma actively controls the extent of Fe²⁺-enrichment in the magma sequence during its evolution (e.g., Larsen, 1976). In this case, the biotite of 590 Ma-S, 570 Ma-E and 560 Ma-N plutons are calculated to be more enriched in Fe²⁺ which is produced from a magma under reducing conditions; whereas the poorer Fe²⁺ biotites of 595 Ma, 590 Ma-N, 570 Ma-W, 560 Ma-S plutons are formed from magma in oxidizing environment (Figure 6.11). On the other hand, the overlap of the concentration range of ferrous iron in Guéra granites indicates as evidence of an internal transition from more oxidizing to reducing condition in the individual intrusion.

Frost and Lindsley (1991) stated that the high O2 magmatic system usually produces Fe-Ti oxide minerals such as ilmenite, hematite, and magnetite especially, the appearance of hematite indicates the culmination in oxidation state reaching in felsic rocks. Nevertheless, this behavior does not clearly show in the studied intrusions. The results of EPMA analysis records no hematite and only extremely low content of ilmenite, magnetite that can be observed as the inclusions of opaque minerals under the microscope in biotite or amphibole. The concentration of SiO2 host rocks is very high ranging from 59.7 to 74.8 wt% for collisional granite, from 69.3 to 76.9 wt% for Guéra post-collisional granites (Pham, 2018) and from 73.7 to 76.9 wt%

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for Lake Fitri post-collisional granites (Shellnutt et al., 2018) which may be the reason for the low abundance of Fe-Ti oxide minerals.

Figure 6.11. The variation of Fe²⁺ content records the transfer of the oxidation state in the course of magma during its evolution. (*) the data using from Shellnutt et al. (2018).

The change in estimated emplacement pressure of biotites from Guéra massif generally shows a decreasing trend from oldest to youngest rocks (Figure 6.8c, g), reflecting the decline of pressure from compression to extension. This feature is consistent with the scenario of the tectonomagmatic evolution in southern Chad (Figure 6.12) suggested by Shellnutt et al. (2017). The 560 Ma-S and 590 Ma-S plutons have the higher biotite pressure (2.76 ± 0.04, 2.49 ± 0.25 kbars, respectively) compared to the 560 Ma-N and 590 Ma-N (1.50 ± 0.06, 1.36 ± 0.04 kbars,

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respectively) (Figure 6.8c). The barometric dichotomy could be due to a pressure gradient in 560 Ma and 590 Ma plutons. The amphibole and biotite pressures of 590 Ma intrusion (Figure 6.8c, g) show a contrast variation trend, in particular, the northern part of 590 Ma intrusion contains higher amphibole pressure (6.17 ± 0.6 kabrs) but lower biotite pressure (1.36 ± 0.04 kbars), in comparison with the southern part (PAm = 2.4 ± 0.7 kbars; PBt = 2.49 ± 0.25 kbars). It indicates a difference in the order of amphibole and biotite crystallization in the two parts of 590 Ma intrusion, i.e., the amphibole of 590 Ma-N likel crystallized earlier (at ~6.7 kbars), the subsequent amphibole and biotite of 590 Ma-S were crystallized at the same time (at ~2.4 kbars), and finally, the biotite of 590 Ma-N crystallized later (at ~1.36 kbars).

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Figure 6.12. A possible tectomagmatic evolution suggesting repeated deformation/relaxation episodes in southern Chad (after Shellnutt et al. (2017). (a) Subduction-related magmatism and deformation

stages (D1 and D2). (b) Transtentional deformation D3 is followed by a first (c) crustal relaxation stage at ~ 570 Ma. (d) The final deformation stages D4 stage is followed by a second (e) (~560 Ma)

and (f) final crustal relaxation stage. SCLM: subcontinental lithospheric mantle. CAFB: Central African Fold Belt.

109 6.5. Biotite composition and parental magma

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 101-0)