Wavelength dispersion and focusing of characteristic X-ray

The EPMA system is equipped with Wavelength Dispersive Spectroscopy (WDS) detectors to detect the characteristic X-rays produced in the interaction volume. The WDS detector is made up of a gas-flow or sealed proportional counter and a few diffracting crystals, by which, diffracting crystals are used to separate wavelengths of characteristic X-rays and direct a specific X-ray wavelength to the gas-flow or sealed proportional counter for measurement. The position of the X-ray source in the sample, the surface of diffracting crystals and detector in spatial define an imaginary circle of constant diameter (Figure 4.1). Because the amount of the possible X-ray photons from the sample reaching the diffracting crystal is relatively low; therefore, the intensity of the X-ray detected by WDS is usually lower than as compared to Energy Dispersive Spectrometer (EDS) with a given beam current (Zhao et al., 2015).

59

Figure 4.1. A sketch illustrating the imaginary Rowland circle.

60 4.2. Source of data and analytical method

To investigate the mineralogical composition of the Neoproterozoic granites from the Guéra Massif (including 590 Ma-, 570 Ma- and 560 Ma granite), twelve samples containing biotite and/or hornblende-biotite-granite were selected for analysis. A JEOL EPMA JXA-8500F equipped with five WDSs at the EPMA laboratory in the Institute of Earth Sciences, Academia Sinica, Taipei was used. The samples were mounted in into epoxy resin and polished until exposed, then facilitated electron conductance by carbon coating (Q150TE, Quorum Technologies Ltd., UK, Picture 4.1). The target positions of minerals were determined based on the secondary- and back-scattered electron images.

The equipment operated at 16 kV voltage and 6 nA current beam with the electron beam was in diameter of 2mm. The ZAF method using the standard calibration of synthetic chemical-known standard minerals with diverse diffracting crystals used to correct the X-ray intensities, list as follows: rutile for Ti (PETJ), periclase for Mg (TAP), corundum for Al (TAP), albite for Na (TAP), wollastonite for Si (TAP), fluorite for F (TAP), Cr-oxide for Cr (PET), tugtupite for Cl (PET), wollastonite for Ca (PET), apatite for P (PET), Mn-oxide fir Mn (PET), orthoclase for K (PET), and anhydrite for S (PET), hematite for Fe (LiF), Zn-oxide for Zn (LiF), Ni-oxide for Ni (LiF), and Co-oxide for Co (LiF). The relative standard deviations (RSD) for F, Cl, S and P were less than 2%, for K, Na and Si were less than 1%, and others were less than 0.5%. The detection limits for F, Cl, S, and P were less than 0.5wt%. The analysis results are shown in the Appendix.

61

The structural formulae of biotite were calculated on the basis of 22 oxygens equivalents ignoring H2O⁺ and 13 cations by using the spreadsheet excel program designed by Tindle and Webb (1990). The partitioning of Fe³⁺ and Fe²⁺ from the total iron (FeO^t) was computed by the program “Fe23” using charge balance (Nenova, 1997). The chemical formulae of amphibole are recalculated on the basis of 23 oxygens equivalents with Fe²⁺/Fe³⁺ estimation assuming, ignoring H2O⁺ and 13 cations. The chemical formula, geochemical features of amphibole and biotite, and their computed stoichiometry are summarized and listed in Appendix 2 and Appendix 3, respectively. Biotite chemistry from the Lake Fitri granites will also be discussed in relation to the Guéra Massif granites (Shellnutt et al., 2018).

62

Picture 4.1. Samples were prepared for biotite and amphibole analysis. (AB): hornblende-biotite-granite; (B): biotite-granite.

63

CHAPTER 5. RESULTS

5.1. Rock classification

The R1-R2 multi-cationic diagram of Batchaler and Bowden (1985) for distinguishing granites from different tectonic settings (Figure 5.1a) indicates the oldest granites of Guéra massif (≥590 Ma) follow either the pre-plate collision (2) or late orogenic (4) granite trend to the post-collision uplift (7) and syn-collisional (6) regions. Therefore, they can be referred to as collisional granite. Meanwhile, the younger granites of Guéra massif (≤570 Ma) and granites from Lake Fitri region (550 Ma) are strictly post-collision (7) with some overlying with the syn-collision field (6).

Furthermore, a commonly useful tool of the immobility of Nb and Y for distinguishing tectonic settings suggested by Pearce et al., (1984) (Figure 5.1b) showing a clear distinction of the studied rocks, in which, the formation of the oldest rocks are related to volcanic arc granites, whereas, the younger rocks fall in the within-plate setting.

Additionally, the collisional granites are magnesian to ferroan, and metaluminous or peraluminous but classify as I-type granite. The Guéra and Lake Fitri post-collisional granites are A-type granites, due to the presence of high silica content, ferroan to slight magnesium, and peraluminous in compositions (Figure 5.2). On the chemical subdivision diagram of A-type granitiods (Figure 5.1c) suggested by Eby (1992), all post-collisional studied intrusions fall in A2 field that produced by magma derived by differentiation of a continental tholeiite, with variable degrees of crustal interaction, or by direct melting of a crustal source that had undergone through a prior melting period. The classification schemes makes the distinction between the

64

collisional and post-collisional granites (including Lake Fitri granites). The classification of the rocks will also be discussed later by using the implication of biotite chemical composition.

Figure 5.1. Tectonic discrimination diagrams for intrusions in this study. (a) after Batchelor and Bowden (1985); (b) after Pearce et al. (1984), WPG: within plate granite, VAG: volcanic arc granite, ORG: oceanic ridge granite, syn-COLG: syn-collisional granite; (c) after Eby (1992), A1, A2 are

sub-groups of A-type granites. * and ** denotes data from Shellnutt et al. (2018) and Pham (2018), respectively.

65

Figure 5.2. Compositional classification diagram of granite for this study. (a) Fe* vs. SiO2 showing the distinction between ferroan and magnesian composition (after Frost et al., 2001); (b) molar Al/(Na+K) vs. Al/(Ca+Na+K) showing the range of metaluminous and peraluminous for granite composition (after Shand, 1943); (c) (Na2O+K2O-CaO) vs. SiO2 showing the ranges of alkalic, alkalic

calcic, calc-alkalic and calcic (after Frost et al., 2001). * and ** denotes data from Shellnutt et al.

(2018) and Pham (2018), respectively.

66 5.2. Mineral classification

5.2.1. Classification of minerals from collisional granite

a. Biotite

Biotite from the collisional granites is classified based on the relative contents of cations per formula unit which are present within the T (tetrahedral) and M (octahedral) positions of the structure after Foster (1960) and Rieder et al. (1998). In general, they range from Fe-biotite to siderophyllite ( Figure 5.3a, b). The biotite of the collisional granite displays variation in the range of Fe# ratio value, consistent with the spatial distribution of sample (Figure 6.7). They can be subdivided into three groups due to the decrease of Fe# ratio (Figure 5.3a): the first group (group I) is characterized by the Fe# ratio between 0.9 and 1.0, corresponding to biotite from granite in the southern part of 590 Ma intrusion (590 Ma-S, 14ZA12D), the second group (group II) ranges from 0.7 to 0.9 of Fe# ratio that represented by biotite from granite in the northern part of 590 Ma intrusion (590 Ma-N, 14ZA16A) and the third group (group III) has Fe# ratio lower than 0.7, belonging to biotite of granites in 595 Ma intrusion (14ZA02, -06, -25C Ma). It is possible that the differentiation of biotite composition is linked to fractional crystallization of the host rocks. Notably, the lowest Fe# ratio values of 595 Ma granites accord with a slight ferroan composition of host rocks (Figure 5.2a). Moreover, the ternary diagram (FeO+MnO)-TiO2-MgO distinguishing for the origin of biotite, indicating the biotite of collisional granites is primary magmatic and re-equilibrated biotite (Figure 5.3c).

67

Figure 5.3. Classification of biotite from collisional granites in Guéra massif. (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).

68 b. Amphibole

Amphibole is a common ferromagnesian mineral in the collisional granites of Guéra massif. Its chemical composition is entirely homogeneous, belonging to the calcic group of amphiboles and formed by magmatic processes (Figure 5.4). All amphiboles of collisional granites show a consistent trend on two amphibole nomenclature diagrams recommended by the International Mineralogical Association (IMA) (Figure 5.4a, b), transferring from ferro-pargasite to ferro-edenite.

Figure 5.4. Classification of amphibole from collisional granites in Guéra massif. (a) Amphibole classification for calcic group

recommended by the International Mineralogical Association (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).

69

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).

70

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

73

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#

76

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).

79

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).

82

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).

84

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

85

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.

86

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

88

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^T in 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,

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^T in 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,

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 74-0)