CHAPTER 3. PETROGRAPHIC METHODS
3.1 Sample collection
Before the analysis can be conducted, samples need to be collected to represent for lithology of the formation in the study. Normally, the sample will be selected in where the weathering is not strong. It has to be clear to observe and identify the minerals by the unaided eye and hand-lense (Fig. 3. 1). There are 19 samples collected with 18 samples from Kannak Complex and 1 sample from Hai Van Complex.
Figure 3. 1. Sample collection in the field with visible coarse grains of granodiorite (a) and leucosome (b).
15 3.2 Sample processing
The sample first must be cut by a rock cutting machine in the most clearly face, which is composed of almost features of samples (mineral assemblage, dyke/veins). Then, the sample will be polished and dried before to stick into a glass slide. Finally, the thin section will be cut by the Petrothin-Thin sectioning system before polish to a standard thickness (usually 0.03mm). At that thickness, some common minerals (normally using quartz) are easily identified under the microscope. Sample processing was conducted in Rock Sample Preparation Room (Fig. 3. 2a, b) and Petrology Lab (Fig. 3. 2c), Department of Earth Sciences, National Taiwan Normal University, GongGuan Campus.
Figure 3. 2. Useful machines to make thin section: a. Rock cutting machine; b. Petrothin-Thin sectioning system; c. Grinder and Polisher Machine.
3.3 Sample analysis
Samples have been analyzed by using Carl Zeiss Axioplan 708 Polarizing Optical Microscope at Magmatic and Volcanic Processes Lab (Fig. 3. 3) to observe the mineral assemblages and the textural relationships within rocks in detail with two purposed:
16 1. Identified the rock-forming minerals;
2. Provide the hypothesis for tectonic or metamorphic evolution for samples in the study based on the microstructures.
Normally, the photograph will be taken under plane-polarized light (PPL) and cross-polarized light (CPL).
Figure 3. 3. Carl Zeiss Axioplan 708 Polarizing Optical Microscope was used throughout the study in Magmatic and Volcanic Processes Lab, National Taiwan Normal University, GongGuan Campus.
3.4 Mineral percentage measurement
To calculate the mineral percentage within the sample, a process is given and proceed as follows:
1. Choose a field within the thin section which is represented for the mineral assemblage of sample and took micrograph of that field (Fig. 3. 4a) under both CPL, PPL;
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Figure 3. 4. a. Big micrograph shows typical mineral assemblage for sample; b. Identify the area of different minerals by hand-draw with distinct color (using Corel Draw software).
2. Normally, Image-Pro Plus will be used directly to identify different minerals in the micrograph (taken in 1st step above) and measure the mineral’s area. In fact, it is not easy for this software to identify the mineral with similar features by itself. Thus, Corel Draw was used to handle it by selecting the mineral’s area by hand draw (Fig. 3. 4b);
3. Apply Image-Pro Plus for micrograph in 2nd step to calculate the mineral’s area and export data to Excel.
4. Use excel to calculate the mineral percentage (MP) by the equation:
MP = Mineral
′s area
The total area of the micrograph
3.5 Composition-paragenesis diagram
There are three types of diagram which are applied in this study, including QAP, ACF and A’FM diagrams.
• QAP diagram
A QAP diagram is a ternary diagram that is used to classify igneous rock by using mineral composition. Each corner of the triangle represents a pure component, such as 100% Quartz, 100% Alkali feldspar, and 100% Plagioclase (Harvey et al., 2006). By using the mineral percentage from section 3.4 above plotted into the QAP diagram, the point/area defining in the diagram will represent for corresponding rock type.
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For example, a sample contains 19.85% Qtz, 39.67% Pl, and 3.8% Kfs in mode.
In order to draw the QAP diagram, there are some necessary steps:
- Recalculate and normalize the percentage of Q-A-P that adds up to 100. Thus, the Q/A/P above will be normalized to 31.35/62.65/6.
- Draw the line to represent the value of Q and P in the diagram (100 at the top and 0 at the bottom). The intersection of Q and P lines will define for that sample.
Therefore, the QAP diagram for this sample can be drawn as follows (Fig. 3. 5):
Figure 3. 5. An example of classifying igneous rock using the QAP diagram.
• ACF diagram
This type of diagram will be applied for this study with purpose, either determine for bulk chemical composition or define chemographic projection for the sample.
Eskola ‘s (1915) diagram (ACF diagram) is defined by mole proportions of Al2O3, Fe2O3, CaO, FeO, MgO, and MnO. The limitation of this diagram is it can be applied to the rocks not containing any muscovite, biotite, or paragonite. Thus, all the accessory
19
minerals, as well as the values of Al2O3, FeO, MgO of biotite, need to be subtracted.
Besides, SiO2, CO2, and H2O are also disregarded. The remarkable thing here is all values need to be converted to mole proportions by dividing the weight percentage of each oxide by its molecular weight. Therefore, the three groups of components in ACF diagram can be calculated below:
A = (Al2O3 + Fe2O3)-(Na2O + K2O) C = CaO
F = FeO + MgO + MnO
In order to graphical purposes, all of these values will be recalculated with A + C + F = 100%.
Similar to the QAP diagram, the values of A, C, F are conveniently plotted on the ACF diagram. The field that the point dropped into will represent for its bulk chemical composition (Fig. 3. 6).
- Pelitic: composed of derivatives of aluminous sedimentary rocks such as shale and mudrocks. It is characterized by the abundance of aluminous minerals, including mica, kyanite, sillimanite, andalusite, and garnet.
- Quartzo-feldspathic: contained mostly quartz and feldspar with a minor amount of aluminous minerals. These rocks are derived from graywacke sandstone/ siltstone or igneous protoliths such as granite, granodiorite, and tonalite.
- Basic: the rocks belong to basic rocks are generally derived from basic igneous rocks such as gabbro or basalt and contained rich in Fe-Mg minerals like biotite, hornblende, chlorite.
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- Calcareous: the composition for calcareous rocks is dominated by calcium-rich minerals such as calcite and dolomite.
Figure 3. 6. An example of using the ACF diagram to define the bulk chemical composition for the sample in this study (modified after Stephen, 2011).
In order to define the chemographic projection for the sample, the mineral assemblage for the sample needs to plot into the triangular diagram‘s edges. The point representing composition in the ACF diagram will indicate for the sample’s paragenesis as well as continuous/discontinuous reactions between minerals (Fig. 3. 7).
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Figure 3. 7. ACF diagram indicated for reactions between minerals in the sample.
• AFM/A’FM diagram
Thompson (1957) has regarded that metapelites are composed of 6 components, such as SiO2, Al2O3, FeO, MgO, K2O, and H2O, with some minor components were ignored (CaO, Na2O, MnO, Fe2O3, TiO2). After removing the unnecessary parts, there are four components Al2O3, FeO, MgO, K2O remain in his diagram. Because muscovite is abundant in metapelites, all other compositions will be projected from muscovite onto the plane of (Al2O3-FeO-MgO), which defined for the AFM diagram.
A modification of AFM projection (A’FM) was given by Reinhardt (1968, 1970) that apply to a broader range of composition for high-grade metamorphic rocks. Many metamorphic minerals can be plotted into this diagram, such as sillimanite, kyanite, cordierite, garnet, biotite, hornblende, and pyroxene.
The parameters for them can be calculated by the following formulae:
A’ = Al2O3-(K2O + Na2O + CaO) F = FeO-Fe2O3
22 M = MgO
An example of A’FM diagram can be shown below for biotite ± muscovite gneiss with 8.1% Bt, 8% Ms, 10% Kfs, 2% Grt, 5.5% Pl, 2% Chl >60% Qtz and a negligible amount of minor minerals (Fig. 3. 8).
Figure 3. 8. A’FM diagram showing the paragenetic relations observed in the sample, green point represented for the composition of that sample.
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CHAPTER 4. RESULTS
In the field, we have observed gneiss, amphibolite, granodiorite, migmatite, and pegmatite, which are constituted the basement of the Kannak Complex. There is only one sample belongs to Hai Van Complex (K16-11-19A1). Samples will be grouped based on the geological map, such as Xa Lam Co Formation, Dak Lo Formation, Kim Son Formation, and Hai Van Complex. The detailed analysis will be conducted and described from old formation to young formation. The metamorphic evolution will be described and interpreted with the metamorphic episodes (Ma, Mb), which may correspond to deformation. Mineral percentages of 19 samples in this study are calculated and summed up in Table 4. 1 in order to apply to figure out the bulk chemical composition. All the descriptions will follow the list of mineral abbreviations of Siivola and Schmid from IUGS.
4.1 Xa Lam Co Formation
There are 6 outcrops of Xa Lam Co Formation with 9 samples sellected (Fig. 4. 1).
Figure 4. 1. Map location for outcrops of Xa Lam Co Formation
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4.1.1 An Trung Commune, An Lao District, Binh Dinh Province
This outcrop is represented for Xa Lam Co Formation with the occurrence of amphibolite (Fig. 4. 2). Sample K16-11-18E1 was collected for this rock type with the purpose of analysis.
Figure 4. 2. The occurrence of amphibolite in the outcrop: a. Big scale; b. Small scale.
The mineral assemblage for this sample contains mainly of hornblende ± pyroxene (clinopyroxene + orthopyroxene) ± plagioclase ± biotite ± chlorite ± quartz (Fig.
4. 3). Accessory mineral includes only opaque.
Figure 4. 3. The mineral assemblage for sample K16-11 -18E1, taken under CPL.
Pre- Mb is defined by the presence of hornblende, plagioclase, and pyroxene.
Hornblende is euhedral to subhedral crystals with pleochroism colors (pale green to
25
brown) (Fig. 4. 4a). Plagioclase is variable about the sizes with muscovite alteration which is caused by Mb (Fig. 4. 4b). Two types of pyroxene are determined in this sample (Fig. 4. 4c). They intergrew with hornblende and formed the reaction rim (corona) with hornblende indicating for syn- deformation.
Figure 4. 4. a. Euhedral to subhedral crystal of hornblende; b. Euhedral crystal of plagioclase intergrows with clinopyroxene and hornblende; c. High interference color of clinopyroxene and orthopyroxene co-existed with hornblende; d. Chlorite grows as the alteration result from biotite.
Muscovite and chlorite are generated by the alteration from plagioclase and biotite, respectively (Fig. 4. 4b, d). This replacement implied that it formed under lower P-T condition around 200°C (Parry and Downey, 1982) and defined for Mb.
ACF diagram was drawn to signify for bulk chemical composition. This amphibolite dropped into basic rocks field that has a relatively high concentration of iron and magnesium (Fig. 4. 5).
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Figure 4. 5. Representative bulk chemical composition for this amphibolite (K16-11-18E1) in the ACF diagram.
4.1.2 An Dung Commune, An Lao District, Binh Dinh Province Outcrop 1
This is first outcrop of Xa Lam Co Formation (Fig. 2. 2) which is represented by the occurrence of migmatite with biotite gneiss (mesosome) and leucosome. There are three samples collected, including K16-11-18C1, K16-11-18C2 (mesosome), and K16-11-18C3 (leucosome) (Fig. 4. 6).
27
Figure 4. 6. The occurrence of three rock types in the outcrop: a. Biotite ± muscovite gneiss;
b. Biotite gneiss; c. Leucosome.
By calculating mineral percentages for each sample, the bulk chemical composition can be given in the following diagram (Fig. 4. 7). Generally, these three samples of migmatite show a similar composition with quartzo-feldspathic rocks, which is represented for convergent plate boundaries (continental crust).
Figure 4. 7. Bulk chemical composition for migmatite in this outcrop by calculating mineral percentage.
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Table 4. 1. Mineral percentages of analyzed samples in this study
No. Sample no. Rock types
Note “+” represented for “not calculated” mineral percentage within the sample; “-” defined for the absence of mineral within the sample.
29 K16-11-18C1 – Biotite ± muscovite gneiss
This sample is dominated by the mineral assemblage of biotite ± muscovite ± garnet ± feldspar ± plagioclase ± microline ± quartz ± chlorite. Opaque occurs as an accessory mineral (Fig. 4. 8).
Figure 4. 8. Mineral assemblage for biotite ± muscovite gneiss (K16-11-18C1) under CPL.
Garnet occurs as high relief subhedral to anhedral porphyroclasts and commonly 0.8 to 1.5mm in diameter. They are entirely surrounded by biotite, which defined for foliation and suggested for Ma (Fig. 4. 9).
Figure 4. 9. High relief grains of garnet intergrew with biotite and quartz and indicated for Ma
under PPL (a) and CPL (b).
30
Post- Ma is represented by the growth of biotite, perthite, myrmekite and microline. Biotite is characterized by the elongated crystal and pleochroism colors under CPL. They aligned with foliation and suggested for syn- deformation (Fig. 4. 10a). The highest temperature in this sample is given by perthite, which is defined by anhedral crystals with albite lamellae and the host is microline or orthoclase (Fig. 4. 10b). It is generated by the intergrowth of two feldspars and suggested the temperature for this intergrowth at 600 to 750ºC, which determined by solid solution between albite and K-feldspar (Heier, 1955; Rollinson, 1982). Myrmekite also occurred in this sample but at a lower temperature around 450-600ºC (Garcia and Roux, 1996) and characterized by vermicular (wormlike) or wartlike quartz intergrow with sodic plagioclase (Fig. 4. 10b).
Microline also formed within this range of temperature by the transformation from orthoclase at 500ºC (Fig. 4. 10b; Lorence and Barbara, 1998; Vernon, 2004). Undulose extinction of quartz was considered to be the evidence for plastic deformation at high-temperatures (Vernon, 2004).
Figure 4. 10. a. Strongly aligned grains of biotite with high interference colors under CPL; b. d.
The intergrowth of two feldspars defined by the occurrence of perthite which is co-existed with microline and myrmekite.
The presence of chlorite and muscovite defined for Mb (Fig. 4. 11). Chlorite partially replaced biotite and shows green color (under PPL) and grey color (under CPL).
This occurrence implied that it formed under low P-T condition around 200°C (Parry and
31
Downey, 1982). Muscovite is distinguished by tiny grain with high interference color, which is caused by the alteration from plagioclase under retrogression (Barker, 1990).
From the above description, the P-T condition is determined to be more than 500ºC and can be inferred to be metamorphosed under amphibolite to granulite facies.
Figure 4. 11. The presence of chlorite and muscovite which are formed by the alteration of biotite and plagioclase, respectively.
K16-11-18C2 – Biotite gneiss
Sample K16 -11- 18C2 was selected to represent for biotite gneiss in this outcrop (Fig. 4. 6b). The mineral assemblage composed mainly of biotite ± quartz ± plagioclase
± garnet ± microline ± muscovite ± chlorite. Opaque occurs as an accessory mineral (Fig.
4. 12).
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Figure 4. 12. Mineral assemblage for this sample, taken under CPL.
Ma might be defined by the appearance of garnet. Garnet shows anhedral crystal with high relief under PPL. In this sample, garnet displays as porphyroclasts and intergrow with biotite and quartz (Fig. 4. 13).
Figure 4. 13. An anhedral grain of garnet intergrows with biotite and quartz.
Post- Ma is represented by the presence of biotite and quartz. Biotite is variable about the sizes, range from 0.1 to 0.6mm in length and pleochroism in colors. It is elongated and aligned with foliation (Fig. 4. 14a). Besides, quartz also shows undulose extinction with bulging recrystallization (Fig. 4. 14b), indicating the temperature condition for deformation around 250 to 400ºC (Drury and Urai, 1990).
33
Figure 4. 14. High interference color of elongated biotite with chlorite alteration within; b.
Undulose extinction of quartz with bulging recrystallization intergrow with tartan twining of microline.
As results of decreasing temperature, chlorite and muscovite grow obviously (Fig.
4. 14). They are partially replaced biotite and plagioclase under hydrothermal environment (Barker, 1990), respectively. This replacement suggested that it formed under a lower P-T condition around 200°C and defined for Mb (Parry and Downey, 1982).
From the above description this biotite gneiss is considered to be metamorphosed under amphibolite facies.
K16-11-18C3 – Leucosome
As a part of migmatite, leucosome defined by light-colored granitic components which is inverse with mesosome-dark colored setting. In this outcrop, sample K16-11-18C3 was collected for leucosome (Fig. 4. 6c). It contains mainly of biotite ± quartz ± garnet ± plagioclase ± feldspar ± pyroxene ± microline ± myrmekite ± muscovite ± chlorite (Fig. 4. 15) and a minor amount of opaque and rutile as accessory minerals.
34
Figure 4. 15. Typical mineral assemblage for leucosome in this outcrop (sample K16-11-18C3), taken under CPL (yellow rectangular will be shown below).
The attendance of garnet might signify either for igneous protolith or Ma which shows high relief with brownish color and pseudomorphs by quartz, biotite and muscovite.
Coarse grains of garnet were broken and filled in the fractures by muscovite (Fig. 4. 16) which may be caused by later events.
Figure 4. 16. Anhedral high relief grains of garnet under PPL; b. Muscovite filled in the fractures within garnet (taken under CPL).
Post- Ma is clearly represented by the presence of biotite, perthite, myrmekite and microline. Biotite grains are elongated with high interference color under CPL. Some grains are bent, which is suggested for the occurrence of later deformation (Fig. 4. 17a).
Perthite occurred in this sample as a result of the intergrowth of two feldspars and
35
characterized by thin and parallel exsolution lamellae (Fig. 4. 17b). This intergrowth formed at temperature 650 to 750ºC, which determined by solid solution between albite and K-feldspar. Myrmekitic intergrowth (Fig. 4. 17c) is common in this rock and easy to distinguished by vermicular (wormlike) texture and normally gave the range of temperature for this growth in the range of 450 - 600ºC (Garcia and Roux, 1996). This range of temperature is also suitable for the growth of microline (Fig. 4. 17b, c) with well-developed tartan twining (Lorence and Barbara, 1998; Vernon, 2004).
Figure 4. 17. a. Bent grain of biotite with chlorite alteration and growth of pyroxene; b.
Intergrowth of perthite and microline with the presence of grain boundary migration recrystallization of quartz; c. Myrmekite grow in the boundary of microline grain.
As result of retrograde metamorphism (Mb), chlorite and muscovite formed by the alteration from biotite and feldspar, respectively (Fig. 4. 17a; b). It is easy to recognize chlorite by pale-green color within biotite, which is normally shown dark color under CPL. Muscovite is clearly visible with fine-grained and high interference color. Hence, this sample is considered to be metamorphosed under amphibolite to granulite facies.
36 Outcrop 2
This outcrop is also represented for Xa Lam Co Formation with the occurrence of pegmatitic muscovite and amphibolite (Fig. 4. 18). There are two samples collected, including K16-11-18D1 and K16-11-18D2.
Figure 4. 18. The occurrence of two rock types in the outcrop: a. Pegmatitic muscovite; b.
Amphibolite
By calculating the mineral percentages, the bulk chemical composition for each of rock type in this outcrop can be given in the diagram below. The QAP diagram (Fig.
4. 19a) shows that pegmatitic muscovite (K16-11-18D1) has a similar composition as granite and amphibolite (K16-11-18D2) tends to have quartzo-feldspathic composition rather than basic rocks (Fig. 4. 19b).
Figure 4. 19. a. Similar composition with granite of pegmatitic muscovite (K16-11-18D1) is shown in the QAP diagram; b. Amphibolite (K16-11-18D2) tends to have quartzo-feldspathic composition in the ACF diagram.
37 K16-11-18D1 – Pegmatitic muscovite
K16-11-18D1 is pegmatitic muscovite (Fig. 4. 18a) and characterized by mineral assemblage of plagioclase ± feldspar ± microline ± muscovite ± quartz (Fig. 4. 20).
Hematite occurs as an accessory mineral. This sample is not only metamorphosed but also deformed.
Figure 4. 20. Typical mineral assemblage for pegmatitic muscovite (K16-11-18D1).
Pre- Ma could be defined by the presence of subhedral to euhedral crystals of plagioclase. They were broken and deformed, which is may be caused by later deformation (Fig. 4. 21).
Figure 4. 21. The presence of pre- deformation of plagioclase, which was broken (a) and deformed (b) by deformation.
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Syn- Ma is identified by muscovite, perthite, microline and quartz. This type of muscovite is characterized by coarse grains with high interference color and well-displayed cleavage. Some grains were bent, which may be caused by the deformation (Fig.
4. 22a). The intergrowth of two feldspars by the solid solution is defined by perthite (Fig.
4. 22b). It is represented by thin and parallel exsolution lamellae and gave the range of this intergrowth at 600 to 750ºC (Heier, 1955). Microline also occurred in this sample with well-developed tartan twining (Fig. 4. 22c) and gave the temperature of growth at least around 500ºC (Lorence and Barbara, 1998). Quartz grains are variable in sizes and show undulose extinction. Subgrain rotation recrystallization and grain boundary migration (Fig. 4. 22d) are common in this sample which grew at high-temperature around 400 to 700ºC (Stipp et al., 2002; Vernon, 2004) and normally at amphibolite facies (Fitz Gerald and Stünitz, 1993; Schmid et al., 1980, 1987; Schmid and Casey, 1986). The growth of fine-grained of muscovite defined for post- deformation as a result of the alteration from plagioclase.
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Figure 4. 22. a. Bent grains of muscovite with high interference colors; b. Intergrowth of two feldspars formed perthite with thin and parallel exsolution lamellae; c. Well-developed tartan twinning of microline; d. Grain boundary migration recrystallization in quartz.
K16-11-18D2 – Gneiss
Samples K16-11-18D1 was first identified in the outcrop as amphibolite based on its color and hardness (Fig. 4. 18b). However, it shows the mineral assemblage of gneiss with biotite ± garnet ± plagioclase ± feldspar ± muscovite ± chlorite ± cordierite ± quartz (Fig. 4. 19b; Fig. 4. 23). Zircon occurs as an accessory mineral.
Figure 4. 23. Typical mineral assemblage for sample K16-11-18D2, taken under CPL.
The occurrence of garnet and some grains of K-feldspar represented for Ma. It is easy to determine garnet in thin section by high relief under PPL. Most of grains of garnet
40
were broken which may be caused by deformation (Fig. 4. 24a). This type of K-feldspar occurred as porphyroclasts with the grain size >0.4mm and surrounded by biotite, tiny
were broken which may be caused by deformation (Fig. 4. 24a). This type of K-feldspar occurred as porphyroclasts with the grain size >0.4mm and surrounded by biotite, tiny