Chapter IV

The Igneous Rocks of the Mogok Stone Tract

 

A Ph.D. Dissertation by Kyaw Thu

Department of Geology, University of Yangon, Myanmar
Copyright © Kyaw Thu, 2007

 

Introduction from Bill Larson, Pala International

Kyaw Thu is a FGA gemologist with a Ph.D. in Geology. Pala International knows him from trips in the mid-1990s as one of the most interesting young gem enthusiasts, among so many wonderful students and collectors we met. Both Will and Bill Larson consider him a special and close friend. He has helped answer many questions regarding locality information for Brendan Laurs of GIA as well as meeting in person Dr. George Harlow of the American Museum of Natural History and corresponding with him for almost a decade. He has opened a gem laboratory and has applied to GIA to further his education. I was given a copy of his PhD thesis last month and Dr. Harlow made it available by scan for interested parties. Pala International is honored to share this with our readers.

 

 

Contents

 

List of Figures (images from Pala International are not listed)
4.1	Sample location map of the Mogok Stone Tract (source; Landsat TM image, 30m resolution, band combination 754)
4.2	Variation diagram of major elements vs SiO2 in igneous rocks of the study area (cont.)
4.3	Na2O/K2O vs SiO2 variation diagram, showing the trend of differentiation (KT-1 to KT-9 are granitic rocks/KT-10 to KT-15 are syenitic rocks and KT-16 to KT-23 are mafic to ultramafic rocks)
4.4	Percent anorthite in normative plagioclase plotted against the Thornton-Tuttle differentiation index (KT-1 to KT-9 are granitic rocks/KT-10 to KT-15 are syenitic rocks and KT-16 to KT-23 are mafic to ultramafic rocks)
4.5	Alkali-lime index diagram for granitic, syenitic, basic and ultramafic rocks (Peacock index)
4.6	Alkali-lime index diagram for granitic, basic and ultramafic rocks (Peacock index)
4.7	Major element discrimination of igneous rocks supposed to be collision-related, subdivided as alkaline, calc-alkaline and strongly peraluminous, and highly fractionated calc-alkaline (after Sylvester, 1989)
4.8	Al2O3 — CaO — (Na2O+K2O) diagram
4.9	Normative Ab-Or-An diagram for the igneous rocks of the study area, with dividing lines according to O’Connor (1965)
4.10	Plot of Wright’s (1969) alkalinity index Al2O3+CaO+Na2O+K2O/Al2O3+ CaO-Na2O+K2O solid line and field names are after Wright, and the dashed line, separates alkaline from calc-alkaline, after Khin Zaw (1986)
4.11	Diagram showing K2O vs SiO2 wt % ration for granitic rocks and syenitic rocks of the study area Calc-alkaline – Jake and Gill (1970) Alkaline (1) – Gried and Lefeure (1972) Alkaline (2) – Lemoitre (1962)
4.12	Fe(t)-Na2O+K2O—MgO (AFM) diagram, in terms of alkalis, total Fe and Mg for the igneous rocks of the study area
4.13	ACF diagram for the granitoids of the study area (after Hyndman, 1985); Molar ratio: A = Al2O3-Na2O-K2O, C = CaO, F = Fe2O3+MgO
4.14	K2O-Na2O-CaO diagram
4.15	General Ca – Na – K compositional difference between the main kinds of alkaline igneous rocks; rocks become more mafic rich towards the Ca corner of the diagram (after Hyndman, 1985)
4.16	SiO2-(Na2O+K2O) – (FeO(t)+MgO) diagram
4.17	SiO2-Na2O-K2O diagram
4.18	Normative Qt-Ab-Or-H2O ratios of igneous rocks from the study area. Diagram showing H2O saturated liquidus field boundaries and isobaric temperature in the system Qtz-Ab-Or-H2O for various water pressures (after Tuttle and Bowen, 1958)
4.19	Temperature – differentiation index diagram for the igneous rocks of the study area, at 2 kb water pressure (after Piwinskii and Wyllie, 1970)
4.20	K2O mol. vs Na2O mol. variation diagram of granitic rocks in the area, showing the fields of syn-kinematic and late-kinematic granites (after Marmo, 1956)
4.21	Schematic depth-temperature relation diagram (after Marmo, 1969)

 

List of Tables
4.1	Chemical composition and norms of the igneous rocks from the Mogok are (in wt%)

Chapter IV Petrochemistry

4.1 Analytic Data

Representative 15 rock samples were selected for analysis, from some common rock types of the study area. Fresh rocks of 1 to 2 lb were crushed using a Sang WC mill crusher, and finely powdered to 300 mesh, using a Katie McGoldrick (X-RF sample preparation method).

Whole rocks geochemical analysis was performed by a wavelength dispersive X-RF spectrometer, using standard calibration method according to Robinson (Analyst), at the University of Tasmania, Australia. The results give Fe₂O₃ as total iron, and comparison with international and L.A.N. Iyer (1953) data are listed in Table 4.1.

4.2 Presentation of the Analytical Data

The weight-percent oxide method, molar-percent and normative-percent methods diagrams were drawn by using the aid of computer (MINITAB-14 GC software) in the present study. Standard C.I.P.W. norms and C.I.P.W. norms with biotite and hornblende were calculated according to the rules of Hutchison (1975), by the use of computer. Differentiation indices, which are also known as “Thornton and Tuttle index,” 1960, which can be used as an indicator of bulk composition, are calculated from the standard C.I.P.W. norms. The results are shown in Table 4.1.

It can be noted from Table 4.1 that the various rock types analysed generally have the common petrochemical characteristics of similar typical rock types. For instance: syenitic rocks have high Al₂O₃ and alkali contents; leucogranites have high SiO₂ and K₂O, but lower content in feromagnesium; mafic and ultramafic rocks have high Fe₂O_3(T) and MgO contents; nepheline rich rocks have high CaO, K₂O and Na₂O contents. In general, it can be said that in modal composition of syenitic rocks, mafic and ultramafic rocks correspond to C.I.P.W. norms with biotite and hornblende. For instance; nepheline syenite and alkali syenite pegmatite have high corundum contents; quartz syenite has high sphene contents; urtite and gabbro have high plagioclase and hornblende; peridotite has high hypersthene (orthopyroxene).

Fig. 4.1. Sample location map of the Mogok Stone Tract (source; Landsat TM image, 30m resolution, band combination 754). (Note: Original is not legible at higher resolution)

Table 4.1. Chemical composition and norms of the igneous rocks from the Mogok area (in wt%).

Table 4.1. Chemical composition and norms of the igneous rocks from the Mogok area (in wt%). (cont.)

Table 4.1. Chemical composition and norms of the igneous rocks from the Mogok area (in wt%). (cont.)

Table 4.1. Chemical composition and norms of the igneous rocks from the Mogok area (in wt%). (cont.)
A-2	Augite-biotite granite, 22° 58' 18.4" N, 96° 28' 8.5" E, peak of Taung-me Taung
P-78	Augite-biotite granite, 22° 57' 33" N, 96° 14' 20" E, On-dan
LG-1	Leucogranite, 22° 56' 32" N, 96° 32' 10" E, Ohn-bin-ywe-twet, East of Mogok
LG-2	Leucogranite, 22° 54' 9.3" N, 96° 23' 55.6" E, Hin-thartaung, west of Kyat-pyin
KB-2	Kabaing Granite, 22° 54' 32" N, 96° 21' 17" E, near Kabaing village
PG-1	Pegmatite, 22° 54' 03" N, 96° 20' 46" E, Sakhan-gyi
AP-1	Pegmatite, 22° 59' 55" N, 96° 27' 48" E, Pyaung-gaung
(46/711)	Augite granite, ENE of 4369', Kyauk-pyat-that area (from L.A.N. Iyer, 1953)
(46/723)	Kabaing Granite (from L.A.N. Iyer, 1953)
OD-5	Sapphire-bearing nepheline syenite, 22° 57' 35" N, 96° 14' 18" E, north of Ondan
B-4	Quartz monzonite, 23° 00' 26.3" N, 96° 30' 19.5" E, Lay-thar, north of Mogok
C-3	Quartz syenite (foliated), 22° 56' 28.2" N, 96° 29' 43.5" E, north of Ohn-gaing
LT-1	Sapphire-bearing alkali syenite pegmatite, 23° 00' 26.0" N, 96° 30' 20" E, JV mine of Lay-thar
(46/721)	Nepheline syenite, north of Kyauk-pyat-that (after L.A.N. Iyer, 1953)
(48/410)	Quartz syenite, west of Pyaung-gaung (after L.A.N. Iyer, 1953)
U-1	Urtite, 22° 54' 34" N, 96° 22' 31" E, near Lone-sho
U-2	Urtite, 22° 54' 12" N, 96° 22' 20" E, Thurein Taung
(46/181)	Urtite, Sinkhwa (after L.A.N. Iyer, 1953)
(46/710)	Hornblende rock, SE of Kyauk-pyat-that (after L.A.N. Iyer, 1953)
P-1	Peridotite, 22° 58' 16" N, 96° 33' 11" E, UMEHL mine, Pyaung-gaung
Pe	Peridotite, 23° 00' 3.4" N, 96° 27' 54" E, NE of Pyaung-gaung
(48/409)	Peridotite, Bernard-myo (after L.A.N. Iyer, 1953)
(46/53)	Hornblende biotite norite, south of Gurka huts, ESE of Bernard-myo (after L.A.N. Iyer, 1953)

 

4.3 Petrogenetic Interpretations

4.3.1 Nature and Chemical Changes in Igneous Differentiation

The igneous rocks of the area are mainly calc-alkaline and only partly alkaline. This is also evident from the bimodal distributions of the petrochemical data plots. According to the (Fig. 4.5 and 4.6), the value of alkali-lime index (Peacock index) is 55 for the igneous rocks of the area, indicating calcic to alkalic-calcic character in this area.

In the normative Ab-Or-An diagram (Fig. 4.9), most of the plots of augite-biotite granite, leucogranite, bioitite microgranite, pegmatite and syenitic rocks fall in the granite field. Hornblende rock, gabbro rock and urtite rocks fall in the tonalite field. However, the two groups may have belonged to two separate igneous events, because the igneous ages probably are different and there are no intermediate rock types present in the area. In the Al₂O₃-CaO-(Na₂O+K₂O) diagram (Fig. 4.8), most of the plots of the igneous rocks fall in the field of per-aluminous and, except urtite rocks, fall in the field of meta-aluminous.

Fe_(T)-(Na₂O+K₂O)-MgO diagram, AFM diagram (Fig. 4.12) shows a rise of Fe_(t) with the decrease of MgO during the initial stage of differentiation. In the later stage of differentiation, there is an increase of alkali with a depletion of Fe_(t). ACF diagram (Fig. 4.13) and some distinctive chemical properties of the granitic rocks of the area indicate that these granitic rocks of are mostly S-type granites.

K₂O-Na₂O-CaO diagram (Fig. 4.14) indicates that in the initial stage of differentiation, there is an addition of K₂O which is subsequently replaced by Na₂O. CaO is decreased from the initial stage but stable in the later stage of differentiation. SiO₂-Na₂O-K₂O diagram (Fig. 4.17) shows the chemical changes in the igneous rocks. It shows that in the later stage of magmatic differentiation Na₂O decreases with an increases in K₂O. Also, SiO₂-(Na₂O+K₂O)-(FeO_t+MgO) diagram (Fig. 4.16) pointed out that, a rise of SiO₂ and a depletion of (FeO_T+MgO) during the entire process of magmatic differentiation. Although, the contents of Na₂O+K₂O indicate no notable change.

Fig. 4.2. Variation diagram of major elements vs SiO2 in igneous rocks of the study area.

Fig. 4.2. Variation diagram of major elements vs SiO2 in igneous rocks of the study area. (cont.)

Fig. 4.3. Na2O/K2O vs SiO2 variation diagram, showing the trend of differentiation (KT-1 to KT-9 are granitic rocks/KT-10 to KT-15 are syenitic rocks and KT-16 to KT-23 are mafic to ultramafic rocks).

Fig. 4.4. Percent anorthite in normative plagioclase plotted against the Thornton-Tuttle differentiation index (KT-1 to KT-9 are granitic rocks/KT-10 to KT-15 are syenitic rocks and KT-16 to KT-23 are mafic to ultramafic rocks).

Fig. 4.5. Alkali-lime index diagram for igneous rocks of the area (Peacock index).

Fig. 4.6. Alkali-lime index diagram for granitic, mafic and ultramafic rocks of the area (Peacock index).

Fig. 4.7. Major element discrimination of igneous rocks supposed to be collision-related, subdivided as alkaline, calc-alkaline and strongly peraluminous, and highly fractionated calc-alkaline (after Sylvester, 1989).

Fig. 4.8. Al2O3 — CaO — (Na2O+K2O) diagram.

Fig. 4.9. Normative Ab-Or-An diagram for the igneous rocks of the study area, with dividing lines according to O’Connor (1965).

Fig. 4.10. Plot of Wright’s (1969) alkalinity index Al2O3+CaO+Na2O+K2O/Al2O3+CaO-Na2O+K2O solid line and field names are after Wright, and the dashed line, separates alkaline from calc-alkaline, after Khin Zaw (1986).

Fig. 4.11. Diagram showing K2O vs SiO2 wt% ratio for granitic rocks and syenitic rocks of the study area. Calc-alkaline – Jake and Gill (1970) Alkaline (1) – Gried and Lefeure (1972) Alkaline (2) – Lemoitre (1962)

Fig. 4.12. Fe(t)-Na2O+K2O-MgO (AFM) diagram, in terms of alkalis, total Fe and Mg for the igneous rocks of the area.

Fig. 4.13. ACF diagram for the granitoids of the area (after Hyndman, 1985); Molar ratio: A = Al2O3–Na2O–K2O, C = CaO, F = Fe2O3+MgO.

Fig. 4.14. K2O–Na2O–CaO diagram.

Fig. 4.15. General Ca – Na – K compositional difference between the main kinds of alkaline igneous rocks; rocks become more mafic rich towards the Ca corner of the diagram (after Hyndman, 1985).

4.3.2 Conditions of the Crystallization of the Igneous Rocks

As already shown in Fig. 4.9, the bulk composition of igneous rocks in the area ranges from granite to peridotite, but majority of the rocks falls in the granite field. All the analysed rocks, except ultramafic, mafic and urtite series, contain greater than 40% normative Ab+Or+Qtz. These ratios are plotted in ternary diagram (Fig. 4.18) with the quartz -feldspar boundaries at water pressures of 2 kb and 10 kb are shown projected onto the anhydrous base of the tetrahedron, after Tuttle and Bowen (1958). The dashed line represents the Ab/An piercing points projected onto the plane Ab-Or-Qtz at 5 kb water pressure (Wiebe, 1974).

According to Fig. 4.18, the points of augite-biotite granite and leucogranite lie between the ternary minimum of 2 kb and 10 kb , and the the two points of biotite microgranite and one point of leucogranite lie at the 5 kb curve of Wiebe (1974). Alkaline rocks and some points of pegmatite lie outside the ternary minimum, and it is in the field of albite and quartz-orthoclase. Assuming P_H2O = P_total, the minimum P_H2O allowed by the scattered points are 2 kb and 5 kb respectively. If the igneous rocks were assumed as crystallization at minimum pressure of 2 kb, their liquidus temperatures could have been cogitated from the diagram showing the relationships between differentiation index and temperature at 2 kb water pressure (Fig 4.19). From this diagram, the liquidus temperatures are: 650°C for pegmatite, 680°C for leucogranite, 690°C for biotite microgranite, 700°C for augite-biotite granite, 700–720°C for syenitic rocks, 785°C for urtite rocks, 950°C for gabbro, and 1180°C for peridotite.

Assuming that they were emplaced at P_H2O of 10 kb, the temperature of crystallization of the rocks would become considerably lower. Depth of crystallization of the igneous rocks from the study area can be deduced from the schematic depth temperature relation diagram (Fig 4.21).

In K₂O mol. vs Na₂O mol. variation diagram (Fig. 4.20), the plots of augite-biotite granite fall within the field of syn-kinematic granite, but leucogranite, biotite microgranite and pegmatite fall within the field of late-kinematic granite.

Fig. 4.16. SiO2–(Na2O+K2O)–(FeO(t) + MgO) diagram.     Fig. 4.17. SiO2–Na2O–K2O diagram.

Fig. 4.18. Normative Qt-Ab-Or-H2O ratios of igneous rocks from the area. Diagram showing H,O saturated liquidus field boundaries and isobaric temperature in the system Qtz-Ab-Or-H2O for various water pressures. (after Tuttle and Bowen, 1958).

Fig. 4.19. Temperature-differentiation index diagram for the igneous rocks of the area, at 2 kb water pressure (after Piwinskii and Wyllie, 1970).

Fig. 4.20. K2O mol. vs Na2O mol. variation diagram of granitic rocks in the area, showing the fields of syn-kinematic and late-kinematic granites (after Marmo, 1956).

Fig. 4.21. Schematic depth-temperature relation diagram (after Marmo, 1956).

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Thus concludes Chapter IV. Access Chapter V here.

References follow.

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