越南中部嵩高岩體高度變質岩之岩石學研究

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(1)Graduate Institute of Earth Sciences College of Science National Taiwan Normal University. Master’s Thesis. 越南中部嵩高岩體高度變質岩之岩石學研究. Petrological analysis for high-grade metamorphic rocks in the Kontum Massif, central Vietnam. Student: Bui Thi Men Supervisor: Prof. Meng-Wan Yeh. 中華民國 109 年 06 月 June, 2020.

(2) ACKNOWLEDGMENT My thesis has been completed at the Department of Earth Sciences, National Taiwan Normal University, under the guidance of Prof. Meng-Wan Yeh. I would like to express my sincere appreciation to my supervisor for her patience, encouragement, and immense knowledge. Besides, I am very thankful to Prof. Hai Thanh Tran at Hanoi University of Mining and Geology, who always encouraged me as well as support me during my study. I am also very grateful to all Professors and staff from the Department of Earth Sciences for their kind help and supports. Especially, I would like to thanks the staff of Magmatic and Volcanic Processes Lab, who permitted me to use the microscope, which is great of value to my thesis. In addition, I thank my labmates (Lulumi, Hank) for their encouragement as well as helpful discussions. Special thanks for Vietnamese and Taiwanese friends who have shared their experience and companion with me to overcome all difficulties in the last four years. Last but not least, I am indebted to my family and relatives as well as friends who always encourage me and motivate me throughout my tough time in Taiwan.. i.

(3) Contents. ACKNOWLEDGMENT ................................................................................................... i Contents ............................................................................................................................ ii List of Figures ................................................................................................................... v List of Tables ................................................................................................................. xiv Abbreviations ................................................................................................................. xv Abstract ............................................................................................................................. 1 CHAPTER 1. INTRODUCTION ..................................................................................... 3 CHAPTER 2. GEOLOGICAL BACKGROUND ............................................................ 8 2.1 Kannak complex ..................................................................................................... 9 2.2 Ngoc Linh complex .............................................................................................. 10 2.3 Kham Duc complex .............................................................................................. 11 2.4 Hai Van complex .................................................................................................. 12 CHAPTER 3. PETROGRAPHIC METHODS ............................................................. 14 3.1 Sample collection ................................................................................................. 14 3.2 Sample processing ................................................................................................ 15 3.3 Sample analysis .................................................................................................... 15 3.4 Mineral percentage measurement ......................................................................... 16 3.5 Composition-paragenesis diagram ....................................................................... 17 CHAPTER 4. RESULTS ................................................................................................ 23. ii.

(4) 4.1 Xa Lam Co Formation .......................................................................................... 23 4.1.1 An Trung Commune, An Lao District, Binh Dinh Province ......................... 24 4.1.2 An Dung Commune, An Lao District, Binh Dinh Province.......................... 26 4.1.3 Ba Tieu Commune, Ba To District, Quang Ngai Province ........................... 42 4.1.4 Ba Chua Commune, Ba To District, Quang Ngai Province .......................... 47 4.2 Dak Lo Formation ................................................................................................ 51 4.3. Kim Son Formation ............................................................................................. 57 4.3.1 Chanh Dao Village, My Tho Commune, Phu My District, Binh Dinh Province ................................................................................................................................ 58 4.3.2 Bong Son Commune, Hoai Nhon District, Binh Dinh Province ................... 70 4.3.3 An My Commune, Hoai Nhon District, Binh Dinh Province ....................... 77 4.4 Hai Van Complex ................................................................................................. 79 CHAPTER 5. INTERPRETATION ............................................................................... 83 5.1. Xa Lam Co Formation ......................................................................................... 83 5.2. Dak Lo Formation ............................................................................................... 85 5.3. Kim Son Formation ............................................................................................. 85 CHAPTER 6. DISCUSSIONS ....................................................................................... 90 6.1. Metamorphic evolution of high-grade metamorphic rocks in the Kontum Massif .................................................................................................................................... 90 6.2. Timing of high-grade metamorphism in the Kontum Massif .............................. 91 6.3. Tectonic evolution of the Kontum Massif ........................................................... 92. iii.

(5) CHAPTER 7. CONCLUSIONS ..................................................................................... 94 References ...................................................................................................................... 95 Appendix. Chemical composition in %mole of minerals calculated from mineral percentage ..................................................................................................................... 107. iv.

(6) List of Figures Figure 1. 1. Topography of present East and Southeast Asia. .......................................... 3 Figure 1. 2. Distribution of metamorphic rocks in Vietnam and adjacent Southeastern Asia. .................................................................................................................................. 4 Figure 2. 1. Simplified map of the Kontum Massif and location of study area……………………………………………………………………………………….8 Figure 2. 2. Simplified geological map of the Kontum Massif……………………….....13 Figure 3. 1. Sample collection in the field with visible coarse grains of granodiorite (a) and leucosome (b)………………………………………………………………………14 Figure 3. 2. Useful machines to make thin section: a. Rock cutting machine; b. PetrothinThin sectioning system; c. Grinder and Polisher Machine. ............................................ 15 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. ....................................................................................................... 16 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..................................... 17 Figure 3. 5. An example of classifying igneous rock using the QAP diagram of Streckeisen (1974). ......................................................................................................... 18 Figure 3. 6. An example of using the ACF diagram to define the bulk chemical composition for the sample in this study. ....................................................................... 20 Figure 3. 7. ACF diagram indicated for reactions between minerals in the sample. ...... 21 Figure 3. 8. A’FM diagram showing the paragenetic relations observed in the sample, green point represented for the composition of that sample. .......................................... 22. v.

(7) Figure 4. 1. Map location for outcrops of Xa Lam Co Formation……………………….23 Figure 4. 2. The occurrence of amphibolite in the outcrop: a. Big scale; b. Small scale. ........................................................................................................................................ 24 Figure 4. 3. The mineral assemblage for sample K16-11 -18E1, taken under CPL. ...... 24 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. .......................................................................................... 25 Figure 4. 5. Representative bulk chemical composition for this amphibolite (K16-11-18E1) in the ACF diagram. ....................................................................................................... 26 Figure 4. 6. The occurrence of three rock types in the outcrop: a. Biotite ± muscovite gneiss; b. Biotite gneiss; c. Leucosome. ......................................................................... 27 Figure 4. 7. Bulk chemical composition for migmatite in this outcrop by calculating mineral percentage. ......................................................................................................... 27 Figure 4. 8. Mineral assemblage for biotite ± muscovite gneiss (K16-11-18C1) under CPL. ........................................................................................................................................ 29 Figure 4. 9. High relief grains of garnet intergrew with biotite and quartz and indicated for Ma under PPL (a) and CPL (b).................................................................................. 29 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 coexisted with microline and myrmekite. .......................................................................... 30 Figure 4. 11. The presence of chlorite and muscovite which are formed by the alteration of biotite and plagioclase, respectively........................................................................... 31 Figure 4. 12. Mineral assemblage for this sample, taken under CPL. ............................ 32. vi.

(8) Figure 4. 13. An anhedral grain of garnet intergrows with biotite and quartz. .............. 32 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. ...................................................................................................... 33 Figure 4. 15. Typical mineral assemblage for leucosome in this outcrop (sample K16-1118C3), taken under CPL. ................................................................................................. 34 Figure 4. 16. Anhedral high relief grains of garnet under PPL; b. Muscovite filled in the fractures within garnet (taken under CPL). .................................................................... 34 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. ..... 35 Figure 4. 18. The occurrence of two rock types in the outcrop: a. Pegmatitic muscovite; b. Amphibolite ................................................................................................................ 36 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 quartzofeldspathic composition in the ACF diagram. ................................................................ 36 Figure 4. 20. Typical mineral assemblage for pegmatitic muscovite (K16-11-18D1). .. 37 Figure 4. 21. The presence of pre- deformation of plagioclase, which was broken (a) and deformed (b) by deformation.......................................................................................... 37 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. Welldeveloped tartan twinning of microline; d. Grain boundary migration recrystallization in quartz. ............................................................................................................................. 39 Figure 4. 23. Typical mineral assemblage for sample K16-11-18D2, taken under CPL. 39. vii.

(9) Figure 4. 24. a. The broken grain of garnet is distinguished by high relief under PPL. b. Porphyroclasts of K-feldspar surrounded by biotite and quartz, taken under CPL. ....... 40 Figure 4. 25. a. Strongly kinked crystal of biotite characterized for plastic deformation; b. High interference color of biotite with microfold; c. Plagioclase crystal with bent polysynthetic twinning; d. The occurrence of cordierite with lamellae twinning at the edges of crystal; e. Subgrain rotation recrystallization and grain boundary migration recrystallization of quartz; f. Chlorite alteration of biotite caused by retrograde metamorphism. ............................................................................................................... 41 Figure 4. 26. The presence of two rock types: a. Migmatitic gneiss; b. Biotite gneiss. . 42 Figure 4. 27. Mineral assemblage for migmatitic gneiss, taken under CPL. ................. 42 Figure 4. 28. a. The dark brown color of biotite; b. Subhedral crystal of garnet. .......... 43 Figure 4. 29. a. Brown elongated crystal of biotite with clear cleavage; b. Quartz with undulose extinction and bulging/grain boundary migration recrystallization. ............... 43 Figure 4. 30. a. Pale-green color of chlorite caused by biotite alteration; b. High interference color of muscovite intergrows with plagioclase, biotite and quartz. .......... 44 Figure 4. 31. Migmatitic gneiss dropped into quartzo-feldspathic field in the ACF diagram. ........................................................................................................................................ 44 Figure 4. 32. The typical mineral assemblage of this biotite gneiss under CPL. ........... 45 Figure 4. 33. a. The dark brown color of biotite in central view; b. Pleochroism in color of hornblende with twinning intergrow with clinopyroxene and orthopyroxene. .......... 46 Figure 4. 34. a. Elongated crystals of biotite with pleochroism color; b. Myrmekite with isolated vermicules of irregular shape. ........................................................................... 46 Figure 4. 35. The occurrence of muscovite within K-feldspar and plagioclase, taken under CPL. ................................................................................................................................ 47. viii.

(10) Figure 4. 36. Bulk chemical composition of sample K16-11 -19D1 in the ACF diagram. ........................................................................................................................................ 47 Figure 4. 37. Granodiorite in the outcrop: a. Big scale; b. Small scale .......................... 48 Figure 4. 38. Typical mineral assemblage for this granodiorite, taken under CPL. ....... 48 Figure 4. 39. a. Subhedral crystal of plagioclase co-existed with myrmekite and quartz; b. Porphyroclast of feldspar surrounded by the growth of biotite and quartz. ................... 49 Figure 4. 40. a. Elongated biotite intergrow with grain boundary migration recrystallization of quartz and aligned with foliation; b. Co-existing of perthite, myrmekite and microline within this granodiorite. ........................................................ 50 Figure 4. 41. The occurrence of chlorite intergrows with biotite, plagioclase and muscovite: a. Under PPL; b. Under CPL........................................................................ 50 Figure 4. 42. QAP diagram signified the bulk chemical composition for sample K16-1119E1. ............................................................................................................................... 51 Figure 4. 43. Map location for the outcrop of Dak Lo Formation. ................................. 51 Figure 4. 44. The occurrence of two rock types in the outcrop: a. Amphibolite; b. Leucosome. ..................................................................................................................... 52 Figure 4. 45. a. The typical mineral assemblage of biotite gneiss rather than amphibolite, taken under CPL; b. Sample K16-11-19B1 dropped into quartzo-feldspathic rock field instead of basic rocks field. ............................................................................................ 53 Figure 4. 46. a. Pleochroism colors of elongated grains biotite; b. Partially altered of biotite and replaced by pale-green color of chlorite. ...................................................... 53 Figure 4. 47. a. Intergrowth of two feldspars generated flame perthite with parallel exsolution lamellae; b. Bulging recrystallization of quartz formed at lower temperature conditions. ...................................................................................................................... 54. ix.

(11) Figure 4. 48. Representative mineral assemblage for leucosome (K16 -11-19B2), taken under CPL. ...................................................................................................................... 55 Figure 4. 49. Euhedral crystal of plagioclase with polysynthetic twinning and muscovite alteration taken under PPL (a) and CPL (b). ................................................................. 55 Figure 4. 50. a. Crosshatched twinning of microline intergrows with flame perthite; b. Vermicular intergrowth of quartz and plagioclase defined myrmekite; c. Undulose extinction with bulging recrystallization and grain boundary migration recrystallization; d. Pale-green color of chlorite grows within biotite as alteration result. ........................ 56 Figure 4. 51. Leucosome (K16-11-19B2) dropped into quartzo-feldspathic rocks field in the ACF diagram. ........................................................................................................... 57 Figure 4. 52. Map location for outcrops of Kim Son Formation. ................................... 58 Figure 4. 53. Sketch diagram for the first outcrop with sample locations for biotite ± muscovite gneiss, amphibolite, and granodiorite (these rectangular will be shown later). ........................................................................................................................................ 58 Figure 4. 54. Bulk chemical composition, as well as protolith for three rock types, are shown in the ACF (a) and QAP diagram (b). ................................................................. 59 Figure 4. 55.a. The occurrence of biotite ± muscovite gneiss in outcrop; b. Sketch diagram for the appearance of biotite ± muscovite gneiss in the outcrop. ................................... 60 Figure 4. 56. Mineral assemblage for sample K16-11-17A1: a. Under plane-polarized light (PPL); b. Cross-polarized light (CPL). ........................................................................... 60 Figure 4. 57. Coarse poikiloblasts of garnet co-existed with amphibole and biotite within gneiss (taken under PPL). ............................................................................................... 61 Figure 4. 58.a. The hexagonal shape of metamorphic garnet intergrows with biotite; b. Co-existing of amphibole, garnet, and biotite (under PPL). ........................................... 62. x.

(12) Figure 4. 59. The clear evidence of deformation within this rock: a. A kinked grain of biotite; b. Bent twinning of plagioclase with the muscovite alteration. ......................... 62 Figure 4. 60. a. The intergrowth of patchy tartan twinning of microline, myrmekite and perthite. Perthite shows coarse grain with exsolution lamellae; b. The green color of chlorite within biotite caused by alteration. The upper left corner show strain-induced grain boundary migration (bulging) in quartz (red arrow). ............................................ 63 Figure 4. 61. Chemographic projections for sample K16-11-17A1. ............................... 64 Figure 4. 62. a. Amphibolite in outcrop cut through by quartz vein; b. Simple sketch diagram for this outcrop. ................................................................................................ 65 Figure 4. 63. Mineral assemblages of sample K16-11-17A2 under PPL. ....................... 65 Figure 4. 64. a. A subhedral grain of hornblende is represented for igneous protolith; b. Upper biotite showed dark brown color and elongated grain indicated for metamorphic biotite. ............................................................................................................................. 66 Figure 4. 65. a. Kink band of biotite, which is represented for syn- Sm; b. Intergrowth of alteration rim of hornblende and biotite with chlorite alteration within as a retrograde result. .............................................................................................................................. 67 Figure 4. 66. Prediction chemographic projection for sample K16-11-17A2 in the ACF diagram. .......................................................................................................................... 67 Figure 4. 67. a. Granodiorite in outcrop; b. Sketch diagram shows the relationship between granodiorite and quartz vein. ............................................................................ 68 Figure 4. 68. Mineral assemblage for sample K16-11-17A3 under CPL ....................... 68 Figure 4. 69. a. Igneous biotite with dark brown color; b. Subhedral crystal of plagioclase intergrows with biotite and quartz. ................................................................................. 69. xi.

(13) Figure 4. 70. a. Partially growth of biotite (small grains) in hornblende; b. Elongated grains of biotite with chlorite alteration (black arrow). .................................................. 69 Figure 4. 71. a. Whole view from outcrop with the occurrence of muscovite ± biotite gneiss with gentle folds; b. Missing part from (a) with the presence of amphibolite rich dark layer within gneiss. ................................................................................................. 70 Figure 4. 72. Mineral assemblage of altered gneiss matrix, taken under CPL. .............. 71 Figure 4. 73. a. Coarse grains of muscovite with random growth defined for pre- Ma; b. Strongly elongated biotite and muscovite aligned with foliation; c. Shiny fine-grained of muscovite intergrow with elongated biotite and muscovite; d. Undulose extinction with bulging recrystallization of quartz. ................................................................................. 72 Figure 4. 74. Muscovite ± biotite gneiss under CPL. ..................................................... 73 Figure 4. 75. a. Microfold of biotite formed during deformation; b. Coexistence of coarse grains and fine-grained of muscovite; c. The presence of undulose extinction of quartz with the growth of hematite. ........................................................................................... 74 Figure 4. 76. Mineral assemblage for amphibolite (K16-11-18A3) under CPL. ............ 74 Figure 4. 77.a. Euhedral crystals of hornblende with pleochroism colors; b. Igneous biotite with dark brown color intergrow with elongated biotite; c. Subhedral crystal of clinopyroxene with two clear cleavage systems; d. High interference color of orthopyroxene under CPL co-existed with hornblende, quartz and plagioclase. ........... 75 Figure 4. 78. Intergrowth of quartz and sodic plagioclase defined for myrmekite. ....... 76 Figure 4. 79. Bulk chemical composition for sample K16-11-18A3 in ACF diagram. .. 77 Figure 4. 80. Muscovite ± biotite gneiss in weathered outcrop: a. Big scale; b. Small scale. ........................................................................................................................................ 77 Figure 4. 81. Mineral assemblage of sample K16-11-18B1, taken under PPL. .............. 78. xii.

(14) Figure 4. 82. a. Elongate crystal of biotite with bent cleavage; b. Microboudinage of kyanite with high interference color under CPL. ........................................................... 79 Figure 4. 83. Map location for outcrop of Hai Van Complex ........................................ 79 Figure 4. 84. The occurrence of granodiorite in the outcrop. ......................................... 80 Figure 4. 85. Mineral assemblages of sample K16-11-19A1, taken under CPL............. 80 Figure 4. 86. a. Deformed plagioclase with bent polysynthetic twinning; b. Fragmented plagioclase grain with quartz and K-feldspar between fragments. ................................. 81 Figure 4. 87. a. Intergrowth of two feldspars shows perthite texture; b. The appearance of tartan twinning of microline co-existed with perthite and plagioclase. .......................... 82 Figure 4. 88. QAP diagram shows granite to granodiorite in the bulk chemical composition of sample K16-11-19A1. ............................................................................ 82 Figure 5. 1. P-T condition for the intergrowth of garnet and biotite by using garnet-biotite thermometer ………………………..…………………………………………………..84 Figure 5. 2. Proposed sketch diagram for tectonic evolution in 1st outcrop if biotite ± muscovite gneiss has sedimentary protolith. .................................................................. 86 Figure 5. 3. Proposed sketch diagram for tectonic evolution in 1st outcrop if biotite ± muscovite gneiss has igneous protolith. ......................................................................... 87 Figure 5. 4. The proposed tectonic setting for Kim Son Formation (1st outcrop) might correlated to the collisiion zone of oceanic-continental crusts………………………… 88 Figure 6. 1. Presumed P-T path for metamorphic rocks from Kannak Complex in the Kontum Massif ……………………………………..…………………………………..90 Figure 6. 2. Active subduction zones between Indochina and South China blocks caused the development of arc magmatism. ............................................................................... 92. xiii.

(15) Figure 6. 3. The closing of Paleo-Tethys during Late Permian-Early Triassic led to the collision of South China and Indochina blocks .............................................................. 93 Figure 6. 4. The development of NW-SE trending shear zones and erosion brought all the rock types to the surface. ................................................................................................ 93. List of Tables Table 1. 1. Recent geochronology results from the Kontum Massif ................................ 7 Table 4. 1. Mineral percentages of analyzed samples in this study................................ 28 Table 5. 1. Summary of petrological analysis results for 19 samples in this study........ 89. xiv.

(16) Abbreviations Abbreviation PPL CPL LA-ICP-MS SHRIMP CHIME U-Pb U-Pb-Th Rb-Sr UHT Ma Ga. Meaning Plane-polarized light Cross-polarized light Laser Ablation Inductively Coupled Plasma Mass Spectrometer Sensitive High-Resolution Ion Microprobe Chemical Th-U-total Pb Isochron Method Uranium-Lead Uranium-Lead-Thorium Rubidi-Strontium Ultra High Temperature Million years Billion years. Mineral abbreviations of Siivola and Schmid from IUGS Grt Ky Bt Ms Kfs Zrn Pl Am Hem. Garnet Kyanite Biotite Muscovite Alkali Feldspar Zircon Plagioclase Amphibole Hematite. Mc Myr Chl Opx Cpx Hbl Qtz Crd Mnz. xv. Microline Myrmekite Chlorite Orthopyroxene Clinopyroxene Hornblende Quartz Cordierite Monazite.

(17) Abstract The Kontum Massif, central Vietnam composed mainly of high-grade metamorphic rocks (amphibolite to granulite facies) and has been traditionally regarded as the ‘Precambrian basement’ of Indochina Block. Recent geochronology studies reported two distinct ultra-high temperature metamorphic events during OrdovicianSilurian and Permian-Triassic. However, the supporting evidence to indicate for ultrahigh temperatures within those studies were poorly documented. The current study conducts detail petrological analysis to reconstruct the metamorphic evolution along with the tectonic evolution of the Kontum Massif. Three lithology units of biotite ± muscovite gneiss interlayered with amphibolite that was later intruded by undeformed granodiorite can be observed from the outcrop. Petrological analysis indicated both gneiss and amphibolite were metamorphosed and deformed under amphibolite facies. Characteristic garnet ± biotite ± K-feldspar ± microline ± myrmekite ± muscovite ± chlorite ± plagioclase ± quartz ± amphibole, biotite ± muscovite ± kyanite ± quartz mineral assemblages are observed for biotite ± muscovite gneiss samples. Hornblende ± biotite ± plagioclase ± pyroxene ± chlorite ± muscovite ± quartz mineral assemblages are observed for amphibolite. The mineral percentages revealed bulk chemical composition of quartzofeldspathic rocks (for gneiss) and basic rocks (for amphibolite), which are represented for the continental crust and oceanic crust, respectively. Rocks in the Kontum Massif might be experienced two metamorphic episodes with the highest peak metamorphic conditions is determined by the presence of garnet or kyanite and equilibrium minerals of Kfs ± Pl ± Bt ± Grt (Ma). The lower condition defined by the occurrence of chlorite and muscovite (Mb). High-grade metamorphic rocks in the Kontum Massif show a clockwise P-T path with peak metamorphic conditions of Ma and Mb as retrograde metamorphism which is. 1.

(18) considered to be occurred ealier than Triassic and Late Triassic, respectively. Therefore, the tectonic evolution for Kontum Massif is most likely related to a collisional orogeny between the Indochina and South China blocks. Keywords: High-grade metamorphic rocks, petrological analysis, Kontum Massif. 2.

(19) CHAPTER 1. INTRODUCTION Present-day East and Southeast Asia is located at the collision zone between the Eurasian, India-Australia, and the Philippine Sea Plates (Fig. 1. 1). It was formed by the amalgamation of many heterogeneous continental blocks, which were derived from Gondwana (Metcalfe, 1988) by continental dispersion, plate tectonic convergence, collision and accretion (Metcalfe, 2013).. Figure 1. 1. Topography of present East and Southeast Asia.. Southeast Asia is the result of micro-continent collisions of South China, Indochina, Sibumasu, West Burma blocks (Fig. 1. 2) and has been considered to assemble during the late Permian to Triassic (Nagy et al., 2001; Carter et al., 2001; Osanai et al., 2001, 2004, 2008; Lan et al., 2003; Nakano et al., 2007; Lepvrier et al., 2008). As a result. 3.

(20) of the collision, high-grade metamorphic rocks (including ultrahigh-T and ultrahigh-P) were formed and exposed along the micro-continent sutures. These metamorphic rocks provide key insights regarding the process of continental collision during their amalgamation. In Vietnam, the metamorphic rocks formed during this collision episode are exposed mainly in Red River shear zone, Song Ma suture zone, Truong Song fold belt and Kontum Massif (Fig. 1. 2).. Figure 1. 2. Distribution of metamorphic rocks in Vietnam and adjacent Southeastern Asia (after Owada et al., 2019).. The Kontum Massif is located in south-central Vietnam, which is considered as the southeastern extension of the Truong Son Fold Belt. It has been long regarded as an “Archean core” complex within the Indochina craton (Hutchinson, 1989; Tien, 1989, 1991; Bao et al., 1994). However, the inferred “Archean” ages and Gondwana-derived. 4.

(21) were based on the petrologic similarities of the Kontum Massif with the Archean granulites terranes in Gondwanaland such as those in East Antarctica, India, Sri Lanka and Australia (Katz, 1993; Lepvrier et al., 1997; Maluski and Lepvrier, 1998; Maluski et al., 1999, 2002; Carter et al., 2001; Nagy et al., 2001; Nam et al., 2001). By using various geochronological methods, recent studies documented Early Paleozoic and Middle Triassic ages in this Massif (Table. 1. 1) and yielded two separate magmatic or tectono-metamorphic events, which are: Ordovician-Silurian (Nam, 1998, 2001; Nagy et al., 2001; Carter et al., 2001; Osanai et al., 2001; Lan et al., 2003; Maluski et al., 2005; Roger et al., 2007; Nakano et al., 2007, 2013; Vuong et al., 2020) and Permian-Triassic (Nam, 1998, 2001; Nagy et al., 2001; Lan et al., 2003; Osanai et al., 2001, 2004, 2008; Maluski et al., 2005; Roger et al., 2007; Usuki et al., 2009; Nakano et al., 2003, 2007, 2013, Tran et al., 2014. Vuong et al., 2020). Besides, the metamorphic evolution of the Kontum Massif has been demonstrated by several previous studies (Nakano et al., 2004, 2007, 2009, 2013; Osanai et al., 2004, 2008; Maluski et al., 2005; Roger et al., 2007; Tich et al., 2013). The P-T condition for high-grade metamorphic rocks in the Kontum Massif has been estimated by geothermobarometry for garnet-biotite at 900-990ºC and 1.03-150GPa (Nakano et al., 2007, 2013). A similar value was also recorded for garnet-orthopyroxene gneiss (Osanai et al., 2008; Owada et al., 2019). Most of these studies suggested that the metamorphic rocks in the Kontum Massif have experienced clockwise P-T paths. Although there are a variety of studies that have been conducted (e.g., Hutchinson, 1989; Nam, 1998; Lo et al., 1999; Maluski et al., 2005; Tich et al., 2007, etc.) but the supporting evidence to indicate for ultra-high temperature metamorphism within the Kontum Massif remained absent.. 5.

(22) Nevertheless, there is a controversy about the metamorphic evolution for the Kontum Massif, whether it is mono- versus poly- metamorphic evolution. Some studies supported for a single high-grade event which might occur during the Ordovician-Silurian (Maluski et al., 2005; Roger et al., 2007; Usuki et al., 2009) or in Permian-Triassic (Nagy et al., 2001; Lan et al., 2003; Osanai et al., 2004; Owada et al., 2019) whereas others preferred two high-grade metamorphic events (Lepvrier et al., 2008; Nakano et al., 2013; Tich et al., 2013; Faure et al., 2018; Vuong et al., 2020). The inconsistency is mostly owing to the dependence on which metamorphic ages were detected through the single dating method such as Zrn U-Pb dating or Mnz U-Th-Pb dating, although metamorphic rocks with various metamorphic conditions occur in the Kontum Massif. The OrdovicianSilurian event was thought to be related to magmatic emplacement (Nagy et al., 2001) while Permian-Triassic ages could be associated with the closing of Paleo-Tethys Sea (Nagy et al., 2001) which led to microcontinental collision tectonics (Chung et al., 1999; Carter et al., 2001; Lan et al., 2003; Osanai et al., 2004; Faure et al., 2018). The most recent studies of Faure et al. (2018) and Owada et al. (2019) suggested that plume-related magma played a significant role as the heat source for high-grade metamorphism in the Kontum Massif in Late Permian to Early Triassic. Most previous studies results were obtained from the western part of the Kontum Massif, while the eastern part remained poorly explored. This study reports the preliminary petrological results for the eastern part of the Kontum Massif. This study carried out a petrological analysis for high-grade metamorphic rocks collected from the eastern portion of Kontum Massif in the hope of determining (1) metamorphic conditions and (2) metamorphic evolution of the Kontum Massif.. 6.

(23) Table 1. 1. Recent geochronology results from the Kontum Massif Loc. No.. Age (Ma). 1 2 3 4. 250 250 405-403 405-403. 5 6 7 8 9 10 11 12 13 14. 270.7 264.3 254 259 240.7 243.3 251.5 249 436 436. 15 16 17. 242 248 405-403 343-325 244-241 237.9 250-247. 18 19 20 21 22 23. 247.8 1404 253 436. Rock type Amphibolite Migmatite gneiss Grt-Bt gneiss Opx-Cpx-Hbl gneiss Grt-Crd-Sil-Bt gneiss Granulites Charnockite Grt-Crd-Sil-Bt gneiss Charnockite Charnockite Charnockite Charnockite Charnockite Biotite gneiss Grt-Bt gneiss. Complex. Method Sr-Nd. References. Formation (in this study’s map). Lan et al., 2003. Sm-Nd Nakano et al., 2007 Ar-Ar. Maluski et al., 2005. Sm-Nd Ar-Ar. Nakano et al., 2007. Kim Son. Kannak. Maluski et al., 2005. U-Pb Sr-Nd. Nagy et al., 2001 Lan et al., 2003. Sm-Nd. Nakano et al., 2007. Xa Lam Co. Ngoc Linh. Grt-Opx-Crd-Sil-Bt gneiss Grt-Opx-Cpx granulite Granulite. Dak Lo. Ar-Ar Maluski et al., 2005. Granulite Charnockite Spl-Crn-bearing Grt-Crd-Sil granulite Grt-Opx-Cpx granulite Granulite Granulite. Kannak U-Pb. Vuong et al., 2020. Ar-Ar Maluski et al., 2005 SHRIMP U-Pb Nam et al., 2001. Cpx-Hbl-Bt gneiss. Ngoc Linh. 7. Nakano et al., 2007. Kon Cot Hai Van.

(24) CHAPTER 2. GEOLOGICAL BACKGROUND The Kontum Massif is considered as the most massive continuous exposure of Precambrian basement within Indochina (Tien et al., 1989) and has been inferred to be the stable continental core of Southeast Asia (Hutchinson, 1989). The Kontum Massif is mainly composed of high-grade metamorphic rocks which are partly covered by Mesozoic volcano-sedimentary formations and Neogene-Quaternary basalts. The metamorphic rocks were later intruded by Paleo-Mesozoic granodiorite bodies such as Ben Giang-Que Son, Van Canh and Hai Van Complexes (Fig. 2. 2, DGMVN, 1989, 1995, Nam, 1998).. Figure 2. 1. Simplified map of the Kontum Massif and location of study area (after Owada et al., 2019).. 8.

(25) The Kontum Massif has been subdivided into three different tectonostratigraphic terranes of the Kannak, Ngoc Linh, and Kham Duc complexes (Fig. 2. 1). 2.1 Kannak complex The Kannak complex occupies the southeastern part of the Kontum Massif (Fig. 2. 1; Fig. 2. 2) and consists mainly of granulite facies metamorphic rocks. The principal lithologies for the Kannak complex are garnet ± cordierite ± biotite gneiss, garnet ± cordierite ± sillimanite ± biotite gneiss, garnet ± orthopyroxene ± cordierite gneiss, orthopyroxene ± clinopyroxene gneiss, and wollastonite-bearing calc-silicate gneiss. These rocks have been subdivided into five subgroups with following sequence of Kon Cot mafic granulite, Xa Lam Co leptynite, Kim Son khondalite, Dak Lo calciphyr granulite and Song Ba orthopyroxene granulite (Figure 2. 2; DGMVN, 1989; Long, 2011). The formation ages of these subgroups were poorly documented. The oldest subgroup (Kon Cot) has been considered to be formed during Precambrian, which evidenced by 1.4Ma detrital zircon ages for granulite (Nam et al., 2001) and 1.4-1.6Ga Rb-Sr ages (Thi, 1985; DGMN, 1989). This formation age of Kon Cot subgroup made up the upper intercept for all the samples which should be younger than 1.4-1.6Ga. The Kannak Complex was considered as an Archean complex based on the similarities in metamorphic grade with other Archean granulite compositions in the world (Bao and Thang, 1979; Luong and Bao, 1982; Trang, 1986; Tien, 1991; Bao et al., 1994). Recent studies documented Neo-Proterozoic to Lower Devonian and Permian-Triassic ages. The Neo-Proterozoic to Lower Devonian age is represented by 401-418Ma SHRIMP U-Pb age of orthogneiss (Carter et al., 2001). Different methods in recent studies also gave similar results. They are 678Ma Sm-Nd age for remaining amphibolite (Nakano et al., 2003), 479Ma U-Th-Pb CHIME age for monazite in UHT granulite (Osanai et al., 2001), 405-403Ma 40Ar-39Ar age for granulite (Maluski et al., 2005). The 9.

(26) Permian-Triassic age was confirmed by 235-246Ma. 40. Ar-39Ar age for metapelitic. granulites (Maluski et al., 2005), 238-250Ma U-Th- Pb CHIME age for UHT granulite (Osanai et al., 2001), 251Ma zircon age (Nagy et al., 2001), 243-258Ma SHRIMP U-Pb age (Carter et al., 2001) and 249-253Ma 40Ar-39Ar biotite age from charnockite (Nagy et al., 2001). The most recent results of Vuong et al. (2020) also confirmed the similar UPb zircon ages of 247-250Ma for granulite in this complex. The P-T conditions for the Kannak complex have been estimated to be 800-850ºC and 8±1kbar, according to garnet-cordierite and two-pyroxene geothermometry (DGMVN, 1989; Roger et al., 2007; Nakano et al., 2007, 2013; Long, 2011). 2.2 Ngoc Linh complex This subterrane occurred in the west of the Kannak subterrane and formed the western part of the Kontum Massif (Fig. 2. 1). The oldest rocks in the area comprise parts of the metamorphosed Ngoc Linh Complex, which exposes mainly along the upper course of the Đắk Mi and Re rivers in the north and northeast of Kontum Province and A Yun Pa area. Petrographically, this complex includes following rock groups: Đắk Mi crystalline schists, Song Ba amphibole gneiss, Ba Điền biotite gneiss, Ia Ban amphibolite, and Đèo Măng Rơi granulite (Long, 2011). The composition of metamorphic rocks consists mainly of garnet ± sillimanite ± biotite gneiss with foliated quartz-feldspar layers corresponding to leucosome (Faure et al., 2018), garnet ± biotite gneiss, amphibolite, garnet amphibolite and hornblende ± biotite gneiss (Nakano et al., 2007; Osanai et al., 2008). These rocks were considered to have metamorphosed under amphibolite facies (Hutchinson, 1989). Some other rock types belong to granulite facies metamorphic rocks are also observed in this complex. They are garnet ± sillimanite ± cordierite ± biotite gneiss, garnet ± orthopyroxene ± biotite. 10.

(27) gneiss, garnet ± orthopyroxene ± clinopyroxene ± hornblende granulite and garnet ± orthopyroxene ± clinopyroxene granulite (Nakano et al., 2004, 2007). The age of the metamorphic rocks of the Ngoc Linh Complex is problematic. Reported isotopic ages obtained by different methods range from Ordovician to Early Triassic (Nam, 1998, 2001; Nakano et al., 2003, 2007). U-Pb age of relict core of zircon grains in some units yielded 1455 and 2541 Ma. Furthermore, according to Lan et al. (2003), the Nd-Sm TDM age of these metamorphic rocks lies in the interval from the Paleoproterozoic to Neoproterozoic (2.4-1.2Ga), and these certainly represent the earliest model age of the protoliths. Two metamorphic events were also recorded by Vuong et al. (2020) by using U-Pb Zrn ages and U-Th-Pb Mnz ages for one mafic granulite and two pelitic gneisses. As such, the interpretation of the age for the Ngoc Linh Complex remains uncertain. 2.3 Kham Duc complex The Kham Duc complex is distributed in the northern and western parts of the Kontum Massif (Fig. 2. 1). It is bounded by a granitic intrusion (Ben Giang-Que Son plutonic suit) in the south and Tra Bong strike-slip fault to the south (Lepvrier et al., 2004). This complex consists mainly of hornblende ± biotite gneiss, epidote amphibolite, biotite ± muscovite schist, and biotite ± sillimanite schist. Garnet ± kyanite ± biotite schist, kyanite ± staurolite ± muscovite ± chlorite schist and garnet ± staurolite ± kyanite ± biotite. schist are also presented as low- temperature and medium- pressure counterparts within this complex. Rocks in the Kham Duc complex is composed of greenschist to blueschist facies and low amphibolite facies metamorphic rocks with moderate pressure conditions (Long, 1995; Nakano et al., 2009; Usuki et al., 2009). The age for the protoliths in the Kham Duc complex is minimal, such as the detrital zircon age for metapelite yields 558±7 Ma (Usuki et al., 2009). This age suggested 11.

(28) that a part of the Kham Duc complex is younger than Late Neoproterozoic. The younger 240Ma Mnz age of Vuong et al. (2020) was interpreted as the timing of the high temperature/ medium pressure metamorphism of Grt ± Ky ± Bt gneiss which was considered to be occurred at 650-700°C and 8-8.6kbar by Nakano et al. (2007). This result is consistent with previous studies of Nakano et al. (2013) and Faure et al. (2018) with the peak metamorphism at 245-250Ma. 2.4 Hai Van complex The Permian-Triassic Hai Van high-Al granite is widely distributed in geological structures that extending along the Truong Son Fold Belt. It located in the northern margin of the Kontum Massif (Fig. 2. 2). In the past, it was considered to belong to many different complexes, such as Late Triassic Phia Bioc complex (Dovjikov, 1965) or Carboniferous Truong Son complex (Tri and Bao, 1977). The most recent study of Faure et al. (2018) considered the Hai Van Complex to be the post- Ngoc Linh plutons. Rocks of Hai Van complex are mainly intrusive phase and exposed with large areas, composed of mediumto coarse-grained biotite granite, fine-grained two-mica granite, and biotite granodiorite. The formation age of the Hai Van complex seems to fall within a short limited time span. The Early Triassic age was recorded based on the Rb-Sr isotopic age of granite in this complex with 236 ± 4.6 Ma (Anh et al., 1995) and 250 Ma (Hurley and Fairbairn, 1972). The similar ages have been acquired from monazite by U-Th-Pb geochronology at 239-219Ma and 237-222Ma (Nakano et al., 2013). Hieu et al. (2015) also reported zircon U-Pb ages within the range of 242-224Ma by using LA-ICP-MS. This formation ages of Hai Van Complex made up a lower intercept for all samples in this study and suggested that they can not be younger than Early Triassic.. 12.

(29) Figure 2. 2. Simplified geological map of the Kontum Massif (after D.G.M.V, 1989).. 13.

(30) CHAPTER 3. PETROGRAPHIC METHODS The petrographic analysis is a fundamental tool for all geologists to identify the minerals and classify the rocks. This analysis must be started in the field and recorded in the field note with the description of hand specimens. However, that is an initial observation with a limited amount of information. Thus, the detailed analysis of minerals plays an important role in understanding the rock by observed the optical mineralogy and the microstructure in the thin section. These data allow us to determine the mineral assemblage and metamorphic facies and also deformation within the rock. Therefore, it contributes to an understanding of the ordinal growth of minerals, forming reactions, and also forming conditions. Consequently, the tectonic as well as metamorphic evolution of the rock can be revealed. 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).. 14.

(31) 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. PetrothinThin 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: 15.

(32) 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;. 16.

(33) 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. 17.

(34) 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. 18.

(35) 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.. 19.

(36) - 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).. 20.

(37) 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. 21.

(38) 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.. 22.

(39) 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. 23.

(40) 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 24.

(41) 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).. 25.

(42) 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 K16-1118C3 (leucosome) (Fig. 4. 6).. 26.

(43) 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.. 27.

(44) Table 4. 1. Mineral percentages of analyzed samples in this study No. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 Note. Major minerals (% in mode) Sample no. Rock types Qtz Pl Kfs Grt Bt Ms Hbl Cpx Opx Chl K16-11-17A1 Bt±Ms Gneiss 64.5 5.5 9.9 2 8.1 8 0.01 2 K16-11-17A2 Amphibolite 27.8 1.2 8.1 7.2 55.6 + K16-11-17A3 Granodiorite 19.8 39.6 3.8 17.1 17.4 2.2 + K16-11-18A1 Altered gneiss matrix 20 20 60 K16-11-18A2 Ms ± Bt Gneiss 35.8 8.2 55.4 K16-11-18A3 Amphibolite 45.6 11.2 0.4 38 2 2 K16-11-18B1 Ms ± Bt Gneiss 45 25 30 K16-11-18C1 Bt ± Ms Gneiss 59.1 1.5 13.9 0.3 21.5 2.8 0.8 K16-11-18C2 Bt Gneiss 74 8.5 5 0.8 7.5 3 1.1 K16-11-18C3 Leucosome 45.1 4.6 27.5 1.7 19 2.3 1.8 K16-11-18D1 Pegmatitic muscovite 26 1 50 23 K16-11-18D2 Bt gneiss 48 14 14 + 22 1 0.2 K16-11-18E1 Amphibolite 55 11 8.9 0.9 3.7 12 1.2 7.2 + K16-11-19A1 Granodiorite 35.6 20 11.7 + K16-11-19B1 Bt Gneiss 40 1 6.2 49 1.5 1.5 K16-11-19B2 Leucosome 75 1.2 17 + 3 1 K16-11-19C3 Migmatitic gneiss 39.5 6 5 + 37 10 2.5 K16-11-19D1 Bt Gneiss 56 2.5 5 26 4 1.5 1.2 1.9 1 K16-11-19E1 Granodiorite 66.3 10 15 + + + “+” represented for “not calculated” mineral percentage within the sample; “-” defined for the absence of mineral within the sample.. 28.

(45) 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).. 29.

(46) 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 Kfeldspar (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 hightemperatures (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. 30.

(47) 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).. 31.

(48) 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).. 32.

(49) 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 K16-1118C3 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.. 33.

(50) 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 34.

(51) 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 welldeveloped 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.. 35.

(52) 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.. 36.

(53) 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.. 37.

(54) Syn- Ma is identified by muscovite, perthite, microline and quartz. This type of muscovite is characterized by coarse grains with high interference color and welldisplayed 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.. 38.

(55) 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. 39.

(56) 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 grains of muscovite and quartz (Fig. 4. 24b).. Figure 4. 24. a. The broken grain of garnet is distinguished by high relief under PPL. b. Porphyroclasts of K-feldspar surrounded by biotite and quartz, taken under CPL.. Post- Ma is defined by the occurrence of biotite, plagioclase, cordierite and quartz. Biotite grains are elongated and aligned with foliation. Some of them were bent/folded which is caused by deformation (Fig. 4. 25a, b). This structure also can be observed in plagioclase which is characterized by polysynthetic twinning (Fig. 4. 25c). There are some grains of cordierite (Fig. 4. 25d) grow in this rock and suggested the temperature for this growth at least 450ºC (Deer et al., 1992). Quartz shows undulose extinction with subgrain rotation recrystallization and grain boundary migration recrystallization (Fig. 4. 25e) which is given the range of temperature for this growth at 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).. 40.

(57) Figure 4. 25. a. Strongly kinked crystal of biotite characterized for plastic deformation; b. High interference color of biotite with microfold; c. Plagioclase crystal with bent polysynthetic twinning; d. The occurrence of cordierite with lamellae twinning at the edges of crystal; e. Subgrain rotation recrystallization and grain boundary migration recrystallization of quartz; f. Chlorite alteration of biotite caused by retrograde metamorphism.. Mb is clearly determined by the presence of muscovite (Fig. 4. 25c) and chlorite (Fig. 4. 25f). They were formed by the alteration from feldspar and biotite in the retrograde process, respectively. Normally, this alteration occurs at low- grade of metamorphism.. 41.

(58) 4.1.3 Ba Tieu Commune, Ba To District, Quang Ngai Province These two outcrops are represented fo Xa Lam Co Formation and belong to Kannak Complex (Fig. 4. 26). There are two rock types that were observed and collected samples, including migmatitic gneiss (K16-11-19C3) and biotite gneiss (K16-11-19D1).. Figure 4. 26. The presence of two rock types: a. Migmatitic gneiss; b. Biotite gneiss.. Outcrop 1 This migmatitic gneiss (K16-11-19C3) consists predominantly of consists of biotite ±garnet ± quartz ± plagioclase ± feldspar ± chlorite ± muscovite (Fig. 4. 27). Hematite occurs as an accessory mineral.. Figure 4. 27. Mineral assemblage for migmatitic gneiss, taken under CPL.. 42.