Ba Chua Commune, Ba To District, Quang Ngai Province

CHAPTER 4. RESULTS

4.1 Xa Lam Co Formation

4.1.4 Ba Chua Commune, Ba To District, Quang Ngai Province

This is the last outcrop of Xa Lam Co Formation that we observed. Granodiorite was found and collected sample K16-11-19E1 for analysis (Fig. 4. 37). It is characterized

by medium to coarse grains rock, which is among the most abundant intrusive igneous rocks.

Figure 4. 37. Granodiorite in the outcrop: a. Big scale; b. Small scale

The typical mineral assemblage for this granodiorite is defined by biotite ± microline ± perthite ± myrmekitic feldspar ± muscovite ± chlorite and quartz (Fig. 4. 38).

Figure 4. 38. Typical mineral assemblage for this granodiorite, taken under CPL.

Defining for pre- Ma and igneous protolith is the presence of plagioclase and feldspar. It is characterized by euhedal to subhedral crystals with polysynthetic twinning and partially replaced by muscovite (Fig. 4. 39a). Feldspar occurs as porphyroclasts which is surrounded by biotite and quartz during metamorphism (Fig. 4. 39b).

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.

In response to changing P-T conditions, they are not only metamorphosed but also deformed. It can be defined by elongated biotite, which is elongated crystal and aligned with foliation (Fig. 4. 40a). The evidence of Ma can be identified by the growth of plenty of minerals. The highest peak of temperature (600 to 750ºC) is given by the occurrence of perthite (Fig. 4. 40b), which was formed by the intergrowth of two feldspars and characterized by thin and parallel exsolution lamellae (Heier, 1955). Microline was formed at lower temperatures by the transformation from orthoclase around 500ºC (Fig.

4. 40b; Lorence and Barbara, 1998; Vernon, 2004). The most common symplectic intergrowth in this rock is myrmekite (Fig. 4. 40b), which is characterized by vermicular (wormlike/wartlike) intergrowth of quartz and sodic plagioclase (Shelley, 1993). The forming of myrmekite along the boundaries of alkali feldspar (Kfs) as the result of the breakdown of Kfs at high temperature (at least 450ºC) (Garcia and Roux, 1996; Passchier and Trouw, 1996; Simpson et al., 1989; Pandit, 2015). Quartz occupied more than 20%

by volume and characterized by undulose extinction and grain boundary migration recrystallization (Fig. 4. 40a). Some grains of quartz are porphyroclast and surrounded by biotite which defined foliation. This dynamic recrystallization texture of quartz suggested the temperature for deformation around 500 to 550ºC (Drury et al., 1990; Hirth et al., 1992; Stipp et al., 2002).

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.

Muscovite and chlorite grew as results of retrograde metamorphism (Mb). They were formed by the alteration from plagioclase/feldspar (Figure 4. 39) and biotite (Fig. 4.

41), respectively. Normally, this replacement suggested the temperature around 200°C (Parry and Downey, 1982; Pichler et al., 1997).

Figure 4. 41. The occurrence of chlorite intergrows with biotite, plagioclase and muscovite: a.

Under PPL; b. Under CPL.

In order to give the bulk chemical composition for this sample, the QAP diagram was drawn to signify this purpose. Because of uncertainty and error when identifying the mineral, this granodiorite showed the composition between granite and granodiorite (Fig.

4. 42).

Figure 4. 42. QAP diagram signified the bulk chemical composition for sample K16-11-19E1. 4.2 Dak Lo Formation

This outcrop is located in Ba Tieu Commune, Ba To District, Quang Ngai Province (Fig. 4. 43).

Figure 4. 43. Map location for the outcrop of Dak Lo Formation.

Migmatite is only one rock type was observed and collected samples (Fig. 4. 44), including mesosome (K16-11-19B1) and leucosome (K16-11-19B2).

Figure 4. 44. The occurrence of two rock types in the outcrop: a. Amphibolite; b. Leucosome.

K16-11-19B1 – Mesosome

Sample K16-11-19B1 was first observed and identified as amphibolite based on its color and hardness (Fig. 4. 44a). But in fact that it shows the mineral assemblages of biotite-rich dark layer (could be biotite gneiss) which is characterized by biotite ± feldspar

± plagioclase ± muscovite ± chlorite ± microline ± quartz (Fig. 4. 45a). Opaque occurs as an accessory mineral. This deduction is also supported by ACF diagram when this sample dropped into quartzo-feldspathic rocks field instead of basic rocks field (Fig. 4. 45b).

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.

Pre- Ma can be recognized by the random distribution of biotite. It occupied 30 to 40% abundance of the mineral mode and shows pleochroism colors (Fig. 4. 46a). In retrograde metamorphism, biotite is normally replasted/replaced by the growth of chlorite as an alteration result (Fig. 4. 46b).

Figure 4. 46. a. Pleochroism colors of elongated grains biotite; b. Partially altered of biotite and replaced by pale-green color of chlorite.

The intergrowth of two feldspars formed perthite defined for syn-Ma and represented for the highest peak of temperature in this sample around 600 to 750 ºC (Heier, 1955). It is characterized by thin and parallel exsolution lamellae (Fig. 4. 47a) and determined by the solid solution between albite and K-feldspar. As a result of temperature decrease, microline grew at a temperature around 500ºC (Lorence and Barbara, 1998) and

defined by cross-hatched twinning under CPL without any difficulty (Fig. 4. 47a). At lower conditions (250 to 500ºC), quartz shows undulose extinction with subgrains or bulging recrystallization (Fig. 4. 47b; Drury et al., 1990; Hirth et al., 1992; Stipp et al., 2002).

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.

The growth of muscovite and chlorite was formed by retrograde metamorphism (Mb) that resulted by alteration from plagioclase/feldspar and biotite, respectively.

K16-11-19B2 – Leucosome

In the field, leucosome is determined by a lighter color part comparing with mesosome of migmatite and characterized by coarse-grained quartzo-feldspathic evein (Fig. 4. 44b). It contains mainly of plagioclase ± feldspar ± biotite ± microline ± myrmekite ± muscovite ± chlorite ± quartz (Fig. 4. 48).

Figure 4. 48. Representative mineral assemblage for leucosome (K16 -11-19B2), taken under CPL.

The appearance of euhedral crystal of plagioclase is defined for post- Mb as well as igneous protolith. They are partially altered and replaced by muscovite which caused by the retrograde process (Fig. 4. 49).

Figure 4. 49. Euhedral crystal of plagioclase with polysynthetic twinning and muscovite alteration taken under PPL (a) and CPL (b).

Syn- Ma can be determined by the growth of perthite, microline, myrmekite and quartz according to temperature decreases. In this sample, perthite was formed at the highest temperature around 600 to 750ºC (Heier, 1955) by the intergrowth of two feldspars (Fig. 4. 50a). Microline and myrmekite were generated at lower range of temperature in the range 450-600ºC (Lorence and Barbara, 1998; Vernon, 2018; Garcia and Roux, 1996). They are clearly distinguished by crosshatched twinning and wartlike

texture, respectively (Fig. 4. 50a, b, c). Quartz commonly shows undulose extinction with bulging recrystallization and grain boundary migration recrystallization (Fig. 4. 50c) which is given the range of temperature from 250 to 550ºC (Drury et al., 1990; Hirth et al., 1992; Stipp et al., 2002).

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.

The presence of muscovite and chlorite defined for retrograde metamorphism-Mb. They were formed by alteration of plagioclase (Fig. 4. 49) and biotite (Fig. 4. 50d) under the hydrothermal environment (Barker, 1990) under lower P-T condition around 200°C (Parry and Downey, 1982). According to the above description, this leucosome is inferred to be metamorphosed under amphibolite facies with quartzo-feldspathic composition (Fig.

4. 51).

Figure 4. 51. Leucosome (K16-11-19B2) dropped into quartzo-feldspathic rocks field in the ACF diagram.

4.3. Kim Son Formation

Kim Son Formation has been recorded by the existence of quartz ± biotite ± sillimanite schist and graphite quartzite (DGMVN, 1989). In this study, three outcrops of this formation were found out and observed as well as collecting samples for analysis (Fig. 4. 52).

Figure 4. 52. Map location for outcrops of Kim Son Formation.

4.3.1 Chanh Dao Village, My Tho Commune, Phu My District, Binh Dinh Province In this outcrop, there are three rock types identified. They are biotite ± muscovite gneiss, amphibolite, and granodiorite, which are represented by sample K16-11-17A1, K16-11-17A2, and K16-11-17A3, respectively (Fig. 4. 53). Cross-cutting relationship defined ordinal formation for these rock types in the following order: biotite ± muscovite gneiss => amphibolite => granodiorite.

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).

The bulk chemical composition for these three rock types is given in the ACF and QAP diagrams (Fig. 4. 54), which is proposed for their protolith. In the ACF diagram (Fig. 4. 54a), sample K16-11-17A1 dropped into quartzo-feldspathic rocks which represented for continental crust, whereas K16-11-17A2 is suggested to have either basalt or gabbro as the parent rock (oceanic crust). Because of the uncertainties when identify K-feldspar and plagioclase under the microscope to calculate the mineral percentage, sample K16-11-17A3 fell into granodiorite to tonalite fields (Fig. 4. 54b).

Figure 4. 54. Bulk chemical composition, as well as protolith for three rock types, are shown in the ACF (a) and QAP diagram (b).

K16-11-17A1 – Biotite ± muscovite gneiss

Biotite ± muscovite gneiss is common and widely distributed metamorphic rock.

Generally, it is considered to be formed under high- temperature and high- pressure metamorphic processes that were acting on igneous or sedimentary rocks. The occurrence of biotite ± muscovite gneiss in this outcrop (yellow rectangular; Fig. 4. 53) shows clearly in Fig. 4. 55.

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.

This sample consists mainly of biotite ± quartz ± K-feldspar ± plagioclase ± garnet

± muscovite ± chlorite ± amphibole ± microline. Accessory minerals include opaque and zircon (Fig. 4. 56).

Figure 4. 56.Mineral assemblage for sample K16-11-17A1: a. Under plane-polarized light (PPL);

b. Cross-polarized light (CPL).

Ma is represented by the occurrence of garnet and some grains of biotite. Garnet is visible in the hand-specimen and thin section (Fig. 4. 56; Fig. 4. 57) because they grow as poikiloblasts with comparatively large grains (about 1 to 3mm in diameter).

Figure 4. 57.Coarse poikiloblasts of garnet co-existed with amphibole and biotite within gneiss (taken under PPL).

Under a microscope with open Nicol, garnets show high relief with light pink color. This type of garnet is represented by coarse grains and commonly truncated by later foliation (Sm). Garnets are partially replaced along with its margin/fractures by secondary biotite and chlorite (Fig. 4. 56; Fig. 4. 57). It is noticeable that there are many grains of amphibole grow only within poikiloblastic garnet (Fig. 4. 57; Fig. 4. 58a). The co-existing of amphibole and garnet has been considered as the production of partial melting on biotite ± quartz ± plagioclase (BQP) assemblage (Gardien et al., 2000) which is common in gneisses, metasediments and metaigneous rocks. Most grains were altered, so the evidence for the previous history was deleted. There are some small grains of garnet with a hexagonal shape and intergrow with biotite, which can be inferred to be formed during metamorphism (metamorphic garnet). Because of uncertainties, it is necessary to conduct more future works such as major and trace element contents as well as oxygen isotope composition to determine precisely the origin of the garnet, which has been examined by

Samadi et al., 2014. The presence of dark brown color and random growth of biotite supported for igneous protolith with rich in Fe (Fig. 4. 58b).

Figure 4. 58.a. The hexagonal shape of metamorphic garnet intergrows with biotite; b. Co-existing of amphibole, garnet, and biotite (under PPL).

Post- Ma is defined by the alignment of biotite, quartz and the growth of perthite, microline and myrmekite. Biotite formed as preferred orientation in the matrix and aligned with foliation (Fig. 4. 56). In response to changing P-T conditions, they are not only metamorphosed but also deformed. The clear evidence for ductile deformation in this sample is the presence of kink band biotite (Fig. 4. 59a) and bent twinning plagioclase (Fig. 4. 59b).

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.

Within the range of temperature around 600 to 750ºC, unmixing of alkali feldspar gives rise to intergrow of two feldspars (albite and K-feldspar), which is represented by flame perthite (Heier, 1955; Spry, 1969; Rollinson, 1982; Pandit, 2015). The most

common symplectic intergrowth in this rock is myrmekite, which is characterized by vermicular (wormlike/wartlike) intergrowth of quartz and sodic plagioclase (Shelley, 1993). This occurrence of myrmekite also suggested the temperature of deformation took place in the range 450-600 ºC (Garcia and Roux, 1996; Passchier and Trouw, 1996; Pandit, 2015). Microline also formed within this range of temperature by the transformation from orthoclase with slow cooling around 500ºC (Lorence and Barbara, 1998; Vernon, 2004) (Fig. 4. 60a).

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).

Mb is proved by the occurrence of muscovite and chlorite. Muscovite is characterized by tiny grain with high interference color under CPL (Fig. 4. 60b). It was formed by the alteration of plagioclase and K-feldspar as a result of retrogression under the hydrothermal environment (Barker, 1990) in the later metamorphic events. Chlorite is replasting biotite and defined by pale-green color within biotite (Fig. 4. 60b). This replacement implied that it formed under a lower P-T condition around 200°C (Parry and Downey, 1982).

If we supposed that tiny garnet (described above) is metamorphic garnet, the primary form of minerals within this sample could be plotted in AFM diagram (Fig. 4.

61) and written as reactions below:

Figure 4. 61. Chemographic projections for sample K16-11-17A1. Poikiloblast garnets:

(1) Bt + Pl + Qtz => Grt + Am + melt (Gardien et al., 2000) Tiny garnet (metamorphic garnet):

(2) Chl + Ms => Grt + Bt + Qtz +H2O (Deer et al., 1992; Spear, 1993)

(3) Chl + Fe-Bt + Qtz => Grt + Mg-Bt + H2O (Deer et al., 1992; Spear, 1993) Growth of muscovite and chlorite (retrogressive processes):

(4) Bt + Qtz +H2O => Chl +Or (Parneix et al., 1985)

(5) Ab + 2H⁺ + K⁺ => Ms + 6Qtz + 3Na⁺ (Launay et al., 2017) (6) 3Kfs + 2H⁺ => Ms + 6Qtz + 2K⁺ (Launay et al., 2017)

K16-11-17A2 – Amphibolite

Amphibolite is a metamorphic rock that contains amphibole, especially hornblende, actinolite as well as plagioclase. It is typically dark-colored and heavy with

a weakly foliated structure (Fig. 4. 62). In the outcrop, amphibolite showed similar foliation with biotite ± muscovite gneiss (black rectangular- Fig. 4. 53).

Figure 4. 62. a. Amphibolite in outcrop cut through by quartz vein; b. Simple sketch diagram for this outcrop.

The mineral assemblage for sample K16-11-17A2 composed predominantly of hornblende ± biotite ± quartz ± plagioclase ± chlorite ± muscovite and small amount of opaque minerals (Fig. 4. 63).

Figure 4. 63.Mineral assemblages of sample K16-11-17A2 under PPL.

Pre- Ma and igneous protolith are defined by a coarse subhedral to euhedral grains of hornblende and some grains of biotite. Hornblende grains vary in sizes from 0.1-1.2mm (in diameter) with green to brown colors (Fig. 4. 64a). They contain lots of inclusions cleavage with random orientation, which does not define foliation. The dark brown color of biotite supported for igneous protolith with rich in Fe (Fig. 4. 64b).

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.

Syn- Ma is determined by the occurrence of elongated hornblende and biotite.

This co-existence defined foliation. This type of biotite has clear cleavage and chlorite alteration (Fig. 4. 64b), which is defined for Mb. In response to deformation, some of them formed the kink band as a strain marker (Fig. 4. 65a). Hornblendes are small, long crystals, and preserved alteration rim (Fig. 4. 65b). During deformation, hornblende formed boundinage and biotite filled in the neck as a result. The ductile deformation of hornblende and growth of biotite suggested the temperature between them around 300 to 500ºC at 1kbar (Brimhall et al., 1985) or 570-700ºC (Brimhall, 1977; Roberts, 1973;

Jacobs and Parry, 1979).

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.

The chemographic projection, as well as mineral forming reactions of sample K16-11-17A2, can be shown in the ACF diagram (Fig. 4. 66) and the following reactions:

Figure 4. 66. Prediction chemographic projection for sample K16-11-17A2 in the ACF diagram.

(1) Hbl +Kfs => Bt + Chl +Pl + Qtz

(2) Hbl + H2O => Chl + Qtz + Ca²⁺ + Na⁺ (Nghia, N. C., 2015) (3) Bt + Qtz +H2O => Chl + Or (Parneix et al., 1985)

K16-11-17A3 – Granodiorite

Granodiorite is represented by sample K16-11-17A3 (blue rectangular- Fig. 4. 53;

Fig. 4. 67) and characterized by medium to coarse grains rock which is the most abundant

intrusive igneous rocks. Because granodiorite is later intrusive event and weakly metamorphism, therefore the below description and interpretation will be followed Mb.

Figure 4. 67. a. Granodiorite in outcrop; b. Sketch diagram shows the relationship between granodiorite and quartz vein.

The mineral assemblages include mainly of plagioclase ± quartz ± biotite ± hornblende ± chlorite ± muscovite (Fig. 4. 68). Accessory minerals consist of opaque and zircon.

Figure 4. 68.Mineral assemblage for sample K16-11-17A3 under CPL (two yellow rectangular will describe below).

Igneous protolith is clearly defined by biotite and plagioclase (Fig. 4. 69). Biotite preserved dark brown color (basal section) which is supported for source rich in Fe.

Plagioclase grains vary in sizes from 0.2mm to 1.5mm in length. Many grains of plagioclase are mostly replaced by muscovite which is caused by later event.

Figure 4. 69. a. Igneous biotite with dark brown color; b. Subhedral crystal of plagioclase intergrows with biotite and quartz.

Syn- Mb is represented by biotite and hornblende (Fig. 4. 70a). This type of biotite shows perfect cleavage with elongated crystal. Hornblende is less common in this rock and determined by greenish to dark green colors under PPL. It is partially replaced by the growth of biotite which suggested the temperature for this intergrowth around 300 to 500ºC at 1kbar (Brimhall et al., 1985).

Figure 4. 70. a. Partially growth of biotite (small grains) in hornblende; b. Elongated grains of biotite with chlorite alteration (black arrow).

The presence of chlorite and muscovite defines post-Mb. Chlorite formed by the alteration of biotite and characterized by pale-green color within biotite crystals (Fig. 4.

70b). Muscovite grains are tiny with clear cleavage (Fig. 4. 69; Fig. 4. 70) and grow as inclusions within plagioclase. This occurrence suggested they may form under low- to

medium- grade (Parry and Downey, 1982). Undulose extinction of quartz with strain-induced grain boundary migration (bulging) also form around 250 to 400 ºC (Drury et al., 1990; Stipp et al., 2002).

4.3.2 Bong Son Commune, Hoai Nhon District, Binh Dinh Province

This outcrop is defined by the presence of altered gneiss matrix, muscovite ± biotite gneiss and amphibolite rich dark layer within gneiss which are represented by sample K16-11-18A1, K16-11-18A2 and K16-11-18A3 (Fig. 4. 71). Because of the similarities in the measurement of foliation, this outcrop can be linked to above outcrop of Kim Son Formation and later the interpretation will be described with Ma/Mb.

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.

71 K16-11-18A1 – Altered gneiss matrix

This is a highly weathered sample (blue rectangular; Fig. 4. 71) which is characterized by fine-grains of muscovite. Mineral assemblages for this sample consist mainly of muscovite ± biotite ± quartz (Fig. 4. 72) with an insignificant amount of zircon and hematite as accessory minerals.

Figure 4. 72.Mineral assemblage of altered gneiss matrix, taken under CPL.

Ma might be defined by coarse grains of muscovite (Fig. 4. 73a). This type of muscovite is randomly grown with high interference colors in thin section (normally 2^nd to 3^rd order). Later this sample may be deformed during the deformation which is characterized by the presence of biotite. During the deformation, biotite and muscovite are elongated and aligned with foliation (Fig. 4. 73b, c). The occurrence of quartz also defined for syn- Ma with undulose extinction and bulging recrystallization (Fig. 4. 73d).

This microstructure suggested the temperature of deformation can be given with the range of 250-400ºC (Drury et al., 1990; Stipp et al., 2002).

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.

The occurrence of fine-grained of muscovite (Fig. 4. 73b, c, d) may defined for syn- Mb. It can be formed by the breakdown of coarser grains of muscovite or caused by the alteration from biotite and can be formed by following reaction:

(1) 3Bt + 0.65O2 + 12.6H⁺ => 2Ms + 3Qtz + K⁺ + 4.8Fe²⁺ + Mg²⁺ + 7.3H2O (Launay et al., 2017)

K16-11-18A2 – Muscovite ± biotite gneiss

Mineral assemblages for this sample consist predominantly of phyllosilicate mineral and quite an amount of quartz (Fig. 4. 71; Fig. 4. 74). Hematite occurs as an accessory mineral.

Figure 4. 74. Muscovite ± biotite gneiss under CPL.

This outcrop was strongly weathered; therefore the evidence for Ma is not clearly shown. However, the presence of biotite and coarser grains of muscovite can be useful indicators for Ma. They are elongated crystals and aligned with foliation which caused by deformation. Some of them were and generated a microfold (Fig. 4. 75a). Without any precise analysis, the coarser grains of muscovite (Fig. 4. 75b) are considered to be either (1) belong to protolith rock or (2) produced from biotite during metamorphism. The occurrence of undulosed extinction and bulging recrystallization of quartz (Fig. 4. 75c) gave the range of temperature for deformation/metamorphism around 250 to 400ºC (Stipp et al., 2002; Law, 2014). Occupying a large area of fine-grained of muscovite is defined for Mb. Similar to sample K16-11-18A1, this type of muscovite can be explained to be formed by the alteration from biotite by retrograde process or from the breakdown of

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