Chanh Dao Village, My Tho Commune, Phu My District, Binh Dinh Province

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).

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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.

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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).

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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

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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

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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:

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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

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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.

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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).

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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

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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.

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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

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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).