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).

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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.

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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 coarser grains of muscovite. The occurrence of hematite (Fig. 4. 75c) indicated for low- to medium- grade metamorphic rocks (Pichler et al., 1997). Thus, this sample is considered to be metamorphosed under greenschist facies.

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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.

K16-11-18A3 – Amphibolite

This amphibolite (Fig. 4. 71-yellow rectangular) contains mainly of hornblende ± biotite ± quartz ± pyroxene (clinopyroxene and orthopyroxene) ± plagioclase ± myrmekite. Opaque and zircon occur as accessory minerals (Fig. 4. 76).

Figure 4. 76. Mineral assemblage for amphibolite (K16-11-18A3) under CPL.

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Igneous protolith is strongly supported by the occurrence of hornblende, biotite and clinopyroxene (Fig. 4. 77). Hornblende grains are subhedral to euhedral and show pleochroism from green to brown color (Fig. 4. 77a). Igneous biotite is defined by dark brown color and intergrows with hornblende (Fig. 4. 77b). There are some grains of clinopyroxene preserved subhedral to euhedral shape with high relief and partially break down into small pieces (Fig. 4. 77c). Orthopyroxene also existed in this sample with pink pleochroism color under PPL with high interference color (Fig. 4. 77d). The coexisting of clinopyroxene and orthopyroxene give the range of temperature from 750 to 1700°C (Deer et al., 1992; Wells, 1977; Mori, 1978; Fonarev and Graphchikov, 1982a; Lindsley, 1983). Because this amphibolite is weakly foliated, therefore these pyroxenes are igneous instead of metamorphic minerals.

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.

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During deformation, hornblende and biotite tend to be elongated and defined for foliation. Within the range of temperature around 450-600 ºC, the intergrowth of quartz and sodic plagioclase generated myrmekite (Garcia and Roux, 1996; Passchier and Trouw, 1996; Abart et al., 2014) which characterized by vermicular (wormlike/wartlike; Fig. 4.

78). According to all the interpretations above, this amphibolite is considered to be metamorphosed under amphibolite facies.

Figure 4. 78. Intergrowth of quartz and sodic plagioclase defined for myrmekite.

In order to determine bulk chemical composition, minenral percentage is applied.

This sample nearly dropped into basic rocks field which is generally derived from basic igneous rocks such as basalts or gabbros (Fig. 4. 79).

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Figure 4. 79. Bulk chemical composition for sample K16-11-18A3in ACF diagram.