Magma Genesis and Arc Evolution at the Indochina Terrane Subduction: Petrological and Geochemical Constraints From the Volcanic Rocks in Wang Nam Khiao Area, Nakhon Ratchasima, Thailand

Volcanic rocks and associated dikes have been exposed in Wang Nam Khiao area, Nakhon Ratchasima Province, northeastern Thailand where complex tectonic setting was reported. These volcanic rocks are classified as rhyolite, dacite, and andesite whereas dikes are also characterized by andesitic composition. These dikes clearly cut into the volcanic rocks and Late Permian hornblende granite in the adjacent area. Rhyolite and dacite are composed of abundant plagioclase and quartz whereas andesite and andesitic dike contain mainly plagioclase and hornblende with minor quartz. The volcanic rocks typically show plagioclase and hornblende phenocrysts embedded in fine-grained quartz and glass groundmass whereas dike rocks contain less glass matrix with more albitic laths. P–T conditions of crystallization are estimated, on the basis of Al-in-hornblende geobarometry and hornblende geothermometry, at about 4.5–5.5 kbar, 861–927°C and 4.8–5.5 kbar, 873–890°C for the magma intrusions that fed volcanic rocks and andesitic dikes, respectively. Whole-rock geochemistry indicates that these rock suites are related to calc-alkaline hydrous magma. The enriched large-ion lithophile elements (LILE; e.g., Rb, Sr) and depleted high-field-strength elements (HFSE; e.g., Nb, Ce, Ti), with similar rare earth element (REE) patterns indicate arc magmatism. The results of this study are comparable with the continental arc magmatism along the Loei Fold Belt (LFB) as a consequence of the Late Permian Palaeo-Tethys subducted beneath Indochina Terrane.

The study area, Wang Nam Khiao in Nakhon Ratchasima province, Central Thailand, is located in the critical junction of several tectonic terranes ( Figure 1A) where volcanic rocks and related dikes are also exposed. Detailed investigation of these rocks has never been reported; therefore, this study provides the results coming from petrology, mineral chemistry, and whole-rock geochemistry of these rocks before reconstruction of magmatic process and tectonic model of the area. The results are used to constrain the magmatic-tectonic scenario of central Thailand in the frame of the Indochina Terrane amalgamation during Carboniferous-Triassic.

MATERIALS AND METHODS
Representative volcanic rocks and related dikes were collected from the unweathered zones of Wang Nam Khiao area, Nakhon Ratchasima, northeast Thailand, for petrographic description, and mineral chemical and whole-rock geochemical analyses.
Microprobe analyses were performed on polished-thin sections using an electron probe micro-analyzer (EPMA JEOL JXA-8100). Analytical conditions were set up at 15 kV and about 2 µA beam current with a focus beam spot of 1 µm. Mineral and pure oxide standards were used for calibration at the same conditions followed by automatic ZAF correction and finally reported as weight percent oxides.
Nine volcanic rocks and two related dikes were subsequently selected and prepared for whole-rock geochemical analysis. Major and minor oxides were analyzed by an X-ray fluorescence spectrometer, Bruker AXS S-4 Pioneer along with rock standards (JG-2, JR-1, JG-1a, JB-1b, JA-2, GSP-2, BHVO-2, STM-1) for  Charusiri et al., 2002;Sone and Metcalfe, 2008;Sone et al., 2012;Metcalfe, 2013;Zaw et al., 2014). (B) Geological map of the study area, Wang Nam Khiao area, Nakhon Ratchasima, and sample locations of this study (modified after Putthaphiban et al., 1989a,b;Kemlheg and Wichidchalermpong, 1992a,b). calibration. Loss on ignition (LOI) was calculated by the weight difference of the sample after heating in a furnace at 1050 • C for 3 h. All research facilities reported previously are based at the Department of Geology, Faculty of Science, Chulalongkorn University.
In addition, the selected rock samples were digested using mixed hydrofluoric acid and nitric acid as suggested by Shapiro (1975) for analyses of trace and rare earth elements (REEs) using an inductively coupled plasma-mass spectrometer (ICP-MS) iCAP Q at the Scientific and Technological Research Equipment Centre, Chulalongkorn University.

Field Survey and Petrographic Description
Volcanic rocks cover an area of about 10 × 12 km 2 in the vicinity of Wang Nam Khiao ( Figure 3A). Dikes are found cutting these volcanic rocks ( Figure 3B) and the adjacent Late Permian hornblende granite ( Figure 3C).
These volcanic rocks are classified as rhyolite, dacite, and andesite whereas the dikes have andesitic composition. Rhyolite and dacite are usually light gray to gray ( Figure 3A) and commonly show porphyritic texture. For andesite, apart from its greenish gray color resulting from enrichment of hornblende phenocrysts, the other appearances are similar to rhyolite and dacite phenocrysts. However, microscopic descriptions and geochemical analyses are meaningful for classification of these volcanic samples.
Microscopically, rhyolite and dacite similarly present abundant plagioclase phenocrysts (≤1 mm) with few hornblende phenocrysts (≤0.3 mm) embedded in fine-grained groundmass of quartz and plagioclase; moreover, their higher proportion of glass matrix (up to 30%) exhibits obviously vitrophyric texture ( Figure 3D). Plagioclase phenocrysts are typically subhedral crystal showing significant albite twins with subordinate carlsbad-albite twins whereas hornblende phenocrysts are subhedral to euhedral. Andesite shows porphyritic texture with less glass matrix in comparison with rhyolite and dacite. Moreover, it comprises abundant phenocrysts (≤1 mm) of plagioclase and hornblende with FIGURE 2 | Map shows distribution of the magmatic rocks in Thailand including the granite belts (modified after Nakapadungrat and Putthapiban, 1992) and volcanic rocks (Jungyusuk and Khositanont, 1992) together with the main tectonic terranes (modified from Charusiri et al., 2002;Sone and Metcalfe, 2008;Metcalfe, 2013;Zaw et al., 2014). Zircon U-Pb ages of the magmatic rocks in Loei Fold Belt are also present in the map. Abbreviations: LFB, Loei Fold Belt; ST, Sukhothai Terrane; CT, Chanthaburi Terrane.
Frontiers in Earth Science | www.frontiersin.org FIGURE 3 | Exposures of (a) dacite, (b) andesite cut by andesitic dike, and (c) andesitic dike cut into Late Permian hornblende granite dated by Fanka et al. (2018). Photomicrographs taken in cross-polarized light showing mineral assemblages of (d) dacite, (e) andesite, and (f) andesitic dike. Descriptions of mineral assemblage and texture are reported in the main text. Mineral abbreviations: Q, quartz; Pl, plagioclase; Hb, hornblende; Opq, opaque minerals. equal proportion ( Figure 3E) surrounded by fine-grained groundmass of quartz, plagioclase, and opaque minerals. Both plagioclase and hornblende phenocrysts are mostly subhedral to euhedral.
Andesitic dikes contain more abundances of subhedraleuhedral hornblende (≤1 mm) and subhedral-euhedral plagioclase laths (0.3-0.5 mm) with accessory opaque minerals and glass ( Figure 3F). They typically show interstitial texture with plagioclase lath and hornblende. These textures are obviously different from the volcanic samples reported previously.

Whole-Rock Geochemistry
Major and minor compositions of the representative rhyolite, dacite, andesite, and related andesitic dikes in the Wang Nam Khiao area, Nakhon Ratchasima, Thailand, are shown in Table 3. These volcanic rocks and related dikes show slightly different major and minor compositions.
On a Nb/Y versus Zr/Ti diagram (Winchester and Floyd, 1977) (Figure 5), the analyzed samples plot in the fields of rhyolite, dacite, and andesite, accordingly. Harker variation diagrams (Harker, 1909) show negative correlation of SiO 2 against TiO 2 , CaO, FeO, and MgO and positive correlations of SiO 2 versus Na 2 O and K 2 O (Figure 6). These chemical charters are compatible with their mineral assemblages, especially increasing proportions of albite and quartz and decreasing proportion of hornblende from the andesite toward dacite and rhyolite, respectively. The linear correlations indicate that these rocks are related through a single magmatic differentiation.
Primitive mantle-normalized spider diagrams ( Figure 7A) of most rock samples reveal almost fractionation patterns from the large-ion lithophile elements (LILE) to high-field-strength elements (HFSE) with slightly positive anomalies of Rb, K, and Sr and negative anomalies of Nb, Ce, and Ti. In addition, the chondrite-normalized REE patterns of all rock types are slightly high in light REE (LREE) when data of chondrite from Sun and McDonough (1989) were used for normalization. Regarding chondrite-normalized La/Yb, (La/Yb) N ratios of dike samples (8.64-9.41) are in the range of volcanic rocks (6.05-14.54 for andesite, 10.77-21.86 for dacite, and 10.77 for rhyolite). In terms of (La/Sm) N ratios, volcanic rocks are 2.58 in the rhyolite, 2.58-5.89 in the dacite, and 2.35-3.06 in the andesite whereas andesitic dikes are 2.57-2.93. Moreover, the primitive mantle-normalized spider diagrams and chondritenormalized REE patterns are comparable with the typical volcanic arc setting reported by Blein et al. (2001) and Reubi and Nicholls (2004); however, the samples' patterns present slightly lower HREE than those typical volcanic arc patterns (Figures 7A,B).

Temperature and Pressure Estimation
Microprobe data have been used to estimate pressure and temperature of crystallization in volcanic rocks as suggested by Kuprubasi et al. (2014). Although some mineral textures (e.g., zoned texture, corona texture, reaction rim, alteration) in volcanic rocks are not in equilibrium, the undisturbed single and/or coexisting minerals as phenocrysts can be used to estimate the P-T condition of crystallization (Foden and Winchester and Floyd, 1977) indicating compositional ranges of andesite, dacite, and rhyolite of the rock samples.
Green, 1992; Kuprubasi et al., 2014). Therefore, mineral compositions of undisturbed phenocrysts were used for the calculation of P-T conditions. Plagioclase compositions are sensitive to melt-water concentration (Lange et al., 2009) and alteration processes (Zhu and Lu, 2009;Hovelmann et al., 2010;Yuan et al., 2019); therefore, hornblende is efficiently used to calculate the P-T conditions (Schmidt, 1992;Ridolfi et al., 2010;Ridolfi and Renzulli, 2012). Al-in-hornblende geobarometer and hornblende geothermometer are employed to calculate the P-T conditions of crystallization in this study. Ridolfi et al. (2010) proposed a hornblende geothermometer for the calc-alkaline volcanic system related to subduction based on the experimental data and natural data from worldwide calc-alkaline volcanoes, with the uncertain calibration of ±57 • C by using the Silicon index (Si * ) of hornblende composition  Table 2). These estimated temperatures are comparable with the crystallization temperature range of calibrated geothermometer for andesite-rhyolite in the calc-alkaline volcanic system (Ridolfi et al., 2010;Walker et al., 2013;Waters and Lange, 2015;Chen et al., 2017).  Sun and McDonough, 1989) and (B) chondrite-normalized REE patterns (chondrite values from Sun and McDonough, 1989) of the studied rocks and shade patterns volcanic rocks from typical arc setting (data from Blein et al., 2001;Jicha et al., 2004;Reubi and Nicholls, 2004;Jicha and Singer, 2006) for comparison.
The Al-in-hornblende geobarometry has been widely used to calculate the pressure of magmatic crystallization (e.g., Hammarstrom and Zen, 1986;Schmidt, 1992;Helmy et al., 2004;Hossain et al., 2009) using the aluminum partition in hornblende, which was provided by experimental studies (Hollister et al., 1987;Johnson and Rutherford, 1989;Schmidt, 1992;Anderson and Smith, 1995). In this study, the calibration equation: Al-in-hornblende geobarometry P (kbar) = 4.76 Al tot -3.01 with error ± 0.6 kbar of Schmidt (1992) in the H 2 O-saturated condition was employed to calculate the pressure of magmatic crystallization. The calculated results are presented in Table 2. The calculated pressures of the volcanic rocks and related dikes fall in similar ranges of 4.5-5.5 and 4.8-5.5 kbar, respectively. As the calculated pressure from hornblende geobarometer has been used to indicate the depths of magma reservoir beneath arc volcanoes (Scott et al., 2012;Costa et al., 2013;Rivera et al., 2017), these calculated pressures can indicate the crystallization pressures of the phenocrysts from a magma reservoir that extruded to the surface.
Moreover, the pressures calculated from hornblende phenocrysts are used to estimate the depth by using the geobarometric gradient of 0.33 kbar/km (Bethel-Thompson et al., 2014). As a result, depths of crystallization are estimated at 14-17 km for the magma intrusion that fed volcanic rocks and dikes ( Table 2). The depths should indicate the crystallization depths of the phenocrysts before complete crystallization of volcanic rocks at the shallower up to the surface. These FIGURE 8 | Zircon U-Pb ages of plutonic rocks and the estimated age of the dike cut into the Late Permian hornblende granite exposed in the Wang Nam Khiao area, Nakhon Ratchasima, Thailand (modified from Fanka et al., 2018). depths (14-17 km) and the calculated temperature range of crystallization (861-927 • C) indicate geothermal gradient of about 50-65 • C/km which is comparable with an active continental margin (Rothstein and Manning, 2003;Annen et al., 2006;Chi and Reed, 2008). The combination of crystallization temperatures, pressure, and depths indicates that their original magmas should have stored and partially crystallized in the middle continental crust, which are the same provenance reported for plutonic rocks (e.g., hornblende granite, biotitehornblende granite, biotite granite) in the adjacent area (Peacock, 1993;Winter, 2001;Kelemen et al., 2003;Richards, 2003;Fanka et al., 2018).

Petrogenesis
Based on petrochemistry, the studied volcanic rocks are classified as rhyolite, dacite, and andesite whereas the related dikes also show andesitic composition. Harker variation diagrams (Figure 6) present close relation of these rock types which reflect the same magmatic differentiation. Moreover, the most typical occurrences of hornblende in all rock types should imply crystallization of a hydrous magma (Foden and Green, 1992;Best, 2003). The hydrous magmatic model of these volcanic rocks and dikes is also supported by the hydrous calc-alkaline magmatism reported by Fanka et al. (2016Fanka et al. ( , 2018 for the plutonic rocks in the Wang Nam Khiao area. Enrichment of LILE (e.g., Rb, Sr) and depletion of HFSE (e.g., Nb, Ce, Ti) presented in Figure 7A suggest an arc-derived magmatic affinity in a subduction zone (Pearce, 1983;Ryerson and Watson, 1987;Kelemen et al., 1990Kelemen et al., , 1993Ringwood, 1990;Blein et al., 2001;Reubi and Nicholls, 2004;Jicha et al., 2004;Jicha and Singer, 2006;Fanka et al., 2016Fanka et al., , 2018. In addition, REE patterns of these rock samples ( Figure 7B) also reflect fractionated pattern of magmatism.
In summary, all characteristics of petrography, geochemistry, and mineral chemistry features of volcanic rocks and related dikes indicate clearly that these rocks are derived from the calcalkaline magmatic affinity related to continental arc margin. Their occurrences are supported by the multiple episodes of arc magmatism in the Wang Nam Khiao area, Nakhon Ratchasima suggested by Fanka et al. (2016Fanka et al. ( , 2018 who have reported crucial evidences in terms of the ages of hornblendite and granitic rocks ( Figure 1B).

Tectonic Implications
According to the field evidence, the dike clearly intrudes into the Late Permian hornblende granite ( Figure 3C); therefore, this dike probably formed during/after the emplacement of the Late Permian hornblende granite. Moreover, the formation of the studied volcanic rocks and dikes can be combined with other magmatism in the study area (Figure 1) reported as multiple arc magmatism comprising Carboniferous biotite granite, Late and Triassic biotite-hornblende granite, and Permian hornblendite and hornblende gabbro (Figure 8). Moreover, the magmatic events of the studied rocks can be comparable with the magmatism from both plutonic rocks (e.g., Zaw et al., 2007Zaw et al., , 2014Khositanont et al., 2008;Salam et al., 2014) and volcanic rocks (e.g., Qian et al., 2015) in other areas belonging to LFB (Figure 2).
The geological data have confirmed that the arc magmatisms along the LFB resulted from the Paleo-Tethys subduction beneath Indochina Terrane in several episodes (Fanka et al., , 2018. According to geochronology of volcanic rocks in LFB, the magmatism might be started from Early Silurian (Khositanont et al., 2008) or Late Devonian (Intasopa and Dunn, 1994) as arc magmatism evidenced from the volcanic rocks in eastern volcanic sub-belt in Loei area. Moreover, these ages are supported by the ages of inherited zircons from Late Permian hornblende gabbro  yielded zircon U-Pb ages of 444 ± 10 Ma, and inherited zircons from Carboniferous biotite granite (Fanka et al., 2018) yielded zircon U-Pb ages of 429 ± 24 Ma in the Wang Nam Khiao area (Figure 8).
Afterward, during the Late Permian, the magmatisms strongly occurred in the LFB evidenced from the exposures of various arc magmatic rocks including volcanic rocks and dikes in this study, Late Permian hornblende granite (zircon U-Pb ages of 253.4 ± 4.3 Ma from Fanka et al., 2018), hornblendite, and hornblende gabbro (zircon U-Pb ages of 257.1 ± 3.4 Ma from Fanka et al., 2016) in the Wang Nam Khiao area (Figures 2, 8). These rocks appear to have crystallized in different depths of the arc-related subduction between Paleo-Tethys and Indochina Terrane ( Figure 9B). Moreover, the magmatisms in this period have also been reported widely along LFB (Figure 2) (e.g., Khositanont et al., 2008;Salam et al., 2014;Zaw et al., 2014). They are composed of plutonic rocks (e.g., zircon U-Pb ages of 254-250 Ma from Khositanont et al., 2008) and volcanic rocks (zircon U-Pb ages of 258.6-245.9 Ma from Salam et al., 2014).

CONCLUSION
Volcanic rocks and related dikes in the Wang Nam Khiao area, Nakhon Ratchasima, Northeast Thailand, provide informative evidences of ancient magmatism and geological process as concluded in the following: 1. Volcanic rocks are petrochemically classified as rhyolite, dacite, and andesite whereas their related dikes yield andesitic composition. Plagioclase, hornblende, and quartz appear to be the main compositions in all rock types, but their proportions are obviously different. 2. Mineral composition, mineral chemistry, and geochemistry indicate the hydrous calc-alkaline magma resulted from continental arc subduction. 3. LILE (e.g., Rb, Sr) enrichment and HFSE depletion (e.g., Nb, Ce, Ti) together with the slightly high LREE patterns indicate arc magmatism. 4. Al-in-hornblende geobarometer and hornblende thermometer indicate that the crystallization P-T conditions of the phenocrysts, which ascend with the magma that subsequently fed the volcanic rocks and the andesitic dikes, range between 4.5 and 5.5 kbar and 861-972 • C, as well as 4.8-5.5 kbar and 873-890 • C, respectively, reflecting the middle crust region. 5. Relationship of the dike cross-cutting into the previously dated Late Permian hornblende granite indicates that the volcanic rocks and dikes have taken place during/after Late Permian. 6. The results of this study well support the model of multiple arc magmatisms in LFB that resulted from the subduction of Palaeo-Tethys beneath Indochina Terrane.

DATA AVAILABILITY STATEMENT
The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

AUTHOR CONTRIBUTIONS
VH contributed to data collection, discussion, comment, and manuscript revision. CS contributed to data collection, comment, and manuscript revision. AF contributed to field investigation, laboratory studies, data collection, discussion, manuscript preparation, and revision. All authors contributed to the article and approved the submitted version.

FUNDING
The first author was supported by the 90th Anniversary of Chulalongkorn University Fund (Ratchadaphiseksomphot Endowment Fund), Graduate School, Chulalongkorn University for her M.Sc. study.