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14 May 2024

Geochemical Characteristics and U–Pb Dating of Granites in the Western Granitoid Belt of Thailand

,
,
and
1
Department of Resources and Environmental Engineering, Faculty of Science and Engineering, Waseda University, Ohkubo 3-4-1, Shinjuku, Tokyo 169-8555, Japan
2
Geochemical Research Center, Graduate School of Science, The University of Tokyo, Hongo 7-3-1, Bunkyo, Tokyo 113-0064, Japan
*
Author to whom correspondence should be addressed.
Geosciences2024, 14(5), 135;https://doi.org/10.3390/geosciences14050135
This article belongs to the Section Geochemistry
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Abstract

This paper presents the integration of magnetic susceptibility measurements and whole-rock geochemical compositional and Nd–Sr isotopic ratio analyses for granite samples collected from the Ranong, Lam Pi, Ban Lam Ru, and Phuket granite bodies in the Western Granitoid Belt of Thailand. In addition, U–Pb dating was performed on zircons extracted from the samples. All samples are proper granites based on their mineralogical and geochemical characteristics. Two samples collected from the Lam Pi granite body were classified as magnetite-series and I-type. The remaining granite samples were classified as ilmenite-series and S- or A-type. Furthermore, all granites were classified as syn-collision granites. Excluding the magnetite-series samples from the Lam Pi granite body, the other samples exhibit enrichment in incompatible elements, such as Nb, Sn, Ta, Pb, Bi, Th, U, Ce, Rb, and Cs. Zircon U–Pb dating yielded ages of ca. 60 Ma for the magnetite-series granites from the Lam Pi granite body, whereas ages of 88–84 Ma were obtained for the other granite bodies. Initial Nd–Sr isotopic ratios indicate a higher contribution of mantle material in the Lam Pi magnetite-series granites and a higher contribution of continental crust material in the other granites. Based on these compositional and zircon U–Pb age data, it is inferred that the 88–84 Ma granites formed as a result of the thickening of the continental crust owing to the collision between the Sibumasu and the West Burma blocks. In contrast, the ca. 60 Ma Lam Pi magnetite-series granites are thought to have been generated via partial melting of the mantle wedge associated with the subduction of the Neo-Tethyan oceanic crust beneath the West Burma Block.

1. Introduction

Southeast Asia consists of four blocks originating from the supercontinent of Gondwana, namely the South China, Indochina, Sibumasu, and West Burma Blocks (Figure 1), and the amalgamation of these blocks has resulted in the formation of many granitic rocks due to associated orogenic processes (e.g., [1,2]). This study aims to investigate the geochemical signatures and the formation age of granitic rocks in western Thailand near the boundary of the Sibumasu Block and the West Burma Block.
Granitic rocks in Thailand can be classified into the Eastern Granitoid Belt, the Central Granitoid Belt, and the Western Granitoid Belt [3]. In the present study, we report the results of magnetic susceptibility measurements, geochemical analyses, and U–Pb dating of zircons from the Ranong, Lam Pi, Ban Lam Ru, and Phuket granite bodies of the Western Granitoid Belt. In western Thailand and Malaysia, numerous tin (Sn) deposits are associated with granitic rocks, making it one of the world’s leading suppliers of Sn (e.g., [4,5,6,7,8]), which occurs primarily as alluvial placer deposits [7]. The Western Granitoid Belt extends northward into Myanmar [8]. The present study also aims to investigate granites of the Western Granitoid Belt to determine the tectonic setting in which the Sn-enriched granites were formed based on their geochemical characteristics and zircon U–Pb ages.
Eastern Thailand is part of the Indochina Block, whereas western Thailand belongs to the Sibumasu Block (Figure 1) [1,9,10,11,12,13,14]. The eastern part of the Indochina Block corresponds to the Loei Fold Belt and the Eastern Granitoid Belt. The Sukhothai Zone occupies the western part of the Indochina Block, and the Inthanon Suture Zone in the eastern part of the Sibumasu Block belongs to the Central Granitoid Belt. The western part of the Sibumasu Block is part of the Western Granitoid Belt. The boundary between the Indochina and Sibumasu blocks corresponds to the boundary between the Sukhothai and Inthanon Suture zones. The Paleo-Tethys Ocean separated the Indochina and Sibumasu blocks before their amalgamation. The West Burma Block lies to the west of the Sibumasu Block, and it is thought that the Meso-Tethys Ocean once separated the two blocks [3,15,16,17,18]. The Indian subcontinent was located to the west of the West Burma Block, separated by the Neo-Tethys Ocean. According to Gardiner et al. [8], the granitic rocks of the Western Granitoid Belt are thought to have formed in association with the subduction of the Neo-Tethyan oceanic crust beneath the West Burma and Sibumasu blocks. The Western Granitoid Belt of Thailand extends northward into the Mogok–Mandalay–Mergui Belt of Myanmar, where numerous granitic bodies are present.
Geosciences 14 00135 g001
Figure 1. Tectonic subdivision of Southeast Asia [1,2,19,20,21,22,23]. The study area is indicated by the red rectangle.

2. Materials and Methods

Granite samples were collected from fifteen locations across the Phuket, Lam Pi, Ban Lam Ru, and Ranong granite bodies at sites 5, 5, 2, and 3, respectively (Figure 2) [24]. Latitude and longitude were measured at the sampling sites using GPS (eTrex Venture HC, Garmin Ltd., Schaffhausen, Switzerland). Magnetic susceptibility measurements were conducted at ten points surrounding each of the sampling sites using a portable magnetic susceptibility meter (SM30, ZM Instruments, Brno, Czech Republic). Measurements were conducted on smooth, fresh surfaces.
Figure 2. Geological map showing a distribution of the studied granites and sampling locations [24].
Thin sections were prepared from the collected samples, and the constituent minerals were identified using a polarizing microscope in transmitted light.
Samples were ground for one minute using a tungsten carbide vibrating mill (TI-100, Heiko Seisakusho, Fukushima, Japan) for whole-rock compositional analysis. After further grinding using an agate mortar, 5 g of each sample was sent to Activation Laboratories Ltd. (Ancaster, ON, Canada) for analysis of major and minor components based on the 5Litho package.
The chemical composition of biotite was determined using a scanning electron microscope (SEM; JSM-6360, JEOL, Tokyo, Japan) equipped with an energy-dispersive X-ray spectrometer (EDS; INCA Energy, Oxford Instruments, Abington, UK). Prior to analysis, sample surfaces were coated with carbon using a carbon coater (QUICK CARBON COATER SC-701C, Sanyu Denshi, Tokyo, Japan). The accelerating voltage was set at 15 kV, and the sample current was adjusted to achieve a total count rate of 2000 counts/s on a Co standard sample. The measurement time was set to 60 s. Analysis was conducted for Si, Ti, Al, Fe, Mg, Mn, Na, and K. Synthetic oxides were used as the standards for Si, Ti, Al, Fe, Mg, and Mn. Natural albite and orthoclase were utilized as standard samples for Na and K, respectively. Prior to analysis, observations were made under a polarizing microscope, and measurements were conducted on fresh, unaltered minerals.
Zircon grains were separated from the granite samples for U–Pb dating using laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS). Samples were crushed in an iron mortar and sieved to a particle size of <250 μm. Then, light minerals were removed by panning in water. After drying, magnetic minerals were removed using a neodymium magnet. Subsequently, a density-adjusted sodium polytungstate solution with 3.0 g/cm3 was used for centrifugation. Heavy particles that settled at the bottom of the container were extracted and deposited in a beaker using a pipette and washed thoroughly with distilled water to remove excess heavy liquid. Magnetic separation using a neodymium magnet was performed again after drying. Zircon grains were handpicked from the heavy fractions using a stereomicroscope and embedded on a glass slide with a thin coating of Petropoxy 154 resin (Burnham Petrographics LLC., Rathdrum, ID, USA) then heat-cured. Next, the zircons were polished using diamond paste in preparation for U–Pb dating. To identify suitable locations for spot analyses, cathodoluminescence images were captured using a field emission scanning electron microscope (FE-SEM; JSM-7001F, JEOL, Tokyo, Japan) equipped with a cathodoluminescence detector (MonoCL3 detector, Gatan, CA, USA) installed at the Kagami Memorial Research Institute for Materials Science and Technology of Waseda University (Tokyo, Japan).
Zircon U–Pb dating was conducted using the LA–ICP–MS facility installed at the Geochemical Research Center of the Graduate School of Science, The University of Tokyo (Tokyo, Japan). To remove contaminants from the surfaces of zircon grains, pre-ablation of each grain surface was performed using a femtosecond laser ablation system over a 40 μm square (Cyber Probe UV Plus, Cyber Laser Inc., Tokyo, Japan). The irradiation conditions for pre-ablation were set to a single repetition with a laser output of 30 mW. Subsequently, a laser with a beam diameter of 10 μm was directed at the center of the pre-ablated area. During measurements, the laser output was set at 30 mW, and a burst of 40 shots was applied. The vaporized samples from laser ablation were introduced into a multi-collector (MC)–ICP–MS (Nu Plasma 2, Nu Instruments, Wrexham, UK) using a mixture of He and Ar with a 0.6 L/min flow rate. The signal intensities of 206Pb, 207Pb, and 235U were detected using Daly ion detectors, and the signal intensities of 202Hg, 204(Hg + Pb), and 208Pb were detected using high-gain ion detectors. The signal intensity of 232Th was detected using a Faraday cup. For the signal intensity of 204Pb, it was assumed that 204Pbblank could be neglected, and the following equation was used:
Pb 204 = ( Hg + Pb ) 204 − Hg 202 × ( Hg + Pb ) 204 Hg 202 blank
The detection of 235U employed the following equation [25]:
U 235 = U 238 × 1 137.88
Calibration of the high-gain ion detectors utilized the glass reference material NIST SRM612, which has an isotopic ratio of 207Pb/206Pb = 0.9073 [26]. Calibration of the 206Pb/238U ratio employed the primary standard Nancy 91,500 zircon, with 206Pb/238U = 0.17928 ± 0.00018 and 207Pb/206Pb = 0.07556 ± 0.00032 [27]. The GJ-1 zircon with a 206Pb/238U age of 600.4 Ma [28] and the OD-3 zircon with a 206Pb/238U age of 33.0 Ma [29] were used as secondary standards. During one measurement cycle, the NIST SRM612 standard sample was measured three times, the 91,500 zircon was measured three times, the GJ-1 zircon was measured once, and the unknown sample was measured up to 13 times. Then, the NIST SRM612 standard sample was measured three times again, followed by another three measurements of the 91,500 zircon.
IsoplotR was used to generate the Wetherill diagrams for the concordant or discordant (TG404) zircon samples [30], which were used to determine U–Pb ages. A zircon grain is considered concordant if its 95% confidence error ellipse (±2σ) intersects with the concordia curve. The concordia ages were computed by employing two-dimensional weighted means of the 207Pb/235U and 206Pb/238U ratios [31].
To determine Sr and Nd isotopic ratios, crushed samples were completely decomposed using a mixed acid (HF–HNO3–HClO4), followed by separation of Sr and Nd using columns packed with Sr and Ln resins (Eichrom Technologies Inc., University Lane Lisle, IL, USA), respectively. The samples were adjusted to ~100 ppb in 1% HNO3 for Sr and Nd. The isotopic ratios of 87Sr/86Sr and 143Nd/144Nd were measured using MC–ICP–MS (Neptune, Thermo Fisher Scientific Ltd., Waltham, MA, USA) installed at the Research Institute for Humanity and Nature (Kyoto, Japan). The 87Sr/86Sr and 143Nd/144Nd ratios were corrected for mass fractionation effects during measurement using stable isotopic ratios in nature, with 86Sr/88Sr = 0.1194 and 146Nd/144Nd = 0.7219, respectively. Simultaneously measured values for the Sr standard sample NIST SRM 987 yielded an 87Sr/86Sr value of 0.710329 ± 0.000032 (2σ; n = 10), and those for the Nd standard sample JNdi-1 yielded a 143Nd/144Nd value of 0.512048 ± 0.000022 (2σ; n = 6). The obtained values were corrected to match the recommended values for the standard samples, which are 143Nd/144Nd = 0.512115 [32] and 87Sr/86Sr = 0.710250 [33], respectively. For further details regarding sample adjustment and measurement, please refer to Uchida et al. [34].

3. Results

3.1. Sample Descriptions

Figure 3 shows photographs of typical rock samples and their photomicrographs taken under a polarizing microscope. None of the samples showed linear structures. Each sample consists mainly of coarse- to medium-grained quartz, potassium feldspar, plagioclase, and biotite (Table 1). Hornblende is observed in samples TG010 from the Phuket granite body and TG401 from the Lam Pi granite body, whereas muscovite is observed in samples TG403-TG404 from the Lam Pi granite body and TG408-TG410 from the Ranong granite body. The accessory minerals include zircon, apatite, and tourmaline, and samples containing hornblende also contain minor titanite.
Figure 3. Photographs (left) and photomicrographs (right) taken using a polarizing microscope under crossed-polars showing representative granite samples: (a) TG013 from the Phuket granite body, (b) TG401 from the magnetite-series Lam Pi granite body, (c) TG405 from the Lam Pi ilmenite-series granite body, (d) TG404 from the Ban Lam Ru granite body, and (e) TG408 from the Ranong granite body. Abbreviations: Pl, plagioclase; Kfs, potassium feldspar; Qz, quartz; Bt, biotite; Hlb, hornblende; Ms, muscovite.
Table 1. Modal composition of the analyzed granite samples.

3.2. Magnetic Susceptibility

Based on their magnetic susceptibilities, granitic rocks are classified as magnetite-series if their magnetic susceptibility is >3 × 10−3 SI units and ilmenite-series if it is lower [35]. This distinction is believed to arise from differences in oxidation–reduction states influenced by the presence of organic matter in the source rocks [36]. Ilmenite-series granitic rocks are believed to form under relatively reducing conditions owing to the reducing action of organic matter in their source rocks or entrained sedimentary rocks. Figure 4 shows the magnetic susceptibilities of the studied samples. Except for two samples (TG401 and TG402 from the Lam Pi granite body), the studied granites exhibit magnetic susceptibilities of <0.1 × 10−3 SI units, and therefore, they can be classified as ilmenite-series rocks. The two samples from the Lam Pi granite body have markedly higher magnetic susceptibility values, averaging 8 × 10−3 SI units, indicating that they can be classified as magnetite-series. However, the Lam Pi magnetite-series granite samples are surrounded by ilmenite-series granite with lower magnetic susceptibility, indicating a transition from magnetite- to ilmenite-series granite.
Figure 4. Magnetic susceptibilities (minimum, mean, and maximum values) at the granite sample locations.

3.3. Whole-Rock Geochemical Compositions

The results of whole-rock geochemical analysis are presented in Table 2.
Table 2. Results of whole-rock geochemical analysis of the studied granite samples.
All samples plot in the granite field using the total alkali vs. SiO2 (TAS) diagram (Figure 5). Furthermore, 11 samples belong to the alkaline series [37,38].
Figure 5. Diagram of Na2O + K2O vs. SiO2 (TAS) for the studied granite samples [37,38]. The dividing line between the alkalic and sub-alkalic fields follows Irvine and Baragar [39].
Excluding the Lam Pi granite samples (TG401 and TG402), which have high magnetic susceptibilities, the other studied samples are enriched in incompatible elements, such as Nb, Sn, Ta, Pb, Bi, Th, U, Ce, Rb, and Cs. In particular, the samples have high contents of Sn, with values ranging from 14 to 57 ppm. In comparison, the Lam Pi magnetite-series granite samples have relatively lower values of 13–14 ppm. The contents for U and Th are 11–35 ppm and 28–234 ppm, respectively, except for sample TG014, which shows higher values of 146 ppm and 229 ppm, respectively.
Most samples plot within the field for S-type granites on a Na2O vs. K2O diagram (Figure 6) [39]. However, on an Al2O3/(Na2O + K2O) vs. Al2O3/(CaO + Na2O + K2O) (A/NK vs. A/CNK) diagram (Figure 7), most samples have A/CNK molar ratios of 1.0–1.1, indicating that they are peraluminous, and most plot within the I-type granite domain [40].
Figure 6. Diagram of Na2O vs. K2O showing the classification of the studied granite samples as I- and S-types [40].
Figure 7. Diagram of Al2O3/(Na2O + K2O) vs. Al2O3/(CaO + Na2O + K2O) showing the classification of the studied samples as metaluminous and peraluminous, as well as I- and S-types [40,41].
In a Zr vs. 10,000 × Ga/Al diagram, the two samples from the Lam Pi granite body that are classified as magnetite-series are categorized as I&S-types, whereas most of the other samples, plot within the field for A-type granites (Figure 8).
Figure 8. Diagram of Zr vs. 10,000 × Ga/Al diagram showing the classification of the studied samples as I&S- and A-types [42].
On the Rb vs. (Yb + Ta) tectonic discrimination diagram, except for one sample (TG014) from the Phuket granite body, the studied samples plot within the field for syn-collision granite (Figure 9) [43]. Of these samples, TG401 and TG402 from the Lam Pi granite body plot closest to the volcanic arc granite field.
Figure 9. Tectonic setting discrimination diagram for the studied granite samples [43]. Abbreviations: syn-COLG, syn-collision granite; VAG, volcanic arc granite; WPG, within-plate granite; ORG, ocean-ridge granite.
On the Sr/Y vs. Y diagram, the studied samples plot within the non-adakitic (calc-alkaline) field (Figure 10) [44]. However, samples TG401 and TG402 from the Lam Pi granite body exhibit slightly higher Sr/Y ratios compared with the other granite samples.
Figure 10. Sr/Y vs. Y diagram for the studied samples showing their classification as adakitic and non-adakitic rocks (calc-alkaline rocks) [44].
The chondrite-normalized [45] rare earth element patterns [45] of the samples exhibit markedly negative Eu anomalies (Figure 11). However, magnetite-series samples TG401 and TG402 from the Lam Pi granite body have smaller negative Eu anomalies compared with the other granite samples.
Figure 11. Chondrite-normalized rare-earth element patterns for the studied samples [45,46].

3.4. Chemistry of Biotite

The results of the chemical composition analysis of biotite are summarized in the Supplementary Materials (Table S1).
Figure 12 shows a plot of Mg/(Mg + Fe) vs. total Al for biotite from the studied samples based on O = 22. Overall, biotite with lower total Al tends to exhibit higher Mg/(Mg + Fe) molar ratios. Biotites from magnetite-series samples TG401 and TG402 from the Lam Pi granite body have the lowest total Al content and the highest Mg/(Mg + Fe) molar ratios of the studied samples.
Figure 12. Diagram of Mg/(Mg + Fe) vs. total Al for biotite from the studied samples based on O = 22.

3.5. Zircon U–Pb Dating

Figure 13 shows representative cathodoluminescence images of the analyzed zircon grains. The results of zircon U–Pb dating analysis of the studied samples are shown in the Supplementary Materials (Table S2) and Figure 14 [30]. All ages fall within the range of 88–60 Ma, indicating that the granites formed during the Late Cretaceous to Early Paleogene.
Figure 13. Cathodoluminescence images of representative zircons used for U–Pb dating. Open circles in the figure indicate measurement locations. Ages shown in the figure are based on 206Pb/238U ratios.
Figure 14. Wetherill diagrams showing the results of U–Pb dating of zircons from the studied granite samples [30].

3.6. Initial Nd and Sr Isotopic Ratios

The measured 143Nd/144Nd and 87Sr/86Sr isotopic ratios for the studied granite samples are presented in Table 3.
Table 3. Nd–Sr isotopic ratios of the studied granite samples.
The initial 143Nd/144Nd and 87Sr/86Sr ratios calculated using the U–Pb ages derived from zircons in each granite body, as determined in Section 3.5, are shown in Table 3 and Figure 15 [33,47].
Figure 15. Initial 143Nd/144Nd and 87Sr/86Sr ratios of the studied granite samples. Abbreviations: PM, primitive mantle; CHUR, chondritic uniform reservoir; DMM, depleted MORB mantle; EM1, enriched mantle 1; EM2, enriched mantle 2; HIMU, high μ. PM, CHUR, DMM, EM1, EM2, and HIMU data are from Faure and Mensing [33] and Schaefer [46].
The initial 143Nd/144Nd ratios range from 0.511741 to 0.512118, with negligible differences observed among the studied granite bodies. In contrast, the samples yield a wide range of initial 87Sr/86Sr ratios from 0.709144 to 0.743762. Among the obtained results, samples TG401 and TG402 from the Lam Pi granite body yield low initial 87Sr/86Sr ratios of 0.711563–0.712490, whereas the remaining samples yield relatively high values of 0.709144–0.743762.

4. Discussion

The studied granite samples from the Phuket, Lam Pi, Ban Lam Ru, and Ranong granite bodies in the Western Granitoid Belt of Thailand tend to have high SiO2 and Na2O + K2O contents and are classified as granites in a TAS diagram (Figure 5). Furthermore, most of the samples belong to the alkalic series. Magnetic susceptibility measurements classified samples TG401 and TG402 from the Lam Pi granite body as magnetite-series, whereas the remaining samples were classified as ilmenite-series (Figure 4). Based on Na2O vs. K2O contents, most of the samples can be classified as S-type granites, except for sample TG104 from the Phuket granite body and sample TG408 from the Ranong granite body, which are classified as I-type granites (Figure 6). However, the A/CNK molar ratios of the samples indicate that two of the samples (TG014 and TG408) are S-type granites, whereas the others are peraluminous I-type granites (Figure 7), which is inconsistent with the classification based on Na2O vs. K2O contents. Furthermore, the classification of the samples into I&S- and A-types using a Zr vs. 10,000 × Ga/Al diagram indicates that most of the samples are A-type granites (Figure 8).
The Lam Pi magnetite-series granites are surrounded by ilmenite-series granites, indicating that the Lam Pi magnetite-series granites intruded older ilmenite-series granites. Except for the magnetite-series samples TG401 and TG402 from the Lam Pi granite body, the other granite samples exhibit enrichment in incompatible elements such as Nb, Sn, Ta, Pb, Bi, Th, U, Ce, Rb, and Cs. In particular, the samples have high contents of Sn (14–57 ppm) (Table 2). Apart from these two samples from the Lam Pi granite body, the rare-earth elements (REE) patterns of the studied samples exhibit markedly negative Eu anomalies (Figure 11), consistent with plagioclase fractionation under reducing conditions. Accordingly, it is suggested that such fractionation processes led to the enrichment of incompatible elements, including Sn. In terms of biotite geochemical compositions within the granite bodies, samples TG401 and TG402 from the Lam Pi granite body have notably higher Mg/(Mg + Fe) ratios and lower total Al contents compared with the other studied samples (Figure 12). The above results for biotite geochemical composition are consistent with trends observed in Japanese and South Korean granitic rocks [48,49]. In addition, the initial Sr isotopic ratios of the Lam Pi magnetite-series granite samples are lower, indicating a substantial influence from mantle material. Conversely, except for sample TG408 from the Ranong granite body, the initial Sr isotopic ratios of the other ilmenite-series samples are high, suggesting derivation from continental crust material (Figure 15).
The results of zircon U–Pb dating indicate that the ilmenite-series granite bodies were emplaced during the Late Cretaceous (88–84 Ma) (Figure 14). In contrast, younger ages of ca. 60 Ma were obtained for the magnetite-series granite samples TG401 and TG402 from the Lam Pi granite body. The zircon ages obtained during this study are consistent with the 40Ar/39Ar ages of 80–50 Ma reported by Charusiri et al. [1].
The Wuntho–Popa Arc, which is part of the West Burma Block, is located to the west of the Western Granitoid Belt in Thailand (Figure 1) and its northern extension, i.e., the Mogok–Mandaley–Mergui Belt in Myanmar. In contrast to the Western Granitoid Belt and the Mogok–Mandaley–Mergui Belt, the Wuntho–Popa Arc contains I-type granites and hosts mineral deposits such as gold, copper, and molybdenum [8,12]. It has been reported that the collision between the West Burma and Sibumasu blocks occurred during the Late Cretaceous [50]. The ilmenite-series and S- or A-type granites distributed in the Western Granitoid Belt are believed to have formed as a result of the collision between the West Burma and Sibumasu blocks at 88–84 Ma, followed by the subduction of the Neo-Tethyan oceanic crust beneath the West Burma and Sibumasu blocks at ca. 60 Ma, leading to the formation of magnetite-series granites (e.g., [51,52]) (Figure 16). Based on Rb vs. (Yb + Ta) tectonic discrimination diagram, it is inferred that the ilmenite-series and S- or A-type granites of the Western Granitoid Belt of Thailand formed in response to the collision between the West Burma and Sibumasu blocks (Figure 9). This collision resulted in the thickening of the continental crust at the western margin of the Sibumasu Block [53,54], leading to the formation of ilmenite-series and S- or A-type granites (Figure 16). The high initial Sr isotopic ratios of these S- or A-type granites indicate that they were derived via partial melting of continental crust material (Figure 15). Furthermore, advanced crystal differentiation processes led to the enrichment of incompatible elements, such as Sn, in the magma that formed the granites. In contrast, it is deduced that the formation of the Lam Pi magnetite-series granites in the Western Granitoid Belt have been associated with the subduction of the Neo-Tethyan oceanic crust, which was located west of the West Burma Blocks. Gardinier et al. [8] reported the occurrence of substantial Sn mineralization in parts of the Andes Mountains of South America that are far from the subduction zones of the Pacific Plate. In contrast, porphyry Cu–Au deposits in the Andes Mountains occur closer to the subduction zones. This relationship is likened to the relationship between the Wuntho–Popa Arc and the Mogok–Mandalay–Mergui and Western Granitoid belts. However, it is difficult to compare the distributions of different deposit types in the Andes Mountains with the mineralization in the Wuntho–Popa Arc and the Mogok–Mandalay–Mergui and Western Granitoid belts because the mineralization in these regions does not occur on the same continental block, as is the case in the Andes Mountains.
Figure 16. Schematic diagram showing the tectonic evolution and granite formation between the Sibumasu and West Burma blocks.

5. Conclusions

(1)
Based on mineralogical and geochemical characteristics, granitic rock samples collected from the Ranong, Lam Pi, Ban Lam Ru, and Phuket granitic bodies of the Western Granitoid Belt, Thailand, can all be classified strictly as granites. Two samples collected from the Lam Pi granite body are magnetite-series and I-type, whereas the rest of the samples are ilmenite-series and S- or A-type.
(2)
Initial Sr isotopic ratios suggest that the magnetite-series granites from the Lam Pi granite body contain a significant contribution from mantle material, whereas the other granites predominantly reflect derivation from continental crust.
(3)
The granite bodies, except for the magnetite-series granite of the Lam Pi granite body, yield ages of 88–84 Ma, indicating that they formed during the thickening of the continental crust that occurred in response to the collision between the Sibumasu and West Burma blocks.
(4)
The magnetite-series granite of the Lam Pi granite body yields an age of ca. 60 Ma and is believed to have formed in response to the subduction of the Neo-Tethyan oceanic crust beneath the West Burma and Sibumasu blocks.
(5)
It is speculated that during the collision between the Sibumasu and West Burma blocks, advanced crystal differentiation of the granite magma led to the formation of granites enriched in incompatible elements such as Sn.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/geosciences14050135/s1, Table S1. Chemical composition of biotite by SEM-EDS; Table S2. Results of zircon U–Pb dating.

Author Contributions

Conceptualization, E.U.; methodology, E.U., T.Y., S.N. and T.H.; formal analysis, E.U., T.Y. and S.N.; investigation, E.U. and T.Y; resources, E.U. and T.Y.; data curation, E.U., T.Y. and S.N.; writing—original draft preparation, E.U.; writing—reviewing and editing, E.U. and T.Y.; visualization, E.U. and T.Y.; supervision, E.U.; project administration, E.U.; funding acquisition, E.U. and T.H. All authors have read and agreed to the published version of the manuscript.

Funding

This research was supported by a funding from the Japan Society for the Promotion of Science (Nos 16K06931 (E.U.), 19K05356 (E.U.), and A2624709 (T.H.)) and by a Waseda University Grant for Special Research Projects (2022R-015).

Data Availability Statement

All data are included/referenced in this article.

Acknowledgments

This research was conducted in part with the support of the Joint Research Grant for the Environmental Isotope Study of Research Institute for Humanity and Nature. The authors express their gratitude to three anonymous reviewers for their useful comments and suggestions that improve the quality of the manuscript. We thank Edanz for editing a draft of this manuscript.

Conflicts of Interest

The authors declare no conflicts of interest.

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