Geochemical characteristics of arc fractionated I-type granitoids of eastern Tak Batholith, Thailand

4.1 Field Survey and Petrographic Description

Field investigation and lithological analysis divided the granitoid into granodiorite and granitic dike, with modal analysis 400-point count of the petrographic data (Figure 3) based on the Q-A-P ternary diagram [25]. Nineteen granodiorites show a medium-grained texture (grain sizes average 2.00 mm across) and have light gray with green colors. Some samples present brecciated texture with quartz and chlorite veins. It is made up of white and pale green minerals. This rock is nonmagnetic and does not react with cold dilute hydrochloric acid. In addition, four granodioritic dikes have a greenish color consisting of white and pale green minerals. It is a medium-grained texture with an average grain size of 2.00 mm across. The rock is nonmagnetic and does not react with cold-diluted hydrochloric acid.

Figure 3

QAP intrusive rocks classification diagram [25].

The granodiorite shows an equigranularity medium-grained texture and is made up mainly of plagioclase (43.25–49.25 vol%) with subordinate orthoclase (6.50–9.00 vol%), quartz (16.25–28.50 vol%), hornblende (7.75–13.00 vol%), and biotite (9.75 vol%). Rarely amounts of apatite and zircon can be observed. Plagioclase is subhedral to euhedral (sizes up to 3.00 mm across) and is highly replaced by sericite, epidotes, and chlorite. Zoning plagioclase has been observed as the same as subhedral hornblende (sizes up to 2.50 mm across). The petrographic data suggest that plagioclase of the studied granitoid ranks between andesine and labradorite (An_41.5–68.5) based on extinction angle (23.00–41.50°). Zoning in this amphibole is common and represented by alteration of the core by fine-grained tremolite–actinolite amphibole (Figure 4a). Plagioclase is present in hornblende crystals. Biotite is anhedral to subhedral with irregular outlines (sizes up to 1.75 mm across). It is completely altered to clay minerals, chlorite, epidotes, titanite, and muscovite (Figure 4b). The inclusion of hornblende, plagioclase, zircon, and apatite is commonly present in biotite crystals. Kink features show that deformed biotite is present in the studied rocks. Orthoclase is anhedral (sizes up to 1.50 mm across) and shows a simple twin. Some crystals are converted to microcline and slightly replaced by clay minerals. Quartz is anhedral with sizes up to 4.00 mm and has a consertal texture, occurring as intergranular crystals with orthoclase. In addition, zircon/apatite is euhedral and occurs as an inclusion in hornblende and biotite (Figure 4c). This granodiorite has an experience of fault/shear in this area, showing brecciated texture (Figure 4d). The crystals are broken into breccia, and fractures were refilled by fine-grained quartz, epidotes, chlorite, calcite, and sericite with small amounts of opaque minerals (Figure 4e).

Figure 4

Photomicrographs of the granitoids from Tak Batholith: (a) alteration of the hornblende core is fine-grained tremolite–actinolite, (b) biotite altered to chlorite and epidotes, (c) zircon inclusion in hornblende, (d) brecciated texture of granodiorite, (e) breccia fractures were refilled by epidotes and chlorite, and (f) mineral compositions and granophyric intergrowth of the granodioritic dike. Pl, plagioclase; Qtz, quartz; Kfs, k-feldspar; Hbl, hornblende; Trm-Act, tremolite–actinolite; Chl, chlorite; Epd, epidote; Apt, apatite; Zrc, zircon; Spn, titanite.

The granodioritic dike shows a highly porphyritic texture under a microscope. The phenocrysts (grain sizes average 2.00 mm across) are composed mostly of plagioclase (38.00–56.00 vol%), with a small amount of orthoclase (8.00–14.00 vol%) and unidentified mafic minerals (10.50–12.25 vol%) as displayed in Figure 4f. They sit in a fine-grained groundmass (average grain size = 0.10 m across), comprising alkali feldspar and quartz with a small amount of plagioclase (27.50–36.25 vol%). The groundmass phase has a granophyric intergrowth. Secondary replacements in these rocks consist of chlorite, epidotes, calcite, quartz, and opaque minerals, especially along veins and fractures. Plagioclase phenocrysts are subhedral (sizes up to 4 mm across) and slightly moderately replaced by epidotes, sericite, and calcite. It is broken into a fragment and refilled by chlorite, epidotes, and fine-grained quartz. Plagioclase groundmass is subhedral with sizes up to 0.15 mm across), displaying an alteration product similar to phenocrysts. Orthoclase phenocrysts are euhedral to subhedral (sizes up to 0.75 mm across). The unidentified mafic mineral is subhedral (up to 1.25 mm across) and completely replaced by clay minerals and fine-grained amphibole. Alkali feldspar is fine grained and occurs as granophyric intergrowth with quartz. Quartz groundmass is fine-grained intergrowth with alkali feldspar as granophyric intergrowth.

4.2 Amphibole characterization

Amphibole is subhedral (sizes up to 3.00 mm across) under the polarized light microscope and also shows a pleochroism as X = pale yellow, Y = pale green, Z = green (Z > Y > X). In both petrography and SEM analyses, hornblende presents perfect cleavage on {110} – intersect at 56 and 124° and also shows partings on {100} and {001} crystal forms (Figure 5a and b). Based on EDS analysis, the hornblende in granodiorites were classified as calcic amphiboles in the exhibit of a small compositional range of XMg ((Mg/Mg + Fe) = 0.26–0.29), (Na + K) (0.17–3.34), and Si (6.14–7.84). These calcic amphiboles mostly fall within the field of Ferro-edenite (based on the nomenclature [26]) in Figure 5c.

Figure 5

SEM images of selected hornblendes: (a) one-direction cleavage and (b) perfect cleavages on {110}. Red rectangular presents the area measured geochemical composition by EDS, and (c) plots of mineral chemistry for calcic amphibole [26].

4.3 Whole-rocks analysis

Major and minor compositions of the representative granodiorite in the Pong Daeng area, Tak province, Thailand, are shown in Table 1. These granitoid show slightly different major and minor compositions including 60.63–64.40 wt% SiO₂, 0.55–0.68 wt% TiO₂, 15.10–16.48 wt% Al₂O₃, 4.40–5.75 wt% Fe₂O₃t, 2.78–4.16 wt% MgO, 2.31–5.73 wt% CaO, 2.30–4.06 wt% Na₂O, 2.42–3.76 wt% K₂O, 0.06–0.25 wt% MnO, and 0.11–0.13 P₂O₅. In addition, LOI demonstrates between 1.55 and 2.54 wt% according to highly composed of altered secondary minerals (hydrous minerals, i.e., chlorite, tremolite–actinolite, epidote, sericite, calcite, and clays) in petrographic analysis.

Major oxides plotted in the total alkali-silica (TAS) diagram [27] suggest that the plutonic rocks mostly rank in granodiorite and two samples are plotted in diorite (PD-10 and PD-15), as shown in Figure 6.

Figure 6

The total alkali-silica (TAS) diagram of the granitoids from Tak Batholith [27].

The relationship between Al³⁺, Na⁺, K⁺, and Ca²⁺ [28] suggests that the studied rocks are metaluminous granitoid (Figure 7a); however, alterations may cause the results of the study inaccurate, especially mobile elements or large ion lithophile elements (LILEs). According to low ratio Nb/Y (0.28–0.39) and low Zr/Ti (0.02–0.03), these granitoid were classified as high-K calc-alkaline similar to the classification by a ratio of K₂O and SiO₂ diagram [29] as shown in Figure 7b.

Figure 7

Chemical classification diagrams of the granitoids from Tak Batholith: (a) A/NK versus A/CNK diagram [28] and (b) K₂O versus SiO₂ diagram [26]. Dark gray shaded area present Eastern belt granite from reported data [12,57] and LFB in light gray shaded area from reported data [37,38,58].

According to Zr versus 10,000 Ga/Al and K₂O + Na₂O/CaO versus Zr + Nb + Ce + Y discriminant diagrams [30] in Figure 8, the granitoid from Tak Batholith is plotted in the I-type fractionated granite field, suggesting that these granitoids are related to crystal fractionation.

Figure 8

Chemical classification diagrams of the granitoids from Tak Batholith: (a) Zr versus 10,000 Ga/Al and (b) Na₂O + K₂O/CaO versus Zr + Nb + Ce + Y discriminant diagram of the granitoids from Tak Batholith [30].

Harker variation diagrams [31] show the negative correlation of SiO₂ against Al₂O₃, TiO₂, MgO, CaO, and Fe₂O_3t, and slightly positive correlations of SiO₂ versus K₂O and Na₂O (Figure 9). These chemical charters are compatible with their mineral assemblages, especially increasing proportions of plagioclase and hornblende. The linear correlations indicate that these rocks are related through a single magmatic differentiation.

Figure 9

Variation diagrams [31] of SiO₂ against other major and Eu of the granitoids from Tak Batholith.

Chondrite normalized REE patterns of the granitoid from Tak Batholith are very high in light REE (LREE) when data of chondrite from Sun and McDonough [32] were used for normalization (Figure 10). Chondrite-normalized La/Yb, (La/Yb)_N ratios of granitoids are 15.13–23.61, and meanwhile, (La/Sm)_N ratios are 6.05–10.12. Moreover, (Sm/Yb)_N ratios of granitoids are 2.27–2.53, and (Ho/Yb)_N ratios rank in 0.35–0.37. Furthermore, the chondrite-normalized REE patterns of these granodiorite present high slope LREE and gentle slope to flat heavy rare earth elements or HREE (Figure 10), comparable with the modern typical arc volcanic granite from southeast Peru [33,34].

Figure 10

Chondrites-normalized map of REE patterns of the granitoids from Tak Batholith (standardized values according to Sun and Mcdonough [32]). The light gray field represents data for the least altered San Rafael megacrystic granite compiled (southeast Peru) [33,34,35] present in the dark gray field.

5.1 Petrogenesis

Based on the texture and crystal relationship, the granodiorite equigranularity is a part of magma fractionation. Petrographic analysis suggests that the crystallization order is plagioclase, zircon–apatite, hornblende, biotite, fluorite, orthoclase, and quartz. Some granodiorite samples were affected by fault movement that preserves a cataclastic texture. Mineral composition and characteristics classify the granodiorite breccia as a part of intrusive, and its order of crystallization is as follows: zircon, hornblende-plagioclase, biotite, allanite, orthoclase, and quartz. Textures and mineral composition indicated the granodioritic dikes as late-stage crystallization of shallow intrusion, presenting the order of crystallization as plagioclase-unidentified mafic mineral, orthoclase, and quartz. Thus, hornblende could be determined by both granodiorites as I-type granitoid, which originate from igneous sources.

Based on petrochemistry, the studied granitoids are classified as I-type fractionated granodiorite and diorite similar to petrographic data. Harker variation diagrams (Figure 9) present close relation between these rock types, which reflect the same magmatic differentiation presenting plagioclase and hornblende crystal fractionation. The enrichment of LILE and depletion of highfield-strength elements presented in Table 1 suggest an arc-derived magmatic affinity in a subduction zone [34,35,36,37,38,39,40,41,42,43]. In addition, REE patterns of these rock samples (Figure 10) also reflect the fractionated pattern of arc magmatism derived from the high K calk alkaline magmatic affinity related to continental arc margin during the Triassic (213–256 Ma) and are grouped in the eastern granite belt [19,20].

5.2 Granite magma source and evolution

The I-type granites result from the partial melting of intermediate-base igneous rocks or metamorphic rocks in the crust [17] and also result from a mantle source during the bark melting process [44]. The geochemical interpretations in the Al₂O₃ + MgO + FeO^t + TiO₂ versus Al₂O₃/MgO + FeO^t + TiO₂ diagram [45] and CaO/MgO + FeO^t versus Al₂O₃/MgO + FeO^t diagram [46] as shown in Figure 11 suggest that the studied I-type granitoids result from the partial melting of intermediate-basic igneous rocks and amphibolite.

Figure 11

Distinguishing diagram of the original rock type of the granitoids from Tak Batholith: (a) CaO/(MgO + FeO^t) versus Al₂O₃/(MgO + FeO^t) diagram [45] and (b) Al₂O₃ + MgO + FeO^t + TiO₂ versus Al₂O₃/(MgO + FeO^t + TiO₂) diagram [44].

According to the MgO and FeO/(FeO + MgO) diagram [47], Nb/Th versus Nb and Th/Y versus Nb/Y [48] suggest that magma of these granodiorites generated in the lower crust (Figure 12a–c) is similar to that of the I-type granites study [17]. Meanwhile, the discriminant diagrams of the tectonic setting (Figure 12d–f), including the Rb/30 versus Hf and 3Ta [49], Y versus Nb [49], and Rb versus Y + Yb [50] suggest that this I-type granitoids are arc volcanic granites related to arc magmatism.

Figure 12

Distinguishing diagram of the sources of the granitoids from Tak Batholith: (a) plots of MgO and FeO/(FeO + MgO) diagram [46], (b) Nb versus Nb/Th, (c) and Nb/Y versus Th/Y diagrams [47], (d) Rb/30 versus Hf and 3Ta diagram [48], (e) Y + Yb versus Rb diagram [49], and (f) Y versus Nb discrimination diagram [48].

According to the regional context and petrological characteristics, the granodiorite and diorite are derived from the partial melting of mafic and/or metamorphic rocks of the Earth’s crust clearly comparable with the study of arc volcanic granite (at Andes Mountain) [51,52,53].

5.3 Tectonic implication

The geological data with the previous tectonic models in Thailand and SE Asia [1,2,3,4,21,22,23,54] were presented, i.e., the tectonic model indicating magmatism of the Tak Batholith related to the volcanic sequence. The geological data have confirmed that the arc magmatism along the Chiang Khong–Lampang–Tak volcanic belt may have resulted from the Paleo-Tethys subduction beneath Indochina Terrane in several episodes from Permo-Triassic, Late Triassic, and Early Jurassic [21,22]. Meanwhile, in the Middle Triassic, the arc magmatism related to the subduction of Palaeo-Tethys of the SIBUMASU, and Indochina Terrane (Figure 13a) still proceeded along with the formation of volcanic arc and resulted in lower crust melting [55,56]. The Tak granitic rocks that occurred in the Sukhothai arc were mainly derived from the partial melting of basaltic juvenile lower crust with potentially chemically weathered ancient crustal residues and basaltic mantle melt, which was induced by hot intruding mantle basaltic magma at the bottom of the Sukhothai continental arc similar to the granites in the Central Asian Orogenic Belt presented a high δ18O feature from the chemical weathered ancient crustal residues that occurred in the Baogutu continental arc [57].

Figure 13

Schematic tectonic model and arc magmatism in Tak Province, Thailand: (a) Paleo-Tethys subduction and Arc Fractionated Granitoids crystallization during the Triassic period [21] and (b) structural complex effect on Tak Batholith during Cenozoic Era [59,60,61].

Subsequently, high K calc-alkaline magma rose to the surface and fractionated minerals, including hornblende and plagioclase, to form a granitic batholith during 213–256 Ma [19,20]. In addition, in the Cenozoic Era, Mae Ping strike-slip faults directly affect the texture and mineral structure of both Chiang Khong–Lampang–Tak volcanic rocks and Tak granitic rocks [58,59,60], presenting as fault evidence in field observation and brecciated texture in granodiorite (Figure 13b).

This work conducts petrographic and geochemical analysis of the Thailand eastern Tak Batholith, an area that is less well studied. However, the results of this study based on the relatively small dataset and geochemistry remain inconclusive. Thus, additional electron probe microanalyses for minerals and isotopes are required for additional resolution. Although the age of the batholith formation is speculated in this article, the exact age of the batholith formation is not specified (which is important).