U-Pb ages of zircon from I- and S-type granites from northern Kon Tum terrane: Implications for late Paleozoic - Mesozoic magmatism in Central Vietnam

1. Introduction

In Central Vietnam, the Tam Ky-Phuoc Son Suture Zone (TPSZ) is extended from the Tam Ky to Phuoc Son areas (Vietnam) over a distance of >100 km (Fig. 1a; e.g., Tran et al., 2014). The suture zone is considered to represent either an Early Paleozoic ocean separating the Truong Son terrane in the north from the Kon Tum terrane in the south (Lepvrier et al., 2008; Tran et al., 2014; Nguyen et al., 2019). Along the TPSZ in Vietnam, numerous lenticular-shaped, metamorphosed peridotites (dunite and harzburgite), cumulate gabbros-pyroxenites, gabbros, and plagiogranites occur in a few meters to several kilometers in length, and have been considered as an ophiolitic complex (e.g., Izokh et al., 2006; Pham et al., 2006). In the southern TPSZ, the Kon Tum terrane is located at the northeastern part of the Indochina terrane (Fig. 1a), and is composed of magmatic and metamorphic rocks (Tran and Vu, 2011). Ordovician-Silurian (e.g., Nagy et al., 2001; Usuki et al., 2009; Nguyen et al., 2019) and Permian-Triassic (e.g. Lepvrier et al., 2004; Osanai et al., 2004; Nakano et al., 2007, 2013; Dinh, 2011) ages were obtained from metamorphic and granitic rocks from the Kon Tum terrane. The magmatic and metamorphic event is attributed to the microcontinental collision tectonics in Asia (e.g., Osanai et al., 2004), but it is still controversial whether the timing of the collision is Ordovician-Silurian (e.g., Maluski et al., 2005; Tran et al., 2014; Nguyen et al., 2019) or Permian-Triassic (e.g., Nagy et al., 2001; Lan et al., 2003; Osanai et al., 2004; Nakano et al., 2013).

Fig. 1.

(a) Simplified tectonic map of Southeast Asia (after Metcalfe, 2013; Metcalfe et al., 2017; SMSZ: Song Ma Suture Zone, TPSZ: Tam Ky - Phuoc Son Suture Zone). (b) Simplified geological map of the northern Kon Tum terrane (Central Vietnam) showing distribution of the Tam Ky-Phuoc Son Suture Zone and lithology. Sample localities are also shown.

The Permian-Triassic granitoid complex in the northern Kon Tum terrane is composed of granite and granodiorite with both I- and S-type geochemical affinities (Tran and Vu, 2011). Three main granitic complexes have been classified: the I-type Ben Giang-Que Son, and S-type Hai Van and Van Canh. Although the Permo-Triassic metamorphic events have been reported (Osanai et al., 2004; Nakano et al., 2007, 2013; Bui et al., 2020), its corresponding magmatic records are rare (Tran and Vu, 2011). In this study, we report the petrology and zircon U-Pb ages of two granitoid samples each from the Ben Giang-Que Son and Hai Van complexes in the northern Kon Tum terrane. Our results contribute to a better understanding of Permo - Triassic magmatism within the Indochina block.

2. Geological setting

The Kon Tum terrane was formerly considered as the Precambrian core of the Indochina block (Hutchison, 1989; Tran and Vu, 2011; Metcalfe et al., 2017). However, the late Archean signature was only recorded in a Nd model age of 2.7 Ga for a pelitic granulite of continental crust origin (Lan et al., 2003). The Kon Tum terrane is subdivided into three litho-tectonic units: the Kham Duc, Ngoc Linh, and Kan Nack complexes southwards (Tran and Vu, 2011). The Kham Duc unit in the northern part consists of metapelite, metapsammite, and amphibolite of greenschist- to amphibolite-facies. Ultramafic and gabbroic rocks occur locally as lenticular shape in the unit, and recorded the ophiolitic geochemical affinity (Izokh et al., 2006). The Ngoc Linh and Ka Nack units are composed of felsic mylonite, pelitic gneiss, and mafic granulite (Nakano et al., 2007). High temperature (HT) to ultra-high temperature (UHT) metamorphic rocks were reported with coeval migmatites (e.g., Osanai et al., 2004; Nakano et al., 2007, 2013). Investigation on the metamorphic ages using the zircon and monazite U-Pb chronometers, and the biotite and muscovite K-Ar and Ar-Ar chronometers revealed at least two metamorphic events: the Ordovician - Silurian (ca. 430-460 Ma; e.g., Nakano et al., 2013; Bui et al., 2020) and the Permian-Triassic (ca. 240-260 Ma; e.g., Osanai et al., 2004; Nakano et al., 2007; Bui et al., 2020).

The early Paleozoic dioritic-granodioritic-plagiogranitic intrusions are locally distributed in the northern Kon Tum terrane, and are classified as the Tra Bong, Dieng Bong, Chu Lai complexes (Fig. 1b). In general, these rocks belong to peraluminous, calc-alkaline series with enrichment in large-ion-lithophile-elements (LILEs) and depletion in highfield-strength-elements (HFSEs, e.g. Nb, Ta) (e.g., Tran and Vu, 2011), which is interpreted as arc-related magmatism within the Kon Tum terrane (e.g., Tran et al., 2014; Gardner et al., 2017; Nguyen et al., 2019). The Dieng Bong complex has geochemical characteristics of island-arc magma with ca. 500-520 Ma emplacement ages (Nguyen et al., 2019). The late Paleozoic to early Mesozoic magmatic intrusions are locally exposed in the northern Kon Tum terrane, and are mainly grouped as the Ben Giang-Que Son, Hai Van, and Van Canh complexes (Tran and Vu, 2011). In general, the Ben Giang-Que Son complex is composed of diorite, granodiorite to granite, and leucogranitic dikes. The K-Ar ages of biotite from these rocks are in the range ca. 232 - 269 Ma. Rare geochemical data of this complex show that the rocks are depleted in HFSE such as Nb, Ta, Ti, and are enriched in Th, U, Cs, which is similar to the modern arc-related magma worldwide (Tran and Vu, 2011). The Hai Van and Van Canh consist of porphyritic granitoids such as granodiorite, granite, and granosyenite dikes. Petrological and geochemical characteristics of these rocks were suggested as S-type granite formed by a crust-thickening event within the Indochina block (Tran and Vu, 2011). The U-Pb isotopic analyses of six representative samples from the Hai Van complex by the LA-ICP-MS show that granites formed at ca. 224-242 Ma (Pham et al., 2015). Only a few K-Ar ages of muscovite from the Van Canh complex were reported to be ca. 140-191 Ma (Tran and Vu, 2011).

3. Sample collection, analytical procedure and result

3.1 Sample collection and petrography

In this study, two representative hornblende-and muscovite-bearing granitic samples were collected from the TPSZ (Fig. 1b). Sample QN-791 was collected in a rock quarry of the Ben Giang-Que Son complex. In the field, the rocks are exposed in a large massif of granodiorite in brown color (Fig. 2a). Sample QN-750 was collected in a white granite block exposed in about 15 m width along the Kham Duc complex. Field investigation suggest the boundary between this granite and the Kham Duc amphibolite is intrusive origin (Fig. 2b).

Fig. 2.

(a) Outcrop photograph of a granodioritic block within the Ben Giang - Que Son complex (sample QN-791). (b) Outcrop photograph showing the occurrence of the white granite intruded the Kham Duc complex (sample QN-750). (c and d) Photomicrographs showing the representative minerals of the granodioritic sample (QN-791) and granitic sample (QN-750), respectively (Qtz: quartz, Kfs: K-feldspar, Pl: plagioclase, Bt: biotite; Ms: muscovite, Hbl: hornblende).

The rock-forming minerals of sample QN-791 are plagioclase, K-feldspar, quartz, biotite, and minor hornblende (Fig. 2c). Plagioclase is subhedral, and is slightly altered to sericite in its rims and cleavages. Hornblende and biotite are mostly anhedral and altered, and biotite is slightly oriented. Accessory minerals include magnetite, pyrite, ilmenite, apatite, sphene, and zircon. Sample QN-750 is composed of quartz, plagioclase, K-feldspar, biotite, and minor muscovite (Fig. 2d). Plagioclase is subhedral, and is highly altered to sericite. Biotite is subhedral, and is partly altered to chlorite. Accessory minerals include magnetite, pyrite, ilmenite, apatite, sphene, and zircon.

3.2 Zircon U-Pb isotopic age dating

Zircon grains were separated by a conventional method, and then were mounted in epoxy resin prior to polishing to expose the interior of the crystals. Cathodoluminescence (CL) images were taken to examine zircon morphology, internal textures, and further to guide spot selection during the U-Pb analyses. The U-Pb dating of zircon was conducted using MC-LA-ICP-MS housed at Korea Basic Science Institute. Zircon 91500 was used as an internal standard for the U-Pb dating. Time-dependent drifts of U-Th-Pb isotopic ratios were corrected using linear interpolation (with time) for every eight analyses according to the variations of standard zircon. U-Th-Pb isotopic ratios for zircon 91500 are from Wiedenbeck et al. (1995). Uncertainty of preferred values for the internal standard 91500 was propagated to the ultimate results of the samples. In order to monitor the external uncertainties of measurements, a zircon standard GJ-1 was analyzed. Eight measurements yielded a weighted mean ²⁰⁶Pb/²³⁸U age of 607 ± 3 Ma, which is in agreement with the recommended age of 608.5 ± 0.4 Ma within uncertainty (Jackson et al., 2004). During the analyses, the spot size of the laser beam was 20 μ m in diameter. The detailed analytical procedure is similar to that described by Lee et al. (2018). Common Pb correction followed the procedure of Andersen (2002). Concordia plots and weighted mean calculations were carried out using the software Isoplot/EX (Ludwig, 2008). Analytical results are listed in Table 1.

Table 1.

Zircon U-Pb dationg results for the late Paleozoic-earlty Mesozoic igneous rocks in the North Kon Tum.

Zircons grains from samples QN-791 and QN-750 are 150-200 μm and 120-250 μm in the longest dimension, respectively. They have morphologies variable from subhedral to euhedral, and length-to-width ratios of 2:1 to 4:1 (Fig. 3a, 3b). The majority of these grains is devoid of inherited zircon cores in the sample QN-791, but a few zircons contain inherited cores in a rounded shape. Moreover, most of the zircon grains show well-developed oscillatory zoning throughout their CL images, indicative of their magmatic origin.

Fig. 3.

Representative cathodoluminescence (CL) images of zircons and U-Pb Concordia diagrams of the analyzed samples.

Twenty-two zircon U-Pb isotopic analyses are within 10 % discordance, and their Th/U ratios are over 0.13 (Table 1). All data points on Tera-Wasserburg Concordia diagrams are located on or close to the U-Pb Concordia, suggesting little Pb loss after zircon crystallization (Fig. 3). Ten concordant analyses from sample QN791 yielded a weighted mean ²⁰⁶Pb/²³⁸U age of 262.0±1.3 Ma (2σ) (MSWD=1.03, Fig. 4a). Eleven analyses within 10 % discordance from sample QN-750 gave a weighted mean ²⁰⁶Pb/²³⁸U age of 243.3±1.0 Ma (2 σ) (MSWD=0.20, Fig. 4b).

Fig. 4.

Diagrams showing weighted mean ages of the analyzed samples.

4. Discussion and conclusions

Within the Kon Tum terrane, two main magmatic events were recognized: (1) the early Paleozoic subduction and collision between the Truong Son and Kon Tum terranes, forming both I- and S-type foliated granites, during the Cambro-Ordovician Caledonian orogeny (e.g., Nguyen et al., 2019; Tran et al., 2020); and (2) the late Paleozoic-early Mesozoic Indosinian orogeny (e.g., Tran and Vu, 2011; Tran et al., 2020). Granitoids in the latter orogeny are widely exposed in the Kon Tum terrane (e.g., Dinh et al., 2011; Ngo et al., 2011, 2014, 2016; Tran and Vu, 2011; Tran et al., 2020), but tectonic model in association with their petrogeneses is attributed either to the Permo-Triassic subduction/collision of the South China block underneath the Indochina block (e.g., Ngo et al., 2016) or to the late Carboniferous-Triassic subduction/collision of the Sibumasu block underneath the Indochina block (e.g., Metcalfe et al., 2017).

The Ben Giang- Que Son magmatic complex in the Kon Tum terrane has two magmatic phases: the phase 1 is represented by small dioritic, quartz-dioritic blocks within the main lithology of the phase 2 that are composed of granodiorite and hornblende-bearing granites. The geochemistry and zircon U-Pb ages of the dioritic rocks reported by Pham et al. (2015) resulted in their calc-alkaline origin and early Ordovician (479±3 Ma) emplacement. Based on the results, the authors attributed petrogenesis of the complex to the result of the Caledonian orogeny. However, the occurrence of phase 1 (diorite) within the main lithology of phase 2 (granodiorite) in the Ben Giang -Que Son magmatic complex suggests that the diorites are xenoliths representing for earlier magmatism within the Kon Tum terrane (Fig. 2a). The ²⁰⁶Pb/²³⁸U mean age of 262.0±1.3 Ma from the granodioritic sample (QN-791) in this study is consistent with ages reported by Dinh (2011) and Tran et al. (2020) (LA-ICP MS U-Pb ages of zircon) from the granite and granodiorite of the Ben Giang-Que Son complex in the northern Kon Tum terrane, which range from 261±2 Ma to 306±2 Ma. These igneous rocks show typical island arc to active continental margin geochemical signatures such as high LILE enrichment relative to HFSE, and high LREE relative to low HREE (Tran et al., 2020), suggesting the existence of active margin during the Carboniferous-Permian within the Kon Tum terrane. The Carboniferous-Early Permian volcanic rocks of andesites, dacites, and basalts with continental arc affinity were also reported in the western and southern part of the Indochina block (e.g., 330-349 Ma andesitic, rhyolitic and tuffaceous rocks, ca. 315 Ma basalts and andesites (Qian et al., 2019), and 288-295 Ma granite (Zaw et al., 2014) in Laos; 278-301 Ma granitoids (Tran and Vu, 2020) in Cambodia and Thai Land). The magmatic events are relatively older than those reported in North Central Vietnam where most of the ages are middle to late Permian (ca. 270-252 Ma). The tectonic setting of middle to late Permian granitoids in the Kon Tum terrane is still unclear, and it is thus considered that the Ben Giang-Que Son complex in the Kon Tum terrane may be a part of the Carboniferous-Permian suites distributed in Vietnam, Laos, Thailand, and Cambodia, which probably resulted from subduction event of the Sibumasu underneath the Indochina blocks (e.g., Metcalfe, 2013; Metcalfe et al., 2017).

Undeformed, muscovite-bearing S-type granite sample QN-750 in the study intruded the Kham Duc amphibolite, and its crystallization age based on the zircon U-Pb dating was 243.3±1.0 Ma. This age is identical to those of S-type granite (ca. 242-224 Ma) in the Hai Van complex (Pham et al., 2015), and the U-Th-Pb chemical ages of monazite (ca. 240-260 Ma) from metamorphic rocks in the Kon Tum terrane (Osanai et al., 2004; Nakano et al., 2007; Bui et al., 2020). Pham et al. (2015) suggested that the geochemical characteristics of the Hai Van granite has geochemical characteristics of S-type granite formed from partial melting of crustal materials. During middle to late Triassic, collision of the Indochina block to the South China blocks in the north (e.g., Tran et al., 2008) and to the Sibumasu block in the west (e.g., Metcalfe, 2013; Metcalfe et al., 2017) had caused highly crust-thickening within the Indochina block that resulted crustal partial melting to form S-type granites and metamorphic rocks within the Indochina block (e.g., Tran et al., 2008). Thus, the middle to late Triassic S-type granite may originated from the latest stage of crustal thickening during the Indosinian orogeny.

Acknowledgments

We thank the Korean authorities to contribute the U-Pb age analyses. The reviewers have greatly allowed the improvement of this manuscript. This research was performed for the Research Project number B2017-MDA-14DT funded by the Ministry of Education and Training of Vietnam.

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