Geochemical and geochronological constrains on the Chiang Khong volcanic rocks (northwestern Thailand) and its tectonic implications

Abstract

Volcanic rocks in northwestern Thailand exposed dominantly in the Chiang Khong area, are commonly considered to be genetically linked to the tectonic evolution of the Paleo-Tethyan Ocean. The volcanic rocks consist mainly of andesitic to rhyolitic rocks and are traditionally mapped as Permian-Triassic sequences. Our zircon U-Pb geochronological results show that two andesitic samples (TL-1-B and TL-31-B), are representative of the Doi Yao volcanic zone, and give a mean weighted age of 241.2±4.6 Ma and 241.7±2.9 Ma, respectively. The rhyolitic sample (TL-32-B1) from the Doi Khun Ta Khuan volcanic zone erupted at 238.3±3.8 Ma. Such ages indicate that Chiang Khong volcanic rocks erputed during the early Middle Triassic period. Seven samples from the Doi Yao and Doi Khun Ta Khuan zones exhibit an affinity to arc volcanics. Three rhyolitic samples from the Chiang Khong area have a geochemical affinity to both arc and syn-collisional volcanic rocks. The Chiang Khong arc volcanic rocks can be geochemically compared with those in the Lampang area in northern Thailand, also consistent with those in Jinghong area of southwestern Yunnan. This indicates that the Chiang Rai arc-volcanic zone might northwardly link to the Lancangjiang volcanic zone in southwestern China.

Keywords

volcanic rocks / geochemistry / zircon U-Pb age / early Middle Triassic / Chiang Khong / northwestern Thailand

Introduction

Southeast Asia and adjacent part of southwestern Yunnan preserve abundant signatures of the Paleo-Tethys evolution. They have tectonically been subdivided into numerous blocks/terranes including the Yangtze, Simao, Baoshan, Tengchong, Indochina, Sukhothai, Inthanon, etc. (Fig. 1). A series of geological investigations on tectonic evolution of these blocks have been carried out. However, there are still many debates, such as the comparison of the Paleo-Tethyan tectonic zones between northwestern (NW) Thailand and southwestern (SW) Yunnan. In northern Thailand, the Nan (or Nan–Uttaradit or Nan River) suture has been regarded by many scholars as the main oceanic basin of the Paleo-Tethyan Ocean and they thought this suture could be connected with the Langcangjiang zone (Metcalfe, 1988; Bunopas, 1994; Hada et al., 1999; Ueno and Hisada, 1999; Ferrari et al., 2008; Sone and Metcalfe, 2008). Other scholars have discovered that the Langcangjiang tectonic zone is the volcanic arc zone of the Changning–Menglian zone and can be linked with the Chiang Rai volcanic arc zone and the Chiang Mai suture, respectively (Wu et al., 1995; Zhong, 1998; Chonglakmani et al., 2001; Metcalfe, 2002; Feng et al., 2004, 2005, 2008; Shen et al., 2009, 2011). Other sutures also have been proposed in northern Thailand including the Loei suture in the east and Yuam suture in the west (Fig. 1) (Chutakositkanon et al., 1997; Chonglakmani and Helmcke, 2001; Charusiri et al., 2002; Hisada et al., 2004; Panjasawatwong et al., 2006; Udchachon et al., 2011).

These differing opinions are mainly a result of poor understanding of the geology of northern Thailand. For this reason, we carried out a geological survey and comparative study. The aim of this paper is to present petrochemical data and U-Pb ages of the volcanic rocks in NW Thailand and to discuss their connection.

Geological Setting and Petrography

Geological reconnaissance in the study area was first made by the German Geological Mission to Thailand and as a consequence, a geological map was published at a scale of 1:250,000 (von Braun and Hahn, 1976). In NW Thailand, the pre-Jurassic volcanic rocks can be subdivided into four zones from west to east: Chiang Rai–Chiang Mai, Chiang Khong–Lampang–Tak, Nan–Uttaradit and Loei–Phetchabun volcanic zones (Fig. 2(a)) (Panjasawatwong et al., 2003). The samples in this paper were taken from northern part of the Chiang Khong–Lampang–Tak volcanic zone, which is composed by the NE-trending Doi Yao and Doi Khun Ta Khuan volcanic zones separated by a Cenozoic basin (Fig. 2(b)). Previous workers (von Braun and Hahn, 1976; Panjasawatwong et al., 2003) considered that the volcanic rocks in the Doi Khun Ta Khuan and Doi Yao zones have erupted at Permian-Triassic period. Barr et al. (2006) further proposed that the volcanic rocks formed in a continental margin arc setting. The mafic-ultramafic rocks are Permian-Triassic and the granitic rocks are Triassic in age. The Permian-Triassic volcanic rocks are composed of rhyolite, andesite, tuff, and agglomerate that are interlayered with the Permian–Triassic sedimentary rocks. The Upper Triassic to Lower Jurassic volcanic rocks include rhyolite, rhyodacite, dacite, andesite, and tuff. The sedimentary sequences in the study area include the following units: the Devonian–Carboniferous sedimentary package consists of limestone, chert, shale, sandstone, and conglomerate; the Permian-Triassic sedimentary sequences comprise gray to dark greenish gray sandstone, siltstone and shale interlayers, with thickly to very thickly bedded, light gray conglomerate, mudstone, and tuff; the lower Jurassic sedimentary rocks comprise reddish brown, purple, and pale yellowish green sandstone, siltstone, shale, and volcanic conglomerate (von Braunand Hahn, 1976).

Sampling and analytical methods

Ten samples involving andesite, rhyolite, dacite and rhyolitic tuff were collected from the Doi Yao, Doi Khun Ta Khuan and Chiang Khong in the Chiang Khong volcanic zone (Fig. 2). Two andesitic samples (TL-1-B and TL-31-B) and one rhyolitic sample (TL-32-B1) were selected for zircon U-Pb dating and ten samples for major and elemental analyses.

Samples were powdered into 200 meshes for elemental analysis. Major elements were analyzed by the titrimetric method at the State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences in Wuhan. Elemental analyses were carried out at the Wuhan Supervision and Test Center for Mineral Resources of Ministry of Land and Resources by an Agilent 7500a ICP-MS. The analytical precision is better than 5% for elements>10 ppm, less than 8% for those<10 ppm, and 10% for transition metals. The analytical results for the typical samples are shown in Table 1.

Zircons were separated from rock samples by standard techniques, mounted in epoxy, and polished. Optical microscopy and cathodoluminescence (CL) images outlined the morphology and internal structure of the grains. CL images were obtained on a JEOL JXA-8100 electron microprobe. Zircon U–Pb dating was undertaken on a LA-ICP-MS at the State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences in Wuhan. The detailed analytical procedure follows Yuan et al. (2004) and involved a 193 nm Geolas 2005M laser-ablation system coupled to an Agilent 7700a ICP-MS. Helium was used as the carrier gas to enhance the transport efficiency of the ablated material. The sport diameter was 32 μm. The data acquisition mode involved peak jumping and raw count rates were measured for ²⁹Si, ²⁰⁴Pb, ²⁰⁶Pb, ²⁰⁷Pb, ²⁰⁸Pb, ²³²Th, and ²³⁵U; U, Th, and Pb concentrations were calibrated using ²⁹Si as an internal standard and NIST SRM 610 as reference standard. Each analysis consists of 30s gas blank and 40s signal acquisition. Off-line selection and integration of background, analyte signals, time-drift correction, and quantitative calibration were conducted by ICPMSDataCal (Liu et al., 2010). Common Pb correction was in accordance with the method of Andersen (2002). U–Pb ages and concordia diagrams were prepared using ISOPLOT3.00 (Ludwig, 2003). All measurements were normalized relative to standard zircon GJ-1 and 91500. Individual analyses in the data table and concordia plots are presented with 1σ errors and uncertainties in ages are quoted at the 95% confidence level (2σ).

Results

LA-ICP-MS zircon U-Pb geochronology

The LA-ICP-MS zircon U-Pb analytical results for the three volcanic samples are presented in Table 2. The ²⁰⁷Pb/²⁰⁶Pb ages were used for older (>1000Ma) zircon grains, while ²⁰⁶Pb/²³⁸U ages were adopted for younger ones. Zircon grains from the samples are mostly euhedral with elongation ratios varying from 1.5∶1 to 3∶1, colorless or light brown, and prismatic with concentric oscillatory zoning (Fig. 3).

Doi Yao andesite (TL-1-B)

Thirteen spots are analyzed and give a relatively wide range in U (480-2608 ppm) and Th (66-1309 ppm) concentration, with Th/U ratios ranging between 0.03 and 0.64. Considering their CL images, the zircons should be of an igneous origin (Wu and Zheng, 2004). Seven analyses of thirteen spots yield a weighted mean ²⁰⁶Pb/²³⁸U age of 241.2±4.6 Ma (MSWD=1.6) (Fig. 4), representing the formation age of the andesite. The apparent ²⁰⁶Pb/²³⁸U ages of the remaining spots range from 387±10 Ma to 276±5 Ma, which can be interpreted as the ages of the xenocrysts.

Doi Yao andesite (TL-31-B)

Th/U ratios for twenty-one analytical zircons range from 0.44 to 1.10, similar to those of magmatic zircons (Wu and Zheng, 2004). These spots form a coherent cluster on the concordia plot and define a weighted ²⁰⁶Pb/²³⁸U mean age of 241.7±2.9 Ma (MSWD=3.0) (Fig. 4), representing the formation age of the andesite.

Doi Khun Ta Khong rhyolite (TL-32-B1)

U and Th concentration of sixteen analysis on sixteen zircons range from 484-2392 ppm and 240-1818 ppm, respectively. Their Th/U ratios are in the range of 0.28-1.21, similar to those of magmatic zircons (Wu and Zheng, 2004). Eight spots yield a weighted ²⁰⁶Pb/²³⁸U mean age of 238.3±3.8 Ma (MSWD=2.0) (Fig. 4), representing the formation age of the rhyolitic sample. The remaining spots (TL-32-B1-1, -2, -3, -4, -5, -11, -13) give the older ages from 1880±27 Ma to 1316±35 Ma, representative of the ages of the xenocrystic grains (Fig. 3).

Geochemical characteristics

Ten samples were selected for whole-rock major oxides and trace elemental analyses (Table 1). All major oxides are volatile-free normalized to 100%. The volcanic rocks have 60.25-84.34 wt.% of SiO₂. Two samples (TL-1-B1 and TL-1-B2) are andesite, with 60.25-61.26 wt.% of SiO₂. Al₂O₃, CaO, MgO, \mathrm{F}{\mathrm{e}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{3}}^{\mathrm{T}}, and TiO₂ show the negative correlation with SiO₂. A scatter trend is shown in the plot of Na₂O+K₂O and SiO₂ (Fig. 5). In the \mathrm{S}\mathrm{i}{\mathrm{O}}_2-({\mathrm{K}}_{\mathrm{2}}\mathrm{O}+\mathrm{N}{\mathrm{a}}_{\mathrm{2}}\mathrm{O}) (TAS) (Irvine and Baragar, 1971; Le Bas et al., 1986) and Zr/TiO₂-Nb/Y diagram (Winchester and Floyd, 1977) (Fig. 6), these samples fall in the field of sub-alkaline rhyodacite/dacite with two samples being falling in the field of andesite. In the AFM diagram (Fig. 6) (Irvine and Baragar, 1971), the samples fall in the field of calc-alkaline.

All samples show similar chondrite-normalized REE patterns with light REE (LREE) enrichment and significant Eu/Eu* anomalies (0.55-0.87, Fig. 7(a)). La_N/Yb_N ratios range from 4.28 to 13.81 and Gd_N/Yb_N ratios from 1.14 to 2.40. All samples have lower Nb and Ta contents and higher Th/Nb, Th/Yb, and La/Yb ratios than those of average MORB (Fig. 7(a)). In the primitive mantle-normalized multielement spidergram (Fig. 7(b)), seven samples from Doi Yao and Doi Khun Ta Khuan zones show depletions in Nb, P, and Ti and high LILE/HFSE ratios, similar to those of arc volcanic rocks (Tatsumi and Maruyama, 1989). Nb_N/La_N ratios are in the range of 0.28–0.78, indicative of a geochemical affinity to arc volcanic rocks. Three samples from the Chiang Khong area exhibit strong depletions in Sr, P, and Ti (Fig. 7(b)) with Nb_N/La_N ratios ranging from 0.14 to 0.28. All samples have lower Nb and Ta contents and higher Th/Nb, Th/Yb, and La/Yb ratios than those of average MORB (Fig. 7).

Discussion

Ages of the Chiang Khong volcanic zone in NW Thailand

The new LA-ICP-MS zircon U-Pb dating of the three samples from Doi Yao zone (TL-1-B and TL-31-B) and Doi Khun Ta Khuan zone in Chiang Khong area (TL-32-B1) indicate the formation age ranging from 238 Ma to 241 Ma in the early Middle Triassic period. Barr et al. (2000, 2006) also reported the similar formation ages of 232.9±0.4 Ma for rhyolitic tuff from Doi Yao zone and 240±1 Ma for the rhyolite of Doi Luang zone in Lampang area. This result indicates that Doi Yao and Doi Khun Ta Khuan volcanic rocks were synchronously erupted. In the sample (TL-32-B1), eight older ages from 1880±27 Ma to 1316±35 Ma are given, which can be interpreted as the ages of inherited zircons. Such Mesoproterozoic and Paleoproterozoic ages are firstly reported from the Sukhothai terrane. Bodet and Schärer (2000) also reported abundant Precambrian ages, including 2.3-2.2 Ga, 2.0-1.9 Ga and 1.2-1.1 Ga from detrital zircons and baddeleyites from the sands of Nujiang, Langcangjiang (Mekong) and Red rivers, respectively. They also suggested the existence of the Precambrian basement in the Southeast Asia area that is similar to the Yangtze basement.

Tectonic setting and implications

As mentioned above, seven samples from Doi Yao and Doi Khun Ta Khuan zones exhibit typical arc geochemical characteristics, e.g., low TiO₂ (0.53-1.04 wt.%), Ni, Cr and higher Al₂O₃ contents, and a marked enrichment in LILE and LREE and a depletion in HFSEs. These characteristics suggest their formation being under an arc setting. Three samples from the Chiang Khong area exhibit higher SiO₂ (75.34-84.34 wt.%) and lower TiO₂ (0.14-0.43 wt.%) contents, and strong depletion in Sr, P and Ti. In the Th/Ta-Yb discrimination diagram (Gorton and Schandl, 2000), all sample plots in the field are of active continental margin (Fig. 8(a)). In the Th/Yb-Nb/Yb discrimination diagram (Pearce and Peate, 1995), the majority of these samples fall into the field of continental arcs (Fig. 8(b)). Similar result is given in the Rb-(Y+Nb) and Nb-Y diagrams (Figs. 8(c) and 8(d)) (Pearce et al., 1984). The synthesis of these data points to the petrogenesis for andesitic samples being related to the continental margin volcanic arc setting, rhyolitic samples from the Chiang Khong area may formed in a transition setting from continental margin arc to syncollision. Barr et al. (2000, 2006) also proposed that the Lampang and Chiang Khong volcanic rocks were formed in an island-arc setting at middle Triassic period.

In the past twenty years, ophiolites, melanges, middle ocean-ridge basalts, oceanic island basalts, and arc volcanic rocks have been discovered in NW Thailand. These rocks are considered to be the products of the Paleo-Tethys evolution in Southest Asia. Chiang Khong and Lampang volcanic zones located between the Chiang Mai and Nan sutures in the Sukhothai terrane (Shen et al., 2009). Previous studies considered that these volcanic rocks in Chiang Khong and Lampang volcanic zones were formed in the Late Permian to Early Triassic period, and can be compared to the Langcangjiang arc volcanic zone in SW Yunnan (Yang et al., 1994; Wu et al., 1995; Barr et al., 2000, 2006; Chonglakmani et al., 2001; Metcalfe, 2002; Feng et al., 2005). Our results further verify the viewpoint of Barr et al. (2000, 2006).

The domestic research focused on the Yunxian and Jinghong arc volcanic rocks which are located in the southern Langcangjiang zone (Yang et al., 1994; Zhong, 1998; Peng et al., 2008; Wang et al., 2012). Yang et al. (1994) first suggested the correlation between the Lampang volcanic zone could compare to Lincang–Jinghong volcanic zone, but the volcanic rocks in that area were reported to range in age from Carboniferous to Late Triassic. Feng et al. (2005) suggested that volcanic rocks in both the Lampang and Chiang Khong areas can correlate with the volcanic rocks in the Simao Basin of Yunnan, who proposed the correlation based on sedimentary units. According to the geochemical and geochronological data of andesitic samples collected from the Manghuai Formation of the Langcangjiang tectonic zone, Peng et al. (2008) also proposed the part of the Manghuai Formation volcanic suite rocks in the southern Langcangjiang zone might be compared with subduction-related arc volcanic rocks and formed in a continental margin volcanic arc setting. Barr et al. (2000, 2006) also thought these rocks in both Lampang and Chiang Khong areas were similar to the Middle Triassic Manghuai Formation volcanic rocks in the southern Lancangjiang zone. Wang et al. (2012) suggested that the Triassic volcanic rocks from southern Lancangjiang tectonic zone formed in a transition-type continental margin orogenic belts and got an age of 236.7±2.2 Ma from a quartz andesitic sample in the Manghuai Formation by the Ar-Ar dating. With the dating results, Wang et al. (2012) suggested the main collision stage of Lancangjiang zone is inferred in Early Triassic. Our data indicate a presence of an early Middle Triassic continental margin in Chiang Khong and Lampang of NW Thailand. The arc volcanic zone in Lampang, through Chiang Khong, Laos, and can link to the Jinghong volcanic zone in SW Yunnan (SW China) (Fig. 9).

Conclusions

1) Three andesitic and rhyolitic samples from the Chiang Khong volcanic zone give the zircon U-Pb ages of 241.2±4.6 Ma, 241.7±2.9 Ma, and 238.3±3.8 Ma, respectively, suggesting an early Middle Triassic origin.

2) Andesitic, dacitic, and rhyolitic samples from Doi Yao and Doi Khun Ta Khuan zones exhibit the geochemical affinity to arc volcanics, suggesting a presence of the an early middle Triassic continental margin in NW Thailand. Rhyolitic samples from the Chiang Khong area might be the product of the tectonic transition from arc to syncollisional stages.

3) Our data indicate the Chiang Khong arc volcanic zone in northern Thailand can link to the Jinghong volcanic zone in SW Yunnan (SW China).

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