Petrogenesis and tectonic implication of the Late Triassic post ...

Lithos 248–251 (2016) 418–431

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Petrogenesis and tectonic implication of the Late Triassic post-collisional volcanic rocks in Chiang Khong, NW Thailand Xin Qian a,b, Yuejun Wang a,⁎, Qinglai Feng b, Jian-Wei Zi c, Yuzhi Zhang a, Chongpan Chonglakmani d a

School of Earth Science and Geological Engineering, Sun Yat-sen University, Guangzhou, 510275, China State Key Laboratory of Geological Processes and Mineral Resources, Faculty of Earth Sciences, China University of Geosciences, Wuhan, 430074, China Department of Applied Geology, Curtin University, Perth, 6102, Australia d School of Geotechnology, Suranaree University of Technology, Nakhon Ratchasima, 30000, Thailand b c

a r t i c l e

i n f o

Article history: Received 8 October 2015 Accepted 25 January 2016 Available online 8 February 2016 Keywords: Elemental and isotopic geochemistry Zircon U–Pb geochronology Late Triassic post-collisional magmatism Chiang Khong–Lampang–Tak igneous zone NW Thailand

a b s t r a c t The volcanic rocks exposed within the Chiang Khong–Lampang–Tak igneous zone in NW Thailand provide important constraints on the tectonic evolution of the eastern Paleotethys ocean. An andesite sample from the Chiang Khong area yields a zircon U–Pb age of 229 ± 4 Ma, significantly younger than the continental-arc and syn-collisional volcanic rocks (ca. 238–241 Ma). The Chiang Khong volcanic rocks are characterized by low MgO (1.71–6.72 wt.%) and high Al2O3 (15.03–17.76 wt.%). They are enriched in LILEs and LREEs and depleted in HFSEs, and have 87Sr/86Sr (i) ratios of 0.7050–0.7065, εNd (t) of −0.32 to −1.92, zircon εHf (t) and δ18O values of 3.5 to −11.7 and 4.30–9.80 ‰, respectively. The geochemical data for the volcanic rocks are consistent with an origin from the enriched lithospheric mantle that had been modified by slab-derived fluid and recycled sediments. Based on available geochronological and geochemical evidences, we propose that the Late Triassic Chiang Khong volcanic rocks are equivalent to the contemporaneous volcanic rocks in the Lancangjiang igneous zone in SW China. The formation of these volcanic rocks was possibly related to the upwelling of the asthenospheric mantle during the Late Triassic, shortly after slab detachment, which induced the melting of the metasomatized mantle wedge. © 2016 Elsevier B.V. All rights reserved.

1. Introduction SW Yunnan (SW China) and its southern extension (e.g., NW Laos and NW Thailand) are the key areas for investigating and understanding the Paleotethys tectonic evolution due to the abundant preservations of the Paleozoic igneous rocks and associated rocks (Fig. 1a). In these areas, there developed several important zones including the Jinshajiang–Ailaoshan and Changning–Menglian suture zones in SW China (Leloup et al., 1995; Liu et al., 1991; Mo et al., 1998; Wu et al., 1995; Zi et al., 2012a, 2012b; Zhong, 1998), the Chiang Mai, Nan– Uttaradit and Loei suture zones in NW Thailand (Charusiri, 1997; Charusiri et al., 2002; Feng et al., 2005; Intasopa and Dunn, 1994; Panjasawatwong et al., 2006; Qian et al., 2015; Sone and Metcalfe, 2008; Sone et al., 2012; Ueno and Hisada, 2001), and the Luang Prabang tectonic zone in NW Laos (Qian et al., 2016), and two igneous zones involving the Lancangjiang zone in SW Yunnan and the Chiang Khong– Lampang–Tak zone in NW Thailand (Barr and Charusiri, 2011; Barr et al., 2000, 2006; Panjasawatwong, 2003; Peng et al., 2006, 2008, 2013; Qian et al., 2013; Srichan et al., 2009; Wang et al., 2010). Previous ⁎ Corresponding author at: School of Earth Science and Geological Engineering, Sun Yat-sen University, No. 135, Xingang Xi Road, Guangzhou, 510275, China. Tel.: +86 20 84111209. E-mail address: [email protected] (Y. Wang).

http://dx.doi.org/10.1016/j.lithos.2016.01.024 0024-4937/© 2016 Elsevier B.V. All rights reserved.

researches on the metamorphic rocks and magmatic rocks along the Lancangjiang igneous zone have established a tectonic history related to the Paleotethys evolution involving eastward subduction during the Permian followed by ocean closure and arc-/continental-continental collision during the Triassic (Dong et al., 2013; Fan et al., 2015; Hennig et al., 2009; Metcalfe, 2002; Mo et al., 1998; Peng et al., 2006, 2008, 2013; Wang et al., 2010; Zhong, 1998). The Chiang Khong–Lampang– Tak igneous zone is located between the Chiang Mai and Nan– Uttaradit suture zones and is a segment of a broader igneous belt. Proposals have been made to link with the Lancangjiang igneous zone in SW Yunnan and further northward with the Weixi-Deqin-Yushu igneous zone in eastern Tibet (Fig. 1a; Barr and Charusiri, 2011; Barr et al., 2000, 2006; Panjasawatwong, 2003; Qian et al., 2013). The Permian– Triassic volcanic rocks are characterized by the mafic–intermediate– felsic volcanic association and the key components of the Chiang Khong–Lampang–Tak igneous zone in NW Thailand. Previous studies on the volcanic rocks in this zone have been focused on the regional lithofacies and stratigraphy, with little attention having been paid to the petrogenesis and tectonic setting of the Triassic volcanic rocks. The absence of precise geochronological and geochemical data has hampered our understanding of the age and tectonic significance of these rocks. In this paper, we present new zircon U–Pb geochronological, whole rock geochemical, and zircon in-situ Hf and O isotopic data for basalt, basaltic andesite, andesite and dacite samples collected

X. Qian et al. / Lithos 248–251 (2016) 418–431

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Fig. 1. (a) Tectonic outline of Southeast Asia (revised after Feng et al., 2005; Leloup et al., 1995; Qian et al., 2015, 2016). (b) Simplified geological map of the study area in northern Thailand (revised after Barr et al., 2006; von Braun and Hahn, 1976). JAS: Jingshajiang–Ailaoshan suture; LZ: Lancangjiang igneous zone; CMS: Changning–Menglian suture; SMS: Song Ma suture; LPZ: Luang Prabang tectonic zone; TSFB: Truong Son Fold Belt; LS: Loei suture; SKS: Sa Kaeo suture; NUS: Nan–Uttaradit suture; CLTZ: Chiang Khong–Lampang–Tak igneous zone; CS: Chiang Mai suture.

from the Chiang Khong area, NW Thailand with the aim to better constrain the age and petrogenesis of the volcanic rocks, and to understand their tectonic significance in the context of the Paleotethyan evolution in SE Asia. 2. Geological setting and petrography Ueno (2003) has proposed several geotectonic units for northern Thailand (from west to east): the Sibumasu Block, Sukhothai Terrane, and Indochina Block, traditionally considered to be separated by the Chiang Mai and Nan-Uttaradit suture zones, respectively (Fig. 1a). Sone and Metcalfe (2008), Hara et al. (2009), Metcalfe (2011, 2013) and Sone et al. (2012) suggested a similar tectonic scheme. Geological records of the Paleotethys in northern Thailand include (1) Carboniferous and Permian mélange, oceanic and oceanic-island basalts along the Chiang Mai suture zone (Barr and Charusiri, 2011; Barr et al., 1990; Feng et al., 2008; Panjasawatwong, 1999; Phajuy et al., 2005; Zhang et al., 2016); (2) Devonian–Triassic shallow-marine carbonates and deepsea sedimentary rocks (e.g., Feng et al., 2004; Thassanapak et al., 2011); (3) the mafic-ultramafic rocks with the U–Pb zircon ages of 311–315 Ma along the Nan-Uttaradit suture zone with a BABB-like geochemical affinity (Ueno and Hisada, 2001; Yang et al., 2009, 2016); (4) Permo–Triassic continental-arc volcanic rocks and granitic rocks

(Barr et al., 2000, 2006; Cobbing, 2011; Panjasawatwong, 2003; Qian et al., 2013). In addition, along the Loei and Luang Prabang zones, there are abundant Late Paleozoic mafic complexes and volcanic rocks showing MORB and/or BABB geochemical affinities, which are coeval with Late Devonian–Early Carboniferous deep-sea sedimentary rocks (Fig. 1a; Boonsoong et al., 2011; Charusiri, 1997; Charusiri et al., 2002; Intasopa and Dunn, 1994; Panjasawatwong et al., 2006; Qian et al., 2015, 2016; Sashida et al., 1993; Udchachon et al., 2011). The Chiang Khong area is located in the eastern part of Chiang Rai Province. According to the early description by von Braun and Hahn (1976), the sedimentary sequences of the region include mainly Carboniferous–Permian limestone, chert, shale, sandstone and conglomerate, Triassic sandstone, siltstone conglomerate, mudstone, tuff and shale interlayer, Lower Jurassic sandstone, siltstone, shale and volcanic conglomerate (Figs. 1b and 2a; e.g., von Braun and Hahn, 1976). The most remarkable geological feature of this area is the NE-trending Doi Yao and Doi Khun Ta Khuan belts separated by a Cenozoic Chiang Khong basin, which are the key elements of the Chiang Khong– Lampang–Tak igneous zone with a length of N 300 km and a width of N40 km (Fig. 1b; e.g., Barr et al., 2006). The plutonic rocks of the igneous zones comprise granite, granodiorite and diorite. The volcanic rocks are composed mainly of basaltic andesite, andesite, dacite, rhyolite, tuff and volcanic breccia with

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belts with the geochemical affinity to arc and syn-collisional volcanic rocks. In this study, we collected various volcanic rock samples including basalt, basaltic andesite, andesite and dacite from the Doi Khun Ta Khuan belt which is located to the west of the Cenozoic Chiang Khong basin in NW Thailand and in fault contact with the Late Triassic– Jurassic red beds and Cenozoic sediments (Figs. 1b and 2a). The basalt is mainly composed of 40% phenocrysts of clinopyroxene, plagioclase, orthopyroxene and 60% matrix of plagioclase, clinopyroxene, orthopyroxene and amphibole. The basaltic andesite and andesite are commonly subaphyric to porphyritic, and are composed of plagioclase and clinopyroxene phenocrysts in an aphanitic matrix of clinopyroxene, plagioclase, quartz and minor opaque oxide minerals. In the andesite, secondary chlorite is also observed. Most of the plagioclase phenocrysts are subhedral to euhedral with low degree of sericitization (Fig. 2b). The dacite shows subaphyric to porphyritic texture with phenocrysts of hornblende, plagioclase and quartz in a groundmass of fine-grained plagioclase, quartz, glass and minor opaque oxides (Fig. 2c). 3. Analytical methods

Fig. 2. Simplified stratigraphic column (a) and microscopic photographs of (b) basaltic andesite sample (TL13A13) and (c) dacite sample (TL11A4) for the Chiang Khong volcanic rocks. Pl: plagioclase, Cpx: clinopyroxene, Q: quartz.

volcanoclastic interlayer (Fig. 2a). These volcanic sequences are in unconformable contact with the Permian–Triassic sedimentary rocks and in fault contact with the Late Triassic–Jurassic red beds and Cenozoic sediments. On the basis of stratigraphic correlations, the volcanic rocks in the Chiang Khong area have been interpreted as products of two magmatic events occurred during the Permian–Middle Triassic and the Late Triassic–Jurassic time intervals (Jungyusuk and Khositanont, 1992). However, this interpretation has been disputed (e.g., Barr et al., 2006; Panjasawatwong, 2003). von Braun and Hahn (1976) and Panjasawatwong (2003) suggested that the volcanic rocks in the Doi Khun Ta Khuan and Doi Yao belts formed in a continental arc during the Permian–Triassic period (Phajuy, 2001). Barr et al. (2006) yielded a zircon age of 232.9 ± 0.4 Ma for the andesite and interpreted that they formed in a continental-arc setting. However, Srichan et al. (2009) concluded that the Chiang Khong volcanic rocks are Late Triassic (220–223 Ma) products in a postcollisional tectonic regime. More recently, Qian et al. (2013) have reported three zircon U–Pb ages of 241.2 ± 4.6 Ma, 241.7 ± 2.9 Ma and 238.3 ± 3.8 Ma for the andesite and rhyolite samples from the Doi Yao and Doi Khun Ta Khuan

Zircon grains were separated by conventional heavy liquid and magnetic techniques. Grains were mounted in epoxy, polished and coated with gold and then photographed in transmitted and reflected light. Their internal texture was examined using cathodoluminescence (CL) imaging at the Institute of Geology and Geophysics (IGG), Chinese Academy of Sciences (CAS), Beijing. Zircon U–Pb dating were analyzed using a Laser ICP–MS at the IGG CAS. The zircon standards CN92-2, 91500 and GJ were used to calibrate the U–Th–Pb ratios. The standard silicate glass NIST 610 was used to optimize the machine. The spot size for data collection was 30 μm. The individual U–Pb analyses are presented with 1σ error and uncertainties in grouped ages are quoted at 95% level. The age calculations and plots were made using Isoplot (version 3.0) (Ludwig, 2003). Detailed analytical procedure for the LA–ICP–MS zircon U–Pb and trace element technique are similar to those described by Yuan et al. (2004). The analytical data are listed in Supplementary Datasets 1. Zircon Lu–Hf isotopic analysis was carried out using a Geolas-193 laser-ablation microprobe, attached to a Neptune multi-collector ICP– MS at the IGG CAS. All of the settings yielded a signal intensity of ~ 10 Vat 180Hf for the standard zircon 91500 with a recommended 176 Hf/177Hf ratio of 0.282293 ± 28 (Wu et al., 2006). Data were normalized to 176Hf/177Hf = 0.7325, using exponential correction for mass bias. The mean βYb value was applied for the isobaric interference correction of 176Yb on 176Hf in the same spot. The ratio of 176Yb/172Yb (0.5887) was also applied for the Yb correction. Zircon oxygen isotopic analysis was measured using the Cameca 1280 at IGG CAS. The Cs+ ion beam was accelerated to 10 kV, with an intensity of ~ 2 nA. The analysis site was the same as for U–Pb dating, and the spot size is about 20 μm in diameter. The normal incidence electron flood gun was used to compensate for sample charging. The NMR (nuclear magnetic resonance) was used for stabilizing magnetic field. Oxygen isotopes were measured in multi-collector mode with two offaxis Faraday cups. Analytical procedures are similar to that described by Li et al. (2010a). The instrumental mass fractionation factor (IMF) was corrected using Penglai zircon standard with δ18O value of 5.31 ‰ (Li et al., 2010b). The internal precision of a single analysis was generally better than 0.20 ‰ (1r standard error) for 18O/16O ratio. The external precision, measured by the reproducibility of repeated analyses of Penglai standard. During the course of this study, an in-house zircon standard Qinghu was also measured as an unknown together with other unknowns (Li et al., 2013). Zircon Lu–Hf and oxygen isotopic analyzed data are listed in Supplementary Datasets 2. Whole-rock samples for elemental and Sr–Nd isotopic analyses were crushed to 200-mesh using an agate mill. The major oxides were analyzed by a wavelength X-ray fluorescence spectrometry at the State


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Fig. 3. LA–ICP–MS zircon U–Pb concordia diagram for sample TL11-A1 in the Chiang Khong area with cathodoluminescence images (CL) of the representative zircon grains.

Key Laboratory of Isotope Geochemistry, Guangzhou Institute of Geochemistry (GIG), Chinese Academy of Sciences (CAS). Trace element analyses were performed at the GIG CAS using a Perkin–Elmer Sciex ELAN 6000 ICP–MS. Detailed sample preparation and analytical procedure followed Li et al. (2002). Sr and Nd isotopic analyses were carried out using a Neptune Plus multi-collection mass spectrometry equipped with nine Faraday cup collectors and eight ion counters. Details of analytical methods are presented by Yang et al. (2006). The total procedure blanks were in the range of 200–500 pg for Sr and ≤50 pg for Nd. The mass fractionation corrections for Sr and Nd isotopic ratios are based on 86Sr/88Sr = 0.1194 and 146Nd/144Nd = 0.7219, respectively. The measured 87Sr/86Sr ratios of the (NIST) SRM987 standard are 0.710265 ± 12 (2σ) and the measured 143Nd/144Nd ratios of the La Jalla standard are 0.511862 ± 10 (2σ). The whole-rock Sr and Nd isotopic data of the selected samples are given in Supplementary Datasets 3. 4. Analytical results 4.1. Zircon U–Pb dating result TL-11A1 is an andesite sample from an outcrop near the Mekong River (20°19′13.98″N, 100°22′47.16″E). The majority of zircon grains

from this sample are euhedral, transparent and light brown in colour and exhibit oscillatory zoning in CL images (Fig. 3). Th and U concentrations of twenty-six analytical spots on 26 zircon grains range from 18 to 805 ppm and 70 to 821 ppm, respectively. The Th/U ratios are in the range of 0.13–0.98. Fifteen spots of analytical grains yield a weighted mean 206Pb/238U age of 229 ± 4 Ma with MSWD = 0.86 (Fig. 3), representing the formation age of the andesite. The remaining grains give apparent ages ranging from 410 Ma to 1918 Ma (Fig. 3), interpreted as inherited ages.

4.2. Zircon Lu–Hf and O isotopic compositions In-situ Lu–Hf and oxygen isotopic analyses were conducted on representative zircon grains from the studied rock. Twenty-four analyses on 24 zircon grains from sample TL-11A1 yield 176Lu/177Hf ratios ranging from 0.0004 to 0.0038 (mostly b 0.002), indicating a negligibly small amount of radiogenic Hf accumulated after zircon crystallization (Griffin et al., 2000). Ten inherited zircon grains have εHf (t) values ranging from +5 to −16.7 with two-stage model ages (TDM2) from 1.47 Ga to 2.26 Ga (Fig. 4a). Fourteen zircon grains with the eruption age of the andesite also exhibit variable Lu–Hf isotopic compositions, with εHf

Fig. 4. Plots of (a) age (Ma) vs εHf (t) and (b) oxygen isotopic compositions for zircon grains from sample TL11-A1. Also shown for MORB (Eiler et al., 2000), altered upper and lower oceanic crust (Cocker et al., 1982; Gregory and Taylor, 1981), mafic rocks and I-, and S-type granites (Eiler et al., 2000). Whole-rock δ18O is calculated using whole rock δ18O = zircon δ18O + 0.0612 × (wt.% SiO2) − 2.5 (Valley et al., 2005).

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Fig. 5. Plots of SiO2 vs (a) Al2O3, (b) Fe2O3t, (c) MgO, (d) CaO, (e) TiO2 and (f) P2O5 for the Late Triassic volcanic rocks in the Chiang Khong area, NW Thailand.

(t) values ranging from + 3.5 to −11.7. The corresponding TDM2 ages are in the range of 0.87–1.60 Ga (Fig. 4a). The measured zircon grains have heterogeneous oxygen isotopic compositions. Ten grains with the eruption age for the andesitic rock yield δ18O values of 4.30–9.80 ‰ (Fig. 4b). The whole rock δ18O values in equilibrium with the zircon δ18O are approximated on the basis of the linear relationship of whole rock δ18O ≈ zircon δ18O + 0.0612*(wt.% SiO2) − 2.5 (Valley et al., 2005). The calculated whole rock δ18O values for these ~229 Ma grains vary from 5.21–10.72 ‰ (Fig. 4b). 4.3. Geochemical characteristics All major oxides are volatile-free normalized to 100%. According to the SiO 2 contents, the analyzed samples can be divided into

mafic and intermediate groups (herein termed Group 1 and Group 2, respectively). The mafic volcanic rocks with SiO 2 of 49.77–55.82 wt.% have TiO2 (1.02–2.02 wt.%), high Al 2 O 3 (16.32–17.76 wt.%) and low MgO (3.31–6.72 wt.%) with mgnumbers (molar Mg × 100/(Mg + Fe) of 40–55. In the Harker diagrams of selected major oxides, Al2O3, Fe2O3t, MgO and CaO decrease with increasing SiO 2, but there are no clear correlations between TiO 2, P2 O 5 and SiO 2 (Fig. 5a–f). In the Zr/TiO 2–Nb/Y diagram (Fig. 6a) (Winchester and Floyd, 1977), all Group 1 samples fall in the field of subalkalic basalt. In the Co–Th diagram (Fig. 6b) (Hastie et al., 2007), most samples fall in the field of high-K calc-alkaline basaltic andesite/andesite. They are similar to the high-Al postcollisional volcanic rocks in the Lancangjiang igneous zone (Fig. 6b) (Wang et al., 2010).


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The Group 2 samples have low Cr and Ni contents ranging from 13 to 29 ppm and 2 to 15 ppm, respectively. They exhibit moderately fractionated light rare-earth elements (LREEs) relative to heavy rare-earth elements (HREEs), with (La/Yb)N ratios ranging from 7.13 to 9.34. These samples give (Gd/Yb)N ratios ranging from 1.54 to 1.77, and Eu/ Eu* from 0.62–0.82 (Fig. 7b). On the Primitive mantle-normalized multi-element spidergram (Sun and McDonough, 1989) (Fig. 7d), these samples are characterized by enrichment in LILEs and depletion in Nb, Ta, Sr and Ti with Th/La = 0.29–0.36 and Nb/La = 0.35–0.48. The samples show an overall profile similar to that of the Chiang Khong Late Triassic volcanic rocks and high-Al post-collisional volcanic rocks in the Lancangjiang igneous zone (Fig. 7d) (Srichan et al., 2009; Wang et al., 2010). The Group 2 samples display depletions in Nb and Ta as observed in Group 1 and they show enhanced negative Sr and Ti anomalies in comparison with Group 1 (Fig. 7c and d) The Sr–Nd isotopic analytical results for the selected samples are presented in Supplementary Datasets 3. The initial Sr–Nd isotopic ratios were calculated using the formation age of 229 Ma. 87 Sr/86Sr (i) ratios of Group 1 range from 0.7050 to 0.7055 and εNd (t) values from − 0.32 to − 1.20 (Fig. 8). 87Sr/86Sr (i) ratios of Group 2 range from 0.7054 to 0.7065 and εNd (t) values from − 1.53 to − 1.92. The εNd (t) values of both groups are similar to those of the high-Al postcollisional volcanic rocks but lower than those of the high-Mg postcollisional volcanic rocks in the Lancangjiang igneous zone (Fig. 8; Wang et al., 2010).

5. Discussion and conclusion 5.1. Petrogenesis of the Chiang Khong volcanic rocks

Fig. 6. (a) Nb/Y vs Zr/TiO2 (after Winchester and Floyd, 1977) and (b) Co vs Th (after Hastie et al., 2007) classification diagrams for the Late Triassic volcanic rocks in the Chiang Khong area, NW Thailand. The data of the Lancangjiang high-Al and high-Mg post-collisional volcanic rocks are from Wang et al. (2010).

The Group 2 samples display values as follows: SiO2 = 58.92–64.14 wt.%, TiO2 = 0.98–1.72 wt.%, Al2O3 = 15.03–17.22 wt.%, MgO = 1.71–4.22 wt.% and mg-numbers of 31–54. Negative correlations are observed between Al2O3, Fe2O3t, CaO, TiO2, P2O5 and SiO2, but there is no clear correlation between MgO and SiO2 (Fig. 5a–f). In the Zr/TiO2–Nb/Y diagram (Fig. 6a) (Winchester and Floyd, 1977), most samples fall in the field of subalkalic to alkalic basalt or andesite, whereas in the Co–Th diagram (Fig. 6b) (Hastie et al., 2007), all samples fall in the field of high-K calc-alcaline basaltic andesite/andesite and dacite/rhyolite series. The Group 1 samples have low Cr and Ni contents ranging from 16 to 150 ppm and 3 to 18 ppm, respectively, and show similar chondritenormalized REE patterns (Fig. 7a). They exhibit REE fractionation with (La/Yb)N (N herein refers to chondrite-normalized value) of 6.81–9.44, (Gd/Yb)N of 1.64–2.15 and Eu/Eu* of 0.85–1.07 (Fig. 7a). On the Primitive mantle-normalized multielement spidergram (Sun and McDonough, 1989) (Fig. 7c), these samples are characterized by enrichment in Large Ion Lithophile Elements (LILEs) and depletion in High Field Strength Elements (HFSEs) (e.g., Nb, Ta and Ti) with Th/La = 0.16–0.30 and Nb/La = 0.32–0.47. The Group 1 samples do not show negative Th–U anomalies observed in the high-Mg post-collisional volcanic rocks in the Lancangjiang igneous zone (Fig. 7c), but display similarities to the high-Al post-collisional volcanic rocks in the Lancangjiang igneous zone (Wang et al., 2010) and Chiang Khong Late Triassic volcanic rocks (Srichan et al., 2009).

The studied samples are generally fresh, although various degrees of sericitic alteration of plagioclase phenocrysts are observed in some samples. The majority of the studied samples have LOI values lower than 3 wt.%, and there is no obvious correlation between Nb/La, Th/La and εNd (t) (not shown) and LOI values. High field strength elements and Nd isotopic compositions are generally considered to be immobile during alteration and weathering, and Zr is often used as an index to test the mobility of other incompatible elements (Rolland et al., 2009). Our samples show positive correlations between Zr and HFSEs (ca. Nb, Th, La, Yb, Nd, Sm, Ti) and LILE (ca. Ba), while no correlations of Zr with Sr and Rb were observed (not shown). All these facts suggest that the behavior of these elements (ca. Nb, Th, Yb, Nd, Sm, La, Ba) is weakly changed by the weather alteration and can be used to discuss the petrogenesis. The negative εNd (t) values (−0.32 to −1.20), along with high Al2O3 content (15.03–17.76 wt.%), and particularly the presence of inherited zircon grains, suggest that crustal contamination was likely effective. In addition, the Nb/La ratios for all samples have weak negative correlations with SiO2 (Fig. 9a). However, the εNd (t) values remain constant with MgO contents (Fig. 9b). These features, together with the lack of correlations between MgO and Ce/Pb and Nb/U, and the similarity of immobile incompatible element patterns, suggest that crustal contamination is weak and insufficient to modify their elemental and isotopic compositions. Group 1 samples are dominated by high-Al basalt and basaltic andesite with Al2O3 values of 16.32–17.76 wt.%. Four scenarios have been proposed for the formation of the high-Al samples: (1) high degree partial melting of the subducted oceanic crust (e.g., Baker and Eggler, 1983; Brophy and Marsh, 1986; Marsh, 1976; Myers, 1988; Myers and Johnston, 1996); (2) extensive interaction of ascending melts with refractory mantle (e.g., Kelemen, 1995; Rivalenti et al., 1998); (3) plagioclase accumulation in low-MgO magmas (e.g., Brophy, 1988; Crawford et al., 1989; Fournelle and Marsh, 1991; Gust and Perfit, 1987); and (4) olivine and clinopyroxene fractionation (e.g., Kersting and Arculus, 1994; Lytwyn et al., 2001; Perfit et al., 1980; Schiano et al., 2004; Wang et al., 2010).

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Fig. 7. (a-b) Chondrite-normalized REE patterns and (c-d) Primitive mantle-normalized trace element spidergrams for the Late Triassic volcanic rocks in the Chiang Khong area, NW Thailand. The N-MORB, E-MORB, OIB, normalized values for chondrite and primitive mantle are from Sun and McDonough (1989). The data of the Chiang Khong Late Triassic volcanic rocks and Lancangjiang high-Al and high-Mg post-collisional volcanic rocks are from Srichan et al. (2009) and Wang et al. (2010), respectively.

Partial melting of the subducted oceanic crust typically generates adakitic magma with high Si, Al, Sr (N 400 ppm) and Sr/Y (N 20) and low Y (b 18 ppm) and Yb (b1.9 ppm). The melt derived from the young oceanic crust would have fractionated LREE/HREE ratios with (La/Yb) N N 20 and depleted Sr–Nd isotopic composition (87Sr/86Sr b 0.705 and εNd N 4; Brophy and Marsh, 1986; Defant and

Fig. 8. Plot of initial 87Sr/86Sr (i) and εNd (t) for the Late Triassic volcanic rocks in the Chiang Khong area, NW Thailand. The fields of Lancangjiang high-Al and high-Mg post-collisional volcanic rocks are from Wang et al. (2010). The fields of EMI and EMII are from Zindler and Hart (1986).

Drummond, 1990; Johnston, 1986; Myers, 1988). Such features are not observed in Group 1. Therefore, scenario (1) can be excluded. The interaction of ascending melts with the refractory mantle can lead to high-Al basaltic magma, as suggested by Kelemen (1995). The magma derived from the interaction between melt and peridotite is commonly characterized by higher mg-number and Cr and Ni contents than the normal peridotite-derived melt. However, Group 1 samples have low mg-numbers of 40–55, Cr and Ni contents ranging from 16 to 150 ppm and 3 to 18 ppm, respectively. Therefore, scenario (2) involving interaction of ascending melts with the mantle is also not likely in the petrogenesis of Group 1. Plagioclase accumulation in low-MgO magma would result in elevated Sr and CaO, and positive Sr and Eu anomalies which are absent from the Group 1 samples (Fig. 7a and c). In addition, petrographic observation does not indicate plagioclase accumulation. Hence, plagioclase accumulation is unlikely the mechanism that controlled the generation of the Group 1. Scenario (4) is the most plausible mechanism to explain the petrogenesis of the studied high-Al basaltic samples based on the following observations: (1) low mg-numbers of 40–55 and low MgO (mostly b7.0 wt.%), Cr (16–150 ppm), Ni (3–18 ppm) contents and the positive correlations of Cr with Ni and V suggest that they were not primary magmas and experienced some fractional crystallization of olivine, clinopyroxene and hornblende (Fig. 10e–f); (2) negative correlations of SiO2 with Al2O3, MgO, CaO, and Fe2O3t (Fig. 5a–d); (3) no clear correlations of SiO2 with TiO2 and P2O5 suggest insignificant fractionation of Fe–Ti oxides and apatite (Fig. 5e–f); (4) the insignificant Eu anomalies indicate that the plagioclase may have


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Fig. 9. Plots of (a) SiO2 vs Nb/La and (b) MgO vs εNd (t) (Wang et al., 2013) for the Late Triassic volcanic rocks in the Chiang Khong area, NW Thailand.

played an insignificant role during magma evolution (Fig. 7a and c). All these facts suggest that the primitive magma of Group 1 is characterized by low MgO and high Al2O3. In addition, experimental studies have demonstrated that the high-Al basalts can be generated by the olivine and clinopyroxene fractionation of primitive melts (e.g., Kersting and Arculus, 1994; Lytwyn et al., 2001; Perfit et al., 1980; Schiano et al., 2004). Two feasible processes have been proposed for the origin of intermediate volcanic rocks: (1) extensive fractional crystallization from a common mantle-derived magma parental to basaltic rocks, coupled with crustal contamination (e.g., Bonin, 2004; Genç and Tüysüz, 2010; Pin and Paquette, 1997); and (2) crustal anatexis due to underplating of mantle-derived mafic magma with distinct isotopic compositions (e.g., Jones et al., 2011; Shellnutt and Zhou, 2007; Zhu et al., 2007). The Harker (Fig. 5) and MgO–Cr, La–La/Sm, La–La/Yb and Yb–La/Yb diagrams (Fig. 10a–d) suggest that the formation of the intermediate volcanic rocks from the Chiang Khong area is controlled by crystallization fractionation rather than the partial melting or source heterogeneity. Decrease of TiO2 and P2O5 with increasing SiO2 (Fig. 5e–f), and negative Ti anomaly (Fig. 7d) might be related to the fractionation of Fe–Ti oxides and apatite. The negative Eu and Sr anomalies suggest that the plagioclase may have played a role during magma evolution (Fig. 7b and d). However, the constant Cr with increasing Ni and V suggests insignificant fractionation of hornblende (Fig. 10e–f). Furthermore, the Group 2 samples have multi-element spidergram and Sr–Nd isotopic compositions similar to those of the Group 1 samples (Figs. 7 and 8). All these evidences suggest that these intermediate volcanic rocks were produced by fractional crystallization from a common magma parental to the Group 1 sample, which is characterized by low MgO and high Al2O3. The question remains about the source of the parental magma. All samples show enrichment in LILEs and depletion in HFSEs with obviously negative Nb, Ta and Ti anomalies (Fig. 7c–d). Zr/Nb ratios range from 11.23 to 15.35 and Nb/La from 0.32 to 0.48, similar to those of typical arc magmas (Brophy and Marsh, 1986; Luhr and Haldar, 2006; Oźerov, 2000). These characteristics, together with the (Ta/La)N ratios of 0.34–0.55 and (Hf/Sm)N ratios of 0.69–1.18, suggest the involvement of a “crustal” component in the source. This is further revealed by the Sr–Nd isotopic compositions with a trend towards the crustal/sedimentary component along the mantle array (Fig. 8). An uncertainty remains as to whether such “crustal” signatures originated from dehydrated fluids/melts from subducted slab/sediments. All samples have narrow variation of Nb/Zr ratios with variable Th/Zr ratios (Fig. 11a). The Ba/ La ratios of these samples show a sharp change in spite of small variations in Th/Yb ratios (Fig. 11b), suggesting the involvement of slab-derived fluids in the mantle source. In addition, our samples give

high Th/La ratios of 0.16–0.36, analogous to that of average crust (Th/ La = ~0.3), suggestive of the input of recycled continental sediments (Plank, 2005). In the Nb/La–εNd (t) diagram (Fig. 11c), the selected samples fall in the field between the crustal end-member and the enriched mantle end-member, indicating a binary mixing source. One component of the binary end-members is characterized by low εNd (b – 2) and Nb/La (b 0.3), similar to the estimates for crustal sediments (e.g., Wang et al., 2010). The other is a high Nb/La (N 0.8) and high εNd (N + 2) component, similar to the enriched lithospheric mantle (e.g., Wang et al., 2010). Fourteen zircon grains giving the crystallization age have εHf (t) values of +3.5 to –11.7 with TDM2 from 0.87–1.60 Ga (Fig. 4a). The wide range of εHf (t) values suggests a heterogeneous source at the time of crystallization of zircon, supporting a mixed crust-mantle in their source. Zircon is extremely retentive of the magmatic O isotopic compositions (Kemp et al., 2007). Igneous zircon grains in equilibrium with mantle magmas have an average δ18O value of 5.3 ± 0.3 ‰ which is insensitive to magmatic differentiation (Valley et al., 1998, 2005). As for the Chiang Khong volcanic rocks, ten zircon grains used to constrain the crystallization age of the andesite (TL-11A1) yield markedly elevated δ18O values (both for zircon and for whole-rock calculated) relative to the normal mantle value, although one analysis records a lower value which might be ascribed to low-temperature alteration (Fig. 4b). These results are consistent with a scenario involving significant input of 18O-enriched supracrustal rocks (e.g., altered upper oceanic crust or clastic sediments) into the magma source of the Chiang Khong volcanic rocks. Studies of erupted lavas and mantle xenoliths from convergent-margin settings have shown evidences for metasomatism of the sub-arc mantle by 18O-enriched materials (Eiler, 2001). The lithospheric mantle beneath the Indochina Block has incorporated a substantial amount of altered oceanic crust/sediments recycled into the sub-arc during the long-lasting subduction of the Paleotethys Ocean (Zi et al., 2012a). All of these facts suggest that these volcanic rocks might have originated from an enriched lithospheric mantle modified by slab-derived fluid and recycled sediments. 5.2. Tectonic implications The Chiang Khong–Lampang–Tak igneous zone is located between the Chiang Mai and Nan–Uttaradit suture zones (Barr and Charusiri, 2011; Barr et al., 2006; Qian et al., 2013). The Chiang Mai suture between the Sibumasu and Indochina blocks has been accepted as the remnant of the Paleotethyan Main Ocean, and it links with the Changning–Menglian suture (SW China) to the north (Feng et al., 2005; Liu et al., 1991; Sone and Metcalfe, 2008; Sone et al., 2012; Wu et al., 1995; Zhang, 2000; Zhong and Zhao, 2000). The Nan–Uttaradit

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Fig. 10. (a) MgO vs Cr, (b) La vs La/Sm, (c) La vs La/Yb, (d) Yb vs La/Yb, (e) Cr vs Ni and (f) Cr vs V diagrams (Wang et al., 2010, 2015) for the Late Triassic volcanic rocks in the Chiang Khong area, NW Thailand.

Fig. 11. Plots of (a) Th/Zr vs Nb/Zr (after Kepezhinskas et al., 1997) and (b) Ba/La vs Th/Yb and (c) Nb/La vs εNd (t) (after Wang et al., 2010) for the Late Triassic volcanic rocks in the Chiang Khong area, NW Thailand.


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suture zone at the eastern margin of Sukhothai Terrane is interpreted as recording a back-arc basin of the Paleotethys (Metcalfe, 2011, 2013; Sone and Metcalfe, 2008; Sone et al., 2012; Ueno and Hisada, 2001). The formation time of the volcanic rocks along the Chiang Khong– Lampang–Tak igneous zone has been controversial. As a result, their tectonic significance has also been debated, with distinct viewpoints having been proposed (e.g., Barr and Charusiri, 2011; Barr et al., 2000, 2006; Jungyusuk and Khositanont, 1992; Panjasawatwong, 2003; Qian et al., 2013; Srichan et al., 2009). The volcanic rocks in the Chiang Khong–Lampang–Tak igneous zone were previously mapped as Permian to Early Triassic sequences (von Braun and Hahn, 1976). They have recently been assigned a Middle Triassic (ca. 233–242 Ma) age (Fig. 12; Barr et al., 2000, 2006; Qian et al., 2013). Our andesite sample (TL-11A1) from the Chiang Khong area yields a zircon U–Pb age of 229 ± 4 Ma. With the assistance of Th/ U ratios and CL images (Wu and Zheng, 2004), this age is interpreted as the formation age of the volcanic rocks. Srichan et al. (2009) have also reported similar but slightly younger ages of 220–223 Ma for the Chiang Khong volcanic rocks. These data confirm the presence of the Late Triassic volcanic rocks along the Chiang Khong– Lampang–Tak igneous zone. Such ages are similar to those of the Xiaodingxi and Manghuihe volcanic sequences and Lincang granite in the Lancangjiang igneous zone in SW China and Kyaing Tong granite in eastern Myanmar (Figs. 12 and 13; Dong et al., 2013; Gardiner et al., in press; Hennig et al., 2009; Kong et al., 2012; Peng et al., 2006, 2013; Wang et al., 2010; Wang et al., 2012). The remaining inherited grains give apparent ages ranging from 410 Ma to 1918 Ma, which indicate the existence of a Precambrian basement in the study area. Our geochemical data for these volcanic rocks in the Chiang Khong area constrain the geodynamic setting of the Late Triassic magmatism along the Chiang Khong–Lampang–Tak igneous zone. As mentioned above, the Chiang Khong volcanic rocks might be the product of low-MgO and high-Al2O3 primitive melts derived from the enriched lithospheric mantle. Considering their geochemical characteristics, we propose that these Late Triassic volcanic rocks formed in a post-collisional tectonic setting, in accordance with the study by Srichan et al. (2009). In combination with the geochronology data, we suggest that these volcanic rocks in the Chiang Khong area are equivalent to the post-collisional volcanic rocks of the Xiaodingxi and Manghuihe formations in the Lancangjiang igneous zone (Wang et al., 2010). Our geochronological data show that the volcanic rocks in the Chiang Khong area erupted at 229 Ma and are younger than the continental arc-related and syn-collisional magmatic events (ca. 238–242 Ma) along the Chiang Khong–Lampang– Tak igneous zone (Barr and Charusiri, 2011; Barr et al., 2000, 2006; Qian et al., 2013). Moreover, the youngest chert and limestone in the mélanges along the Chiang Mai and Changning–Menglian suture zones to the west of the Chiang Khong–Lampang–Tak igneous zone, and southern extension as the Bentong–Raub suture zone in Malaysia are Middle Triassic in age (Metcalfe, 2002; Wakita and Metcalfe, 2005; Ito et al., 2016). In addition, the studied volcanic sequences are in fault contact with the Late Triassic–Jurassic red beds and Cenozoic sediments (Jungyusuk and Khositanont, 1992; Srichan et al., 2009; von Braun and Hahn, 1976). This stratigraphic relationship constrains the Late Triassic as the time of collisional orogen. This time gap is consistent with a post-collisional magmatism (e.g. volcanic eruptions), which is usually 15–20 Ma after the syn-collisional event (Coulon et al., 2002; Cvetković et al., 2004; Liegeois, 1998; Wang et al., 2010). Srichan et al. (2009)

Fig. 12. Simplified geological map showing the crystallization ages of the Triassic igneous rocks along the Lancangjiang, Chiang Khong–Lampang–Tak igneous zones and Kyaing Tong granite.

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Fig. 13. Summary of the reported crystallization ages of the Triassic igneous rocks along the Lancangjiang, Chiang Khong–Lampang–Tak igneous zones and Kyaing Tong granite. Data source are from (1) Peng et al. (2008), (2) Hennig et al. (2009), (3) Wang et al. (2012), (4) Peng et al. (2013), (5) Dong et al. (2013), (6) Peng et al. (2006), (7) Kong et al. (2012), (8) Wang et al. (2010), (9) Qian et al. (2013), (10) Barr et al. (2000), (11) Barr et al. (2006), (12) this study, (13) Srichan et al. (2009) and (14) Gardiner et al. (2015).

suggests that the genesis of these Late Triassic volcanic rocks might be related to gravitational collapse of the thickened crust on the basis of geochemical similarity between rift-related Whitsunday volcanic province and the Chiang Khong suites. However, the gravitational collapse is a slow passive process and usually occurs in the continental lower crust and does not cause the extensive magma activity (Zhang et al., 2006). Therefore, this model is not considered appropriate for explaining the formation of the Chiang Khong volcanic rocks. It is well known that continuous subduction can result in the HP-UHP metamorphism of the subducted slab to form eclogite, and slab detachment can then be developed due to the density increase of the eclogitic slab (e.g., Bird, 1979; Cooke and O'Brien, 2001). We propose that the uprising of the asthenospheric mantle in response to the local slab detachment (Davis and von Blanckenburg, 1995; Orozco-Esquivel et al., 2007; Wang et al., 2010; Whalen et al., 2006) is a potential mechanism governing the genesis of these volcanic rocks. The upwelling of asthenospheric mantle not only supplies heat, but also causes the thermo-mechanical erosion of subduction-modified lithospheric mantle. The Chiang Khong–Lampang–Tak igneous zone is commonly proposed to link with the Lancangjiang igneous zone to the north (Barr et al., 2000, 2006; Feng et al., 2005; Gardiner et al., in press; Peng et al., 2008, 2013; Qian et al., 2013; Srichan et al., 2009; Wang et al., 2010; Yang et al., 1994). Barr et al. (2000, 2006), Phajuy (2001) and Panjasawatwong (2003) suggested that these volcanic rocks formed in a mature continental volcanic arc. Qian et al. (2013) also suggested that the rhyolite samples might be the product of the tectonic transition from arc to syn-collisional stages. Therefore, the collision of the Sibumasu with Indochina blocks along the Chiang Khong–Lampang–Tak volcanic zone can be constrained to the Middle Triassic period and the syn-collisional volcanic rocks might be equivalent to the Manhuai volcanic sequence and its equivalents in the Lancangjiang igneous zone (e.g., Mo et al., 1998; Peng et al., 2006, 2008, 2013; Zhong, 1998). A recent study on mafic blueschists exposed in the Lancangjiang igneous zone led Fan et al. (2015) to conclude that the continent–continent collision

occured in the Middle–Late Triassic. In addition, Gardiner et al. (in press) suggested that the final closure of Palaeotethyan main ocean occurred at ca. 230 Ma in northern Thailand and eastern Myanmar. Our new geochemical and geochronological data presented in this paper allow us to propose that the Chiang Khong–Lampang– Tak igneous zone can link with the Lancangjiang igneous zone to the north and that the volcanic rocks in the zone formed in two magmatic phases involving the Middle Triassic subduction to syn-collision and subsequent Late Triassic post-collision (Barr et al., 2006; Fan et al., 2015; Metcalfe, 2011, 2013; Peng et al., 2008; Sone and Metcalfe, 2008; Wang et al., 2010). Incorporating all the data above allow us to propose the following tectonic scenario for the temporal evolution of the Chiang Khong–Lampang–Tak igneous zone as illustrated in Fig. 14. During the Middle Triassic, the continental subduction and collision between the Sibumasu and Indochina blocks occurred and formed a series of continental arc-like volcanic rocks (Fig. 14a). At the early Late Triassic, the Chiang Khong–Lampang–Tak igneous zone underwent post-collision extensional setting and formed a series of post-collisional volcanic rocks in response to the slab detachment (Fig. 14b). Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.lithos.2016.01.024.

Acknowledgements We are grateful to Prof. Michael Crow and an anonymous reviewer for their critical and constructive reviews on this paper, and Prof. Sun-Lin Chung for his comments and editorial advice. This work was jointly supported by the National Natural Science Foundation of China (41190073 and 41172202), the China Geological Survey (1212011121256), the National Basic Research Program of China (2014CB440901), the Fundamental Research Funds for the Central Universities to SYSU and the State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences in Wuhan (MSFGPMR201402).


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Fig. 14. Schematic tectonic cartoons showing the Paleotethyan temporal evolution along the Ching Khong–Lampang–Tak igneous zone: (a) Middle Triassic (242–238 Ma) eastward subduction and subsequent collision of the Sibumasu with Indochina blocks, (b) Late Triassic (229–220 Ma) post-collisional collapse in response to slab detachment following the initial uprising of asthenospheric mantle.

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