Journal of Asian Earth Sciences 74 (2013) 50–61

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Journal of Asian Earth Sciences

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U–Pb ages of detrital zircons within the Inthanon Zoneof the Paleo-Tethyan subduction zone, northern Thailand: Newconstraints on accretionary age and arc activity

1367-9120/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.jseaes.2013.06.006

⇑ Corresponding author. Tel.: +81 298 613981; fax: +81 298 613653.E-mail address: hara-hide@aist.go.jp (H. Hara).

Hidetoshi Hara a,⇑, Yoshiaki Kon a, Tadashi Usuki b, Ching-Ying Lan b, Yoshihito Kamata c,Ken-ichiro Hisada c, Katsumi Ueno d, Thasinee Charoentitirat e, Punya Charusiri e

a Geological Survey of Japan, AIST, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8567, Japanb Institute of Earth Sciences, Academia Sinica, Taipei 115, Taiwanc Graduate School of Life and Environmental Sciences, University of Tsukuba, Ibaraki 305-8572, Japand Department of Earth System Science, Fukuoka University, Fukuoka 814-0180, Japane Department of Geology, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand

a r t i c l e i n f o

Article history:Received 12 February 2013Received in revised form 7 June 2013Accepted 13 June 2013Available online 25 June 2013

Keywords:Accretionary complexDetrital zirconU–Pb datingSandstoneMélangeChertArcPaleo-Tethys

a b s t r a c t

U–Pb dating of detrital zircons was performed on mélange-hosted lithic and basaltic sandstones from theInthanon Zone in northern Thailand to determine the timing of accretion and arc activity associated withPaleo-Tethys subduction. The detrital zircons have peak ages at 3400–3200, 2600–2400, 1000–700, 600–400, and 300–250 Ma, similar to the peaks ages of detrital zircons associated with other circum-Paleo-Tethys subduction zones. We identified two types of sandstone in the study area based on the youngestdetrital zircon ages: Type 1 sandstones have Late Carboniferous youngest zircon U–Pb ages of 308 ± 14and 300 ± 16 Ma, older than associated radiolarian chert blocks within the same outcrop. In contrast,Type 2 sandstones have youngest zircon U–Pb ages of 238 ± 10 and 236 ± 15 Ma, suggesting a MiddleTriassic maximum depositional age. The youngest detrital zircons in Type 1 sandstones were derivedfrom a Late Carboniferous–Early Permian ‘missing’ arc, suggesting that the Sukhothai Arc was activeduring sedimentation. The data presented within this study provide information on the developmentof the Sukhothai Arc, and further suggest that subduction of the Paleo-Tethyan oceanic plate beneaththe Indochina Block had already commenced by the Late Carboniferous. Significant Middle Triassic arcmagmatism, following the Late Carboniferous–Early Permian arc activity, is inferred from the presenceof conspicuous detrital zircon U–Pb age peaks in Type 2 sandstones and the igneous rock record of theSukhothai Arc. In contrast, only minimal arc activity occurred during the Middle Permian–earliest Trias-sic. Type 1 sandstones were deposited between the Late Permian and the earliest Triassic, after thedeposition of associated Middle–Late Permian cherts that occur in the same mélanges and during a hiatusin Sukhothai Arc magmatism. In contrast, Type 2 sandstones were deposited during the Middle Triassic,coincident with the timing of maximum magmatism in the Sukhothai Arc, as evidenced by the presenceof abundant Middle Triassic detrital zircons. These two types of sandstone were probably derived fromdiscrete accretionary units in an original accretionary prism that was located along the western marginof the Sukhothai Arc.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

The Paleo-Tethys Ocean covered a large equatorial regionduring the Devonian and Triassic, and is characterized by thedeposition of pelagic carbonates and cherts. The Eastern Tethyanbasin in Southeast Asia closed during Triassic collision betweenthe Sibumasu and Indochina blocks (e.g., Metcalfe, 1999, 2011,2013). A proposed tectonic scheme for northern Thailand suggests

that the Inthanon Zone represents a convergence zone where Pa-leo-Tethyan rocks were subducted beneath the western marginof the Indochina Block (Ueno, 1999, 2003; Ueno and Hisada,2001; Sone and Metcalfe, 2008; Kamata et al., 2009; Metcalfe,2011, 2013). This scheme indicates that the geotectonic subdivi-sion in northern Thailand is subdivided from east to west intothe Indochina Block, an interpreted Permian–Triassic continentalisland arc system (the Sukhothai Arc), the Inthanon Zone, andthe Sibumasu Block (Fig. 1). The Inthanon Zone represents a con-vergence zone containing an accretionary complex, and consists

Fig. 1. Tectonic map of Thailand and surrounding areas (modified from Ueno,1999).

H. Hara et al. / Journal of Asian Earth Sciences 74 (2013) 50–61 51

of tectonic outliers of Paleo-Tethyan rocks that were thrust west-ward over the eastern margin of the Sibumasu Block.

The accretionary complex within the Inthanon Zone is charac-terized by a typical ocean plate stratigraphy, including basalts, pe-lagic radiolarian cherts and limestones, mudstones, and turbidites(Wakita and Metcalfe, 2005). We recently investigated clasticrocks within this convergence zone in order to understand theconvergence and accretionary tectonism that occurred duringPaleo-Tethys subduction in northern Thailand. Hara et al. (2009)indicated that mélanges within the accretionary complex in theInthanon Zone developed during this subduction event. Inaddition, Hara et al. (2012) used petrographic and geochemicalevidence to indicate that clastic rocks within these mélanges havedetrital sources from continental island arc and continental marginsettings, with clastics derived from felsic volcanic rocks within theSukhothai Arc, and quartz-rich fragments derived from within theIndochina Block, respectively. The depositional age of the Paleo-Tethyan oceanic rocks and the timing of granitoid emplacementduring Paleo-Tethyan subduction indicates that accretion relatingto this subduction occurred between the latest Carboniferous andthe Middle Triassic (Sone and Metcalfe, 2008; Metcalfe, 2011,2013), although the precise timing of this event has not previouslybeen determined, primarily because of poor geochronological con-straints from fossils within clastic rocks in these areas.

Detrital zircon geochronology has developed over the past twodecades, coincident with a significant increase in the use of ionprobes or laser ablation techniques for analysis. In particular,U–Pb dating of detrital zircons within sandstones has been usedto estimate maximum depositional ages by identifying the youngestage or peak age within a sandstone, thereby enabling the recon-struction of the tectonic evolution of sediment provenance (Fedoet al., 2003; Weislogel et al., 2006; Shibata et al., 2008; Dickinsonand Gehrels, 2009; Chen et al., 2012; Cho et al., 2013). Here, wepresent new U–Pb dating of detrital zircons from four mélange-hosted sandstones and a basaltic sandstone from the InthanonZone, and use these data to determine the timing of accretion of

Paleo-Tethyan rocks, to identify magmatic activity associated withthe Sukhothai Arc, and to estimate spatial and temporal variationsin accretionary units within the Inthanon Zone.

2. Geological outline of northern Thailand

Northern Thailand is here divided into the following four geo-tectonic units (from west to east): the Sibumasu Block, the Inth-anon Zone, the Sukhothai Arc, and the Indochina Block (Fig. 1).

The Sibumasu Block, which constitutes an eastern part of theCimmerian continent (Sengör, 1979), is characterized by theperi-Gondwanan stratigraphy, with Lower Permian glaciogenicdiamictites and cool/cold-water faunas, and Middle–UpperPermian platform carbonates with temperate–subtropical Cimmerianfaunas (Metcalfe, 1988, 2006; Shi and Archbold, 1998; Ueno, 2003).These rocks are distributed in western to southern Thailand,eastern Myanmar, western Peninsular Malaysia, and Sumatra(Metcalfe, 2013; Ridd and Watkinson, 2013). The oldest datedsedimentary rocks within the Sibumasu Block are MiddleCambrian–Early Ordovician sediments within Peninsular Malaysiaand southern and western Thailand (Metcalfe, 2013). Recentdetrital zircon studies in Peninsular Malaysia indicate that thebasement of the Sibumasu Block has a peak age of 2000–1900 Ma,with subordinate peaks at 3000–2800 and 1600 Ma (Sevastjanovaet al., 2011; Hall and Sevastjanova, 2012).

The Inthanon Zone, originally proposed by Barr and Macdonald(1991), is characterized by scattered Paleo-Tethyan oceanic rocks,pre-Devonian basement rocks belonging to the Sibumasu Block,and Late Triassic and Early Jurassic S-type granitoids and gneissicrocks. The Paleo-Tethyan rocks consist of pelagic Carboniferous–Permian seamount-type carbonates (the Doi Chiang Dao Lime-stone) associated with basaltic rocks, Middle Devonian–Middle Tri-assic radiolarian chert (the Fang Chert), and mélange-type rocksproduced by the Paleo-Tethys subduction (Caridroit et al., 1992;Ueno, 1999; Ueno and Hisada, 2001; Wakita and Metcalfe, 2005;Wonganan et al., 2007; Hara et al., 2009; Kamata et al., 2009;Hara et al., 2010; Ueno et al., 2010). Caridroit et al. (1992) andWonganan et al. (2007) previously described mélange-type rocksas olistostromal sediments. The Sibumasu basement in theInthanon Zone comprises metamorphic rocks, Cambrian sandstone,Ordovician limestone, and Carboniferous quartzose sediments(Barber et al., 2011; Ueno and Charoentitirat, 2011). The Cambriansandstone, Ordovician limestone, and Carboniferous quartzose sedi-ments within the Sibumasu Block are imbricated with Paleo-Tethyanrocks in the Inthanon Zone. The Inthanon Zone is interpreted torepresent nappes of Paleo-Tethyan rocks thrust westward over amarginal part of the Sibumasu Block (Caridroit et al., 1992; Uenoand Hisada, 1999, 2001).

The Sukhothai Arc, which largely corresponds to the SukhothaiZone of Barr and Macdonald (1991) and the Sukhothai fold belt ofBunopas (1981), is dominated by deformed Paleozoic–Mesozoicsedimentary rocks, volcanic rocks, and Early Permian to TriassicI-type granitoids. The Sukhothai Arc is considered to represent acontinental island arc, produced by subduction of the Paleo-Tethys(Ueno and Hisada, 1999, 2001; Sone and Metcalfe, 2008; Metcalfe,2013). The Nan-Uttaradit Suture Zone, dividing the Sukhothai Arcand the Indochina Block, is interpreted as the remnant of a back-arc basin (Ueno and Hisada, 1999, 2001; Sone and Metcalfe, 2008).

The Indochina Block is part of the South China–IndochinaSuperterrane of Cathaysian domain (Metcalfe, 2002, 2006, 2011,2013) and has remained within the paleo-equatorial region sinceits Early Devonian breakaway from Gondwana. In eastern Thailand,Upper Paleozoic shallow-marine carbonate rocks, containinghighly diverse Tethyan faunas, are widely distributed over themargin of the Indochina Block (Wongwanich and Boucot, 2011;Ueno and Charoentitirat, 2011).

52 H. Hara et al. / Journal of Asian Earth Sciences 74 (2013) 50–61

3. Sandstone occurrence and description

A number of lithic sandstones within mélange-type rocks in theInthanon Zone were selected for detrital zircon U–Pb analyses,based on the petrographic and geochemical data presented by Haraet al. (2012). In addition, radiolarian fossils from cherts in themélanges within the study area were identified in order to deter-mine the upper age-limit of mélange formation. Four lithic sand-stone samples and one basaltic sandstone sample from theInthanon Zone were analyzed in this study (Fig. 2). The lithic sand-stones are dominated by quartz and volcanic rock fragments, andminor feldspar in argillaceous matrices (Hara et al., 2012). Theyplot within ‘recycled orogen’ fields on Qt–F–L and Qm–F–Lt ternarydiagrams (Dickinson et al., 1983), with a continental island arc geo-chemical provenance (Hara et al., 2012). The basaltic sandstoneanalyzed in this study was described by Kamata et al. (2012b). Alist of samples is given in Table 1.

3.1. Sample BPK14

Sample BPK14 was collected from an outcrop described by Haraet al. 2009, their Fig. 7) and Kamata et al. 2009, their Fig. 4). It is arepresentative section of mélange that contains chert andsandstone, with lens-shaped blocks of bedded chert and sandstoneembedded within a sheared argillaceous matrix. We performedU–Pb dating on zircons from a lithic sandstone block from thismélange outcrop. Cherts at this locality are characterized by glassyfresh surfaces, and consist of densely packed radiolarian testswithin microcrystalline quartz in a clay mineral matrix, and areinterpreted to be pelagic chert (Kamata et al., 2009). A chert

Fig. 2. Simplified geological map of the area between Chiang Dao and Mae Hong SonGeological map based on 1:1,000,000 scale geological map of Thailand (Department of M

sample examined in this study yielded moderately well-preservedradiolarians, including Albaillella excelsa, A. lauta, A. flexa, A. angus-ta, Neoalbaillela sp., Latentifistula? sp., Ishigaum sp., and Raciditor sp.(Kamata et al., 2012a). These radiolarians are indicative of the lat-est Permian Changhsingian age (Kuwahara et al., 1998; Kuwahara,1999).

3.2. Sample BMS05

Sample BMS05 is a lithic sandstone, which was collected frommélange outcrop with rectangular-shaped chert block and shale.The sandstone is slightly enriched in quartz compared with otherlithic sandstones of the Inthanon Zone (AC02 of Hara et al.,2012). Chert at this locality is black and well bedded, with individ-ual beds around 3–5 cm in thickness (Kamata et al., 2012a). Part ofthe chert block is tightly folded with chevron-type folds. Kamataet al. (2012a) reported Follicucullus scholasticus, F. cf. scholasticus,F. ventricosus, and F. dilatatus from this chert. These Follicucullusspecies are all diagnostic of the Capitanian (late Middle Permian)F. scholasticus–F. ventricosus zone (De Wever et al., 2001), indicat-ing that mélange formation occurred at least after the MiddlePermian.

3.3. Sample Th07-121203

Sample Th07-121203 was collected from a broken sandstonebed within shale in an outcrop showing block-in-matrix texture,as described by Hara et al. (2012, their Fig. 3a and b). The sand-stone occurred as disrupted, isolated, and fractured clasts in an

areas, showing location of samples used in this study. See Fig. 1 for map location.ineral Resource, 1999).

Table 1Sandstone samples used for detrital zircon U–Pb dating during this study.

Samplenumber

Location Laboratory Rock type Lithology Geochem Remark

BPK14 19�38053.7400N 98�13032.4600E NTU Lithic sandstone Mélange with Late Permian chert and sandstone block AC03a –a

BMS05 19�3200300N 98�0605200E GSJ Lithic sandstone Mélange with Middle Permian chert and sandstone block AC02aTh07-121203 19�36014.000N 99�07031.000E GSJ Lithic sandstone Broken interbedded sandstone and shale AC07 –b

Th10-120103 19�39018.8300N 98�43046.9900E NTU Lithic sandstone Sandstone adjacent to Late Permian chert AC05BMH02 19�39021.0600N 98�43047.1200E NTU Basaltic

sandstoneInterbedded basaltic sandstone and Late Permian chert –c –d

Analytical Laboratory; GSJ: Geological Survey of Japan, NTU; National Taiwan University, Geochem.: Sample number used during petrological and geochemical analysis ofHara et al. (2012).

a Outcrop photograph is shown in Fig. 7 of Hara et al. (2009) and Fig. 4 of Kamata et al. (2009).b Outcrop photograph is shown in Fig. 3 of Hara et al. (2012).c Geochemical data were provided by Kamata et al. (2012b).d Outcrop description in Kamata et al. (2012b).

a b

c d

Fig. 3. Photomicrographs of basaltic sandstones in the Inthanon Zone; (a and b) Basaltic sandstone textures; arrow points to terrigenous microcrystalline quartz (Qtz). (c andd) Microcrystalline quartz with zircon. Ba: basalt, Chl: chlorite, Zr: zircon; (a and c) were taken under plane polarized light, (b and d) under crossed polarized light.

H. Hara et al. / Journal of Asian Earth Sciences 74 (2013) 50–61 53

argillaceous matrix. This outcrop yielded no age information, asboth fossils and chert were absent.

3.4. Samples BMH02 (basaltic sandstone) and Th10-120103

A small section at BMH02 was described by Kamata et al.(2012b), and is characterized by a basaltic sandstone–chertsuccession that provides important information on the deep-seaPaleo-Tethys paleo-environment. In this outcrop, basaltic sand-stone conformably underlies and is partly intercalated with UpperPermian radiolaria-bearing chert. The basaltic sandstone is domi-nated by sand-sized glasses and basaltic rock fragments with achlorite matrix (Fig. 3a and b), and rarely contains zircon-bearingmicrocrystalline quartz fragments (Fig. 3c and d). We separatedzircons, although a few, from this basaltic sandstone (BMH02) forU–Pb dating to constrain the maximum depositional age of thisunit. Kamata et al. (2012b) stated that the geochemistry of basalticfragments within the sandstone have an oceanic island basalt affin-ity. Chert in this outcrop is gray to bluish gray and well bedded. It isabsent from sand grain size fraction, indicating a pelagic in origin.The chert yielded Late Permian radiolarians, including Follicucullus

scholasticus, F. dilatatus, F. charveti, Albaillella protolevis, A. cf. flexa,A. sp., and Neoalbaillella sp., based on which Kamata et al. (2012b)correlated the section with the Wuchiapingian to the earlyChanghsingian. A lithic sandstone is also exposed about 10 m tothe west of the chert and basaltic sandstone outcrop. Detrital zir-cons were also separated from this sandstone (sample Th10-120103) for U–Pb dating in this study.

4. Analytical techniques

Detrital zircons were separated using isodynamic magnetic andheavy liquid separation techniques. Zircons are usually 50–100 lmin size, and are generally transparent and light violet to light yel-low in color, with euhedral, prismatic, and subrounded shapes.Cathodoluminescence (CL) imaging was performed to observe theinternal structure and zonation pattern of zircons and to selectsuitable sites for U–Pb dating. Representative CL images of zirconsanalyzed in this study are shown in Fig. 4. Most zircons containclear oscillatory zoning and faint zoning and high Th/U ratios(mostly >0.2, Supplementary Table A), indicating magmatic growth(Corfu et al., 2003; Hoskin and Schaltegger, 2003).

Fig. 4. Cathodoluminescence images of detrital zircons from samples BMS05 and Th07-121203 analyzed in this study. Circles show locations of analyses spots, whilenumbers indicate zircon U–Pb ages collected from these spots.

54 H. Hara et al. / Journal of Asian Earth Sciences 74 (2013) 50–61

Zircon U–Pb isotope analysis was undertaken using laserablation–inductively coupled plasma–mass spectrometry (LA–ICPMS),with zircons from sandstone samples BMS05 and Th07-121203analyzed using an Agilent 7500cx instrument with a Ti:S femtosec-ond laser ablation system at the Geological Survey of Japan (GSJ,Tsukuba, Japan) following the analytical method of Kon and Takagi(2012), and zircons from sandstone samples BPK 14, Th10-120103,and BMH02 analyzed using an Agilent 7500s instrument with aNew Wave UP213 laser ablation system at the National TaiwanUniversity (NTU, Taipei, Taiwan) following the analytical methodof Chiu et al. (2009). Operating conditions and data acquisitionparameters at both laboratories are summarized in Table 2. Cali-bration and data quality control were undertaken using standardzircons of 91500 (Wiedenbeck et al., 1995), NIST SRM 610 (Walderet al., 1993) and Plešovice (Sláma et al., 2008) at GSJ, and GJ-1(Jackson et al., 2004) and 91500 and MT (Australian Mud Carbon-atite zircon) at NTU. Common Pb correction was performed usingmethod by Anderson (2002). 206Pb/238U ages are given for zirconsyounger than 1000 Ma, and 207Pb/206Pb ages for those formed at1000 Ma or older, with a maximum of 20% discordancy between206Pb/238U and 207Pb/206Pb ages.

5. Results

The results of U–Pb dating in this study are shown on concordiadiagram (Fig. 5) and relative age probability diagram (Fig. 6), andare summarized in Supplementary Table A in the supplementarydata that accompanies this paper. Analytical errors present at 2r.The majority of U–Pb ages for zircons from both lithic and basalticsandstones are concordant.

Detrital zircons from sample BPK14 have U–Pb ages rangingfrom 308 ± 14 Ma to 3283 ± 40 Ma (n = 156), with age peaks at312, 550, 852, 966, and 2468 Ma. Those from sample BMS05yielded U–Pb ages between 300 ± 16 Ma and 3160 ± 94 Ma(n = 87), with the majority between 300 and 1000 Ma, and withpopulations at 300, 514, and 782 Ma. Samples BPK14 and BMS05are characterized by a small amount of youngest Late Carbonifer-ous zircons and abundant older zircons. Sample Th07-121203exhibits detrital zircon U–Pb ages between 207 ± 9 Ma and

3369 ± 57 Ma (n = 85), with the majority between 250 and1000 Ma, and with peak ages of populations at 240 and 417 Ma,and subordinate peaks at 280, 690 and 820–890 Ma. The youngest207 ± 9 Ma U–Pb age is discordant. The second youngest U–Pb ageof 236 ± 15 Ma is interpreted as youngest age in this sample. Detri-tal zircon U–Pb ages of sample Th10-120103 vary from238 ± 10 Ma to 2951 ± 32 Ma (n = 79), with a significant peak ofpopulation at 243 Ma. The BMH02 basaltic sandstone yielded var-ious U–Pb ages between 217 ± 8 Ma and 2535 ± 46 Ma (n = 20),with the majority of zircons clustering around 255 Ma(255 ± 10 Ma, 256 ± 10 Ma). The youngest ages of detrital zirconsfrom lithic sandstones are at 308 ± 14 Ma for BPK14, 300 ± 16 Mafor BMS05, 236 ± 15 Ma for Th07-121203, and 238 ± 10 Ma forTh10-120103.

6. Interpretation of detrital zircon U–Pb and accretionary ages

It is essential to understand ocean plate stratigraphy (OPS) inorder to decipher the history of an oceanic plate. These plates re-cord a stratigraphic succession from initial ocean-floor spreadingto subduction at an oceanic trench (Matsuda and Isozaki, 1991;Wakita and Metcalfe, 2005). In particular, the presence of an OPSin an area allows the depositional age of the uppermost trench-fillclastic rocks within the stratigraphy to be interpreted as nearlyidentical to the age of accretion (Lash, 1985). This means that thesedepositional ages have been used to determine the timing andduration of accretionary tectonics (Matsuda and Isozaki, 1991).Paleo-Tethyan rocks that have a preserved OPS were subductedbeneath the western margin of the Indochina Block during thePermian–Triassic (e.g., Metcalfe, 2011). In comparison, the youn-gest detrital zircon age of these sediments is usually interpretedas a maximum depositional age (e.g., Nelson, 2001; Fedo et al.,2003; Dickinson and Gehrels, 2009). Here, we discuss detrital zir-con geochronology of sandstones and the radiolarian dating ofchert blocks within a mélange as constraints on the timing ofaccretion associated with Paleo-Tethys subduction.

Lithic sandstone samples BPK14 and BMS05 were collectedfrom mélanges containing both chert and sandstone blocks, andhave maximum depositional ages, as defined by the youngest

Table 2LA–ICP–MS operating conditions and data acquisition parameters.

GSJ NTU

ICP–MSInstrument Agilent 7500cx Agilent 7500sForward power 1600 W 1500 W

Gas flow rateCool 15 L/min 15 L/minAuxiliary 1 L/min 1 L/minNebulizer 1–1.1 L/min 1.3–1.4 L/minCarrier (He) 0.8 L/min 0.65–0.7 L/min

Laser ablationInstrument Ti:S femtosecond New wave UP213Wavelength 780 nm NIR 213 nm UVPulse energy 20 lJ/cm2 8–14 J/cm2

Repetition rate 10 Hz 4 and 5 HzSpot size 15 lm 30 lm

Data acquisitionData acquisition

protocolTime-resolved analysis Time-resolved analysis

Scanning mode Peak jump Peak jump

Dwell time per isotope206Pb 50 ms 15 ms207Pb 50 ms 30 ms208Pb 30 ms 30 ms232Th 30 ms 10 ms238U 30 ms 15 msData acquisition

time40 s 130 s

(10 s gas blank and 30 sablation)

(60 s gas blank and 70 sablation)

H. Hara et al. / Journal of Asian Earth Sciences 74 (2013) 50–61 55

detrital zircon U–Pb age within each sample, of 308 ± 14 Ma (LateCarboniferous, BPK14) and 300 ± 16 Ma (latest Carboniferous,BMS05) (Fig. 6). In comparison, chert blocks within the samemélange have yielded latest Permian (Changhsingian, sampleBPK14) and late Middle Permian (Capitanian; sample BMS05) radi-olarian ages that approximate to 254–253 and 265–260 Ma (Grad-stein et al., 2012), both of which are younger than the youngestdetrital zircon U–Pb ages of these samples. The fact that these sed-iments form part of an OPS indicates that the depositional age ofclastic rocks in this area should be younger than the age of chertblocks within the same mélange. However, samples BPK14 andBMS05 have lithic sandstone depositional ages—as inferred fromthe youngest detrital zircon U–Pb age within a sample—that areolder than the radiolarian ages of the same sample, indicating thatthe former does not represent the potential maximum depositionalage of these samples. This chert block dating indicates that themixed sandstones and cherts within an argillaceous matrix thatis exposed in these outcrops formed at least after the latest Perm-ian and Middle Permian, respectively.

The Th10-120103 lithic sandstone is exposed close to the LatePermian chert and basaltic sandstone succession at site BMH02,although the poor exposure in this area conceals the stratigraphicrelationship between these two units. If the lithic sandstone, to-gether with the Late Permian chert and basaltic sandstone at siteBMH02, constitutes part of the OPS, the former should be youngerthan the Late Permian chert, as evidenced by the general OPS. Thisis consistent with detrital zircon U–Pb dating of sample Th10-120103 that yielded a youngest Middle Triassic age of238 ± 10 Ma. This youngest age could be regarded as the maximumdepositional age for this lithic sandstone. In addition, detrital zir-con U–Pb dating of the Th07-121203 lithic sandstone, a unit thatis not associated with chert blocks, also yielded a cluster of youn-gest ages around 236 ± 15 Ma that is similar to the Th10-120103peak age. We interpret that this 236 ± 15 Ma date also representsthe maximum depositional age of this sample, and therefore both

Th10-120103 and Th07-121203 lithic sandstones have Middle Tri-assic maximum ages of accretion.

7. Significance of basaltic sandstone detrital U–Pb age

Detrital zircon U–Pb ages of the BMH02 basaltic sandstone arehighly variably between 217 ± 8 and 2535 ± 46 Ma. This age pat-tern is similar to that of the lithic sandstones, in that both have zir-con populations with ages of 2500, 1600–1800, 1300, and<1000 Ma (Fig. 6). The basaltic sandstone conformably underliesand is partly intercalated with Upper Permian radiolarian-bearingcherts (Kamata et al., 2012b), and has younger detrital zircon U–Pbages of 217 ± 8, 255 ± 10, and 256 ± 10 Ma. The two detrital zirconU–Pb ages of 255 ± 10 and 256 ± 10 Ma are coincident with theLate Permian Wuchiapingian (260–254 Ma) radiolarian age of thechert, all of which probably indicates the timing of basaltic activityin this area. We also obtained a single zircon grain with a U–Pb ageof 217 ± 8 Ma, younger than the age of the chert; this age may re-late to hydrothermal alteration, although further detailed exami-nation of this zircon is needed.

Kamata et al. (2012b) noted that the geochemistry of basalticfragments within this sandstone indicates an ocean island basaltaffinity, and concluded that these basaltic fragments were origi-nally transported by a gravity current from an oceanic topographichigh, such as a seamount or an oceanic plateau. Thus, oceanicbasaltic volcanism activity is consistent with a setting associatedwith slow accumulation of radiolarian chert within the pelagicPaleo-Tethyan Ocean. However, this basaltic sandstone also con-tains rare continental-derived terrigenous quartz and zircon thatsuggests temporary terrestrial influxes during deposition. Inaddition, the data presented here indicate that Proterozoic detritalzircons within this basaltic sandstone were most likely derivedfrom an older continental mass (Fig. 6). Although we cannotexplain the exact processes involved in transporting thesepre-Late Permian detrital zircons from a continental domain intoa deep-sea environment, the presence of these zircons doessuggest that the basaltic sandstone is likely to have a similarcontinental provenance to the lithic sandstones in the study area,as evidenced by the presence of similar detrital zircon age-peakdistributions within both units. Our data suggest that the deposi-tion of the basaltic sandstone involved input from both oceanicand continental sedimentary sources.

8. Detrital zircon age-dating evidence for the timing of arcmagmatism within the Sukhothai Arc

U–Pb detrital zircon ages within the Inthanon Zone yield peaksat ca. 3400–3200, 2600–2400,1000–700, 600–400, and 300–250 Ma, similar to other sediments associated with circum-Paleo-Tethys subduction zones, and indicative of an orogenic tectonicprovenance (Pullen et al., 2008; Li et al., 2010; Robinson et al.,2012). In addition, similar patterns are present in detrital zirconU–Pb populations from younger sediments that formed during orafter collision between the Indochina and Sibumasu blocks, includ-ing the Mesozoic Khorat Group in eastern Thailand (Carter and Bri-stow, 2003), Upper Triassic continental deposits in Laos (Blanchardet al., 2013), Upper Triassic shallow marine sediments in Singapore(Oliver et al., 2013), and modern river sediments in Southeast Asia(Bodet and Schärer, 2000; Rino et al., 2008; Sevastjanova et al.,2011). Hara et al. (2012) concluded that lithic sandstones withinmélanges of the Inthanon Zone were sourced from volcanic rocksin the Sukhothai Arc and from quartz-rich continental rocks inthe Indochina Block, and that older detrital zircons (having peakU–Pb ages of 3400–3200, 2600–2400, 1000–700, and 600–400 Ma) were potentially derived from the latter.

Fig. 5. Concordia diagrams of detrital zircons from lithic and basaltic sandstones.

56 H. Hara et al. / Journal of Asian Earth Sciences 74 (2013) 50–61

Here, we classify lithic sandstones in the study area into twotypes based on youngest detrital zircon U–Pb ages: Type 1 sand-stones (samples BPK14 and BMS05) contain Late Carboniferous zir-cons, whereas Type 2 sandstones (samples Th10-120103 andTh07-121203) contain Middle Triassic zircons (Fig. 6). These zir-cons also allow us to understand temporal changes in arc activitywithin the Sukhothai Arc; these changes were caused by subduc-tion of the Paleo-Tethys beneath the Indochina Block during theperiod between the Late Paleozoic and Early Mesozoic. This datingrelies on the fact that younger detrital zircons were derived from

arc-related igneous rocks, meaning that the ages of these zirconsare proxies for the timing of arc activity and associated plate sub-duction. Combining the youngest zircon U–Pb ages with the knownmagmatism in this area enables us to further our understanding ofarc activity within the Sukhothai Arc (Fig. 7).

Zircons within Type 1 sandstones have youngest U–Pb ages of308 ± 14 and 300 ± 16 Ma, suggesting Late Carboniferous arc activ-ity. The Dan Lan Hoi Group of the Sukhothai Arc is distributed inthe southern part of the zone, and is thought to represent a marineCarboniferous succession that contains thick volcaniclastic beds

Fig. 6. Relative probability plots, and age distribution histograms with bin widths of 100 m.y. showing ages of detrital zircons from lithic and basaltic sandstones. 206Pb/238Uages are given for zircons younger than 1000 Ma, and 207Pb/206Pb ages are for those at 1000 Ma or older.

H. Hara et al. / Journal of Asian Earth Sciences 74 (2013) 50–61 57

(Bunopas, 1981), although the assigned Carboniferous age is basedsolely on stratigraphic relationships with adjacent probableSilurian–Devonian formations, and is therefore questionable(Vimuktanandana, 2008; Ueno and Charoentitirat, 2011). The old-est unequivocally dated succession that records magmatism in theSukhothai Arc is the Early Permian Kiu Lom Formation of the NgaoGroup, as established by Piyasin (1971, 1972). This formation isdominated by sandstones and shales with thin beds or lenses oflimestone and rhyolitic to andesitic volcaniclastics, and has been

broadly assigned to the Early Permian (late Asselian to Sakmarian;Ueno and Charoentitirat, 2011). A Recent investigation of fusulinewithin these sediments (Miyahigashi et al., 2012) indicates thatdeposition of the sediments continued until at least the Artinskian(late Early Permian). The volcanic source rocks for the sediments inthe study area, including the source of the Dan Lan Hoi Group, havenow been eroded away. This indicates that detrital zircons withinthe Type 1 sandstones that record the youngest U–Pb ages(308 ± 15 and 300 ± 16 Ma), combined with the volcanic activity

Fig. 7. Stratigraphic summary of mélange and subdivision of sandstone types inInthanon Zone, and supposed arc activity in Sukhothai Arc. Type 1 sandstones arecharacterized by Late Carboniferous youngest detrital zircon U–Pb ages, whereasType 2 sandstones have Middle Triassic youngest ages. Age data of I-type granitoidsintrusion and volcanic rocks within the Sukhothai Arc are based on Charusiri et al.(1993), Searle et al. (2012), Barr et al. (2000, 2006), Srichan et al. (2009), and Barrand Charusiri (2011). Ages of volcaniclastic rocks within marine sediment in theSukhothai Arc are based on Ueno and Charoentitirat (2011) and Miyahigashi et al.(2012). L: Lower, M: Middle, U: Upper. See text for details.

58 H. Hara et al. / Journal of Asian Earth Sciences 74 (2013) 50–61

recorded in the Early Permian Ngao Group, suggest the presence ofa ‘missing’ Late Carboniferous to Early Permian arc that has disap-peared from the direct geological record. In addition, a subordinatedetrital zircon U–Pb peak age around Early Permian (280 Ma) wasidentified in sample Th07-121203, a sample that has characteris-tics typical of Type 2 sandstones as described later; this youngdetrital zircon was also derived from this missing arc. Type 1 sand-stones contain a small number of Late Carboniferous detrital zir-cons that were probably supplied from the active Sukhothai Arc,as well as abundant older detrital zircons that were potentially de-rived from the Indochina Block. It is generally accepted that mag-matic activity in the Sukhothai Arc had commenced by the LateCarboniferous (Barr and Macdonald, 1991; Ueno and Hisada,1999; Sone and Metcalfe, 2008; Metcalfe, 2011, 2013; Ueno andCharoentitirat, 2011; Sone et al., 2012), and the detrital zircon datapresented here indicate that the Sukhothai Arc was active duringthe Late Carboniferous. However, the scarcity of young detrital zir-cons within Type 1 sandstones, and the lack of direct evidence ofmagmatism, would probably suggest that only minor magmatismwas occurring in the Sukhothai Arc during the period betweenthe Late Carboniferous and the Early Permian.

Subsequent intrusion of I-type granitoids associated with sub-duction of Paleo-Tethyan oceanic crust beneath the IndochinaBlock within the Sukhothai Arc occurred at 245–210 Ma (Ar–Arage; Charusiri et al., 1993). I-type granitoids were also intrudedwithin the Central and Eastern Belts of the East Malaya Block(the southern geotectonic extension of the Sukhothai Arc; Met-calfe, 2013) at 267–227 Ma, as evidenced by zircon U–Pb agescompiled by Searle et al. (2012). The majority of I-type granitoidswere intruded during the Middle Triassic (245–227 Ma), althoughsome Late Permian ages have been reported from the East Malaya

Block. Additionally, volcanic and volcaniclastic rocks of the Lamp-ang Volcanic Belt have been identified within the Sukhothai Arc(Barr et al., 2000, 2006; Panjasawatwong et al., 2003; Srichanet al., 2009); these volcanics and volcaniclastics are associated withMiddle–early Late Triassic (245–220 Ma) subduction-induced arcmagmatism, as indicated by petrographic and geochemical evi-dence and by zircon U–Pb dating. Type 2 sandstones with youngestdetrital zircon U–Pb ages of 238 ± 10 and 236 ± 15 Ma are consis-tent with the intensive Middle–early Late Triassic arc activity evi-denced by the presence of intrusive and volcanic rocks within theSukhothai Arc. This intensive arc magmatism is also supported bydetrital zircon U–Pb ages obtained from syn- or post-collisionalsediments associated with collision between the Indochina andSibumasu blocks, and from modern river sediments. Oliver et al.(2013) determined the U–Pb ages of detrital zircons from UpperTriassic Jurong Formation shallow-marine sediments in Singapore;these sediments yielded Carboniferous, Middle Triassic, and LateTriassic peak ages of 350, 245 and 217 Ma, respectively, in additionto rare Permian detrital zircons. Sevastjanova et al. (2011) also ob-tained numerous Triassic detrital zircons from modern river sedi-ments in Peninsular Malaysia, although two analyzed samplesalso yielded Permian detrital zircons. These Triassic detrital zirconswithin younger sediments were also sourced from the relict activeSukhothai Arc.

A combination of direct geological evidence of the SukhothaiArc and detrital zircon data from mélanges of the Inthanon Zonehas enabled the identification of two significant periods of LateCarboniferous–Early Permian and Middle Triassic magmatismwithin the Sukhothai Arc. The Late Carboniferous–Early Permianarc activity is still poorly understood, and the only evidence forthis event to date has been obtained from volcaniclastic sedimen-tary successions and youngest detrital zircon U–Pb ages. The datapresented here support a model whereby subduction of Paleo-Tethyan oceanic crust beneath the Indochina Block commencedduring the Late Carboniferous in Thailand and surrounding areas,and significant Middle Triassic arc magmatism is evidenced notonly by detrital zircons, but also by the emplacement of high-levelI-type granitoids and volcanism within the Sukhothai Arc (Barrand Macdonald, 1991; Sone and Metcalfe, 2008; Barr and Charu-siri, 2011).

Relatively little magmatic activity occurred between the MiddlePermian and earliest Triassic in the Sukhothai Arc, as corroboratedby a lack of Permian detrital zircons that also reflects a period ofquiescence within this arc (Oliver et al., 2013; this study). Exten-sive Permian to Triassic forearc and/or shallow-marine basin sedi-mentary successions formed within the Sukhothai Arc and thesouthern extension of this arc in central and east Thailand, formingunits that include the Ngao, Phrae, and Lampang groups that aredominated by sandstones, shales, conglomerates, and limestones(Charoenprawat et al., 1994; Ishibashi et al., 1994; Chaodumrongand Burrett, 1997; Singharajwarapan and Berry, 2000; Feng et al.,2005; Kobayashi et al., 2006; Chonglakmani, 2011; Ueno andCharoentitirat, 2011; Sone et al., 2012; Ueno et al., 2012). In addi-tion, the mainly Triassic (and supposedly Permian in part) volcanicsuccessions that form the Lampang Volcanic Belt are also wide-spread throughout the Sukhothai Arc (Barr and Charusiri, 2011).These Permian–Triassic units include precisely dated Middle–LatePermian carbonate platforms within the Pha Huat and Huai Thakformations of the Ngao Group, although Permian volcanic rocksare rare in this group (Ueno and Charoentitirat, 2011). The overallPermian–Triassic stratigraphic record essentially agrees with thedata presented here in terms of secular magmatic variations withinthe Sukhothai Arc (see also Sone et al., 2012, their Fig. 10 for strati-graphic compilation). Subduction of the Paleo-Tethyan oceaniccrust led to the formation and evolution of a continental islandarc (the Sukhothai Arc) in tandem with the intrusion of Middle

H. Hara et al. / Journal of Asian Earth Sciences 74 (2013) 50–61 59

Triassic plutonic units, Late Middle–early Late Triassic volcanism,and Triassic forearc basin development.

9. Temporal and spatial variations of accretionary units withinthe Inthanon Zone

Recent studies of the Inthanon Zone indicate that Paleo-Tethyanoceanic rocks are scattered extensively in what are interpreted asnappes (tectonic outliers) that form a peculiar tectono-sedimentarydomain within the Sibumasu Block (Ueno and Charoentitirat,2011). These oceanic rocks are incorporated within mélange sedi-ments similar to those examined in this study, although theserocks originally formed accretionary complexes along the westernmargin of the Sukhothai Arc during subduction of Paleo-Tethyanoceanic lithosphere beneath the Indochina Block. This indicatesthat Inthanon Zone mélange units exposed at the present-day sur-face should be regarded as exotic geological bodies that have beenlaterally transported considerable distances from their original siteof formation during collision between the Sibumasu and Indochinablocks. Consequently, it is difficult to determine the major proper-ties of each accretionary complex, including younging polaritiesand structural positions within an accretionary prism, directlyfrom field relationships between individual mélange units.

The Type 1 sandstones described above are characterized byyoungest detrital zircon ages that are older than the radiolarianages of cherts in the same mélange outcrops (Fig. 7). There aretwo possible explanations for the presence of Type 1 sandstonesin the mélange: (1) the youngest detrital zircon U–Pb ages (LateCarboniferous for samples BPK14 and BMS05) are close to thedepositional age of these sandstones, indicating that older sand-stone blocks that were derived from the continental side of thetrench as debris or olistostromal sediments may also be presentwithin the mélange; or (2) the sandstones are almost coeval withthe sheared argillaceous matrixes, thus constituting the topmostpart of the OPS. This latter possibility suggests that these sedi-ments were formed during a hiatus in magmatism, leading to onlyolder detrital zircons being present within trench-fill clastic sedi-ments. In this case, the Type 1 sandstones were formed in a trencharea along the Sukhothai Arc after the deposition of Middle–LatePermian cherts in the same mélanges during a period of magmaticquiescence, and probably before the voluminous Triassic magmaticevent within the Sukhothai Arc. Furthermore, the Late Permian–earliest Triassic accretionary age of the Type 1 sandstones (Fig. 7)suggests that the oceanic lithosphere associated with the forma-tion of accretionary units that yielded these sandstones was sub-ducted over a relatively short time (less than 10 Myr) afterpelagic chert accumulation, similar to the accretionary complexwithin the Permian Akiyoshi Belt of southwest Japan (Uchiyamaet al., 1986; Sano and Kanmera, 1988).

In contrast to the Type 1 sandstones, Type 2 sandstones wereaccreted after the Middle Triassic magmatic event within the Suk-hothai Arc, as evidenced by the presence of abundant detrital zir-cons of this age (Fig. 6). In addition, Type 1 sandstones werederived from the western part of the Inthanon Zone, away fromthe Sukhothai Arc, whereas Type 2 sandstones were derived fromsource areas near the Sukhothai Arc (Fig. 2). The differences inaccretionary ages of these sandstones suggest that they were de-rived from discrete accretionary units in the original accretionaryprism at the margin of the Sukhothai Arc.

10. Conclusions

U–Pb dating of detrital zircons from lithic and basaltic sand-stones within mélanges in the Inthanon Zone of northern Thailandhas allowed the reconstruction of the timing of accretion and arc

activity during subduction of Paleo-Tethyan oceanic lithospherebeneath the Indochina Block. In turn, this has enabled the clarifica-tion of variations in accretionary units within this peculiar geotec-tonic unit, and the main results of this study are summarizedbelow.

(1) Detrital zircon U–Pb ages within the Inthanon Zone havepeaks at 3400–3200, 2600–2400, 1000–700, 600–400, and300–250 Ma, similar to those from other circum-Paleo-Tethys subduction zones.

(2) Two types of lithic sandstone were identified using youngestzircon U–Pb ages: Type 1 sandstones are characterized byLate Carboniferous youngest detrital zircon U–Pb ages(308 ± 14 and 300 ± 16 Ma) that are older than the radiolar-ian ages of chert blocks within mélanges in the same out-crops, indicating that these peak ages do not correspond tothe age of accretion. In contrast, Type 2 sandstones haveMiddle Triassic youngest detrital zircon ages (238 ± 10 and236 ± 15 Ma) that probably represent the maximum deposi-tional age, and therefore indicate a Middle Triassic maxi-mum age of accretion.

(3) The youngest zircon U–Pb ages yielded by Type 1 sandstonesindicate relatively Late Carboniferous magmatism in theSukhothai Arc, and suggest that these zircons were derivedfrom an unexposed ‘missing’ Late Carboniferous–EarlyPermian arc. In addition, the abundant older zircons withinsandstones were supplied from the continental IndochinaBlock.

(4) Intensive Middle Triassic arc magmatism is inferred fromconspicuous detrital zircon U–Pb ages in Type 2 sandstonesand the igneous rock record of the Sukhothai Arc, whereasthe Middle Permian–earliest Triassic was a period of mag-matic quiescence within the arc.

(5) The Type 1 sandstones have Late Permian–earliest Triassicaccretionary ages, post-dating the deposition of Middle–LatePermian cherts in the same mélanges and coinciding with amagmatic hiatus within the Sukhothai Arc, probably beforethe significant Triassic magmatic event within this arc. Incontrast, the maximum accretionary age of the Type 2 sand-stones is coincident with this Middle Triassic magmaticevent within the Sukhothai Arc, as these sandstones containabundant detrital zircons that formed during this event. Inaddition, these two types of sandstone were collected fromdifferent parts of the Inthanon Zone, suggesting that differ-ences in accretionary age and distribution are indicative ofderivation from discrete accretionary units in the originalaccretionary prism along the margin of the Sukhothai Arc.

Acknowledgments

We would like to thank Dr. S.-L. Chung for providing the zirconsdating facilities at NTU, Dr. T. Danhara of Kyoto Fission Track Co.Ltd., Japan, for assistance during zircon separation for U–Pb dating,and Mr. A. Owada, Mr. T. Sato, and Mr. K. Fukuda of the GeologicalSurvey of Japan for their expertise in thin section preparation. Thisis a contribution to IGCP516 and IGCP589. Thanks are also due tothe Professor I. Metcalfe and Dr. I. Sevastjanova for their construc-tive and valuable comments of the manuscript.

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.jseaes.2013.06.006.

60 H. Hara et al. / Journal of Asian Earth Sciences 74 (2013) 50–61

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