Implications for Paleo-Tethys Subduct

Journal of Asian Earth Sciences 61 (2012) 2–15

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Journal of Asian Earth Sciences

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Petrography and geochemistry of clastic rocks within the Inthanon zone, northern Thailand: Implications for Paleo-Tethys subduction and convergence ⇑ Hidetoshi Hara a, , Miyuki Kunii b, Ken-ichiro Hisada b, Katsumi Ueno c, Yoshihito Kamata b, Weerapan Srichan d, Punya Charusiri e, Thasinee Charoentitirat e, Megumi Watarai f, Yoshiko Adachi g, Toshiyuki Kurihara h a Geological Survey of Japan, AIST, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8567, Japan b Graduate School of Life and Environmental Sciences, University of Tsukuba, Ibaraki 305-8572, Japan c Department of Earth System Science, Fukuoka University, Fukuoka 814-0180, Japan d Department of Geological Sciences, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand e Earthquake and Tectonic Geology Research Unit (EATGRU), Chulalongkorn University, Bangkok 10330, Thailand f Meikei High School, Tsukuba, Ibaraki 305-8502, Japan g Center for Transdisciplinary Research, Niigata University, Niigata 950-2181, Japan h Graduate School of Science and Technology, Niigata University, Niigata 950-2181, Japan article info abstract

Article history: The provenance, source rock compositions, and sediment supply system for a convergence zone of the Available online 28 June 2012 Paleo-Tethys were reconstructed based on the petrography and geochemistry of clastic rocks of the Inth- anon Zone, northern Thailand. The clastic rocks are classiﬁed into two types based on ﬁeld and micro- Keywords: scopic observations, the modal composition of sandstone, and mineral compositions: (1) lithic Geochemistry sandstone and shale within mélange in a Permo–Triassic accretionary complex; and (2) Carboniferous Sandstone quartzose sandstone and mudstone within the Sibumasu Block. Geochemical data indicate that the clastic Accretionary complex rocks of the mélange were derived from continental island arc and continental margin settings, which Mélange correspond to felsic volcanic rocks within the Sukhothai Zone and quartz-rich fragments within the Indo- Continental island arc Paleo-Tethys china Block, respectively. The results of a mixing model indicate the source rocks were approximately 35% volcanic rocks of the Sukhothai Zone and 65% craton sandstone and upper continental crust of the Indochina Block. In contrast, Carboniferous quartzose sedimentary rocks within the Sibumasu Block originated from a continental margin, without a contribution from volcanic rocks. In terms of Paleo-Tethys subduction, a continental island arc in the Sukhothai Zone evolved in tandem with Late Permian–Triassic forearc basins and volcanic activity during the Middle–early Late Triassic. The accretionary complex formed contemporaneously with the evolution of continental island arc during the Permo–Triassic, supplied with sediment from the Sukhothai Zone and the Indochina Block. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction on Paleozoic and Mesozoic biostratigraphy and paleo-biogeography (e.g., foraminifers and radiolarians), as well as correlations between The Paleo-Tethys, which opened in response to the Devonian northern Thailand and the western Yunnan area of south China. separation of the North China, South China, and Indochina blocks This scheme indicates that the Inthanon Zone represents a conver- from Gondwana, occupied a large area around the equator from gence zone between the Indochina and Sibumasu blocks, compris- the Devonian to the Triassic, where carbonates, chert were depos- ing nappes of Paleo-Tethyan rocks thrust westward over the ited in a pelagic domain (e.g., Metcalfe, 1999). These Paleo-Tethyan marginal part of the Sibumasu Block. Hara et al. (2009) reported rocks, characterized by an ocean plate stratigraphy, were subducted that mélanges within an accretionary complex evolved during sub- beneath the Indochina Block during the Permian–Triassic (Wakita duction of the Paleo-Tethys, as indicated by the occurrence of a con- and Metcalfe, 2005; Metcalfe, 2011). A tectonic scheme has recently vergence zone in the Inthanon Zone. The mélanges in the Inthanon been proposed for northern Thailand (Ueno, 1999, 2003; Ueno and Zone are characteristically chaotic rocks showing block-in-matrix Hisada, 2001; Sone and Metcalfe, 2008; Kamata et al., 2009), based structure, composed mainly of sandstone and chert blocks within an argillaceous matrix. Sandstone blocks within the mélange are angular and lenticular, and range in size from several millimeters ⇑ Corresponding author. Tel.: +81 298 61 3981; fax: +81 298 61 3653. E-mail address: [email protected] (H. Hara). to several meters, showing a wide range of compositions.

The petrography and geochemistry of clastic rocks have been granitoids and gneissic rock. The Paleo-Tethyan rocks consist of pe- utilized in determining their provenance, tectonic setting, and sed- lagic Carboniferous–Permian seamount-type carbonate rocks (the iment recycling (e.g., Dickinson et al., 1983; Bhatia and Crook, Doi Chiang Dao Limestone) associated with basaltic rocks, Middle 1986; Roser and Korsch, 1986). Based on analyses of petrography Devonian–Middle Triassic radiolarian chert, and mélange-type and geochemistry, the tectonic evolution of convergence zones rocks induced by the Paleo-Tethys subduction (Caridroit et al., has been reconstructed for a Jurassic accretionary complex in 1992; Ueno, 1999; Ueno and Hisada, 2001; Wonganan et al., Southwest Japan (Joo et al., 2007), for arc–continent collision in 2007; Hara et al., 2009; Kamata et al., 2009; Ueno et al., 2010). the Central Philippines during the Miocene (Gabo et al., 2009), Metamorphic rocks of unknown age, Cambrian sandstone, Ordovi- and for arc–continent collision in the Southern Altaids (Guo cian limestone, and Carboniferous quartzose sediments correspond et al., 2012). Geochemical variations in clastic rocks are useful in to the rocks of the Sibumasu Block (Barber et al., 2011; Ueno and terms of understanding the tectonic evolution of a convergence Charoentitirat, 2011). The Cambrian sandstone, Ordovician lime- zone; however, the provenance and source rocks of mélange in- stone, and Carboniferous quartzose sediments within the Sibu- duced by Paleo-Tethys subduction have yet to be investigated. masu Block are imbricated with Paleo-Tethyan rocks in the The aim of this paper is to clarify the provenance of clastic rocks Inthanon Zone. The Inthanon Zone is interpreted to represent nap- within a Paleo-Tethyan subduction convergent zone, as recorded in pes of Paleo-Tethyan rocks thrust westward over a marginal part of the Inthanon Zone, based on the petrography and geochemistry of the Sibumasu Block (Caridroit et al., 1992; Ueno and Hisada, 1999, clastic rocks. In addition, we discuss the system of sediment supply 2001). into the convergence zone in relation to subduction of the Paleo- The Sukhothai Zone, which largely corresponds to the Sukho- Tethys. thai Zone of Barr and Macdonald (1991) and the Sukhothai fold belt of Bunopas (1981), is dominated by deformed Paleozoic–Mesozoic sedimentary rocks, volcanic rocks, and Early Permian to Triassic I- 2. Geological outline of northern Thailand type granitoids. The Sukhothai Zone is considered to represent an continental island arc with back-arc basin, induced by subduction Northern Thailand is here divided into the following four of the Paleo-Tethys (Ueno and Hisada, 2001; Sone and Metcalfe, geotectonic units (from west to east): the Sibumasu Block, the 2008). The Nan-Uttaradit Suture Zone, dividing the Sukhothai Zone Inthanon Zone, the Sukhothai Zone, and the Indochina Block and the Indochina Block, is interpreted as the remnant of a back- (Fig. 1). arc basin (Ueno and Hisada, 2001; Sone and Metcalfe, 2008). The Sibumasu Block, which is the eastern part of the Cimmerian The Indochina Block is part of the South China–Indochina continent (Sengör, 1979), is characterized by a Gondwanan stratig- Superterrane (Metcalfe, 2002, 2006) and has remained within the raphy, Lower Permian glaciogenic diamictites with Gondwanan paleo-equatorial region since its Early Devonian breakaway from fauna and flora, and Middle–Upper Permian platform carbonates Gondwana. In eastern Thailand, Upper Paleozoic shallow-marine (Metcalfe, 1988, 2006; Ueno, 2003). These rocks occupy western carbonate rocks, containing highly diversified Tethyan faunas, are to southern Thailand, eastern Myanmar, western Peninsular widely distributed over the margin of the Indochina Block. Malaysia, and Sumatra. The Inthanon Zone, originally proposed by Barr and Macdonald 3. Petrography of clastic rocks of the Inthanon Zone (1991), is characterized by Paleo-Tethyan oceanic rocks, pre-Devo- nian basement rocks, and Late Triassic and Early Jurassic S-type Sandstones of the Inthanon Zone in northern Thailand occur between Chiang Dao in the east and Mae Hong Son in the west (Fig. 2). Previous studies have described sandstones in the Inth- 100˚E 105˚E anon Zone as part of a Carboniferous–Permian sedimentary succession (Hesse and Koch, 1979; Department of Mineral Resources, Nan—Uttaradit SouthSouth CChinahina Chiang Rai Suture Zone BlockBlock 1999), as olistostromal sediments (Caridroit et al., 1992; Wonga- Tectonic Line CHINA nan et al., 2007), and as Paleo-Tethyan mélange and clastic rocks (Hara et al., 2009). Based on field and microscopic observations, Mae Yuam Fault Song Ma Suture Zone the modal composition of sandstone, and mineral compositions, Fig.Fig. 2 clastic rocks within the Inthanon Zone are classified into two MYANMARMYANMAMYANMAR 20˚N types: (1) lithic sandstone and shale associated with mélange; and (2) Carboniferous quartzose sandstone and mudstone within LAOLAO PPDRDR SibumasuSSibumasuSiibbuummasu ChiangChiang MMaiai the Sibumasu Block. Figs. 3 and 4 show field photographs and representative photo- BlockBBlockBlloocck LoeiLoei micrographs of the main rock types of the Inthanon Zone. The min- IndochinaIndochina eral compositions of the clastic rocks were determined by X-ray diffractometer (XRD) analysis, using RINT2000 at the Geological BlockBlock 15˚N Survey of Japan, Tsukuba, Japan. The obtained XRD patterns are Sibumasu BangkokBBangkokBaannggkkookk THAILANDTHAILAND shown in Fig. 5. For modal analysis of eighteen sandstones, more Block than 250 points (generally 300–500 points) were counted per thin VIETVIET section. The results of the modal analysis are shown in Table 1 and Inthanon CAMBODIACAMBODIA NAMNAM Zone Fig. 6. Lithic sandstones and shales were collected from mélange with- Sukhothai in a Permo–Triassic accretionary complex (Hara et al., 2009). The Zone 10˚N sandstones generally occur as disrupted, isolated, and fractured Indochina Gulf of Thailand clasts in an argillaceous matrix, frequently showing block-in-ma- Block 500 km trix structure (Fig. 3A). The sandstone clasts within mélange are angular to subrounded, with both sharp and gradational margins. Most of the lithic sandstones are poorly sorted, with the grain size Fig. 1. Tectonic map of Thailand and surrounding region (after Ueno, 1999). ranging from very fine to medium sand (Fig. 3B). The sandstone 4 H. Hara et al. / Journal of Asian Earth Sciences 61 (2012) 2–15

Fig. 2. Simplified geological map of the area between Chiang Dao in the east and Mae Hong Son in the west, showing sample localities (see Fig. 1 for map location). The map is based on the Geological Map of Thailand (1:1,000,000) published by Department of Mineral Resources (1999). composition is mainly quartz, rock fragments (generally volcanics), sandstones (Table 1). The quartz grains are subrounded to well and feldspar within predominantly argillaceous matrix (Table 1; rounded, and are medium to coarse sand with tangential contacts Fig. 4A). Plagioclase is occasionally saussuritized and fractured. Lo- (Fig. 4C). Sutured contacts were frequently observed between cally, calcite occurs in pore spaces, probably of secondary origin quartz grains, with contacts marked by clay minerals with a (Fig. 4B). The shale of the argillaceous matrix is black in color, shape-preferred orientation (Fig. 4D). Mudstones composed of clay and usually with a scaly foliation (Fig. 3C). The shale contains clay minerals and silt-sized quartz grains, are light to dark gray in color, minerals with a shape-preferred orientation and black pressure- and are weakly laminated and fissile (Fig. 3E). The mudstones con- solution seams oriented parallel to the scaly foliation. Composi- tain Late Carboniferous ammonoids such as Pronorites arkansasen- tionally, the shale is dominated by silt-sized detrital grains, mainly sis, Cravenoceras (?) sp. (Fujikawa and Ishibashi, 1999). We also quartz, with the local development of asymmetric fabrics around found ammonoids and bivalves fossils in some of the sandstone quartz grains due to shear deformation. and mudstone samples collected for geochemical analysis as part XRD analyses of the sandstones and shales from the mélange of the present study (samples QS02, QS03, and QS09-QS11), but indicate similar mineral compositions, consisting mainly of quartz, species identifications have not been undertaken. XRD analyses illite, and chlorite (Fig. 5), although the chlorite peak in the shale is indicate that the sandstone is solely quartz, whereas the mudstone short and broad. The ages of the clastic rocks are poorly con- consists of quartz, illite, and chlorite (QS04, QS09 in Fig. 5). The strained because of a lack of age-diagnostic fossils, but are likely quartzose sandstones are classified as quartzose arenite and to be Permian–Triassic, based on a reconstruction of Paleo-Tethyan quartzose wacke (Table 1; classification after Okada, 1971). On rocks accreted during this period (Sone and Metcalfe, 2008; Hara Qt–F–L and Qm–F–Lt ternary diagrams, the quartzose sandstones et al., 2009). The lithic sandstones of the mélange are classified plot in the ‘craton interior’ and ‘quartzose recycled’ fields, reflect- as lithic wacke, with some lithic arenite (Table 1; classification ing their high quartz content (Fig. 6). after Okada, 1971). Based on Qt–F–L and Qm–F–Lt ternary diagrams with the tectonic fields proposed by Dickinson et al. 4. Methods of geochemical analysis (1983), the lithic sandstones plot in the ‘recycled orogen’ field in the former and range widely from the ‘lithic recycled’ to ‘quartzose Thirty samples of clastic rocks were collected from the Inthanon recycled’ fields in the latter (Fig. 6). Zone for geochemical analyses. Major elements were analyzed by The quartzose sandstones are massive and interbedded with X-ray fluorescence (XRF) on fused glass beads using a PANalytical mudstone, occurring as a coherent sequence (Fig. 3D). Disrupted Axios PW4400/40 housed at the Geological Survey of Japan, and sandstones occur locally, associated with slumping structures. Int- a Rigaku RIX3000 at Niigata University, Japan. Concentrations of erbedded sandstone and mudstone locally contain beds of quartz- trace elements were determined by analyses of fused glass beads ite conglomerate. Quartz is the dominant framework grain in the by XRF (Rigaku RIX3000) at Niigata University. Concentrations of H. Hara et al. / Journal of Asian Earth Sciences 61 (2012) 2–15 5

Fig. 3. Outcrop photographs of clastic rocks of the Inthanon Zone. (A) Lithic sandstone presenting block-in-matrix structure (AC07). (B) Close-up view of sandstone with rip- up clasts of shale. (C) Close-up view of shale with scaly foliation. (D) Carboniferous quartzose sandstone interbedded with mudstone (QS11). (E) Close-up view of mudstone with ﬁssility.

Fig. 4. Photomicrographs of clastic rocks of the Inthanon Zone. (A) Lithic sandstone from mélange (AC03a). (B) Lithic sandstone with calcite in pore space (AC01). (C) Carboniferous quartzose sandstone with tangential contact (QS01). (D) Suture contact between quartz grains (QS07). All photographs were under crossed polarized light. Lv: Volcanic lithic fragment, Lm: metamorphic lithic fragment, Ca: calcite, HM: heavy mineral. 6 H. Hara et al. / Journal of Asian Earth Sciences 61 (2012) 2–15

the equivalent values for quartzose sandstone are 1.89% and 0.37%, respectively, reﬂecting a lower proportion of matrix minerals. Shales within mélange and Carboniferous mudstone contain

mean Al2O3 contents of 17.78% and 20.58%, and mean K2O contents of 4.19% and 4.76%, respectively. Fig. 7A shows the mean values of major elements normalized to Post-Archean Australian Shale (PAAS; Taylor and McLennan, 1985; McLennan, 1989). Relative to PAAS, CaO is enriched in lithic sandstones and shales from mélange, due to the presence of secondary

calcite. K2O is enriched in all samples except quartzose sandstones, in which all elements except SiO2 are depleted. Na2O is depleted in all samples (Table 1; Fig. 7A), probably reﬂecting a lack of plagioclase (Joo et al., 2007). Compared with PAAS, lithic sandstones and shales from mélange have slightly low concentrations of trace elements (Fig. 7B). Within lithic sandstones, the concentrations of high ﬁeld strength elements (HFSEs; Pb, Y, Th, U, Zr, Nb, and Hf) are higher than those of other trace elements. In quartzose sandstones, all

the trace elements are generally depleted due to the high SiO2 contents, and HFSEs are enriched relative to other trace elements. In addition, large-ion lithophile elements (LILEs; Ba, Rb, and Sr) and compatible elements (Cr, V, Sc, and Ni) are strongly depleted relative to PAAS. Carboniferous mudstones have high concentrations of trace elements relative to quartzose sandstones, and are enriched in HFSEs relative to PAAS. The lithic sandstones and quartzose sandstones contain high concentrations of Zr and Hf, probably due to the presence of detrital heavy minerals such as zircon (McLennan et al., 1993). Fig. 7C shows REE patterns normalized to chondrite for median values obtained from each clastic rock type. The REE patterns of the lithic sandstones and shales from mélange are similar to that of PAAS, showing enrichment in light REEs, ﬂat patterns of heavy REEs, and negative Eu anomalies (Eu/EuÃ). The mean Eu/EuÃ values for sandstone and shale are 0.70 and 0.63, respectively. The REE of the quartzose sandstones are strongly depleted relative to PAAS. The quartzose sandstone also contains strongly negative Eu anomalies (Eu/EuÃ = 0.55). In contrast, the total REE contents of Carbon- iferous mudstones are high relative to those of quartzose sandstones. The quartzose sandstones, which are composed mainly of quartz (Fig. 5), are depleted in trace elements and REEs relative Fig. 5. X-ray diffractometer patterns of the clastic rocks of the Inthanon Zone. Qtz: to mudstone. Chemical sorting, due to the grain-size effect, possi- quartz, Chl: chlorite, and Sme: smectite. bly occurred between quartzose sandstone and mudstone.

Sc, Hf, U and rare earth elements (REEs) were analyzed by induc- tively coupled plasma–mass spectrometry (ICP–MS) using an Agi- 6. Provenance and tectonic setting of clastic rocks of the lent 7500a housed at Niigata University. Samples were prepared Inthanon Zone using a combined acid digestion procedure (HCl and HF) and alkali fusion by dissolution with a combination of HF–NHO3 and HF–HCl The provenance and tectonic setting of the clastic rocks were at 150 °C after adding anhydrous Na2CO3. Analytical accuracy, as investigated based on their geochemical composition. In the SiO2 estimated by deviation values using the geological reference mate- versus K2O/Na2Odiagram(Roser and Korsch, 1986), most of the lithic rial W-2 (US Geological Survey; Eggins et al., 1997) and JB-1a (Geo- sandstones from mélange plot between the ‘passive margin’ and ‘ac- logica Survey of Japan; Imai et al., 1995), was less than 8%. tive continental margin’ fields, showing a wide spread, whereas the Analytical precision estimated by relative deviation values was less Carboniferous quartzose sandstones plot in the ‘passive margin’ field than 5%. due to their high SiO2 contents (Fig. 8A). Kiminami et al. (1992) proposed a basicity diagram, with Al2O3/SiO2 plotted against the basicity index, defined as (FeO + MgO)/(SiO2 +K2O+Na2O), based on 5. Results of geochemical analyses Permian to Cretaceous sandstones from the Japanese Island Arc. On this diagram, lithic sandstones from mélange of the present study

The results of geochemical analyses for major elements are plot outside of the proposed ﬁelds, due to their low Al2O3/SiO2 values listed in Table 2, and those for trace elements and REEs are listed (Fig. 8B). The Al2O3/SiO2 value is thought to be controlled by the pro- in Table 3. SiO2 contents of sandstones vary from 70% to 85% for portion of quartz relative to feldspar (Kiminami et al., 1992, 2000); lithic sandstones from mélange, and exceed 95% for Carboniferous however, our data suggest that samples with >10% feldspar (AC02a, quartzose sandstones. SiO2 contents of Carboniferous mudstones AC05, AC06, and AC07) do not have high Al2O3 contents (Tables 1 (63–74%) are slightly higher than those of shale from mélange and 2). The low Al2O3/SiO2 values are probably due to the dominance (55–69%). Aluminous clay minerals in lithic sandstones have a of quartz in the analyzed samples. In addition, the values of the mean Al2O3 content of 9.73% and a mean K2O content of 1.55%; basicity index for the present samples, which reﬂect the petrologic Table 1 Modal analysis of sandstone collected from the Inthanon Zone.

Quartz Feldspar Lithic fragment Matrix HM Counts Qt–F–L (%) Qm–F–Lt (%) Sandstone type Qm Qp Kf Pl Lv Lp Ls Lm Qt F L Qm F Lt Lithic sandstone from mélange AC01 44.6 5.2 0.8 4.4 10.0 13.9 1.2 1.2 18.7 0.0 251 61.3 6.0 32.4 54.9 6.4 38.7 Lithic wacke AC02a 43.5 4.4 1.4 6.9 8.1 5.0 3.4 0.4 26.0 0.8 496 65.6 11.3 23.1 59.5 11.3 29.2 Lithic wacke

AC03a 34.4 2.6 1.2 4.6 14.4 7.4 6.6 1.2 26.8 0.8 500 51.1 8.0 40.9 47.5 8.0 44.5 Lithic wacke 2–15 (2012) 61 Sciences Earth Asian of Journal / al. et Hara H. AC04a 36.8 2.0 1.8 8.4 8.2 5.8 5.0 0.4 30.6 1.0 500 56.7 14.9 28.4 53.8 14.9 31.3 Lithic wacke AC05 27.8 1.4 0.6 10.5 33.1 4.3 3.9 3.4 15.0 0.0 507 34.3 13.0 52.7 32.7 13.0 54.3 Lithic arenite AC06a 17.9 2.7 0.2 2.7 41.7 6.5 3.6 12.4 11.8 0.4 475 23.5 3.4 73.1 20.4 3.4 76.3 Lithic arenite AC07 28.0 3.1 0.0 7.9 13.9 8.9 4.3 5.8 26.5 0.6 483 44.0 10.8 45.2 39.8 10.8 49.4 Lithic wacke AC08 12.2 4.9 0.0 4.5 19.1 16.7 23.3 6.9 12.5 0.0 288 19.4 5.2 75.4 13.9 5.2 81.0 Lithic arenite AC09 23.3 2.7 0.0 4.7 52.2 4.5 0.4 2.5 9.5 0.2 515 28.8 5.2 66.0 25.8 5.2 69.0 Lithic arenite AC10 35.3 4.3 0.0 1.2 20.5 6.0 0.6 8.9 23.1 0.0 484 51.6 1.6 46.8 46.0 1.6 52.4 Lithic wacke AC11 23.7 3.3 1.3 1.7 36.7 7.3 0.0 0.3 25.0 0.7 300 36.3 4.0 59.6 31.8 4.0 64.1 Lithic wacke Mean 29.87 3.3 0.66 5.2 23.4 7.9 4.8 4.0 20.5 0.4 436 43.0 7.6 49.2 38.7 7.6 53.7 Lithic wacke

Carboniferous quartzose sandstone QS01 79.0 2.3 0.0 0.6 5.5 1.0 2.6 0.0 8.4 0.6 310 89.4 0.7 9.9 86.9 0.7 12.4 Quartzose arenite QS02a 81.5 3.0 0.3 0.0 0.7 2.3 1.0 0.0 10.6 0.7 302 95.1 0.4 4.5 91.8 0.4 7.8 Quartz arenite QS03 76.7 5.7 0.0 0.0 0.3 4.3 1.7 0.7 10.3 0.3 300 92.2 0.0 7.8 85.8 0.0 14.2 Quartzose arenite QS04 63.3 3.7 0.7 0.3 4.7 3.3 0.7 0.7 22.7 0.0 300 86.6 1.3 12.1 81.9 1.3 16.8 Quartzose wacke QS05 60.3 1.7 0.3 0.3 5.3 2.0 1.7 2.0 26.0 0.3 300 94.2 0.9 14.9 81.9 0.9 17.2 Quartzose wacke QS06 76.8 4.2 0.0 0.3 6.1 1.9 0.3 0.0 9.7 0.6 310 90.3 0.4 9.4 85.6 0.4 14.0 Quartzose arenite QS07 83.5 3.2 0.0 0.3 1.9 1.3 3.2 0.0 6.1 0.3 310 92.8 0.3 6.9 89.3 0.3 10.3 Quartzose arenite Mean 74.4 3.4 0.2 0.3 3.5 2.3 1.6 0.5 13.4 0.4 305 91.5 0.6 9.4 86.2 0.6 13.2 Quartzose arenite

Qm: Monocrystalline quartz; Qp: polycrystalline quartz; Kf: potassium feldspar; Pl: plagioclase; Lv: lithic fragment of volcanic rock; Lp: lithic fragment of plutonic rock; Ls: lithic fragment of sedimentary rock; Lm: lithic fragment of metamorphic rock; HM: heavy minerals. Qt = Qm + Qp, F = K + Pl, L = Lv + Lp + Ls + Lm, Lt = L + Qp, sandstone type is based on a classiﬁcation proposed by Okada (1971). 7 8 H. Hara et al. / Journal of Asian Earth Sciences 61 (2012) 2–15

Fig. 6. Qt–F–L and Qm–F–Lt diagrams with tectonic ﬁelds of Dickinson et al. (1983). Qt: total quartz (mono- and polycrystalline grains), Qm: monocrystalline quartz, F: feldspar (plagioclase and K-feldspar), L: lithic fragment, Lt: lithic fragment and polycrystalline quartz.

Table 2 Major element compositions of clastic rocks from the Inthanon Zone (wt.%).

SiO2 TiO2 Al2O3 Fe2O3 MnO MgO CaO Na2OK2OP2O5 Total LOI Lithic sandstone from mélange AC01 77.95 0.30 4.85 4.07 0.12 1.28 9.09 0.02 0.54 0.09 98.30 8.6 AC02a 84.89 0.37 7.43 0.69 0.00 0.29 0.10 1.82 2.05 0.04 97.68 1.4 AC03a 86.51 0.34 6.40 1.65 0.12 0.50 0.11 1.51 0.73 0.04 97.91 2.0 AC04a 83.16 0.34 8.23 1.83 0.02 0.84 0.11 3.14 0.37 0.04 98.08 1.8 AC05 69.94 0.85 13.58 6.26 0.05 1.69 0.29 3.55 1.51 0.11 97.83 2.9 AC06a 66.33 0.77 15.94 8.00 0.12 2.62 2.07 0.06 2.07 0.09 98.07 8.7 AC07 78.82 0.56 9.20 4.38 0.04 1.64 0.63 0.94 1.81 0.16 98.17 3.0 AC08 71.83 0.38 5.17 3.12 0.15 1.31 14.56 0.66 0.75 0.13 98.06 12.2 AC09 70.48 0.74 15.47 5.26 0.02 2.34 0.21 0.54 3.13 0.14 98.32 6.6 AC11 71.19 0.63 11.06 4.71 0.03 1.49 0.80 1.14 2.53 0.14 98.68 5.2 Mean 76.11 0.53 9.73 4.00 0.07 1.40 2.80 1.34 1.55 0.10 98.11 5.2

Shale from mélange AC02b 68.69 1.17 17.66 2.10 0.00 1.10 0.13 1.34 5.43 0.07 97.69 11.1 AC03b 64.26 0.96 20.03 5.21 0.01 1.74 0.00 0.06 5.20 0.08 97.56 8.2 AC04b 68.59 0.71 16.86 5.57 0.03 1.81 0.03 0.06 4.48 0.08 98.23 6.7 AC06b 63.41 0.82 19.60 7.36 0.05 2.39 0.69 0.16 4.13 0.16 98.76 7.3 AC12 59.09 0.68 15.00 5.98 0.06 2.13 11.81 0.78 3.14 0.19 98.85 12.4 AC13 55.42 0.65 14.23 4.34 0.03 1.57 19.89 0.12 2.95 0.13 99.34 18.9 AC14 60.88 0.76 21.09 9.25 0.03 1.48 0.19 0.18 3.97 0.15 97.98 9.5 Mean 62.91 0.82 17.78 5.69 0.03 1.75 4.68 0.39 4.19 0.12 98.34 10.6

Carboniferous quartzose sandstone QS01 95.96 0.21 1.39 0.16 0.00 0.13 0.02 0.00 0.40 0.01 98.27 0.6 QS02a+ 99.35 0.11 1.18 0.25 0.00 0.04 0.09 0.02 0.14 0.01 101.19 0.7 QS03+ 100.72 0.09 0.97 0.45 0.00 0.03 0.02 0.02 0.06 0.02 102.37 0.6 QS04+ 94.81 0.14 3.51 0.20 0.00 0.09 0.02 0.02 0.56 0.03 99.38 1.2 QS05+ 96.17 0.22 4.05 0.19 0.00 0.15 0.01 0.02 0.76 0.01 101.58 1.3 QS06 96.50 0.13 1.26 0.12 0.00 0.12 0.00 0.00 0.35 0.01 98.49 0.6 QS07 96.61 0.08 0.85 0.23 0.00 0.13 0.00 0.00 0.30 0.01 98.20 0.5 Mean 97.16 0.14 1.89 0.23 0.00 0.10 0.02 0.01 0.37 0.01 99.93 0.8

Carboniferous mudstone QS02b 72.58 0.81 17.66 1.90 0.00 1.30 0.01 0.06 3.94 0.05 98.30 6.0 QS08 65.52 0.94 20.29 4.01 0.02 1.65 0.01 0.08 5.19 0.09 97.80 7.0 QS09 73.50 0.69 17.09 1.38 0.00 1.00 0.01 0.05 4.82 0.05 98.58 4.0 QS10 64.61 1.05 24.48 1.93 0.00 1.14 0.01 0.06 4.71 0.11 98.09 9.6 QS11 70.55 0.71 18.77 2.04 0.00 1.40 0.00 0.06 4.57 0.10 98.21 6.4 QS12 62.75 0.99 25.19 2.34 0.00 1.49 0.00 0.06 5.32 0.15 98.29 8.2 Mean 68.25 0.87 20.58 2.27 0.01 1.33 0.01 0.06 4.76 0.09 98.21 6.9

LOI: Weight loss on ignition. +: Samples analyzed at Niigata University; other samples analyzed at Geological Survey of Japan. Table 3 Trace element and REEs compositions of clastic rocks collected from the Inthanon Zone (ppm). P TiÃ CrÃ NiÃ Sc V Ã RbÃ SrÃ YÃ ZrÃ NbÃ BaÃ La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Hf Th Ã PbÃ U Eu/Eu Ã REE Lithic sandstone from mélange AC01 1757 38.7 31.9 6.7 53.4 27.4 84.9 19.7 108.9 4.8 589.8 13.0 26.4 3.2 12.4 3.2 0.9 3.8 0.6 3.3 0.6 1.6 0.2 1.4 0.2 2.8 6.2 9.9 1.3 0.78 70.7 AC02a 2498 37.5 0.0 4.8 35.0 62.3 61.3 14.0 243.7 9.4 396.5 18.2 35.3 3.9 14.3 2.6 0.5 2.0 0.3 2.1 0.4 1.3 0.2 1.4 0.2 6.0 8.6 16.3 1.0 0.67 82.8 AC03a 2198 36.3 28.2 3.7 47.9 29.4 43.1 11.7 217.3 9.1 233.6 16.0 29.6 3.5 12.6 2.3 0.4 1.9 0.3 1.7 0.4 1.2 0.2 1.3 0.2 5.6 7.3 40.3 1.8 0.57 71.7 AC04a 2318 39.7 0.4 5.2 44.0 15.0 63.8 14.6 186.8 9.4 74.8 19.4 35.1 4.1 15.2 2.8 0.6 2.3 0.4 2.2 0.5 1.3 0.2 1.4 0.2 4.8 8.5 19.7 1.0 0.69 85.6 AC05 6195 59.2 9.7 15.2 146.8 64.5 99.1 26.0 206.3 9.2 189.5 24.8 46.6 5.8 21.4 4.3 1.1 3.9 0.6 3.6 0.8 2.2 0.4 2.2 0.3 4.7 8.6 10.0 2.4 0.79 118.0 AC06a 5038 60.3 18.5 18.6 137.6 79.5 53.7 24.5 156.1 7.4 571.9 23.4 46.8 5.7 22.4 4.6 1.0 4.2 0.7 4.1 0.9 2.6 0.4 2.4 0.4 4.1 7.2 20.8 2.1 0.69 119.4 AC07 3752 69.8 30.6 9.4 76.9 84.2 66.6 24.7 280.4 11.1 284.3 34.4 66.2 7.7 28.5 5.2 0.9 4.7 0.7 4.3 0.9 2.6 0.4 2.5 0.4 7.8 12.7 11.6 2.8 0.58 159.5 AC08 2185 41.0 27.0 6.5 76.2 30.2 265.8 15.9 66.4 6.0 109.2 12.4 26.1 3.2 12.9 2.9 0.8 2.8 0.4 2.4 0.5 1.4 0.2 1.2 0.2 1.5 3.5 7.0 1.9 0.90 67.4 AC09 4909 81.9 57.0 16.9 138.7 141.0 32.7 35.7 202.6 12.8 475.2 35.1 67.5 8.2 31.6 6.1 1.3 5.7 0.9 5.3 1.2 3.4 0.5 3.3 0.5 5.9 12.8 6.3 3.6 0.69 170.7 AC11 3995 71.5 35.0 11.1 84.6 124.9 107.9 29.4 199.9 13.1 394.0 35.2 68.6 7.9 28.8 5.3 1.0 4.8 0.7 4.6 1.0 2.7 0.4 2.6 0.4 5.1 13.6 11.6 3.1 0.61 164.2 Mean 3485 53.6 23.8 9.8 84.1 65.8 87.9 21.6 186.8 9.22 331.9 23.2 44.8 5.3 20 3.9 0.8 3.6 0.6 3.4 0.7 2.0 0.3 2.0 0.3 4.8 8.9 15.4 2.1 0.70 111.0

Shale from mélange

AC02b 7578 110.8 12.3 17.4 194.8 184.8 80.8 35.6 340.7 28.3 582.8 64.2 118.6 13.9 49.4 7.7 1.4 6.2 1.1 6.8 1.6 4.7 0.8 4.8 0.7 9.6 15.1 49.5 3.0 0.60 281.6 2–15 (2012) 61 Sciences Earth Asian of Journal / al. et Hara H. AC03b 6203 110.6 34.5 19.3 166.8 216.8 12.1 41.4 190.8 23.6 533.9 68.8 152.4 15.5 57.6 11.0 2.2 9.7 1.4 8.0 1.6 4.2 0.6 3.9 0.6 5.3 21.0 14.6 2.6 0.65 337.4 AC04b 4538 52.1 78.5 14.8 108.3 182.0 20.2 30.4 143.7 18.9 493.3 47.3 186.6 11.7 45.0 8.9 1.7 8.1 1.1 5.9 1.2 3.2 0.5 2.8 0.4 4.1 11.9 8.7 1.2 0.60 324.2 AC06b 5320 80.5 28.3 20.7 136.4 156.9 48.8 34.0 176.5 13.0 290.3 33.3 67.9 8.1 31.0 6.3 1.3 5.6 0.9 5.4 1.2 3.4 0.6 3.4 0.5 5.0 11.2 18.9 3.0 0.67 169.0 AC12 3851 81.5 47.0 16.0 134.5 131.7 458.9 32.1 153.2 12.7 335.5 39.7 75.8 9.3 34.7 6.7 1.3 6.2 0.9 5.5 1.1 3.3 0.5 3.1 0.5 4.3 10.2 13.2 4.4 0.61 188.5 + 3895 n.a. n.a. 13.7 107.0 138.5 335.3 25.5 149.4 12.5 253.1 33.7 62.0 7.6 28.4 5.1 1.0 4.6 0.7 4.1 0.9 2.6 0.4 2.4 0.4 3.8 12.8 9.9 3.1 0.62 153.8 AC13 AC14 5103 64.4 38.4 22.6 144.4 165.4 39.9 39.2 163.1 11.4 612.8 36.3 65.1 8.4 31.7 6.4 1.4 6.2 1.0 5.9 1.2 3.8 0.6 3.6 0.6 4.6 10.4 15.6 2.6 0.65 172.0 Mean 5213 83.3 39.8 17.8 141.7 168.0 142.3 34.0 188.2 17.2 443.1 46.2 104.0 10.6 39.7 7.4 1.4 6.7 1.0 5.9 1.3 3.6 0.6 3.4 0.5 5.2 13.2 18.6 2.8 0.63 232.4

Carboniferous quartzose sandstone QS01 1319 26.5 0.0 1.8 10.2 24.4 6.0 8.0 230.7 5.2 0.0 12.4 23.3 2.7 9.6 1.8 0.2 1.5 0.2 1.3 0.3 0.9 0.1 0.9 0.2 5.9 11.4 7.5 0.8 0.39 55.5 QS02a 635 7.6 0.0 0.9 10.0 12.7 6.9 5.4 110.0 3.1 0.0 7.0 14.3 1.6 5.6 1.0 0.1 0.8 0.1 0.8 0.2 0.6 0.1 0.6 0.1 2.8 8.6 3.1 0.7 0.50 32.9 QS03 551 7.5 0.0 1.0 10.1 9.3 1.7 5.6 100.0 3.1 0.0 7.0 13.8 1.6 5.7 0.9 0.1 0.7 0.1 0.7 0.2 0.5 0.1 0.5 0.1 2.4 7.7 3.8 0.6 0.50 32.0 QS04 871 10.4 0.0 2.4 11.1 22.9 13.5 9.2 189.8 3.9 193.0 15.1 29.3 3.9 14.8 2.6 0.5 2.1 0.3 1.6 0.3 1.0 0.2 1.0 0.2 5.1 6.3 10.3 0.6 0.71 73.0 QS05 1411 19.2 0.0 3.7 21.3 37.5 9.3 13.5 252.8 5.1 31.8 12.0 23.2 2.8 10.5 2.3 0.4 1.9 0.3 1.9 0.4 1.2 0.2 1.4 0.2 5.9 8.3 7.6 0.8 0.63 58.7 QS06 841 15.6 0.0 1.3 6.3 14.4 3.5 7.5 132.5 2.5 35.1 6.6 15.1 1.6 5.9 1.1 0.2 1.0 0.2 0.9 0.2 0.5 0.1 0.6 0.1 3.2 7.0 5.7 0.5 0.50 34.2 QS07 450 12.9 0.0 0.8 5.2 13.0 7.0 5.3 78.6 1.8 31.7 5.3 10.7 1.2 4.4 0.8 0.2 0.7 0.1 0.7 0.1 0.4 0.1 0.4 0.1 1.8 4.9 6.1 0.5 0.60 25.2 Mean 868 14.2 0.0 1.7 10.6 19.2 6.8 7.8 156.4 3.5 41.7 9.4 18.5 2.2 8.1 1.5 0.3 1.3 0.2 1.1 0.2 0.7 0.1 0.8 0.1 3.9 7.8 6.3 0.6 0.55 44.5

Carboniferous mudstone QS02b 5260 87.5 9.8 15.1 99.0 163.9 35.1 33.9 161.6 19.1 379.1 71.8 138.1 15.1 55.1 10.7 2.1 10.3 1.5 7.8 1.4 3.5 0.5 3.1 0.5 4.3 24.4 60.7 4.0 0.60 321.3 QS08 6092 109.4 62.1 18.3 133.0 239.6 43.3 66.1 161.5 22.4 921.8 125.8 241.1 27.6 102.4 19.0 3.2 16.1 2.2 12.1 2.4 6.2 0.9 5.0 0.8 4.6 29.2 39.8 5.1 0.56 564.9 QS09 4497 81.1 18.1 14.9 95.1 224.8 31.8 48.8 167.7 17.2 594.4 90.1 154.6 18.0 64.0 11.7 1.8 10.2 1.4 8.1 1.7 4.7 0.7 4.3 0.6 4.7 21.4 25.1 2.1 0.51 372.0 QS10 6905 122.8 16.5 22.8 183.6 198.5 90.4 89.8 216.2 29.8 667.8 111.1 194.7 24.6 97.4 23.5 5.6 24.1 3.5 18.6 3.5 8.9 1.3 7.2 1.0 6.2 26.3 119.1 3.8 0.72 524.9 QS11 4616 83.9 29.6 16.5 99.6 193.2 38.4 45.8 156.6 16.6 808.8 99.4 200.6 20.2 74.9 13.5 2.5 12.1 1.6 8.8 1.8 4.6 0.6 3.7 0.5 4.5 23.4 42.1 2.3 0.59 444.9 QS12 6420 100.7 25.6 20.5 122.8 198.0 108.0 67.4 224.9 22.2 759.1 150.9 265.9 38.7 151.6 32.2 6.2 27.2 3.5 16.8 2.9 6.8 1.0 5.4 0.8 6.3 28.2 99.8 3.5 0.64 709.8 Mean 5632 97.6 27.0 18.0 122.2 203.0 57.8 58.6 181.4 21.2 688.5 108.2 199.2 24.0 90.9 18.4 3.6 16.7 2.3 12.0 2.3 5.8 0.8 4.8 0.7 5.1 25.5 64.5 3.5 0.60 489.6 ⁄ + : Twelve elements (Ti, Cr, Ni, V, Rb, Sr, Y, Zr, Nb, Ba, Th and Rb) were analyzed by XRF; other elements were analyzed by ICP–MS. AC13 : Concentration of Ti was calculated from TiO 2; other elements were determined by ICP-MS. Ã n.a.: Not analysis. Eu/Eu=Eun/[(Smn)(Gdn)1/2] (McLennan, 1989). Subscript n means normalization against chondrite. 9 10 H. Hara et al. / Journal of Asian Earth Sciences 61 (2012) 2–15

A A

B B

Fig. 8. Major element diagrams to determine tectonic setting. (A) SiO2 versus K2O/

Na2O diagram (after Roser and Korsch, 1986). (B) Basicity diagram (after Kiminami et al.,1992, 2000). See text for detail.

quartzose sandstones within the Sibumasu Block have a passive margin origin. Bhatia and Crook (1986) proposed tectonic discrimination diagrams based on immobile trace elements to gain an understanding of the tectonic setting of sedimentary basins, discriminating among the fields of ‘oceanic island arc’, ‘continental island arc’, ‘active continental margin’, and ‘passive margin’. Their diagrams are relevant to sandstone and argillaceous rock (Gabo et al., 2009; Lee, 2009; Wani and Mondal, 2011). Based on the La–Th–Sc and Th–Sc–Zr/10 ternary diagrams of Bhatia and Crook (1986), most of the lithic sandstones and shales from mélange of the present Fig. 7. Mean major element compositions (A) and trace element compositions (B) study cluster within the ‘continental island arc’ field, with a few normalized against Post-Archean Australian Shale (Taylor and McLennan, 1985; samples plotting in the ‘passive continental margin’ field (Fig. 9). McLennan, 1989). Chondrite-normalized rare earth element plot (C). The provenance of clastic rocks within the mélange is interpreted to have been a continental island arc, corresponding to the continental island arc developed in the Sukhothai Zone (i.e., the Sukho- evolution of the contributing arc, show a wide range from the ‘imma- thai Arc, Sone and Metcalfe, 2008). ture island arc’ field to the ‘continental and dissected arc’ field. The In contrast, Carboniferous quartzose sandstones plot in the ‘pas- discrimination diagrams based on major elements in sandstone indi- sive continental margin’ field, and mudstones in the ‘active conti- cate the lithic sandstones from mélange contain both continental- nental margin’ field. The quartzose sandstones and mudstones and arc-derived fragments; however, details are obscured by the plot in different parts of the ‘continental margin’ field, probably wide variations in major element composition. The Carboniferous due to chemical sorting via the grain-size effect. The Carboniferous H. Hara et al. / Journal of Asian Earth Sciences 61 (2012) 2–15 11

Fig. 10. Zr/Sc versus Th/Sc diagram to determine source rock compositions (after McLennan et al., 1993). PhG: Phanerozoic granite (data from Condie, 1993). SK-Bas: Basalt, SK-And: andesite, and SK-Fel: felsic volcanic rock of the Sukhothai Zone (the average values from Srichan et al., 2009).

are dominant in maﬁc and ultramaﬁc rock (Feng and Kerrich, 1990; McLennan et al., 1990). The present data show that trace elements in clastic rocks from the mélange have no consistent trend from incompatible to compatible elements relative to PAAS (Fig. 7B), probably indicating the mixing of sources with different compositions. The negative Eu anomaly (Eu/EuÃ = 0.60–0.90) of clastic rocks from mélange indicates that the source rocks were from a passive margin or active margin setting, rather than from a young, undif- ferentiated arc setting (McLennan et al., 1990, 1993), and also indicate a mixture of source compositions. McLennan et al. (1993) proposed the Zr/Sc–Th/Sc diagram for determining the source composition, using Th as a typical incompatible element, Sc as a compatible element in igneous differenti- ation systems, and Zr (which is enriched in zircon) as a recycling element. This diagram was used to investigate the compositions Fig. 9. Ternary diagrams to determine tectonic setting using trace elements (after of the source rock of the clastic rocks examined in this study. In Bhatia and Crook, 1986). (A) La–Th–Sc diagram. (B) Th–Sc–Zr/10 diagram. OIA: addition, the volcanic rock trend of the Sukhothai Zone was esti- Oceanic island arc, CIA: continental island arc, ACM: active continental margin, mated from the average Zr/Sc and Th/Sc ratios of basalt, andesite, PCM: passive continental margin. See text for detail. and felsic rock, based on geochemical data provided by Srichan et al. (2009). Volcanic rocks within the Sukhothai Zone vary from quartzose sedimentary rocks within the Sibumasu Block were de- basalt to felsic volcanic rocks (which are dominant), corresponding rived from a continent, without any contribution from an island to arc magmatism induced by subduction during the Middle–early arc. Late Triassic, as indicated by petrographic and geochemical data, and U–Pb dating of zircon (Barr et al., 2000, 2006; Panjasawatwong et al., 2003; Srichan et al., 2009). On a Zr/Sc–Th/Sc diagram, most of 7. Source rock compositions the lithic sandstones and shales from the mélange plot around felsic volcanic rock (Fig. 10). A comparison of geochemical data for The geochemistry of clastic rocks is also useful for understand- clastic rocks from mélange and volcanic rocks of the Sukhothai ing the composition of the source rock (McLennan et al., 1993; Zone indicates the source composition of mélange is consistent Gabo et al., 2009). Trace elements in clastic rocks can be used to with volcanic rocks within the Sukhothai Zone, characterized by constrain the nature of the source rock, because incompatible ele- mainly felsic volcanic rock. Some of the lithic sandstones from ments are enriched in felsic volcanic rock, large ion lithophile ele- the mélange plot along the sediment recycling trend in Fig. 10, sug- ments are abundant in continental crust, and compatible elements gesting a contribution by continental rocks. 12 H. Hara et al. / Journal of Asian Earth Sciences 61 (2012) 2–15

Table 4 Results of mixing calculation for lithic sandstone from mélange and chemical compositions of end members.

UCC PrCS PhG Sk-BA Sk-Fel Measurement values Mixing results Major elemetns (wt.%)

SiO2 66.79 91.15 73.8 52.66 68.66 76.11 75.89

TiO2 0.62 0.17 0.25 1.21 0.60 0.53 0.48

Al2O3 14.99 3.87 13.4 17.53 14.93 9.73 10.73 FeO 4.51 1.32 2.2 10.05 3.94 3.93 3.53 MnO – – – 0.19 0.09 0.07 – MgO 1.96 0.55 0.4 10.31 1.45 1.40 1.93 CaO 3.36 0.45 1.2 8.62 2.66 2.80 2.43

Na2O 3.35 0.51 3.5 3.72 4.23 1.34 2.47

K2O 3.1 0.88 4.8 0.86 3.26 1.55 2.07

P2O5 0.13 0.03 0.09 0.28 0.19 0.10 0.12 Trace elements and REEs (ppm) Sc 11 2.4 5 30.24 12.47 9.80 9.56 Th 10.7 4.2 18 3.46 17.20 8.90 8.67 La 27.3 10.1 40 15.65 34.94 23.20 20.17 Ce 59.3 21.6 94 33.62 70.43 44.80 42.14 Nd 26.6 9 46 19.91 34.37 20.00 19.87 Sm 5.43 1.75 8.8 4.50 6.89 3.92 4.03 Eu 1.01 0.36 0.9 1.38 1.48 0.85 0.88 Gd 5.11 1.52 7.63 4.61 6.13 3.61 3.68 Tb 0.8 0.23 1.15 0.74 0.96 0.57 0.57 Yb 2.36 0.84 3.2 2.50 3.15 1.96 1.89 Lu 0.43 0.13 0.54 0.38 0.50 0.30 0.30

UCC: Upper continental crust (0.2–0.8 Ga), PrCS: Proterozoic cratonic sandstones, PhG: Phanerozoic granite (after Condie, 1993). Sk-BA: Basalt and andesite, Sk-Fel: felsic volcanic rock of the Sukhothai Zone (the average values from Srichan et al., 2009). Mixing ratio is the following proportions; UCC:PrCS:PhG:Sk-BA:Sk-Fel = 26:40:0:8:26. The mixing ratio was achieved by the mass balance calculation using Th, Sc, and REEs. The mixing result of major elements was calculated using the estimated mixing ratio.

2009). In using the mixing model in the present study, ﬁve candi- date source rocks were selected as end-members: Paleozoic and latest Proterozoic upper continental crust (UCC, 0.2–0.8 Ga), Prote- rozoic craton sandstones (PrCS), Phanerozoic granite (PhG), basalt and andesite of the Sukhothai Zone (Sk-BA), and felsic volcanic rocks of the Sukhothai Zone (Sk-Fel). Geochemical data are based on Condie (1993) for UCC, PrCS, and PhG; and on Srichan et al. (2009) for Sk-BA and Sk-Fel. The best-ﬁt mixing model was calculated based on the mass balance calculation when considering Th, Sc, and REEs (Table 4), using the mass balance calculator proposed by Tsune (2005). The results suggest that the source rock of the mélange is PrCS (40%) and Sk-Fel (26%), accompanied by UCC (26%) and Sk-BA (8%), with no PhG (0%). Fig. 11 shows the chondrite-normalized REE patterns of lithic sandstone from the mélange, the mixing result, and end members. The mixing model indicated multiple sources for the lithic sandstone, including volcanic rocks of the Sukhothai Zone and continental materials such as

Fig. 11. Results of mixing calculation for the REEs plotted with lithic sandstones PrCS and UCC. A key input material was craton sandstone, charac- from mélange. UCC: Upper continental crust (0.2–0.8 Ga) and PrCs: proterozoic terized by high SiO2 concentrations and depleted REEs. craton sandstones (data from Condie, 1993). SK-BA: Basalt and andesite, and SK-Fel: For the Carboniferous quartzose sandstones within the Sibu- felsic volcanic rock of the Sukhothai Zone (the average values from Srichan et al., masu Block, the Zr/Sc and Th/Sc ratios are close to or higher than 2009). those for Phanerozoic granite (Fig. 10). The Carboniferous sandstones and mudstones plot in different fields, probably reflecting A component of the lithic sandstones originated from a passive the relatively high concentration of trace elements in the mud- continental margin setting (Fig. 9) and sediment recycling (Fig. 10). stone. Therefore, the source rock of Carboniferous quartzose sand- In addition, the samples of lithic sandstone from the mélange are stones and mudstones was derived from continental fragments generally dominated by monocrystalline quartz (Table 1). These such as granite and recycled sediments, without a contribution findings suggest the lithic sandstones from the mélange were de- from volcanic rocks. rived from not only a continental island arc but also a continental margin. To aid in analyzing sediment supply from multiple source 8. Sediment supply system for Paleo-Tethys subduction and rocks, mixing models were proposed to assess the relative contri- convergence butions of different source rock compositions (Fedo et al., 1996; Hassan et al., 1999; Roser et al., 2002; Joo et al., 2007; Lee, The sediment supply system for the Paleo-Tethys subduction 2009). A mass balance calculation using the least squares method and convergence was reconstructed based on the petrography is useful when considering a mixing model for multiple source and geochemistry of clastic rocks from the mélange within the rocks (Joo et al., 2007; Lee, 2009). In particular, REEs, Th, and Sc Inthanon Zone. A schematic model of the sediment supply system are known to be good indicators of the bulk composition when with evolution of the continental island arc during the Permo is using a mixing model (Fedo et al., 1996; Hassan et al., 1999; Lee, shown in Fig. 12. H. Hara et al. / Journal of Asian Earth Sciences 61 (2012) 2–15 13

Fig. 12. Schematic model of sediment supply system with evolution of continental island arc during the Early to Middle Permian. See the text for details.

The features of clastic rocks from the mélange of the present clastics were not transported from the continental margin to study suggest that provenance of the accretionary complex for Pa- the hemipelagic environment. In other words, clastic rocks of leo-Tethys subduction was dominated by continental island arc the Sibumasu Block were not transported to the subduction zone and continental margin settings. Geochemical analyses indicate along the Sukhothai Zone over the deep ocean of the Paleo- that the continental island arc corresponds to the Sukhothai Zone Tethys. Clastic rocks from the mélange were derived from both with volcanic activity during the Middle to early Late Triassic. In a continental island arc (mainly felsic magmatism) of the Sukho- addition, Singharajwarapan and Berry (2000) reported that Late thai Zone, and Proterozoic quartzite and continental crust of the Permian to Triassic turbidite sequences were deposited on forearc Indochina Block (Fig. 12). During the Early to Late Permian, basins developed in the Sukhothai Zone. These sequences, which back-arc basin was opened behind the continental island arc comprise the Ngao, Phrae, and Lampang groups, are composed of (Sone and Metcalfe, 2008). Quartzite and continental crust of sandstone, shale, conglomerate, and limestone (Charoenprawat the Indochina Block supplied into the convergence zone, probably et al., 1994; Ishibashi et al., 1994; Chaodumrong and Burrett, passed through undeveloped area of the continental island arc. 1997; Singharajwarapan and Berry, 2000; Feng et al., 2005; Kobay- The present results document the mixture of multiple supply sys- ashi et al., 2006). During subduction of the Paleo-Tethys, the con- tems to the convergence zone, although this interpretation needs tinental island arc in the Sukhothai Zone evolved in conjunction to be clarified based on the timing of accretion and the tectonic with Late Permian to Triassic forearc basins and volcanic activity evolution of continental island arc. during the Middle to early Late Triassic. The accretionary complex was probably contemporaneous with the evolution of the conti- 9. Conclusions nental island arc during the Permo–Triassic, supplied with sediment from the Sukhothai Zone. Based on the petrography and geochemistry of clastic rocks The geochemical data and mixing model strongly suggest that within the Inthanon Zone, the source rock compositions of the the source rocks of the mélange were continental margin rocks, provenance of the clastic rocks were reconstructed, as well as the such as craton sandstone with high SiO2 contents. The craton nature of the sediment supply system for the convergence zone sandstone may correspond to Proterozoic quartzite in the Indo- of the Paleo-Tethys, as recorded in northern Thailand. Based on china Block (Metcalfe, 1988), now eroded away. In the subduction field and microscopic observations, modal analyses of sandstone, zone of the Paleo-Tethys, quartz-rich fragments from the conti- and mineral compositions, the clastic rocks of the Inthanon Zone nental margin may have been sourced from the Indochina Block were classified into two types: (1) lithic sandstone and shale with- rather than from the Sibumasu Block, because the Paleo-Tethys in mélange in the Permo–Triassic accretionary complex; and (2) was a deep and broad ocean in which pelagic chert was deposited Carboniferous quartzose sandstone and mudstone within the Sibu- between the Indochina Block and the Sibumasu Block during the masu Block. Geochemical data indicate that the source rocks of the Permian to Middle Triassic (Sashida and Igo, 1999; Sashida et al., clastic rocks within the mélange were mainly felsic volcanic rocks 1993, 2000; Wakita and Metcalfe, 2005; Kamata et al., 2009; of the Sukhothai Zone, and continental crust rocks of the Indochina Thassanapa et al., 2011). In addition, the Sibumasu Block contains Block. The results of a mixing model indicate the source rocks com- Triassic marine sediments (the Mae Sariang Group; e.g., Srinak prised felsic volcanic rocks (26%) and basalt–andesite (8%) of the et al., 2007) consisting of sandstone, mudstone, conglomerate, Sukhothai Zone, and craton sandstone (40%) and upper continental chert, and limestone. Kamata et al. (2009) classified the chert crust (26%) of the Indochina Block. In contrast, Carboniferous within the Mae Sariang Group as ‘hemipelagic chert’, character- quartzose sedimentary rocks within the Sibumasu Block originated ized by both siliceous organisms (radiolarians) and calcareous from the continental margin, without any input from an island arc. organisms (e.g., foraminifers and thin-shelled bivalves) within a microcrystalline quartz and clay-rich matrix, deposited on the Acknowledgments continental slope and rise along the eastern margin of the Sibumasu Block during the Early to early Late Triassic. The We would like to thank Dr. K. Wakita, Dr. P. Chaodumrong and existence of ‘hemipelagic chert’ suggests that coarse-grained Dr. Uno for their valuable comments on the geology and tectonics 14 H. Hara et al. / Journal of Asian Earth Sciences 61 (2012) 2–15 of northern Thailand; Ms. R. Nohara for her support with ICP-MS provenance and tectonism at the Gondwana margin. Chemical Geology 158, and XRF analyses; Dr. A. Sardsud, Dr. C. Montri, Ms. S. 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