Journal of Asian Earth Sciences 61 (2012) 2–15 Contents lists available at SciVerse ScienceDirect Journal of Asian Earth Sciences journal homepage: www.elsevier.com/locate/jseaes 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 orig- inated 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, sup- plied 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. 1367-9120/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jseaes.2012.06.012 H. Hara et al. / Journal of Asian Earth Sciences 61 (2012) 2–15 3 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 be- tween 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 succes- sion (Hesse and Koch, 1979; Department of Mineral Resources, Nan—Uttaradit SouthSouth ChinaChina 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