Sandstone provenance and U–Pb ages of detrital zircons from Permian–Triassic forearc sediments within the Sukhothai Arc, northern Thailand: Record of volcanic-arc evolution in response to Paleo-Tethys subduction

Highlights

• Geochemistry and detrital zircon U–Pb ages of Permian–Triassic forearc sediments.

• The Sukhothai Arc was active during the Late Carboniferous–Triassic.

• Provenance of forearc sediments aid our understanding of arc evolution.

Abstract

Provenance analysis and U–Pb dating of detrital zircons in Permian–Triassic forearc sediments from the Sukhothai Arc in northern Thailand clarify the evolution of a missing arc system associated with Paleo-Tethys subduction. The turbidite-dominant formations within the forearc sediments include the Permian Ngao Group (Kiu Lom, Pha Huat, and Huai Thak formations), the Early to earliest Late Triassic Lampang Group (Phra That and Hong Hoi formations), and the Late Triassic Song Group (Pha Daeng and Wang Chin formations). The sandstones are quartzose in the Pha Huat, Huai Thak, and Wang Chin formations, and lithic wacke in the Kiu Lom, Phra That, Hong Hoi and Pha Daeng formations. The quartzose sandstones contain abundant quartz, felsic volcanic and plutonic fragments, whereas the lithic sandstones contain mainly basaltic to felsic volcanic fragments. The youngest single-grain (YSG) zircon U–Pb age generally approximates the depositional age in the study area, but in the case of the limestone-dominant Pha Huat Formation the YSG age is clearly older. On the other hand, the youngest cluster U–Pb age (YC1σ) represents the peak of igneous activity in the source area. Geological evidence, geochemical signatures, and the YC1σ ages of the sandstones have allowed us to reconstruct the Sukhothai arc evolution. The initial Sukhothai Arc (Late Carboniferous–Early Permian) developed as a continental island arc. Subsequently, there was general magmatic quiescence with minor I-type granitic activity during the Middle to early Late Permian. In the latest Permian to early Late Triassic, the Sukhothai Arc developed in tandem with Early to Middle Triassic I-type granitic activity, Middle to Late Triassic volcanism, evolution of an accretionary complex, and an abundant supply of sediments from the volcanic rocks to the trench through a forearc basin. Subsequently, the Sukhothai Arc became quiescent as the Paleo-Tethys closed after the Late Triassic. In addition, parts of sediments of supposed Devonian–Carboniferous age within the Sukhothai Arc were revised as the Triassic Lampang Group, and the Early Cretaceous Khorat Group.

Introduction

The Sukhothai Arc in northern Thailand developed in response to the subduction of Paleo-Tethyan oceanic crust beneath the present western margin of the Indochina Block, and it is characterized by volcanic successions, the intrusion of I-type granitoids, and the deposition of Permian–Triassic forearc basin sediments (Barr and Macdonald, 1991, Ueno and Hisada, 2001, Sone and Metcalfe, 2008, Metcalfe, 2013). Reconstruction of igneous activity associated with Paleo-Tethys subduction is important in understanding Permian–Triassic convergence tectonics and the magmatism that occurred between the Indochina and Sibumasu blocks in Southeast Asia. However, the nature of the igneous activity and its precise timing has not been clarified in this area. In northern Thailand, the Permian–Triassic volcanic successions have been named the Chiang Khong and Lampang volcanic belts (e.g., Barr et al., 2000, Singharajwarapan and Berry, 2000, Panjasawatwong et al., 2003, Barr and Charusiri, 2011). In these belts, U–Pb dating of zircons from the volcanic rocks has only been done for the Middle to Late Triassic volcanic rocks (e.g., Barr et al., 2000, Srichan et al., 2009, Qian et al., 2013), and there are no data for the Permian to Early Triassic volcanic rocks still now, as these rocks have largely been eroded away. I-type granitoids have provided some evidence of the magmatic activity, but the data are few because of the limited distribution of these granitoids in the Sukhothai Arc (Gardiner et al., 2016). In contrast, Permian to Triassic sedimentary successions, which formed in the forearc basin of the Sukhothai Arc, are widely distributed in northern Thailand. The sediments are mainly sandstone, mudstone, and limestone, and are known as the Permian Ngao and Triassic Lampang and Song groups (e.g., Charoenprawat et al., 1994, Ishibashi et al., 1994, Chaodumrong and Burrett, 1997, Singharajwarapan and Berry, 2000, Kobayashi et al., 2006, Chonglakmani, 2011, Ueno and Charoentitirat, 2011, Sone et al., 2012). In this study, we focus on the provenance of these forearc sediments, as they possibly preserve continuous records of the Permian–Triassic magmatic evolution of the missing Sukhothai Arc.

Sandstone petrography and geochemistry are useful in determining provenance, tectonic setting, and sediment recycling (e.g., Bhatia and Crook, 1986, Dickinson et al., 1983), and U–Pb dating of detrital zircons from sandstone enable the maximum depositional age to be determined from the youngest or peak age, and enable reconstruction of the tectonic evolution of sediment provenance (e.g., Fedo et al., 2003, Dickinson and Gehrels, 2009, Ernst et al., 2009, Hampton et al., 2010 Beranek and Mortensen, 2011, Cawood et al., 2012). Combining these methods potentially provides not only the maximum depositional age but also information on tectonic setting and the temporal and spatial variations in provenance (Eizenhöfer et al., 2015, Hu et al., 2015, Lee et al., 2016). Hara et al., 2012, Hara et al., 2013 studied the U–Pb ages of detrital zircons and the provenance of an accretionary complex in northern Thailand that developed during the time of Paleo-Tethys subduction, and they discussed the supply system of the sediment from provenance (continent and arc) to trench as well as the maximum depositional age of the accretionary complex during Late Permian to Middle Triassic. Hara et al. (2013) also outlined the activity of the Sukhothai Arc, based on detrital zircon U–Pb ages. In addition, the chemistry of detrital chromian spinels, obtained from several tectonic units in Thailand, provided specific information on the tectonic setting with respect to the occurrence of mafic and ultramafic rocks (Chutakositkanon et al., 2001, Hisada et al., 2004).

Here, we adapt these techniques for a study of the Permian–Triassic forearc sediments within the Sukhothai Arc. In addition, we examined the ages of zircons from sediments of supposed Devonian and Carboniferous age within the Sukhothai Arc in an attempt to understand the initial tectonic framework of arc evolution; previously the depositional ages of these sediments were uncertain due to the lack of age-diagnostic fossils. The aim of this paper is to determine evolution of the Sukhothai Arc, as recorded in continuous sedimentary successions of Carboniferous and Permian–Triassic age, and to reconstruct the evolution of the arc system associated with Paleo-Tethys subduction and closure, by combining provenance analysis from petrography and geochemistry with detrital zircon U–Pb dating.