Paleogeography and tectonic evolution of a late Paleozoic to earliest Mesozoic magmatic arc in East Asia based on U-Pb ages of detrital zircons from the Early Triassic Shingai Unit, Kurosegawa Belt, Southwest Japan
Introduction
Permian–Triassic strata in Japan are widely distributed in the Akiyoshi, Maizuru, Ultra-Tamba, Hida–Gaien, Kurosegawa, and South Kitakami belts (Fig. 1 and Table 1). The tectonic settings of the depositional basins of these strata include a fore-arc shelf for the Hida–Gaien Belt, part of the Kurosegawa Belt, and the South Kitakami Belt; a trench for the Akiyoshi Belt, the Ultra-Tanba Belt, and part of the Kurosegawa Belt; and a back-arc basin for the Maizuru Belt. The depositional basins of these shelf deposits have been identified by paleontological studies as being located near the South China (Ehiro, 2001) and Sino-Korean (e.g., Tazawa, 1991, Tazawa, 2002) blocks. Trench-fill deposits in the Akiyoshi, Kurosegawa, and Ultra-Tanba belts are also considered to have been deposited around the Sino-Korean Block on the basis of chemical compositions of detrital garnets (Takeuchi et al., 2008) and sandstone compositions (Yoshida and Machiyama, 2004), and in an island arc region near the South China Block from U–Pb ages of detrital zircons (Nakama et al., 2010).
The Kurosegawa Belt (or Kurosegawa Tectonic Belt; Ichikawa et al., 1956) in Southwest Japan forms a > 1000-km-long discontinuous narrow belt (Fig. 1) of tectonic mélange containing a wide variety of lithological types (e.g., early Paleozoic metamorphic and granitic rocks, Paleozoic to Mesozoic shelf deposits, and late Paleozoic to Mesozoic metamorphic rocks and accretionary complex) that are wholly or partly set in a serpentinite matrix and which were formed in various geodynamic settings (Hada et al., 2001, Murata, 2016). These rocks differ from those of the tectonically underlying Chichibu Composite Belt (Jurassic to Cretaceous accretionary complex), with regard to their origin and tectonic development (e.g., Ichikawa et al., 1956; Isozaki and Itaya, 1991). Recently, Hara et al., 2018, Isozaki et al., 2017 reported U − Pb ages for detrital zircons from the Paleozoic shelf deposits and accretionary complexes in the Kurosegawa Belt. Hara et al. (2018) proposed that these sediments were deposited along the eastern margin of the South China Block. In contrast, Isozaki et al. (2017) suggested that they were deposited in the subduction zone between the South China and Khanka blocks. However, those previous studies did not discuss the relationship between stratigraphy and age spectra, that is, a possible change in provenance over time. Identifying change in provenance through time in each geological body should improve understanding of Permian to Triassic tectonics and sedimentation in East Asia.
The Shingai Unit of the late Permian accretionary complex in the Kurosegawa Belt (Isozaki, 1985, Isozaki, 1986, Wakita et al., 2007) is located in the south-central Shikoku region (Fig. 1). Because the accretionary complex is composed mainly of mixed rocks that show block-in-matrix structure, it has thus far proved difficult to investigate and identify changes in provenance with time. However, Takeuchi, 1996, Takeuchi, 1998 found sedimentary strata in the Shingai Unit and preliminarily reported that sandstone compositions and detrital garnet compositions in this unit varied upwards through the stratigraphic column and suggested that these variations represented changing provenance. However, the depositional age of the Shingai Unit is unknown because of the lack of fossils from the mudstone matrix, and chronological control for constraining possible changes in provenance is lacking. Here, we present new U–Pb ages for detrital zircons from sandstones of the Shingai Unit, investigate change in provenance through the Shingai Unit, and infer the regional tectonic evolution during the deposition of this unit.
Section snippets
Geological setting
The Shingai Unit is located in the Shirakidani area of the south-central Shikoku region (Fig. 2a). It is bounded from the Tosayama Unit (late Permian accretionary complex) in the north and from Cretaceous sediments in the south by faults (Wakita et al., 2007, Hara et al., 2018). The Shingai Unit is subdivided into sedimentary zone (mainly sandstone) and mélange zone (mainly mudstone and mixed rock) (Takeuchi, 1998). In addition, the Yasuba conglomerate that characteristically accompanies
Sandstone petrography
Sandstones in the study area are classified into three types: feldspathic arenites, lithic wackes, and lithic arenites that contain abundant volcanic rock fragments (Fig. 2a, b, 4e, f, and 5). Feldspathic arenites occur in the upper part of the sedimentary zone and in massive sandstones in the mélange zone. Lithic wackes are found in the lower part of the sedimentary zone and entire mélange zone. Lithic arenites containing abundant volcanic rock fragments are contained in the mélange zone,
Methods
The sandstone samples were broken with a hammer and subsequently crushed with a stamp mill for several tens of minutes and sieved through #60 (250 μm) mesh. Then, heavy minerals were separated from the powder samples by water panning, and magnetic minerals were removed with a magnet. After heavy mineral grains had dried, zircons were randomly extracted and mounted into epoxy resin (petropoxy) on a microscope slide under a stereoscope. After the epoxy resin had cured, the zircons were polished
U–Pb ages of detrital zircons
Measurements of 212 spots on 211 grains yielded 141 concordant ages from sample S2 (Fig. 8, Fig. 9 and Table 2). The YSG age is 244.8 ± 3.3 Ma, and the YC1σ [2+] age is 248.7 ± 3.1 Ma (n = 7). The oldest 207Pb/206P age is 2385 ± 61 Ma. The age spectrum of S2 has a major cluster at 259 Ma and minor clusters at 308 and 279 Ma. In addition, S2 has scattered age components through the range of 1250–340 Ma (Fig. 10c). Most of the analyzed zircon grains show oscillatory zoning in CL images. However,
Depositional ages of the Shingai Unit
YC1σ [2+] ages of samples S2–S6 along route 1 (Fig. 2b), which shows continuous stratigraphy, overlap at 1σ and have a range of 252–245 Ma (mean age of 249 Ma). According to Cohen et al. (2019), the Permian–Triassic boundary has an age of 251.902 ± 0.024 Ma, and the Early–Middle Triassic boundary is 247.2 Ma. The maximum depositional age of the Shingai Unit based on the mean YC1σ [2+] ages is thus Early Triassic. Hara et al. (2018) reported a YC1σ [2+] age of 256 ± 3 Ma from detrital zircons of
Conclusion
In this study, we inferred the provenance and tectonic evolution of the Shingai Unit, an accretionary complex in the Kurosegawa Belt of Southwest Japan, on the basis of U–Pb ages for detrital zircons obtained from sandstones. Our main conclusions are as follows.
- )A.
Age spectra for detrital zircons from six sandstone samples from the Shingai Unit have main late Paleozoic to earliest Mesozoic clusters (ca. 320–300, 290–270, and 260–250 Ma) and minor Proterozoic–early Paleozoic (ca. 2300–700 and
CRediT authorship contribution statement
Masahiro Ohkawa: Conceptualization, Methodology, Software, Formal analysis, Investigation, Writing - original draft, Writing - review & editing, Visualization. Makoto Takeuchi: Investigation, Supervision, Project administration, Writing - review & editing. Yuaxiao Li: Methodology, Formal analysis. Shimon Saitoh: Investigation, Formal analysis. Koshi Yamamoto: Resources.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
We thank H. Yoshida and K. Tsukada of Nagoya University for fruitful discussions and advice. We are also grateful to Y. Kouketsu of Nagoya University for technical support during cathodoluminescence measurements.
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Magmatism and tectonic setting of proto-Japan during the Early Carboniferous: Constraints from the geochemical characteristics of mafic volcanic rocks in the Hida Gaien Belt, SW Japan
2022, Journal of Asian Earth SciencesCitation Excerpt :The Japanese Islands have affinity to Phanerozoic orogens (Fig. 1) and provide a record of long-term tectonics in East Asia during the Early Paleozoic to Cenozoic (Charvet, 2013; Isozaki et al., 2010; Maruyama et al., 1997; Wakita et al., 2021). Numerous studies aimed at clarifying the tectonic evolution of East Asia have involved investigation of the Japanese Islands, including structural geology (e.g., Faure et al., 1986; Faure and Charvet, 1987; Tsukada, 2003; Wakita et al., 2013; Yamakita and Otoh, 1998), paleogeography (e.g., Isozaki, 2019; Tazawa, 1993, 2002; Tominaga et al., 2019; Tsukada et al., 1999; Ueno, 2006), geochemistry and chronology of volcanic–plutonic rocks (e.g., Aoki et al., 2015; Arakawa et al., 2000; Isozaki et al., 2015; Tominaga and Hara, 2021; Tsutsumi et al., 2014), and provenance based on compositions of detrital heavy minerals and U–Pb ages of detrital zircons (e.g., Isozaki et al., 2017; Kimura et al., 2021; Li and Takeuchi, 2022; Nakama et al., 2010; Ohkawa et al., 2021; Suzuki and Kurihara, 2021; Takeuchi et al., 2008). However, the petrogenesis, tectonic evolution, and paleogeography of the Paleozoic rocks in the Japanese Islands remain debated because these rocks are distributed in narrow and limited regions as a result of post-Paleozoic deformation, including strike-slip faulting (e.g., Ehiro, 1977; Taira et al., 1983; Tazawa, 2004; Tsukada, 2003) and thrusting (e.g., Isozaki and Itaya, 1991; Isozaki and Maruyama, 1991).
U–Pb ages and sandstone provenance of the Permian volcano-sedimentary sequence of the Hida Gaien belt, Southwest Japan: Implications for Permian sedimentation and tectonics in Northeast Asia
2021, Journal of Asian Earth SciencesCitation Excerpt :Of these studies, Yoshida et al. (1994) and Yoshida and Machiyama (2004) showed a clear change in provenance for the Permian strata in the South Kitakami belt from an undissected arc source area during the Early to mid-Middle Permian to an uplifted arc basement source area during the late Middle to Late Permian. Trench-fill sediments of Middle–Late Permian and Early Triassic ages in the Kurosegawa and Akiyoshi belts show similar trends in terms of input from volcanic/plutonic rock detritus, as recognized from geochemical analyses and modal compositions (Takeuchi et al., 2008; Hara et al., 2018a; Zhang et al., 2018; Ohkawa et al., 2021). For the Hida Gaien belt, Yoshida and Tazawa (2000) conducted provenance analysis in the type section of the Moribu Formation, which partly corresponds to units MB2–MB4 in the present study, based on petrographic signatures and major-element-oxide contents.
Pb age distributions, and three main age groups: 200–300 Ma, 400–500 Ma, and 700–1000 Ma. The primary provenances of the study area are the Yunkai Massif to the west, Hainan Island to the south, and northern Fujian, eastern Hunan, Wuyi Terrane, and Jiangnan Orogen to the north. The distribution of detrital zircons in the southeast margin of the SCB and Proto-Japan is further compared by applying various methods such as multidimensional scaling and Chi-square tests, which shows significant differences in the age distribution patterns of detrital zircons between the two areas. Further provenance analysis reveals significant differences between the southeast margin of the SCB and Proto-Japan during the Late Triassic, suggesting a lack of affinity between the two regions. Therefore, the Late Triassic Proto-Japan likely not located to the southeast margin of the SCB and moving closer to its current position.