ORIGINAL ARTICLE
A new finding of mylonite along the Median Tectonic Line near Sakurabuchi Park in Shinshiro City, Aichi Prefecture
2025 年 120 巻 1 号 論文ID: 241015b
詳細
2025 年 120 巻 1 号 論文ID: 241015b
We report an outcrop of tonalitic mylonite along the Median Tectonic Line (MTL) in the Ryoke Belt near Sakurabuchi Park in Shinshiro City, Aichi Prefecture. The mylonite has a foliated porphyroclastic texture with coarse (up to a few millimeters) feldspar grains in a greenish/brownish matrix. The strike of the foliation is approximately parallel to that of the MTL. The mylonite consists of plagioclase (locally K-feldspar) porphyroclasts in a fine-grained foliated matrix that consists mainly of dynamically recrystallized quartz aggregates, along with plagioclase and biotite/chlorite. The feldspar porphyroclasts contain quartz-filled fractures and have a asymmetric shape that indicates dextral shear. The quartz c-axis fabric has an intense Y-maxima with a weak cross-girdle pattern. These features of the mylonite fabric are similar to those of porphyroclastic mylonites, such as the Kashio mylonite, which occur along the MTL in other regions.
The Median Tectonic Line (MTL) is the most prominent tectonic boundary in SW Japan and separates the Ryoke high-temperature/low-pressure (high-T/low-P) metamorphic belt from the Sanbagawa low-T/high-P metamorphic belt (Fig. 1a; Brown, 1998; Wallis and Okudaira, 2016; Endo et al., 2024a; Okudaira et al., 2024). In the Shinshiro area of Aichi Prefecture, outcrops around the MTL contain the Ryoke metamorphic rocks and granitoids, the Sanbagawa schists, the Shitara sedimentary and volcanic rocks, and the Mikabu greenstones (Fig. 1b; e.g., Makimoto et al., 2004). Fault rocks in this area occur in a narrow zone along the MTL (Fig. 1b; e.g., Saito, 1955; Ui, 1980; Ui et al., 1988; Ohtomo, 1993, 1996; Makimoto et al., 2004; Yokoyama, 2007).
In this study, we report a newly discovered outcrop of mylonite along the MTL near Sakurabuchi Park in Shinshiro City, Aichi Prefecture (Fig. 1c), which is the southwestern-most occurrence of mylonite along the MTL in the Chubu district, except for the mylonite reported in a drillcore recovered from Cape Irago on the Atsumi Peninsula (Fig. 1a; Yamada et al., 1984). We present the microstructures of the mylonite and discuss their development in comparison with those reported previously along the MTL.
The MTL trends NE-SW in the Shinshiro area in the southeastern part of Aichi Prefecture (Fig. 1b). The Ryoke Belt, a Cretaceous high-T/low-P metamorphic belt, comprises mainly pelitic, psammitic, and siliceous schists and gneiss derived from Jurassic accretionary complexes (e.g., the Mino-Tamba Belt; Wallis and Okudaira, 2016; Endo et al., 2024a; Okudaira et al., 2024). The Ryoke Belt is also intruded by many Cretaceous granitoids. The Ryoke granitoids have traditionally been grouped into the gneissose Older Ryoke granitoids and massive Younger Ryoke granitoids (Brown, 1998; Wallis and Okudaira, 2016). However, recent geochronological studies have shown that this division is inconsistent with the U-Pb zircon ages of granitoids (Takatsuka et al., 2018a, 2018b). Instead, the granitoids in the Mikawa area have been divided into three groups with ages of 99-95, 81-75, and 75-69 Ma (Takatsuka et al., 2018a, 2018b; Kawakami et al., 2022; Okudaira et al., 2024).
The Ryoke Belt has been divided into several regional metamorphic zones (e.g., the biotite, sillimanite-K-feldspar, and garnet-cordierite zones) and contains contact metamorphic aureoles around younger granitoid intrusions (Miyazaki, 2010; Endo and Yamazaki, 2013). In the Shinshiro area, the Ryoke metamorphic rocks are mainly schistose and gneissose pelitic, psammitic, and siliceous metasediments, all of which strike ENE-WSW and dip moderately to the north. The regional metamorphic grade increases from north to south (Endo and Yamazaki, 2013). Kawakami et al. (2022) reported an age of ∼ 97-89 Ma for the peak of the Ryoke regional metamorphism, based on U-Pb zircon dating, compared with an age of 100 Ma proposed by Suzuki and Adachi (1998). Contact metamorphic aureoles (K-feldspar-cordierite zone, >600 °C and 230-240 MPa) associated with the Shinshiro Tonalite are widely distributed (Endo and Yamazaki, 2013).
The Ryoke granitoids in the Shinshiro area consist mainly of Shinshiro Tonalite, which is a gneissose hornblende-biotite tonalite that was emplaced in the middle crust at 8.5-9.0 km depth (Endo and Yamazaki, 2013). The tonalite yields a CHIME monazite age of 86.0 ± 4.7 Ma (Morishita and Suzuki, 1995), compared with U-Pb zircon ages of 69.5 ± 0.3 and 70.6 ± 1.0 Ma (Takatsuka et al., 2018a).
The Shitara sedimentary and volcanic rocks in the Shinshiro area consist of clastic sediments of the Hokusetsu Subgroup that are unconformably overlain by volcanic rocks of the Nansetsu Subgroup (Kato, 1962). This area contains the Shitara volcanic rocks as well as many dike swarms, intrusions, and the Otoge cauldron (Sawai, 1979; Geshi, 2003).
Seki et al. (1959) and Makimoto et al. (2004) reported the Sanbagawa schists and Mikabu greenstones in this area. The Sanbagawa Metamorphic Belt, a Cretaceous low-T/high-P metamorphic belt, consists mainly of basic, pelitic, and siliceous schists derived from a Late Jurassic accretionary complex (Wallis and Okudaira, 2016; Endo et al., 2024a). The Mikabu greenstones consist mainly of basaltic pillow lava (e.g., metabasalt and metagabbro) accompanied by ultramafic rocks and gabbro (e.g., Shioya et al., 2021).
Mylonite was newly found in an outcrop near the MTL in the vicinity of Sakurabuchi Park (Fig. 1c). The mylonite occurs along the Tamachigawa River, close to the confluence with the Toyogawa River (Fig. 1c), and occurs in outcrops that are pale green to whitish and dark brown, with white feldspar (Fig. 2a). The foliation in the mylonite (Figs. 2a and 2b) strikes ∼ 060° and dips ∼ 70° to the SE, and the lineation is subhorizontal.
The outcrops of mylonite do not contain the MTL or cataclasite. The MTL appears to be located close to the confluence between the Tamachigawa and Toyogawa rivers (Fig. 1b). Greenschists of the Sanbagawa Belt are exposed on the true right of the Toyogawa River at a site located ∼ 60 m upstream of the mylonite outcrops.
Microstructural observations of the mylonite were performed on polished thin sections using optical and scanning electron microscopy. Mylonite samples were cut perpendicular to the foliation (XY plane) and parallel to the lineation (X-axis) (i.e., the XZ plane).
The crystallographic preferred orientations (CPOs) of quartz were analyzed on polished thin sections using a scanning electron microscope (SEM; HITACHI S-3400N Type II) equipped with electron backscatter diffraction (EBSD; Oxford Instruments, INCA Synergy AZTec 4.0) at the Rock and Mineral Laboratory, Nagoya University (e.g., Endo et al., 2024b). The operating conditions were as follows: acceleration voltage of 20 kV, low-vacuum conditions with an internal pressure of 30 Pa, probe current of 80-100 µA, working distance of 28 mm, and specimen tilted at 70°. The acquired data were analyzed using AZTec 4.0 to determine the crystal orientations. We used MATLAB MTEX Toolbox software (version 5.8.2; http://mtex-toolbox.github.io/; Bachmann et al., 2010, 2011) to process the mapping data. We then determined grain boundaries in the mineral-phase maps, using a critical grain boundary misorientation angle of 10°.
The CPOs were plotted as one point per grain in equal-area lower-hemisphere stereonets within the XZ structural reference frame. We calculated the fabric strength (i.e., the J, pfJ, and M indexes; Mainprice and Silver, 1993, Michibayashi and Mainprice, 2004; Skemer et al., 2005) to quantify the intensity of a given CPO using the MATLAB MTEX Toolbox (version 5.8.2). The J index has a value of 1 for randomly oriented crystals and ∼ 92 for a single quartz crystal. The pfJ index has a value of 1 for a random distribution and a maximum value of ∼ 23 for the c-axis and ∼ 7 for other axes in the present case for a single quartz crystal. The M index has a value of 0 for a random fabric and ∼ 0.89 for a single quartz crystal.
Microstructures and quartz CPOsThe mylonite is characterized by whitish feldspar porphyroclasts in a brownish/greenish fine-grained matrix (Figs. 2a and 2b). Quartz, plagioclase, and biotite are the main minerals, with minor K-feldspar, allanite, zircon, and monazite.
The mylonite has a typical porphyroclastic texture, consisting mainly of coarse-grained plagioclase (locally K-feldspar) porphyroclasts in a fine-grained matrix of quartz, plagioclase, and biotite (Figs. 2c-2f). The foliation in the matrix wraps around the porphyroclasts that have asymmetric δ- and σ-type fabrics (Passchier and Trouw, 2005) that indicate dextral (top-to-the-southwest) shear (Figs. 2c-2f).
The plagioclase porphyroclasts are largely subhedral, have igneous textures (e.g., twins and chemical zoning), and contain quartz-filled fractures (Figs. 2e and 2f). Some plagioclase grains are fine-grained, but they are commonly coarser than other minerals such as quartz and biotite. Quartz ribbons are up to 50 µm long (Figs. 2c-2f). The fine-grained biotite forms a planar structure that defines the foliation; however, most of the biotite is partially altered to chlorite or transformed into clay minerals.
Among the quartz CPOs with crystallographic axes [c], <a>, and {r}, the c-axes are mostly aligned with the Y-axis, whereas a few c-axes are weakly aligned with the Z-axis, defining a weak crossed girdle with an intense Y-maximum (Fig. 3).
The tonalitic mylonite that occurs along the MTL, which was termed the Kashio gneiss (i.e., Kashio mylonite) by Harada (1890), is characterized by feldspar and amphibole porphyroclasts in a fine-grained matrix of quartz and feldspar (Hara et al., 1980; Takagi, 1984, 1986; Michibayashi and Masuda, 1993; Yamamoto, 1994; Shimada et al., 1998; Michibayashi et al., 1999; Okudaira and Shigematsu, 2012; Suzuki et al., 2015; Czertowicz et al., 2019; Nakamura et al., 2022; Endo et al., 2024b). In the Takato-Oshika area, gneissose hornblende-biotite tonalite and quartz diorite (the so-called Hiji tonalite) are the protoliths of the mylonite along the MTL (e.g., Takagi, 1984, 1986; Nakamura et al., 2022).
In the Shinshiro area, the ∼ 70 Ma Shinshiro Tonalite intruded the Ryoke metamorphic rocks (Takatsuka et al., 2018a). Ikeda et al. (1974) reported that the Shinshiro Tonalite is in direct contact with schists of the Sanbagawa Belt across the MTL. Therefore, the Shinshiro Tonalite may be the protolith of the mylonite reported in this study. Alternatively, Takatsuka et al. (2018b) reported three magmatic pulses with different spatial distributions and ages in the Mikawa area, where 81-75 Ma granitoids occur as elongated belts oriented parallel to the NE-SW striking regional foliation in the Ryoke metamorphic rocks along the MTL. These granitoids might also be the protolith of the mylonite described in this study.
Quartz c-axis fabric and inferred deformation conditions of the myloniteMichibayashi and Masuda (1993) recognized two stages of ductile deformation, at high and low temperatures, respectively, in the Kashio shear zone along the MTL near Kamimura, Nagano Prefecture (∼ 80 km NE along the MTL from the present study area; Fig. 1a). The former deformation event resulted from an earlier and weaker stage of plastic deformation that formed the gneissosity in the granites, and the latter resulted from a later and stronger stage of plastic-brittle deformation that occurred mainly within a narrow zone along the margin of the granites.
Nakamura et al. (2022) also identified two stages of mylonitization (D1 and D2) in the Kashio shear zone in the Oshika area, Nagano Prefecture (∼ 100 km NE along the MTL from the present study area; Fig. 1a), where the Kashio mylonite was reported for the first time by Harada (1890). The D1 and D2 mylonites were formed during two phases of ductile deformation that correspond approximately to the high-T and low-T mylonites of Michibayashi and Masuda (1993), respectively. The quartz c-axis fabrics of the D1 mylonites are divided into two groups: Y-maxima and girdle distributions. The Y-maxima distribution changes to an asymmetric single girdle or cross-girdle in areas close to the MTL, whereas the D2 mylonites have asymmetric Type I crossed girdles along with some weak and random fabrics.
The dominant slip system of quartz depends on the temperature at the time of deformation, as follows: at lower temperatures (∼ 300-450 °C), basal <a> and rhomb <a> slip are most important, and quartz c-axes define Type I crossed girdle and single girdle patterns; at medium to high temperatures (450-600 °C), prism <a> slip becomes more important and the girdle tends to Y-maxima; and at very high temperatures under hydrous conditions (≥600 °C), prism <c> slip operates and X-maxima appear (Passchier and Trouw, 2005).
Quartz c-axis fabrics in mylonites within the Kashio shear zone along the MTL near Urakawa in NE Shizuoka Prefecture (∼ 30 km NE along the MTL from the present study area; Fig. 1a) show Type I crossed girdle, single girdle, and Y-maxima patterns (Endo et al., 2024b). Since both Type I and single-girdle fabrics tend to have a concentration of c-axes parallel to the Y-axis, Endo et al. (2024b) proposed that the deformation was dominantly under low-T conditions (lower greenschist facies), whereas the Y-axis concentration of c-axes develops under high-T conditions (450-600 °C) and is preserved during subsequent low-T deformation.
In the present study, the quartz c-axis fabric of the mylonite near Sakurabuchi Park shows a weak crossed girdle with an intense Y-maximum (Fig. 3), which is similar to some of the quartz c-axis fabrics reported by Endo et al. (2024b). Moreover, plagioclase porphyroclasts have δ- and σ-type asymmetric fabrics (Figs. 2c-2f; Passchier and Trouw, 2005) and contain quartz-filled fractures (Figs. 2e and 2f).
In general, the plastic-to-brittle transition of feldspar occurs at higher temperatures than that of quartz (Tullis and Yund, 1977; Simpson, 1985). The fractured porphyroclasts of this study suggest that the deformation conditions of plagioclase fell below the temperature of the plastic-brittle transition during progressive retrogression, as discussed by Michibayashi and Masuda (1993). Consequently, the mylonite near Sakurabuchi Park is likely to have been formed by plastic shear deformation during the progressive retrogression from high-T to low-T conditions, similar to other mylonites in the Kashio shear zone along the MTL (e.g., Michibayashi and Masuda, 1993; Nakamura et al., 2022; Endo et al., 2024b).
Shear sense of the myloniteIn the Chubu district, the Kashio shear zone changes in dip from vertical to sub-horizontal from north to south along a distance of ∼ 100 km (Hara et al., 1980; Takagi, 1984, 1986; Hayashi and Takagi, 1987; Masuda et al., 1990; Yamamoto and Masuda, 1990; Michibayashi and Masuda, 1993; Yamamoto, 1994; Michibayashi et al., 1997, 1999; Nakamura et al., 2022; Endo et al., 2024b). The shear zone is truncated by the MTL between Kamimura in Nagano Prefecture (Michibayashi and Masuda, 1993) and Misakubo in Shizuoka Prefecture (Yamamoto and Masuda, 1990; Yamamoto, 1994; Michibayashi et al., 1999). In the Misakubo area (∼ 40 km NE along the MTL from the present study area; Fig. 1a), the shear zone is sub-horizontal and the mylonitic foliation defines a macroscopic antiform (Yamamoto and Masuda, 1990; Michibayashi et al., 1999). Asymmetric fabrics around feldspar grains and mica fish indicate a regional sinistral strike-slip or top-to-the-south sense of shear (Takagi, 1984, 1986; Hayashi and Takagi, 1987; Yamamoto and Masuda, 1990; Michibayashi and Masuda, 1993; Yamamoto, 1994; Michibayashi et al., 1999; Nakamura et al., 2022; Endo et al., 2024b). In contrast, dextral shear (Shimada et al., 1998) has been reported in the Iinan-Iidaka area in Mie Prefecture (∼ 120 km SW along the MTL from the present study area; Fig. 1a). This could be due to the formation of an antiform-synform pair that trends ENE-WSW, as almost all the sense of shear indicators in the mylonites suggest a top-to-the-west sense of movement (Shimada et al., 1998).
In the mylonite of this study, the shear sense was determined from asymmetric feldspar fabrics that indicate a dextral sense of shear, in contrast to the other mylonites along the MTL. This shear sense might also be explained by the synform-antiform pair in the Iinan-Iidaka area of Mie Prefecture (Shimada et al., 1998), although further study of the mylonite outcrops is needed to test this hypothesis. Nonetheless, analysis of the mylonite in the southwestern-most part of the MTL in the Chubu district could yield a better understanding of the nature of the shear zone and the tectonic setting in association with the MTL.
We thank Yui Kouketsu and members of the Rock and Mineral Laboratory at Nagoya University for helpful support and discussions, and Tetsuo Kawakami, Takamoto Okudaira, and Yoshihiro Nakamura for thoughtful comments. We also thank Natsuko Takagi and Aoi Harada for help with preparing thin sections. This research was supported by the Co-Creation of Knowledge Program of the Graduate School of Environmental Studies, Nagoya University to MN and a research grant from the Japan Society for the Promotion of Science (Kiban-B 23H0172) to KM.
