2023 年 118 巻 1 号 論文ID: 220630
The metamorphic and plutonic rocks within a serpentinite mélange in the Nagato Tectonic Zone, Yamaguchi prefecture, southwest Japan have been known as a part of the early Paleozoic rock record in Japan. Thus, they play an important role in deciphering the tectonic evolution of proto-Japan. This study determined their U-Pb zircon ages to deduce the origin and geological significance of the Nagato Tectonic Zone. The results of the petrographic observation and LA-ICP-MS U-Pb zircon dating revealed the occurrences of the massive amphibolite originated from the 460 Ma mafic igneous rock, two-mica tonalites formed at 470-460 Ma in association with some xenocrystic zircon grains, and low-grade metasedimentary rocks including detrital zircon grains of 2460-400 Ma. The lithology, petrographic characteristics, and U-Pb zircon ages of these constituent rocks in the Nagato Tectonic Zone are well comparable with the blocks within serpentinite mélange in the Kurosegawa Tectonic Belt, rather than the Hida Gaien Belt which has been considered to form a single geological unit with the Nagato Tectonic Zone as the Sangun-Renge metamorphic belt or Nagato-Renge belt. It implies the Nagato Tectonic Zone could be regarded as an isolated fragment of the Kurosegawa Tectonic Belt. The features of detrital age spectra and Ordovician magmatism in the Nagato Tectonic Zone and Kurosegawa Tectonic Belt provide the possibility of the development of their granitoids and protoliths of metamorphic rocks at the active continental margin of the South China craton.
The Nagato Tectonic Zone (NTZ; Matsumoto, 1949) is located in the Yamaguchi prefecture, the western end of Honshu Island in Japan (Fig. 1a), and is discontinuously distributed with an NNE-SSW trend (Fig. 1b). The NTZ consists of Carboniferous to Permian sedimentary rocks, schists, and a serpentinite mélange including gabbro, diorite, granite, and metamorphic rocks such as granitic gneiss, and amphibolite (e.g., Isozaki and Tamura, 1989; Igi, 2002). The occurrence of a Silurian granitic rock within the serpentinite mélange has made the NTZ as one of the oldest terranes in Japan (Kawano et al., 1966). Thus, understanding the tectonic background of the NTZ contributes to reconstructing the development of the Japanese Islands in the early Paleozoic. Kawano et al. (1966) pointed out the lithological similarity between the NTZ in the inner zone and the Kurosegawa Tectonic Belt (KTB) in the outer zone of southwest Japan (Fig. 1a) in terms of the occurrence, petrography, and Silurian age of granitoids in the serpentinite. The KTB is a serpentinite mélange that contains blocks of various early Paleozoic rocks including Silurian granitoids (e.g., Ichikawa et al., 1956). However, Shibata and Nishimura (1989) reported numerous K-Ar ages of muscovite and barroisite, and Rb-Sr ages of muscovite from schists in the Sangun metamorphic belt (Nishimura et al., 1977), including pelitic schist from the Toyogadake area in the NTZ. Their results imply that the schists in the NTZ belong to the Sangun-Renge Belt of which radiometric age of around 300 Ma is comparable with those in the Hida Gaien Belt (HGB) (Fig. 1a). The HGB is composed of serpentinite associated with schist, jadeitite, eclogite, blueschist, metagabbro, and amphibolite (e.g., Tsujimori, 2002). Moreover, the late Carboniferous radiolarian fossils were discovered from a tuffaceous mudstone in the NTZ by Isozaki and Tamura (1989). Based on the occurrences and lithological assemblages, they concluded its correlation with an undated tuffaceous sedimentary rock nearby the Carboniferous to Permian sequence in the HGB. Their correlation was supported by Kametaka (2006) who reported the early Permian radiolarian fossils from mudstone in the alternated beds with sandstone in the NTZ. Isozaki et al. (2010) proposed the Nagato-Renge belt, which almost corresponds to the Sangun-Renge Belt of Shibata and Nishimura (1989), as a serpentinite mélange including blocks of 450-340 Ma high pressure (HP) metamorphic rocks and 400 Ma granitic gneisses.
However, the terrain division in Japan has been reconsidered and modified on the basis of zircon U-Pb chronology (e.g., Isozaki et al., 2010; Osanai et al., 2014, 2021). It provides reliable and precise ages of rock formation due to the durability of zircon against secondary alteration or weathering (e.g., Harley and Kelly, 2007). As zircon ages can reflect the detritus provenances for sedimentary rocks, period of magmatic activity for igneous rocks, and timing of metamorphism, as well as protolith formation for metamorphic rocks, the database of zircon U-Pb ages is a powerful criterion to reveal the timeline of tectonic events and define or distinguish the chronological affinities of some geological units. The presently known thermal history in the NTZ was based on the K-Ar and Rb-Sr chronologies, which are prone to modifications during the secondary events. Hence, these methods yield the tentative ages, though reliable ages for some geological events, lead to the necessity of more robust method to concretize the thermal history of the NTZ. Nevertheless, no geochronological studies utilizing the zircon U-Pb dating method have been conducted in the NTZ. The absence of U-Pb zircon chronology hinders understanding the origin and thermal history of the NTZ, as well as its controversial correlation with the HGB and KTB. It also leads to the ambiguity of the tectonic framework for the development of proto-Japan in the early Paleozoic. Therefore, the present study performed the petrographic observation and U-Pb zircon dating as the first report of tonalites and metamorphic rocks including amphibolite, pelitic schists, and actinolite rocks in the NTZ to discuss the geological and geochronological correlation among the NTZ, HGB, and KTB, and the geological significance.
The NTZ is a complex mixture of the Carboniferous to Permian coherent formation, Permian accretionary complex, schists, and serpentinite mélange accompanied by various blocks of plutonic and metamorphic rocks such as granitoid, metagabbro, and amphibolite (e.g., Murakami et al., 1977; Isozaki and Tamura, 1989; Igi, 2002; Kametaka, 2006) (Fig. 2). The high angle faults bound its margin from the Permian Akiyoshi terrane in the northeast, Triassic Mine Group in the east (e.g., Kabashima et al., 1993; Igi, 2002; Matsuura et al., 2007) (Fig. 2). The NTZ is covered by the Jurassic Toyora Group in the southwest, and by Cretaceous Kanmon Group in the center to the north, and partly intruded by late Cretaceous granitoids (Murakami and Mikami, 1975; Kabashima et al., 1993; Igi, 2002; Matsuura et al., 2007) (Fig. 2). The geology of the NTZ in the present study areas of Misumi, Dai, and Toyogadake areas (Fig. 1b) is described below.
The Misumi area in the north of the NTZ consists of Paleozoic sedimentary rocks of greenish tuffaceous mudstone to sandstone including various lenses of chert, limestone, basaltic green rock, serpentinite, granodiorite, quartz diorite, gabbro, and amphibolite (Murakami and Mikami, 1975; Murakami et al., 1977; Igi, 2002) (Fig. 2a). The lower Permian fusulinid fauna was reported from the limestone lens within a tuffaceous sandstone by Murakami and Mikami (1975). The serpentinite occurs as a huge lenticular body in contact with or including lenses of amphibolites and plutonic rocks (Murakami and Mikami, 1975; Murakami et al., 1977), or intrudes into the Paleozoic mudstone and sandstone (Igi, 2002). The dykes of Cretaceous granitoid intrude into these sedimentary rocks (Murakami and Mikami, 1975; Igi, 2002) (Fig. 2a), which caused the transformation to hornfels (Igi, 2002).
The main lithologies in the Dai area are serpentinites and sedimentary sequences of mudstone and sandstone (Isozaki and Tamura, 1989; Igi, 2002; Kametaka, 2006) (Fig. 2b). The former includes blocks of metagabbro, amphibolite, and trondhjemite to tonalite with several centimeters to tens of meters in size (Ishizaka and Yanagi, 1975; Shibata and Murakami, 1975; Isozaki and Tamura, 1989; Igi, 2002; Kametaka, 2006). The tonalites, composed mainly of plagioclase and quartz with minor biotite, muscovite, and primary subhedral epidote without K-feldspar, were classified as oceanic plagiogranite (Ishizaka and Yanagi, 1975; Murakami et al., 1977). The muscovite K-Ar age of 424 Ma (Kawano et al., 1966) and muscovite Rb-Sr age of 406 Ma (Hayase and Ishizaka, 1967) were reported from the gneissose garnet-bearing muscovite granodiorite within serpentinite. The edenitic hornblende-rich amphibolite is found as an angular block in serpentinite and yielded the hornblende K-Ar age of 373 ± 23 Ma (Shibata and Murakami, 1975). The mudstone in this area is associated with a limestone block that contains the early Permian fusulinids (Toriyama, 1954). Kametaka (2006) also discovered the early Permian radiolarian fossils from the fractured mudstone alternated with sandstone. The late Carboniferous radiolarian fossils were found from an andesitic tuffaceous mudstone in the coherent sequence named Higashihirano Formation (Isozaki and Tamura, 1989).
Toyogadake area is located in the southern part of the NTZ (Fig. 2c). The weakly metamorphosed accretionary complex of Toyohigashi Group which mainly occupies this area is sandwiched by schists in the east and west (Isozaki and Tamura, 1989; Kabashima et al., 1993; Igi, 2002) (Fig. 2c). The Toyohigashi Group consists of mudstones associated with rubbles of sandstone, chert, green rock, limestone as well as metagabbro, metadolerite, metagranite, schist, and serpentinite in various sizes of 1 mm to several tens of meters (Isozaki and Tamura, 1989; Igi, 2002). Kabashima et al. (1993) suggested the accretionary complex underwent low-grade metamorphism not exceeding prehnite-pumpellyite-facies based on the mineral parageneses of chlorite + muscovite for pelitic and psammitic rocks and chlorite + epidote + pumpellyite for metamorphosed mafic rocks. The Carboniferous to Permian fusulinids and smaller foraminifera have been found in the limestone rubbles within mudstones (Fujii, 1972; Murakami and Nishimura, 1979). Isozaki and Tamura (1989) also reported the early Permian radiolarians from an allochthonous block of bedded chert hosted by coarse-grained clastic rocks in the Toyohigashi Group. The schist is composed of pelitic schist and greenschist intercalating with psammitic and siliceous schists (Murakami and Nishimura, 1979; Kabashima et al., 1993; Igi, 2002). The mineral assemblages of greenschist are indicative of the pumpellyite-actinolite- to blueschist-facies grade (Murakami and Nishimura, 1979; Kabashima et al., 1993; Igi, 2002, and references therein). The schist shows two modes of occurrences; mappable elongated bodies trending NE-SW in contact with the Toyohigashi Group via high angle fault or Toyogadake thrust named by Kabashima et al. (1993) (Fig. 2c), or unmappable rubbles in the clastic rocks of Toyohigashi Group (Igi, 2002). The pelitic schists of the former type yielded 264 and 274 Ma of muscovite K-Ar ages (Nishimura et al., 1983) and 303 ± 9 Ma of muscovite Rb-Sr age (Shibata and Nishimura, 1989). The similar ages of 292 ± 22 Ma as amphibole K-Ar age and 285 ± 9 Ma as muscovite K-Ar age were obtained from greenschist and quartz schist of the latter type, respectively (Igi, 2002, and references therein). Thus, Igi (2002) concluded the same origin of these schists. Due to the intrusion of Cretaceous granitoids, the contact metamorphism partly affected the Permian accretionary complex and schist in the southern part of the Toyogadake area (Kabashima et al., 1993; Igi, 2002).
Most of the analyzed samples were collected as angular boulders of which sizes are upto around 40 cm in diameter, except for tonalite (11902D) from the Dai area and pelitic schist (12001) from the Toyogadake area (Figs. 2b and 2c). The shape and size of boulders imply short transportation from the sampling localities, as long-distance transportation makes boulders rounder and smaller. The tonalite (11902D) occurs as a rounded block in the serpentinite at the locality of 11902 in the Dai area (Fig. 3a). A small outcrop of pelitic schist was found from the locality of 12001 in the Toyogadake area (Fig. 3b), though the relationship with surrounding rocks is unclear in the field. The petrography of analyzed samples is described below and their mineral assemblages are summarized in Table 1. The abbreviations of mineral names in figures and tables in this study follow Whitney and Evans (2010).
| Sample No. | Rock type | Grt | Hbl | Act | Bt | Ms | Ep/Zo | Chl | Pl | Kfs | Qz | Aln | Opq | Ttn | Ap | Zrn | Secondary phase |
| Misumi area (North) | |||||||||||||||||
| 119T03C | Amphibolite | ○ | ○ | + | + | + | + | + | Act, Chl | ||||||||
| Dai area (Center) | |||||||||||||||||
| 11902D | Grt-bearing Bt-Ms tonalite | + | + | △ | ○ | ○ | + | + | + | Chl | |||||||
| 119T06D | Ep-bearing Bt-Ms tonalite | + | △ | △ | ○ | ○ | + | + | + | + | Chl | ||||||
| Toyogadake area (South) | |||||||||||||||||
| 12001 | Ep-Ms-Chl schist | ○ | ○ | △ | △ | + | ○ | + | + | + | |||||||
| 120T01O | Chl-Ms schist | ○ | ○ | △ | ○ | + | △ | + | + | ||||||||
| 120T01S | Act rock | ○ | + | ○ | + | △ | + | + | |||||||||
| 120T01U | Act rock | ○ | △ | + | △ | + | + | Chl | |||||||||
○, abundant; △, moderate; +, minor.
Amphibolite (119T03C). This amphibolite shows granoblastic texture and consists mainly of hornblende and plagioclase (Fig. 4a) with accessory minerals of quartz, titanite, apatite, opaque minerals, and zircon (Table 1). Hornblende is subhedral to anhedral, ~ 1.4 mm in length, and deep green (Fig. 4a). Some hornblende grains exhibit the brownish-colored portion in the core (Fig. 4a). Secondary actinolite and chlorite partly replace hornblende (Fig. 4a). Although the minor subhedral plagioclase is present (Fig. 4a), almost all the plagioclase is anhedral, saussuritized, and forms subgrains in association with anhedral fine-grained quartz. Zircon occurs in the matrix.
Garnet-bearing biotite-muscovite tonalite (11902D). The tonalite is leucocratic and weakly deformed (Fig. 4b). It is composed mainly of medium- to coarse-grained subhedral plagioclase and fine- to medium-grained anhedral quartz with subordinate muscovite and minor garnet, biotite, apatite, opaque minerals, and zircon (Fig. 4b, Table 1). The fine-grained subhedral garnet which is surrounded by chlorite occurs in the matrix (Fig. 4b). The muscovite is weakly oriented (Fig. 4b). Almost all the biotite grains are replaced by chlorite. The plagioclase is ~ 2.5 mm in length and saussuritized. Zircon is found in the matrix.
Epidote-bearing biotite-muscovite tonalite (119T06D). This tonalite exhibits a similar texture to the tonalite of 11902D, except for the presence of epidote and absence of garnet. The epidote has euhedral to subhedral shape, and a length of 0.1-1.7 mm (Fig. 4c). A few epidote grains include metamictized allanite (Fig. 4c). The orientated muscovite and biotite define a weak foliation (Fig. 4c). Zircon grains are recognized in the matrix and inside plagioclase.
Toyogadake area (south of NTZ)Epidote-muscovite-chlorite schist (12001). The schist contains fine-grained epidote, chlorite, muscovite, plagioclase, K-feldspar, and quartz (Fig. 4d). The accessory minerals of opaque minerals, apatite, and zircon are also present. The lepidoblastic chlorite and muscovite define the schistosity (Fig. 4d). The epidote grains are euhedral to subhedral and accompanied by the orientated muscovite (Fig. 4d). Zircon is present in the matrix.
Chlorite-muscovite schist (120T01O). This rock is characterized by plagioclase porphyroclast and well-developed schistosity (Fig. 4e). Its major mineral assemblage is chlorite + muscovite + plagioclase + quartz (Fig. 4e) with minor titanite, opaque minerals, apatite, and zircon (Table 1). Plagioclase grains are subhedral to anhedral, medium to fine-grained (~ 1.2 mm in length), and include abundant carbonaceous materials (Fig. 4e). Other minerals are fine-grained. The oriented muscovite and chlorite form the schistosity (Fig. 4e). Zircon can be seen in the fine-grained matrix.
Actinolite rocks (120T01S, 120T01U). These rocks are foliated by ductile and cataclastic deformations and mainly consist of fine-grained actinolite and plagioclase (Figs. 4f and 4g). Minor quartz, zoisite, titanite, opaque mineral, apatite, and zircon are also observed (Table 1). Actinolite forms radially orientated grains that fill a veinlet (Fig. 4f), or an aggregate (Fig. 4g). The zircon grains in these rocks were observed in the matrix.
The zircon grains separated from powders of seven samples were mounted on a disc of epoxy resin and polished with a diamond paste following the procedures described in Kitano et al. (2014). Observing the cathodoluminescence (CL) images of mounted zircon grains and identifying their inclusions were carried out by using a scanning electron microprobe (JEOL JSM-5310S-JED2140) equipped with energy-dispersive spectroscopy and a CL detector (Gatan MiniCL) in Kyushu University. The U-Pb dating of zircon was operated by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) combined with an Agilent 7500cx quadrupole ICP-MS with a New Wave Research UP-213 laser in Kyushu University. The analytical procedure followed Adachi et al. (2012). The isotopes of 202Hg, 204Hg + 204Pb, 206Pb, 207Pb, 208Pb, 232Th, and 238U were monitored during the dating operation. The measured 202Hg and 204Hg + 204Pb used for the correction of common Pb in zircon were negligibly low in this study. The spot sizes of 25-40 µm were applied for the laser during analysis. The zircon standards of TEMORA 1 (417 Ma; Black et al., 2003) and FC 1 (1099 Ma; Paces and Miller, 1993) were used for the calibration and accuracy check standards, respectively. The Th/U ratio of zircon was determined by using NIST SRM-611 glass standard. The isotopic ratios and ages were calculated by GLITTER software (Griffin et al., 2008). The Isoplot/Ex 3.7 software (Ludwig, 2008) was used to draw concordia diagrams and estimate weighted mean ages. The kernel density estimate with histograms and automatic peak search for the age spectra were operated using IsoplotR (Vermeesch, 2018). The concordant data in this study was defined as the data plotted on a concordia curve within its 2 sigma error and with low discordance [−5% < discordance = (207Pb/235U age − 206Pb/238U age) / (206Pb/238U age) * 100 < 5%]. The analysed FC 1 provided a weighted mean 206Pb/238U age of 1105.3 ± 9.5 Ma (n = 11, MSWD = 0.30) during this measurement. The analytical results are summarized in Supplementary Table S1 and listed in Supplementary Tables S2-S8 (Supplementary Tables S1-S8 are available online from https://doi.org/10.2465/jmps.220630).
ResultsAmphibolite (119T03C). The dominant morphology of zircon in amphibolite is elongated and rounded with minor ovoid shaped grains. It has 25-90 µm in length and aspect ratios of 1.2-2.9. The zircon grains exhibited a faint sector zoning or unzoned texture in the CL image (Fig. 5a). The analyses for the zircon provided 7 concordant and 5 discordant data. The 206Pb/238U ages of concordant data were spread between 479 ± 48-432 ± 48 Ma with Th/U ratios of 0.13-0.47 (Table S2). The clustered data yielded a weighted mean age of 457 ± 10 Ma (n = 7, MSWD = 0.55) (Fig. 5a).
Garnet-bearing biotite-muscovite tonalite (11902D). Zircon in this tonalite represents the euhedral to subhedral shape, length ranging from 20 µm to 125 µm and aspect ratios of 1.3-4.0. The internal textures of these zircon grains showed clear to faint oscillatory, banding, sector zoning, or some without zoning (Fig. 5b). 29 concordant and 5 discordant data were obtained from the analyzed zircon. The concordant data showed a spread in 206Pb/238U ages of 485 ± 21-441 ± 22 Ma and Th/U ratios of 0.17-0.54 (Table S3), from which the weighted mean age of 469.4 ± 3.9 Ma (n = 29, MSWD = 1.4) was calculated (Fig. 5b).
Epidote-bearing biotite-muscovite tonalite (119T06D). Euhedral zircon grains which have 35-90 µm in length and aspect ratios of 1.2-3.9 are extracted from the sample of 119T06D. These grains displayed the internal textures of the oscillatory zoning and minor core-rim texture composed of high CL intensity core and low CL intensity rim (Fig. 5c). The rim was too thin (~ 10 µm at most) to be analyzed. The dating of the zircon resulted in 18 concordant data and 5 discordant data. Three older ages of 534 ± 26-520 ± 20 Ma and dominantly younger ages of 485 ± 18-449 ± 16 Ma were obtained from concordant data with Th/U ratios of 0.26-0.36 and 0.07-0.43, respectively (Table S4). The younger concordant data yielded a weighted mean age of 459.3 ± 7.3 Ma (n = 15, MSWD = 2.3) (Fig. 5c).
Epidote-muscovite-chlorite schist (12001). Zircon is characterized by an euhedral shape and length of 20-60 µm with aspect ratios of 1.2-3.3. The clear to faint oscillatory zoning was observed in the CL image (Fig. 5d). 62 spot analyses of the zoned zircon produced 43 concordant data and 19 discordant ones. The 206Pb/238U ages and Th/U ratios of concordant data varied from ~ 670 Ma to 410 Ma and from 0.09 to 1.09, respectively (Table S5).
Chlorite-muscovite schist (120T01O). The shape and internal textures of zircon in this sample resemble those in the schist of sample 12001. Euhedral zircon grains which exhibit 20-100 µm in length and aspect ratios of 1.3-2.6 were extracted from this sample. They preserved the CL texture of the concentric oscillatory zoning (Fig. 5e). 16 concordant data obtained from these grains represented a range of 206Pb/238U ages between 553 ± 30 Ma and 420 ± 22 Ma with high Th/U ratios of 0.46-1.24 (Table S6).
Actinolite rocks (120T01S, 120T01U). The zircon crystals in the actinolite rock (120T01S) are dominantly euhedral with minor ovoid ones. These zircon grains have lengths of 40-120 µm and aspect ratios of 1.1-2.8. The CL images indicated several internal textures of the oscillatory zoning, sector zoning, and unzoned texture (Figs. 5f and 5g). The minor grains consist of brighter CL clear oscillatory zoned core and darker CL faint oscillatory zoned rim. The dated zircon grains gave 76 concordant data and 18 discordant ones from 94 spots. The 206Pb/238U ages of concordant data were scattered between 2460-2430 Ma (2 points), 1900-910 Ma (38 points), and 660-400 Ma (36 points) (Figs. 5f and 5g) with a wide range of Th/U ratios of 0.004-2.02 (Table S7). The other rock (120T01U) contained a small number of zircon grains which are characterized by the euhedral shape, lengths of 30-60 µm, aspect ratios of 1.1-3.2, and oscillatory zoned to unzoned CL textures (Figs. 5h and 5i). The analyses for these grains yielded 6 concordant and 7 discordant data. The concordant data showed the ages of 1340-1040, 470-420 Ma (Figs. 5h and 5i), and Th/U ratios of 0.20-1.01 (Table S8).
The petrography of amphibolite (119T03C) from the Misumi area in the NTZ was characterized by the granoblastic texture and presence of relic brownish hornblende (Fig. 4a). Shibata and Murakami (1975) reported the occurrence of blueish green pargasitic hornblende replaced by green edenitic hornblende and actinolite from the foliated amphibolite in the Dai area of the NTZ and pointed out the possibility of epidote amphibolite- to amphibolite-facies metamorphism. The petrographic descriptions in this study are broadly consistent with those reported by Shibata and Murakami (1975) and references therein. The tonalite of sample 11902D showed the occurrence of a rounded block in serpentinite (Fig. 3a). The quartzofeldspathic and aluminous mineral assemblages of muscovite + biotite + plagioclase + quartz ± garnet ± epidote without K-feldspar were confirmed from both tonalite samples analyzed in this study (Figs. 4b and 4c, Table 1), which were well consistent with the previous reports by Kawano et al. (1966), Ishizaka and Yanagi (1975), and Murakami et al. (1977). The pelitic schists (12001, 120T01O) and actinolite rocks (120T01S, 120T01U) in the Toyogadake area of the NTZ exhibited the foliated textures and low-grade metamorphic mineral assemblages of chlorite + muscovite and actinolite + plagioclase, respectively (Figs. 4d-4g, Table 1). One of the pelitic schists (12001) was collected from an outcrop (Fig. 3b), which is located within the Toyohigashi Group of the Permian accretionary complex according to the geological map (Fig. 2c). The pelitic schists (12001, 120T01O) and actinolite rocks (120T01S, 120T01U) might be derived from blocks of schists in the Toyohigashi Group, as described by Kabashima et al. (1993) and Igi (2002). All samples in this study were affected by either retrograde hydration, cataclastic deformation, or secondary veins, which possibly modified their original textures, mineral assemblages, and chemical compositions.
The U-Pb dating provided the clustered ages from faint sector zoned to unzoned zircon in amphibolite (119T03C), and oscillatory or sector zoned zircon in the tonalites (11902D, 119T06D), as well as variation of internal textures and ages of zircon grains in pelitic schists (12001, 120T01O), and actinolite rocks (120T01S, 120T01U) (Fig. 5, Supplementary Table S1). The analyzed zircon grains of the former were characterized by the dominance of euhedral to subhedral forms and high Th/U ratios over 0.20, which indicates their igneous origin (e.g., Hoskin and Schaltegger, 2003). These results suggest that the zircon ages obtained from amphibolite and tonalites reflect the magmatic age for the igneous protolith of amphibolite at 457 Ma and emplacement ages of tonalites at 469-459 Ma. In the tonalite of 119T06D, three older ages of 530-520 Ma were obtained from the oscillatory zoned inherited cores of zircon (Fig. 5c, Supplementary Table S4). We suggest these ages as the age of xenocrystic zircon grains trapped in the tonalitic magma. The U-Pb ages of magmatic zircon in amphibolite and tonalites were older than the hornblende K-Ar age of 373 ± 23 Ma (Shibata and Murakami, 1975), muscovite K-Ar age of 424 Ma (Kawano et al., 1966) and muscovite Rb-Sr age of 406 Ma (Hayase and Ishizaka, 1967), respectively. It indicates the younger K-Ar, and Rb-Sr ages may reflect the timing of cooling or resetting of the isotopic compositions through secondary events such as cataclastic deformation, hydration, or thermal recrystallization by the granitic intrusion. The scattered ages with a variety of the morphology, internal textures, and Th/U ratios of zircon in pelitic schists and actinolite rocks could be regarded as detrital zircon ages derived from their sedimentary protoliths such as mudstone and marl, respectively. The age spectra of their detrital zircon were characterized by the predominant ages of 500-400 Ma and subordinate older ages of early Paleoproterozoic to late Neoproterozoic (Figs. 6 and 7a). The youngest detrital zircon age at ~ 400 Ma was consistently older than the reported radiometric ages of schists at 303-264 Ma which may reflect their metamorphic ages (Nishimura et al., 1983; Shibata and Nishimura, 1989; Igi, 2002 and references therein).
Consequently, this study revealed that amphibolite derived from an Ordovician mafic protolith, Ordovician two-mica tonalites with some inherited zircon and metasedimentary rocks containing Paleoproterozoic to early Devonian detrital zircon, together occur in the NTZ (Table 2).
| Rock type | Nagato Tectonic Zone | Hida Gaien Belt | Kurosegawa Tectonic Belt |
| Amphibolite, Metagabbro Protolith igneous age |
Amphibolite: ~ 460 Ma (This study) |
Amphibolite: ~ 540 Ma, ~ 480 Ma (Ichiyama et al., 2020) |
HP-type metagabbro: ~ 490 Ma (Osanai et al., 2014; Sawada et al., 2020) HT-type metagabbro: ~ 450 Ma (Osanai et al., 2014) |
| Granitoid Magmatic age |
Granitoid: ~ 470-460 Ma + inherited ages (This study) |
- | Granitoid: ~ 470-440 Ma + inherited ages (Osanai et al., 2014; Aoki et al., 2015; Sawada et al., 2020) |
| Metasedimentary rock Detrital zircon age |
Pelitic schist & calcsilicate rock: ~ 2460-400 Ma (This study) |
Blueschist: ~ 3200-390 Ma (Yoshida et al., 2020) |
Pelitic schist & quartzite: ~ 3330-420 Ma (Yoshimoto et al., 2013; Matsunaga et al., 2021) |
The NTZ has been considered as the extension of the HGB (Fig. 1a) on the basis of late Carboniferous to early Permian radiolarian fossils present in the mudstone or chert (Isozaki and Tamura, 1989; Kametaka, 2006) and late Carboniferous to early Permian ages of HP schists (Nishimura et al., 1983; Shibata and Nishimura, 1989). This study clarified the petrographic and geochronological characteristics of amphibolite, metasedimentary rocks, and granitoids in the NTZ, which could be key factors to verify the geological correlation between the NTZ and HGB. In the HGB, the HP metamorphic rocks including jadeitite, eclogite, blueschist, and schist, metagabbro, and amphibolite are present as enclaves in the serpentinite body (e.g., Tsujimori, 2002; Kunugiza et al., 2017; Ichiyama et al., 2020; Yoshida et al., 2020). Ichiyama et al. (2020) determined the U-Pb zircon ages of clinozoisite-bearing gneissose amphibolites which occur as huge bodies within the serpentinite. Their protoliths of oceanic gabbro likely formed at ~ 540 Ma or 480 Ma (Ichiyama et al., 2020). The hydrothermal activity at ~ 520 Ma during the subduction of the oceanic crust produced jadeitite in the HGB (Kunugiza et al., 2017). The blueschist in the HGB was characterized by the U-Pb zircon ages of 3200-390 Ma from its relic detrital cores and ~ 350 Ma from the metamorphic rim (Yoshida et al., 2020). The age spectrum of the detrital cores represented a strong peak at 455 Ma (Fig. 7b). The detrital zircon ages of metasedimentary rocks in the NTZ exhibited a broad concentration between 550 Ma and 400 Ma with the trimodal distribution centered at 498 Ma, 456 Ma, and 420 Ma (Fig. 7a). In addition, the amphibolite (119T03C) in the NTZ showed the protolith igneous age of 457 Ma which is younger than the zircon ages from clinozoisite-bearing amphibolite and jadeitite in the HGB (Table 2). These chronological data in the HGB are likely to differ from those in the NTZ (Figs. 7a and 7b, Table 2). Furthermore, a common lithology of granitoid in the NTZ (e.g., Murakami et al., 1977) is absent in the HGB (Table 2). Therefore, the comparison of lithology and zircon geochronology between the NTZ and HGB indicates gap in their geological correlation. Alternatively, serpentinite mélange associated with early Paleozoic rocks in southwest Japan is the KTB (Fig. 1a, Table 2). The KTB is composed of HP-type metagabbro (e.g., glaucophane-bearing metagabbro, jadeite-glaucophane-bearing metagabbro, and lawsonite-glaucophane-bearing metagabbro), blueschist, high temperature (HT)-type metagabbro (e.g., garnet-clinopyroxene granulite, garnet-bearing or -free amphibolite), granitoids, schists, metachert, Silurian to Devonian sedimentary rocks within serpentinite (e.g., Ichikawa et al., 1956; Osanai et al., 2014; Aoki et al., 2015; Yabuta and Hirajima, 2020). The Permian accretionary complex accompanies the KTB with the fault boundary (e.g., Ichikawa et al., 1956). Osanai et al. (2014) reported the bulk chemistry and zircon geochronology of the metagabbros and granitoids from the Kyushu Island, Shikoku Island, and Kii Peninsula in the KTB. They revealed the magmatic activities of aluminous and S-type granitoids formed at 470-450 Ma, MORB-like magmatism at ~ 490 Ma attributed to form oceanic gabbro as the protoliths of HP-type metagabbros, and gabbroic volcanic arc magmatism producing the precursor of HT-type metagabbros at ~ 450 Ma. The equivalent rocks of the KTB from the Kanto Mountains were dated by Sawada et al. (2020). They also confirmed the evidence of granitic magmatism at ~ 440 Ma and zircon xenocrysts of 2400-500 Ma from leucocratic granitoids which could be equivalent to plagiogranite and garnet granite described in Shimamura et al. (2003). Besides, a fragment of 490 Ma layered gabbro in the oceanic crust was discovered from the lawsonite vein in the blueschist (Sawada et al., 2020). Moreover, the pre-Devonian detrital zircon ages of 3330-420 Ma were obtained from pelitic schists and quartzite throughout the KTB (Yoshimoto et al., 2013; Matsunaga et al., 2021). Their detrital age spectra between 700 and 350 Ma exhibit dominant peaks at 597, 505, 460, and 437 Ma (Fig. 7c). The protolith igneous ages of HT-type metagabbros at ~ 450 Ma in the KTB (Osanai et al., 2014) are well comparable with that of amphibolite in the NTZ (Table 2). The aluminous mineral assemblages such as garnet, biotite, and muscovite, magmatic ages of 470-440 Ma, and older xenocrystic zircon in the S-type granitoids of the KTB (Osanai et al., 2014; Aoki et al., 2015; Sawada et al., 2020) are also similar to those of tonalites (11902D, 119T06D) in the NTZ (Table 2), except for the occurrence of subhedral epidote. In addition, the common patterns of detrital zircon age spectra between the KTB and NTZ can be observed in terms of the strongest trimodal peaks at ~ 500, 460, and 440-420 Ma, as well as various ages ranging from Paleoproterozoic to Neoproterozoic (Figs. 7a and 7c, Table 2), with difference in the peak of detrital zircon age at ~ 600 Ma which is present in the KTB (Fig. 7c). Therefore, the lithological, petrological, and chronological characteristics of HT-type metagabbros, granitoids, and schists in the KTB resemble those in the NTZ (Figs. 7a and 7c, Table 2). Their comparability indicates the NTZ is possibly regarded as an equivalent of the KTB, rather than an extension of the HGB. The equivalents of the KTB HP-type metagabbros could not be found so far in the NTZ. This necessitates detailed investigation and zircon geochronology of metagabbros in the NTZ. The presence of early Permian radiolarian fossils from mudstone in the NTZ is one of the premises for comparison with the Mizuyagadani Formation in the HGB (Kametaka, 2006). However, as Kuwahara et al. (2009) reported and reviewed, these fossils have been discovered from mudstones accompanied by the KTB in Kyushu Island, Shikoku Island, and the Kii Peninsula. It implies that not only early Paleozoic plutonic rocks and metamorphic rocks but also late Paleozoic sedimentary rocks in the NTZ can be compared with those in the KTB. According to Ota and Sakai (1999) who investigated the depositional environments of the Jurassic to Cretaceous sedimentary sequences contacting with the KTB via fault, and covering the NTZ, the same formational history of the sedimentary sequences and compositional transition of their sandstones were recognized in both areas. Ota and Sakai (1999) concluded the NTZ and KTB were originally a continuous terrane along the eastern margin of the Asia continent before the early Cretaceous. The integration of the results of this study and recent stratigraphic studies allow us to suggest the common origin and tectonic evolution of constituent rocks between the NTZ and KTB. Nevertheless, the common prominent peak at 460-455 Ma and various older components of Neoproterozoic to Paleoproterozoic in the detrital age spectra were detected from the HP schists in the NTZ, HGB, and KTB (Fig. 7). The features may indicate their sedimentary precursors might have formed as trench-fill sediments under the Ordovician-Silurian arc-trench system at the margin of the South China craton where Cambrian to Paleoproterozoic basement and extensive Ordovician-Silurian granitoids occur, as proposed by previous studies (e.g., Isozaki et al., 2010; Yoshimoto et al., 2013; Osanai et al., 2014; Aoki et al., 2015; Yoshida et al., 2020; Matsunaga et al., 2021). The similar dominant ages at 505-498, 460-456, and 437-420 Ma of detrital zircon in the NTZ and KTB imply their similar detritus provenances (Fig. 7), but a different one for sedimentary protoliths of the HGB of which detrital zircon is characterized by a single strong peak at 455 Ma (Fig. 7). Furthermore, the discrepancies of constituent rocks within serpentinite between the NTZ-KTB and HGB (Table 2) may reflect the different crustal evolution associated with the HT or HP metamorphism, subduction-related magmatism, and development of accretionary complex and coherent volcano-sedimentary sequences at the margin of east Asia continent before the serpentinite upwelling to form serpentinite mélanges. Therefore, the constituent rocks within serpentinite mélanges of the NTZ-KTB and HGB might have formed under the synchronous arc-trench system with different hinterlands and processes at the continental active margin of the South China craton.
Further field investigation and multi-analyses studies on petrology, geochemistry, and geochronology are required from the NTZ as well as KTB and HGB to reconsider the tectonic framework of the Japanese basement in the early Paleozoic. Especially, the future task of trace elements geochemistry including the Hf isotope ratios of the inherited zircon in amphibolites or metagabbros and detrital zircon in metasedimentary rocks from these early Paleozoic serpentinite mélanges may play a critical role in reconstructing their tectonic environment.
The petrography and U-Pb zircon dating in this study revealed the Nagato Tectonic Zone is an early Paleozoic terrane associated with amphibolite derived from a 460 Ma igneous precursor, 470-460 Ma leucocratic two-mica tonalites which contain older inherited zircon, metasedimentary rocks of pelitic schists and calcsilicate rocks with detrital zircon of 2460-400 Ma in the serpentinite mélange. Compared to the serpentinite mélanges including early Paleozoic blocks in southwest Japan, these results suggest the geological correlation between the Nagato Tectonic Zone and Kurosegawa Tectonic Belt, though the Nagato Tectonic Zone has been considered the western extension of the Hida Gaien Belt. The constituent rocks in the Nagato Tectonic Zone and Kurosegawa Tectonic Belt probably formed with similar detritus provenances and tectonic processes under the arc-trench system at the margin of the South China craton.
We thank M. Owada, T. Adachi, E.S. Rezaei., B. Tsogtbaatar, G.N. Abdulzada, T. Battogtokh, M. Bold, and U. Sergelen for helping with the fieldwork. T. Adachi also gave some comments which improved the manuscript. We are grateful to K. Das for his helpful English correction of our manuscript. We greatly appreciate K. Horie and an anonymous reviewer for their constructive reviews, especially about chronological data and introduction, and T. Kawakami for the editorial support. This work was supported by JSPS KAKENHI Grants Numbers JP21253008, JP22244063, and JP16H02743 to Y. Osanai and JP15K05345, JP18H01316, and JP21K18381 to N. Nakano.
Supplementary Tables S1-S8 are available online from https://doi.org/10.2465/jmps.220630.
