Cambrian Plutonism in Northeast Japan and Its Significance for The

Journal of Asian Earth Sciences 108 (2015) 136–149

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Journal of Asian Earth Sciences

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Cambrian plutonism in Northeast Japan and its signiﬁcance for the earliest arc-trench system of proto-Japan: New U–Pb zircon ages of the oldest granitoids in the Kitakami and Ou Mountains ⇑ Y. Isozaki a, , M. Ehiro b, H. Nakahata a, K. Aoki a,1, S. Sakata c, T. Hirata c a Department of Earth Science & Astronomy, The University of Tokyo, Komaba, Meguro, Tokyo 153-8902, Japan b The Tohoku University Museum, Sendai 980-8578, Japan c Department of Geology & Mineralogy, Kyoto University, Sakyo, Kyoto 606-8502, Japan article info abstract

Article history: In order to clarify the early history of Japan, particularly during the Early Paleozoic, pre-Devonian gran- Received 31 October 2014 itoids in the South Kitakami Belt (SKB), NE Japan were dated by U–Pb zircon age by laser-ablation induc- Received in revised form 16 April 2015 tively coupled plasma-mass spectrometry (LA-ICP-MS). Two samples of diorite/tonalite from the Accepted 23 April 2015 Nagasaka area (Shoboji Diorite) and two of mylonitic tonalite from the Isawa area (Isawagawa Available online 30 April 2015 Tonalite) yielded late Cambrian ages (500–490 Ma) for the primary magmatism. These ages newly identify a ca. 500 Ma (late Cambrian) arc plutonism in central NE Japan, which has not been recognized pre- Keywords: viously and has the following geological signiﬁcance. The Cambrian granitoids are the oldest felsic Zircon U–Pb age plutonic rocks in NE Japan, which are independent of the previously known ca. 450 Ma (latest Granitoid Arc Ordovician) Hikami Granite in SKB. The Cambrian granitoids are extremely small in size at present but Cambrian likely had a much larger distribution primarily, at least 30 km wide and potentially up to 80 km wide NE Japan in a cross-arc direction. Their southerly extension was recognized in the Hitachi area (ca. 200 km to the south) and in central Kyushu (ca. 1500 km to SW). They likely represent remnants of the same mature arc plutonic belt in Early Paleozoic Japan, which developed in western Panthalassan (paleo-Paciﬁc) margin, as well as the Khanka block in Far East Russia. The extremely small size of the Cambrian granitoids at present can be best explained by intermittent, severe tectonic erosion since the Paleozoic. This Cambrian arc granitoid belt likely developed from Primorye possibly to eastern Cathaysia (South China) via Japan. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction include the occurrence of ca. 520 Ma (early Cambrian) metaso- matic zircon in gneiss (Kunugiza and Goto, 2010), ca. 510– Before the end of the last century, the overall geotectonic 500 Ma (middle-late Cambrian) granitoid (Sakashima et al., 2003; framework of Japan and its over 500 million year-long history Tagiri et al., 2010), ca. 480 Ma (earliest Ordovician) ophiolites was documented mainly by virtue of the identiﬁcation of various (Ozawa, 1988; Nishimura and Shibata, 1989), and ancient subduction-related orogenic units in Southwest Japan, mid-Ordovician felsic volcaniclastics (Tsukada and Koike, 1997; such as accretionary complex, blueschist, ophiolite, arc granite, Nakama et al., 2010a; Shimojo et al., 2010). Although these rocks and arc-related basin (e.g., Maruyama and Seno, 1986; Isozaki, indicate that the arc-trench system of proto-Japan had developed 1996; Maruyama et al., 1997). Proto-Japan was born as a part of by the mid-Cambrian (ca. 520–500 Ma) (Isozaki et al., 2010), a passive continental margin by the breakup of the supercontinent details of the onset of timing of subduction, in particular, that of Rodinia during the late Neoproterozoic, and it experienced a major the tectonic inversion, still remain a mystery, owing mostly to tectonic turnover from a passive margin to an active one sometime the sporadic/fragmentary occurrence of these rocks in much in the earliest Paleozoic (Isozaki et al., 2010). The main lines of evi- younger units. dence for the oldest subduction along the proto-Japan margin The data for the Early Paleozoic evolution of Northeast Japan are more limited because of the relatively thick/extensive cover of ⇑ Cenozoic volcanic rocks and sediments (Fig. 1). Nonetheless, as in Corresponding author. SW Japan, the development of an Early Paleozoic arc-trench system E-mail address: [email protected] (Y. Isozaki). in NE Japan has been suggested by the occurrence of 524–498 Ma 1 Current address: Department of Applied Science, Okayama University of Science, Okayama 700-0005, Japan. (Cambrian) amphibolite (Kanisawa et al., 1992), Ordovician– http://dx.doi.org/10.1016/j.jseaes.2015.04.024 1367-9120/Ó 2015 Elsevier Ltd. All rights reserved. Y. Isozaki et al. / Journal of Asian Earth Sciences 108 (2015) 136–149 137

Fig. 1. Index map of pre-Cenozoic rocks in NE Japan (modiﬁed from Oide et al., 1989), showing the localities of interest; e.g., Kitakami Mountains, Ou Mountains, and the Hitachi area. The Nagasaka and Isawa areas are located in the west-central part of the South Kitakami Belt (SKB). Note that the current volcanic front extends in a N–S direction along the middle of the Ou Mountains. 138 Y. Isozaki et al. / Journal of Asian Earth Sciences 108 (2015) 136–149

Silurian arc-related igneous and volcaniclastic sedimentary rocks; Nameirizawa formations (the oldest strata in the SKB; Ehiro e.g., ca. 480 Ma ophiolite (Ozawa, 1988), ca. 460–450 Ma et al., 1988), whereas the ca. 450 Ma Hikami Granite is uncon- (middle-late Ordovician) granitoids (e.g., Sasada et al., 1992; formably overlain by the Silurian Kawauchi and Okuhinotsuchi for- Kanisawa and Ehiro, 1997; Asakawa et al., 1999), and Silurian mations, i.e. the oldest fossil-bearing strata in NE Japan (Murata fossil-bearing shallow marine strata (e.g., Kitakami Paleozoic et al., 1974, 1982; Kitakami Paleozoic Research Group, 1982; Research Group, 1982) from the South Kitakami Belt (SKB). These Kawamura et al., 1990). igneous and metamorphic rocks were only dated by conventional Besides the Hikami Granite in the central part of the SKB, there hornblende K–Ar and whole-rock Rb–Sr methods, and thus require is an isolated occurrence of similar dioritic/gabbroic rocks in the more reliable modern ages by e.g. single zircon laser-ablation Nagasaka area on the western margin of the Kitakami Mountains inductively coupled plasma-mass spectrometry (LA-ICP-MS) or (Figs. 1, and 2A), which are structurally sandwiched between sensitive high-resolution ion microprobe (SHRIMP). apparently underlying 500 Ma metamorphic rocks (Motai meta- However, some granitoids do have U–Pb zircon ages, for exam- morphic rocks associated with amphibolite and serpentinite) and ple the largest (8 km N–S and 14 km E–W) Early Paleozoic granitic the overlying unmetamorphosed Upper Devonian Tobigamori body (traditionally called the Hikami Granite) (Watanabe et al., Formation. This plutonic unit exposed in a small area of 4 km 1995; Shimojo et al., 2010; Sasaki et al., 2013) in the central N–S by 3 km E–W was distinguished from the Hikami Granite Kitakami Mountains of NE Japan has a U–Pb age of 470–410 Ma (with the name of Shoboji Diorite) by K–Ar hornblende ages of (Watanabe, 1950; Murata et al., 1982; Kitakami Paleozoic 446–432 Ma (latest Ordovician to Early Silurian) at 5 localities Research Group, 1982; Fig. 1), updated recently to ca. 450 Ma by (Kanisawa and Ehiro, 1997). Sasaki et al. (2013). Furthermore, there is a much smaller (500 m 500 m) myloni- In the Hitachi area at the southern tip of NE Japan, more than tic Isawagawa Tonalite (Kobayashi et al., 2000) in the Isawa area on 200 km to the south of the Kitakami Mountains (Fig. 1), a notable the southern flank of Mt. Yakeishi-dake in the Ou Mountains, ca. meta-granitoid has a U–Pb zircon age of 510–500 Ma (Sakashima 30 km to the west of the Shoboji area (Figs. 1, 2B). This tonalite et al., 2003; Tagiri et al., 2010); however, its direct connection with is associated with amphibolites, which are correlated with the the main Hikami Granite remains unclear. Motai metamorphic rocks (Kitamura and Kanisawa, 1971). The On the other hand, detrital zircons in Lower-Middle Paleozoic rock type and rock association of the tonalite and amphibolite terrigenous clastics in NE Japan include those with ca. 500 Ma are basically identical to those in the Shoboji area described above. U–Pb ages (e.g., Okawa et al., 2013; Isozaki et al., 2014), suggesting Very thick Cenozoic volcanic/sedimentary rocks in the Ou that there was once a much older (>450 Ma) arc granite belt Mountains conceal most of their basement, except for small win- exposed in proto-Japan in the Middle Paleozoic (Isozaki et al., dows of pre-Cenozoic rocks; nonetheless, on the eastern side of 2010). Nonetheless, direct documentation of such an older grani- the HTL, the basement is considered to form the western extension toid body in the main part of NE Japan appears to be necessary of the Paleozoic–Mesozoic rocks of the SKB (Fig. 1). Sasada (1985) in order to prove the putative development of the initial arc crust described the basic petrology of the mylonitic tonalite in the Isawa in proto-Japan. area, and Sasada et al. (1992) determined K–Ar hornblende ages of The present study aims to date U–Pb ages of co-magmatic zir- 457 Ma (Late Ordovician), which are similar to those of the Shoboji con crystals from the pre-Devonian granitoids in central NE Diorite. Japan, in order to identify older granitoids and/or confirm/re-evaluate previously reported K–Ar hornblende ages. This article reports our new geochronological data from two gran- 3. Samples itoid bodies that have a much older late Cambrian age, which enables us to discuss their geological significance not only in In order to check the U–Pb zircon ages of the Shoboji Diorite and Japan but also in East Asia. Isawagawa Tonalite, we collected and analyzed 4 samples of granitoids, two each from the Nagasaka (Fig. 2A) and Isawa areas (Fig. 2B); brief descriptions are given below. 2. Geologic setting

NE Japan extends from northern Kanto to Hokkaido with its centre in the Tohoku region (Fig. 1). Regarding the pre-Cenozoic 3.1. Shoboji Diorite (Fig. 2A) basement geology, the main source of information has been the SKB in the southern half of the Kitakami Mountains, which is sep- 3.1.1. SB-0 arated on the west from the Gosaisho Belt (equivalent to the Ryoke Hornblende gabbroic diorite collected just beneath a road 0 00 0 00 Belt in SW Japan) by the NS-trending Hatagawa Tectonic Line bridge near the Shoboji temple (N39°03 59 , E141°13 26 ; the (HTL). The SKB is unique in Japan in having a thick pile of the same outcrop as S-8 and S-9 of Kanisawa and Ehiro, 1997). Paleozoic–Mesozoic sedimentary rocks of shelf facies (Kawamura Analyzed zircons were separated from a coarse-grained, et al., 1990); this is in remarkable contrast to the rest of Japan that plagioclase-rich diorite with semi-euhedral to anhedral phe- is composed mostly of subduction-related accretionary complexes nocrysts of plagioclase and hornblende. The major and trace ele- and their metamorphosed equivalents (Isozaki, 1996; Isozaki et al., ment chemistry of the diorite from this outcrop was previously 2010). Details of the basement geology of the SKB have not yet reported by Kanisawa and Ehiro (1997) as cited in Table 1. been fully clariﬁed; however, the age of ca. 480 Ma (Early Ordovician) for the Hayachine ophiolite and ca. 450 Ma (latest Ordovician) for the sub-Devonian Hikami Granite were already 3.1.2. SB-2 determined by U–Pb zircon dating (Shimojo et al., 2010; Sasaki Hornblende-biotite tonalite collected along a forest road et al., 2013). The 480 Ma Hayachine ophiolite (Hayachine (N39°0403400, E141°1401900; the same outcrop as S-4 of Kanisawa Complex of Ehiro et al., 1988) with arc-related geochemical charac- and Ehiro, 1997). Zircons were separated from a quartz-rich tona- teristics (Ozawa, 1988) is mainly composed of serpentinized peri- litic part of heterogeneous gabbro/tonalite, with apatite, zircon, dotite, gabbro, dolerite, trondhjemite and basalt, and covered magnetite, and ilmenite. A hornblende K–Ar age of 434 ± 20 Ma conformably by the Ordovician–Silurian Yakushigawa and was reported from this outcrop (Kanisawa and Ehiro, 1997). Y. Isozaki et al. / Journal of Asian Earth Sciences 108 (2015) 136–149 139

141 15’ 30’’ E

39° 04’ 00’’ N

Fig. 2. Geologic sketch maps and photographs of rocks in the Nagasaka area in the Kitakami Mountains and the Isawa area in the Ou Mountains. (A) Geologic sketch map of the Nagasaka area near the Shoboji Temple with sample localities of SB-0 and SB-2 (modiﬁed from Kanisawa and Ehiro, 1997), (B) geologic sketch map of the Isawa area with sample localities YK-2 and YK-4 (modiﬁed from Sasada et al., 1992), (C) polished surface of the Shoboji Diorite (SB-2), (D–F) roadside outcrop, chipped surface, and thin section view of the Isawagawa Tonalite (YK-2, 4).

Table 1 Major and trace element composition of the Shoboji Diorite and Isawagawa Tonalite (compiled from Kanisawa and Ehiro, 1997; Kobayashi et al., 2000). Note the mutual similarity between the two granitoids, which is concordant with their almost identical U–Pb zircon ages.

Unit Shoboji Diorite (Kanisawa and Ehiro, 1997) Isawagawa Tonalite (Kobayashi et al., 2000) Sample No. S2 S3 S9 93112123e 93112125e Yak-2 Yak-5

SiO2 55.17 56.96 52.35 59.29 59.59 61.70 56.87

TiO2 0.79 0.72 1.94 0.49 0.48 0.45 0.58

Al2O3 17.07 13.94 16.22 15.33 15.20 14.53 15.84 ⁄ Fe2O3 7.56 8.78 10.53 7.60 7.37 7.25 8.85 MnO 0.12 0.13 0.19 0.14 0.13 0.13 0.16 MgO 5.12 6.03 4.47 3.94 3.73 3.46 4.09 CaO 5.93 6.54 6.64 5.19 5.48 5.64 6.50

Na2O 5.00 4.71 5.19 3.19 3.38 3.70 3.36

K2O 1.20 0.75 0.85 1.66 1.12 1.31 1.06

P2O5 0.09 0.12 0.11 0.23 0.21 0.24 0.30 Total 100.00 99.99 100.00 96.96 96.69 98.41 97.59

Ba 310 200 191 510.3 402.1 439.0 399.5 Nb 6.3 4.7 6.5 6.8 6.3 5.9 5.1 Zr 92 7 52 105.3 132.5 97.0 116.9 Y 23 7 18 13.4 12.6 12.3 13.0 Sr 545 319 476 419.3 402.8 377.8 471.4 Rb 12 10 10 27.1 17.0 17.8 16.7 Zn – – – 70.2 70.1 56.3 68.1 Cu – – – 95.6 78.0 48.5 109.4 Ni 45 110 10 13.1 14.0 5.4 11.1 Co 29.7 38.0 28.7 22.8 21.8 110.2 79.4 Cr 102 246 12 57.4 61.5 46.3 43.0

Major elements in wt%; Trace elements in ppm. 140 Y. Isozaki et al. / Journal of Asian Earth Sciences 108 (2015) 136–149

3.2. Isawagawa Tonalite (Fig. 2B) for each analysis were collected over 80 and 8 s, respectively. Acquired data consisted of multiple groups of 10 unknowns sample 3.2.1. YK-2 bracketed by quartets of analyses of the 91500 zircon standard Mylonitic hornblende tonalite collected along the main high- (Wiedenbeck et al., 1995, 2004) following a single background way #397 (N39°604000; E140°5100000; the same outcrop as Loc. analysis. 8990603 of Sasada et al., 1992). Zircons were separated from a part Throughout the entire analysis, 202Hg was monitored to correct dominated by deformed hornblende and plagioclase, with poly- the isobaric interference of 204Hg on 204Pb. Zircon ages can be cor- crystalline quartz, biotite, chlorite, epidote, and sericite. A horn- rected for common Pb contamination based on measurement of blende K–Ar age of 457 ± 10 Ma was reported from this outcrop 204Pb (e.g. Stern, 1997; Storey et al., 2006; Chew et al., 2014). In (Sasada et al., 1992). this study, due to the very low 204Pb counts, it was not possible to apply the common Pb correction with sufficient precision. 3.2.2. YK-4 Therefore, we rejected the data which showed a significant level 206 204 Mylonitic hornblende tonalite collected along the main high- of the contribution from common Pb ( Pb/ Pb)total < 1000, 207 204 way at the eastern entrance of a tunnel (N39°604200, E140°5005500; ( Pb/ Pb)total < 100) and thus no common Pb correction was between the outcrops of 8990603 and 8990702 of Sasada et al., made. 1992). Zircons were separated from a part dominated by deformed All uncertainties are quoted at the 2 sigma level to which hornblende and plagioclase, with polycrystalline quartz, biotite, repeatability of measurements of the primary standard is propa- 235 238 chlorite, epidote, and sericite. gated based on quadratic addition. U was calculated from U 238 235 Although the sample localities were not specifically shown, using a U/ U ratio of 137.88 (Jaffey et al., 1971). During the Kobayashi et al. (2000) reported major and trace element chem- analysis procedure, Plešovice (337.13 ± 0.37 Ma; Sláma et al., istry of this tonalite (cited in Table 1), which is exposed along 2008) and AS-3 (1099.1 ± 0.2 Ma; Schmitz et al., 2003) zircons the main highway (Fig. 2B). were measured as secondary standards for quality control. The summary of instrumental settings and the resulting ages of secondary zircons are shown in the appendix (Table A). For U–Pb dat- 4. Analytical procedures ing of individual zircon by LA-ICP-MS, 18, 37, 17, and 41 data were eventually obtained from the samples SB-0, SB-2, YK-2, and YK-4, Zircon extraction was performed with the mineral separation respectively. system at the Tokyo Institute of Technology. After crushing and sieving of a 500 g rock sample, magnetic and heavy-liquid sepa- 5. Results rations were performed. Then, separated zircons were mounted on a 5–10 mm acrylic disc and were polished. Internal structures of Measurement results are shown in Tables 2–5 and in Figs. 3 and the zircons and the presence of inclusions were checked by trans- 4. The weighted average ages of studied samples were calculated mitted and reflected optical microscopy and cathodoluminescence by means of Isoplot/Ex 3 (Ludwig, 2003). The sample SB-0 has all (CL) imaging. CL images were obtained using a JEOL JSM-5310 analyzed zircons properly plotted on the Concordia line, whereas scanning electron microprobe combined with an Oxford CL system the sample SB-2 has 31 zircons out of 37 plotted on the at the Tokyo Institute of Technology. Concordia line (Fig. 3). Age distributions of individual zircons indi- In situ zircon U–Pb dating was carried out using a Nu AtttoM cate that two samples have almost identical weighted average ages single-collector ICP-MS (Nu instruments, Wrexham, UK) coupled with similar error ranges; i.e. 493.2 ± 4.8 Ma (95% confidence, to a NWR-193 laser-ablation system (ESI, Portland, US), that uti- MSWD = 1.3) for sample SB-0 and 493.2 ± 4.0 Ma (95% confidence, lizes a 193 nm ArF excimer laser at the Department of Geology MSWD = 2.5) for SB-2. Judging from the clear oscillatory zoning in and Mineralogy, Kyoto University. The laser was operated with all analyzed zircons, the obtained weighted average age of the zir- an output energy of 9 mJ per pulse, repetition rate of 8 Hz and cons is regarded as the primary crystallization age of the host laser spot size of 15 lm in diameter, providing an estimated power Shoboji Diorite. density of the sample of <2.5 J cm2. The total count of the laser In contrast, the two mylonitic tonalite samples from the Isawa pulse during the ablation was 180 shots. area (YK-2 and YK-4) contain zircons with wider age ranges, partic- The ablation took place in helium gas within the sample cell ularly with many zircons with ages that plot far from the Concordia of <1 ml, and then the ablated sample, aerosol and helium gas were line (Fig. 4). Seven grains out of 27 from YK-2, and 30 of 49 from mixed with argon gas downstream of the cell. The helium minimizes YK-4 plot properly on the Concordia line, and their weighted aver- redeposition of ejecta or condensates, while argon provides efficient age age is 494.7 ± 7.7 Ma (95% confidence, MSWD = 2.2) for YK-2, sample transport to the ICP-MS (Eggins et al., 1998; Günther and and 497.4 ± 4.4 Ma (95% confidence, MSWD = 2.4) for YK-4, respec- Heinrich, 1999; Jackson et al., 2004). A signal-smoothing device tively (Fig. 4). These zircon ages suggest that the tonalite magma was used (Tunheng and Hirata, 2004). To reduce the isobaric inter- crystallized around 490–500 Ma. These igneous ages of zircon from ference, an Hg-trap device with an activated charcoal filter was the Isawa area are very similar within error ranges to those from applied to the Ar make-up gas before mixing with the He carrier the Shoboji Diorite in the Nagasaka area. gas (Hirata et al., 2005). Prior to each individual analysis, regions of interest were pre-ablated using a few pulses of the laser with a laser spot size diameter of 20 or 30 lm to remove potential surface con- 6. Discussion tamination, dramatically reducing common Pb contamination (Iizuka and Hirata, 2004). The ICP-MS was optimized using continu- 6.1. Cambrian plutons in NE Japan ous ablation of a 91500 zircon standard (Wiedenbeck et al., 1995, 2004), a NIST SRM 610 to provide maximum sensitivity while main- The present study identified for the first time the occurrence of taining low oxide formation (232Th16O+/232Th < 1%). Data were Cambrian plutonic bodies in the Kitakami and Ou mountains in acquired on six isotopes, 202Hg, 204Pb, 206Pb, 207Pb, 232Th and 238U central NE Japan. The Shoboji Diorite at two localities in the by measuring the signal intensity at the peak top. Nagasaka area (Fig. 2A) yielded ca. 493 Ma igneous zircons The ages and isotope ratios of zircon samples were calculated (Fig. 4A and B). Much younger K–Ar ages of 446–432 Ma (latest using an in-house excel spreadsheet. Gas blank and ablation data Ordovician–Silurian) were previously reported for hornblende in Y. Isozaki et al. / Journal of Asian Earth Sciences 108 (2015) 136–149 141

Table 2 LA-ICP-MS U–Pb isotopic analytical data for separated zircons in sample SB-0 from the Shoboji Diorite in the Nagasaka area, South Kitakami Belt (SKB).

Grain number 206Pb/238U 207Pb/235U 207Pb/206Pb Age (Ma) 206Pb/238U 207Pb/235U 1 0.0778 ± 0.00312 0.6348 ± 0.04320 0.0033 ± 0.00369 482.8 ± 18.70 499.1 ± 27.19 2 0.0820 ± 0.00332 0.6619 ± 0.04640 0.0034 ± 0.00287 508.0 ± 19.81 515.8 ± 28.75 3 0.0782 ± 0.00303 0.6247 ± 0.03750 0.0027 ± 0.00256 485.4 ± 18.17 492.8 ± 23.71 4 0.0800 ± 0.00309 0.6320 ± 0.03732 0.0026 ± 0.00272 496.2 ± 18.48 497.3 ± 23.49 5 0.0790 ± 0.00306 0.6216 ± 0.03715 0.0026 ± 0.00253 490.3 ± 18.32 490.9 ± 23.53 6 0.0798 ± 0.00309 0.6165 ± 0.03664 0.0025 ± 0.00300 494.8 ± 18.45 487.6 ± 23.28 7 0.0799 ± 0.00310 0.6283 ± 0.03779 0.0026 ± 0.00307 495.4 ± 18.53 495.1 ± 23.84 8 0.0799 ± 0.00310 0.6379 ± 0.03842 0.0027 ± 0.00324 495.3 ± 18.54 501.0 ± 24.10 9 0.0795 ± 0.00315 0.6168 ± 0.04057 0.0030 ± 0.00323 493.0 ± 18.87 487.9 ± 25.80 10 0.0795 ± 0.00306 0.6221 ± 0.03633 0.0025 ± 0.00236 493.1 ± 18.32 491.1 ± 23.00 11 0.0835 ± 0.00262 0.6371 ± 0.05754 0.0047 ± 0.00595 517.1 ± 15.60 500.5 ± 36.33 12 0.0785 ± 0.00246 0.5986 ± 0.05408 0.0047 ± 0.00228 487.2 ± 14.73 476.3 ± 34.95 14 0.0772 ± 0.00252 0.6162 ± 0.05821 0.0051 ± 0.00412 479.1 ± 15.09 487.4 ± 37.25 16 0.0790 ± 0.00293 0.5830 ± 0.06461 0.0056 ± 0.00315 489.9 ± 17.53 466.4 ± 42.31 17 0.0792 ± 0.00240 0.6247 ± 0.05419 0.0046 ± 0.00726 491.6 ± 14.38 492.8 ± 34.44 18 0.0810 ± 0.00258 0.5916 ± 0.05467 0.0046 ± 0.00301 502.0 ± 15.41 471.9 ± 35.49 19 0.0803 ± 0.00246 0.6129 ± 0.05392 0.0046 ± 0.00396 497.9 ± 14.72 485.4 ± 34.52 20 0.0778 ± 0.00241 0.5920 ± 0.05271 0.0046 ± 0.00396 483.0 ± 14.43 472.1 ± 34.19

All errors are quoted at 2r.

Table 3 LA-ICP-MS U–Pb isotopic analytical data for separated zircons in sample SB-2 from the Shoboji Diorite in the Nagasaka area, SKB. Measurements plotted not on Concordia line are shown in italics (the same for the following tables).

Grain number 206Pb/238U 207Pb/235U 207Pb/206Pb Age (Ma) 206Pb/238U 207Pb/235U 1 0.0792 ± 0.002 0.603 ± 0.045 0.0555 ± 0.0012 491.3 ± 12 481 ± 21 2 0.0789 ± 0.0024 0.614 ± 0.041 0.057 ± 0.0019 490 ± 14 484 ± 24 3 0.0797 ± 0.0028 0.624 ± 0.028 0.056 ± 0.0017 494 ± 16 486 ± 20 4 0.0774 ± 0.0026 0.62 ± 0.025 0.0588 ± 0.0022 480 ± 15 491 ± 16 5 0.0766 ± 0.0023 0.563 ± 0.027 0.0542 ± 0.0022 476 ± 14 452 ± 18 6 0.0777 ± 0.0024 0.555 ± 0.033 0.052 ± 0.0031 482 ± 14 452 ± 20 7 0.0795 ± 0.0025 0.62 ± 0.031 0.0575 ± 0.0025 493 ± 15 488 ± 20 8 0.0809 ± 0.0025 0.682 ± 0.03 0.0634 ± 0.0033 501 ± 15 527 ± 18 9 0.085 ± 0.0026 0.674 ± 0.034 0.0576 ± 0.0026 526 ± 15 525 ± 20 10 0.08 ± 0.0026 0.638 ± 0.034 0.0603 ± 0.0032 496 ± 16 504 ± 19 11 0.0826 ± 0.0032 0.643 ± 0.029 0.0561 ± 0.0026 511 ± 19 502 ± 19 12 0.0822 ± 0.0027 0.641 ± 0.026 0.0566 ± 0.0019 509 ± 16 502 ± 17 13 0.0841 ± 0.003 0.658 ± 0.028 0.0572 ± 0.0022 520 ± 18 504 ± 24 14 0.0819 ± 0.0028 0.647 ± 0.03 0.0573 ± 0.0033 508 ± 17 504 ± 20 15 0.0794 ± 0.0028 0.652 ± 0.033 0.059 ± 0.0029 492 ± 17 511 ± 22 16 0.0785 ± 0.003 0.667 ± 0.036 0.0591 ± 0.0035 487 ± 18 516 ± 23 17 0.0827 ± 0.0026 0.692 ± 0.029 0.0603 ± 0.0029 512 ± 16 532 ± 18 18 0.0791 ± 0.0022 0.633 ± 0.025 0.058 ± 0.0023 490 ± 13 500 ± 16 19 0.0803 ± 0.0027 0.615 ± 0.021 0.0562 ± 0.0022 498 ± 16 486 ± 15 20 0.0778 ± 0.0029 0.611 ± 0.028 0.0577 ± 0.0025 483 ± 17 487 ± 22 21 0.0783 ± 0.0023 0.622 ± 0.027 0.0569 ± 0.0028 486 ± 13 489 ± 18 22 0.0787 ± 0.0029 0.649 ± 0.065 0.0588 ± 0.0067 488 ± 17 508 ± 35 23 0.0781 ± 0.0026 0.606 ± 0.028 0.0554 ± 0.0023 485 ± 16 483 ± 17 24 0.0792 ± 0.0024 0.617 ± 0.033 0.0564 ± 0.003 491 ± 14 493 ± 19 25 0.0799 ± 0.0022 0.624 ± 0.028 0.0564 ± 0.0021 496 ± 13 494 ± 18 26 0.0772 ± 0.0026 0.6 ± 0.038 0.0541 ± 0.0027 479 ± 15 475 ± 24 27 0.0785 ± 0.0026 0.598 ± 0.059 0.0551 ± 0.0042 487.1 ± 15 475 ± 31 28 0.0783 ± 0.0025 0.637 ± 0.032 0.0578 ± 0.0027 486 ± 15 498 ± 20 29 0.0845 ± 0.0028 0.637 ± 0.03 0.0552 ± 0.0023 523 ± 17 503 ± 20 30 0.0808 ± 0.0029 0.643 ± 0.038 0.0566 ± 0.0034 501 ± 17 501 ± 24 31 0.0809 ± 0.0025 0.642 ± 0.025 0.057 ± 0.002 501 ± 15 505 ± 15 32 0.0786 ± 0.0023 0.619 ± 0.029 0.0556 ± 0.0024 488 ± 14 488 ± 18 33 0.0775 ± 0.0023 0.629 ± 0.03 0.057 ± 0.0026 481 ± 14 494 ± 19 34 0.0793 ± 0.0031 0.62 ± 0.03 0.0559 ± 0.0024 492 ± 19 488 ± 19 35 0.079 ± 0.0025 0.608 ± 0.029 0.0558 ± 0.0026 490 ± 15 487 ± 19 36 0.0794 ± 0.0027 0.677 ± 0.03 0.0613 ± 0.0028 493 ± 16 524 ± 19 37 0.0772 ± 0.0023 0.599 ± 0.033 0.0557 ± 0.0028 479 ± 14 474 ± 21

All errors are quoted at 2r. the gabbro-diorite from the same unit in the same area (Kanisawa around 493 Ma. According to the latest geological timescale, the and Ehiro, 1997); however, on the basis of the robustly consistent age of 493 Ma corresponds to the late Cambrian (Peng et al., 2012). zircon U–Pb ages that are nearly 50–60 million years older, the The current exposure of the Shoboji Diorite in the Nagasaka K–Ar ages with lower closure temperature are considered to area is limited merely to an area of 3 km in N–S and 2 km in represent cooling ages after the primary magma consolidation E–W (Kanisawa and Ehiro, 1997; Fig. 2). Nonetheless the present 142 Y. Isozaki et al. / Journal of Asian Earth Sciences 108 (2015) 136–149

Table 4 LA-ICP-MS U–Pb isotopic analytical data for separated zircons in sample YK-2 from the Isawagawa Tonalite in the Isawa area, SKB.

Grain number 206Pb/238U 207Pb/235U 207Pb/206Pb Age (Ma) 206Pb/238U 207Pb/235U 1 0.0769 ± 0.0016 0.655 ± 0.037 0.0624 ± 0.0034 477.5 ± 9.4 509 ± 23 2 0.0814 ± 0.0021 0.635 ± 0.028 0.0573 ± 0.0022 505 ± 13 503 ± 20 3 0.0766 ± 0.0023 0.631 ± 0.034 0.0589 ± 0.0032 476 ± 14 512 ± 28 4 0.0788 ± 0.0021 0.63 ± 0.028 0.0576 ± 0.003 489 ± 13 494 ± 18 5 0.08 ± 0.0014 0.62 ± 0.024 0.0582 ± 0.0028 496.3 ± 8.6 492 ± 16 6 0.0786 ± 0.0016 0.65 ± 0.027 0.0597 ± 0.0023 487.8 ± 9.5 507 ± 17 7 0.0804 ± 0.002 0.644 ± 0.027 0.059 ± 0.0028 498 ± 12 503 ± 16 8 0.0819 ± 0.002 0.698 ± 0.033 0.0628 ± 0.0032 508 ± 12 546 ± 21 9 0.0795 ± 0.0036 0.713 ± 0.051 0.0662 ± 0.0048 493 ± 21 543 ± 30 10 0.0782 ± 0.003 0.66 ± 0.042 0.0619 ± 0.0028 485 ± 18 511 ± 25 11 0.0836 ± 0.0019 0.634 ± 0.028 0.0562 ± 0.0023 517 ± 11 497 ± 17 12 0.0818 ± 0.0017 0.724 ± 0.032 0.0646 ± 0.0031 507 ± 10 562 ± 18 13 0.0786 ± 0.002 0.669 ± 0.034 0.0612 ± 0.0031 488 ± 12 518 ± 21 14 0.0807 ± 0.0016 0.627 ± 0.03 0.0561 ± 0.0025 500.4 ± 9.8 495 ± 18 15 0.0789 ± 0.0019 0.639 ± 0.024 0.0606 ± 0.0023 489 ± 12 501 ± 15 16 0.0783 ± 0.0017 0.648 ± 0.03 0.0608 ± 0.0031 486 ± 10 506 ± 19 17 0.0878 ± 0.0025 0.78 ± 0.042 0.063 ± 0.0033 542 ± 15 583 ± 23

All errors are quoted at 2r.

Table 5 LA-ICP-MS U–Pb isotopic analytical data for separated zircons in sample YK-4 from the Isawagawa Tonalite in the Isawa area, SKB.

Grain number 206Pb/238U 207Pb/235U 207Pb/206Pb Age (Ma) 206Pb/238U 207Pb/235U 1 0.0797 ± 0.0024 0.633 ± 0.033 0.0573 ± 0.0032 494 ± 14 496 ± 21 2 0.0812 ± 0.0025 0.63 ± 0.03 0.0567 ± 0.003 503 ± 15 498 ± 20 3 0.0774 ± 0.0022 0.613 ± 0.029 0.0582 ± 0.003 480.4 ± 13 490 ± 18 4 0.0805 ± 0.0024 0.63 ± 0.026 0.0567 ± 0.0022 499 ± 14 497 ± 15 5 0.0817 ± 0.0025 0.661 ± 0.03 0.0594 ± 0.0034 506 ± 15 514 ± 19 6 0.0774 ± 0.0025 0.649 ± 0.042 0.0599 ± 0.004 480 ± 15 506 ± 26 7 0.0819 ± 0.0026 0.665 ± 0.031 0.0591 ± 0.0026 507 ± 15 520 ± 20 8 0.0826 ± 0.0027 0.656 ± 0.048 0.0576 ± 0.0031 511 ± 16 511 ± 26 9 0.0788 ± 0.003 0.626 ± 0.055 0.0572 ± 0.0025 489 ± 19 493 ± 25 10 0.0792 ± 0.003 0.649 ± 0.068 0.058 ± 0.003 491 ± 18 507 ± 29 11 0.0788 ± 0.0031 0.603 ± 0.092 0.0561 ± 0.0047 489 ± 18 477 ± 41 12 0.0789 ± 0.003 0.611 ± 0.079 0.0562 ± 0.0035 489 ± 18 486 ± 36 13 0.0794 ± 0.0026 0.623 ± 0.061 0.0567 ± 0.0028 493 ± 15 494 ± 26 14 0.0834 ± 0.0028 0.718 ± 0.12 0.0621 ± 0.0048 517 ± 17 545 ± 45 15 0.0793 ± 0.0026 0.649 ± 0.049 0.0597 ± 0.0035 492 ± 16 507 ± 26 16 0.0803 ± 0.0027 0.661 ± 0.049 0.0593 ± 0.003 498 ± 16 513 ± 25 17 0.0809 ± 0.0026 0.636 ± 0.028 0.0564 ± 0.0029 502 ± 16 498 ± 18 18 0.0799 ± 0.0023 0.622 ± 0.028 0.0564 ± 0.0029 495 ± 14 490 ± 18 19 0.0775 ± 0.0018 0.617 ± 0.035 0.0574 ± 0.0031 481 ± 11 494 ± 22 20 0.0825 ± 0.0027 0.647 ± 0.033 0.056 ± 0.003 511 ± 16 506 ± 20 21 0.0858 ± 0.0029 0.671 ± 0.033 0.0569 ± 0.0023 530 ± 17 521 ± 19 22 0.0822 ± 0.0028 0.634 ± 0.036 0.0564 ± 0.0024 509 ± 16 498 ± 20 23 0.0834 ± 0.0026 0.641 ± 0.047 0.0553 ± 0.0033 516 ± 15 505 ± 28 24 0.0828 ± 0.0027 0.64 ± 0.044 0.0538 ± 0.0028 513 ± 16 501 ± 26 25 0.0907 ± 0.0035 0.783 ± 0.048 0.0618 ± 0.0029 560 ± 21 585 ± 26 26 0.0771 ± 0.0044 0.598 ± 0.03 0.0563 ± 0.0036 479 ± 25 473 ± 57 27 0.0817 ± 0.0061 0.638 ± 0.015 0.0567 ± 0.0032 506.1 ± 34 499 ± 22 28 0.0813 ± 0.015 0.65 ± 0.01 0.0583 ± 0.0031 504 ± 71 507 ± 13 29 0.085 ± 0.0092 0.681 ± 0.0069 0.0582 ± 0.0031 526 ± 98 527 ± 8.3 30 0.084 ± 0.0083 0.644 ± 0.0085 0.0554 ± 0.0029 520 ± 72 504 ± 10 31 0.0772 ± 0.0065 0.651 ± 0.01 0.0603 ± 0.0031 479 ± 54 508 ± 13 32 0.0803 ± 0.0065 0.647 ± 0.017 0.0582 ± 0.0029 498 ± 53 509 ± 25 33 0.079 ± 0.0022 0.629 ± 0.025 0.058 ± 0.0026 490 ± 13 495 ± 17 34 0.0801 ± 0.0022 0.625 ± 0.026 0.0567 ± 0.0027 497 ± 13 492 ± 17 35 0.078 ± 0.0026 0.609 ± 0.041 0.0571 ± 0.0042 484 ± 16 481 ± 25 36 0.0792 ± 0.0022 0.584 ± 0.032 0.0537 ± 0.0026 492 ± 13 469 ± 20 37 0.0767 ± 0.0022 0.66 ± 0.055 0.0618 ± 0.0043 476 ± 13 512 ± 31 38 0.0813 ± 0.0023 0.612 ± 0.038 0.0545 ± 0.0026 504 ± 14 484 ± 22 39 0.0793 ± 0.002 0.626 ± 0.04 0.0574 ± 0.003 492 ± 12 496 ± 22 40 0.0809 ± 0.0024 0.641 ± 0.042 0.0568 ± 0.0033 502 ± 14 501 ± 24 41 0.0776 ± 0.0021 0.611 ± 0.043 0.0558 ± 0.0032 482 ± 13 482 ± 25

All errors are quoted at 2r. identiﬁcation of a 493 Ma granitoid body in the SKB is geologically well-described ca. 450 Ma Hikami Granite exposed about 30 km signiﬁcant in two aspects. First, it is clear that the Shoboji Diorite to the east (Murata et al., 1974; Kitakami Paleozoic Research forms an independent igneous body separated distinctly from the Group, 1982; Watanabe et al., 1995; Sasaki et al., 2013). Second, Y. Isozaki et al. / Journal of Asian Earth Sciences 108 (2015) 136–149 143

Fig. 3. U–Pb Concordia diagrams for igneous zircons from the two analyzed samples of the Shoboji Diorite (SB-0 and SB-2) in the Nagasaka area in NE Japan. Representative cathodoluminescence (CL) images of zircons are also shown.

Fig. 4. U–Pb Concordia diagrams for igneous zircons from the two analyzed samples of the Isawagawa Tonalite in the Isawa area (YK-2 and YK-4) in NE Japan. Representative cathodoluminescence images of zircons are also shown. the Shoboji Diorite has unusual but distinctive geochemical char- correlative, and suggest that they once belonged to the same acteristics similar to those of arc-related igneous rocks (Kanisawa Cambrian granitoid belt. Independently, Tsuchiya et al. (2014c) and Ehiro, 1997). Thus, this granitoid body belongs to an older orally reported their preliminary U–Pb zircon dates of the same arc pluton, which is the oldest known so far in NE Japan. bodies, which are more or less the same as ours. On the other hand, zircons separated from the Isawagawa The Nagasaka area is situated about 30 km to the east of the Tonalite at two localities (Fig. 2B) yielded weighted average U–Pb Isawa area (Fig. 1), and these two areas are currently separated ages of ca. 500–490 Ma (Fig. 4), which reflect the primary crystal- from each other by the geomorphological feature called the lization of the Isawagawa Tonalite in the late Cambrian. Much Kitakami Lowland between the Kitakami and Ou mountains, in younger K–Ar ages of 457, 403, and 381 Ma were previously which no pre-Quaternary basement is exposed (Fig. 5). reported for hornblende from the same body (Sasada et al., Nonetheless, the latest seismic profile beneath the Kitakami 1992); however, these ages more likely represent the time of cool- Lowland clearly illustrates a subsurface listric, NS-trending, normal ing after the magma consolidation at 497–495 Ma. Indeed, Sasada fault system (Abe et al., 2008); these faults formed during the et al. (1992) already considered that these K–Ar ages were partly Miocene back-arc opening of the Japan Sea (Yamaji, 1990; Sato, affected by later mylonitization. 1994). After the Miocene extensional tectonics that attenuated The identification of a 500–490 Ma granitoid in the Isawa area the pre-Cenozoic crust in an E–W direction, tectonic inversion in the Ou Mountains is particularly noteworthy. Its exposure is put the regime into contraction. The pre-rift relative distance more limited than that of the Shoboji Diorite, i.e. several hundred between the Isawagawa and Shoboji bodies was, therefore, more meters along the Isawagawa River (Fig. 2B); however, their ages or less similar to the present distance, i.e. about 30 km. This pro- are almost identical. Except for the mylonitic texture recognized vides a minimum across-arc dimension for the ca. 500 Ma in the Isawagawa Tonalite, the petrological characteristics and Shoboji–Isawagawa granitoids. major/trace element compositions of these two bodies are similar Lately, a ca. 500 Ma tonalite xenolith, 2 m in diameter, was (Kanisawa and Ehiro, 1997; Sasada et al., 1992). The present reported by Tsuchiya et al. (2014a,b) (Figs. 1 and 5) within results, therefore, confirm that these two intrusions are mutually Cretaceous volcanic rocks from the Ryori area on the Pacific coast 144 Y. Isozaki et al. / Journal of Asian Earth Sciences 108 (2015) 136–149

Fig. 5. E–W seismic profile across the NE Japan arc (above; modified from Zhao et al., 2009) and a more detailed profile across the Kitakami Lowland with sample localities (below; modified from Abe et al., 2008). Note that the locations of the discussed areas (I, S, and R for the Isawagawa, Nagasaka, and Ryori, respectively) are on the trench side of the modern volcanic front. The subsurface structure of the Nagasaka and Isawa areas is controlled by the re-activated listric normal fault system (Yamaji, 1990; Sato, 1994). The current distance between these two areas suggests that the subsurface Cambrian plutonic rocks potentially extend for at least 30 km in an across-arc direction. When the coeval granitoid xenolith in the Ryori area (Fig. 1) is taken into consideration, the potential across-arc extent of the 500 Ma granitoid in NE Japan becomes much greater, i.e. over 80 km.

along the eastern margin of the SKB. This indirectly suggests that documented from the intervening area, which extends for ca. the 500 Ma granitoid in the SKB potentially had a greater subsur- 1000 km along the arc on the main island of Honshu. face extent; i.e. the primary size of the Cambrian granitoids in Nonetheless, the occurrences of ca. 500 Ma detrital zircons in the SKB was probably more than 80 km in an across-arc direction. younger sedimentary rocks and high-P/T psammitic schists in var- Moreover, there is also a correlative 500 Ma meta-granitoid with ious areas in Japan (Nakama et al., 2010a,b; Isozaki et al., 2010, an arc signature in the Hitachi area at the southern tip of NE 2014; Tsutsumi et al., 2011; Yoshimoto et al., 2013; Okawa et al., Japan, nearly 200 km to the south of the Nagasaka-Isawa area 2013) and of ca. 500 Ma xenocrysts in younger granitoids (Fujii (Fig. 1; Sakashima et al., 2003; Tagiri et al., 2010, 2011). The simi- et al., 2008; Osanai et al., 2014; Aoki et al., 2015) suggest the larity of the Paleozoic geology between the Hitachi area and the potential development of the coeval granitoid belt throughout SKB has been discussed for years, despite the absence of a conter- early Paleozoic Japan. All of these lines of evidence, although still minous surface trace. The Hitachi area is situated in the Takanuki fragmentary, imply that the 500 Ma granitoid belt formed a major Belt on the southwest of the Gosaisho Belt (correlative to the mature arc with a minimum along-arc length of up to 1500 km, at Ryoke Belt in SW Japan), which is separated by the NS-trending least comparable with the modern Izu-Bonin–Mariana arc strike-slip HTL from the SKB (Fig. 1), although the putative amount (Suyehiro et al., 1996), along the western margin of Panthalassa of left-lateral strike-slip dislocation is unknown. Given the ca. (=paleo-Pacific). 500 Ma meta-granitoid in the Hitachi area correlated with the Shoboji Diorite/Isawagawa Tonalite, the primary size of the 6.2. Lost arc: where have all the granitoids gone? Cambrian arc granitoids in NE Japan may reach 100 km wide or more across arc, and more than 200 km along arc. Such a dimen- The potentially huge extent of the late Cambrian arc batholith sion suggests the development of a mature regional arc. in both NE and SW Japan provides significant evidence for the first In general, it is not easy to estimate the total spatial dimension oceanic subduction in proto-Japan, i.e. the initial development of a of ancient geologic entities, including the case of the putative mature arc-trench system with a granitoid batholithic belt (Fig. 6). 500 Ma arc granite belt in proto-Japan. There is another isolated Along active continental margins in general, a subduction-related occurrence of ca. 500 Ma meta-granitoid in Kyushu, SW Japan arc-granite belt develops on a scale of >2000 km along-arc (Sakashima et al., 2003), which likely represents the western and >100 km across-arc (e.g., Takahashi, 1983; Suyehiro et al., extension of the Shoboji–Isawagawa–Hitachi granitoids in NE 1996; Ito et al., 2009). In contrast, the overall volume of the Japan. These coeval granitoids possibly formed in the same arc of exposed 500 Ma granitoids in present-day Japan is small. This size proto-Japan, although no correlative unit has been hitherto difference naturally suggests that the 500 Ma arc granite belt in Y. Isozaki et al. / Journal of Asian Earth Sciences 108 (2015) 136–149 145

Fig. 6. Schematic diagram showing the proto-Japan arc immediately after its birth in the earliest Paleozoic, ca. 520–480 Ma (modified from Isozaki et al., 2010). proto-Japan was originally much larger (not necessarily of huge 6.3. Unavoidable fate: tectonic erosion batholithic proportions), and that a considerable proportion has completely disappeared secondarily after residing on the surface The latest studies of detrital zircon chronology of Paleozoic– long enough for its erosion to feed terrigenous clastics to Mesozoic sandstones in Japan have demonstrated a remarkable arc-related basins. difference between the occurrence of abundant Paleozoic zircons During the 1980s and 1990s, so-called strike-slip tectonics in pre-Cretaceous sandstones and the total amount of extant became popular in Japan (e.g., Taira et al., 1983; Yamakita and Paleozoic granitic bodies exposed in SW Japan (Nakama et al., Otoh, 2000), and under the contemporary influence of such an idea, 2010b; Isozaki et al., 2010). In general, the main source of detrital Sakashima et al. (2003) explained the isolated occurrence of the zircons is felsic plutonic rocks, i.e. granitoids, rather than 500 Ma granite in Hitachi and Kyushu as the consequence finer-grained volcanic rocks. The age spectra of detrital zircons of >1000 km large-scale strike-slip dislocation along the Median from Silurian to Jurassic sandstones demonstrate that Tectonic Line (MTL) without mentioning its primary size. There Early-Middle Paleozoic granitoids were exposed extensively in are two difficult issues, however, in their interpretation. First, proto-Japan, and they provided the main source of terrigenous strike-slip dislocation itself cannot completely remove/erase a clastics to feed Middle Paleozoic to Triassic basins, whereas the large batholith belt from the surface, and dissect/translate it to Paleozoic granitic plutons disappeared almost totally from the sur- somewhere else, as in the case of the Salinian block dislocated by face after the end-Triassic. In Japan and nearby areas in Far East the San Andreas Fault in western North America (e.g., Wallace, Asia, there are no Paleozoic–Triassic sedimentary basins that could 1990). In fact, in Japan and in conterminous Far East Asia, we can- have accommodated such abundant granitic clastics of corre- not identify any large Cambrian granitoid body that was putatively sponding volume. In other words, Paleozoic arc granitic bodies dislocated along the MTL. Similar problems arise in interpreting indeed existed prominently in proto-Japan (Fig. 6); however, they the many, extremely small, occurrences of Paleozoic orogenic com- were totally removed from the surface afterwards. Can large grani- ponents in SW Japan, such as accretionary complexes and blues- tic bodies disappear from the planet’s surface and subduct into the chists that likely had similar dimensions to their coeval mantle, despite their relatively lower density with respect to man- granitoids, as pointed out by Isozaki et al. (2010). Surface erosion tle rocks? For the present data from SKB, we need to check the can be an efficient process to remove a granitic belt; however, in background plate tectonic regime and physical conditions for this order to totally remove an entire batholith, large-scale sedimen- granitoid-disappearance episode in NE Japan. tary basins are required to accommodate the equivalent amounts To explain the disappearance of such a large Paleozoic arc plu- of terrigenous clastics derived from the relevant batholith belt. ton, the process of tectonic erosion is the most promising (Isozaki Without identifying such a ‘‘graveyard’’ of older granitoids, there- et al., 2010; Suzuki et al., 2010). Tectonic erosion or subduction fore, the claimed strike-slip interpretations cannot reasonably erosion is a process occurring in many modern active continental explain the almost total disappearance of the Early Paleozoic oro- margins, which can remove a significant amount of crustal mate- genic components, which include the Cambrian arc granite belt. rial from the bottom of a fore-arc (e.g., Scholl et al., 1980; von Second, none of the strike-slip faults in Japan records clear evi- Huene and Scholl, 1991, 1993; Scholl and von Huene, 2010). This dence of >1000 km strike-slip dislocation with well documented mechanism has been operating much more frequently than previ- reference points. For example, the MTL in SW Japan was ously realized (e.g., Vannucchi et al., 2008; Yamamoto et al., 2009; re-evaluated as a strike-slip fault activated not during the Clift et al., 2009; Aoki et al., 2012). Cretaceous as previously believed, but merely in the Quaternary It is noteworthy that the Cambrian arc granitoids newly recog- (Isozaki, 1996; Sato et al., 2015). A recent seismic profile clearly nized in the Kitakami and Ou mountains are located on the trench illustrated the north-dipping sub-horizontal structure of the MTL side of the present volcanic front of the active NE Japan arc (Figs. 1 that transected and dissected the entire arc crust of SW Japan and 5). A brand-new granitoid is under construction beneath the (Ito et al., 2009), and this post-late Cretaceous structure is inconsis- current volcanic front, about 300 km to the west of the modern tent with the claimed Cretaceous oblique subduction and resultant Japan trench. In other words, the fore-arc of NE Japan has not large-scale dislocation. The MTL is better explicable as a low-angle gained any extra across-arc breadth during the last 500 million thrust that was activated during the Miocene back-arc (Japan Sea) years, regardless of the long-continued subduction processes cou- opening associated with across-arc contraction in the fore-arc pled with intermittent granitoid production in large amounts ever (Isozaki, 1996; Isozaki et al., 2010; Aoki et al., 2011; Sato et al., since the Early Paleozoic; e.g., in the Cambrian, Ordovician– 2015). Under the circumstances, we need another feasible and Silurian, Devonian–Carboniferous, Cretaceous, and Paleogene. more viable explanation for the disappearance of the old arc grani- This positively indicates that large amounts of pre-Cenozoic tic belt. fore-arc crust, mostly composed of granitoids, have been 146 Y. Isozaki et al. / Journal of Asian Earth Sciences 108 (2015) 136–149 transported from the surface into the deep mantle via a subduction granitoids in SW Japan (e.g., Fujii et al., 2008; Osanai et al., 2014; zone. Along the modern active Japan Trench, tectonic erosion is Aoki et al., 2015) likely shared a similar fate, i.e. the mature arc plu- indeed currently in operation, as confirmed by seismic profiles tons were totally removed and disappeared by tectonic erosion. (von Huene and Scholl, 1991, 1993; Scholl and von Huene, 2010). Marine geological research and direct GPS observations in the 6.4. Regional connections in Far East Asia Japanese trench have documented the absence of any modern accretionary complex, but the presence of a Cretaceous accre- At the end of this article, we discuss possible correlations of the tionary complex obliquely truncated by the trench axis, and of a early Paleozoic granitoids in Far East Asia. There are sporadic regional unconformity that records a major subsidence in the occurrences of Early Paleozoic granitoids in Primorye (Russia) fore-arc (Nasu et al., 1980; Heki, 2004). and in South China, with more or less the same ages as the The Paleozoic granitoids in NE Japan (and probably also in SW Shoboji–Isawagawa granitoids; e.g., ca. 500 Ma plutons in the Japan) have been totally eroded from the bottom of a fore-arc crust, Khanka and Jiamsi blocks and so-called Caledonian granitoids in in other words they have been subducted into the mantle by tec- the Cathaysia block (Fig. 7). tonic erosion since the Paleozoic. As preliminarily speculated by All the circum-Japan Sea 500 Ma granitoids, i.e. those in the SKB Isozaki et al. (2010) and Suzuki et al. (2010), the Paleozoic (present study), the Kuroseagwa belt in SW Japan (Sakashima et al.,

Fig. 7. The distribution of Early Paleozoic granitoids in Far East Asia (A: compiled from Li et al., 2012; Shu et al., 2014; Khanchuk et al., 1996, and Bi et al., 2014), and of the Japanese ca. 500 Ma zircons (detrital and xenocryst) and >400 Ma granitoids (B: compiled from Sakashima et al., 2003; Fujii et al., 2008; Nakama et al., 2010a,b; Isozaki et al., 2010, 2014; Tsutsumi et al., 2011; Okawa et al., 2013; Yoshimoto et al., 2013; Osanai et al., 2014; Aoki et al., 2015; this study). Although their current distribution is patchy, the Early Paleozoic arc-related granitoids in Japan (B) likely belonged to a much larger arc crust that extended to the northeast to the Khanka block in Primorye, Far East Russia, and possibly further to the eastern Cathaysian margin of South China (A). The ca. 500 Ma granitoids in the Jiamsi block and in Cathaysia (blank star) are of non-arc afﬁnity. Nonetheless, an ‘‘early Paleozoic granitoid belt’’ probably developed along an active western Panthalassan (=paleo-Paciﬁc) margin before the Pangean time, as suggested by detrital zircons in younger sedimentary rocks and high-P/T psammitic schists in South China, Taiwan, and Japan (Isozaki et al., 2010, 2014; Tsutsumi et al., 2011; Yui et al., 2012; Okawa et al., 2013; Hu et al., 2015) and by zircon xenocrysts in younger granitoids (Fujii et al., 2008; Osanai et al., 2014; Aoki et al., 2015). Note that the general trend of ca. 500 Ma arc granitoid extends in a N–S direction, almost perpendicular to the E–W oriented Central Asian Orogenic Belt (CAOB). Y. Isozaki et al. / Journal of Asian Earth Sciences 108 (2015) 136–149 147

2003), and in the Khanka-Jiamsi blocks (Khanchuk et al., 1996), probably continued into Paleozoic Japan and further to Paleozoic probably formed a single belt along the Paleozoic Pacific Khanka in Far East Russia, thus the Paleozoic South China was (Panthalassan) margin because the Japan Sea is a relatively young much larger than the currently exposed mainland South China ca. 20 Ma (Miocene) back-arc basin surrounded by relevant land- (to form the Greater South China by Isozaki et al., 2014). masses with continental crust, which were previously connected On the other hand, the Central Asian Orogenic Belt (CAOB; each other prior to the Miocene rifting. In particular, the Fig. 7A) contains many Paleozoic granitoids (e.g., Xiao et al., Tafuinsky granite in the southern Khanka block (Fig. 7A) has 2004), although their occurrence is rather restricted to the western 493–491 Ma U–Pb zircon ages and geochemistry of island arc affin- part of the CAOB, and the general structural E–W trend of the CAOB ity (Khanchuk et al., 1996; Nevolin et al., 2010), therefore, likely is highly oblique or almost perpendicular to the general trend of formed a dissected counterpart of the ca. 500 Ma granitoids in the putative paleo-Pacific margin ‘‘granitoid belt’’, which extends Japan. On the other hand, apparently coeval granitoids also occur mostly in a N–S direction. Thus it seems reasonable to consider in the southern Jiamsi block; however, these have trace element that the apparently coeval arc granitoids in the CAOB were not geochemistry totally different from the island arc type granitoids genetically related to those in Far East Asia. At any rate, in order (e.g., Bi et al., 2014; Yang et al., 2014) thus independent of the coe- to reconstruct the paleogeography of Paleozoic Japan and its sur- val arc-type granitoids in Japan. Likewise the 500 Ma granitoids in roundings, we need further detailed geological, geochemical and Cathaysia have distinctive geochemistry of atypical arc affinity (Li isotopic studies of the tectonic setting of individual pre-Pangean et al., 2012; Shu et al., 2014), and thus they cannot be directly cor- continental blocks. related with those in Japan. Nevertheless, the intimate consanguinity between Paleozoic 7. Conclusions Japan and South China is suggested by the following various crite- ria. (1) Zircon xenocrysts in mid-Paleozoic granitoids in Japan The present study presents four new U–Pb ages of zircons sep- range in age-range of 3200–500 Ma (Fujii et al., 2008; Osanai arated from pre-Devonian granitoids in the Nagasaka and Isawa et al., 2014; Aoki et al., 2015) that overlap the ages of the areas in the SKB, NE Japan. All the 4 analyzed samples have late Yangtze basement; thus suggest the significant geological connec- Cambrian ages (ca. 500–490 Ma), which document the time of tion between the South China crust and the site of crystallization of the granitic magmatism. These new ages identify granitoid-dominant arc of Paleozoic Japan. (2) Paleozoic sand- a late Cambrian arc-related regional plutonic belt in NE Japan, the stones and Paleozoic-Triassic high-P/T metamorphic rocks in geological significance of which is summarized as follows: Japan often contain abundant Proterozoic detrital zircons (e.g., Nakama et al., 2010a,b; Shimojo et al., 2010; Isozaki et al., 2010, 1. The newly recognized Cambrian granitoids correspond to the 2014; Tsutsumi et al., 2011; Yoshimoto et al. 2013; Okawa et al., oldest felsic plutonic rocks in NE Japan, which are independent 2013), in particular, Neoproterozoic (1200–600 Ma) zircons have of, and younger by ca. 50 million years than, the previously the same ages as the basement of South China (Fig. 7B). (3) The known ca. 450 Ma Hikami Granite. occurrence of Paleozoic shallow marine fossils in Japan, such as 2. The Cambrian granitoid pluton has an across-arc spatial width Silurian to Carboniferous rugose corals and Permian mollusks of 40 km, and potentially of 80 km in the SKB. Together with (e.g., Kato, 1990; Nakazawa, 1991; Ehiro, 1997; Isozaki and Kase, coeval granitoids in the Hitachi area in southernmost NE 2014) suggests a significant faunal similarity with South China Japan and in central Kyushu, SW Japan, the Cambrian granitoids and accordingly an intimate geotectonic connection during the in central NE Japan likely represent the remnants of the oldest Paleozoic. Japanese arc crust, which probably formed a highly matured In addition, some indirect data support the development of a ca. arc granitic belt, possibly with batholithic aspect. 500 Ma granitoid belt along the western active margin of early 3. The small size of these granitoids today is due to intermittent, Paleozoic Panthalassa, which includes the eastern margin of severe tectonic erosion along subduction zones since the Cathaysia. For example, (4) the occurrence of ca. 500 Ma zircons Paleozoic. from a Permo-Triassic basin in southwestern Cathaysia, the main 4. The early Paleozoic granitoid belt likely developed along an clastic source of which was located to the southeast (Fig. 7A; Hu active margin of the western Panthalassa (paleo-Pacific) et al., 2015). (5) The occurrence of ca. 500 Ma detrital zircons from Ocean, and was dissected by the Mesozoic–Cenozoic tectonics a Cretaceous accretionary complex in Taiwan off Cathaysia (Yui into several pieces currently exposed in Japan, Primorye, and et al., 2012) also suggests the clastic delivery from an adjacent possibly in eastern Cathaysia. provenance with a relict granitoid belt of similar age. The occurrences require the exposure of ca. 500 Ma granitic crust in the late Paleozoic to Mesozoic in the corresponding domain of the modern Acknowledgements South China Sea, i.e. eastern Cathaysia. By summarizing all these, we speculatively propose the devel- Three anonymous reviewers gave helpful comments to the orig- opment of an Early Paleozoic arc-trench system featuring a inal MS, in particular on the granitoids in South China. Nobutaka trench-parallel granitoid belt along the western margin of Tsuchiya (Iwate University) kindly provided information about Panthalassa. Owing to the Mesozoic-Cenozoic tectonic modifica- his study material in the SKB. Brian Windley (University of tions, this largely linear granitoid belt was probably dissected into Leicester) corrected the English language. R. Hayashi and M. smaller fragments that currently occur in Japan and Primorye Shimatsuka helped in the sample collection. This research was (Fig. 7A). The current uncertainty in the geotectonic connection funded by the Japan Society for the Promotion of Science (no. between Japan and South China derives mostly from obscured 26016005 to YI). information of the Paleozoic geology of eastern Cathaysia caused by the severe overprint of Mesozoic magmatism, and from the absence of information of pre-Cenozoic basement under the East Appendix A. Supplementary material China Sea. Nonetheless we predict the future finding of small remnants of ca. 500 Ma arc granitoids from the eastern Cathaysia, e.g., Supplementary data associated with this article can be found, in in the coastal mountains and/or the basement of the East China the online version, at http://dx.doi.org/10.1016/j.jseaes.2015.04. Sea. In other words, the northeastern extension of Cathaysia 024. 148 Y. Isozaki et al. / Journal of Asian Earth Sciences 108 (2015) 136–149

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