Journal of Asian Earth Sciences 108 (2015) 136–149 Contents lists available at ScienceDirect Journal of Asian Earth Sciences journal homepage: www.elsevier.com/locate/jseaes 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 iden- tify 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) mar- gin, 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.