Oceanic-arc subduction, stagnation, and exhumation: zircon U–Pb geochronology and trace-element geochemistry of the Sanbagawa eclogites in central Shikoku, SW Japan

Journal Pre-proof Oceanic-arc subduction, stagnation, and exhumation: zircon U–Pb geochronology and trace-element geochemistry of the Sanbagawa eclogites in central Shikoku, SW Japan

Shogo Aoki, Kazumasa Aoki, Tatsuki Tsujimori, Shuhei Sakata, Yuta Tsuchiya PII:

S0024-4937(20)30015-3

DOI:

https://doi.org/10.1016/j.lithos.2020.105378

Reference:

LITHOS 105378

To appear in:

LITHOS

Received date:

1 October 2019

Revised date:

6 January 2020

Accepted date:

11 January 2020

Please cite this article as: S. Aoki, K. Aoki, T. Tsujimori, et al., Oceanic-arc subduction, stagnation, and exhumation: zircon U–Pb geochronology and trace-element geochemistry of the Sanbagawa eclogites in central Shikoku, SW Japan, LITHOS(2020), https://doi.org/ 10.1016/j.lithos.2020.105378

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Oceanic-arc subduction, stagnation, and exhumation: zircon U–Pb geochronology and trace-element geochemistry of the Sanbagawa eclogites in central Shikoku, SW Japan Shogo Aokia* , Kazumasa Aokib, Tatsuki Tsujimoric, d, Shuhei Sakatae and Yuta Tsuchiyaa Department of Biosphere-Geosphere Science, Okayama University of Science,

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a

Okayama, Japan

Department of Applied Science, Okayama University of Science, Okayama, Japan

c

Center for Northeast Asian Studies, Tohoku University, Sendai, Japan

d

Department of Earth Science, Tohoku University, Sendai, Japan

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Department of Chemistry, Gakushuin University, Tokyo, Japan

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*corresponding author: Shogo Aoki

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b

Department of Biosphere-Geosphere Science, Okayama University of Science,

Abstract

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E-mail: [email protected]

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Okayama 700-0005, Japan

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In the Iratsu and the quartz-bearing eclogite bodies of the Sanbagawa high-pressure type metamorphic belt, southwest Japan, zircon U–Pb dating and trace-element analysis of the mafic gneiss combined with its geologic structure revealed that the protolith basaltic rock constituted the topographic high on a seafloor in relation to intra-oceanic arc magmatism at ca. 195 Ma. Moreover, the metamorphic zircon U–Pb data and

the rare-earth element patterns obtained

from the subordinated

metasedimentary rocks of the Iratsu and the quartz-bearing eclogite bodies indicate that both bodies were subducted from a trench at ca. 120 Ma and underwent the eclogite facies metamorphism between ca. 120 Ma and ca. 90 Ma. This study, combined with

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previous studies for the Sanbagawa rocks and the Jurassic-Cretaceous accretionary complexes in Japan, identifies the following constraints that led to the tectonic evolution of the Sanbagawa eclogites: 1) the metamorphic unit including the Iratsu and the quartz-bearing eclogite bodies (the Besshi unit) was subducted from a trench at ca. 120 Ma. 2) This unit was stagnated at the depth of the eclogite- facies condition between ca.

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120 Ma and ca. 90 Ma. 3) The eclogites in the Besshi unit was exhumed with the younger metamorphic rocks which were subducted at ca. 100–90 Ma (Asemi- gawa unit).

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4) The Besshi unit is a high-pressure metamorphic equivalent of the non- or weakly

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metamorphic Sanbosan accretionary complex and the Mikabu greenstones from a

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standpoint of age similarity on accretion. The probable mechanism for the stagnation of the Besshi unit at the depth of the eclogite- facies condition needs 1) the detachment of

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oceanic-arc material from the subducting slab, driven by the resistance against the

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subduction of the topographic-high part underneath the forearc, and 2) the oceanward

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movement of the entire arc-trench system, which might have depressed the subduction of the Besshi unit into a deeper depth than its eclogite depth. Keywords: Sanbagawa metamorphic belt; eclogite; zircon; U–Pb dating; rare-earth element composition

1. Introduction Geochronological and

geochemical studies of the high-pressure (HP)

metamorphic rocks in the Pacific-type orogeny (e.g., Matsuda and Uyeda, 1971; Maruyama et al., 1996, 2010; Maruyama, 1997; Agard et al., 2009; Aoki et al., 2019a, b) can provide us with the geologic information such as the temporal and spatial scales

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of the HP rocks within the orogenic cycle. Among radiogenic isotope analyses, laser ablation-inductively coupled plasma- mass spectrometer (LA-ICP-MS) zircon U–Pb and trace element analyses are very useful tools for clarifying the information because of its high spatial resolution and rapid data obtainment (e.g., Hirata and Nesbitt, 1995; Schoene, 2014).

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In Japan, most basement rocks are composed of subduction-related geologic units, such as accretionary complexes (ACs), HP parts of ACs, calc-alkaline granites,

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and arc-related basins; these rocks are the products of numerous Pacific-type orogenic

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events that occurred along the western margin of the Paleo-Pacific Ocean throughout

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the past 500 million years (e.g., Uyeda and Miyashiro, 1974; Isozaki, 1996; Maruyama, 1997; Maruyama et al., 1997; Isozaki et al., 2010). Of those units, the Cretaceous

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Sanbagawa metamorphic belt (SMB) in southwest Japan is one of the best-preserved HP

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rocks in the world (Fig. 1a), enabling numerous studies on it, which significantly

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improve our understanding of its tectonic evolution. Nevertheless, some issues remain unresolved. One of them is the areal distribution of ca. 120 Ma eclogites in the SMB. Recently, Aoki et al. (2019a, 2019b) estimated that the SMB consists of at least three distinct metamorphic units in light of different peak metamorphic ages based on the detrital and metamorphic zircon U–Pb dating and their REE patterns, i.e., ca. 120–110 Ma, ca. 90 Ma and ca. 80–60 Ma metamorphic units. Nagata et al. (2019) also suggest a time gap between ca. 90Ma and ca. 80–60 Ma metamorphic units, based on the zircon U–Pb dating. However, the areal distribution of the oldest unit (ca. 120–110 Ma) has not been constrained because the peak metamorphic age has been reported only from a

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small outcrop which is the quartz-bearing eclogite body of the Besshi area in central Shikoku (Okamoto et al., 2004; Arakawa et al., 2013) (Fig. 1b, c, d). In addition, the timing of the eclogite- facies metamorphism of the oldest unit itself has been controversial. Endo et al. (2009) showed an age of 115.9 ± 0.5 Ma from the eclogite in the Western Iratsu body, next to the quartz-bearing eclogite body (Fig. 1), by using

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garnet-omphacite Lu–Hf isochron analysis, and they interpreted its age as the timing of the pre-eclogite facies metamorphism in combination with the petrological data.

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Moreover, the protolith origin and tectonic setting of these eclogites has not well

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been constrained even though their geological, petrological and geochemical studies

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have been performed for the constraints so far (e.g., Kugimiya and Takasu, 2002; Terabayashi et al., 2005; Aoya et al., 2006; Utsunomiya et al., 2011; Aoki et al., 2019c ).

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For example, the metagabbros of the Seba body in Besshi area have whole-rock

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geochemical affinities with the hanging-wall materials, based on the Cr vs. Sr

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discrimination diagram (Aoya et al., 2006). On the other hand, Utsunomiya et al. (2011) and Aoki et al. (2019c) argue that the metagabbros of the Iratsu and Tonaru bodies were originally formed by an oceanic- island arc magmatism based on whole-rock and zircon trace-element analyses. Determining whether the 120–110 Ma eclogite is distributed over a wide region in the SMB or not and where the protolith of its eclogite was formed will be critical to gain a deeper understanding of the tectono- metamorphic evolution of the whole Sanbagawa metamorphic rocks. In this study, we applied LA-ICP-MS zircon analysis to the Iratsu and

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quartz-bearing eclogite bodies (Fig. 1b, d), because this method is expected to offer key insights to solving these problems. We present new zircon U–Pb and REE data, and discuss the areal distribution of the 120–110 Ma eclogite in central Shikoku. And, zircon trace-element abundance could constrain tectonic setting of magmas, from which they were crystallized (e.g., Grimes et al., 2007, 2015). We discussed the tectonic

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setting responsible for the protoliths of the metamafic rock in the bodies. Finally, the implications of the tectono- metamorphic history of the eclogite from subduction to

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exhumation are discussed based on our new data and previous studies.

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2. Geological background

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2.1. Sanbagawa metamorphic belt in central Shikoku The SMB extends along the MTL from the Kanto Mountains to the eastern part

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of Kyushu Island for over 800 km (Fig. 1a). The SMB is tectonically overlain by the

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Jurassic AC (Chichibu composite belts) and underlain by the Late Cretaceous Shimanto

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AC (Northern Shimanto belt). In the southernmost part of the SMB, the Jurassic-Cretaceous metabasalts with subordinate amounts of metasedimentary rocks, collectively called the Mikabu greenstones, are exposed (e.g., Iwasaki et al., 1984; Sawada et al., 2019). The greenstones suffered subduction metamorphism at the same time with the Sanbagawa metamorphic rocks, based on radiometric K–Ar and Ar–Ar ages (e.g., Watanabe et al., 1982; Isozaki et al., 1992; Kurimoto et al., 1993, 1995; de Jong et al., 2000). In the SMB of central Shikoku, especially the Besshi and Asemi- gawa areas, variations of the metamorphic grades from pumpellyite-actinolite through blueschist to eclogite facies are well observed (e.g., Kunugiza et al., 1986;

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Banno et al., 1986; Takasu, 1989; Ota et al., 2004; Aoya et al., 2013). Hence, those areas have significantly improved our standing of the tectonic evolution of the SMB. On the basis of the tectono-stratigraphic aspect of the Sanbagawa metamorphic rocks in the Besshi and Asemi- gawa areas, Aoki et al. (2019a) has proposed that the SMB comprises three distinct metamorphic units (Fig. 1b, c), i.e., Oboke, Asemi- gawa, and Besshi units.

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From here, we would like to briefly describe the geochronological characteristics of each unit.

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The Oboke unit, which is situated at the structurally lowermost part, is

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composed mainly of psammitic, pelitic and mafic schists preserving the prograde

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metamorphism ranging from the pumpellyite-actinolite to blueschist facies (e.g., Aoki et al., 2008, 2019a; Aoya et al., 2013). The LA-ICP-MS U–Pb dating from detrital zircons

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in this unit shows that the deposition age in a trench was after 80 Ma (Aoki et al., 2007;

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Otoh et al., 2010; Nagata et al., 2019). The phengite K–Ar dating indicates the prograde

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metamorphic age was ca. 80–60 Ma (e.g. Aoki et al., 2008, 2011; Nagata et al., 2019). The Asemi- gawa unit overlying the Oboke unit is mainly composed of mafic, pelitic and siliceous schists (e.g., Aoya et al., 2013; Aoki et al., 2019a). Numerous geochronological and petrological studies have been conducted in this unit. The metamorphic grade in the unit varies from the lower greenschist through blueschist transition facies and epidote-amphibolite facies to the eclogite- facies (e.g., Takasu, 1989; Aoki et al., 2009; Aoya et al., 2013; Kouketsu et al., 2010; Taguchi and Enami, 2014a, b; Taguchi et al., 2019). The LA-ICP-MS detrital zircon U–Pb ages obtained from the psammitic and pelitic schists indicate that the protoliths were subducted from a

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trench after ca. 100–90 Ma (Endo et al., 2018; Aoki et al., 2019a; Aoki et al., 2019b; Knittel et al., 2019). The LA-ICP-MS metamorphic zircon U–Pb datings and garnet omphacite Lu–Hf isochron analyses show that the peak metamorphic age of U–Pb ages of ca. 90 Ma (Aoki et al., 2009, 2019a; Wallis et al., 2009; Aoki et al., 2019b). The Ar – Ar and K–Ar datings of whole rock and white mica in this unit yield ca. 90–80 Ma,

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which is interpreted as the retrograde and cooling ages during the exhumation to the surface (Itaya and Takasugi, 1988; Takasu and Dallmeyer, 1990; Dallmeyer and Takasu,

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1991; Maruyama et al., 2004; Aoki et al., 2008; Itaya and Tsujimori, 2015). Recently,

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Knittel et al. (2019) reported some zircon grains younger than 90 Ma in this area.

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However, those ages are clearly equal to or younger than the K–Ar and Ar–Ar metamorphic ages from this unit. Such age discrepancies will arise from analysis

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artifacts, and so they are not considered in this study.

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The Besshi unit is mainly composed of the metamafic rocks, pelitic, psammitic

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and siliceous schists, and marble, and its peak metamorphic condition reached the eclogite facies (e.g., Takasu, 1989; Ota et al., 2004; Endo et al., 2009, 2010, 2012; Aoya et al., 2013; Aoki et al., 2019a). The Higashi- Akaishi peridotite body preserves relicts of the eclogite–ultra HP metamorphic conditions (e.g., Enami et al., 2004). Although the origin of the peridotite body has been controversial (Mizukami et al., 2004; Terabayashi et al., 2005; Hattori et al., 2010), some small peridotites are concurrently enclosed within the metamorphic body near the boundary between the meta-mafic and Higashi- Akaishi bodies (Kugimiya and Takasu, 2002; Ota et al., 2004). Hence, the Higashi- Akaishi body is included in this unit. The Besshi unit is situated at the

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intermediate structural level within the Asemi- gawa unit (Fig. 1b, c; Ota et al., 2004; Yamamoto et al., 2004; Terabayashi et al., 2005; Aoya et al., 2013). The radiometric U– Pb dating of zircons show the sediment protoliths were subducted from a trench at ca. 130–120 Ma, and underwent peak metamorphism at ca. 120–110 Ma (Okamoto et al., 2004; Arakawa et al., 2013; Aoki et al., 2019a). The K–Ar datings of white mica yield

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ca. 80 Ma, which is interpreted as retrograde age during exhumation along with the Asemi-gawa unit (Maruyama et al., 2004; Aoki et al., 2008; Itaya and Tsujimori, 2015).

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As described above, each unit has distinct age history, but the spatial distribution

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of the Besshi unit is still no better than a conjecture, because the metamorphic zircons

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showing the 120–110 Ma peak metamorphism have been reported from only one rock type, i.e., quartz-bearing eclogite (Okamoto et al., 2004; Arakawa et al., 2013). To

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understand the tectonic evolution of the whole of SMB, the identification of the spatial

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distribution of the Besshi unit will be needed. To understand the tectonic evolution of

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the whole of the SMB, it is critical to find metamorphic zircons from 120 to 110 Ma from other localities within the Besshi unit proposed by Aoki et al (2019a). This study focuses on the Iratsu and quartz-bearing eclogite bodies in the Besshi unit (Fig. 1d), where a lot of evidence for eclogite metamorphism has been reported (Aoya et al., 2013 and references there in). 2.2 Iratsu and quartz-bearing eclogite bodies The Iratsu and quartz-bearing eclogite bodies are situated at the intermediate structural level of the SMB in the Besshi area (Fig. 1b, c) (e.g., Ota et al., 2004). The eastern part of the Iratsu body is predominantly composed of metagabbro having

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diopside, omphacite, garnet and kyanite (e.g., Tsujimori et al., 2000; Ota et al., 2004). On the other hand, the western part of the body consists predominantly of mafic gneiss and diopside hornblendite with subordinate amounts of metasedimentary rocks (Fig. 1d; e.g., Endo et al., 2010; Aoya et al., 2013). The mafic gneiss exhibits layered structures composed mainly of garnet, amphibole and epidote (e.g., Aoya et al., 2013). The

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diopside hornblendite contains garnet, clinopyroxene and epidote, and has no or slight schistosity (e.g., Enami, 2000; Ota et al., 2004; Aoya et al., 2013). Metasedimentary

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rocks in the western part of the body include marble, and psammitic and pelitic schists

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(Fig. 1d). The existence of thick marble in this body suggests that the original tectonic

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setting was a reef-capped topographic high in a basaltic seafloor, such as a seamount or oceanic plateau (e.g., Kugimiya and Takasu, 2002; Terabayashi et al., 2005; Aoya et al.,

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2006). On the other hand, Utsunomiya et al. (2011) postulated that those mafic gneisses

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had been formed on an oceanic- island arc based on their Nb anomalies and Sr-Nd

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isotopic data. Zircons from the metagabbros in the Tonaru body, which is situated at a similar structural level to the Western Iratsu body in the Besshi unit, also have trace-element characteristics seen in those crystallized from arc- like magmas (Aoki et al., 2019c). In some places of the Iratsu body, omphacite and its symplectite minerals (albite and hornblende) can be observed within the metamafic and metasedimentary rocks (e.g., Ota et al., 2004; Endo et al., 2009, 2010, 2012; Aoya et al., 2013). Moreover, quartz included in the garnet show large Raman frequency shifts (Δω 1 > 10 cm-1 ) (Mouri and Enami, 2008; Aoya et al., 2013). Those mineralogical data suggest that the Iratsu body was subducted into a depth of the eclogite- facies conditions. The

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eclogite-facies condition of them are estimated to be T = 500–800 °C and P = 1.4–2.5 GPa (Ota et al., 2004; Endo et al., 2009, 2010, 2012). After the metamorphism, they suffered the retrograde overprinting with hydration during exhumation, ranging from the epidote-amphibolite or amphibolite facies to greenschist facies (Ota et al., 2004; Endo et al., 2010). Some mafic gneisses in this body preserve the relicts of high-T

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metamorphism (amphibolite-granulite facies) before eclogite- facies metamorphism (Endo et al., 2009, 2010, 2012). The same relicts can be also observed in the metamafic

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and metaultramafic rocks of the Tonaru body and Eastern Iratsu body (e.g., Yokoyama

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et al.,1980; Takasu et al., 1994; Ota et al., 2004; Terabayashi et al., 2005; Miyagi and

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Takasu, 2005). Endo et al. (2009, 2010, 2012) estimate that the high-T metamorphism of the body occurred in connection with subduction; on the other hand, Terabayashi et al.

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(2005), Utsunomiya et al. (2011) and Aoki et al. (2019c) insisted that the metamorphism

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the protolith subduction.

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occurred at lower crustal level of thick oceanic arc, seamount or oceanic plateau prior to

The quartz-bearing eclogite body is exposed between the Iratsu and the Higashi- Akaishi bodies. It is mainly composed of quartz, omphacite, garnet, kyanite, phengite, epidote and amphibole (e.g., Okamoto et al., 2004; Miyamoto et al., 2007). The peak metamorphic condition of the eclogite- facies metamorphism is estimated to be as T = 700–750 °C and P = 2.0–2.5 GPa (Enami, 1996; Miyamoto et al., 2007). Banno and Yokoyama (1977) suggested that the quartz-bearing eclogite was originated from the igneous rock crystallized from the Si-rich residual melt during fractional crystallization. On the other hand, Takasu et al. (1994) suggested that the protolith is

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deep-sea sediment with volcaniclastics based on the field observations. Okamoto et al. (2004) performed the zircon U–Pb dating of the quartz-bearing eclogite, and yielded variable ages of the igneous core domains from 1900 to 130 Ma, suggesting that those zircons are detrital zircons from trench-fill sediments. Utsunomiya et al. (2011) argued the protolith is volcaniclastic sediments formed in a setting close to an oceanic island

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arc based on the whole-rock compositions in combination with the presence of detrital zircons shown by Okamoto et al. (2004).

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Okamoto et al. (2004) and Arakawa et al. (2013) reported metamorphic zircon

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U–Pb ages from 120 to 110 Ma in the quartz-bearing eclogite, which were interpreted as

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the timing of the eclogite- facies metamorphism. On the other hand, Endo et al. (2009) reported 115.9 ± 0.5 Ma of garnet-omphacite Lu–Hf isochron age in the mafic gneiss of

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the Iratsu body, which was interpreted as the timing of the epidote-amphibolite

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metamorphism before the eclogite- facies metamorphism because the analyzed garnets

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with high Lu contents were grown at the timing. Thus, one of our targets is to identify what the 120–110 Ma mean from the point of view of the LA-ICP-MS zircon geochronology and geochemistry. 3. Sample description 3.1. Petrography In this study, we collected three mafic gneisses (sample No. TN18-06, TN18-07 and TN18-09) and three psammitic schists (sample No. TN18-05, TN18-08 and TN18-10) in the western part of the Iratsu body, and one eclogite (sample No. MS-06) in the quartz-bearing eclogite body (Fig. 1d).

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The mafic gneisses show weak gneissose structures of melanocratic and leucocratic parts (Fig. 2a, b). The former part is composed mainly of amphibole and epidote. The latter contains quartz and albite. Those minerals are elongated in parallel to the gneissosity. The garnet grains form subhedral porphyroblasts (varying in size from 100 to 500 μm) irrespective of the gneiss structures. In addition, phengite, rutile, apatite

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and zircon occur as accessory phases.

The psammitic schists are interlayered or enclosed within the mafic gneiss (Fig.

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2c, d). They show schistosity parallel to the gneissosity of the enclosing hornblende-rich

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mafic gneiss. The psammitic schists are composed mainly of quartz, phengite, albite,

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garnet and amphibole with subordinate amounts of epidote, apatite, rutile, titanite and zircon. The preferred orientation of phengite, amphibole, quartz, and albite in the matrix

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define the schistosity. The garnets occur as euhedral porphyroblasts (up to 1mm across).

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and quartz as inclusions.

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Some albite grains occur as porphyroblasts (up to 5 mm across) that contain phengite

As described above, it is known that all parts of the Iratsu body underwent eclogite-facies metamorphism during subduction (e.g., Ota et al., 2004; Endo et al., 2009, 2010, 2012; Aoya et al., 2013). However, index minerals for the eclogite metamorphism, such as omphacite, was not found in our studied samples. Thus, they indicate that the two studied samples preserve the mineral parageneses formed by retrograde metamorphism during exhumation. The quartz-bearing eclogite sample is composed mainly of garnet, omphacite, quartz, kyanite, barroisite, and phengite (Fig. 2e, f). In addition, rutile, apatite, and

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zircon occur as accessory minerals. The garnet forms euhedral varying from 500 to 5000 μm in size, and include rutile and quartz. The omphacite and kyanite occur as anhedral grains varying 500 to 2000 μm in size. Some omphacites are enclosed by granular barroisite, and contain rutile and quartz. The quartz and phengite occur in the matrix as elongated or granular forms between garnet, omphacite and kyanite.

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3.2. Internal structures of zircons

For separating zircon grains from samples TN18-06, TN18-08, and MS-06, each

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rock sample was crushed into fragments in an iron mortar, which were subsequently

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passed through a sieve of 250 μm. Subsequently, zircons were concentrated by

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conventional panning and magnetic separation. Finally, the separated zircons were mounted on an acrylic disc and polished. Their internal structures were observed with a

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XM-26740PCLI cathodoluminescence (CL) spectrometer coupled to a JXA-8230

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Science (OUS).

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electron probe microanalyzer (JEOL Ltd., Tokyo, Japan) at the Okayama University of

The zircons in TN18-06 show variable grain size from 100 to 600 μm in size, and usually rounded subhedral shapes. Most of them have internal zonal structures in their CL images, which are composed of core and rim domains (Fig. 3a). The core domains have homogeneous, mosaic or weak zoning textures in their CL colors. The rim domains exhibit a different luminescence from the core domain in the CL images. The core–rim boundaries are usually rounded. The zircons in TN18-08 range from 50 to 200 μm in size, and are usually rounded subhedral shape. Distinctive zonal structures characterized by the core and rim

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domains are observed in the CL images (Fig. 3b). The core domains have dark-to-bright luminescence, and show oscillatory zoning. The rim domains also have dark-to-bright luminescence, and show homogeneous or disordered textures. Some rim domains apparently penetrate the core domains. The core–rim boundaries are usually irregular. The zircons in MS-06 range from 50 to 200 μm in size, and are usually rounded

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subhedral shape. Most of them show homogeneous or disordered textures in the CL images (Fig. 3c). A few grains have zonal structures characterized by the core and rim

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domains in the images (grain No. 2, 7 and 13 in Fig. 3c).

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4. Analytical procedures

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4.1. Zircon U–Pb dating

In situ zircon U–Pb dating of above three samples (TN18-06, TN18-08 and

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MS-06) was carried out using an iCAP-RQ single-collector quadrupole ICP-MS

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(Thermo Fisher Scientific, Waltham, USA) coupled to an Analyte G2 laser ablation

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(LA) system that utilizes a 193 nm ArF excimer laser (Teledyne Cetac Technologies, Omaha, USA) in the OUS.

The zircon mounts were set in the two- volume HelEx2 sample cell of the LA system. The areas free of cracks and inclusions in each zircon were chosen for the analysis using a LA camera, utilizing transmitted and reflected light. In this study, the core domains of zircons in TN18-06 were analyzed by a laser of 25-μm diameter with fluence of 2.0 J/cm2 and repetition rate of 5 Hz. The U–Pb ages of the rim domains could not be calculated because of low U and Pb signal intensities; this was also the case in the previous analysis of the zircon rim domains from the metagabbroic rock in

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the Tonaru body (Aoki et al., 2019c). In contrast, the zircon core and rim domains in TN18-08 were analyzed by a laser of 15- or 12-μm diameter with fluence of 1.8 J/cm2 and repetition rate of 4 Hz, respectively. The zircons in MS-06 were analyzed by a laser of 25- or 15-μm diameter with fluence of 2.0 or 1.8 J/cm2 , and repetition rate of 5 or 4 Hz, respectively.

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The analytical areas were ablated in He carrier gas which was introduced into the HelEx2 sample cell and its arm part. The ablated materials of the samples in the He

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carrier gas were passed through the signal-smoothing device “squid” and mixed with Ar

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make-up gas prior to the ionization at the ICP-MS.

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The ICP-MS was optimized using continuous ablation of a NIST SRM 612 glass standard to provide maximum sensitivity while maintaining low oxide formation

U) were analyzed. The 91500 zircon (Wiedenbeck et al., 1995, 2004) for correcting

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238

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(232 Th16 O/232 Th < 1%). On the ICP-MS, 6 nuclides (202 Hg, 204Pb, 206 Pb, 207 Pb, 232 Th and

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mass bias of Pb/U and Th/U ratios, and NIST SRM 612 for Pb/Pb isotopic ratios were analyzed as calibration materials. The detailed method of the analysis is described in Appendix 1.

The age data to be discussed below was in the following way: ages of the zircons younger than 1000 Ma were based on their zircons were based on

207

206

Pb/238 U ratios, whereas older

Pb/206 Pb ratios. The concordance is defined as the value of

100% × (206 Pb/238 U age)/(207 Pb/235 U age) for the former and that of 100% × (206 Pb/238 U age)/(207 Pb/206 Pb age) for the latter. Eventually, the ages of the zircons with a concordance range of ≥ 90 % and ≤ 110 % were adopted as concordant ages.

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4.2 Zircon trace-element analysis In-situ trace-element analyses of the zircons from the mafic gneiss (the core domains of TN18-06 zircons) and quartz-bearing eclogite (the core and rim domains of MS-06 zircons) were performed by using LA-ICP-MS. In this study, the zircons from the psammitic schist (TN18-08) were not analyzed because their rims were

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unfortunately too small to analyze.

For constraining the protolith tectonic setting, the trace-element analysis o f

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TN18-06 zircons was performed by using an Agilent 8800 single-collector triple

e-

quadrupole ICP-MS (Agilent Tech., Santa Clara, USA) coupled to a NWR-213 LA

Pr

system that utilizes a 213-nm Nd:YAG laser (ESI, Portland, USA) at the Gakushuin University.

al

The zircons were set in the two- volume sample cell. The areas adjacent to the

rn

pits generated by the U–Pb dating within the core domains were chosen for the analysis.

Jo u

The laser was operated with fluence of 2.0 J/cm2 , repetition rate of 5 Hz, and laser spot size diameter of 25 μm. The ablated materials were carried to the ICP-MS by He carrier gas with Ar make-up gas through the signal-smoothing device (a 500- mL polypropylene bottle filled with quartz beads). The ICP-MS was optimized using the continuous ablation of a NIST SRM 610 glass standard to provide maximum sensitivities before the analysis. For removing potential interference from polyatomic ions (e.g., ions (e.g.,

90

Zr2+ to

45

29

Si16 O+ to

45

Sc+) and doubly-charged

Sc+), oxygen gas was introduced into the collision/reaction cell

between the tandem quadrupole mass spectrometer as a reaction gas, resulting in a mass

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shift of +16 amu for some analyzed elements (e.g., Nakano, 2015). As a primary calibration standard and internal standard element, NIST SRM 610 and

28

Si16 O were

used. More detailed analytical procedures are given in Appendix 2. For estimating the coexisting minerals at the time of zircon growth, in-situ trace-element analysis of the zircon from MS-06 was performed by using the same

oo f

LA-ICP-MS as that used for U–Pb dating in the OUS. The same areas as those yielding the concordant U–Pb ages and core domains of grain No. 2 were chosen for this

pr

analysis. The laser was operated with fluence of 1.7 J/cm2 , repetition rate of 5 Hz, and

e-

laser spot size diameter of 25 μm. Other laser conditions were same as the analyses of

Pr

U–Pb dating.

The ICP-MS was optimized using continuous ablation of a NIST SRM 612 glass

al

standard to provide maximum sensitivity while maintaining low oxide formation

rn

(232 Th16 O/232 Th < 1%). As primary calibration standard and internal standard element,

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NIST SRM612 and 28 Si were used, respectively. More detailed analytical procedures are given in Appendix 3.

Analyzed REE compositions of the TN18-06 and MS-06 zircons were normalized by CI-chondrite compositions (McDonough and Sun, 1995). Anomalous behaviors of Ce and Eu in chondrite-normalized (CN) patterns were expressed as Ce/Ce* = CeCN/(LaCN×PrCN)1/2 and Eu/Eu* = EuCN/(SmCN×GdCN)1/2 , respectively. 5. Results 5.1. Mafic gneiss Seventeen concordant data were obtained from the core domains of 17 zircon

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grains. All data are listed in Table 1. Representative analyses with their CL images are also shown in Fig. 3a. And,

206

Pb/238 U–207 Pb/235 U concordia curve (Wetherill concordia

diagram) made with Isoplot v.4.15 (Ludwig, 2003, and its update) is shown in Fig. 4a. Their 206 Pb/238 U ages range from 187.2 ± 2.9 Ma to 199.1 ± 3.4 Ma, and form a tight cluster at the concordia at ca. 195 Ma (Fig. 4b). The weighted mean

206

Pb/238 U age of

oo f

them is 194.0 ± 1.7 Ma (95% confidence level, MSWD = 4.4; Fig. 4b). Their Th/U ratios range from 0.23 to 0.64.

pr

Trace-element compositions of the core domains are shown in Table. 2 and Fig.

e-

5. Chondrite-normalized REE patterns commonly show positive Ce anomalies (Ce/Ce*

Pr

> 1), negative Eu anomalies (Eu/Eu* from 0.17 to 0.40) and enrichment HREE relative

5.1.2 Psammitic schist

al

to LREE and MREE (Ybcn /Smcn from 45 to 226 and Ybcn /Gdcn from 13 to 40) (Fig. 5a).

rn

The U–Pb ages with their isotopic and elemental ratios are listed in Table 3. The

207

Jo u

representative analyses with their CL images are also shown in Fig. 3b. The

206

Pb/238 U–

Pb/235U concordia curve is shown in Fig. 6a, b. In the core domains of 56 zircon grains, 56 concordant data were obtained. The

core domains have the following age groups: Early-to-Middle Cretaceous (ca. 140-120 Ma; 5 data of 5 grains), Late Triassic to Middle Jurassic (ca. 210-170 Ma; 23 data of 23 grains), Late Triassic (ca. 230-220 Ma; 6 data of 6 grains), Middle-to-Late Permian (ca. 270-250 Ma; 5 data of 5 grains) and Paleo-to-Middle Proterozoic (ca. 2500-1100 Ma; 17 data of 17 grains). The youngest 206 Pb/238 U age is 121.3 ± 4.0 Ma of grain No. 125. The Th/U ratios of the core domains range from 0.03 to 2.2. Except for grain No. 31 and

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121, the ratios are higher than 0.1. On the other hand, in the rim domains, two concordant data of grain No. 8 and 74 were obtained (Table 3, Figs. 3b and 6a, b). Their 206Pb/238 U ages are 120.4 ± 7.7 and 118.7 ± 7.7 Ma, respectively. Both of their Th/U ratios are lower than 0.1. 5.1.3 Quartz-bearing eclogite

oo f

Sixteen concordia data of 13 zircon grains were obtained from rim domains in zircons with zonal textures, and homogeneous or disordered zircons. Their U–Pb

206

Pb/238 U–207 Pb/235 U

Pb/238 U ages vary from 122.4 ± 4.9 Ma to 92.6 ± 8.8 Ma.

Pr

The obtained

206

e-

concordia curve is shown in Fig. 7a.

pr

isotopic data and ages, and Th/U ratios are listed in Table 4. And,

Apparently, those ages are irrespective of their domains and textures. In addition, their

al

Th/U ratios are consistently lower than 0.1.

rn

Trace-element compositions including their REE are listed in Table 5. In

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addition, their chondrite-normalized REE patterns are shown in Fig. 7b. Three data show positive Ce anomalies (Ce/Ce* > 1), even though the anomalies of another 11 data could not be calculated due to low intensities of La and/or Pr. The core domain of grain No. 2 shows higher Th/U ratios over 0.1, negative Eu anomalies (Eu/Eu* = 0.33) and prominent HREE enrichment relative to LREE and MREE (Yb cn /Smcn = 211 and Ybcn /Gdcn = 34) (Fig. 7b). On the other hand, rim domains of zircons having zonal textures and zircons with disordered or homogeneous areas consistently show slight or no negative Eu anomalies (Eu/Eu* from 0.74 to 1.11) and reduced HREE enrichment relative to LREE (Ybcn /Smcn from 2.3 to 35.9) with lower Th/U ratios less than 0.1.

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Especially, six data show HREE depletion relative to MREE (Yb cn /Gdcn < 1). 6. Discussion 6.1. The magmatic age and tectonic setting of the Iratsu body The weighted mean

206

Pb/238 U age obtained from the core domains of zircons

separated from the mafic gneiss (TN18-06) in the Iratsu body is 194.0 ± 1.7 Ma (Fig.

oo f

4b). Even though their core domains do not show oscillatory zoning in the CL images which are typical of those crystallized from melts (e.g., Corfu et al., 2003), their Th/U

pr

ratios are within the range of the typical igneous zircons (Table 1; e.g., Rubatto 2002).

e-

In the diagram of Ta vs. Nb (Fig. 5b), the analyzed zircons are plotted in the same

Pr

domains as zircons crystallized from mafic melt compiled by Belousova et al. (2002). It is known that some zircons formed from the mafic melt have broad zonings or

al

homogeneous textures in the CL image (e.g., Dai et al., 2012). These visual and

rn

geochemical characteristics lead us to the conclusion that the core domains are igneous

Jo u

in origin and the weighted- mean age of 194.0 ± 1.7 Ma represents the timing of the magmatism forming the substantive mafic protoliths of the Iratsu body. The trace-element abundance of the zircon plotted on the bivariate diagrams of U/Yb vs. Y, U/Yb vs. Hf, Nb/Yb vs. U/Yb and Nb/Yb vs. Sc/Yb could be proxies for the tectono-magmatic settings where the parental magma of the zircons was formed (e.g., Grimes et al., 2007, 2015; Sawada et al., 2019; Aoki et al., 2019c). The analyzed zircons tend to have higher U/Yb ratios (Fig. 5c and 5d) and are plotted on the “Continental zircon” domain (Grimes et al., 2007). In addition, the analyzed zircons have higher U/Yb and Sc/Yb ratios than those of the “MOR- ” (middle oceanic ridge)

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and “OI-type” (oceanic island) zircons, and their compositions are mainly plotted within the domain of the “Arc-type” in the diagrams of Nb/Yb vs. U/Yb and Nb/Yb vs. Sc/Yb (Grimes et al., 2015) (Fig. 5e and 5f). The series of geochemical characteristics of the Iratsu zircons indicate that the protolith of the Iratsu mafic gneiss was originally formed in an arc-related magmatic setting. Previous structural and geologic studies for the

oo f

Iratsu body indicated that the metaclastic and metacarbonate rocks within the body were originally deposited in an oceanic environment (e.g., Wada et al., 1984) (Fig. 8a). Hence,

pr

the depositional- magmatic setting for the Iratsu body is reasonably regarded as an

e-

oceanic-arc environment rather than a continental-arc one. The metaclastic and

Pr

metacarbonate rocks were probably deposited on the oceanic-arc basalts at a topographic high shallower than the Carbonate Compensation Depth (CCD). This

al

interpretation is supported by the whole-rock geochemical signature of the metamafic

rn

rocks in the Iratsu body showing a setting proximal to the oceanic island arc

Jo u

(Utsunomiya et al., 2011).

6.2. Timing of deposition in and subduction from a trench The core domains of the zircons in the psammitic schist (TN18-08) of the Iratsu body show oscillatory zonings in their CL images (Fig. 3b). And, most of their Th/U ratios are higher than 0.1. Thus, they suggest that those core domains were crystallized from melts, and their U–Pb ages indicate the crystallization ages. The youngest igneous 206

Pb/238 U ages of the detrital zircons in accretionary prisms can place an upper limit on

the timing of the deposition of the rocks in a trench (e.g., Dickinson and Gehrels, 2009). The youngest age of the igneous core domains (grain No. 125) in TN18-08 is 121.3 ±

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4.0 Ma (Table 3 and Fig. 6a, b). Thus, the protolith of the psammitic schist was deposited in and subducted from a trench after 121.3 ± 4.0 Ma. Even though this study could not show concordant U–Pb ages of igneous zircons from the quartz-bearing eclogite body, Okamoto et al. (2004) also showed igneous 238

U/206 Pb and

207

Pb/206 Pb ages of ca. 1900–130 Ma, which are similar to the age range

oo f

of the igneous core ages of the psammitic schist of the Iratsu body. The age similarities between the metaclastic rocks of the Iratsu and quartz-bearing eclogite bodies indicate

pr

that both bodies likely have the same tectono-deposition history.

Pr

a trench after ca. 120 Ma (Fig. 8b).

e-

Thus, the protoliths of both bodies were likely deposited in and subducted from

6.3. Duration of the eclogite metamorphism

al

The analyzed areas of zircons from the quartz-bearing eclogite MS-06 show

rn

homogeneous or disordered textures in their CL images (Fig. 3c). Moreover, their Th/U

Jo u

ratios are also extremely low (< 0.1) (Table 4). Thus, those zircons were also grown at the metamorphism because metamorphic fluids are generally enriched in U relative to Th (e.g., Rubatto, 2002). Additionally, those U–Pb ages ranging from 122.4 ± 4.9 Ma to 92.6 ± 8.8 Ma represent the timing of the subduction-related metamorphism (Table 4 and Figs. 3c, 7a). It is worth noting that the age range is wider than the previously-reported age range (ca. 120–110 Ma) in the metamorphic rim domains (Okamoto et al., 2004; Arakawa et al., 2013). In MS-06, the core domains of grain No. 2 show negative Eu anomalies (Eu/Eu* = 0.33) and HREE enrichment relative to LREE and MREE (Yb CN /SmCN = 210.6,

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YbCN/Gd CN = 33.7) (Fig. 7b). In addition, it has high Th/U ratio (> 0.1). Thus, those REE chemical characters suggest that the domain has been crystallized from a magma with a reactive bulk composition of negative Eu anomalies and HREE enrichment at plagioclase-stable and garnet-unstable conditions (e.g., Murali et al. 1983; Hoskin and Ireland 2000). On the other hand, all analyzed disordered or homogeneous domains

oo f

including rim parts show slight or no negative Eu anomalies (Eu/Eu* from 0.74 to 1.11) with lower Th/U ratios less than 0.1 (Fig. 7b and Table 5). In addition, they commonly

pr

show reduced HREE enrichment relative to LREE and MREE (Ybcn /Smcn from 2.3 to

e-

35.9, Ybcn /Gdcn from 0.6 to 7.6) (Fig. 7b), compared with that of the igneous domain.

Pr

Those REE characteristics of zircons indicate that the zircons formed from the metamorphic fluids with no or minimal negative Eu anomalies and reduced HREE

al

enrichments at the depth of the eclogite- facies conditions under breakdown of

rn

plagioclase, and existence of pyroxene and garnet minerals (e.g., Rubatto, 2002;

Jo u

Spandler et al., 2003; Korh et al., 2009). Although there is a possibility that the U–Pb system of the metamorphic zircon under the eclogite-facies conditions was altered by the subsequent high- temperature metamorphism up to 950 °C (Štípská et al., 2016), previous petrological studies for the Sanbagawa eclogites show that the eclogite had not been suffered from such a high-temperature metamorphism (e.g., Enami, 1996; Miyamoto et al., 2007). Therefore, the metamorphic zircon U–Pb ages obtained from MS-06 represent the timing of the eclogite- facies metamorphism of the quartz-bearing eclogite body from ca. 120 to 90 Ma at least (Fig. 8c, d, e). Around the Iratsu area, the metamorphic rocks preserve relicts of the peak

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eclogite-facies metamorphic conditions, similar to the quartz-bearing eclogite body (e.g., Ota et al., 2004; Endo et al., 2009, 2010). In the psammitic schist (TN18-08) of the Iratsu body, the rim domains of the zircons show homogeneous textures in their CL images (Fig. 3b). Additionally, the Th/U ratios of concordant data are extremely low (< 0.1) (Table. 3). Therefore, those textures and Th/U ratios suggest that the rim domains

oo f

were formed during metamorphism, and their 206 Pb/238 U ages of 120.4 ± 7.7 and 118.7 ± 7.7 Ma (grain No. 8 and 74) represent the timing of the subduction-related

pr

metamorphism (Figs. 3 and 6, and Table 3). As described above, the time when the

206

Pb/238 U ages of the Iratsu body are consistent

Pr

Moreover, the obtained metamorphic

e-

protoliths of the Iratsu and quartz-bearing eclogites reached in a trench is almost same.

with those of the quartz-bearing eclogite body. Thus, it is reasonable that all parts of the

al

Iratsu and quartz-bearing eclogite bodies were subducted from a trench at ca. 120 Ma,

rn

and underwent eclogite-facies metamorphism from ca. 120 to 90 Ma (Fig. 8c, d, e).

Jo u

6.4. Spatial distribution of the Besshi unit, and its contrast to an AC In the Besshi area, the metamafic body called the Tonaru body is distributed at the same structural level as the Iratsu and quartz-bearing eclogite bodies (e.g., Ota et al., 2004; Yamamoto et al., 2004; Terabayashi et al., 2005; Aoya et al., 2013; Miyagi and Takasu, 2005). This body also preserves relicts of eclogite-facies metamorphism in some parts (e.g., Miyagi and Takasu, 2005). In addition, the zircon U–Pb dating and trace-element abundance of the metagabbroic rock in the body show that the protolith was formed by intra-oceanic arc magmatism from 200 to 180 Ma (Aoki et al., 2019c). These geological, geochemical, and geochronological similarities among the Tonaru,

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Iratsu and quartz-bearing eclogite bodies suggest that the three bodies had been experienced the same tectono-metamorphic history, and there is no problem with regarding the three bodies as a part of same metamorphic unit (Besshi unit) proposed by Aoki et al. (2019a) (Fig. 1b, c). As described above, because the Higashi- Akaishi peridotite body has a geological connection with those metamafic bodies, the peridotite

oo f

body is considered to be part of the Besshi unit in this study.

As for the corresponding AC to the Besshi unit, the Late Jurassic-to-Early

pr

Cretaceous radiolarians in the acidic tuffs within the mudstones have been reported

e-

from the Sanbosan AC of the southern part of the Chichibu composite belts in Shikoku

Pr

(Okuda et al., 2005). This AC is situated at the structurally lowermost part of the composite belts. In addition, Aoki et al. (2012) report the youngest U–Pb age of 157.7 ±

al

5.2 Ma from the detrital zircons of the sandstone in this AC. The Mikabu greenstones

rn

are tectonically overlain by the northern part of the Chichibu composite belts, and the

Jo u

Late Jurassic radiolarians have been reported from the red volcanic phyllite interbedded with the greenstones (e.g., Iwasaki et al., 1984; Aoki et al., 2007). Sawada et al. (2019) report a weighted mean age of 154.6 ± 1.6 Ma calculated from igneous zircon in the greenstones. Those structural and age data suggest that the Sanbosan AC and Mikabu greenstones were formed and subducted in a trench at similar timing to the Besshi unit. Thus, there is considerable validity in that the Sanbosan AC and Mikabu greenstones are regarded as the original AC of the Besshi unit (Aoki et al., 2019a). 6.5. Tectonic implications for the Sanbagawa metamorphic rocks This study reveals that the Besshi unit was originally a topographic-high part on

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a seafloor formed by the ca. 200–180 Ma intra-oceanic arc magmatism, and the unit underwent the prolonged eclogite-facies metamorphism from ca. 120 to 90 Ma after subduction. Such a 30-Myr age span suggests the detachment from the subducting slab and stagnation of the Besshi unit at the depth of the eclogite-facies conditions. It is well-known that the subduction of the topographic-high parts such as

oo f

seamounts, plateau and ridge are subducted underneath the forearc in modern subduction zone such as Japan and Andes (e.g., Yamazaki and Okamura, 1989; Uchida

pr

et al., 2010; Gutscher et al., 1999; Baudino and Hermoza, 2014 ), and those parts are left

e-

behind in mantle wedge at a depth of 25–50 km due to detachment from the subducting

Pr

slab driven by the resistance against their subductions (Uchida et al., 2010). Thus, it is inferred from these studies that the subduction of the topographic-high material like

al

intra-oceanic arcs and the resistance against its subduction occurred underneath the

rn

forearc in the western margin of the Cretaceous Paleo-Pacific Ocean during the

Jo u

subduction of the Besshi unit. This is likely the reason why the subducted Besshi unit could be detached from the subducting slab, possibly at the depth of the eclogite- facies conditions (Fig. 8c). Moreover, not only the detachment from the slab but also the trench movement toward the ocean side might have caused the stagnation of the Besshi unit at the eclogite depth (Fig. 8d). Intermittent oceanward growth of ACs during the Cretaceous has been confirmed in Japan (e.g., Isozaki et al., 2010; Aoki et al., 2012). This means that the entire arc-trench system moved oceanward at the time. It is quite likely that this oceanward movement depressed the function of subduction to drag the detached Besshi unit into a deeper depth than its eclogite depth.

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The Besshi unit is tectonically sandwiched by the Asemi- gawa unit that exists as a younger unit with a subduction age of 100–90 Ma (Aoki et al., 2019a; this study). As described above, some metamorphic rocks in the oligoclase-biotite and albite-biotite zones in the Asemi- gawa unit preserve the relicts of the eclogite- facies metamorphism that occurred at ca. 90 Ma, and the metamorphic P–T conditions of the Besshi unit are

oo f

equivalent to or higher than those of the Asemi- gawa unit (e.g., Wallis et al., 2009; Aoki et al., 2009; Taguchi and Enami et al., 2014a, b; Aoki et al., 2019a; Aoki et al., 2019b).

pr

The eclogite rocks in both units are situated at the intermediate structural level within

e-

the whole Sanbagawa metamorphic rocks (Yamamoto et al., 2004; Ota et al., 2004;

Pr

Terabayashi et al., 2005). These field observations require juxtaposition with the Besshi and Asemi- gawa along the subduction zone at the depth of the eclogite- facies condition

al

(over ca. 40 km depth) at ca. 90 Ma (Fig. 8e; Wallis et al., 2009; Aoya et al., 2013; Aoki

rn

et al., 2019a; Aoki et al., 2019b). On the other hand, HP-type metamorphic rocks

Jo u

formed by the subduction from 120 to 90 Ma must have existed between the Besshi and Asemi- gawa eclogites. However, such rocks are presently not confirmed in the Besshi- Asemi- gawa regions. Thus, the juxtaposition of the both units at the depth of the eclogite facies conditions could have been caused by erosion of pre-existing HP rocks along the subduction zone like tectonic erosion occurred at ca. 90 Ma (e.g., von Huene and Lallemand, 1990; Vannucchi et al., 2008; Aoki et al., 2012). In the Asemi- gawa unit, U–Pb dating of metamorphic zircon including retrograde phengite yields an age of 85.6 ± 3.0 Ma, suggestive of the timing of retrograde metamorphism during exhumation (Aoki et al., 2009). White mica K–Ar and

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Ar–Ar datings also yield ca. 90–80 Ma of retrograde metamorphism in the Besshi and Asemi- gawa units (Dallmeyer and Takasu, 1991; Itaya and Tsujimori, 2015). The similar retrograde metamorphism ages in both units suggest that the units were coevally exhumed to the surface soon after the juxtaposition (Fig. 8f). In other words, it suggests that exhumation mechanisms of HP rocks were more activated during ca. 90–80 Ma.

oo f

The mechanisms might be related to the subduction angle of slab and fluid injection into hanging wall materials (Maruyama et al., 2010; Aoki et al., 2019b).

pr

7. Conclusions

e-

In the Iratsu body of the Besshi area in the SMB, igneous zircon U–Pb dating of

Pr

the metamafic rock yields an age of ca. 195 Ma, which is within the age range (ca. 200– 180 Ma) of the igneous zircons of the metagabbroic rock of the Tonaru body, which is

al

situated at the same structural level. Moreover, the trace-element characters are similar

rn

to those crystallized by arc magmatism. Combined with the field observations, the

Jo u

Iratsu and Tonaru bodies were originally the topographic high of the mafic rocks formed by the oceanic-arc magmatism, on which the sedimentary protoliths of the Iratsu and Tonaru bodies were deposited. Moreover, igneous and metamorphic zircon U–Pb dates in the psammitic schist of the Iratsu body are ca. 2500–120 Ma and ca. 120 Ma, respectively. The zircon U–Pb datings of quartz-bearing eclogite yield metamorphic ages of ca. 120 to 90 Ma; their metamorphic zircons show no or slight negative Eu anomalies and suppressed HREE enrichment. The U–Pb data from this and previous studies suggest that the Besshi unit was deposited in and subducted from a trench at ca. 120 Ma. The deposition and

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accretion age similarity suggest that the Besshi unit is the HP metamorphic equivalent of the non- and weakly metamorphic Sanbosan AC and the Mikabu greenstones. Following subduction, the Besshi unit was stagnated at the depth of the eclogite- facies conditions from ca. 120 to 90 Ma. The stagnation processes might require the detachment of the Besshi unit from the subducting slab and the oceanward retreat of the

oo f

subduction zone. The detachment might have been driven by the high resistance against the subducting motion of the high topographic materials of the Besshi unit on the

pr

seafloor underneath the forearc.

e-

At ca. 90 Ma, the Besshi unit was juxtaposed with the younger subducted

Pr

Asemi- gawa unit at the eclogite- facies condition. After ca. 90 Ma, both units were coevally exhumed.

al

Acknowledgments

rn

We thank T. Ohno for his supports for LA-ICP-MS analyses. We are also grateful to M. Scambelluri, T. Itaya and I. Y. Savonova for their helpful comments on this manuscript.

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This study was financially supported by Private University Research Branding Project (OUS International Research Project on Mongolian Dinosaurs) and JSPS KAKENHI Grant Number JP16K21531 (K.A.). Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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metamorphic terrain. Contributions to Mineralogy and Petrology 73, 1–13. Figure Captions

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Figure 1 (a) Distribution of the Sanbagawa metamorphic belt in Japan, and simplified geological map of the Shikoku island. (b) Metamorphic zones of the Besshi and

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Asemi- gawa areas based on the present mineral parageneses of pelitic schists; the chlorite, garnet, albite-biotite and oligoclase-biotite zones in ascending order of

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metamorphic grade (modified from Higashino, 1990; Aoya et al., 2013, 2017). Distribution of the Besshi, Asemi- gawa, and Oboke units (modified from Aoki et al., 2019 (a)) is also shown. (c) Cross section A-B-C in Fig. 1(b) modified from Ota et al. (2004) and Yamamoto et al. (2004). (d) Geological map of the Iratsu body (the western part) and the quartz-bearing eclogite showing the sample locality (modified from Aoya et al., 2013). Figure 2 Polished slab and microphotographs showing representative textures and minerals of (a), (b) the mafic gneiss (TN18-06), (c), (d) the psammitic schist (TN18-08) and (e), (f) the quartz-bearing eclogite (MS-06). Mineral abbreviations: Qtz, quartz; Amp, amphibole; Ep, epidote; Grt, garnet; Ab, albite; Phg, phengite; Omp, omphacite; Ky, kyanite Figure 3 Representative cathodoluminescence (CL) images showing internal structure of the analyzed zircons from (a) the mafic gneiss (TN18-06), (b) the psammitic schist

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(TN18-08) and (c) the quartz-bearing eclogite (MS-06). The

206

Pb/238 U ages are shown

for each analysis spot (white circles). Dashed lines indicate the boundaries between the core and rim domains. Figure 4 Summary of the concordant zircon U–Pb ages from the mafic gneiss (TN18-06). (a) Wetherill concordia diagram of with error bars (2 sigma). (b) Weighted mean of 206 Pb/238U ages with error bars (2 sigma). Figure 5 Summary of trace-element compositions of zircons from the mafic gneiss (TN18-06). (a) Rare-earth element (REE) concentrations normalized by that of C1

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chondrite (McDonough and Sun, 1995). (b) The plot of Ta va. Nb with the fields of zircon compositions for different rock types (Belousova, 2002). The plot of (c) Y vs.

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U/Yb, (d) Hf vs. U/Yb, (e) Nb/Yb vs. U/Yb, (f) Nb/Yb vs. Sc/Yb for discrimination of the tectono- magmatic source of the igneous zircons. The compositional variations of

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“Continental”, “Oceanic crust zircon” and “Kimberlite” shown in (c) and (d) are according to Grimes et al. (2007). Those of “Continental/Oceanic Arc-type”,

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“MOR-type” and “OI-type” in (e) and (f) are according to Grimes et al. (2015). The chemical compositions of the zircons from the metagabbro of the Tonaru body (Aoki et

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al., 2019c) are also shown in (c), (d), (e) and (f).

Figure 6 Wetherill concordia diagrams of (a) all concordant zircons and (b) < 200 Ma

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concordant zircons with error bars (2 sigma) from the psammitic schist (TN18-08). Figure 7 Summary of U–Pb dating and rare-earth element (REE) compositions of the

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zircons in the quartz-bearing eclogite (MS-06). (a) Wetherill concordia diagram of all concordant data in rim domains of zircons with zonal structure, or zircons with homogeneous or disordered textures. (b) REE concentrations normalized by that of C1 chondrite (McDonough and Sun, 1995). Red: the core domain of zircon (grain No.2 in Fig. 3 (c)) with zonal structures showing Th/U > 0.1. Green: the rim domains of zircon with zonal structures, or zircons with homogeneous or disordered textures, showing Th/U < 0.1. Figure 8 Tectonic history of the Besshi and Asemi- gawa units in the Sanbagawa metamorphic belt. (a) At ca. 200–180 Ma, the basaltic rocks were formed by intra-oceanic arc magmatism in the Paleo-Pacific Ocean (formation of the protolith of the mafic gneiss and metagabbro of the Iratsu and Tonaru bodies). The carbonate rocks (protolith of the marble) were formed on the topographic high of basaltic rocks on the seafloor. (b) At ca. 120 Ma, the clastic sediments (protolith of the psammitic schist

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TN18-08 and the quartz-bearing eclogite MS-06) were deposited on or close to the oceanic-arc rocks. After that, the basaltic rocks and the sedimentary rocks (the Besshi unit) were subducted from the paleo-Japan trench. (c) At ca. 120–110 Ma, the Besshi unit was subducted into a depth of eclogite- facies metamorphism. The subducting motion of the Besshi unit could be resisted at that depth due to its topographic characteristics, and finally, the unit was detached from the subducting slab. (d) At ca. 100–90 Ma, the protolith of the pelitic schists of the Asemi- gawa unit were deposited, and they were subducted from a trench. The Besshi unit was stagnated at the depth of

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the eclogite- facies metamorphism. (e) At ca. 90 Ma, a part of the Asemi- gawa unit was subducted into eclogite- facies metamorphism depth, and juxtaposed with the Besshi

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e-

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unit. (f) After ca. 90 Ma, the Besshi and the Asemi-gawa unit were exhumed coevally.

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Table 1 U-Pb data and Th/U ratios of mafic gneiss "TN18-06" zircon

Isotopic Ages (Ma) ratios grain

dom

207Pb/

No.

ain

235U

206Pb/

2σ

238U 0.0

6

7

core

core

09

7

048

0.0

0.0308

0.00

13

1

055

0.0

0.0304

0.00

core

12

5

051

0.0

0.0301

0.00

0.2165 12

9

0.0

0.0299

12

13

14

15

core

core

core

core

core

0.0312

0.00

al

052

0.0

0.0306

0.00

9.

049

0.0302

0.00

0.2222

8 10

193.3

5

047

0.0

0.0300

0.00

0.2154

.5 8.

6

048

0.0

0.0294

0.00

0.2163

9 9.

6

047

0.0

0.0301

0.00

0.2191

1 8.

9

058

0.0

0.0301

0.00

0.2206

5 8.

3

053

0.0

0.0310

0.00

0.2346

7 8.

8

052

0.

0.

4

47

02

3.

0.

0.

2

24

01

3.

0.

0.

2

33

01

3.

0.

0.

3

23

01

3.

0.

0.

1

43

01

3.

0.

0.

1

61

02

3.

0.

0.

0

52

02

3.

0.

0.

0

64

02

2.

0.

0.

9

58

02

3.

0.

0.

6

28

01

3.

0.

0.

3

31

01

3.

0.

0.

2

60

02

187.2 7 12 191.7 .5 10 191.4 .9 10

214.0 12

3.

190.9

202.5 13

01

192.1

201.1 15

33

194.3

198.8 10

0

198.1

198.1

10

0.

190.1

203.8

10

0.

191.8

207.8 0

3.

193.4

7

199.7 050

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11

2

0

0.0

core

9.

0.00

2σ

195.6

.1

051

11

11

10

/U

11

199.0

0.2097

0.2271

σ

9

202.3

rn

9

238U 196.6

199.5

0.2205

0.2173

Th

7.

0.2171

0.0 core

2

195.1

13 8

206Pb/

oo f

core

0.00

pr

5

core

0.0309

0.2119

e-

2

core

2σ

235U

Pr

1

207Pb/

2σ

197.3 .1

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0.0 10

9

046

0.0

0.0307

0.00

7.

0.2050

189.4 09

4

044

0.0

0.0313

0.00

12 192.7

15

7

055

0.0

0.0311

0.00

14 206.2

8

058

.4

pr

8

45

00

2.

0.

0.

7

44

00

3.

0.

0.

4

37

00

3.

0.

0.

7

35

00

197.9

oo f

17

0.

199.1 .4

0.2251

0.

195.2 9

0.2089

2. 196.1

4

e-

core

8. 187.7

Pr

19

core

0.00

al

18

core

0.0308

0.2031

rn

17

core

Jo u

16

Journal Pre-proof

Table 2 Trace-element compositions of mafic gneiss "TN18-06" zircon

Concentrati ons (ppm) Y

Y

b

b

C

C

N

N

/

/

S

G

g d

C E

a

o

T e u

i

m

S T

n

a

c i

N

i

o

n

N L C P N S E G T D H E T Y L H T T Y d m u d b y o r

0 0 2 0 1 0 .

5

5 6 0

1

.

1

Jo u 0

4

3 o

1 5

2

.

.

.

.

3

0 1

.

C

C

N

N

. .

3 .

5

. 1 3

1 8

0

0 6

.

.

4 2

5 0 2 8 1

7 3 1 8 4

2 0

. .

2 5 .

1 0

0 3

. .

5

8

1 2

7 6

2 3 9 0

9 0 2 1 2 9 .

. .

.

.

1 .

9 1 4

8 6 8 5 8 2

.

0

7

2

.

.

9

1

6

2

.

.

1

5

6

6

0

0

.

1 1 0 7 9 6 .

0 1 5

3 . .

3 0 0 5

7

5 9

2 1 5 1 1 1

.

6

1

*

4

0 5 4 2

7 e

.

0

6 2

.

2 r

U e u

0 3 3 8 0

0 9

0 0 1 0

5 1

C E m d

2 2 3 1

.

0

rn

9 9 0 7

.

0

5 0 8 5 6 1 0 7 8 8

c

/

*

0

1 9

.

5 0 5

2

.

0 7 0 4 .

5 8 e

.

0 .

0

.

4

. .

. r

U

a h

1

5 1

al

5

/

ePr

0

o

m b u f

h /

pr

b a e r

.

c

oo f

r

8 7 8 9

2 3 7 0 9 1

0 3

0 0 0 0 0 c

3 1

0 .

2

2 o

5 4

5

. .

r

.

.

1 1

.

. .

3 0 9

.

.

2 3

.

2

8 2

3 .

.

7 6

2 6 4 0 7

3 5 5

7

.

2

3 2

. .

2 6 8 2

4

2

6 0 .

.

3 9

8

1 6 0

6 2

.

5 3 1

1 8 9 8

1 1 9 9

6

.

2 0 9 4 4 4 7

3 7

8

0 2 3 1

6 e

.

0 .

2

1 3 1

. .

6

0

6 1

5 5

3 6

c

5 6 3 0 0 2 0 0 1 0 5 2 2 1 5 1 1 2 1 0 1 4

0 8 0

1

2

Journal Pre-proof

o

7 .

r

.

e

2

2 .

.

.

.

.

.

.

.

.

8 0 5 3 4 3 0 .

2 0 3 0 6 0 2 0 2 9 2 .

.

.

.

1 .

4 0 8 3 8 4 7 0 6 6 9 2 0 . 1

8 9

9 1 .

.

8

.

2

9

.

3

3

4

.

8 1 2 6 2

8

4

.

5

0 3 0 7

7 1 7 1 0 2 3

0 2 5 0 2

8 3

9 2

6

8 0 6 1

0 .

6

9 o

8 0

7

.

.

.

.

.

5 .

3 1 0

9 1

4 8 1 .

.

6 6 2 2 9 5

9 2 6

.

8

3

2

7 2 7

. .

r

. 3

7

.

9 .

.

0 1 5 1 9

7 3 e

.

.

5

.

.

4 5 1 8 9 3

5

1

7 3

0 3

o

7 3

9

. .

0 .

1 r

.

.

9 0

.

.

3 0 1

.

2 1 1

8 1

.

.

4

4

4

1

4

0

0

2

6 .

8

9

1 0 3 .

.

2 2

. 2

2 3 0

4

8 1

7 9

2 6

. .

0

1

2 1

2

9

.

9 2

1 5

4 6

4

.

2 3

5

3 7

2

. .

1

.

0

6

1

3

5 2 2

4 1

4 2 .

5

0 3

4 3 5 0

.

. .

1 1

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8

5

.

8 8

.

7 2

8

0

5

.

.

3

0 1

4 3 9 2 9 3

6

.

2

5 2 .

6

.

0 5 2 4

Jo u

e

.

0 2 6 1

1

1

3 1

rn

2

al

0 .

6

2 7

3

4

5

0 0 0 0 3 1

1 .

1

8

0 2 7 7

c

.

.

1

0

3 0 1 8 1 .

5

0

4 1 1 1 1

.

0

4

3

8

Pr

o 8

1 3 1

2

e-

0

0

.

2 7

pr

0 0 0 0 3

5

7 7 8 8

8

2 8

. 7 .

9 .

9 0

5 c

0 .

7 7

9 8 5 6 2

6

1 1 8 1 5

1 5 0 8 .

7 e

.

0 .

3 r

7 .

1 3 3 4 1

oo f

c

1

8 6

2 9

9

. .

8

4 6

8 7

4 0

1

0 4 7 1 3 1

c

7

1 0 .

6

1

o

7

.

2

6

.

6 7 2

.

.

2

1

6

7

3

0 0 6 0 1 3 0 2 6 7 2 1 2 2 3 1 0 6 1

0

0

6

1

.

.

.

3

5

6 6 2

.

.

6

3

5

2 .

r

.

.

2 0 .

9 5 2 8

5

3 7

3 7 4 3 .

6 e

.

0 .

. 0

.

7 2 9 4

5 .

5

. 2 9

2 6

8

0

3

.

1

. 3

2 .

4 7

. .

3 6

6 6

9

. 0 4 2 4 2 0

0 3 2 4 0

.

5 8 5 0 4 2 c

9

8 5

.

1

1 4 4 6 1

0

7 1

1 4

6 4

6 9 7 8

9

4

7

7 8 1

o

0 .

1

r

.

.

.

.

.

.

0 .

3 6 1 5 5 8 0 .

7 0

9

1

.

5 0 7 0 7 7 6 .

0 .

.

9 .

5 .

9 2 .

1

2 9 e

1

4 0 9 9 4 0 2 3 4 8 7 .

6 .

2 7 7 3 .

2

Journal Pre-proof

3

8 2 5 5 5

1 5 9 4 1 5 0

0

6 4

2

7

0 0 0 1 0 1 c

6 1

0 .

4

5 1

o

9 3

. .

r

.

.

.

.

1 .

6 0 6

2 7

.

.

.

0

0 2 9 0 8 9 9

0 0 0 0

1

8

.

.

3 6

4 2 1 4 7

1 7

3 1 8 7 5

2 0

. .

2 3 6 4 9 0 .

0

9 3

9 .

6

1 0

9 . .

3 1 9 6 4 8 5 3

3 9 9 0

0 9 3 1 1 4

0 6 8 2 .

8 e

.

0 .

8 2

5 1 9 2 2 3 3

. .

2

7

1 2

7 5

7 0

2

2 1

o

7 2

. .

r

.

.

2

7 7

.

. .

6

.

.

7 2

3

7 0

0 0 0 0 0 0 .

3

8 2 1

o

3

4

r

. .

.

.

.

.

3 0 4

0

.

5 5

rn

7 0

0 0 2 0 1

5 1

0 .

6

Jo u

c

6 1

o

1 0

.

.

5

r

.

.

8 6

.

.

2 . 3

0 9

0

9 7 4

1

4 0 2

.

9 .

8 3 0

8

0

9

2

.

2

3

2

.

.

0

9

5

9

1

0

0

4

1

6

.

.

4

3

.

6 7 2

.

.

5

9

4

7

9

0 3 0

5

1

1 .

.

5 7

8

1 1 2 9 7

8

0 .

2 3 9 6 8 .

. . 3

9 8

8

0

6 9 8 9 8

1

. .

2 7

5 9

4 0 9 2 2 6 1 .

6 5

9 8

0 .

.

4

4

2 2

1

3 0

9 4

.

3

.

1 7 5

0

. .

3

1 0

. .

9 5

0 0

5 2 9 2 2 3 1

0 8 2 4 .

7 e

.

8

4

0 .

1

0

1

5 1

0 6 4 8

.

. .

2 5

5 2

.

2

4

0

0 9

0 1

1 1

8

.

2

1

9 9

.

.

1 8 2 6 4 0 4

3

.

9 4

5 9

3 7

.

1

9 4

8

1

0 1 7 1

8 5 e

.

0 .

4

.

2

4 1

Pr

4

al

c

.

7 3

1

0

3 8

3

4 .

1 0

.

.

4

1

8

2

e-

9 2 3 6

1 5

0

4 1

.

. 8

8

1 1

8 9

5

.

0 8 7 2 0

3 6

7

0 0 4 1

9 e

.

.

1 3

3 1

oo f

3 1

pr

c

0 6

2 5 8

8 2 6 9 6 3 3

6

8 4

3 0

1 0 3 7 1 3 1

c

8 1 1 0 .

7

1 .

1

o

3 1 2 .

.

.

.

3 0

0 .

r

.

.

.

3 5 1 0

e

1

c

3

5 0

0 2

0 5

0 1 2 8

3

. 2

1 .

8

0 .

8 .

.

6 6

0 .

0

4 3 6 0 4 4

8

9 7 5 0 3

.

8 8 5 3 1 0 1

8 .

8 3 7 9 6 5

1 3 3 5 1

1 8

1 6 7 2 . 6

1 4

7 7

8 1 1 0 0 7 0 4 6 1 3 1 1 4 1 3 3 5 1 0 7 1

Journal Pre-proof

6

o

3 0 2 .

(

r

.

r

e

8 7 3 3 1 9 1 7 6 8 5 1 .

.

.

.

.

.

.

.

5 0 1 0 7 7 8 7 0 .

2 5 0 1 2 0 9 1 .

.

8 .

9 .

1 .

2 .

4 .

6 1

4 2 .

.

0

.

0

3

0

6

2

.

.

4 9 4 5 .

9

3

4

3

0

0

e

6

6 7 9 6 5 3 1 2 1 3 0 2 9

p

0

4

2 5

4

2

e ti ti

oo f

o n

pr

) 0

1

0 0 2 0 1 0 .

5

7 1

o

3 2

. .

r

.

.

.

.

7

1

0 3 4 0 2 5

1 0 .

6

8 1

o

9

.

0 .

0 .

. r

.

.

.

.

9

0

2

7 3

4

0

0

7

2

5

.

.

3

0

.

6 2 2

.

.

5

6

6

3

9

0

8

2

.

2

0

3

.

.

2

8

1

3

. 2

9 9

8

7

. .

6 .

6 .

0 .

1

2

0

.

5 9

2

1

.

6

8 0 8 2 0

9

3

4 .

3 6 3 2

3 3 5 1

9 5

3 0

7 0

9 7 6 1

.

6 0 1 8 3

3

7

0

. 2

6

3 9 4 6

1 1

4

7

1 1 7 8 .

.

2 9

1

7 0

2 .

.

1 8

9

3

2

9 5 1 2 8

e

.

Jo u

8

rn

c

9 2

0 .

1

8 7 .

9 .

.

5 3

al

0

.

9 5

9 2 2 7 0 3

6 5 0 0

.

2

5 9 0

8

.

2 5 7 2 3 6 1 9

2 2 4 1

1

9 5

.

4 0 6

7 5

6

0 7 0 4 .

3 e

.

0 .

8 7

5

e-

6 1

Pr

c

6 2

5 6

4 4 9 4

5

3

9

0

0 0 1 0 c

5 1

0 .

3

2 1

o

2 5

. .

9

r

.

.

4 1

.

. .

3 0 4

.

.

2

1 2

1 .

.

1

6 3

3 3 2

9 0

1 .

.

3 5 7 2

0

0 2 3

. .

4 5

9

1 3 0

4 1

.

5 0 6

3 5 3 7 0

.

6 8

1 1 9 9

0

.

1 9 1 4 4 2 7

4 8

1

0 3 0 2

7 e

.

0 .

5

2 5 1

6 7

5 7

Journal Pre-proof

Table. 3 U-Pb data and Th/U ratios of psammitic schist "TN18-08" zircon

Ages

Isotopic

(Ma)

ratios do

207

206

ain

m

Pb/

2

Pb/

No

ai

235

σ

238

.

n

U

207

207

2

Pb/2 2

Pb/

2

Pb/

2

Pb/2

2

h

2

σ

06P

σ

235

σ

238

σ 06P

σ

/

σ

U

b

U

059

0 8

000

0

2

2

0

co

0.1

0

re

996

0

285 5

co

4.8

0

re

737

8 6

134

0

b

U

1

2

0

0

175

4

169

4

182

6

.

.

0.3

.

1.4

.

1.5

.

3

0

1

9

2

0

0

.

.

5

0

4

3

2

0

0

2

184

0

T

.8

0

4

2

4

1

.

181 .5

.

4

8

1

1

3

0.

6

0

Jo u

5

0.0

0

al

4

0.

0.11

1

rn

0.

9

0.

e-

re

4.6

0

Pr

co 3

0.3

207

U

pr

0. 0.

206

oo f

gr

0.

0.3

0

132

0

7

3

0. 0.11

0

179

4

175

4

184

6

.

.

283

0

7.7

.

6.8

.

5.5

.

3

0

7

2

1

2

9

8

1

1

0

0

0 0. 0. co 8

re

0.1 910

0 1

0.0

0

284

0

3

1

8

9

177

5

180

1

.

.

.5

.

.7

.

1

0

4

9

1

3

8

co

5.1

0.

0.3

0.

0.11 0.

184

1

184

1

184

2

0

0

re

634

0

316

0

292

6.6

4

6.3

5

7.0

6

.

.

0

Journal Pre-proof

9

3

0

0

0

.

.

.

2

0

3

2

8

4

2

2

1

6

2

0

0

.

.

5

0

6

3

0

0

.

.

4

0

4

2

0

0

.

.

2

0

4

1

1

0

0

2 0.

0

325

0

2

0

8

0. 14

co

0.2

0

re

285

1 4

0.1

0

re

767

0 5

7

206 .3

. 3

0. 0.0

0

348

0

9

0

1

0. 0

266

0

2

0

208

1

221

.9

.

.1

165 .2

4 . 0

.

2

5

5

0.0

3

169 .3

1 . 7

al

co

.9

.

4

0. 18

205

oo f

248

0

pr

re

0.2

0

e-

co 10

0.0

Pr

0.

3

re

0.1

0

0.0

0

262

0

5

1

Jo u

co 19

rn

0.

0.

961

2 3

0. 20

co

4.9

0

re

380

8 5 0.

21

co

0.2

0

re

097

0 7

0. 0

198

0

8

3 1

299 6

181

9

167

0

.

.

.8

.

.1

.

2

0

4

8

8

1

1

1

2

0

0

7

0.3

0.0

1

0. 0.11

0

180

4

178

5

183

6

.

.

196

0

8.8

.

9.2

.

1.5

.

6

0

1

0

3

0

0

.

.

8

0

6

4

2

0. 0

193

0

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0

6

0

6

2

. 1

190 .3

. 1

Journal Pre-proof

3 0.

0. 22

co

4.7

1

re

601

8 1

0.3

0

059

0

2

9

0.

3

4

4

0

0

0.11

0

177

2

172

4

184

3

.

.

285

0

7.9

.

0.6

.

5.8

.

9

0

6

3

5

0

0

.

.

1

0

5

1

4

1

0

3

0

6

7

5

0 0.

0

277

0

8

0

8

27

co

4.2

1

re

808

6 4

0. 0

839

0

7

8 4 0.

0. co

8.7

3

re

883

2

0.

co 31

re

3

Jo u

6

120

0.1 834

0 0

37

co

0.1

0

re

824

0 9

38 co

6.6

0.

1 2

0. 0.10 934

0

.

176

.

0

0

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3

2

4

168

1

161

2

178

4

.

.

9.7

.

1.3

.

8.4

.

3

0

3

5

1

2

6

7

0.

3

5

3

0

0

0.15

0

231

3

222

5

239

8

.

.

470

0

6.3

.

4.2

.

8.5

.

1

0

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0

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.

0

0

9

0

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0

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3

0

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0

0

3

8

3

6

5

1

0.

0.0

0

274

0

9

0

8

0.

0

rn

30

0.4

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2

8

0.2

181

0

al

0.

0

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953

0

0.

pr

re

0.1

0

e-

co 24

0.0

Pr

0.

171 .0

. 5

174 .8

. 1

8 0. 0.0

0

277

0

1

0

7 170 .2

.

5 176 .2

9

. 3

8 0.3

0.

0.12 0.

207

3

209

5

204

4

Journal Pre-proof

re

869

2

841

0

5

3

1

0

.

1

3

0

0

625

0

0.9

3

5.5

2

6.5

0

.

.

.

.

3

0

7

7

2

2

0

0

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.

4

0

4

2

0

0

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.

6

0

1

3

0

0

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4

0

4

2

0

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4

0

6

1

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2

0

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1

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.

1

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0. 44

co

0.2

0

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852

1 3

0.0

0

422

0

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1

1

288

0

9

0

3

co

0.2

0

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491

1 5

56

0.0

0

334

0

1

co

0.1

0

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814

0 9

.

8

183 .6

1

. 8

2

211

.9

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.9

1

5 . 7

6

225

. 5

6

0

0.

0.0

0

269

0

6

0

Jo u

0.

9 0.

0

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al

49

185

rn

0.

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pr

001

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co 46

0

266

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0. 0.0

0

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3 0.

254

7

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7 . 6

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8 0.

0. co 57

re

0.1 999

0 0

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0

281

0

8

0

9

68

co

5.0

re

482

.0

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5 . 4

9

0.

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0

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7

3

0

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0. 0 0

182 7.4

3 4 .

180 6.9

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185 0.9

Journal Pre-proof

5

9

3

5

7

2

5

0

0.

3

5

4

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0

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re

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2

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4 . 4

137 .9

4 . 1

Journal Pre-proof

0.

3

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0

4

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0. 84

co

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re

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8

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9

0.

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5

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0

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3

al

co 88

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rn

0.

0.0

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0

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3

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2

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598

2

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pr

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9

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0

288

0

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.

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.

.

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Journal Pre-proof

1

6

0

1

5

2

2

0

7

1

9

0

0

1 0 0.

0

938

0

1

3

3

0. 10 co 1

re

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481

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390

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8

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8

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2

2

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1

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pr

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0.

4

6

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1

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0

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0

1

5

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10 co

rn

0.

0.

907

9 9

0. 10 co 7

re

5.4

1

463

6 7 0.

10 co 9

re

0.2

0

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580

0

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200

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1

5

. 2

Journal Pre-proof

2 0.

0. 11 co 0

re

0.1

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0 8

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2

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11 co 2

re

0.2

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0 9

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0

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7

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0.

4

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0.2

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1

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11 co 5

re

309

5

Jo u

1

0.0

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0 1

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re

0.2

0

961

1 3

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0.2

0.

0 1

185

.

7

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7

198

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.

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.

7

9

9

6

. 5

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0. 0

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1

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0. 0.0

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0

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1

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1

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0

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7

4

3

0.0

.

0

0.0

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0.

0

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11 co

.1

al

0.

179

5

oo f

re

0

7

pr

1

0.1

0

e-

11 co

0.0

Pr

0.

227

6

Journal Pre-proof

7

re

508

0

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0

1

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1

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3

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0

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.

.

.

9

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0

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.

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0

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1

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9

5

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0

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1

4

0

1 0.

0. 11 co 8

re

0.2

0

140

1 0

0.0

0

296

0

9

0

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0.

re

611

0 1

363

0

7

1

1

3

re

0.2

0

572

1 9

12 co 4

re

0

362

0

4

0.1

0

824

0 9

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.

1

230 .3

1

. 7

6 . 9

7

232

5

229

.4

.

.5

1

. 4

6

2

0.

0.0

0

279

0

7

0

Jo u

0.

0.0

al

12 co

0.

.6

rn

0.

1

235

9

pr

1

0.2

0

3

188

e-

12 co

0.0

Pr

0.

.

5

oo f

9

8

170 .1

7 . 7

177 .8

5 . 4

9 0.

0. 12 co 5

re

0.1 144

0 1

0.0

0

189

0

9

0

1

12 co 6

re

0.1 232

110 .0

0.0

0.

0

194

0

2

0

0 .

121 .3

4 . 0

0

6

0.

0

1

118 .0

5 . 5

124 .0

Journal Pre-proof

6

0

0

1

0

0

.

.

0

0

6

0

0

0

.

.

0

0

3

0

5

ri

0.1

0

m

249

1 3

0.0

0

188

0

6

1

ri

0.1

0

m

283

1 4

1

120

.5

.

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0

185

0

8

1

1 122

2

118

.6

.

.7

5

rn

al

Pr

e-

2

Jo u

. 7

4

0. 0.0

7

119

2

0. 74

1

7

oo f

8

0.

pr

0.

.

7

Journal Pre-proof

Table 4 U-Pb data and Th/U ratios of quart-bearing eclogite "MS-06" zircon

Isotopic Ages (Ma) ratios grain

dom

207Pb/

No.

ain

235U

206Pb/

2σ

238U 0.0

2

0.0144

rim

27

7

126

0.0

0.0180

0.00

10

2

104

0.0

0.0191

0.00

20

8

0.0

0.0188

0.0159

13

14

15

15 18

4

al

0.0

0.00

0.0153

0.0

0.0184

0.00

rn

093

.6 18

0

135

0.0

0.0153

0.00

.6 37

5

109

0.0

0.0177

0.00

.1 21

2

115

0.0

0.0186

0.00

0.1152

.3 22

6

093

0.0

0.0182

0.00

0.1238

.3 16

6

109

0.0

0.0179

0.00

0.1144

.1 28

3

082

0.0

0.0155

0.00

.6 17

6

01

00

4.

0.

0.

9

03

00

4.

0.

0.

4

02

00

9.

0.

0.

0

03

01

5.

0.

0.

9

03

00

8.

0.

0.

6

01

00

6.

0.

0.

9

02

00

7.

0.

0.

3

04

00

5.

0.

0.

9

09

00

6.

0.

0.

9

09

00

5.

0.

0.

2

01

00

9.

0.

0.

114.6 .1

101.1

0.

116.7

110.0 19

0.

119.2

118.5 32

6.

113.2

110.7 18

00

98.2

111.2 24

04

117.5

93.7

23

0

98.1

124.5

41

0.

102.0

102.8

4

0.

120.1 27

0.00

0.1157

0.1047

.8

99.4

142

8.

122.4

9

0.00

0.0966

rim

17

7.

07

20

Jo u

12

078

2σ

115.1

4

Pr

1

30

0.1304

/U

9.

112.0

0.1028

rim

σ

.1

119.7

0.0

8

238U 92.6

111.6

0.1252

0.1066

Th

25

0.1162

09

rim

2

88.0

0.1166

6

206Pb/

oo f

rim

5

7

0.00

0.0906

pr

2

2σ

235U

e-

1

207Pb/

2σ

29

99.2

Journal Pre-proof

0.0

0.0150

0.00

.3 25

0.0998

96.6 27

4

105

0.0

0.0167

0.00

28 99.8

31

0

099

0.0

0.0161

0.00

21 106.9

3

073

.1

pr ePr

01

6.

0.

0.

6

08

01

6.

0.

0.

3

07

00

4.

0.

0.

7

08

00

103.2

oo f

23

07

106.8 .8

0.1110

7 96.2

.0

0.1033

al

33

153

rn

33

0

Jo u

29

32

Journal Pre-proof

Table 5 Trace-element compositions of quartz-bearing eclogite "MS-06" zircon

Concentra tions (ppm) d

r

o

ai

m

n

a

N

i

o.

n

oo f

Y g

L

C P

N S E G T D H E

T Y L

a

e

d

m b u

Y

E

Y

T

e

u

bC

h

/

/

N/

/

C

E

S

U

e

u

m

*

*

CN

b

U

0

.

.

4

0

5

1

0

8

3

0

0

1 0 5 0 8

.

1 .

.

0

1

1

.

3

Jo u .

0

.

0

r .

3

.

.

0

0

.

pr .

5 2

f

h

0

CN

/ G d CN

0

9

0

9

0

0

.

4

.

.

.

.

4.

1.

9

7

1

5

0

7

0

3

4

2

7

3

2

5

1

1

1

2 0

0

2

1

1

0

7 .

.

1

3

3

0.

7

3

6

6 . . 8 1

8 7

4

2 5 6 0 .

.

7

7

.

. 2

0

9 9 6 0

5

1 5

.

0

3

r

4

2 0 4 9

0

5

.

5

8

e

.

0 0 2 0

.

o

.

1 0 5 8 8

1 4

.

4 7 7 8 4

3

c

.

rn

4

o

e-

0 6

m u d b y

Pr

r

al

5

2

H T

C

.

6 4

5

2 9

. 0

.

7

1 1 6 1 8

1 5

8

8

5

0

9

2

3.

4

6 0

9

3

7

7

0 5 ri

9

. 0

6

. 5

0 0

.

0

3

2

7

4

6

.

2 m

.

3 4 .

4

.

.

.

.

.

.

.

7 6

.

8

0

.

6

7

6 9

3

.

3

7

0

6

.

3.

0.

0

4

9

5

4 5

1 3

.

9 1 5 3 9 2

0 4

. 3 0 6 6 9

2

0

9 7

9

1

8 5

0

2 0

0

1 0 6 1 7

1 3

0 3 0

1

7

3

0

1

0

3.

0.

6

.

.

.

.

.

.

2

.

7

.

2

.

1

7

5 .

.

.

.

.

.

.

.

Journal Pre-proof

.

0

2 0

1

0 6 2 7 8

5 7

5 4 4

5

9

4

0

5

0

7 1

3

0 6 8 4 8

3 4

1

0

6

.

2

1

1

2

7

0

0

6

0

8

8

6

6

7

0

1

. 0

9

0 5

1 1 0 4 0 5

.

.

0

3

2

2

0

8

.

6 .

0

3

.

.

.

.

.

.

0 1

.

9 3

3

0 . .

m

5 .

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.

0

2

5

1 2 8 3

7

8 7 4 9

6

0

0 0 2 0 7

.

.

. 1

1 0

.

4

.

.

2 2 3 8 5

2

8

.

5

1

.

0 . 4

6

8

8

3

2

9

0

0

.

.

6.

2.

0

8

4

1

6

8

0

0

.

.

6.

1.

0

8

9

0

1

7

0

1

.

.

2 4

5

1

2 1 3

0

0

8

0

1 .

.

.

5 .

2

.

4 0

3

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0

2

9

5 2 4 9 3

0

.

8

5 3 1

1.

9

1

4

3.

3

2

.

.

6

1

2 1 2

8

1

.

Jo u

m

.

rn

ri

8 7

al

0

8

2

2

. .

6

7

2

0

.

0 8

2

5

.

. .

0

6

4

.

5

.

9

e-

0 8

4 9

Pr

ri 7

1 2 1 8 2

2

. 7

pr

0 6

3

3

2

.

.

7

1 0

3

0 0 9 4 3 9

1

0 .

2 6 6 7 3

0

0 5

.

6

1 3

oo f

0 4

4 3

7

8 2

4

9 1

6

0

9 9

2

. 8

1

0 0 6 1

.

1

0

4

.

.

. 0

5

0

0

7

7

2

0 .

.

.

2

.

.

5

6

.

9

4

.

5

1

0 0 3 1 8

2 6

ri

9

.

.

.

.

.

.

.

.

.

1 3

m

.

2

5 4 9 0 5

4 6

9

0

2 7 1 7 0 2

7 0

0

.

2

4

0

1

.

.

1 0

0

5 8

9 4

2. 3.

3 9

1

1

.

6 7

8

7

0

7

6 9

1 1

.

1

3

6

.

7 3

1 7

0

1

7. 5.

3

1

. 1

8 9

0

3

.

7

0 5

.

6 9 1 1

0

9

2

7

6 4

1

.

5 4 7 1 6

1 2 3 8

.

9 8 0 1

4

1 3

0

Journal Pre-proof

2

3

8

3

7

0

0

8

.

.

0

3

1

5

0

8

0

0

8

.

1

0

7

2

.

.

0 2

8

.

6

1

0

.

.

.

.

4 8

4

8 3

5

1 7

.

.

.

.

.

.

.

.

9 5 8 0 2

6

.

.

8 3

.

6

6 6

.

.

9

1 2

0

4 .

2 2

0 1

8

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Journal Pre-proof

Highlights: 

・LA-ICP-MS zircon analysis was applied to the Sanbagawa eclogites in SW Japan



・Protolith of the mafic eclogites was formed by ca. 195 Ma intra-oceanic arc

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magmatism ・The rocks were stagnated at the eclogite- facies depth during 120–90 Ma

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・This stagnation was related to the subduction of intra-oceanic arc

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・Oceanward movement of trench was also related the stagnation

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

Figure 1

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Figure 7

Figure 8