Nature of Late Mesoproterozoic to Early Neoproterozoic magmatism in the western Gyeonggi massif, Korean Peninsula and its tectonic significance

    Nature of Late Mesoproterozoic to Middle Neoproterozoic magmatism in the western Gyeonggi massif, Korean Peninsula and its tectonic significance Seung-Ik Park, Sung Won Kim, Sanghoon Kwon, M. Santosh, Kyoungtae Ko, Weon-Seo Kee PII: DOI: Reference:

S1342-937X(16)30440-3 doi: 10.1016/j.gr.2016.11.006 GR 1713

To appear in:

Gondwana Research

Received date: Revised date: Accepted date:

29 March 2016 3 November 2016 7 November 2016

Please cite this article as: Park, Seung-Ik, Kim, Sung Won, Kwon, Sanghoon, Santosh, M., Ko, Kyoungtae, Kee, Weon-Seo, Nature of Late Mesoproterozoic to Middle Neoproterozoic magmatism in the western Gyeonggi massif, Korean Peninsula and its tectonic significance, Gondwana Research (2016), doi: 10.1016/j.gr.2016.11.006

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Nature of Late Mesoproterozoic to Middle Neoproterozoic magmatism

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significance

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in the western Gyeonggi massif, Korean Peninsula and its tectonic

Seung-Ik Parka, Sung Won Kima,*, Sanghoon Kwonb, M. Santoshc,d, Kyoungtae Koa,

Geological Research Division, Korea Institute of Geoscience and Mineral Resources,

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Weon-Seo Keea

Department of Earth System Sciences, Yonsei University, Seoul 120-749, Republic of

Korea c

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b

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Daejeon 305-350, Republic of Korea

School of Earth Sciences and Resources, China University of Geosciences, Beijing, 29

Xueyuan Road, Beijing 100083, China d

Centre for Tectonics, Resources and Exploration, Department of Earth Sciences,

University of Adelaide, SA 5005, Australia ∗ Corresponding author; E-mail address: [email protected] (S.W. Kim)

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Abstract

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The western margin of the Gyeonggi massif, southern Korean Peninsula, has preserved

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N-S trending Neoproterozoic and sporadic Late Mesoproterozoic metaigneous rocks.

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Here we present the results from systematic field mapping, sensitive high-resolution ion microprobe (SHRIMP) zircon U–Pb dating, and whole-rock geochemical analyses of

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the Mesoproterozoic and Middle Neoproterozoic metaplutonic rocks in the Hongseong

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area, together with previously published data from the western Gyeonggi massif. The

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SHRIMP ages of these rocks are categorized into three groups: (1) Late

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Mesoproterozoic (ca. 1.25–1.15 Ga), (2) Early to Middle Neoproterozoic (ca. 900– 770

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Ma), and (3) Late Middle Neoproterozoic (ca. 762–730 Ma). The geochronological and geochemical features of the Late Mesoproterozoic rocks suggest that they were possibly formed in associated with convergent plate motion. The Early to Middle and late Middle Neoproterozoic rocks are interpreted to arc-related orogenic and rift-related post-orogenic environments, respectively. These age results and the tectonic signatures provide insight into the convergence process along the margins of the Rodinia supercontinent.

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Keywords: Korean Peninsula, Mesoproterozoic and Neoproterozoic magmatism,

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Rodinia, SHRIMP zircon U–Pb geochronology, post-orogenic A-type granite

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1. Introduction

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The Gyeonggi massif in the Korean Peninsula led to diverse collision tectonic

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models, illustrating the architecture of the eastern extension of the Triassic Dabie-Sulu

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collision belt in China (e.g., Ernst et al., 2007; Oh and Kusky, 2007; Zhai et al., 2007; Kwon et al., 2009). This is because of the absence of precise evidence of collision-

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related products such as eclogites and high-pressure rocks that can be used to define

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suture zones (e.g. Kwon et al., 2009). Although the suture zone itself may not be

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preserved within the Korean Peninsula, these collision models can be tested indirectly

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by comparing evolutionary histories of two different continents before the collision. In

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this context, recently reported Neoproterozoic igneous rocks from the Gyeonggi massif are considered important evidence of tectonic affinity (e.g., Lee et al., 2003; Song, 2010; Kim et al., 2008, 2013). However, a lack of understanding of the distribution and nature of the Neoproterozoic magmatic rocks makes it difficult to present a realistic tectonic scenario for the Proterozoic crustal evolution in the Korean Peninsula.

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The western margin of the Gyeonggi massif in the Korean Peninsula has recently

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been interpreted as being located in active continental margins during the

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Neoproterozoic, Paleozoic, and Early Mesozoic, preserving evidence of subduction-

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collision tectonics (e.g., Oh et al., 2005, 2009, 2010, 2012, 2014; Kim et al., 2006b; 2008; 2011a, b, c, 2013, 2015, 2016; Seo et al., 2010, 2013; Kwon et al., 2009, 2013;

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Park et al., 2014a, b). It is notable that Neoproterozoic arc-related magmatism,

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Paleozoic arc-related magmatism and granulite-facies metamorphism, and Mesozoic

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near eclogite-facies metamorphism appear only on the western side of the Gyeonggi

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massif. Thus, it is worth considering this area part of an orogenic belt preserving

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various tectonic events related to the evolution of the Asian continent, like the Central Asian Orogenic Belt (CAOB) between the Tarim-North China and Siberian blocks (Safonova et al., 2011; Wihem et al., 2012; Kröner et al., 2014) and the Central China Orogenic Belt (CCOB) bounded by the North and South China blocks (Fig. 1; Ratschbacher et al., 2006; Zhai et al., 2007; Dong et al., 2011; Wu and Zheng, 2013; Dong and Santosh, 2016).

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The Neoproterozoic metaigneous rocks recognized along the southwestern and

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northwestern margin of the Gyeonggi massif (Lee et al., 2003; Kwon et al., 2013; Kim

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et al., 2008, 2013; Kim and Kee, 2015) have been considered important evidence,

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indicating its tectonic affinity to the South China block (SCB). This interpretation is based on the suggestion that the Neoproterozoic magmatic activities may be related to

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the amalgamation and disruption of the Rodinia supercontinent (e.g., Li and Powell,

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2001; Ling et al., 2003; Rogers and Santosh, 2004; Li et al., 2005, 2008a, b), and are

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seldom reported from the North China block (NCB) (Lu et al., 2008; Zhai et al., 2015).

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Previous studies on the Neoproterozoic metaigneous rocks along the western Gyeonggi

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massif revealed that the area preserves evidence of magmatism of ca. 900–700 Ma (e.g., Song, 2010; Kwon et al., 2013; Kim et al., 2008, 2013; Park et al., 2014b). Among them, ca. 900–770 Ma arc-related igneous rocks have been interpreted as possibly having been formed in association with the assembly of the Rodinia supercontinent (e.g., Kim et al., 2008, 2013; Kwon et al., 2013), while ca. 750–700 Ma rift-related rocks were formed during its disruption(e.g., Kim et al., 2013; Park et al., 2014b). The late stage of the Neoproterozoic magmatism is temporally correlated with the rift-related volcanic suite

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of ca. 760 Ma preserved within the Okcheon belt (Lee et al., 1998; Kim et al., 2006a).

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Here, we present detailed SHRIMP zircon U-Pb ages and geochemical data for the

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Late Mesoproterozoic as well as the Middle Neoproterozoic metaplutonic rocks in the Hongseong area of the southwestern Gyeonggi massif (Figs. 2 and 3). Although the

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Late Mesoproterozoic and Middle Neoproterozoic plutonisms have been reported from

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the area previously (e.g. Kim et al., 2008, 2013), we report new bodies defined based on

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detailed geological mapping and systematic age dating. The results, together with

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previously reported data from the area, will help elucidate the nature of the Late

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Mesoproterozoic to Middle Neoproterozoic magmatic events along the western margin of the Gyeonggi massif, and the paleogeography of the western Gyeonggi massif in the Rodinia supercontinent.

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2. Geological setting

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The Korean Peninsula, located at the eastern margin of the Asian continent, is

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composed of three major Precambrian basement units: the Nangnim, Gyeonggi, and

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Yeongnam massifs (Fig. 1). These massifs are separated by two narrow orogenic belts, namely the Okcheon and Imjingang belts, are predominantly composed of

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Neoproterozoic to Phanerozoic metasedimentary, metavolcanic, and sedimentary rocks

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(Cho and Kim, 2005; Cho et al., 2007). These tectonic provinces are considered to

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represent the assembly of continental terranes that amalgamated during the Permo-

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Triassic collision in East Asia, although the final configuration before the Jurassic and

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Cretaceous disruptions remains controversial (e.g., Yin and Nie, 1993; Ree et al., 1996; Chough et al., 2000; Kim et al., 2006b; Oh and Kusky, 2007; Kwon et al., 2009; Oh et al., 2015).

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The major lithological units in the southwestern Korean Peninsula, including the

orthogneisses,

paragneisses,

schists,

amphibolites,

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Neoproterozoic

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western Gyeonggi massif and the Imjingang belt (Fig. 2), are Paleoproterozoic and and

other

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metasedimentary rocks, as well as Early to Middle Paleozoic ortho- and paragneisses, schists, metasedimentary rocks, metavolcanic rocks, and Middle to Late Paleozoic

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metasedimentary rocks (Fig. 2: Cho et al., 2007; Kim et al., 2014a, c, 2015; Koh et al.,

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2015). These rocks have been affected in places by Triassic upper amphibolite- to near

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eclogite-facies metamorphism(s) (e.g., Cho et al., 2007; Oh et al., 2005; Kim et al.,

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2006b; Kwon et al., 2009, 2013; Sajeev et al., 2010). In addition, the Triassic post-

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collisional gabbro–monzonite–syenite–mangerite–granitoid series (ca. 245–226 Ma) are sparsely distributed in this region (e.g., Peng et al., 2008; Williams et al., 2009; Seo et al., 2010; Kim et al., 2011a). Several tectonic models that have recognized the Triassic tectonomagmatic/metamorphic events in the southwestern Korean Peninsula indicate that the Imjingang belt (e.g., Ree et al., 1996; Chough et al., 2000; Cho et al., 2007; Kwon et al., 2009) and/or the western Gyeonggi massif (e.g., Oh, 2006; Kwon et al., 2009; Oh et al., 2015) can be considered part of the Mesozoic collision belt.

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The Hongseong area, the target area for this study, is located in the southwestern

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region of the Gyeonggi massif (Fig. 2). The area began to attract attention after the first

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discovery of Triassic (retrogressed) eclogite in the Korean Peninsula (Oh et al., 2005;

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Kim et al., 2006b; Kwon et al., 2009). Since then, Neoproterozoic to Mesozoic magmatism, metamorphism, migmatization, and serpentinization in relation to the

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subduction/collision/post-collision tectonics have been reported from the Hongseong

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area (e.g. Kim et al., 2008, 2011a, b, c, 2013, 2015, 2016; Park et al., 2014a, b; Oh et al.,

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2009, 2010, 2012, 2014; Williams et al., 2009; de Jong et al., 2015). This makes it

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possible to infer that the area was a part of the convergent continental margin prior to

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the final amalgamation of East Asia. A recent published tectonostratigraphic map of the Hongseong area (Kim et al., 2014c, d) showed that the area can be divided into three domains (Fig. 3).

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The eastern domain is dominated by Paleoproterozoic gneiss (viz. Yugu gneiss)

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affected by Neoproterozoic and Paleozoic magmatic activities (Fig. 3; Kim et al.,

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2014c). A portion of the dismembered Neoproterozoic metabasites in this domain

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preserves an evidence of a possible Triassic eclogite facies metamorphism (Oh et al.,

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structural trend (Kim et al., 2014c).

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2005; Kim et al., 2006b). The lithological units of this domain show an overall NE-SW

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The central domain is mainly composed of Paleoproterozoic or Mesoproterozoic (?)

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metasedimentary rock (viz. Gonam schist), Neoproterozoic metaplutonic rock, and

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Early to Middle Paleozoic gneiss (viz. Gwangcheon gneiss) showing an overall N–S to NE–SW structural trend (Fig. 3; Kim et al., 2008, 2011b, c, 2013). It is notable that the Mesoproterozoic metaplutonic rocks (Fig. 4), part of the focus of this study, occur as outcrop-scale dioritic xenoliths preserved within the Neoproterozoic and Paleozoic intrusives. In addition, dismembered mafic and serpentinized ultramafic lenses also occur in this domain. Detailed descriptions of the main lithological units in the central domain are presented below.

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Paleoproterozoic or Mesoproterozoic (?) Gonam schist consists of biotite schist,

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quartz schist, quartzite, and marble, which are invaded by Neoproterozoic and Triassic

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intrusive rocks (Fig. 4). The upper and lower limits for the protolith age of the

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metasedimentary rock are constrained by the youngest detrital zircon age of ca. 1819

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Ma and the oldest intrusive rock of ca. 821 Ma (Kim et al., 2014c).

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Neoproterozoic metaplutonic rock, the main focus of this study, can be subdivided

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into five units (Kim et al., 2014c): Deokjeongri granodiorite, Janggokri gneiss,

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Seonggokri granite, Bugiri granite, and alkali granite. Some Neoproterozoic rocks

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occasionally contain marble–amphibolite xenoliths. The Deokjeongri granodiorite mainly consists of granodiorite and tonalite with a minor volume of trondhjemite showing magmatic foliation (Fig. 4). This unit has been reported to have been formed in an arc tectonic setting during ca. 851–832 Ma, and metamorphosed during ca. 235–229 Ma (Kim et al., 2008, 2013). The Janggokri gneiss predominantly consists of orthogneiss with a minor volume of paragneiss, including garnet-biotite gneiss, biotite gneiss, biotite-amphibole gneiss, and clinopyroxene-biotite-amphibole gneiss (Fig. 4).

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The Seonggokri granite is mainly composed of massive or weakly foliated leucogranites

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(Fig. 4). The Bugiri granite consists of biotite granite, partly showing foliation defined

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by preferred alignment of biotite and elongated quartz grains (Fig. 4). Alkali granite

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with strong foliation occurs sporadically as a small discrete stock (diameter of ca. 50 to 100 m) or a dyke (width of ca. 30-50 m) (Figs. 3 and 4). The protolith and metamorphic

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age range from ca. 760 to 740 Ma and ca. 255 to 230 Ma, respectively. The formation of

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the alkali granite has been interpreted as being emplaced in association with extensional

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tectonics during disruption of the Rodinia (Kim et al., 2011c, 2013).

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Early to Middle Paleozoic Gwangcheon gneiss is divided into three subunits (viz. paragneiss unit: OSg1, metavolcanic unit: OSg2, metadolerite unit: OSg3) (Fig. 3; Kim et al., 2014c). The paragneiss unit mainly consists of garnet-sillimanite gneiss and biotite gneiss, with a small volume of biotite schist. The paragneiss unit shows the youngest detrital zircon of ca. 476 Ma, and metamorphic zircon overgrowth of ca. 441 to 403 Ma (Kim et al., 2011b, 2014c, 2015, 2016). The metavolcanic unit is mainly composed of calc-alkaline mafic metavolcanic rock with a banded structure defined by

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alternating layers rich in amphibole-quartz-feldspar and epidote-titanite-feldspar-quartz

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(Kim and Kee, 2010; Kim et al., 2011b). It is occasionally intercalated with metabasite.

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The banded metavolcanic rock has been interpreted as having been formed prior to ca.

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440 Ma in a subduction-related environment, and metamorphosed during ca. 420–370 Ma (Kim and Kee, 2010). The metadolerite unit is mainly composed of fine- to

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medium-grained metagabbroic rock. It is notable that the Neoproterozoic metaplutonic

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relics, possibly corresponding to the Deokjeongri granodiorite in lithology, occur

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sparsely within the metadolerite unit as a mappable inclusion (Fig. 3). It has been

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interpreted as having been formed during ca. 440 to 425 Ma under subduction-related

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volcanic arc settings (Kim et al., 2015). The subunits of the Gwangcheon gneiss are also affected by Devonian mafic to intermediate magmatisms (Kim et al., 2015). The western domain of the Hongseong area exposes the Late Paleozoic low grade metasedimentary rocks (viz. the Taean Formation; Fig. 3) showing an overall N-S structural trend. Available ages from detrital zircon, metamorphic mineral, and intrusive rock constrain the depositional age of the Taean Formation, ranging between ca. 400 Ma to ca. 230 Ma (Cho et al., 2010; Kim et al., 2014a; de Jong et al., 2015).

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3. U-Pb geochronology

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Pb–Th–U isotopic analyses of zircons were performed using a SHRIMP-IIe/MC ion

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microprobe at the Korea Basic Science Institute (KBSI). Zircon was separated from

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rocks using standard crushing and magnetic, water-based panning, and heavy liquid techniques. Hand-picked zircon grains were mounted, together with chips of the FC1

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(Duluth gabbro of 1099 Ma; Paces and Miller, 1993) and SL13 (Sri Lankan gem zircon,

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U = 238 ppm) reference zircons, in a 25.4-mm epoxy disk. Before the analyses, the

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grains were photographed under an optical microscope, and their internal zoning was

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imaged by cathodoluminescence (CL) using a JEOL 6610LV scanning electron

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microscope at KBSI (Fig. 5). The analytical conditions and data acquisition procedures were similar to previous studies (Williams, 1998; Williams et al., 2009; Kim et al., 2013). U–Pb isotopes were sputtered from the zircons using a primary oxygen ion (O 2-) beam of 3.0–4.0 nA intensity at 10 keV and an elliptical spot approximately 30-µm-in diameter. Secondary ions, accelerated to 10 keV, were measured by cycling the analyzer magnet five times through the U, Th and Pb masses of interest. The mass resolution at 1% of the

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U16O+ peak height was greater than 4500, and the total Pb sensitivity was

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Pb/238U ratio was

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within the range of 14–20 counts/s/ppm Pb/nA O2-. The measured

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calibrated using FC1 zircon. Concentrations of U and Th were calculated with reference

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to SL13. Ages and concordia diagrams were produced using the Squid 2.50 and Isoplot

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3.71 programs of Ludwig (2008, 2009). The U–Pb results and sample locations are listed in Supplementary Table 1 and illustrated on concordia plots in Figures 6–8.

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Uncertainties, which are listed in the data tables and plotted on the concordia diagrams,

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are 1σ and include the measurement errors and the errors in the common Pb corrections.

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Some analyses with abnormally high common Pb were corrected assuming a model Pb

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composition appropriate to the age of the analysed spot (Cumming and Richards, 1975).

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For most analyses, common Pb contents were estimated using 204Pb. Uncertainties in the calculated mean zircon ages followed by t-test are 95% confidence limits and include the uncertainty in the Pb/U calibration for each analytical session (0.45–0.85%).

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Two samples from Deokjeongri granodiorite (HS-163 and 131108-6), six samples

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from Janggokri gneiss (090319-1B, 130313-A3, 130313-A4, 120531-1A, 120531-1B,

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and 120531-1C), one sample from Seonggokri granite (HS-70B), three samples from

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dykes intruding the Gonam schist (110715-1A, 110715-2A, and 110715-2B), and two samples from alkali granite (130313-A5 and 131108-7) were selected for dating (Fig. 3).

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One sample of Late Mesoproterozoic metaplutonic rock (100223-4) was from an

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outcrop of xenolith in the Janggokri gneiss in the central part of the Hongseong area that

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is tens of meters wide, but not mappable.

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The zircons from these rocks commonly show euhedral shapes, are up to 100–350 µm in diameter, and have aspect ratios ranging from 1 to 4. About 40% of the grains were stubby prisms with rounded terminations. Most of the grains from the felsic orthogneisses

including

granodiorite,

garnet–biotite

gneiss,

biotite

gneiss,

clinopyroxene-biotite-amphibole gneiss, biotite-amphibole gneiss, leucogranite, and alkali granite, occurred as prismatic subhedral to euhedral zoned grains with oscillatory growth zoning (Fig. 5). In contrast, zircons from the tonalites and mafic intrusions

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occurred as blocky grains and grain fragments with texturally-distinct sector zoning (Fig.

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5). Many zircons showed distinct overgrowths. The strongly light or dark luminescent

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overgrowths had mostly low Th/U (Fig. 5 and Supplementary Table 1). Analyses of the

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zircon overgrowths yielded ages of ca. 255–227 Ma suggesting regional metamorphism

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during the Permo-Triassic collisional event in the Korean Peninsula.

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3.1. Late Mesoproterozoic metaplutonic rock

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A Late Mesoproterozoic metaplutonic rock sample (100223-4) was collected from

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the dioritic xenolith within the Janggokri gneiss. Most analyses of the blocky sector-

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zoned grains from this sample showed a wide range of Th/U ratios (0.35–3.41) (Supplementary Table 1). Excluding statistical outliers or data affected by age discordance due to Pb loss, 14 analyses from sector zoned domains yielded a weighted mean

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Pb/238U age of 1148 ± 68 Ma, although radiogenic

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Pb/238U values showed

significant scatter (Fig. 6). The Late Mesoproterozoic age is concordant with the zircon SHRIMP U-Pb age of ca. 1197 Ma formerly reported from a dioritic xenolith (090402-2) in Early to Middle Paleozoic Gwangcheon gneiss (Kim et al., 2011b).

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3.2. Middle Neoproterozoic Deokjeongri granodiorite

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One mafic tonalite sample (131108-6) was collected from Deokjeongri granodiorite.

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The blocky sector-zoned zircon grains from this rock showed wide range of Th/U ratios

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(0.24–1.18) (Supplementary Table 1). Excluding spots that showed scatter due to Pb loss, 14 analyses from sector zoned grains gave a weighted mean

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Pb/238U age of 843

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± 8 Ma (Fig. 7). One granodiorite sample (HS-163), corresponding to Deokjeongri

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granodiorite in lithology, was also collected from the mappable relic within Paleozoic

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metadolerite of Gwangcheon gneiss. Most analyses (n = 7 of 9) of euhedral-zoned

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zircon grains in this rock showed a moderate range of Th/U ratios (0.26–0.88) and

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yielded a weighted mean age of 828 ± 7 Ma (Fig. 7).

3.3. Middle Neoproterozoic Janggokri gneiss Six samples of garnet-biotite, biotite, biotite-amphibole, and clinopyroxene-biotiteamphibole gneisses were collected from Janggokri gneiss (090319-1B, 130313-A3, 130313-A4, 120531-1A, 120531-1B, and 120531-1C). Zircons from these rocks had Th/U ratios ranging from 0.21 to 1.48 (Supplementary Table 1). Nine analytical points

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Pb/238U age of 829 ± 43 Ma (Fig. 7). All analyses of euhedral-zoned

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weighted mean

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of euhedral-zoned zircons from a garnet–biotite gneiss sample (090319-1B) gave a

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grains from biotite gneiss (120531-1B) fell along a discordia adjacent to the Middle

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Neoproterozoic concordant cluster with a straight regression line (mean square of weighted deviation [MSWD] = 1.0). Their upper intercept age, indicating the

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emplacement age, was 850 ± 29 Ma (95% confidence level). Excluding some of the

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analyses that showed scattering, the zircons from two biotite-amphibole gneiss samples

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(130313-A3, and 130313-A4) had Th/U ratios ranging from 0.59 to 1.09

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(Supplementary Table 1), and yielded weighted mean

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Pb/238U ages of 832 ± 11 Ma

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and 844 ± 13 Ma, respectively (Fig. 7). Zircons from two samples of clinopyroxenebiotite-amphibole gneiss (120531-1A and 1C) had Th/U ratios ranging from 0.40 to 1.04 (Supplementary Table 1). The results yielded weighted mean ages of 800 ± 9 Ma (n = 14; MSWD = 2.2) and 789 ± 9 Ma (n = 7 of 15; MSWD = 1.4), respectively, excluding some scattered data (Fig. 7).

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3.4. Middle Neoproterozoic Seonggokri granite

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One leucogranite sample (HS-070B) was collected from Seonggokri granite. Most of

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the analyses of euhedral-zoned grains from this sample had a relatively wide range of

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Th/U ratios (0.35–1.31) (Supplementary Table 1). Excluding statistical outliers according to the t-test, the results gave a weighted mean

Pb/238U age of 820 ± 7 Ma

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(Fig. 7).

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3.5. Middle Neoproterozoic metaplutonic rocks in the Gonam schist

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Leucogranite occurs frequently as dykes in the central and southern parts of the

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Gonam schist. Most analyses of euhedral-zoned zircon grains from two leucogranites samples (110715-1A and 110715-2A) had Th/U ratios ranging from 0.12 to 1.50. Excluding the analyses that appeared to be statistical outliers or that showed age discordance due to Pb loss, the results gave weighted mean

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Pb/238U ages of 785 ± 9

Ma and 821 ± 6 Ma, respectively (Supplementary Table 1) (Fig. 7). One sample of a lentoid-type mafic intrusive (110715-2B) in association with the leucogranite body (110715-2A) was collected in the southern part of the Gonam schist unit. Analyses of

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Pb/238U age of ca. 812 ± 8 Ma excluding

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five analyses that fell along discordia (Fig. 7).

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0.04 to 1.08, and yielded a weighted mean

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blocky sector-zoned zircon grains from this sample indicated Th/U ratios ranging from

3.6. Late Middle Neoproterozoic alkali granite

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One sample (130313-A5) of alkali granite stock was collected from the central-

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southern margin of Janggokri gneiss. Another sample of alkali granite dyke (131108-7)

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was also collected around a serpentinized ultramafic body (viz. Bibong ultramafic body;

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Oh et al., 2010) in Yugu Paleoproterozoic gneiss of the eastern domain in the

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Hongseong area. Analyses of euhedral-zoned grains from the two alkali granites (130313-A5 and 131108-7) revealed a wide range of Th/U ratios of 0.46–1.78 and 0.10–2.19, respectively (Supplementary Table 1). Although some analyses showed discordant ages, presumably due to radiogenic Pb loss during deformation, the remaining data yielded weighted mean 206Pb/238U ages of 762 ± 10 Ma and 741 ± 15 Ma, respectively (Fig. 8).

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4. Geochemistry

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Thirty samples from Late Mesoproterozoic and Middle Neoproterozoic metaplutonic

SC R

rocks were analyzed for whole-rock major, trace, and rare earth element (REE)

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abundances (Supplementary Table 2) using inductively coupled plasma atomic emission spectrometry (ICP-AES; Thermo Jarrel-Ash ENVIRO II) and ICP mass spectrometry

MA

(ICP-MS; Perkin-Elmer Optima 3000) at Activation Laboratories, Ltd., Ontario, Canada.

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Analytical uncertainties ranged from 1% to 3%. To synthetically evaluate the

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geochemical characteristics of Late Mesoproterozoic and Middle Neoproterozoic

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metaplutonic rocks in the western margin of the Gyeonggi massif synthetically, the

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geochemical data obtained in this study were interpreted together with available geochemical data from Deokjeongri granodiorite and alkali granite (Lee et al., 2003; Kim et al., 2008, 2013).

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The samples from Deokjeongri granodiorite and Janggokri gneiss (ca. 851–789 Ma)

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varied considerably in composition from subalkalic granite through granodiorite to

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syenodiorite/diorite, as classified on Na2O + K2O versus SiO2 diagram (Cox et al., 1979;

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Irvine and Baragar, 1971) (Fig. 9a). Those from Bugiri, Seonggokri granites, and intrusive rock in Gonam schist (ca. 822–785 Ma) were found to be subalkalic granite,

MA

whereas the alkali granites (ca. 774-741 Ma) have alkali granite to syenite compositions

D

(Fig. 9a). A K2O versus SiO2 diagram indicated that metaplutonic rocks from

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Deokjeongri granodiorite, Janggokri gneiss, Bugiri and Seonggokri granites, and

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Gonam schist are mostly calc-alkaline to high-K calc-alkaline in composition (Fig. 9a).

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The alkali granite and some Janggokri gneiss were found to be shoshonitic. On the other hand, Late Mesoproterozoic metaplutonic rocks (ca. 1197 and 1148 Ma) had calcalkaline to high-K calc-alkaline diorite compositions (Fig. 9a, b). According to a Y + Nb vs. Rb classification diagram (Pearce et al., 1984; Pearce, 1996), Middle Neoproterozoic metaplutonic rocks of ca. 851–785 Ma and the Late Mesoproterozoic metaplutonic rock of ca. 1148 Ma plotted into the volcanic arc granitoid field (Fig. 9c). Meanwhile, another Late Mesoproterozoic metaplutonic rock of ca. 1197 Ma and the

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Late Middle Neoproterozoic alkali granites of ca. 774–741 Ma plotted into the within-

SC R

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plate granitoid field (Fig. 9c).

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Late Mesoproterozoic and Middle Neoproterozoic metaplutonic rocks mostly have light rare earth element (LREE)-enriched patterns (Fig. 10 and Supplementary Table 2),

MA

according to the chondrite-normalized diagrams of Sun and McDonough (1989).

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Characteristically, ca. 774–741 Ma alkali granites showed a slightly LREE-enriched

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pattern with a strong negative Eu anomaly (Eu/Eu* = 0.25 – 0.58) (Fig. 10). In addition,

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most of the Neoproterozoic metaplutonic rocks from Deokjeongri granodiorite,

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Janggokri gneiss, Bugiri and Seonggokri granites, and Gonam schist showed moderate to weak negative Eu anomalies (Eu/Eu* = 0.50 – 0.99). However, some samples from Deokjeongri granodiorite and Janggokri gneiss showed weak positive Eu anomalies (Eu/Eu* = 1.02 – 1.19). According to spider diagrams normalized to primitive mantle (Sun and McDonough, 1989), samples from Janggokri gneiss, Bugiri and Seonggokri granites, Gonam schist, and alkali granite are depleted in Nb, Ta, Sr, P and Ti (Fig. 11 a, c–f), while those from Deokjeongri granodiorite are mostly depleted in Nb, Ta, P and Ti

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but nor Sr (Fig. 11b). Such geochemical characteristics are interpreted as representing

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the geochemical trends caused by subduction (Kim et al., 2008, 2013), supporting the

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tectonic discrimination revealed by the Y + Nb vs. Rb classification diagram (Fig. 9d).

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The alkali granites, however, have an overall enriched pattern with strong depletion of Sr, P, and Ti (Fig. 11g) indicating a rift-related environment. Late Mesoproterozoic

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metaplutonic rock of ca. 1197 Ma showed LREE-enriched and humped trace-element

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patterns comparable to those of basalts derived from oceanic island basalt (OIB)-like

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sources in the intracontinental rift environment (Fig. 11a). On the other hand, Late

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Mesoproterozoic metaplutonic rock of ca. 1148 Ma showed depletion of Th, U, Ta, Nb

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and P and enrichment in K and Sr, corresponding to arc-like sources (Fig. 11a).

5. Discussion

5.1 Neoproterozoic magmatic chain along the western Gyeonggi massif Based on geological investigations and reconnaissance geological mapping (1: 100,000 tectonostratigraphic map of the Hongseong area; Kim et al., 2014c), Middle Neoproterozoic metaplutonic rocks in the Hongseong area mainly occur as lithological

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units about 5–10 km in wide (viz. Deokjeongri granodiorite, Bugiri granite, Seonggokri

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granite, Janggokri gneiss), mostly composed of diorite, granodiorite, and granite with

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minor amount of tonalite and monzonite, together with small-scale alkali granite stocks.

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Although the lithological architecture in the central Hongseong area is quite highly revised (Fig. 3; Kim et al., 2014c, d), the area also preserved the Paleozoic units

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including Gwangcheon gneiss (Fig. 3; Kim et al., 2015, 2016), and has been referred to

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as evidence of Paleozoic subduction (Kim et al., 2011b). The main Middle

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Neoproterozoic lithological units (ca. 850–785 Ma; Fig. 7), together with a minor

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volume of Late Mesoproterozoic plutonic xenoliths (ca. 1197 Ma and ca. 1148 Ma; Fig.

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6), and the late Middle Neoproterozoic alkali granite stocks (ca. 762 Ma and ca. 741 Ma; Fig. 8), are considered a part of a magmatic chain formed during the Neoproterozoic along the western side of the Gyeonggi massif. This interpretation is supported by available geochronological, petrological, and geochemical data reported from the western Gyeonggi massif over the last decade (Fig. 2; Kim et al., 2008, 2013, 2014c; Song, 2010; Choi et al., 2014; Koh et al., 2015). The Neoproterozoic magmatic chain can be traced from the Mal and Myeong islands (Song, 2010; Kim et al., 2013), through

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the Hongseong-Dangjin-Hwaseong area (Kim et al., 2008, 2013; Choi et al., 2014) to

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the Gamak Mountain area (Lee et al., 2003), parallel to the overall N–S structural trend

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of the western Gyeonggi massif (Fig. 2). The magmatic chain preserves several groups

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of igneous events, based on their occurrence, geochemical and geochronological features, as follows (Fig. 12): 1) ca. 1.25–1.15 Ga arc- or rift-related metaplutonic rocks

MA

and metabasite occurred as sporadic xenolith or dykes (Kim et al., 2011b; Choi et al.,

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2014; Koh et al., 2015; this study), 2) ca. 900–770 Ma arc-related metaplutonic rocks

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and metabasite as major intrusive masses or sporadic xenolith (or dykes) (Song, 2010;

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Kim et al., 2008, 2011b, 2013, 2014c; Kwon et al., 2013; Choi et al., 2014; Park et al.,

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2014b; Koh et al., 2015; this study), and 3) ca. 762–730 Ma rift-related metagranites as sporadic intrusives (Lee et al., 2003; Kim et al., 2011c, 2013, 2014c; Choi et al., 2014; Park et al., 2014b; Koh et al., 2015; this study).

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These age groups are also recognized from the detrital zircons in sedimentary and

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metasedimentary rocks. Kim et al. (2014a) reported that the detrital zircons from the

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Paleozoic metasedimentary rocks (viz. Yeoncheon Group and Taean Formation) in the

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western Gyeonggi massif record Late Mesoproterozoic to Middle Neoproterozoic ages. Jeon et al. (2007) also showed that some detrital zircons collected from the Mesozoic

MA

Nampo Group in the southwestern Gyeonggi massif yield ages of ca. 1.2–0.6 Ga. These

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ages suggest that Mesoproterozoic to Neoproterozoic igneous rocks have been possibly

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exposed as provenances during the sedimentation of the Paleozoic to Mesozoic cover

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along the present-day western Gyeonggi massif, although the possibility of zircon

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recycling should also be considered. In summary, the overall N-S trending Neoproterozoic magmatic chain enclosing Late Mesoproterozoic bodies and xenoliths are prominent tectonic features preserved along the western Gyeonggi massif, providing important insight into the paleogeometry of a Neoproterozoic continental margin. These observations further suggest that the magmatic chain can play an important informative role when exploring the architecture of the Phanerozoic continental collision zone in and around the Korean Peninsula.

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5.2. Tectonic framework of the Late Mesoproterozoic to Middle Neoproterozoic

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magmatism in western margin of the Gyeonggi massif

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The Late Mesoproterozoic to Middle Neoproterozoic magmatic records are of

paleocrustal

fragments

of

the

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particular interest because of their importance in solving possible discrimination of the Rodinia

supercontinent.

In

particular,

Late

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Mesoproterozoic to Middle Neoproterozoic magmatism is known to have occurred

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mainly in the SCB among the East Asian continental terranes, (Fig. 1; e.g., Zhou et al.

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2002; Wang and Li, 2003; Li et al., 2002, 2008a; Wang et al., 2010 and references

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therein; Dong et al., 2011; Xia et al., 2012), although it has also been reported from the

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NCB (e.g. Zhai et al., 2003; Zhao et al., 2006).

In the SCB, Late Mesoproterozoic and Early Neoproterozoic volcanic and plutonic rocks occur sporadically as independent small masses at the northern margin (e.g., ca. 1103–897 Ma arc-related volcanic rock; Ling et al., 2003; Qiu et al., 2011), western margin (e.g., ca. 1142 Ma rift-related volcanic rock; Greentree et al., 2006), and southeastern margin (e.g., ca. 1159 Ma rift-related volcanic rock and ca. 970–890 Ma

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arc-related volcanic and intrusive rocks; Ye et al., 2007; Li et al., 2013) of the Yangtze

IP

block. These igneous activities have been considered typical Grenville events, and are

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interpreted as having been formed in association with the amalgamation of the Rodinia

NU

supercontinent (e.g. Greentree et al., 2006; Li et al., 2013). This tectonic model is based on the interpretation that rift-related intraplate magmatism also occurred during

D

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amalgamation of the supercontinent (Hanson et al., 2004).

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Subsequent Middle Neoproterozoic igneous rocks mainly occur along the margins of

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the Yangtze and Cathaysia blocks with rare occurrences within the interior, and have

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ages ranging predominantly from ca. 860 to 740 Ma (e.g. Zhou et al., 2002, 2006; Ling et al., 2003; Wang and Li, 2003; Wang et al., 2010). However, the tectonic environment and process regarding the formation of Middle Neoproterozoic igneous rocks in the SCB are still controversial. Some authors have suggested that the Middle Neoproterozoic igneous record is related to the upwelling of a superplume prior to the breakup of the Rodinia supercontinent (e.g. Li et al., 1999; Wang and Li, 2003; Wang et al., 2010), while others have interpreted the observations as suggesting a subduction-

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related process along the margins of the supercontinent (e.g. Zhou et al., 2002; Zhao et

SC R

IP

al., 2008; Dong et al., 2011).

NU

In the western margin of the Gyeonggi massif, Late Mesoproterozoic to Middle Neoproterozoic magmatic rocks are temporally well correlated with those from the

MA

margins of the continental blocks comprising the SCB. Based on these occurrences,

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Kim et al (2008, 2013) suggested that the southwestern Gyeonggi massif can be

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tectonically correlated with the margins of the Yangtze block (Fig. 1). Taking this into

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account, we discuss the tectonic implications of Late Mesoproterozoic to Middle

AC

Neoproterozoic magmatism along the western Gyeonggi massif.

The metaplutonic rocks and metabasites ranging from ca. 1.25 to 1.15 Ga in age (Fig. 12), in the Hongseong area (ca. 1197 Ma and ca. 1148 Ma dioritic xenolith; Kim et al., 2011b; this study), the Hwaseong area (ca. 1201 Ma leucocratic granite dyke; Choi et al., 2014), and Dongman Island (ca. 1259 Ma tonalite dyke and ca. 1241 Ma amphibolite dyke; ca. Koh et al., 2015) are the products of typical Grenville-age magmatic events.

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The geochemical data from the ca. 1197 Ma and ca. 1148 Ma dioritic rock indicate rift-

IP

and arc-related signatures, respectively (Fig. 9), although it is difficult to discriminate

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these processes because of the limited data and considerable error ranges (Fig. 6). As

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the ca. 1.25–1.15 Ga magmatism along the western Gyeonggi massif occurred simultaneously with the worldwide Grenvillian-age orogens (Mosher, 1998; Greentree

MA

et al., 2006; Li et al., 2008b, 2009), both arc- and rift-related metaigneous rocks are

D

likely to have been formed in association with convergent tectonics during the

TE

amalgamation of the Rodinia supercontinent. Geodynamic processes such as

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extensional crustal deformation occurring orthogonally to the orogenic front (e.g.

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Sengor et al., 1978; Greentree et al., 2006), mantle plumes during subduction kinematics (e.g. Hanson et al., 2004; Li et al., 2013), and extension induced by orogenic collapse (Doglioni, 1995) are possible causes of rift-related magmatism during convergent plate motion. Although further studies are required to gain a precise understanding of the significance of Late Mesoproterozoic magmatism, it is clear that the small xenoliths and dykes also present a meaningful opportunity to track the Grenvillian-age tectonic events along the western Gyeonggi massif.

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Subsequent Early to Middle Neoproterozoic magmatism spanning from ca. 900 Ma

IP

to 770 Ma (Fig. 12) has been interpreted as having occurred in an arc-related

SC R

environment during the amalgamation of the Rodinia supercontinent (e.g., Kim et al.,

NU

2008, 2013; Kwon et al., 2013; Park et al., 2014b). However, as the Rodinia supercontinent was almost assembled through worldwide orogenic events between ca.

MA

1300 Ma and ca. 900 Ma (Li et al. 2008b and references therein), the ca. 900–770 Ma

D

arc-related magmatism was most likely due to subduction of oceanic lithosphere

TE

beneath the (present) western Gyeonggi massif after its completion. This suggest that

CE P

the western Gyeonggi massif has been a part of the active margin of the Rodinia

AC

supercontinent, far from the interior of the supercontinent, where widespread mantle superplumes and continental rifting occurred during the same period (ca. 825–740 Ma; Li et al. 2003, 2008a, b; Zhang et al., 2013).

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Late Middle Neoproterozoic magmatism (Fig. 12) lasting from ca. 762 Ma to ca. 730

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Ma is characterized by rift-related alkali (A-type) granite. Previous studies have

SC R

suggested that the alkali granite in the northwestern (Gamak Mountain area) and

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southwestern (Hongseong area) Gyeonggi massif is the product of plume activity and continental rift within Rodinia (Lee et al., 2003; Kim et al., 2013), but this remains

MA

controversial for several reasons. For instance, global continental rifting and related A-

D

type magmatism are typically represented by bimodal granitoids (or volcanics)

TE

(Grebennikov, 2014), and are generally separated from periods of orogenic

CE P

(compressional) events by 50–100 million years or more (Eby, 2006). This feature is

AC

also well recognized in worldwide magmatic rocks related to rifting and disruption of Rodinia due to mantle plumes 40–60 million years after the completion of supercontinent assembly (Li et al., 2008b and references therein). In the western Gyeonggi massif, on the other hand, the rift-related Neoproterozoic magmatism (ca. 762–730 Ma), following the arc-related (orogenic) magmatism (ca. 900–770 Ma) with a negligible transitional period, is characterized only by felsic alkali granite.

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A-type granite occur in diverse tectonic settings generally featuring 1) dehydrated

IP

crustal source, 2) high heat flux sufficient to melt the crustal source, and 3) effective

SC R

pathway from the source to high level (Whalen et al., 1987; Bonin, 2007). These

NU

conditions are not restricted to within-plate continental rifting environment, and can be satisfied even under subduction setting characterized by high heat flux and granitic

MA

magmatism (Whalen et al., 1987; Eby, 1990, 1992). In the western Gyeonggi massif,

D

the A-type magmatism immediately follows a long period of arc-related (orogenic)

TE

magmatism. The temporal relationship makes it possible to infer that the rift-related

CE P

alkali granite was formed during the waning stage of subduction-related arc magmatism,

AC

rather than during within-plate global continental rifting, as similar to those in South China (Zhao et al., 2008; Wong et al., 2009), Papua-New Guinea and New Zealand (Smith et al., 1977).

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This alternative interpretation is supported by the geochemical features expressed on

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discrimination diagrams for A-type granite (ternary diagrams Nb-Y- Ga*3 and Nb-Y-Ce)

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proposed by Eby (1992). The Late Middle Neoproterozoic alkali granites in the Gamak

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Mountain and Hongseong area straddle the boundary between A1 (within-plate continental rift environment) and A2 (post-orogenic or post-collisional environments).

MA

This data set is in marked contrast contrasts to the within-plate continental rift related

D

metavolcanic rock (ca. 760 Ma) in the Okcheon metamorphic belt reported by Kim et al.

TE

(2006a) (Fig. 13). Bonin (2007) noted that the data set used by Eby (1992) showed a

CE P

continuous shift from post-collision (plotted within A2 suite) through post-orogenic

AC

(straddling at the A1/A2 boundary) to within-plate (plotted within the A1 suite). In this context, the Late Middle Neoproterozoic alkali granites along the western Gyeonggi massif were estimated to have been formed in association with post-orogenic extension imprinted on a former arc system. The post-orogenic extension may have been due to break off or roll back of an oceanic slab (e.g., Bonin, 1990; Zhao et al., 2008; Wong et al., 2009).

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In summary, Early to Middle Neoproterozoic magmatisms (ca. 0.9–0.73 Ga) along

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the western Gyeonggi massif are likely to have occurred in former convergent margin(s)

SC R

of the Rodinia supercontinent. This suggests that the (present) western Gyeonggi might

MA

related to disruption of the supercontinent.

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not be located in the interior of Rodinia, where it was affected by superplume activity

work

was

supported

as

a

Basic

Research

Project

(GP2016-005;

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This

TE

D

Acknowledgments

Tectonostratigraphy of the Mid-west Korean Peninsula and construction of the

AC

intergrated geoscience information system) of the Korea Institute of Geoscience and Mineral Resources (KIGAM), funded by the Ministry of Science, ICT (Information, Communication and Technology), and Future Planning, Korea. This research was also supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2015-11-1315) to SK.

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Figure captions

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Figure 1. Tectonic map of East Asia showing the distribution of Mesoproterozoic to

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Neoproterozoic rocks, Early Neoproterozoic mafic dykes, and Neoproterozoic plutonic rocks in China and the Korean Peninsula. NM, Nangnim massif; IB,

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Imjingang belt; GM, Gyeonggi massif; OB, Okcheon belt; YM, Yeongnam

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massif; PB, Pyeongnam basin; GB, Gyeongsang basin.

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Figure 2. Map of the western margin of the Gyeonggi massif showing the distribution of

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Neoproterozoic metaigneous rocks, Early to Middle Paleozoic gneiss, Middle Paleozoic (meta)sedimentary rocks, and Late Paleozoic metasedimentary rocks.

Figure 3. Geological map of the Hongseong area in the western margin of the Gyeonggi massif, central-western Korean Peninsula (modified from Kim et al., 2014a).

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Fig. 4. Outcrop photographs showing various metaplutonic rocks in the Hongseong area,

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granodiorite, Janggokri gneiss, Seonggokri granite, Bugiri granite, intrusive rocks

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in Gonam schist, and alkali granite.

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Figure 5. Scanning electron microscope (SEM) cathodoluminescence images of

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metaplutonic rocks in the Hongseong area.

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Figure 6. Concordia plots of SHRIMP U-Pb isotopic analyses of zircons from Late Mesoproterozoic metaplutonic xenoliths in the Hongseong area.

Figure 7. Concordia plots of SHRIMP U-Pb isotopic analyses of zircons from Middle Neoproterozoic metaplutonic rocks in the Hongseong area.

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Figure 8. Concordia plots of SHRIMP U-Pb isotopic analyses of zircons from Late

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Middle Neoproterozoic alkali granites in the Hongseong area.

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Figure 9. Diagrams of a) SiO2 wt.% versus Na2O + K2O wt.% (Cox et al., 1979; Irvine and Baragar, 1971), b) normative quartz–orthoclase–plagioclase composition

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(Streckeisen, 1976), c) SiO2 wt.% versus K2O wt.%, and d) Rb/(Y + Nb) (Pearce

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et al., 1984; Pearce, 1996) for Late Mesoproterozoic and Neoproterozoic

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plutonic rocks in the Hongseong area.

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Figure 10. Chondrite-normalized rare earth element (REE) patterns (Sun and McDonough, 1989) of Late Mesoproterozoic and Middle Neoproterozoic metaplutonic rocks in the Hongseong area. Symbols are the same as those in Figure 9.

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Figure 11. Primitive mantle-normalized trace element distribution diagrams (Sun and

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McDonough, 1989) of Late Mesoproterozoic and Middle Neoproterozoic

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metaplutonic rocks in the Hongseong area. Symbols are the same as those in

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Figure 9.

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Figure 12. Distribution diagrams of SHRIMP zircon U–Pb ages for Late

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Hongseong area, together with those in other areas along the western margin of

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Figure 13. Nb-Y-Ga and Nb-Y-Ce ternary diagrams for the subdivision of A1- and A2-

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type magmatism (Eby, 1992). Late Middle Neoproterozoic alkali granites in the

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western Gyeonggi massif (Hongseong and Gamak Mountain areas), together

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with metavolcanic rocks in the Okcheon metamorphic belt, are plotted in the diagrams. A1-type magmatic rock is invariably associated with an anorogenic

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Figure 12

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Figure 13

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Graphical abstract

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Research highlights

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► Late Mesoproterozoic to Middle Neoproterozoic arc magmatism in the western

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Gyeonggi massif, Korean Peninsula

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► Late Mesoproterozoic to Middle Neoproterozoic metaplutonic rocks display evidence of the evolution of the Rodinia supercontinent

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► Late Mesoproterozoic to Middle Neoproterozoic metaplutonic rocks provide insight

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into the convergence process along the margins of the Rodinia supercontinent

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