Gondwana Research 20 (2011) 852–864

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Forearc serpentinite mélange from the Hongseong suture, South Korea

Sung Won Kim a, M. Santosh b, Nari Park c, Sanghoon Kwon c,⁎a Korea Institute of Geoscience and Mineral Resources, Daejeon 305-350, Republic of Koreab Division of Interdisciplinary Science, Faculty of Science, Kochi University, Akebono-cho 2-5-1, Kochi 780-8520, Japanc Department of Earth System Sciences, Yonsei University, Seoul 120-749, Republic of Korea

⁎ Corresponding author. Tel.: +82 2 2123 5666; fax:E-mail address: skwon@yonsei.ac.kr (S. Kwon).

1342-937X/$ – see front matter © 2011 International Adoi:10.1016/j.gr.2011.01.012

a b s t r a c t

a r t i c l e i n f o

Article history:Received 27 June 2010Received in revised form 23 January 2011Accepted 25 January 2011Available online 12 March 2011

Handling Editor: R.D. Nance

Keywords:Dismembered serpentinite mélangeHydrated forearc mantle peridotiteChromian spinelZircon SHRIMPWolhyeonri complexKorean Peninsula

The signature of a prolonged subduction–accretion history from Paleozoic to Early Mesozoic is preservedwithin the dismembered serpentinite mélanges within the Hongseong suture. Here we present major andtrace element data from the mafic fragments/blocks within the Baekdong serpentinite mélange revealingtheir arc-like tholeiite affinity within a suprasubduction zone tectonic setting. Chromian spinel compositionsfrom the Baekdong hydrated mantle peridotite (serpentinite) are characterized by high Cr# (0.53–0.67) andFe2+/Fe3+ ratio, medium Mg# (0.42–0.55), and Al2O3 contents (17–25 wt.%) indicating a forearc tectonicenvironment for the hydrated mantle peridotite. The estimated melting degree (N17.6%) and FeO/MgO of theparental melt (0.9–1.3) are consistent with that of forearc magmas. SHRIMP zircon U–Pb ages from a high-grade mafic rock and an anorthosite from the study area give protolith ages of ~310 Ma and ~228 Ma,respectively. Zircons from an associated orthogneiss block within the mélange yield a Neoproterozoiccrystallization age of ~748 Ma. These results, together with the recent SHRIMP zircon ages from otherdismembered serpentinite mélanges within the Wolhyeonri complex, suggest that Paleozoic to EarlyMesozoic subduction and subsequent collision events led to the exhumation of the hydrated forearc mantleperidotites from a metasomatized mantle wedge. The Hongseong region preserves important clues to a long-lived subduction system related to global events associated with the final amalgamation of the Pangaeasupercontinent.

© 2011 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.

1. Introduction

Serpentinite mélanges enclosed within high-grade metamorphicbelts preserve critical clues on successive subduction and accretionevents along suture zones developed through the closure of oceanbasins (e.g., Volkova et al., 2008; Brueckner et al., 2009; Maruyamaet al., 2009; Ali et al., 2010; Braid et al., 2010; Saumer et al., 2010;Wakabayashi and Dilek, 2003; Saha et al., 2005; Xiao et al., 2010; Zhanget al., 2010). The disrupted fragments from various oceanic complexes,including oceanic crust, seamounts and guyots and volcanic islandstogether with pelagic, trench and forearc sediments preserved withinserpentinite mélanges, provide important clues for understanding theevolutionary history of continents (e.g., Volkova et al., 2008; Brueckneret al., 2009; Saumer et al., 2010). The paleogeography reconstructedfrom suchmélanges and subduction complexes provide information onthe paleo-ocean from its initiation at a mid-ocean ridge or suprasub-duction zone setting to the successive subduction and accretion ofoceanic materials from the oceanic plate through off-scraping and

+82 2 2123 8169.

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underplating to form a subduction complex (e.g., Matsuda and Isozaki,1991; Isozaki and Blake, 1994; Brueckner et al., 2009; Wakita, 2000;Santosh et al., 2009; Isozaki et al., 2010; Santosh, 2010a,b).

In the Hongseong area of the southwestern Gyeonggi massif ofSouth Korea (Fig. 1), serpentinite mélanges occur as dismemberedlenticular bodies in several localities such as those at Hongseong,Gwangcheon, Wonnojeon, Baekdong, Gyeweol and Singok (Fig. 2).Many of these are chaotic in nature and incorporate different types ofexotic blocks. Among these, eclogite/high pressure granulite has beenreported from Bibong and Baekdong, both from within serpentinitemélanges (Oh et al., 2004, 2005; Kim et al., 2006). The Wolhyeonricomplex is considered to be part of the “Imjingang-Hongseong suturezone” related to the “Korean collision belt” (Kwon et al., 2009; Sajeevet al., 2010).

In this paper, we present new geochemical data, chromian spinelcompositions and SHRIMP zircon geochronology from the Baekdongserpentinite mélange. Combined with the available data from thisregion, we attempt to trace the Paleozoic to Early Mesozoicsubduction and accretion history related to crustal evolution in aconvergent margin in East Asia. Our results also provide importantinsights into the probable tectonic link with the assembly of thePangaea supercontinent.

Published by Elsevier B.V. All rights reserved.

Fig. 1. (a) Regional tectonic map of Far East Asia (modified after Ernst et al., 2008)showing the main tectonic boundaries in eastern Asia. (b) Simplified geologic map ofthe Korean Peninsula modified after the 1/1,000,000 Geological Map of Korea (KIGAM,1995).

853S.W. Kim et al. / Gondwana Research 20 (2011) 852–864

2. Regional Geology

The Hongseong area has been traditionally described as thesouthwestern part of the Gyeonggi massif, which consists ofPaleoproterozoic (ca. 2.1–1.8 Ga) basement rocks and Neoproterozoic(ca. 900–742 Ma) supracrustal rocks (e.g., Lee and Cho, 2003; Lee et al.,2003; Sagong et al., 2003; Kim et al., 2006, 2008). Orthogneiss, bandedparagneiss, migmatite and quartzo-feldspathic gneiss with minorslate, schist and quartzite constitute the dominant lithological units in

the Paleoproterozoic basement (e.g., Lee and Cho, 2003; Lee et al.,2003; Sagong et al., 2003; Kim et al., 2006, 2008). The Neoproterozoicrocks scattered throughout the western and central Gyeonggi massifconsist of granitic gneiss with amphibolite, marble, pelitic schist andquartzite (e.g., Lee et al., 2003; Kim et al., 2006). However, recentdetrital SHRIMP zircon analysis indicates that the supracrustalparagneiss, metavolcanics, quartzites and limestones that wereconsidered part of the western Gyeonggi massif (viz., WolhyeonriFormation; Fig. 2) may have actually formed in a continental marginsetting during the Paleozoic (Kim et al., 2010). Within theWolhyeonriFormation, blocks of dismembered, lens-shaped serpentinized bodiesoccur in association with mafic and felsic rocks, such as those atHongseong, Gwangcheon, Wonnojeon, Baekdong, Gyeweol andSingok (Fig. 2). The peak metamorphic temperatures and pressuresof themafic and felsic rockswithin the serpentinite bodies varywidelyfrom place to place. The Neoproterozoic tonalite–trondhjemite–granodiorite (TTG) gneisses, such as the Deokjeongri granitic gneiss(Fig. 2), yield magmatic emplacement ages of ~810–850 Ma with aprominent metamorphic overprint at 223–235 Ma (Kim et al., 2008).The serpentinite bodies show faulted, sheared or normal contactrelationships where exposed, and possibly represent serpentinitemélanges incorporated within a tectonic complex, newly termed the“Wolhyeonri complex”. In addition, several occurrences of high-grademetamorphic rocks, such as the Bibong eclogite and the Baekdonggarnet granulite (eclogite), have also been reported (Oh et al., 2004,2005; Kim et al., 2006; Kwon et al., 2009). These blocks/fragmentswithin the serpentinite mélanges show clockwise metamorphic P–Tpaths (Kwon et al., 2009), supporting development in a collisionalregime (e.g., Liou et al., 1996).

Within the outcrop exposure of the Baekdong body (ca. 50 m inlength) (Fig. 3a), serpentinized mantle peridotites (dunite andharzburgite) (Seo et al., 2005) and serpentinite (Fig. 3b) occur inassociation with anorthosite (Fig. 3c and d), gabbroic anorthosite(Fig. 3e), and garnet-bearing mafic rocks (Fig. 3f), together represent-ing the Baekdong serpentinite mélange. For this study, we collectedrepresentative samples of the high-pressure mafic granulite, anor-thosite and mylonitized orthogneiss blocks from within the mélange(Fig. 3). The contact relations of the mélange and surrounding rocksare obscure (Fig. 3g). The Paleozoic Wolhyeonri complex is juxta-posed against the Paleoproterozoic basement, such as the Yuguorthogneiss complex to the east of the study area, along a thrust/shearzone contact (Fig. 2), which may be the most suitable place for thesuture zone boundary. Serpentinized ultramafic complexes occurwithin the Paleoproterozoic orthogneiss at Yugu and at Bibong,although their age and contact relationships have not yet been studiedin detail. Both the Paleozoic Wolhyeonri complex and the Precam-brian basement gneiss are partly overlain by Early to Middle Mesozoic(Jeon et al., 2007; Kim et al., 2008) sedimentary rocks (viz., DaedongFormation), suggesting the possibility of a wedge-top basin. Syn- topost-collisional igneous activity related to Triassic collision (Choiet al., 2009; Williams et al., 2009) is also reported from the hangingwall (i.e., Wolhyeonri complex) side of the thrust/shear zone (Fig. 2).The low-grade metasedimentary successions on the western side ofthe Wolhyeonri complex record maximum depositional ages ofMiddle to Late Paleozoic (Cho, 2007). These rocks belong to theTaean Formation and comprise mostly sandstones and quartzitesformed in a deltaic environment. The boundary with the Wolhyeonricomplex is not known, but that with the Paleoproterozoic supracrus-tal rocks (Fig. 2) is reported as an unconformity (Lee et al., 1999).

3. Petrography

Although the ultramafic rocks preserved within the Baekdongbody (Fig. 3) are metasomatized to serpentinite, these rocks can begrouped as weakly to strongly serpentinized dunite and harzburgite(corresponding to hydrated mantle peridotite) based on the modal

Fig. 2. Detailed geologic map of the Hongseong area, southwestern Gyeonggi massif, South Korea, showing the major structures and the locations of ultramafic bodies.

854 S.W. Kim et al. / Gondwana Research 20 (2011) 852–864

abundance of olivine, orthopyroxene, clinopyroxene, spinel, amphi-bole, phlogopite, serpentine and chlorite (Seo et al., 2005). In theserpentinized harzburgite, large porphyroclasts of orthopyroxeneand olivine with strong wavy or undulatory extinction are set in amatrix of medium- to fine-grained olivine, spinel, clinopyroxeneand amphibole (Fig. 4a). They are commonly surrounded byrecrystallized fine-grained olivine (Fig. 4a) with the developmentof a mylonitic fabric. The ultramafic rocks have been altered toserpentinite (Fig. 4b) and the assemblage includes mainly lizardite,chrysotile, chlorite, talc and magnetite. The fibrous or platyserpentine occurs along irregular cracks to produce an anastomos-ing network of veinlets.

The mafic rocks occur as blocks within the serpentinite mélangeand comprise weakly to strongly foliated gabbroic rocks with anassemblage of amphibole, plagioclase, epidote, biotite, quartz andminor titanite (Fig. 4c). The degree of retrograde metamorphismincreases towards the rim of the gabbro bodies with near-completetransformation to amphibolite, where garnet and clinopyroxene reactto produce amphibole and plagioclase. However, the core domains ofthese rocks often preserve hypidomorphic, granular and ophitictextures. Themetagabbro/metadiabase shows extensive retrogressionto garnet amphibolite or granulite, and consists of garnet, augiticclinopyroxene, amphibole, plagioclase and quartz with minor rutileand ilmenite (Fig. 4d). The common occurrence of symplectite ofamphibole and plagioclase around garnet suggests retrogradebreakdown of garnet.

The anorthosite block consists mainly of anorthite (Fig. 4e) andquartz with minor clinopyroxene and muscovite. Fractures developedwithin the anorthosite are filled with serpentines.

The orthogneiss block in the Baekdong serpentinite mélange ismedium- to coarse-grained and exhibits strong foliation. The typicalmineral assemblage in the gneiss is biotite, plagioclase, K-feldspar,

amphibole and quartz with or without garnet (Fig. 4f). Garnet grainscontain plagioclase, biotite and quartz inclusions. Most of the garnetswere partially replaced by biotite and plagioclase during retrogrademetamorphism.

4. Geochronology

Zircon grains for this study were separated from the high-pressuremafic granulite (090319-3D), anorthosite (BDA-1) and mylonitizedorthogneiss (100428-2) within the Baekdong serpentinite mélange(36°33′28.73″N, 126°44′02.36″E), using standard crushing, magneticand water-based panning techniques. The hand-picked zircon grainswere mounted in epoxy with zircon standards SL13 (U=238 ppm)and Temora (206Pb*/238U=0.06683) or FC1 (206Pb*/238U=0.1859)and polished to expose cross sections for analysis. Before analysis, thegrains were photographed under an optical microscope, and theirinternal zoning was imaged by cathodoluminescence (CL) using aHitachi S-2250 N scanning electron microscope and a JEOL 6610LVscanning electron microscope at the Korea Basic Science Institute(KBSI), Ochang Campus, South Korea. The zircons were analyzed forPb–Th–U isotopes using the SHRIMP II ion microprobe at theAustralian National University and the KBSI. The instrumentalconditions and data acquisition procedures were similar to thosedescribed by Williams and Claesson (1987) and Williams (1998). ThePb isotopic compositions were measured directly, without correctionfor the small (ca. 2‰/amu) mass-dependent fractionation. Correc-tions for much larger inter-element fractionations were made byreference to the Temora or FC1 standard using a power-lawrelationship between Pb+/U+ and UO+/U+. For most analysesplotted, common Pb contents were estimated using 204Pb. Concentra-tions of Pb, U, and Th were calculated with reference to SL13. Eachanalysis consisted of five scans through the Zr, Pb, U, and Th species of

Fig. 3. Outcrop photographs showing Baekdong serpentinite mélange in the Hongseong area: (a) Field photograph showing the overall outcrop and the locations of detailedphotographs. Hydrated mantle peridotite (serpentinite) (b) encloses blocks of anorthosite (c), (d), gabbroic anorthosite (e) and garnet-bearing mafic rock (f). (g) Photographshowing the contact between serpentinite mélange and the mylonitized orthogneiss.

855S.W. Kim et al. / Gondwana Research 20 (2011) 852–864

interest and took about 9 min. Uncertainties are listed in the datatable and are plotted on the concordia diagrams; they are 1σ andinclude the measurement errors and those in the common Pbcorrections. The uncertainties in the mean 206Pb/238U ages are the95% confidence limits (tσ, where ‘t’ is Student's t), and include thosein the Pb/U calibration for each analytical session (0.25–0.40%). Ageswere calculated using the constants recommended by the Interna-tional Union of Geosciences (IUGS) Subcommission on Geochronol-ogy (Steiger and Jäger, 1977). The U–Pb results are listed in Table 1and are illustrated on concordia plots in Fig. 5.

The zircon grains from the high-pressure mafic granulite(090319-3D) are ca. 50–200 μm in diameter. CL imaging showsthat some of the subhedral to euhedral prismatic grains are ofcomposite nature, consisting of a medium CL zoned core of a wholegrain or grain fragments, surrounded by a thin bright CL or dark tomedium CL unzoned overgrowth (Fig. 5a). The remaining grains aresubhedral to anhedral, mostly rounded, medium to bright CL grainsand show textures typical of recrystallization and resorbed cores orgrains (Fig. 5a). Most grains are surrounded by texturally very thinunzoned overgrowths with light luminescence. Three analyses ofold, euhedrally zoned grain cores showed a wide range of U contents(199–365 ppm) with Th/U of 0.24–0.93 (e.g., Table 1 and Fig. 5a,grains 2.1, 6.1 and 10.1). One intermediate older overgrowth has a Ucontent of 772 ppm and Th/U of 0.08 (grain 9.1). Most cores yieldold 207Pb/206Pb ages, with an oldest age of 1848 Ma (95% confidencelevel). One dark CL intermediate unzoned overgrowth and onemedium CL intermediate unzoned overgrowth had U contents of641 and 75 ppm and Th/U of 0.23 and 0.03, respectively (e.g., Table 1

and Fig. 5a, grains 14.1 and 16.1). Their isotope data define mid-Paleozoic 206Pb/238U ages of 395 Ma and 423 Ma (Fig. 5b). Thesubhedral to anhedral, bright CL grains, which had experiencedrecrystallization, yield 206Pb/238U apparent ages of ~310 Ma (e.g.,Table 1 and Fig. 5, spots 4.1, 5.1, 12.1, 15.1, 17.1 and 18.1). Themedium to dark CL overgrowths are all characterized by low-Th/U(0.007–0.009) with U contents of 53–421 ppm with one exception(unzoned medium CL grain 11.1) of high Th/U (0.74). The four spotsanalyzed (grains 3.1, 7.1, 8.1 and 13.1) gave a weighted mean 206Pb/238U age of 236±5 Ma (95% confidence level, Fig. 5b) with the largeuncertainties reflecting the low radiogenic Pb contents (1–13 ppm).In a recent study, Kim et al. (2010) analyzed zircons from a maficrock within the paragneiss around Baekdong, which yielded agessimilar to those from the high-grade mafic rocks analyzed in thisstudy. These results indicate that both the Baekdong body and themetasediments experienced the same high-grade metamorphicepisode following the Early Paleozoic.

The zircon grains from the anorthosite (BDA-1) are mediumsized with 100–150 μm diameters. 70% of the zircon grains arestubby with short pyramids and long prisms (aspect ratio 1–3),colorless to pale brown, and subhedral to euhedral with prismaticterminations. CL imaging revealed that these grains show euhedraloscillatory zoning typical of magmatic zircon (Fig. 5c). The analyses(n=10) of the zircons revealed a range of U contents (247–790 ppm) and Th/U (0.66–1.21) ratios. The eleven spots analyzedgave a weighted mean 206Pb/238U age of 228±2 Ma (MSWD=3.2;Fig. 5d), which is taken to indicate the crystallization age of thisrock. The remaining 30% of the zircon grains are exotic and mostly

Fig. 4. Photomicrographs (plane polarized light) showing representative mineral assemblages of the serpentinite and related blocks from the Baekdong serpentinite mélangeand the orthogneiss: (a) partly serpentinized harzburgite with large orthopyroxene surrounded by recrystallized fine-grained olivine, (b) serpentinized peridotite, (c) gabbroicrock, (d) gabbroic diabase with assemblage of augitic clinopyroxene, amphibole, plagioclase and quartz, (e) coarse-grained anorthosite that mainly consists of labradorite andbytownite plagioclase, and (f) medium- to coarse-grained biotite orthogneiss with strong foliation. Abbreviations: Opx, orthopyroxene; Ol, olivine; Amph, amphibole; Pl,plagioclase; Qtz, quartz; Ser, Serpentine.

856 S.W. Kim et al. / Gondwana Research 20 (2011) 852–864

rounded or embayed with anhedral to subhedral habits, consistentwith partial dissolution (Fig. 5c). These zircons show a wide range ofU contents (348–1486 ppm) and Th/U (0.01–0.56) ratios. The 206Pb/238U apparent ages of these grains range from ~668 to ~422 Ma,with a weighted mean 206Pb/238U age of 433±11 Ma (n=6,MSWD=3.2; Fig. 5d).

The zircons from the mylonitized orthogneiss (100428-2) occur asmedium to large (100–300 μm diameter), stubby, rounded, slenderprismatic (aspect ratio 1–5), colorless to mid-brown, subhedralgrains, mostly with rounded terminations. CL imaging revealed thatmost grains possess oscillatory zoningwith no core, a common featureof zircons from felsic igneous rocks (Fig. 5e). The analyses (n=14) ofthe zircons revealed a narrow range of U contents (119–350 ppm) andTh/U (0.37–0.76) ratios with two exceptions (spots 6.1 and 12.1) thatshow discordant ages, presumably through radiogenic Pb loss duringmylonitization. Omitting two analyses with larger errors from theconcordia, the remaining 12 analyses yield a more precise 206Pb/238Uage of 746±4.9 Ma (MSWD=1.8; Fig. 5f). Thus, the crystallizationage of the mylonitized orthogneiss is interpreted to be about 746 Ma.Some CL-dark overgrowths (spots 4.1 and 5.1) show a range ofmedium U contents (277–660 ppm) and low Th/U (ca. 0.01). The206Pb/238U apparent ages of these grains range from ~244 to ~229 Ma,reflecting the time of the Middle Triassic metamorphic event (Fig. 5f).

In summary, the minimum protolith age of the serpentinisedperidotite is inferred to be Late Paleozoic from the age of the high-

grade mafic block. In addition, cracks developed within the TriassicBaekdong anorthosite are filled with serpentinites indicating that theperidotite was not hydrated until the Triassic collision, the lattermarked by the presence of the Bibong eclogite in the Hongseong area(Fig. 5g and h) (Kim et al., 2006).

5. Major and trace element geochemistry

The high-grade mafic rocks occurring as blocks in and around theBaekdong serpentinite mélange were analyzed for whole-rockmajor, trace, and rare earth element (REE) abundances usinginductively coupled plasma atomic emission spectrometry (ThermoJarrel-Ash ENVIRO II) and inductively coupled plasma massspectrometry (Perkin-Elmer Optima 3000) at Activation Laborato-ries, Ltd., Canada. Analytical uncertainties range from 1 to 3%. Wealso compiled some of the geochemical data on the high-grade maficrocks from the Baekdong area reported by Oh et al. (2009). Themajor element data for high-grade mafic rocks from the Baekdongserpentinite mélange are given in Table 2. The SiO2 contents of thesesamples vary between ~43.17 and 50.03 wt.%, and the Na2O+K2Ocontents fall between ~1.21 and 4.59 wt.%, similar to the non-MORB-type basaltic rocks of orogenic belts, although the possibility ofseafloor alteration cannot be excluded (Saha et al., 2005). The FeO*/MgO ratios (where FeO* is total Fe as FeO), when compared to TiO2

(wt.%) of the high-grade rocks, lie within a restricted range from

Table 1SHRIMP U–Pb zircon data for the high-grade mafic and anorthosite blocks from the Baekdong serpentinite mélange and the country rock of mylonitized gneiss.

Grainspot

Zircontypea

Pbb

(ppm)U(ppm)

Th(ppm)

Th/U 204Pb206Pb

± 206Pbb/238U

± 207Pbb/206Pb

± Apparent ages (Ma)

206Pb/238U ± 207Pb/206Pb ±

Sample 090319-3D (high-grade mafic block)Older cores and rim

3D−2.1 MZC 63 199 49 0.24 0.00002 0.00001 0.3247 0.0049 0.0936 0.0025 1812.6 24.0 1499.1 51.73D−6.1 MZC 133 336 313 0.93 0.00002 0.00002 0.3334 0.0038 0.1135 0.0008 1855 18 1856 133D−9.1 MIO 119 365 89 0.24 0.00006 0.00002 0.3189 0.0041 0.1129 0.0008 1784 20 1846 143D−10.1 DZC 264 772 58 0.08 0.00000 0.00000 0.3507 0.0043 0.1127 0.0007 1938 21 1843 12

Intermediate unzoned overgrowths3D−14.1 MIO 5 75 2 0.031 0.00064 0.00030 0.0678 0.0019 0.0574 0.0029 423.0 11.7 – –

3D−16.1 DIO 39 641 147 0.23 0.00009 0.00006 0.0632 0.0007 0.0536 0.0014 394.8 4.4 – –

Recrystallized grains3D−4.1 MIO 1 23 19 0.83 0.00222 0.00132 0.0489 0.0022 0.0242 0.0215 307.5 13.7 – –

3D−5.1 MUG 5 76 85 1.11 0.00025 0.00029 0.0493 0.0014 0.0535 0.0055 310.2 8.3 – –

3D−12.1 MUG 10 158 213 1.35 0.00055 0.00032 0.0487 0.0010 0.0479 0.0053 306.3 6.1 – –

3D−15.1 MUG 6 90 160 1.78 0.00002 0.00002 0.0496 0.0011 0.0561 0.0025 311.9 6.7 – –

3D−17.1 MUG 11 198 161 0.81 0.00002 0.00002 0.0507 0.0007 0.0530 0.0016 318.8 4.5 – –

3D−18.1 MUG 6 112 102 0.90 0.00051 0.00037 0.0500 0.0011 0.0461 0.0061 314.6 6.5 – –

Unzoned overgrowths and unzoned grain3D−3.1 MUO 2 53 0 0.007 0.00185 0.00146 0.0374 0.0017 0.0565 0.0053 236.7 10.8 – –

3D−7.1 MUO 2 77 1 0.007 0.00029 0.00021 0.0355 0.0012 0.0555 0.0040 224.6 7.7 – –

3D−8.1 DUO 13 421 4 0.009 0.00009 0.00011 0.0352 0.0006 0.0484 0.0013 223.1 3.7 – –

3D−11.1 MUG 1 27 20 0.74 0.00359 0.00197 0.0416 0.0025 0.0597 0.0126 262.9 15.7 – –

3D−13.1 DUO 4 103 1 0.007 0.00002 0.00002 0.0385 0.0010 0.0486 0.0027 243.3 6.1 – –

Sample BDF-1 (anorthosite block)FD−1.1 OZG 17 423 358 0.85 0.00026 0.00019 0.0357 0.0005 0.0483 0.0033 226.3 3.3 113.7 152.2FD−2.1 OZG 12 317 209 0.66 0.00031 0.00014 0.0350 0.0005 0.0474 0.0026 221.6 3.1 68.8 127.6FD−3.1 OZG 16 402 286 0.71 −0.00017 0.00037 0.0357 0.0005 0.0542 0.0059 226.2 3.3 377.6 263.2FD−4.1 OZG 30 399 191 0.48 0.00009 0.00004 0.0721 0.0011 0.0536 0.0015 448.7 6.4 355.8 64.4FD−5.1 UOC 62 1016 5 0.01 0.00000 0.00000 0.0677 0.0008 0.0553 0.0005 422.0 4.9 424.9 20.9FD−6.1 OZG 28 658 670 1.02 0.00011 0.00009 0.0361 0.0005 0.0478 0.0019 228.4 2.8 90.0 91.6FD−7.1 SZC 60 846 247 0.29 0.00002 0.00001 0.0715 0.0010 0.0556 0.0010 444.9 6.0 435.3 40.1FD−8.1 SZC 28 431 62 0.14 −0.00006 0.00011 0.0689 0.0010 0.0550 0.0023 429.6 6.3 413.6 94.5FD−9.1 SZC 40 599 110 0.18 0.00067 0.00015 0.0693 0.0010 0.0550 0.0027 432.0 5.9 413.8 111.8FD−10.1 OZG 10 247 204 0.83 0.00008 0.00005 0.0366 0.0006 0.0523 0.0020 232.0 3.8 296.2 88.9FD−11.1 OZG 22 549 462 0.84 0.00058 0.00017 0.0359 0.0005 0.0491 0.0029 227.1 2.9 152.2 140.0FD−12.1 OZG 14 316 384 1.21 0.00017 0.00009 0.0362 0.0005 0.0519 0.0029 229.4 3.4 281.0 132.8FD−13.1 OZG 35 790 904 1.14 0.00044 0.00008 0.0363 0.0005 0.0477 0.0020 229.9 2.8 86.3 94.0FD−14.1 OZG 13 316 257 0.81 −0.00029 0.00064 0.0365 0.0006 0.0552 0.0102 230.8 3.5 419.0 418.9FD−15.1 SZC 39 348 135 0.39 0.00003 0.00002 0.1092 0.0015 0.0642 0.0008 667.8 8.6 749.1 27.6FD−16.1 SZC 118 1486 839 0.56 0.00000 0.00000 0.0750 0.0008 0.0563 0.0004 466.3 4.8 462.2 14.6FD−17.1 SZC 34 535 46 0.09 0.00007 0.00003 0.0679 0.0008 0.0562 0.0008 423.4 5.0 461.5 33.5FD−18.1 OZG 12 294 237 0.81 0.00092 0.00025 0.0362 0.0006 0.0365 0.0042 229.0 4.0 0.0 0.0

Sample 100482–2 (mylonitized gneiss)2–1.1 OZG 31 257 100 0.39 0.00008 0.00003 0.1188 0.0017 0.0628 0.0016 723.5 9.8 702.4 54.52–2.1 OZG 16 128 80 0.63 0.00021 0.00015 0.1161 0.0025 0.0625 0.0030 708.0 14.5 689.8 104.42–3.1 OZG 36 267 204 0.76 0.00003 0.00002 0.1218 0.0022 0.0639 0.0014 741.0 12.7 737.9 48.32–4.1 OZG 22 660 7 0.01 −0.00006 0.00004 0.0362 0.0005 0.0511 0.0010 229.3 3.1 247.3 43.52–5.1 UO 10 277 2 0.01 −0.00001 0.00001 0.0386 0.0007 0.0542 0.0012 244.4 4.3 380.9 49.42–6.1 OZG 34 424 70 0.17 0.00007 0.00004 0.0830 0.0013 0.0605 0.0013 513.9 8.0 622.2 46.62–7.1 SZC 19 150 86 0.57 0.00007 0.00002 0.1215 0.0029 0.0651 0.0015 739.4 16.5 776.6 49.82–8.1 OZG 19 148 75 0.51 0.00015 0.00009 0.1247 0.0021 0.0627 0.0019 757.4 11.8 699.3 64.12–9.1 OZG 44 350 153 0.44 0.00006 0.00004 0.1226 0.0021 0.0628 0.0011 745.7 11.8 700.3 36.72–10.1 OZG 18 131 85 0.65 0.00001 0.00002 0.1243 0.0022 0.0651 0.0012 755.1 12.5 777.1 37.72–11.1 OZG 34 261 132 0.51 0.00009 0.00004 0.1233 0.0017 0.0634 0.0010 749.4 10.0 721.8 33.72–12.1 OZG 6 71 39 0.55 0.00055 0.00028 0.0732 0.0019 0.0626 0.0052 455.3 11.2 693.1 188.32–13.1 OZG 7 52 53 1.02 0.00001 0.00002 0.1190 0.0041 0.0657 0.0019 725.0 23.6 795.8 62.62–14.1 OZG 44 342 155 0.45 0.00007 0.00003 0.1243 0.0018 0.0629 0.0008 755.2 10.2 704.3 28.62–15.1 OZG 41 331 121 0.37 0.00002 0.00001 0.1227 0.0018 0.0636 0.0008 746.1 10.4 729.8 27.12–16.1 OZG 16 119 73 0.62 0.00001 0.00000 0.1206 0.0025 0.0658 0.0011 734.2 14.4 799.5 36.02–17.1 OZG 30 235 128 0.55 −0.00003 0.00002 0.1221 0.0018 0.0646 0.0010 742.7 10.2 760.2 34.22–18.1 OZG 30 242 103 0.42 −0.00001 0.00001 0.1215 0.0016 0.0662 0.0008 739.1 9.1 812.4 24.7

aZircon Textures: MZC, Medium CL, euhedrally zoned core; DZC, Dark CL zoned core; DIO, Dark CL, intermediate overgrowth; SZC, sector zoned core; OZG, Oscillatory zoned grain;MIO, Medium CL, intermediate overgrowth; MUG, Medium CL grain; MUO, Medium CL, unzoned overgrowth; DUO, Dark CL, unzoned overgrowth.bRadiogenic Pb, corrected for common Pb using 204Pb or 208Pb, and a common Pb composition of laboratory Pb or model rock Pb (see text for details).cPercentage of total 206Pb contributed by common 206Pb.

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0.75 to 2.59, whereas the TiO2 values vary from 0.44 to 2.20 wt.%.Shervais (2001) use such variation to define fields for the northernand southern Coast Range Ophiolite on the basis of the slopes of theobserved trends. The Coast Range Ophiolite, parts of which arebelieved to have formed in an arc setting, show similar chemical

variations to those of the high-grade samples examined in this study.The Franciscan high-grade rocks indicate both arc-tholeiite-like andMORB-like major element chemistry, based on their TiO2 and FeO*/MgO variations. Many of these features, however, may not bediagnostic because of the effect of fO2 variations during arc magma

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Table 2Major and trace element analyses of the high-grade mafic blocks in and around the Baekdong serpentinite mélange.

Rock typeAreaSample no.

Mafic rockWithin Baekdong090319-3A

Mafic rockWithin Baekdong090319-3B

Mafic rockWithin Baekdong090319-3BL

Mafic rockWithin Baekdong090319-3C

Mafic rockWithin Baekdong090319-3D

Mafic rockAround Baekdong090312-2B

Mafic rockAround Baekdong090312-2

SiO2 (wt.%) 49.15 43.17 45.37 48.12 45.96 47.91 50.03TiO2 1.31 0.81 0.44 0.62 0.34 1.33 2.20Al2O3 14.78 13.9 14.98 15.28 14.13 14.11 20.46Fe2O3* 11.99 11.89 7.85 8.46 9.63 14.06 9.12MnO 0.28 0.31 0.14 0.10 0.10 0.22 0.27MgO 8.38 12.29 10.5 10.28 11.84 7.21 3.52CaO 12.94 13.49 16.09 12.29 13.26 10.03 7.15Na2O 1.07 1.57 1.18 1.73 1.17 2.19 3.8K2O 0.14 0.39 0.61 0.83 0.89 1.33 0.79P2O5 0.05 0.09 0.02 0.05 0.04 0.12 0.49LOI 0.12 1.58 1.69 1.98 2.12 1.22 1.52TOTAL 99.95 99.5 98.88 99.73 99.47 99.73 99.36Ba (ppm) 20 48 49 76 37 482 287Sr 202 93 509 442 202 298 603Zr 62 44 501 30 18 53 55Ni 116 383 284 234 252 47 33Ga 15 10 10 8 10 18 15V 343 286 247 275 206 329 228Hf 1.8 1.3 11.1 0.8 0.5 1.8 1.4Nb 1.6 1.7 1.6 1.4 0.5 2.9 13Ta 0.11 0.12 0.14 0.03 0.02 0.17 1.12Rb 7 9 24 27 29 21 23Y 27 26 11 16 10 26 20Cs 0.9 0.2 0.8 0.5 0.4 0.1 0.6U 0.01 0.12 0.91 0.85 0.11 0.13 0.68Th 0.05 0.6 0.9 0.34 0.27 0.29 0.56Pb 5 5 5 5 5 21 12Sc 47.7 50.3 42.1 49.7 41.3 45.7 21.5Be 1 2 1 2 1 2 2Cr 345 392 511 476 1060 177 23.7Co 46.9 67.8 70 61.7 61.8 42.6 25.7Cu 27 15 55 45 24 42 7Zn 37 27 27 35 39 86 49Ge 1.9 2.2 0.7 0.6 1.4 1.3 1La 1.14 2.07 9.31 3.15 3.91 8.03 19.1Ce 3.57 5.05 17.5 6.69 9.2 18.4 41Pr 0.66 0.67 1.93 0.83 1.11 2.57 5.02Nd 3.98 3.1 7.76 3.65 4.65 12 21.2Sm 2.24 1.23 2.03 1.33 1.27 3.51 5Eu 0.939 0.399 0.937 0.63 0.547 1.46 2.49Gd 3.68 1.82 1.9 1.54 1.28 4.33 4.32Tb 0.73 0.57 0.4 0.4 0.28 0.83 0.78Dy 4.56 4.03 2.35 2.47 1.72 5.32 4.07Ho 1.01 1 0.49 0.56 0.36 1.08 0.75Er 2.88 3.14 1.4 1.56 1.01 3.23 1.84Tm 0.457 0.556 0.226 0.262 0.163 0.486 0.245Yb 2.83 3.5 1.46 1.59 0.94 3.1 1.54Lu 0.411 0.529 0.229 0.225 0.139 0.456 0.214Eu/Eu* 1.00 0.82 1.46 1.35 1.31 1.14 1.64

Eu* represents the normalized values between Sm and Gd.Fe2O3* represents total Fe.

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generation in the subduction environment, which can also generatesuch trends. Slab-derived fluids also contribute significantly to thenature of arc magmas (e.g., Nakamura and Iwamori, 2009). Thus,major element chemical variations are not robust criteria forprotolith characterization in this case (Saha et al., 2005).

The chondrite-normalized REE patterns of the high-grade maficrocks from the Baekdong serpentinite mélange are shown in Fig. 6,which shows relatively flat patterns with concentrations varyingbetween 5 and 40 times that of chondrite. The mafic rocks show bothslight LREE-enriched and depleted patterns, with (La/Sm)n between0.33 and 2.96, and (Gd/Lu)n between ~0.43 and 1.14. They alsogenerally display slightly LREE-enriched patterns, with (La/Sm)nbetween 1.48 and 2.47, and (Gd/Lu)n between ~1.17 and 2.50. The

Fig. 5. Scanning electron microscope cathodoluminescence (CL) images of sectioned zircon gmafic rock (a), (b), anorthosite (c), (d), orthogneiss (e), (f), and high-pressure eclogite (g)

overall REE patterns are similar to those of arc basalts, such as thosefrom the South Sandwich Islands and the Marianas (Hawkesworthet al., 1977; Tatsumi and Eggins, 1995). In Fig. 6, the Western Pacificarc tholeiite REE data from Jakes and Gill (1970) are shown in shadingfor a broad comparison. The data from the high-grade mafic rocks ofBaekdong overlap with the Western Pacific arc tholeiite REE data.

On the Ti/100-Zr–Y×3 diagram of Pearce and Cann (1973), thehigh-grade mafic rocks plot in the field of island arc tholeiites (Fig. 7).The analyzed samples have Ce concentrations of ~3.57 to 41 ppm, andCe/Pb varies from ~0.7 to 3.5. These are compared with the globalreservoirs of bulk-earth, MORB, OIB, average continental crust, andthe global arc tholeiites (or basalts) fields on a Ce/Pb versus Ceconcentration plot (Fig. 8). Pb is deficient in continental crust and

rains and concordia plots of SHRIMP U–Pb isotopic analyses of zircons from high-grade, (h) from the Bibong block reported by Kim et al. (2006).

Fig. 6. Chondrite-normalized rear earth element (REE) patterns of high-grade maficfragments/blocks in and around the Baekdong serpentinite mélange. For comparison,the shaded region shows the general range of Western Pacific arc tholeiite (Jakes andGill, 1970).

Fig. 8. Ce/Pb ratios versus Ce concentrations in high-grade mafic rocks in and aroundthe Baekdong block, together with those in the Baekdong serpentinite mélangereported by Oh et al. (2009), in comparison with fields for oceanic basalts (MORB andOIB), global arc lavas, average continental crust, and the chondritic bulk-earthcomposition.

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subduction-related lavas (Briqueu et al., 1984). Nb concentrations inthe samples vary between 0.5 and 13.0 ppm (Table 2). Ce/Pb ratiosfrom the Baekdong samples are generally lower than MORB and OIBvalues (Ce/Pb=20), varying between 0.7 and 3.5. Other incompatibleand compatible trace element patterns, including the REE contents ofthe high-grade mafic rocks normalized to N-MORB, are shown inFig. 9. All the samples show prominent LILE enrichment. Pb also showsenrichment as illustrated in Fig. 9. Nb depletion is prominent in someof the samples.

6. Chromian spinel chemistry

The compositions of spinels in ultramafic rocks is a useful indicatorof magma petrogenesis, geotectonic environments, degree of partialmelting and fractional crystallization (e.g., Dick and Bullen, 1984;

Fig. 7. Tectonic discrimination diagrams using Ti, Zr and Y from high-grade mafic rocksin and around the Baekdong serpentinite mélange, together with those from theBaekdong block reported by Oh et al. (2009). The protoliths of the high-grade maficrocks are mostly island-arc tholeiites.

Kimball, 1990; Burkhard, 1993; Barnes, 2000). Electron microprobeanalyses of primary chromian spinels within the hydrated mantleperidotites from the Baekdong body were carried out at OkayamaUniversity of Science using a JEOL JXA-8900R, Japan. The quantitativeanalyseswere performedwith 15 kV accelerating voltage, 12 nA beamcurrent and 3 μmbeam size. Natural and synthetic silicates and oxideswere used for calibration, and the ZAF method (oxide basis) wasemployed for matrix corrections. Fe2+ and Fe3+ contents areestimated from total FeO based on the charge balance usingstoichiometic criteria (Droop, 1987). Representative compositions ofthe chromian spinels from present study are listed in Table 3.

The red-brown to dark-brown primary igneous chromian spinelsin our samples show homogeneous compositions from core to rim,except for alterations along cracks and margins in the black-coloredchromian spinels (Fig. 10a). The spinels are characterized bymoderate Cr2O3 (42.09–51.61 wt.%), Al2O3 (17.38–25.45 wt.%), FeO(18.95–124.17 wt.%) and MgO (8.38–12.74 wt.%), and very low TiO2

(0.00–0.19 wt.%). Cr# [Cr/(Cr+Al)] andMg# [Mg/(Mg+Fe2+)] rangefrom 0.53 to 0.66 and from 0.43 to 0.67, respectively. The moderate tohigh Cr# (some higher than 0.6) is typical of chromian spinels that are

Fig. 9. Multiple trace element concentrations normalized to N-MORB for high-grademafic rocks in and around the Baekdong serpentinite mélange. The overall traceelement signatures of these rocks are clearly unlike MORB but similar to arc basalts.

Table 3Representative chemical compositions of the chromian spinels from the Baekdong hydrated mantle peridotite.

Sample no. BDS

SiO2 0.02 0.01 0.11 0.04 0.05 0.04 0.04 0.04 0.02 0.02 0.03 0.07 0.04 0.07 0.01 0.01 0.07 0.00 0.05 0.01TiO2 0.14 0.10 0.15 0.06 0.06 0.09 0.07 0.11 0.04 0.08 0.10 0.09 0.07 0.06 0.12 0.05 0.05 0.08 0.14 0.07Al2O3 18.83 19.16 22.77 25.38 18.95 24.51 24.25 23.68 23.63 23.97 23.32 22.91 23.69 20.67 19.84 20.44 17.38 18.80 19.42 18.96Cr2O3 50.02 49.04 44.12 42.67 49.18 43.20 43.44 43.66 44.37 43.49 44.83 45.67 43.55 46.04 45.15 48.27 52.31 49.61 45.58 49.12FeO 18.43 17.76 16.05 16.52 17.97 16.96 16.98 16.66 17.06 16.38 16.47 17.53 16.89 18.88 17.18 17.53 20.03 18.46 17.48 17.91Fe2O3 1.73 2.34 2.91 1.59 2.05 2.48 2.44 2.21 2.74 2.76 2.14 2.11 2.40 2.34 4.74 2.20 1.15 2.25 5.63 2.22MgO 10.92 11.30 12.56 12.52 11.00 12.32 12.24 12.22 12.25 12.59 12.47 11.95 12.09 10.42 11.46 11.63 9.84 10.86 11.50 11.10NiO 0.05 0.01 0.03 0.05 0.08 0.05 0.06 0.07 0.05 0.02 0.08 0.01 0.05 0.05 0.06 0.04 0.02 0.04 0.04 0.01Total 99.97 99.48 98.42 98.66 99.13 99.41 99.26 98.42 99.89 99.03 99.22 100.14 98.53 98.29 98.10 99.95 100.73 99.88 99.28 99.18Cr 1.25 1.23 1.09 1.04 1.24 1.05 1.06 1.07 1.08 1.06 1.10 1.11 1.07 1.16 1.14 1.19 1.32 1.24 1.14 1.23Ti 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00Al 0.70 0.71 0.84 0.92 0.71 0.89 0.88 0.87 0.86 0.87 0.85 0.83 0.87 0.78 0.74 0.75 0.65 0.70 0.72 0.71Fe3+ 0.04 0.06 0.07 0.04 0.05 0.06 0.06 0.05 0.06 0.06 0.05 0.05 0.06 0.06 0.11 0.05 0.03 0.05 0.13 0.05Fe2+ 0.49 0.47 0.42 0.43 0.48 0.44 0.44 0.43 0.44 0.42 0.43 0.45 0.44 0.50 0.46 0.46 0.53 0.49 0.46 0.48Mg 0.51 0.53 0.58 0.57 0.52 0.56 0.56 0.57 0.56 0.58 0.57 0.55 0.56 0.50 0.54 0.54 0.47 0.51 0.54 0.53Ni 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00Mg/(Mg+Fe2+) 0.51 0.53 0.58 0.57 0.52 0.56 0.56 0.57 0.56 0.58 0.57 0.55 0.56 0.50 0.54 0.54 0.47 0.51 0.54 0.52Cr/(Cr+Al) 0.64 0.63 0.57 0.53 0.64 0.54 0.55 0.55 0.56 0.55 0.56 0.57 0.55 0.60 0.60 0.61 0.66 0.64 0.61 0.63Fe2+/Fe3+ 11.82 8.43 6.13 11.53 9.76 7.60 7.74 8.39 6.91 6.59 8.55 9.25 7.81 8.96 4.03 8.84 19.38 9.12 3.45 8.97F melt ne ne 17.65 19.31 ne 17.94 18.07 18.15 18.00 18.13 17.75 18.35 18.87 18.02 18.21 ne ne ne ne ne(FeO/MgO)melt 1.52 1.42 1.22 1.34 1.47 1.36 1.37 1.34 1.35 1.27 1.28 1.41 1.37 1.69 1.35 1.39 1.79 1.52 1.34 1.45

ne: Not estimate.

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formed in a forearc setting, where themantle suffered a high degree ofpartial melting (Dick and Bullen, 1984). On the Mg# versus Cr#diagram (Fig.10b), the primary chromian spinel compositions plotwithin the forearc field. On the Al2O3 versus Fe2+/Fe3+ diagram(Fig. 10c), all of the chromian spinel compositions plot within thesuprasubduction zone (SSZ) peridotite field. These features suggestthat the chromian spinels in the Baekdong serpentinites formed in asuprasubduction zone setting related to a forearc tectonic environ-ment (Kamenetsky et al., 2001). The estimated degree of partialmelting using the equation of Hellebrand et al. (2001), is greater than17.6%. This is similar to the melting degree of harzburgite in a forearcand is relatively higher than that from the mid-ocean ridge(Hellebrand et al., 2001). The FeO/MgO ratio of the parental meltwas determined from Maurel and Maurel (1982), and the represen-tative data are given in Table 3. The (FeO/MgO) parent melt ratio ofthe Baekdong hydratedmantle peridotite ranges from 0.9 to1.3, whichfalls in the range of forearc boninitic magmas (0.7–1.4; Wilson, 1989;Uysal et al., 2007, 2009). Hence, the primary igneous chromian spinelcompositions from the Baekdong serpentinites show the character-istics of those derived from a highly partial-melted upper mantle.

7. Tectonic evolution of the Baekdong serpentinite mélange

The Wolhyeonri complex in the Hongseong area of the centralwestern Korean Peninsula preserves the signature of Paleozoic toEarly Mesozoic subduction and accretion in East Asia (e.g., Kröneret al., 1993; Xue et al., 1996; Zhai et al., 1998; Tsujimori and Itaya,1999; Tsujimori et al., 2000; Sun et al., 2002; Yang et al., 2005;Ratschbacher et al., 2006; Metcalfe, 2006, 2011; Ernst et al., 2008;Rogers and Bernosky, 2008; Wu et al., 2009; Isozaki et al., 2010). TheLate Paleozoic crystallization age (Fig. 5a) of the high-grade maficblock suggests the existence of a mantle wedge above a “Pacific-type”subduction zone (e.g., Maruyama et al., 2009; Santosh et al., 2009;Isozaki et al., 2010) prior to ca. 310 Ma (Fig. 5a), followed by collisionduring the Early Mesozoic. The serpentinite-filled anorthosite(~228 Ma; Figs. 3d and 5b) further suggests that the peridotiteswere not hydrated until the Early Mesozoic collision stage. Theabsence of antigorite suggests that these serpentinites were derivedfrom relatively shallow depths of less than 35 km from the mantlewedge (Dungan, 1977; Evans, 1977).

Evidence for northward subduction is recorded in the Wolhyeonricomplex, where Paleozoic metamorphic events and igneous activity arerecorded in SHRIMP zircon data (Kim et al., 2010). Geochemical data onthe mafic rocks from the Baekdong serpentinite mélanges indicate anisland arc-like tholeiite affinity (Figs. 6, 7, 8 and 9) with none of thesamples possessing the chemical signature of continental source rocks(Fig. 8). The primary chromian spinel compositions from the Baekdonghydrated mantle peridotite show the characteristics of those fromhighly partial-melted upper mantle in a forearc setting (Fig. 10). This isalso supported by the passive margin interpretation for the Baekdonghydrated mantle peridotite (Seo et al., 2005) based on the Cr# (spinel)vs. Al2O3 (wt.%) relationship. The high degree of mantle melting in thiszone can be explained by the involvement of fluids released from asubducted slab and the injection of wet sediments into the mantlewedge in a forearc environment (e.g., Stern and Bloomer, 1992). Wetherefore consider the Baekdong serpentinite mélange to have formedby hydrous metasomatism of the mantle wedge above a subductionzone by fluids released from subducted slabs, and that it comprisesdisrupted fragments of oceanic complexes, such as island arcs, producedduring the prolonged subduction–accretion–collision regime. Severallens-shaped dismembered bodies, such as those at Hongseong,Gwangcheon, Wonnojeon, Gyeweol and Singok within the PaleozoicWolhyeonri complex (Fig. 2), may represent serpentinite mélange thatenclosed the fragments of oceanic crust and/or island arc, etc.,representing a dismembered ophiolite suite, during ocean closure.

Based on bulk chemical characters, chromian spinel chemistry andgeochronological data presented in this study in conjunction with theavailable ages from the region, we propose a plate tectonic model forthe development of the Baekdong serpentinite mélange (Fig. 11),following the conceptual models for subduction-related processesproposed by Wakabayashi and Dilek (2003), Saha et al. (2005),Brueckner et al. (2009) and Maruyama et al. (2009). Ocean closureduring the Paleozoic is correlated with the northward subduction ofthe Gyeonggi massif (corresponding to the south China Craton)(Fig. 11a). Adiabatic decompression from an upwelling astheno-sphere, combined with the dehydration of the downgoing lithospherecausedwidespreadmelting (Wakabayashi and Dilek, 2003; Saha et al.,2005). The geochemical characteristics of rocks generated in thissetting would be comparable to the “Alpine-type” peridotites (Dickand Bullen, 1984) as proposed for the Baekdong hydrated mantle

Fig. 10. (a) Representative photomicrograph of a chromian spinel grain from theBaekdong hydrated mantle peridotite showing original igneous (in red-brown to dark-brown) and altered domains (in black). (b) Cr# vs. Mg# for the analyzed spinels andtheir alteration products. Field boundaries are from Dick and Bullen (1984), Bloomeret al. (1995) and Ohara et al. (2002). (c) Al2O3 vs. Fe2+/Fe3+ diagram showing thefields of supra-subduction zone (SSZ) vs. mid ocean ridge (MOR) peridotite (afterKamentsky et al., 2001).

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peridotite (Seo et al., 2005). The hydrated forearc mantle peridotite ofthe Baekdong serpentinite represents fragments of the subductedlithosphere on the Gyeonggi massif side as well as the hanging wallmantle wedge, formed by serpentinization above the “Pacific-type”subduction margin (Bruecknet et al., 2009) during the Paleozoic(Fig. 11a), prior to the final Late Paleozoic to Early Mesozoic “Alpine-type” collision in East Asia (Fig. 11b). The episodic metamorphic

events during the prolonged subduction are clearly recorded in thePaleozoic metamorphic episodes within the Wolhyeonri complex(Kim et al., 2010). This is also supported by the early Silurian to lateDevonian sedimentary hiatus in the foreland fold-thrust belt (viz., theOkcheon belt). The detrital zircon SHRIMP data from paragneiss,metavolcanics, quartzites and limestones of the Wolhyeonri complexsuggest that they were deposited in a suprasubduction zone settingprobably during the Early toMiddle Paleozoic (Kim et al., 2010). Thesedata, together with the Late Paleozoic age from the high-grade maficblock (Fig. 5a) in this study, attest to the Paleozoic subduction history(Fig. 11a). A portion of Wolhyeonri complex, which includes theserpentinite mélanges, was exhumed to the surface during thesubsequent collision event (Fig. 11b) between the Gyeonggi massif(corresponding to the South China block) and the Nangnim massif(corresponding to the North China block) as suggested by Seo et al.(2005). The disrupted fragments/blocks, which may include oceaniccrust, seamounts/guyots, and volcanic islands, as well as continentalcrust, were trapped within the mélange during the subduction–accretion–collision orogeny.

8. Conclusions

Bulk chemical data, chromian spinel compositions and SHRIMPzircon U–Pb ages from the Baekdong serpentinite mélange in theHongseong area trace the Paleozoic subduction–accretion–collisionhistory in this region. The forearc mantle peridotites were hydratedand detached from the subducting lithosphere and its hanging wallmantle wedge. The hydrated forearc mantle peridotites witnessedaqueous metasomatism above a “Pacific-type” subduction zone. ThePaleozoic to Early Mesozoic subduction and accretion events wereprobably related to the worldwide orogenic processes associated withthe amalgamation of the Pangaea. The Hongseong belt preservesimportant clues on the possible link between the Korean Peninsula andthe east-central orogenic belt in China in relation to the Permo-Triassiccollision that finally assembled the East Asian collage.

Acknowledgements

We thank Drs. Sung-Tack Kwon, Wenjiao Xiao and an anony-mous reviewer for constructive reviews and Dr. Damian Nance forvaluable comments and editorial handling. We also thank Dr. NgoThanh for chromian spinels. This work was supported by the BasicResearch Project (GP2009-012; “Tectonic Evolution of Major CrustalUnits in the Midwestern Part of the Korean Peninsula”) of the KoreaInstitute of Geoscience and Mineral Resources, funded by theMinistry of Knowledge and Economy of Korea to S.W. Kim. Thispaper represents part of N. Park's M.Sc. research performed atYonsei University. This work was supported by Grant in-Aid NRF2010-0011102 to S. Kwon.

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