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Spinel compositions and tectonic relevance of the Bibong ultramafic bodies in theHongseong collision belt, South Korea

C.W. Oh a,b, V.J. Rajesh b,c,⁎, J. Seo d, S.-G. Choi d, J.H. Lee a,b

a Department of Earth and Environmental Sciences, Chonbuk National University, Chonju 561-756, South Koreab Basic Science Research Institute, Chonbuk National University, Chonju 561-756, South Koreac Department of Earth and Space Sciences, Indian Institute of Space Science and Technology, Thiruvananthapuram 695 022, Indiad Department of Earth and Environmental Sciences, Korea University, Seoul 136-701, South Korea

a b s t r a c ta r t i c l e i n f o

Article history:Received 7 September 2009Accepted 24 February 2010Available online 6 March 2010

Keywords:Chromian spinelUltramafic rocksBack-arc basinHongseongNeoproterozoicTriassic collision

The Hongseong area in the western Gyeonggi Massif, South Korea, is characterized by the occurrence ofseveral isolated ultramafic lenses in close association with metabasites within the granitic gneiss. Most ofthese bodies suffered various degrees of serpentinization and also have been highly deformed andmetamorphosed. The Bibong ultramafic rock is assumed to be a mantle section of a probable Neoproterozoicsupra-subduction zone (SSZ) and its evolution involved two igneous stages overprinted by later stagemetamorphism. The compositions of the chromian spinel cores and olivines representative of the igneousstages were used to deduce the petrogenesis and tectonic environments for the formation of the Bibongultramafic rocks. Spinel Cr# (100⁎Cr/(Cr+Al)) for the igneous stages in lherzolite, harzburgite andchromitite are 24.5–33.3, 37.4–61.2 and 37.3–50.9, respectively. During the first stage, magma formed in thenascent back-arc basin spreading center by a small degree of partial melting of wedge mantle intruded thepreexisting lithospheric mantle causing the melt/rock interaction that resulted in lherzolite with Al-richchromian spinel. During the second stage more evolved magma formed in the mature back-arc basin by highdegrees of partial melting due to the increased supply of fluid or melt from the subducting oceanic crust andintruded the lherzolitic mantle forming harzburgite with chromian spinel (with high Cr#) through thecoupled process of olivine precipitation and dissolution of pyroxene. The Bibong ultramafic rocks have laterundergone three metamorphic episodes. The first stage is evidenced by development of medium to finegrained olivine, orthopyroxene, clinopyroxene and amphibole around orthopyroxene porphyroblasts. Thesecond stage is serpentinization which is followed by the third stage amphibolite facies metamorphism asevidenced by Fe-rich rims around Al-rich chromian spinel due to the diffusional exchange of Al, Mg, Cr, andFe between chromian spinel and adjacent silicate minerals. The spinel composition shows that the transitionfrom the nascent back-arc basin to mature back-arc basin tectonic setting may have been caused by trenchroll back during the Neoproterozoic in the Hongseong area, and the serpentinization and amphibolite faciesmetamorphism could have occurred during uplift after Triassic collision between the North and South Chinablocks.

© 2010 Elsevier B.V. All rights reserved.

1. Introduction

Recent studies have suggested that the Hongseong belt in thesouthwestern Gyeonggi Massif, South Korea, is the extension of theTriassic Dabie–Sulu collision belt that occurs between the North andSouth China blocks (Oh et al., 2005; Kim et al., 2006). Ultramafic rocksin close association with eclogite are considered as one of the typicalfeatures of many collision/suture zones. In the Hongseong collisionbelt, metabasites and ultramafic rocks occur as lensoidal bodies. Themetabasites in the Bibong and Baekdong areas within the Hongseong

collision belt originated in an arc tectonic setting during the lateNeoproterozoic and underwent eclogite facies high-P/T metamor-phism during the Permo-Triassic collision between the North andSouth China blocks (Oh et al., 2004, 2005; Kim et al., 2006). Theoccurrence of the high-P/T metabasites of island-arc tholeiite (IAT)affinities with spatially associated ultramafic rocks suggest that atleast some ultramafic rocks in the Hongseong area may also bederived from an ancient arc tectonic setting, probably of Neoproter-ozoic age. The origin and tectonic affinity of the ultramafic rocks areunclear because few studies had been done on them and the originaligneous assemblages have been almost obliterated by several laterstages of metamorphism (including serpentinization and amphibolitefacies metamorphism). Spinel-group minerals occur as commonaccessory minerals in ultramafic rocks of diverse tectonic settings.

Lithos 117 (2010) 198–208

⁎ Corresponding author. Department of Earth and Space Sciences, Indian Institute ofSpace Science Technology, Thiruvananthapuram 695 022, India.

E-mail address: rajeshvj@iist.ac.in (V.J. Rajesh).

0024-4937/$ – see front matter © 2010 Elsevier B.V. All rights reserved.doi:10.1016/j.lithos.2010.02.015

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They display a wide range in composition reflecting their primarymagmatic or secondary origin. Unaltered, primary chromian spinel inserpentinite can be used as a petrogenetic and geotectonic indicatorbecause its chemical composition depends on the petrogenesis andphysical conditions of the host peridotites and the primary chromianspinel composition is well preserved during later stage serpentiniza-tion or metamorphism (Ozawa, 1983; Arai, 1992, 1994; Zhou et al.,1994; Zhou and Robinson, 1997; Karipi et al., 2007). In the Hongseongcollision belt, most ultramafic lenses are strongly serpentinized butcontain unaltered spinel (e.g., Seo et al., 2005; Arai et al., 2008).Therefore, unaltered spinel composition can be used to interpret thepetrogenetic and tectonic history of the ultramafic rocks in theHongseong collision belt.

The ultramafic lens in the Bibong area is one among the ultramaficrock exposures in the Hongseong collision belt. The Bibong ultramaficrocks occur in close associationwithmetabasites (Oh et al., 2005; Seo etal., 2005). These metabasites had undergone eclogite and/or amphib-olite facies metamorphism during Triassic collision (Oh et al., 2005).The present study focuses on the petrography andmineral chemistry ofchromian spinel in the Bibong ultramafic rocks in order to determinetheir petrogenesis and tectonic environment.

2. General geology

The threemain Precambrian blocks in the Korean Peninsula are theNangrim, Gyeonggi and Yeongnam Massifs (Fig. 1A). The GyeonggiMassif is separated from the Nangrim Massif to the north by thePaleozoic Imjingang fold-thrust belt, and from the Yeongnam Massifto the south by the Late Precambrian–Paleozoic Okcheon (Ogcheon)fold-thrust belt. The Hongseong area is situated in the southwesternpart of the Gyeonggi gneissic complex (henceforth, Gyeonggi Massif)in the Korean Peninsula and is the extension of the Dabie–Sulucollision belt between the North and South China blocks in China(Fig. 1A; Oh et al., 2005; Kim et al., 2006). The Hongseong collision beltis suggested to extend towards the Odesan Area in the eastern part ofthe Gyeonggi Massif (Fig. 1A) indicating that the collision beltbetween the North and South China blocks goes all the way acrossKorean Peninsula (Oh et al., 2006; Oh, 2006; Oh and Kusky, 2007). TheHongseong area includes the Wolhyeonri Formation and theDeokjeongri granitic gneiss (Fig. 1B). The predominant lithologicalunits of the Wolhyeonri Formation are closely related biotite schists,metabasites, and marbles. Amphibole- and biotite-bearing graniticgneiss and leucogranite are the dominant orthogneisses of the

Fig. 1.Generalized geological map of the Hongseong area, Gyeonggi massif, depicting the isolated occurrence of serpentinized harzburgites (modified fromOh et al., 2004). A tectonicmap of the Korean Peninsula is shown as an inset.

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Deokjeongri granitic gneiss (Oh et al., 2004, 2005; Kim et al., 2006).Zircons from the Deokjeongri granitic gneiss gave an intrusion age of850–820 Ma and a metamorphic age of ca. 235 Ma based on sensitivehigh-resolution ion microprobe (SHRIMP) U–Pb zircon geochronolo-gy (Kim et al., 2006, 2008; Cho, 2001).

Both the Wolhyeonri formation and the Deokjeongri graniticgneiss are characterized by many lensoid-shaped metabasite andultramafic lenses (200–1000 m long and 10–80 m wide; with varyingdegrees of serpentinization) within the major lithological units (e.g.,Song et al., 1997; Choi et al., 1998; Seo et al., 2005). They are elongatedin a NNE direction except the Baekdong ultramafic lens which iselongated in a roughly E–W direction. The metabasites in theHongseong collision belt are divided into two types based on theintrusion age and tectonic origin (Oh et al., 2008). Type I metabasite(e.g. Gwangcheon metabasite) has an intrusion age of 760 Ma andoriginated in a within plate tectonic setting. The Type II metabasites(e.g., Bibong and Baekdong metabasites) are formed in an arc tectonicsetting at ca. 890–870 Ma (Guo et al., 2004; Oh et al., 2008). Based onthe two types of metabasites, Oh et al. (2008, 2009) suggested that thetransition from arc to within plate tectonic setting occurred in theHongseong collision belt between 830 and 760 Ma.

In the Bibong area, lensoidal shaped ultramafic bodies are found inclose association with metabasites (Fig. 1B). The bodies are serpenti-nized to variable degrees and extend ∼600×60 m in a NNE direction.Eclogite facies metamorphism (17.0–20.9 kbar and 835–860 °C) isidentified from the metabasites in the Bibong area (Guo et al., 2004;Oh et al., 2005; Kim et al., 2006). A 231 Ma metamorphic age wasobtained for the Bibong metabasites using SHRIMP zircon age dating(Guo et al., 2004; Kim et al., 2006). This age is interpreted as the timewhen themetabasite underwent eclogite facies metamorphismwhichwas subsequently overprinted by granulite and amphibolite faciesmetamorphic events (Guo et al., 2004; Oh et al., 2005; Kim et al.,2006). The 231 Ma metamorphic age is rather well correlated to themetamorphic age of the Deokjeongri granitic gneiss (235 Ma)indicating that the regional metamorphism was due to Triassiccontinental collision (Kim et al., 2008). The Daedong Formation(Mesozoic sedimentary rocks) unconformably overlies all of thesedominant lithologies in the Hongseong belt. Although the Bibongultramafic rocks are serpentinized in most parts, slightly serpenti-nized ultramafic rocks are still observed. They show strong foliation.Porphyroclastic and mylonitic fabrics are observed in places. In thestrongly foliated part, minerals such as olivine and spinel areelongated. The Bibong ultramafic rocks consist mainly of harzburgitewith chromitite bands (a few centimeters thick as measured in rockslabs) parallel to the tectonite fabric. Lherzolites occur as relictswithin harzburgite. Lherzolite relicts are identified under themicroscope but are difficult to identify in the outcrop due to smallsize of the relict and serpentinization. As the chromitite band isalways strongly serpentinized, spinel is the only visible primarymineral preserved in chromitite band.

3. Petrography

Harzburgite is coarse-grained with equigranular and porphyro-blastic texture and consists of olivine, orthopyroxene, amphibole andspinel (Fig. 2). Olivine occurs as a main mineral with subhedral grainsand has clinopyroxene and spinel inclusions. Orthopyroxenes andclinopyroxenes are also subhedral and orthopyroxenes also includeclinopyroxene and spinel and sometimes occur as porphyroblasts.Spinels also occur as inclusions in clinopyroxene and amphibole. Ourmineral chemical analysis revealed that primary clinopyroxenes occuras relicts in amphiboles indicating that clinopyroxenes were retro-graded into amphiboles during later stage metamorphism. Lherzolitehas the same mineral assemblage and texture as the harzburgite buthas lower modal abundance of orthopyroxene (Fig. 2). The clinopy-roxene in lherzolite is very low in content when compared to the

abundant amphibole and a few smaller sized relict grains ofclinopyroxene in amphibole indicate the retrogression of clinopyrox-ene into amphibole during metamorphism as occurred in theharzburgite. More abundant amphibole in lherzolite than harzburgitesuggests higher modal percentage of clinopyroxene during theigneous stage in lherzolite. Further, the chromian spinels in thelherzolite samples are more Al-rich than in the harzburgite. Inharzburgite, medium to fine grained secondary olivine, orthopyrox-ene, clinopyroxene and amphibole were formed around orthopyrox-ene porphyroblasts during the first stage metamorphism. During thisstage most primary clinopyroxene might have recrystallized intoamphibole. The primary igneous and secondary metamorphic olivinesand orthopyroxenes were partly or completely serpentinized formingmagnetite during the second stage metamorphism. The serpentinesshow mesh texture replacing olivine and orthopyroxene. It is difficultto investigate the primary mineral assemblage of the chromititebecause all primary minerals except spinel were changed intoserpentine (Fig. 2G, J and I). During the final stage metamorphism,serpentinization showing ribbon texture occurred with the develop-ment of magnetite and minor chlorite.

Chromian spinels in lherzolite show bright brown color and occuras vermicular grains (Fig. 2). On the other hand, in the harzburgite,chromian spinels show dark brown color and mainly occur assubhedral grains (Fig. 2). Some spinel grains have inclusions of tinyamphibole and/or phlogopite, which might suggest the hydrousnature of the melts responsible for their formation. In the chromitite,spinels occur as subhedral to anhedral grains larger (length up to2 mm) than those in lherzolite and harzburgite, and show black colorwith dark brown core (Fig. 2). In the chromitite, Al-rich chromianspinels are extensively replaced by Fe-rich spinels along rims andfractures and are rimmed by magnetite overgrowths (Fig. 2). Veintype fined grained magnetite is observed within chromitite.

4. Mineral chemistry

Mineral compositions were determined using a JEOL JXA-8600 SXmicroprobe situated at the Korea University in Seoul. The acceleratingvoltage, beam current, and probe diameter were 15.0 kV, 3.0 nA, and3 μm respectively. Tables 1 and 2 list representative mineralcompositions.

4.1. Spinel

The primary spinels are chromian spinels (here after Cr-spinel) inlherzolite, harzburgite and chromitite. Cr-spinels representative of theigneous stage in lherzolite have Al-rich core compositions (Table 1,Figs. 3 and 4). Core compositions of Cr-spinels have low Cr# ([100⁎Cr/(Cr+Al)]=24.5–33.3) and high Mg# ([100⁎Mg/(Mg+Fe)]=69.3–74.5). These cores are rimmed by Cr-spinels with high Cr# (37.3–55.3) and lowMg# (53.8–67.4), suggestive of compositional variationdue to either later stage melt/rock interaction and/or post igneousprocesses. Cr-spinels in harzburgite have high Cr# (37.4 to 61.2) andlowMg# (54.6 to 67.7) (Table 1, Fig. 3). The alteration rims developedaround harzburgite Cr-spinels (Fig. 3, Table 1) are too thin to beanalyzed by EPMA. Fe-rich chromite or magnetite formed along thefractures in Cr-spinels in both rocks indicating later stage metamor-phism. TiO2 contents in the Cr-spinels are mostly below detectionlimit (the detection limit of TiO2 is 0.01 wt.%). Cr-spinels in chromititebands are more extensively altered than those in lherzolite andharzburgite (Fig. 4). BSE images combined with compositional datashow that Cr-spinels in chromitite bands display concentric zoningfrom an Al-rich dark Cr-spinel core (Cr#=37.3–50.9, Mg#=57.7–67.9) to a Fe-rich chromite (ferrichromit) rims (Cr#=89.4–98.3,Mg#=6.3–15.2; Table 1, Figs. 2, 4). Between the core and rim, atransitional zone (Cr#=69.3–90.7, Mg#=27.5–79.0) is observed. In

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many grains, Cr-magnetite overgrowths were formed around theferrichromit.

4.2. Other minerals

There is very little compositional variation between the primaryand secondary olivines in lherzolite and harzburgite (Table 2). Theyare forsterite with almost constant Mg# of 90–92. The NiO content ofprimary olivine in harzburgite ranges from 0.37 to 0.42 wt.%, which iswithin the range of the olivine mantle array of Takahashi (1986).Higher values of NiO (up to 0.55 wt.%) is noticed in olivines inharzburgites. The orthopyroxene in both rock types is enstatite andhas rather homogenous composition (En 89.8–91.4). The clinopyrox-ene is diopside with MgO of 17.34–18.31 wt.%. The primaryorthopyroxene and clinopyroxene have slightly higher Mg# thanthe secondary orthopyroxene and clinopyroxene (Table 2). Amphi-boles are magnesio-hornblende with Mg# of 93–97 and Si (a.f.u) of6.64–7.18. Generally, serpentines have very low Al2O3 (lower than0.71 wt.%) but the second stage serpentines formed near or aroundFe-rich chromite (ferrichromit) have higher Al content than the otherserpentines (Table 2). The second stage Cr-magnetites, formed asovergrowths along the rim of Cr-spinel or as individual grains around

ferrichromit, have higher Cr content than the other magnetites. TheAl-rich serpentine and Cr-magnetite around ferrichromit indicate thatthere was a local Al, Cr, Fe exchange between Cr-spinel and thesurrounding minerals during the second stage serpentinization.Chlorites are pennite and contain Cr2O3 of 0.47–1.19 wt.% and Al2O3

of 8.24–9.16 wt.%.

5. Discussion

Studies on ultramafic rocks have confirmed that Cr-spinel canserve as a potential petrogenetic and tectonic indicator due to itsability to survive during later stage metamorphism (Ahmed et al.,2001, 2005; Hellebrand et al., 2002). This is true especially with thecore compositions. The use of Cr-spinel composition as a petroge-netic and geotectonic indicator requires a rigorous petrographicstudy to identify episodes of magmatic and post-magmatic eventsexperienced by the rock (e.g., Rollinson, 1995; Suita and Streider,1996). The samples analyzed in this work normally preserveunaltered chromian spinel grains or unaltered cores. Only theanalyses performed on the unaltered chromian spinels have beenconsidered in the interpretation of the igneous petrogenesis.However, the zones of altered chromite were also analyzed to

Fig. 2. Photomicrographs and back scattered images of chromian spinel and other minerals in the ultramafic rocks; lherzolite (A–C), harzburgite (D–F) and chromitite (G–I). Spl:chromian spinel, Opx: orthopyroxene, Ol: olivine and Mt: magnetite.

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Table 1Compositions of representative spinels from the Bibong ultramafic rocks.

Lherzolite Harzburgite Chromitite

Location Core Core Rim Rim Inclusion Core Core Middle Middle Rim Rim

Sample bb119 bb130 bb121 bb126 bb163 bb190 bb271 bb228 bb226 bb260 bb033 bb087 bb081 bb011 bb097 bb111

TiO2 n.d n.d n.d n.d n.d n.d 0.21 n.d n.d n.d 0.24 n.d n.d n.d n.d 0.14V2O5 n.d n.d n.d n.d n.d n.d 0.29 n.d n.d 0.24 0.20 n.d n.d n.d n.d n.dAl2O3 41.87 45.18 28.95 32.76 31.96 33.85 21.48 34.43 28.87 32.45 34.75 30.05 4.74 2.21 1.39 0.81Cr2O3 27.07 24.09 40.20 35.94 36.46 35.1 47.39 33.21 39.17 35.26 32.2 339.38 23.70 26.69 31.47 31.43MnO n.d n.d n.d n.d n.d n.d n.d 0.26 n.d 0.27 0.57 n.d 2.44 1.93 1.94 2.17MgO 16.90 17.83 12.66 14.17 12.97 14.72 12.26 14.61 13.04 14.19 15.47 14.23 5.39 1.96 2.45 1.04ZnO 0.22 n.d n.d 0.17 0.18 n.d n.d 0.22 n.d n.d n.d n.d 0.83 0.74 0.21 0.24FeO 12.41 11.74 17.35 15.23 16.98 14.71 17.06 14.41 16.55 15.16 13.58 14.86 18.58 24.82 25.56 27.87Fe2O3 0.91 0.62 1.13 1.24 1.09 1.11 1.32 2.08 2.05 1.88 2.86 0.94 39.50 39.09 36.05 36.57SiO2 n.d n.d n.d n.d n.d n.d n.d n.d n.d n.d n.d n.d 2.28 n.d n.d n.dNiO n.d n.d n.d n.d n.d n.d n.d n.d n.d n.d n.d n.d 0.96 0.87 n.d n.dTotal 99.38 99.46 100.29 99.51 99.64 99.49 100.01 99.22 99.68 99.45 99.90 99.45 98.42 98.31 100.25 100.27Cr 0.599 0.523 0.952 0.836 0.856 0.810 1.162 0.768 0.931 0.821 0.736 0.926 0.693 0.794 0.927 0.928Ti 0.000 0.000 0.000 0.000 0.000 0.000 0.005 0.000 0.000 0.000 0.005 0.000 0.000 0.000 0.000 0.004V 0.000 0.000 0.000 0.000 0.000 0.000 0.006 0.000 0.000 0.005 0.004 0.000 0.000 0.000 0.000 0.000Al 1.382 1.464 1.022 1.136 1.119 1.165 0.785 1.187 1.023 1.127 1.183 1.053 0.207 0.098 0.061 0.036Fe3+ 0.019 0.013 0.025 0.027 0.024 0.024 0.031 0.046 0.046 0.042 0.062 0.021 1.100 1.107 1.011 1.028Fe2+ 0.290 0.270 0.435 0.375 0.422 0.359 0.442 0.352 0.416 0.373 0.328 0.369 0.575 0.782 0.797 0.871Mn 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.006 0.000 0.007 0.014 0.000 0.076 0.062 0.061 0.069Mg 0.705 0.730 0.565 0.621 0.574 0.641 0.567 0.637 0.584 0.623 0.666 0.631 0.297 0.110 0.136 0.058Zn 0.005 0.000 0.000 0.004 0.004 0.000 0.000 0.005 0.000 0.000 0.000 0.000 0.023 0.021 0.006 0.007Ni 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.029 0.026 0.000 0.000Mg# 0.71 0.73 0.57 0.62 0.58 0.64 0.56 0.64 0.58 0.63 0.67 0.63 0.34 0.12 0.15 0.06Cr# 0.30 0.26 0.48 0.42 0.43 0.41 0.60 0.39 0.48 0.42 0.38 0.47 0.77 0.89 0.94 0.96F 12.04 10.66 15.64 15.09 18.84 14.65 16.58 15.36 14.41 16.40

n.d = not detected; F = degree of partial melting after Hellebrand et al. (2001).

Table 2Compositions of representative minerals from the Bibong ultramafic rocks.

Lherzolite Harzburgite Chromitite

Mineral ol opx cpx amp ol opx cpx amp chl sps sps mt

Location 1st 1st 1st 2nd 1st 2nd 1st 2nd 1st 2nd 2nd * *

Sample bb304 bb314 bb301 bb312 Ji005 bb317 bb192 ji013 bb331 bb322 bb318 Ji032 bb136 160 138 19 148SiO2 40.86 56.18 54.96 45.81 40.84 40.95 57.02 57.14 54.23 53.67 46.77 37.41 40.46 42.68 37.85 n.d. n.d.TiO2 n.d. n.d. n.d. n.d. 0.02 n.d. n.d. n.d. n.d. n.d. 0.45 0.02 n.d. n.d. n.d. n.d. n.d.Al2O3 n.d. 2.53 0.72 11.65 n.d. n.d. 1.85 1.13 1.04 0.84 10.69 0.09 n.d. 0.71 9.11 n.d. n.d.FeO 8.40 5.64 1.44 3.20 8.19 8.88 6.02 5.45 1.68 1.69 2.42 4.98 3.03 2.47 2.69 28.70 25.56Fe2O3 n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. 68.47 39.88Cr2O3 n.d. 0.29 n.d. 1.19 n.d. n.d. 0.43 0.21 0.26 n.d. 1.33 0.04 n.d. n.d. n.d. 0.46 30.4MnO 0.18 0.18 n.d. n.d. 0.12 n.d. n.d. 0.11 n.d. n.d. n.d. 0.02 n.d. n.d. n.d. n.d. 3.16MgO 50.01 34.52 17.92 19.36 50.33 50.13 34.22 35.1 17.96 17.78 19.47 40.81 38.79 40.62 37.59 0.56 1.99CaO n.d. 0.22 24.80 12.62 n.d. n.d. 0.22 0.25 24.64 24.61 12.78 0.06 n.d. n.d. n.d. n.d. n.d.NiO n.d. n.d. n.d. n.d. 0.41 0.42 n.d. 0.04 n.d. n.d. n.d. 0.16 n.d. n.d. n.d. 1.38 n.d.Na2O n.d. n.d. 0.36 2.17 n.d. n.d. n.d. 0.12 n.d. n.d. 1.90 n.d. n.d. 0.52 n.d. n.d. n.d.K2O n.d. n.d. n.d. 1.00 n.d. n.d. n.d. n.d. n.d. n.d. 0.41 n.d. n.d. n.d. n.d. n.d. n.d.Cl n.d. n.d. n.d. 0.20 n.d. n.d. n.d. n.d. n.d. n.d. 0.22 n.d. n.d. n.d. n.d. n.d. n.d.Total 99.45 99.56 100.20 97.20 99.91 100.38 99.76 99.55 99.81 98.59 96.44 83.59 82.28 87.00 87.24 99.57 101.00Si 1.000 1.938 1.981 6.492 0.994 0.996 1.969 1.968 1.968 1.971 6.63 77.466 2.005 1.995 1.765 0.000 0.000Ti 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.003 0.000 0.000 0.000 0.000 0.000Al 0.000 0.103 0.030 1.944 0.000 0.000 0.075 0.046 0.044 0.036 1.786 0.021 0.000 0.039 0.500 0.000 0.000Fe3 0.000 0.013 0.033 0.246 0.000 0.000 0.000 0.021 0.013 0.023 0.158 0.000 0.000 0.000 0.000 1.986 1.111Fe2 0.172 0.150 0.010 0.133 0.167 0.181 0.174 0.136 0.038 0.029 0.128 0.831 0.126 0.097 0.105 0.925 0.791Cr 0.000 0.008 0.000 0.133 0.000 0.000 0.012 0.006 0.007 0.000 0.149 0.006 0.000 0.000 0.000 0.014 0.889Mn 0.004 0.005 0.000 0.000 0.002 0.000 0.000 0.003 0.000 0.000 0.000 0.003 0.000 0.000 0.000 0.000 0.099Mg 1.824 1.775 0.963 4.090 1.827 1.818 1.762 1.802 0.972 0.974 4.119 12.142 2.865 2.831 2.613 0.032 0.110Ni 0.000 0.000 0.000 0.000 0.008 0.008 0.000 0.001 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.043 0.000Ca 0.000 0.008 0.958 1.916 0.000 0.000 0.008 0.009 0.958 0.968 1.943 0.013 0.000 0.000 0.000 0.000 0.000Na 0.000 0.000 0.025 0.597 0.000 0.000 0.000 0.008 0.000 0.000 0.522 0.062 0.000 0.047 0.000 0.000 0.000K 0.000 0.000 0.000 0.181 0.000 0.000 0.000 0.000 0.000 0.000 0.074 0.000 0.000 0.000 0.000 0.000 0.000Cations 3.000 4.000 4.000 15.732 2.998 3.003 4.000 4.000 4.000 4.001 15.516 20.547 4.996 5.009 4.983 3.000 3.000Mg# 0.91 0.92 0.99 0.97 0.92 0.91 0.91 0.93 0.96 0.97 0.97 0.94 0.96 0.97 0.96

n.d. = not detected, ol = olivine, opx = orthopyroxene, cpx = clinopyroxene, amp = amphibole, chl = chlorite, sps = serpentine, mt = magnetite, * located near the Fe-richchromite.

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investigate the compositional changes that occurred during ser-pentinization, metamorphism and/or hydrothermal alteration.

5.1. Igneous petrogenesis

The Cr# of chromian spinel can be used to determine the degree ofpartial melting experienced by chromian spinel-bearing peridotites(e.g. Dick and Bullen, 1984; Michael and Bonatti, 1985; Arai, 1994;Hellebrand et al. 2001, 2002). The Fo content of olivine and the Cr# ofspinel do not change during subsolidus recrystallization (Ozawa,1988; Arai, 1994). The Cr# of the spinel group increases withprogressive degrees of partial melting, which reduce the Al contentsof orthopyroxene and the host rock (Jaques and Green, 1980; Oharaand Ishi, 1998). The chemical compositions of chromian spinels ofprimary origin are plotted in various discrimination diagrams basedon above cited concepts. The primary spinel data for lherzolite,harzburgite and chromitite plot within the mantle array on an Al2O3

versus Cr2O3 diagram (Fig. 3A). In the Mg# versus Cr# diagram(Fig. 3B) the primary Cr-spinels from lherzolite plot in the lower endof abyssal peridotite field indicative of its undepleted nature (e.g. Arai,1994). Most primary Cr-spinel data for harzburgite and chromititesplot in an overlap area between abyssal and arc peridotite fields andsome Cr-spinel data for harzburgite plot in the arc area indicating thatthe harzburgite formed in an arc tectonic setting instead of an abyssaltectonic setting. On an Al2O3 versus Fe2+/Fe3+ diagram (Fig. 3C) theCr-spinels in lherzolite plot within the MORB-peridotite field whereasthose in harzburgite and chromitite plot mainly in the supra-subduction zone (SSZ) peridotite field.

The olivine spinel mantle array (OSMA) diagram (Fig. 5) proposedby Arai (1994) based on Fo of olivine and Cr# of Cr-spinel isextensively employed to characterize mantle-derived, spinel-bearingultramafic rocks, especially mantle restites on the basis of the meltingtrend for a fertile MORB mantle (FMM: annotated by percentage ofmelting). Pearce et al. (2000) incorporated the SSZ peridotite field andpassive continental margin field in the OSMA diagram. In the OSMAdiagram (Fig. 5), data for lherzolites plot in the lower end of theabyssal peridotite field further indicating the fertile nature of thelherzolites. On the other hand, data for harzburgite mostly plot in thearea between the higher end of the abyssal peridotite and the lowerend of SSZ peridotite field, but some of them also plot in the SSZperidotite field. The samples plot very near to the melting trend offertile mantle composition (FMM) (Fig. 5). This observation furthersuggests the primary nature of the spinel and olivine compositionsand reveals the various degrees of partial melting suffered by the rock.From this diagram, it is also evident that the lherzolite relictunderwent ca. 10–12% partial melting and harzburgite underwentca. 15–30% partial melting (Fig. 5). We also calculated the degrees ofpartial melting based on an equation proposed by Hellebrand et al.(2001) for spinel Cr# values between 0.10 and 0.60. The calculatedvalues (Table 1) are more or less the same as deduced from the OSMAdiagram. These data indicate that Bibong ultramafic rocks are mantlerestite which experienced ca. 10–12% partial melting and thenexperienced second stage partial melting. The low Fe3+ contents ofthe Cr-spinels indicate relatively low oxygen fugacity conditions oftheir primary source.

The TiO2 content of spinel in the magma varies depending on thetectonic site of magma separation: TiO2 is extremely low in arc/back-arc magma, intermediate inmaturemid-oceanic ridgemagma and highin intraplatemagma (Arai, 1992; Beccaluva et al., 1983). Jan andWindly(1990) suggested that Alaskan-type complexes and mafic–ultramaficrocks from deeper levels of island arcs generally contain N0.3 wt.% TiO2.The low TiO2 content, less than 0.3 wt.% in spinels from the Bibongultramafic rocks, indicates that the Bibong ultramafic rocks formed inan arc tectonic setting. The Al2O3 contents of the melts in equilibriumwith the chromian spinelswere calculated using the equation ofMaureland Maurel (1982). The calculated values show the high Al nature of

Fig. 3. Plot of chromian spinels on the discrimination diagrams; A) Al2O3 versus Cr2O3 (wt.%)diagram (after Franz and Wirth, 2000), B) Mg/(Mg+Fe2+) versus Cr/(Cr+Al) diagram(after Arai, 1994; Barnes and Roeder, 2001). The range of Cr# in chromian-spinels in back-arcperidotites is after Monnier et al. (1995) and C) Al2O3 versus Fe2+/Fe3+ diagram (afterKamentsky et al., 2001). SSZ, Supra-subduction zone.

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the parental melts. The data together with Al2O3 contents in chromianspinel lie very close to the evolution trend of MORB in the diagramAl2O3 in melts versus Al2O3 in spinel (Fig. 6; after Kamenetsky et al.,2001 and Rollinson, 2008) and proceed towards the arc setting withincreasing degree of partial melting. The back-arc spinel compositionsspread across island-arc and MORB fields indicating the presence ofvarying melting conditions with involvement of subduction-relatedand MORB mantle and magma components in a back-arc tectonicsetting (Kamenetsky et al., 2001; Rollinson, 2008). MORB-likeperidotite is reported to be formed in nascent spreading centers inthe back-arc or fore-arc basin (e.g., Mirdita ophiolite; Dilek and Polat,2008). We therefore, consider that the Bibong ultramafic rocks initiallywere formed in the nascent back-arc basin spreading center related tosupra-subduction.

The compositions of primary Cr-spinels in lherzolite, harzburgiteand chromitite suggest that 1) all these ultramafic units in Bibong

were indeed part of mantle sections, 2) the higher Cr# in Cr-spinels inharzburgite and chromitites than lherzolite indicates that Cr-rich Cr-spinels formed under higher degree partial melting than Al-rich Cr-spinel, 3) the compositions of Cr-spinels in the Bibong ultramaficrocks reveal that the rocks experienced two partial melting events(e.g., Choi et al., 2008; Python et al., 2008), and 4) the two partialmelting events may have occurred during the transition from nascentto mature stages of back-arc basin in the supra-subduction zone.

5.2. The last stage metamorphic effects on Cr-spinel

Two compositional domains in the Fe-rich rim areas around Al-rich chromian spinels in chromitites (Fig. 4) have been identified:Type I has a very restricted composition with Mg# less than 15 andCr# higher than 85 and Type II has a wide compositional range withMg# (20–80) and Cr# between 75 and 85. The BSE image shows thatthe Type II Cr-rich area in chromian spinels is the transitionalcomposition between the relict Al-rich core and Type I Fe-rich rim.Farahat (2008) mentioned that Mg–Fe2+ and Al(Cr)–Fe3+ exchangesbetween chromian spinel and the surrounding silicate minerals occurduring amphibolite facies regional metamorphism. The composi-tional difference between Type I Fe-rich rim and Al-rich core indicatesthat Fe-rich chromite may be formed by Mg-Fe+2 and Al(Cr)-Fe+3 àMg-Fe2+ and Al(Cr)-Fe3+ exchanges between chromian spinels andthe surrounding silicate minerals during amphibolite facies meta-morphism. This suggestion is well supported by the following facts:1) serpentines around the chromian spinels have higher Al contentthan serpentines in thematrix, 2)magnetite overgrowths around thechromian spinel rim have higher Cr content than chromian spinels inthe matrix, and 3) Type I Fe-rich rim compositions are similar to thecompositions of Fe-rich chromites formed by later stage metamor-phism in Iti and Kallidromon ophiolites and El Ideid–El Sodmeinserpentinite (Karipi et al., 2007; Farahat, 2008). The texture of theType I Fe-rich rim shown by BSE image is similar to the typical meta-morphic texture identified from the El Ideid–El Sodmein serpentinite(Farahat, 2008). The Mg/(Mg+Fe2+) values in the metamorphicspinel-group minerals are dominantly controlled by the metamorphicgrade (Barnes, 2000). Spinels metamorphosed under greenschist faciesdisplay Mg/(Mg+Fe2+) values ranging from 0.4 to 0.7 while thosemetamorphosed under amphibolite facies have Mg/(Mg+Fe2+)values lower than 0.35 (Barnes, 2000). Hence, the lowMg/(Mg+Fe2+)values, less than 0.1 in the Fe-rich chromian spinel rims, presumablysuggest that the Bibong ultramafic rock were metamorphosed under ametamorphic grade higher than amphibolites facies. The relativelyhigher Fe3+ contents of the Fe-rich Cr-spinel (Fe3+/Fe2+=0.66–0.69)compared to Al-rich Cr-spinel (Fe3+/Fe2+=0.36–0.04) indicate thatthe last stage metamorphism occurred under higher oxidation con-ditions than the igneous stages. The first stage serpentines arecharacterized by low Al contents (Table 2). As a result, the meta-morphic Fe-rich spinel rims, which are highly depleted in Al2O3,could not have formed during the first stage serpentinization process.On the other hand, the second stage serpentines around the Fe-richspinels contain higher Al and Mg/Fe ratio than olivines or pyroxenesand Cr-magnetite overgrowths around the spinel rim contain higherCr than other magnetites in thematrix (Table 2). Accordingly, the Al-rich serpentine and Cr-magnetite could not have formed byisochemical metamorphism of olivine, pyroxene or even serpentineimplying that Al-rich chromian spinel is the only mineral that couldhave supplied the Al, Mg and Cr necessary for their formation. It islikely that metamorphic Fe-rich spinel rims formed during the laststage regional metamorphism after the first stage serpentinization.During the process, olivines broke down to serpentine and Cr-magnetite overgrowth and supplied Fe into spinel by diffusion whileMg and Al in spinel diffused into Al-rich serpentine and Cr in spinelinto Cr-magnetite resulting Fe-rich chromian spinel.

Fig. 5. Characterization of the Bibong ultramafic rocks in the olivine–spinel mantlearray (OSMA: olivine [Fo or Mg#] versus spinel [Cr#]) diagram follow Arai (1994) andmodified by Pearce et al. (2000). FMM represents the fertile MORBmantle. The range ofCr# in chromian spinels in back-arc peridotites is after Monnier et al. (1995).

Fig. 6. The compositional relationship of spinel–melt expressed in terms of Al2O3

contents. The arc and MORB evolution trends are adapted from Kamenetsky et al.(2001) and Rollinson (2008).

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The Bibong metabasite located near the Bibong ultramafic rocksintruded during the Neoproterozoic and underwent eclogite faciesmetamorphism during Triassic collision between the North and SouthChina blocks (Oh et al., 2005; Kim et al., 2006). After eclogite faciesmetamorphism, the Bibong metabasite experienced retrogrademetamorphism first by granulite facies metamorphism (830–850 °C,12–15 kb) and then by amphibolites facies metamorphism (570–740 °C, 7–11 kb). Considering the metamorphic evolution of theBibong metabasite, the medium to fine grained olivine, orthopyrox-ene, clinopyroxene and amphibole around the orthopyroxeneporphyroblasts in the Bibong ultramafic rocks can be considered tobe formed by the granulite facies metamorphism (Seo et al., 2005).After granulite facies metamorphism, the Bibong ultramafic rockssuffered first stage serpentinization defined by serpentine with low Alcontent. Finally, the Bibong ultramafic rocks underwent amphibolitesfacies metamorphism indicated by the formation of Fe-rich rimsaround the Al-rich core in chromian spinels, serpentine with high Al-content and Cr-magnetite.

5.3. Tectonic implications

The Bibong ultramafic rocks are closely associated with metaba-sites. Oh et al. (2009) proposed that the Bibong metabasites havegeochemical characteristics similar to MORB-like back-arc basinbasalts. They interpreted the Bibong metabasites as having formedduring back-arc opening at 890–860 Ma from upwelling astheno-spheric mantle which was metasomatized by LILE enriched melt orfluid derived from the subducted slab and/or subducted sedimentbeneath the arc axis (Oh et al., 2009). The back-arc basin tectonicsettings deduced from primary chromian spinel and olivine composi-tions in the Bibong ultramafic rocks of the present study are in goodagreement with the proposed model of Oh et al. (2009).

In the area of spreading centers of nascent back-arc basins, thesubduction zone is weakly developed and fewer volatiles are added tothe wedge melts. Under this condition, lower percentage partial meltscan be generated in themantle wedge above the subduction zone. Thelower degrees of partial melting will produce MORB-like magmasfrom which high Al chromian spinels crystallize (Zhou and Robinson,1994). Melts generated by partial melting will rise into preexistinglithospheric mantle which is thinned during spreading. As the meltspass through the lithospheric mantle they will react with the wallrocks and precipitate high Al chromian spinels. However as thesemelts are not so far out of equilibrium with their host rocks, lessreaction will take place compared to the island-arc area where meltsand host rock are far out of equilibrium. The residual peridotitesformed in this environment would be moderately depleted or fertilelherzolites and clinopyroxene-bearing harzburgites, similar to thoseof the Yakuno-type opholites which are characterized by high Altholeiite (Ishiwatari, 1985). Such bodies are similar to mid-oceanridge basalt ophiolites as defined by Pearce et al. (1984).

On the other hand, beneath a mature back-arc basin, thesubduction system is well developed and there is high input ofvolatiles from the subducted slab into the overlying mantle wedgeleading to higher degrees of partial melting. In these environmentsthe preexisting lithospheric mantle immediately beneath the crust isinvaded by large volumes of melts generated at depth by melting ofdepleted harzburgites. As the intruded melts are no longer inequilibrium with the invaded preexisting mantle the melts wouldreact with the mantle producing Al-poor chromitite and dunite pods.

The continuous transition from nascent to mature back-arc basinenvironments is well documented from the Semail ophiolite in Oman,one of the old supra-subduction zones (SSZ). The Semail ophioliteformed by back-arc rifting that occurred due to arc-trench rollbackwhich was faster than the convergence rates (Dilek and Polat, 2008).During the beginning stage of the back-arc basin, MORB-like tholeiitesformed with lower partial melting of lithospheric mantle due to a

minor supply of fluid from the weakly developed subduction zone.The beginning stage was followed by extensive subduction under theback-arc basin which supplied large amounts of fluid and melt to theupper lithosphere causing higher degrees of partial melting. As aresult, arc-type tholeiites formed (Dilek and Polat, 2008).

Considering the studies by Zhou and Robinson (1997) and Dilekand Polat (2008) and the mineral chemistries of spinel and olivine inthe Bibong ultramafic rock, the Bibong ultramafic rocks can beinterpreted to have formed in the supra-subduction zone where back-arc basin rifting occurred by arc-trench rollback. The lherzolite relictwith Al-rich chromian spinel indicates that the Bibong ultramafic rockwas originally mantle part of a SSZ ophiolite formed in the spreadingcenters in nascent back-arc basin. The lherzolite relicts were formedby the interaction of preexisting lithospheric mantle with intrudedmagma formed by lower partial melting of the mantle wedge abovethe subduction zone at deeper depth. The primary Al-rich chromianspinel and Fo-rich olivine in the Bibong lherzolite is comparable tothose in spinel-bearing peridotites reported from the Mariana troughand East Sulawesi ophiolite (Figs. 3B and 5) which are considered tobe typical of back-arc basins (e.g., Ohara et al., 2002; Monnier et al.,1995; Michibayashi et al., 2009).

After the first stage igneous event, a transition from nascent back-arc basin stage to mature back-arc basin stage might have occurred.During themature back-arc basin stage, more hydrousmagma formedby high degrees of partial melting of depletedwedgemantle and thesemelts intruded and reacted with lherzolitic mantle. During theinteraction, all pyroxene in the host rock would be dissolved toproduce a dunite envelope (e.g. Quick, 1981; Kelemen et al., 1992).Farther outward, only clinopyroxene would be dissolved producing azone of harzburgite, which in turn, would grade into the hostlherzolite. Such gradations from dunite to harzburgite to lherzolite areclearly visible around chromitite pods in a number of ophiolites andappear to be the result of melt–rock reaction (Zhou et al., 1996, 1994;Arai and Yurimoto, 1994). In the Bibong ultramafic rocks, lherzoliteonly occurs as a relict in harzburgite. Melting of pyroxenes from thehost rocks would add silica and Cr to the magma (Fisk, 1986; Arai andYurimoto, 1995), driving the melt into the field of chromitecrystallization. Incompatible components (Na, Ti and H2O) are alsoconcentrated through the process of partial zone melting (e.g.,Kushiro, 1968) and are used to form hydrous minerals such asamphibole. The enriched melt with Cr and Si was mixed with asubsequently supplied relatively primitivemelt within amain conduitprecipitating chromian spinels due to the supersaturation of Crcomponents in the mixed melt (Irvine, 1975; Zhou et al., 1994; Araiand Yurimoto, 1994). Finally the melt/rock interaction led to theformation of the high Al chromian spinels and depleted the hostperidotites in modal pyroxene but enriched the peridotites inincompatible elements. The Bibong ultramafic rocks consist mainlyof harzburgite with relict lherzolite. Considering the melt–rockinteraction model (Arai and Yurimoto, 1994; Zhou et al., 1994), thechromitite in the Bibong ultramafic rock is a kind of cumulate from a Siand Cr enriched mixed melt that resulted from the interactionbetween lherzolite and primary melt which passed through thelherzolite. The harzburgite can be regarded as a product formed by acoupled process of olivine precipitation and dissolution of pyroxeneduring melt–rock interaction. No chromitites would be formed inmid-oceanic ridges (Zhou and Robinson, 1997). Therefore, theoccurrence of chromitite bands in the ultramafic rocks of the Bibongarea can be correlated with a back-arc basin in a supra-subductionzone instead of a mid-oceanic ridge.

Currently, we have no age data for the serpentinized Bibongultramafic rock. However, the Bibong metabasites in the study areayielded inherited core ages of 887±14, which is considered as theprotolith formation age (Guo et al., 2004; Kim et al., 2006). Therefore,one possible age for the Bibong ultramafic rock is middle Neoproter-ozoic which is the time of formation of the Rodinia supercontinent.

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Guo et al. (2004) and Kim et al. (2006) also reported a 230 Mametamorphic age for the metabasite which had undergone eclogitefacies metamorphism first and then granulite and amphibolite faciesretrograde metamorphism during uplift. The 230 Ma eclogite faciesmetamorphism occurred during collision of the North and SouthChina blocks. The first serpentinization in the Bibong ultramafic rocksmight have occurred between the granulite and amphibolites faciesmetamorphism during uplift as discussed in the above section. Ohet al. (2009) reported a transition from arc to rift tectonic settingbetween 887 Ma and 760 Ma in the study area. The transition isinterpreted to match the amalgamation and subsequent breakup ofthe Rodinia supercontinent. One possible tectonic history is asfollows: 1) the Bibong ultramafic rocks formed in the SSZ andoverthrusted onto continental margin respectively before and duringthe amalgamation of the Rodinia supercontinent, 2) the continentalcrust including the Bibong ultramafic rocks became a passive marginduring breakup and rifting of the Rodinia supercontinent, and 3) thecontinental crust including the Bibong ultramafic rocks subductedunder the North China block and then were uplifted during or afterTriassic collision between the North and South China blocks. Furtherstudies are needed to confirm the suggested tectonic model.

6. Conclusion

The Bibong ultramafic rock was a mantle part of the SSZ ophioliteand its evolution involves the transition from the nascent to matureback-arc basin tectonic setting. During the first stage, magma formedby small degree partial melting of wedge mantle in the spreadingcenters in the nascent back-arc basins and intruded the preexistinglithospheric mantle causing the melt/rock interaction which resultedlherzolite with Al-rich Cr-spinel. During the second stage, moreevolved magma formed by high degrees of partial melting in themature back-arc basin tectonic setting and these magmas intrudedthe lherzolitic mantle and caused the second stage melt/rockinteraction that formed harzburgite through the coupled process ofolivine precipitation and dissolution of pyroxene. The transition fromthe nascent to mature back-arc basin tectonic setting may have beencaused by trench roll back. During the second stage interaction, theprimary melt was changed into a Cr and Si enriched melt. Finallychromitites formed from the melt as a kind of cumulate. After theigneous stage, the Bibong peridotite underwent metamorphism.During the final metamorphism, Al-rich chromite altered into Fe-rich chromite due to the diffusional exchange of Al, Mg, Cr, and Febetween chromite and the surrounding silicate minerals.

Acknowledgments

This research was supported in part by the Post-Doctoral Programof Chonbuk National University (2004 and 2005), the NationalResearch Foundation of Korea (RO1-2007-000-20051-0, KRF-2008-313), the Human Resource Development Program of the KoreaInstitute of Energy Technology Evaluation and Planning (KETEP;2009EAPHMP1800002010) and CNSF (90714003). We thank Prof.Shoji Arai (Kanazawa University, Japan) for critical suggestions. Wealso thank two anonymous reviewers and journal editor Prof. NelsonEby for formal reviews and editorial efforts.

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