Metamorphic evolution of the northwestern Ogcheon metamorphic belt, South Korea

Ž .Lithos 43 1998 31–51

Metamorphic evolution of the northwestern Ogcheonmetamorphic belt, South Korea

Kyoungwon Min 1, Moonsup Cho )

Department of Geological Sciences, Seoul National UniÕersity, Seoul 151-742, South Korea

Received 31 July 1997; revised 27 January 1998; accepted 27 January 1998

Abstract

Ž .The Ogcheon metamorphic belt OMB comprises Late Proterozoic to Paleozoic metasedimentary and metavolcanicsequences which are intruded by Mesozoic granitoid plutons. To delineate the metamorphic evolution of the OMB, we haveinvestigated mineral parageneses, metamorphic reactions and P–T conditions of pelitic and mafic schists in the Chungjuarea, northwestern part of the OMB. The regional metamorphic grade increases north-westward from biotite to garnet zones,although low-pressure assemblages have developed in contact aureoles around Jurassic granites. Garnet crystals showchemical zoning, typical for prograde metamorphism, with decreasing Mn, and increasing Fe and Mg from core to rim.Compositions of amphiboles constituting the common assemblage in the mafic schist are consistent with those of biotite- andgarnet-zones documented in the medium-pressure metabasites. P–T conditions of the garnet zone, estimated fromgarnet–biotite, garnet–chlorite and amphibole–plagioclase geothermometers together with garnet–plagioclase–biotite–muscoviterquartz geobarometers, are in the range of 5–8 kbar and 520–5908C. Retrograde P–T path based on fluidinclusion studies suggests that the exhumation of the OMB has passed through the P–T range of 1–3 kbar and 350–5008C,following the isochore curves of the CO inclusions. In conjunction with structural and geochronologic data, we conclude2

Ž .that the OMB has experienced a polycyclic P–T evolution characterized by 1 crustal thickening during the MiddleŽ .Paleozoic time and 2 regional retrograde metamorphism in the Triassic. Our result further suggests that the Triassic

collision belt in east-central China does not pass through the OMB. q 1998 Elsevier Science B.V.

Keywords: Ogcheon belt; Regional metamorphism; Geothermobarometry; Fluid inclusions; P–T path

1. Introduction

The Korean Peninsula, situated between the Asianmainland and the Pacific active margin, consists ofseveral metamorphic terrains: from north to south,these are Nangrim massif, Imjingang belt, Gyeonggi

) Corresponding author. Tel.: q82-2-880-6732; fax: q82-2-871-3269; e-mail: [email protected].

1 Present address: Department of Geology and Geophysics,University of California, Berkeley, CA 94720, USA.

Žmassif, Ogcheon belt, and Yeongnam massif Fig..1a . The three massifs, primarily composed of Pre-

cambrian basement gneisses and schists, are sepa-rated by two Phanerozoic mobile belts. The tectonichistory of these orogenic belts has been the focus ofrecent investigations in light of the discovery ofcoesite and microdiamond from the Qinling–

ŽDabie–Sulu belt in east-central China Yin and Nie,1993, 1996; Ernst and Liou, 1995; Kim, 1996; Otoh

.and Yanai, 1996; Ree et al., 1996 . In particular, ReeŽ .et al. 1996 suggested on the basis of structural,

0024-4937r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved.Ž .PII S0024-4937 98 00006-1

( )K. Min, M. ChorLithos 43 1998 31–5132

Ž .Fig. 1. a General sketch map of the Korean Peninsula and China, showing major tectonic provinces and location of the study area. NM,Ž .Nangrim massif; GM, Gyeonggi massif; YM, Yeongnam massif. b Geologic map of the study area, modified from a compilation of

Ž .1:50,000 geologic maps Kim and Lee, 1965; Lee and Park, 1965; Kim et al., 1967; Park and Yeo, 1971 . Stratigraphy of the OgcheonŽ .Group follows Lee et al. 1980 .

petrological, and geochronological analyses that theultrahigh-pressure collisional belt in China extendsinto the Korean Peninsula as the Imjingang belt.Nevertheless, the Ogcheon belt or the Gyeonggimassif is often regarded as a candidate for the suturezone extending from the Qinling–Dabie–Sulu beltŽLiu, 1993; Yin and Nie, 1993, 1996; Ernst and

.Liou, 1995 . It is thus apparent that tectono-meta-morphic evolution of the Ogcheon belt remains asone of the key issues for deciphering east Asiantectonics.

The NE-trending Ogcheon belt is a fold-and-thrustbelt, consisting of the Ogcheon metamorphic beltŽ .OMB in the southwest and the Taebaeksan zone in

Ž .the northeast Fig. 1a . The OMB, truncated by adextral strike-slip shear zone to the south, consistsprimarily of metasedimentary rocks together withsubordinate amounts of bimodal metavolcanic rocks.Stratigraphic relationships among these volcano-sedimentary sequences are equivocal mainly becauseof the absence of fossils and the obliteration ofprimary structures. Further ambiguities in delineating

( )K. Min, M. ChorLithos 43 1998 31–51 33

tectonic evolution of the OMB stem from the lack ofgeochronologic data available. In spite of these un-certainties, some previous workers have attempted tocorrelate the OMB with other Phanerozoic mobilebelts in eastern Asia, including the Hida metamor-phic belt in Japan and the Qinling–Dabie–Sulu belt

Žin China Hiroi, 1981; Yin and Nie, 1993, 1996;.Ernst and Liou, 1995 . Various tectonic models have

been also proposed for the evolution of the OMB byŽmany workers e.g., Reedman and Um, 1975;

Chough, 1981; Lee, 1987; Cluzel et al., 1990, 1991;.Kim, 1996 . Polyphase tectonism in the OMB was

Ž .first proposed by Reedman et al. 1973 who recog-nized three separate generations of penetrative minorstructures and attributed them to the Jurassic Daebo

Ž .orogeny. Cluzel et al. 1990, 1991 , however, sug-gested that the major tectonic event responsible forthe NE-trending structures in the OMB occurredduring the Ogcheon orogeny in early middle Paleo-zoic time. They further envisaged that the OMBrepresents a stack of synmetamorphic nappes pro-duced by intracontinental extension and subsequentclosure processes of an aborted rift. On the other

Ž .hand, Kim 1996 inferred that the Triassic Songrimorogeny is responsible for the NE-trending regionalstructure in the OMB.

Regional metamorphism of the intermediate P–TŽ .type Miyashiro, 1961 in the OMB has long been

Ž .known since Kim 1971 documented a progradezonal sequence of chlorite, biotite, garnet and kyan-

Ž .ite zones. Reedman and Um 1975 further noted thatthe metamorphic zones trend parallel to the OMBand the metamorphic grade increases towards itsnorthwestern margin. Quantitative determinations ofpeak metamorphic conditions in the OMB became

Žavailable only recently Cho et al., 1994; Kim et al.,.1995; Oh et al., 1995 . For example, in the central

Ž .OMB, Kim et al. 1995 reported a progressiveincrease in P–T conditions from 4.2–8.2 kbar and490–5408C in the southeast to 5.4–9.4 kbar and520–6308C in the northwest. However, the P–T–tevolutionary path of the OMB remains uncertain.

In this study, we have determined the P–T his-tory of the northwestern OMB using phase relations,geothermobarometry and fluid inclusion data. Peliticschists were studied in detail because they containmineral assemblages appropriate for determining themetamorphic P–T conditions. In addition, P–T

conditions of mafic schists were estimated and com-pared with those of metapelites. Finally, by incorpo-rating available geochronologic data, we propose asimplified P–T–t path of the OMB and discuss itstectonic implications.

2. Geological setting

The Ogcheon belt has been divided into the north-eastern Taebaeksan zone and the southwesternOgcheon zone or Ogcheon metamorphic belt, based

Žon lithology and metamorphic grade Fig. 1a:.Kobayashi, 1953; Lee, 1987 . The Taebaeksan zone

consists of unmetamorphosed to weakly metamor-phosed, fossiliferous sedimentary strata of EarlyCambrian to Jurassic age, whereas the Ogcheon zonecomprises non-fossiliferous metasedimentary andmetavolcanic rocks ranging in their protolith ages

Žfrom either Late Proterozoic Kim, 1970; Reedmanand Um, 1975; Kwon and Lan, 1991; Min et al.,

. Ž1995 or Early Paleozoic Lee, 1974; Son, 1986;.Cluzel et al., 1990 to Late Paleozoic. A recent

U–Pb zircon age of 756"1 Ma obtained from aŽ .metamorphosed felsic tuff Lee et al., in press ,

however, supports the former interpretation.Metamorphic rocks of the OMB generally strike

NE–SW and dip to the northwest. Their regionalstructures are primarily dominated by large-scale,

ŽSE-vergent thrust faults Cluzel et al., 1990; Koh and. Ž .Kim, 1995 . In particular, Cluzel et al. 1990 di-

vided the OMB into five thrust-bounded units: theŽstructurally upper units Chungju and Pibanryeong

.units represent the deep axial domain containingdistal flyschoids and large olistoliths, whereas the

Ž .lower units Turungsan, Poeun and Iwharyeong unitsrepresent the transitional zone between the deep riftdomain and the shallow platform. Among the fourmajor deformation episodes described by Cluzel et

Ž .al. 1990 , the D to D phases are synmetamorphic1 3

and have produced regional structures. D and D1 2

structures have been progressively formed in a sub-horizontal shear regime related to the south-eastward

Ž .stacking of nappes. Cluzel et al. 1991 further sug-gested that the medium-pressure parageneses of theOMB are related to the D –D tectonism.1 2

Metavolcanic rocks of mafic and felsic composi-tions are widespread particularly in the lower and


upper units, respectively, of the OMB. Their occur-rences are attributed to bimodal volcanic activityaccompanying the formation of an intra-continental

Ž .rift Cluzel, 1992; Kwon and Lee, 1992 .The study area, located in the northwestern part of

Ž .the OMB Fig. 1 , comprises the Chungju, Tu-Ž .rungsan and Poeun units of Cluzel et al. 1990 . The

Chungju unit comprises Daehyangsan Quartzite,Hyangsanri Dolomite and Kyemyungsan Formations,in ascending order of present structural position,whereas the Turungsan and Poeun units consist ofMunjuri, Seochangri and Hwanggangri Formations.The overall lithology consists of interlayered peliticto psammitic schist, mafic to felsic schist, quartzite,marble and pebble-bearing phyllite. All of thesesupracrustal rocks were intruded by the Jurassic bi-

Ž .otite granite Fig. 1b .

3. Petrography and metamorphic zones

More than 400 samples were collected in thestudy area, and about 500 thin sections were exam-ined in order to determine mineral parageneses andmetamorphic zones. Three metamorphic zones, thebiotite, garnet and Al-silicate zones, were defined inthe study area on the basis of the first appearance of

Ž .garnet and andalusite in metapelites Fig. 2 . Theformer two zones are related to regional dynamother-mal metamorphism, whereas the Al-silicate zone re-sults from thermal metamorphism occurring at highercrustal level at much later time, i.e., during theJurassic. The Al-silicate isograd is nearly parallel tothe granite boundary and overprints the regionalmetamorphic assemblages. Towards the granite con-tact in the Al-silicate zone, andalusite coexists withfibrolitic sillimanite. Kyanite was previously re-ported near the Jurassic granite by Reedman et al.Ž .1973 , but was not found in this study. However,kyanite porphyroblasts rarely occur in the Piban-ryeong unit which forms the southwestern extension

Ž .of the study area Kim et al., 1995 .

3.1. Metapelite

3.1.1. Biotite zoneDiagnostic mineral assemblages of the biotite zone

occur in pebble-bearing phyllite and phyllitic schist

of pelitic and calcareous compositions from theHwanggangri Formation. Pebbles are primarily com-posed of limestone, granite, quartzite and slate, andare strongly deformed to produce sub-rounded toelongated shapes. Biotite porphyroblasts, replacingprecursor minerals such as K-feldspar, occur both inthe matrix and pebbles. In the calcareous matrix,lepidoblastic biotite coexists with calciteqdolomiteqquartz q muscovite q plagioclase " K-feldspar "

actinolite " clinozoisite " chlorite. Pelitic assem-blages consist of chloriteqbiotiteqmuscoviteqquartz q opaque minerals " plagioclase " epidote.Opaque minerals include ilmenite, hematite, pyriteand pyrrhotite. Most of biotite crystals are oriented

Ž .parallel to the regional foliation S , whereas some2

biotites appear as pre-S porphyroblasts.2

3.1.2. Garnet zonePsammitic and felsic schists predominate in the

garnet zone, and commonly consist of high-varianceassemblages. In the northeastern part of the studyarea, however, pelitic to semi-pelitic schists of theMunjuri Formation contain two distinct types of

Žgarnet-bearing assemblages qquartzqmuscovite.q ilmeniteqmagnetite : garnetqchloriteqepidote

in the chlorite schist; and garnet q biotite "

plagioclase"epidote"chlorite in the biotite schistŽ .Table 1 . In contrast to the chlorite schist, chloriteof the biotite schist occurs as retrograde phase. Whenthe mineral compositions of chlorite and biotite

Ž .schists are plotted on the AFM diagram Fig. 3 , tielines do not cross, attesting to the attainment ofchemical equilibrium. Thus two contrasting assem-blages result from the variation in major elementchemistry of protoliths, as confirmed by our bulkrock analyses of representative schists.

Garnet in the chlorite schist occurs as euhedral tosubhedral porphyroblasts, and is commonly alteredto chlorite. The majority of garnet grains are synk-inematically grown and contain sigmoidal inclusiontrails consisting of chlorite, muscovite, quartz, epi-

Ž .dote, magnetite and ilmenite Fig. 4 . The outer rimsof the garnet porphyroblasts are commonly free ofinclusions and represent the post D growth. Pres-2

sure shadows of quartz and chlorite are commonlydeveloped around garnet porphyroblasts. Chlorite oc-curs as pale green, euhedral to subhedral plates andits mode reaches up to 40%. The main schistosity is


Ž . Ž . Ž .Fig. 2. Mineral assemblage map showing the locality of biotite squares , garnet circles and Al-silicates triangles . Stars denote samplelocations of the analyzed mafic schists. Two isograds are defined by the first appearances of garnet and Al-silicate, respectively, in pelitic

Žrocks. Average pressures estimated from garnet–plagioclase–biotite–muscoviterquartz geobarometers Hodges and Crowley, 1985; Hoisch,. Ž Ž . Ž ..1990 , and temperatures from garnet–biotite Ferry and Spear 1978 with the activity model of Berman 1990 , garnet–chlorite

Ž . Ž .Grambling, 1990 and hornblende–plagioclase Holland and Blundy, 1994 geothermometers for each sample of pelitic and mafic schistsare also shown.


Table 1Ž .Mineral assemblages and visually-estimated modes % of metapelite and amphibole schist

aSample no. Qtz Bt Mus Chl Grt Opq Epi Pl Kfs And Sil Crd Amp Others

METAPELITEGarnet zoneChlorite schist

bM761 30 35 25 2 4 trM763 20 50 25 2 2 tr TurM764 20 30 40 3 6 trBiotite schistM716 20 25 1 tr 4 8 2 40 ApM768 35 30 5 3 1 3 -1 20M956 60 30 -1 2 3 2M756 55 30 7 tr 2 3 2M749 35 30 20 8 3M417 40 20 10 4 3 2 3 15M934 30 30 25 13 tr trM958 70 10 5 10 -1 3M755 13 15 70 trM302 25 20 40 5 4 tr

Al-silicate zoneM919 20 10 35 3 2 tr 25 3 TurM927 20 25 15 5 2 2 30M733 30 15 10 2 10 1 10 20 5 -1 Tur

AMPHIBOLE SCHISTM416 10 3 25 10 3 25 25 Cc, TtnM819 8 1 3 15 5 8 60 Cc, TtnM853 25 tr 30 15 1 8 20 Cc, TtnM120 25 12 1 8 5 3 55 Cc, TtnM711 15 tr 7 1 1 70 TtnM951 10 tr 1 7 1 5 75 Cc, Ttn

a Ž .Opaque minerals Opq consist of magnetite, ilmenite and trace hematite.b Minerals present in trace amount are indicated as tr.

Ž .Mineral abbreviations mostly from Kretz, 1983 : Qtz, quartz; Bt, biotite; Mus, muscovite; Chl, chlorite; Grt, garnet; Epi, epidote; Pl,plagioclase; Kfs, K-feldspar; And, andalusite; Sil, sillimanite; Crd, cordierite; Amp, amphibole; Tur, tourmaline; Ap, apatite; Cc, calcite;Ttn, titanite.

defined by the preferred orientation of primary chlo-rite of uniform grain size together with muscoviteand flattened quartz in the matrix.

In the biotite schist, garnet porphyroblasts up to 7mm in the maximum dimension commonly occur.These porphyroblasts show no textural or chemicaldiscontinuity, except for those of metapelites occur-ring in the vicinity of the Al-silicate isograd. Someporphyroblasts contain inclusion trails, consisting ofquartz, epidote, ilmenite and magnetite, that are of-ten oblique to the external foliation. Epidote com-monly occurs as inclusions in garnet porphyroblastsbut is rare in the matrix. On the other hand, biotite

and plagioclase are present as abundant matrixphases. It is thus likely that garnet was produced bythe reaction: chloriteqmuscoviteqepidoteqquartz

ŽsgarnetqbiotiteqplagioclaseqH O Selverstone2.and Spear, 1985; Menard and Spear, 1993 .

3.1.3. Al-silicate zonePelitic schists in the Al-silicate zone consist of

andalusite, fibrolitic sillimanite, cordierite, biotite,garnet, plagioclase, chlorite, muscovite, quartzandror K-feldspar. Andalusite is modally abundantŽ .up to 40% , whereas minor sillimanite commonlyoccurs at the margins of andalusite or biotite. Por-


Fig. 3. An AFM diagram showing the phase relation amongcoexisting minerals in each sample of biotite and chlorite schists,

Ž .and Al-silicate zone dashed line . Note the crossing tie-linerelationship between the chlorite schist and the Al-silicate zonesamples.

phyroblasts of andalusite and cordierite are randomlyoriented and crosscut the regional foliation. In thevicinity of the Al-silicate isograd, most garnet grainsare partly replaced by andalusite and biotite but

preserve helicitic textures of regional metamorphism.Together with the phase relationship shown in Fig. 3,this textural evidence suggests that the appearance ofandalusite is accounted for by the reaction:garnet q chlorite q muscovite q quartz s andalusite

Ž .biotiteqH O Spear et al., 1995 .2

3.2. Metabasite

Metabasites, occurring primarily in the MunjuriFormation and the lower member of the Kye-myungsan Formation, consist of well-foliated amphi-bole schist and massive amphibolite. The formercontains medium-grained nematoblasts of amphibole,whereas amphibole in the latter occurs as coarse-grained porphyroblast. Typical mineral assemblagesare hornblendeqchloriteqepidoteqplagioclaseq

Ž .quartzq magnetiteq ilmeniteq titanite Table 1 .Minor actinolite and biotite commonly occur as sec-ondary minerals. Textural relationships in amphiboleschists suggest that the peak assemblages of theamphibolite facies were attained during the D –D1 2

deformation.

Ž . Ž .Fig. 4. A thin-section photomicrograph of garnet–chlorite schist M763 in the Munjuri Formation plane polarized light . Sigmoidalinclusion trails in garnet crystal consist of quartz, epidote, chlorite, muscovite and opaque mineral.


4. Mineral chemistry

Compositions of minerals from 21 pelitic and fivemafic schists were analyzed by an automated JEOL733 JXA electron microprobe at Seoul National Uni-versity. Analytical conditions were 15 kV accelerat-ing voltage, 10 nA beam current, and 5 mm beamdiameter. Data acquisition and reduction were per-formed using the ZAF calculation. Natural and syn-thetic minerals were used as standards. Amphiboleformulae were calculated on the basis of 13 cationsexclusive of Ca, Na and K, following the method of

Ž .Robinson et al. 1982 .

4.1. Phyllosilicates

Muscovite occurs as fine-grained aggregates andcoarse-grained platy porphyroblasts. In metapelites,the average Si content of primary muscovite is 3.08

Ž . Ž ."0.05 1s atoms per formula unit a.p.f.u. based

Ž .on 11 anhydrous oxygens Table 2 . The Si contentŽ .of muscovite in the biotite schist 2.97–3.23 is more

Ž .variable than that in the chlorite schist 3.01–3.13 .Ž .On the other hand, Nar NaqK of muscovite isŽ .higher in the chlorite schist 0.18"0.05 than in the

Ž .biotite schist 0.04"0.03 containing sodic plagio-clase.

Biotite is chemically homogeneous on a thin-sec-Ž .tion scale. Fer FeqMg of biotite ranges from 0.55

to 0.94 in the garnet and biotite zones, and from 0.42Ž .to 0.67 in the Al-silicate zone Table 3 . The Al and

Ti contents of biotite are 2.94–3.70 and 0.10–0.48a.p.f.u., respectively.

Chlorite occurs as pale green, euhedral to subhe-dral plates and its mode reaches up to 40% in thechlorite schist. Most of the analyzed chlorites areripidolite and pseudothuringite, according to the clas-

Ž . Ž .sification of Hey 1954 . Fer FeqMg of chloriteŽ .ranges from 0.43 to 0.64 Table 4 and its value for

inclusions in garnet is identical to that of the matrixchlorite. The Mn contents of chlorite in the chlorite

Table 2Representative analyses of muscovite

Sample no. Garnet zone Al-silicate zone

Chlorite schist Biotite schist M733 M919-1

M761 M763 M764 M716 M768 M302

SiO 45.59 45.63 46.21 46.99 46.16 45.95 45.36 44.902

Al2O 35.26 35.38 34.90 34.49 35.81 32.28 34.06 33.913

TiO 0.25 0.22 0.22 0.05 0.18 0.40 0.09 0.402

MgO 0.42 0.44 0.63 0.27 0.45 0.81 0.57 0.67aFeO 2.11 2.24 2.77 2.46 1.04 3.98 3.03 3.31

MnO 0.05 0.00 0.03 0.05 0.05 0.05 0.06 0.01CaO 0.11 0.10 0.02 0.13 0.19 0.00 0.00 0.06Na O 1.46 1.19 1.66 0.04 1.03 0.32 0.93 0.712

K O 9.08 9.49 8.88 11.23 10.32 10.18 9.60 10.142

Total 94.34 94.69 95.31 95.72 95.22 93.96 93.70 94.10

Cations per 11 oxygensSi 3.06 3.06 3.08 3.13 3.07 3.14 3.09 3.06Al 2.80 2.80 2.74 2.71 2.81 2.60 2.74 2.73Ti 0.01 0.01 0.01 0.00 0.01 0.02 0.01 0.02Mg 0.04 0.04 0.06 0.03 0.04 0.08 0.06 0.07Fe2q 0.12 0.13 0.15 0.14 0.06 0.23 0.17 0.19Mn 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00Ca 0.01 0.01 0.00 0.01 0.01 0.00 0.00 0.01Na 0.19 0.16 0.21 0.01 0.13 0.04 0.12 0.09K 0.78 0.81 0.76 0.96 0.88 0.89 0.83 0.88

a Total Fe as FeO.


Table 3Representative analyses of biotite used for thermobarometry


M716 M768 M956 M749 M934 M417 M302 M733 M919 M927

SiO 34.81 34.81 33.94 32.94 31.80 34.62 34.00 34.42 35.56 34.192

Al O 18.14 19.15 17.02 17.52 18.93 15.58 17.99 19.24 19.36 18.952 3

TiO 2.64 1.45 3.59 2.96 1.83 2.06 2.22 2.13 1.72 1.372

MgO 9.11 7.98 5.88 1.71 4.63 5.92 6.32 6.90 11.26 7.86aFeO 20.92 20.98 22.64 31.33 27.10 25.19 24.41 23.06 16.33 20.99

MnO 0.18 0.03 0.71 0.14 0.25 1.23 0.57 0.43 0.34 0.25CaO 0.03 0.01 0.07 0.00 0.07 0.03 0.03 0.01 0.06 0.00Na O 0.16 0.08 0.11 0.05 0.01 0.02 0.05 0.36 0.22 0.312

K O 9.34 9.74 9.27 8.74 8.63 9.73 9.15 8.48 9.81 9.032

Total 95.31 94.23 93.23 95.40 93.26 94.38 94.74 95.02 94.65 92.87

Cations per 11 oxygensSi 2.68 2.71 2.71 2.67 2.59 2.78 2.68 2.67 2.69 2.69Al 1.65 1.76 1.60 1.67 1.82 1.48 1.68 1.76 1.73 1.76Ti 0.15 0.09 0.22 0.18 0.11 0.12 0.13 0.12 0.10 0.09Mg 1.04 0.93 0.70 0.21 0.56 0.71 0.74 0.80 1.27 0.94Fe 1.35 1.36 1.51 2.12 1.84 1.69 1.61 1.50 1.03 1.36Mn 0.01 0.00 0.05 0.01 0.02 0.08 0.04 0.03 0.02 0.02Ca 0.00 0.00 0.01 0.00 0.01 0.00 0.00 0.00 0.01 0.00Na 0.02 0.01 0.02 0.01 0.00 0.00 0.01 0.06 0.03 0.05K 0.92 0.97 0.94 0.90 0.90 1.00 0.92 0.84 0.95 0.94

bX 0.56 0.60 0.68 0.91 0.77 0.71 0.68 0.65 0.50 0.59Fe

a Total Fe as FeO.b Ž .X sFer FeqMg .Fe

Ž .and biotite schists are 0.02"0.01 1s and 0.06"Ž .0.01 1s a.p.f.u., respectively.

4.2. Plagioclase

Fine to medium granoblasts of plagioclase arecommon in pelitic and mafic schists except in thechlorite schist. Its composition in pelitic schists is

Ž .variable An , but no systematic change with0 – 57Ž .increasing grade is apparent Table 5 . Many plagio-

clase grains are slightly zoned with decreasing Ancontents from core to rim, and their zoning patternsoften show inconsistent relationships. In mostmetapelites, the rim compositions of plagioclaserange from An to An , and the An contents vary22 52

less than 5 mol% within a single specimen.Ž .In an amphibole schist M416 adjacent to the

Ž .garnet isograd, albite An is modally abundant0 – 4Ž .and coexists with rare oligoclase An . Most mafic23

schists, however, lack albite and contain oligoclase–Ž .andesine An as commonly reported in15 – 42

Žmedium-grade metabasites Laird and Albee, 1981;.Maruyama et al., 1983; Begin, 1992 .´

4.3. Garnet

Medium- to coarse-grained garnet porphyroblastscommonly occur in pelitic and semi-pelitic schists,but are absent in mafic schists. All of the analyzedgarnet grains are chemically zoned and rich in al-

Ž . Žmandine X s0.4–0.6 , spessartine X s0.1–Alm Sps. Ž . Ž .0.4 and grossular X s0.05–0.2 Table 6 . Zon-Grs

Žing patterns of garnet in the chlorite schist Fig. 5a.and b are characterized by a bell-shape profile with

the spessartine component decreasing toward the rim,and the pyrope and almandine contents increasingfrom core to rim. These features are indicative ofgrowth zoning. Abrupt increases in the spessartine

Ž .content and the Fer FeqMg value at the outer-Ž .most rim -50 mm in width of garnet result from

Žthe late-stage resorption or diffusional zoning Tracy,.1982 .


Table 4Representative analyses of chlorite in pelitic schists

Sample no. Chlorite schist Biotite schistaŽ .M761 M763 M763 i M764 M716 M768

SiO 23.59 23.56 23.33 25.01 35.26 26.882

Al O 22.93 23.20 23.85 24.23 18.24 22.852 3

TiO 0.09 0.09 0.11 0.03 0.07 0.002

MgO 12.15 11.26 11.29 15.89 8.61 11.18bFeO 27.20 29.61 29.00 21.46 21.79 25.39

MnO 0.67 0.47 0.51 0.73 0.22 0.15CaO 0.01 0.01 0.06 0.00 0.06 0.05Na O 0.00 0.01 0.03 0.00 0.22 0.722

K O 0.06 0.03 0.00 0.00 0.02 0.062

Total 86.69 88.23 88.17 87.34 84.47 87.28

Cations per 14 oxygensSi 2.55 2.53 2.50 2.59 3.67 2.83Al 2.93 2.94 3.02 2.96 2.24 2.84Ti 0.01 0.01 0.01 0.00 0.01 0.00Mg 1.96 1.80 1.80 2.45 1.34 1.75Fe 2.46 2.66 2.60 1.86 1.90 2.23Mn 0.06 0.04 0.05 0.06 0.02 0.01Ca 0.00 0.00 0.01 0.00 0.01 0.01Na 0.00 0.00 0.01 0.00 0.04 0.15K 0.01 0.00 0.00 0.00 0.00 0.01

cX 0.56 0.60 0.59 0.43 0.59 0.56Fe

a Inclusion in garnet.b Total Fe as FeO.c Ž .X sFer FeqMg .Fe

In contrast to smooth zoning patterns in the chlo-rite schist, a garnet porphyroblast of M716, near theAl-silicate isograd, shows a reversal in the almandine

Ž .zoning Fig. 5c . In the inner part, the almandinecontent increases and the spessartine content de-creases. In the outer part, however, the spessartine

Ž .content slightly increases, whereas Fer FeqMgand the almandine content decrease. This complexpattern in the biotite schist may be attributed to achange in garnet-producing reactions as well as acomplicated growth history such as the coalescenceof separate nuclei of garnet or open system behavior

Ž .during the growth of garnet Whitney, 1996 .

4.4. Amphibole

Amphiboles in metabasites vary from magnesio-and ferro-hornblende to tschermakitic hornblende,

Ž .according to the nomenclature of Leake 1978 . The

compositions of amphiboles can be used to delineatemetamorphic grade and facies series, inasmuch asthe analyzed metabasites contain the common assem-blage, amphiboleqchloriteqepidoteqplagioclase

ŽqquartzqTi-phase Table 1; Laird and Albee,.1981 . According to the scheme of Laird and Albee

Ž .1981 , amphiboles in the OMB belong to themedium-pressure facies series. Furthermore, most ofthe amphibole compositions correspond to those of

Ž .the garnet zone defined by Laird and Albee 1981 inVI IV Ž . Ž .the Al –Al and Nar Ca q Na –Alr Si q Al

plots of Fig. 6. Amphiboles of M416, occurring inthe vicinity of the garnet isograd, however, are plot-ted over the transitional field between biotite andgarnet zones. These results are consistent with thosefound in pelitic schists.

The overall change in the analyzed compositionsof amphiboles can be represented by the pargasiteŽ . Ž .NaAl I Mg Si exchange vector Fig. 6 .3 y1 y1 y2

The AlIV content of hornblende ranges from 0.6 to


Table 5Representative analyses of plagioclase in metapelite and amphibole schist of the garnet zone

Sample no. Metapelite Amphibole schistaŽ . Ž . Ž . Ž .M716 c M716 r M768 c M768 r M956 M416 M853 M819 M120 M711

SiO 58.22 58.40 61.98 62.28 59.12 67.92 63.48 61.65 60.72 61.382

Al O 26.70 26.77 23.70 23.52 25.81 19.83 22.50 23.64 25.28 24.782 3bFe O 0.03 0.00 0.05 0.11 0.22 0.00 0.17 0.07 0.10 0.072 3

MnO 0.01 0.04 0.00 0.05 0.08 0.00 0.00 0.02 0.00 0.00CaO 8.54 8.41 5.09 5.03 7.03 0.16 3.47 4.56 6.37 5.71Na O 6.75 7.09 8.80 9.19 7.89 12.30 10.30 9.40 8.15 8.732

K O 0.04 0.10 0.09 0.07 0.13 0.06 0.06 0.05 0.05 0.072

Total 100.29 100.81 99.71 100.26 100.30 100.28 99.98 99.38 100.67 100.74

Cations per 8 oxygensSi 2.60 2.59 2.76 2.76 2.64 2.97 2.81 2.75 2.68 2.71Al 1.40 1.40 1.24 1.23 1.36 1.02 1.18 1.25 1.32 1.29

3qFe 0.00 0.00 0.00 0.00 0.01 0.00 0.01 0.00 0.00 0.00Mn 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00Ca 0.41 0.40 0.24 0.24 0.34 0.01 0.17 0.22 0.30 0.27Na 0.58 0.61 0.76 0.79 0.68 1.04 0.88 0.81 0.70 0.75K 0.00 0.01 0.01 0.00 0.01 0.00 0.00 0.00 0.00 0.00

cX 0.41 0.39 0.24 0.23 0.33 0.01 0.16 0.21 0.30 0.26An

a Ž . Ž .Analyses from core c and rim r of plagioclase grain.b Total Fe as Fe O .2 3c Ž .X sCar CaqNaqK .An

Ž .1.7 a.p.f.u. and is lowest in M416 Table 7 . Theapparent scatter in M120 suggests the partial retro-gression of hornblende during the thermal metamor-phism.

5. Metamorphic temperature and pressure

Peak metamorphic P–T conditions were esti-mated for pelitic and mafic schists of the garnetzone, and their localities are shown in Fig. 2. Noprograde P–T trajectory could be determined pri-marily because the compositional variations of pla-gioclase and biotite in equilibrium with zoned garnetare unknown. Results of P–T calculations are sum-marized in Tables 8–10, and shown in Figs. 2 and 7.Temperatures and pressures calculated from geother-mobarometers were rounded off to the nearest 58Cand 0.5 kbar, respectively.

Ž .Garnet is moderately to strongly zoned Fig. 5 ,and the rim composition of garnet at the position of

Ž .the ‘trough’ Kohn et al., 1993 was used to deter-

mine peak temperature and pressure conditions ofregional metamorphism. We used the most sodic rimcomposition of plagioclase for geothermobarometric

Ž .calculation Kohn et al., 1993 . Biotite, chlorite andmuscovite do not show systematic chemical changeon thin section scale, and the compositions of the

Ž .minerals in the matrix Tables 2–4 , not in directcontact with garnet, were used for geothermobarome-try. All iron in biotite, chlorite and garnet wasassumed to be Fe2q.

5.1. Geothermometry

Metamorphic temperatures were estimated for thegarnet–biotite pairs using the calibrations of Ferry

Ž .and Spear 1978 with the garnet activity models ofŽ . Ž .Hodges and Spear 1982 and Berman 1990 . Both

calibrations yield temperatures identical within theŽ .error range for each sample Table 8 . In the follow-

ing discussion and figures, we will use the resultŽ .based on the activity model of Berman 1990 . Aver-

age temperatures estimated from the rim composi-tions of garnet in the garnet zone range from 5208C

()

K.M

in,M.C

hor

Lithos

431998

31–

5142

Table 6Representative analyses of garnet in pelitic schists


Biotite schist Chlorite schist M733 M919 M927aŽ . Ž . Ž . Ž . Ž . Ž . Ž . Ž .M716 c M716 r M768 c M768 r M956 c M956 r M417 c M417 r M934 M958 M302 M761 M763 M764

SiO 37.13 37.47 37.78 37.30 37.27 37.30 36.57 36.78 36.82 36.65 36.25 36.75 35.48 37.58 37.32 36.88 37.232

Al O 21.18 21.49 21.59 21.41 21.20 20.88 20.35 20.56 21.42 20.92 20.56 21.36 21.07 21.64 21.35 21.07 20.932 3

TiO 0.14 0.13 0.10 0.09 0.17 0.14 0.23 0.10 0.00 0.09 0.06 0.07 0.04 0.02 0.05 0.17 0.062

MgO 0.98 1.67 1.12 1.25 0.71 0.93 0.50 0.57 0.95 0.32 1.14 1.96 1.52 2.16 1.41 1.87 1.54bFeO 26.34 27.60 29.39 28.28 23.07 22.53 16.73 18.12 33.11 24.28 26.01 25.07 30.38 24.35 24.97 19.11 26.95

MnO 8.60 7.19 3.66 4.11 12.36 12.39 19.60 16.57 4.54 9.19 13.57 9.76 6.45 11.73 13.74 16.78 7.91CaO 6.05 5.71 7.20 6.56 6.00 5.89 6.12 6.43 3.33 7.99 2.32 4.48 5.28 4.17 0.71 3.44 5.11Total 100.42 101.26 100.84 99.00 100.77 100.06 100.10 99.12 100.16 99.44 99.91 99.49 100.22 101.65 99.53 99.32 99.74

Cations per 12 oxygensSi 2.99 2.98 3.00 3.01 2.99 3.01 2.98 3.00 2.98 2.98 2.97 2.97 2.89 2.98 3.03 2.99 3.01Al 2.01 2.02 2.02 2.04 2.01 1.99 1.96 1.98 2.05 2.01 1.99 2.04 2.03 2.02 2.04 2.02 1.99Ti 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.00 0.01 0.00 0.00 0.00 0.00 0.00 0.01 0.00Mg 0.12 0.20 0.13 0.15 0.08 0.11 0.06 0.07 0.11 0.04 0.14 0.24 0.19 0.25 0.17 0.23 0.19

2qFe 1.77 1.83 1.95 1.91 1.55 1.52 1.14 1.24 2.24 1.65 1.78 1.69 2.07 1.61 1.69 1.30 1.82Mn 0.59 0.48 0.25 0.28 0.84 0.85 1.35 1.15 0.31 0.63 0.94 0.67 0.45 0.79 0.94 1.15 0.54Ca 0.52 0.49 0.61 0.57 0.52 0.51 0.53 0.56 0.29 0.70 0.20 0.39 0.46 0.35 0.06 0.30 0.44X c 0.94 0.90 0.94 0.93 0.95 0.93 0.95 0.95 0.95 0.98 0.93 0.88 0.92 0.86 0.91 0.85 0.91Fe

dX 0.59 0.61 0.66 0.66 0.52 0.51 0.37 0.41 0.76 0.55 0.58 0.57 0.65 0.54 0.59 0.44 0.61AlmdX 0.04 0.07 0.04 0.05 0.03 0.04 0.02 0.02 0.04 0.01 0.05 0.08 0.06 0.08 0.06 0.08 0.06PrpdX 0.20 0.16 0.08 0.10 0.28 0.28 0.44 0.38 0.11 0.21 0.31 0.22 0.14 0.26 0.33 0.39 0.18SpsdX 0.17 0.16 0.21 0.20 0.17 0.17 0.17 0.19 0.10 0.23 0.07 0.13 0.15 0.12 0.02 0.10 0.15Grs

a Ž . Ž .Analyses from core c and rim r of garnet grain.b Total Fe as FeO.c Ž .X sFer FeqMg .Fed X sFerM; X sMgrM; X sMnrM; X sCarM, where MsFeqMgqMnqCa.Alm Prp Sps Grs


Fig. 5. Zoning profiles of garnet grains in pelitic schists of theŽ . Ž . Ž .garnet zone: a M763; b M764; and c M716. Analysis points

Ž .follow the sigmoidal traverse defined by inclusion trails. Both aŽ .and b of the chlorite schist show continuous changes in alman-

Ž .dine and spessartine contents, whereas c is characterized bycompositional reversal between inner and outer part of the grain.Abrupt changes in the outermost rims are attributed to the retro-grade metamorphism.

Ž . IV VI Ž . Ž . Ž .Fig. 6. a Al vs. Al and b Alr SiqAl vs. Nar CaqNaplots of hornblende in the amphibole schists. Compositional fieldsof amphiboles from biotite, garnet and staurolite–kyanite zones

Ž .defined in Vermont Laird and Albee, 1981 are shown forcomparison. Reference line denotes the substitution towards parg-

Ž .asite Parg .

Ž .to 5858C Table 8 . On the other hand, temperaturesof the Al-silicate zone samples and a few thermally-affected schists of garnet zone close to the Al-silicateisograd were estimated at a nominal pressure of 2kbar. A range of temperatures of 565–5958C wasobtained, except for M919. The low temperature for

Ž .M919 5058C probably results from partial retro-gression of garnet–biotite pairs.

Temperatures were also estimated from the Fe–Mg exchange reaction between garnet and chlorite in

Ž .three adjacent chlorite schists Table 9 , using theŽ . Ž .calibrations of Ghent et al. 1987 , Laird 1988

Ž .modified from Dickenson and Hewitt 1986 , andŽ .Grambling 1990 . These calibrations yield tempera-

tures which are consistent within the 1s range ofeach sample, and the result based on Grambling


Table 7Representative analyses of amphibole in amphibole schists

Sample no. M416 M853 M819 M120 M711

SiO 46.15 44.20 41.81 42.60 41.392

Al O 9.56 11.31 14.06 16.65 14.412 3

TiO 0.32 0.32 0.34 0.35 0.552

MgO 12.12 11.19 9.21 8.60 6.00aFeO 13.80 14.97 16.01 14.85 21.07

MnO 0.51 0.35 0.28 0.31 0.37CaO 11.54 11.65 11.53 11.25 11.90Na O 1.89 1.58 1.93 1.78 1.262

K O 0.21 0.23 0.27 0.27 0.582

Total 96.09 95.81 95.43 96.66 97.53

Cations per 23 oxygensSi 6.89 6.66 6.38 6.34 6.33Al 1.68 2.01 2.53 2.92 2.60Ti 0.04 0.04 0.04 0.04 0.06Mg 2.70 2.52 2.10 1.91 1.37Fe 1.72 1.89 2.04 1.85 2.70Mn 0.06 0.04 0.04 0.04 0.05Ca 1.85 1.88 1.88 1.79 1.95Na 0.55 0.46 0.57 0.51 0.37K 0.04 0.05 0.05 0.05 0.11

bX 0.39 0.43 0.49 0.49 0.66Fe

a Total Fe as FeO.b Ž .X sFer FeqMg .Fe

Ž .1990 gives the smallest range of temperatures,510–5758C. Although these temperatures show awide range, they are compatible with those of gar-net–biotite geothermometer.

Temperatures of three amphibole schists in thegarnet zone were estimated using edeniterrichteriteexchange reactions between amphibole and plagio-

Ž .clase Holland and Blundy, 1994 . Although theanalyzed samples are distributed throughout the studyarea, they yield consistent temperatures of 575–

Ž .5908C Table 10 . This result corroborates the tem-peratures obtained from pelitic schists.

The temperature estimates of the garnet zoneappear to range up to those of staurolite zone re-

Žported in Barrovian type schists e.g., Whitney and.Ghent, 1993; Menard and Spear, 1994 . However,

preferential partitioning of Mn into garnet may ex-pand the stability field of Mn-rich garnet at theexpense of other phases like staurolite. Such anexpansion is demonstrated by the P–T grid of Spear

Ž .et al. 1995 , where the garnet–chlorite assemblageŽ .persists up to 6208C at 6.5 kbar Fig. 7 . This result

based on phase equilibria is compatible with ourtemperature estimates in the garnet zone, and theabsence of staurolite in the study area is attributed to

Žthe high Mn content of garnet X s0.15;0.38;Sps.Table 6 .

5.2. Geobarometry

Metamorphic pressures were calculated with theŽgarnet–plagioclase–biotite–muscovite GPBM:.Hodges and Crowley, 1985; Hoisch, 1990 and gar-

Žnet–plagioclase–biotite–quartz GPBQ: Hoisch,.1990 geobarometers. Pressures were estimated for

three pelitic schists, in conjunction with temperaturesestimated from the garnet–biotite geothermometer of

Ž .Ferry and Spear 1978 with the activity model ofŽ .Berman 1990 . Pressures using the rim composi-

tions of garnet and plagioclase range approximatelyŽ .from 5 to 8 kbar Table 8 . Both GPBM and GPBQ

geobarometers yield pressures that agree within theŽ .range of uncertainty Fig. 7 , although the GPBMŽ .calibration of Hoisch 1990 gives lower pressures

Ž .than others Table 8 .

6. Fluid inclusions

Quartz veins are rare in the study area, and areeither deformed or undeformed. The undeformedveins, probably produced during the emplacement ofthe Jurassic granite, contain abundant low-densitygaseous and aqueous inclusions. On the other hand,the veins which are commonly boudinaged and foldedcontain three types of fluid inclusions: CO -rich2

Ž .inclusions with no detectable H O carbonic ; CO –2 2Ž .H O inclusions carbonic–aqueous ; and H O-rich2 2

Ž .inclusions with no detectable CO aqueous . Using2Ž .the textural criteria of Roedder 1984 , two different

Ž .generations of inclusions are distinguishable: 1 iso-lated ‘older’ inclusions of carbonic, carbonic–aque-

Ž .ous and aqueous compositions; and 2 smaller,‘younger’ aqueous inclusions occurring along healedmicrofractures.

The CO -rich inclusions are present as scattered,2

equant inclusions generally ranging from 3 to 13 mmin diameter. Some inclusions show evidence of de-crepitation such as small satellite inclusions around a


Table 8Temperatures and pressures estimated from various geothermobarometry

Ž . Ž .Sample no. T 8C P kbara b c d en FSB HS HC H

Fe Mg

Garnet zonef fM716 6 550 550 7.0 5.0 5.5f fM768 6 520 515 7.5 6.0 6.5f fM956 3 585 600 7.5 7.5g gM417 9 540 560h hM934 3 595 585h hM302 6 565 575

Al-silicate zoneh hM733 8 580 595h hM919 5 505 520h hM927 12 565 565

a Number of the analyzed pairs of garnet and biotite.b Ž . Ž .FSB, garnet–biotite geothermometer of Ferry and Spear 1978 with garnet activity model of Berman 1990 .c Ž .HS, garnet–biotite geothermometer of Hodges and Spear 1982 .d Ž .HC, garnet–plagioclase–biotite–muscovite geobarometer of Hodges and Crowley 1985 .e Ž .H, garnet–plagioclase–biotite–muscovite geobarometer of Hoisch 1990 , except for M956 based on the garnet–plagioclase–biotite–quartz

Ž .geobarometer of Hoisch 1990 .f Pressure used for calculating temperature is the average of HC and H pressures.g Pressure is assumed to be 6.5 kbar during the growth of garnet.h Pressure of thermal metamorphism is assumed to be 2 kbar.

large solitary inclusion. Diameters of H O-rich in-2

clusions are 1–20 mm, but most are less than 10mm. The inclusions are scattered or aligned. Most ofthe aqueous inclusions contain a gas phase occupy-ing 10–20 vol.% of the inclusion at room tempera-ture.

For fluid inclusion studies, doubly polished thinsections of three quartz veins were examined on apetrographic microscope equipped with a gas flowheating–freezing stage. Fluid isochores and densitieswere calculated using MacFlinCor computer pro-

Ž .gram of Brown and Hageman 1994 . The equationsof state used for the isochore calculation are adopted

Ž .from Bowers and Helgeson 1983 and Kerrick andŽ . Ž .Jacobs 1981 for the H O–NaCl KCl and CO2 2

systems, respectively.Fluid inclusions from a deformed quartz vein

Ž .M413 in the chlorite schist was analyzed in detailand the results are shown in Fig. 8. Melting tempera-tures of CO in the isolated ‘older’ CO -rich and2 2

CO –H O inclusions range from y58.78C to2 2Ž .y55.48C Fig. 8a . Their frequency maximum agrees

Table 9Temperatures estimated from the garnet–chlorite geothermometer at 6.5 kbar

Ž .Sample no. MgrFe T 8CaŽ . Ž . Ž .Garnet Chlorite Ghent et al. 1987 Laird 1988 Grambling 1990

Ž . Ž .M761 0.143 ns2 0.869 ns18 590 615 575Ž . Ž .M763 0.091 ns2 0.640 ns15 545 560 540Ž . Ž .M764 0.161 ns2 1.294 ns11 500 510 510

Temperatures are calculated using the average MgrFe values of garnet and chlorite: nsnumber of analyses.a Ž .Modified from Dickenson and Hewitt 1986 .


Table 10Ž .Temperatures estimated from the amphibole–plagioclase geothermometer of Holland and Blundy 1994 at 6.5 kbar

Ž .Sample no. Plagioclase Hornblende T 8Ca bŽ . Ž . Ž .Car CaqNa X M4yNa X M2yAl

Ž . Ž . Ž . Ž .M120 0.306 ns7 0.075 ns9 0.480 ns9 575"10 Ns6Ž . Ž . Ž . Ž .M711 0.262 ns4 0.049 ns5 0.403 ns5 590"10 Ns2Ž . Ž . Ž . Ž .M853 0.172 ns6 0.091 ns7 0.372 ns7 590"5 Ns6

nsnumber of analyses; Nsnumber of pairs.a Ž . Ž .X M4yNa s 2yCaycm r2; cms‘cummingtonite substitution’.b Ž . Ž . Ž .X M2yAl s AlqSiy8 r2; calculations are based on site allocation method of Holland and Blundy 1994 .

Ž .with the melting temperature y56.68C of pureCO . The gaseous and liquid phases of CO in the2 2

CO -rich and CO –H O inclusions homogenize to2 2 2

the liquid phase in a relatively wide range ofy16.28C to q27.48C, but mostly at 68C to 268CŽ .Fig. 8b corresponding to the equivalent CO den-2

sity of 0.69–0.89 grcm3.Solitary H O-rich inclusions homogenize to the2

liquid phase at 177–2998C but commonly at 239–Ž .2718C Fig. 8c . In conjunction with the freezing

temperatures of y48C to y28C for these inclusions,homogenization temperatures indicate an approxi-mate density of 0.79–0.87 grcm3. Total homoge-

nization temperature of CO –H O inclusions may2 2

provide a minimum temperature for the entrapmentof inclusion, but could not be determined because ofearly decrepitation at ;3608C.

Isochores obtained from the majority of fluidinclusions in the deformed quartz vein intersect ap-

Žproximately at about 350–5008C and 1–3 kbar Fig..9 . These P–T estimates may represent density equi-

libration during the decompressional cooling associ-ated with exhumation or during the thermal activitysubsequent to the regional, medium-P metamor-phism. In either case, the retrograde path passesthrough the P–T condition defined by fluid inclu-

Fig. 7. A diagram showing the P–T ranges estimated from three pelitic schists of the garnet zone. The garnet–biotite geothermometer ofŽ . Ž .Ferry and Spear 1978 with the activity model of Berman 1990 and the garnet–plagioclase–biotite–muscoviterquartz geobarometers of

Ž . Ž .Hodges and Crowley 1985 and Hoisch 1990 were used for the P–T calculation. Thin curves denote the KFMASH reactions adoptedŽ .from Spear et al. 1995 . Dashed curves represent the displacement of reactions, garnetqchloriteqmuscovitesstauroliteqbiotiteq

quartzqH O and stauroliteqmuscoviteqquartzsgarnetqAl SiO qbiotiteqH O, when the X values are 0.1 and 0.2, respectively2 2 5 2 SpsŽ .Spear et al., 1995 . Dotted curve represents the FASH reaction of Fe–chloriteqquartzsalmandineqH O. Reaction curves among2

Ž .Al-silicates are from Holdaway 1971 .


Ž . Ž . Ž . Ž .Fig. 8. Melting temperatures Tm of CO -rich and CO qH O inclusions a and homogenization temperatures Th of CO qH O b2 2 2 2 2Ž .and H O-rich c inclusions analyzed from M413. N and n represent the frequency and the number of analyses, respectively.2


Fig. 9. Simplified P –T – t path of northwestern Ogcheon meta-morphic belt. Approximate retrograde path is represented by two

Ž .segments dashed and solid curves , because of the polyphasemetamorphism accompanying significant temporal gap betweenpeak and retrograde P –T conditions. Isochores of H O and2

CO -rich fluid inclusions are also shown, covering the range of2

the observed densities. Thermal metamorphism has occurredaround Jurassic and Cretaceous granitoids, but its effect on theP –T – t path is not shown for clarity. Prograde P –T path wasnot determined in this study, and a representative prograde path

Ž .from the central OMB thin dashed line; Kim et al., 1995 isshown for reference.

sion data. Moreover, the wide range in the P–Testimates suggests various stages of entrapment orreequilibration for each fluid inclusion.

7. Discussion

Quantitative calibration and application of theŽ .pressure–temperature–time P–T–t path are essen-

tial for understanding tectonic processes in metamor-Ž .phic belts e.g., Spear, 1993; Kohn et al., 1995 . Our

petrologic and fluid inclusion data are combinedwith available radiometric data to delineate the P–T–t path of the OMB, and the result is shown in Fig.9. Peak metamorphic conditions in the study area,5–8 kbar and 520–5908C, are similar to those re-

Žported in the central 4.2–9.4 kbar and 490–6308C;. ŽKim et al., 1995 and southwestern 3.5–7.4 kbar

.and 430–6158C; Oh et al., 1995 parts of the OMB.Peak metamorphic assemblages are associated with

Ž .the D –D deformation of Cluzel et al. 1991 , and1 2

thus medium-pressure regional metamorphism maybe attributable to ductile stacking of large-scalenappes. This interpretation is preliminarily supported

Ž .by a steep prograde P–T path Fig. 9 that has beenŽ .reported in the central OMB by Kim et al. 1995 .

After the peak metamorphism, the OMB evolvedthrough a retrograde P–T condition of 1–3 kbar and350–5008C as determined from the fluid inclusionstudy. Final exhumation may closely follow the P–Tcondition of isochores for fluid inclusions, in order

Žto prevent autodecrepitation Crawford and Hollister,.1986 .

The timing of regional metamorphism in the OMBhas been traditionally construed as Late Paleozoic to

Ž .Early Mesozoic because: 1 the Permian strata ofthe Pyeongan Supergroup are metamorphosed and

Ž . Ž .deformed Kim, 1971 ; and 2 Early to MiddleJurassic granitoids intruding the OMB are not af-fected by regional metamorphism. In addition, radio-metric dating of metamorphic muscovite and biotite

Žhas commonly yielded Early Mesozoic ages see.Min et al., 1995, for review . For example, Cliff et

Ž .al. 1985 suggested a Late Triassic metamorphismfollowed by slow cooling, based on a muscoviteRb–Sr age of 219"3 Ma and younger biotite agesof 148–163 Ma. The Triassic age of metamorphismis further supported by a 40Ar– 39Ar amphibole age of

Ž .230 Ma, reported by Lee 1988 in the southeasternOMB.

Recent radiometric age dating and structural anal-yses, however, suggest that the OMB experiencedpeak metamorphic conditions prior to the Early

ŽMesozoic metamorphism Kim, 1990; Cluzel et al.,.1990, 1991; Kwon and Lan, 1991; Min et al., 1995 .

Ž .For example, Cluzel et al. 1990, 1991 concludedfrom structural and geological syntheses that themain deformation, D –D , of the OMB occurred in1 2

the Late Silurian to Early Devonian. Moreover, ourpreliminary 40Ar– 39Ar dating of hornblende in an

Ž .amphibolite Cho et al., unpublished data defines aplateau age of 408"2 Ma, corroborating the ‘Cale-donian’ orogenic event in the OMB. Although fur-ther radiometric data are needed to delineate the T–tpath precisely, it is likely that the OMB experiencedits peak metamorphic episode in the Middle Paleo-

Žzoic time. Thus the Triassic ages Cliff et al., 1985;.Lee, 1988 are interpreted to result from the isotopic

reequilibration during the D orogeny.3


Retrograde metamorphism has affected the OMBon a regional scale, as inferred from various lines ofevidence. Retrograde phases such as chlorite, mus-

Ž .covite and margarite Kim et al., 1995 arewidespread in pelitic schists. In particular, Cluzel et

Ž .al. 1991 suggested that the D deformation accom-3

panying the Triassic retrograde metamorphism hasproduced the pressure shadow of quartz and chlorite

Ž .around D –D garnet porphyroblasts cf. Fig. 4 . On1 2

the other hand, hornblendes in amphibolites are com-monly replaced by actinolite–actinolitic hornblende,epidote and chlorite. Microstructural analyses usingthe transmission electron microscope also revealedubiquitous intercalations of chlorite, biotite and pyri-

Ž .bole within hornblende crystals Ahn and Cho, 1996 .All of these results are compatible with the P–Tconditions estimated from fluid inclusions in thedeformed quartz vein. Thus we interpret that green-schist to low-rank amphibolite facies metamorphismof the low-pressure type has prevailed in the studyarea during the D or the Triassic Songrim orogeny3Ž .Lee, 1987; Cluzel et al., 1991 .

After the Triassic retrogression, a Jurassic to Cre-taceous thermal overprint has occurred in the OMB,

Ž .as demonstrated by Kim 1990 and Cho et al.Ž . Ž .1995 . Cho et al. 1995 concluded that the biotiteK–Ar system in central OMB was strongly affectedby both Jurassic and Cretaceous granitoids. A similar

Žresult was also found in the study area Min et al.,.1995 . Thermal events during the Jurassic to Creta-

ceous have not only produced low-pressure mineralssuch as andalusite and cordierite in contact aureoles,but also reset the K–Ar ages of biotite even inmetapelites several kilometers away from the con-tact.

In summary, the P–T–t evolution of the north-Ž . Ž .western OMB Fig. 9 is characterized by: 1 pro-

grade metamorphism at 5–8 kbar and 520–5908C, inassociation with crustal thickening during the Sil-

Ž .urian–Devonian time; 2 regional retrograde meta-morphism at 1–3 kbar and 350–5008C in the Trias-

Ž .sic; and finally 3 thermal metamorphism aroundJurassic to Cretaceous granitoids. Our result furtherstrengthens the polycyclic evolution model of the

Ž .OMB, proposed by Cluzel et al. 1990 , but the LateProterozoic zircon age in an acidic metavolcanic

Ž .rock Lee et al., in press suggests a basin evolutionmore protracted and complex than their model. The

tectono-metamorphic evolution of the OMB eluci-dated in this study can be accounted for by intracon-tinental extension followed by the closure of an

Žaborted rift Cluzel et al., 1990, 1991; Cho et al.,.1994 rather than an intercontinental collision pro-

cess which involves high-pressure metamorphismŽ .e.g., Ernst and Liou, 1995 . Hence, our result

Ž .corroborates the suggestion of Cluzel et al. 1991that the Ogcheon belt is not the continuation of theTriassic collision belt between Sino–Korean andYangtze cratons in east-central China.

Acknowledgements

We gratefully acknowledge critical reviews andhelpful comments provided by D.A. Carswell, W.R.Fitches, S. Foley, E.D. Ghent, S.-T. Kwon, T. Rei-necke, and an anonymous reviewer. This study wassupported by the Basic Science Research Institute

Ž .Program, Ministry of Education BSRI-96-5404 .

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Metamorphic evolution of the northwestern Ogcheon metamorphic belt, South Korea