Precambrian Research 102 (2000) 207–220

Pb and Nd isotopic constraints on Paleoproterozoic crustalevolution of the northeastern Yeongnam massif, South

Korea

Chang-Sik Cheong a,*, Sung-Tack Kwon b, Kye-Hun Park c

a Isotope Research Team, Korea Basic Science Institute, 52 Eoeun Dong, Yusung Ku, Taejeon 305-333, South Koreab Department of Earth System Sciences, Yonsei Uni6ersity, Seoul 120-749, South Korea

c Department of Applied Geology, Pukyong National Uni6ersity, Pusan 608-737, South Korea

Received 18 May 1999; accepted 25 February 2000

Abstract

We report Pb isotopic ages and Nd isotopic signatures of Paleoproterozoic basement rocks from the Pyeonghaearea, northeastern Yeongnam massif, South Korea. The PbSL (lead step-leaching) garnet data of the Wonnam group(Precambrian metasediments) yield a 207Pb/206Pb age of 1840926 Ma, which can be regarded as the timing ofamphibolite to upper amphibolite facies metamorphism and associated garnet growth. Whole rock data for thePyeonghae gneiss intruding the Wonnam group give a 207Pb/206Pb age of 2093986 Ma, denying the possibility of adirect link between the intrusion of the Pyeonghae gneiss and the regional metamorphism of the Wonnam group. Ourresults confirm the significance of the 2.1 Ga and 1.8 Ga episodes that have been broadly constrained in theYeongnam massif. The depleted mantle Nd model ages of metasedimentary rocks from the Wonnam group(2.63–2.47 Ga) are slightly younger than those of the Pyeonghae gneiss samples (2.71–2.57 Ga). This Nd isotopicsignature also precludes a direct derivation of the Pyeonghae gneiss from the Wonnam Group, instead implying thepresence and involvement of the older, probably late Archean crustal materials during the 2.1 Ga magmatism in thenortheastern Yeongnam massif. Compiled Pb and Nd isotope data from the Yeongnam and Gyeonggi massifs suggesta similar geologic history for them, arguing against the conventional idea that the Gyeonggi and Yeongnam massifsare separate continental blocks respectively correlated to the South and North China blocks. The whole rock Pbisotope data of basement rocks from the two massifs form a well defined 207Pb/206Pb linearity of around 2.0 Ga,suggesting their common crustal evolution process for the past two billion years. A broad coincidence of majortectonic episodes in the two massifs is confirmed by reviewed geochronological data. The Nd model ages of basementrocks from the two massifs support a probable existence of Archean crusts in South Korea. The Nd model ages, bothArchean and Proterozoic, of the Gyeonggi and Yeongnam massifs agree with neither those of the North China block(predominantly Archean) nor those of the South China block (predominantly Proterozoic). Our compiled isotopedata together with recent estimation for the age of the Honam shear zone appear to refute the presence of suture zonebetween the two South Korean Precambrian massifs, which leaves the Imjingang belt as the possible suture zone.

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* Corresponding author: Tel.: +82-42-8653446; fax: +82-42-8653419.E-mail address: ccs@comp.kbsi.re.kr (C.-S. Cheong)

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Keywords: Yeongnam massif; Gyeonggi massif; Pb–Pb age; Suture zone

1. Introduction

Recent studies on the continent collision be-tween the North (Sino-Korean) and South China(Yangtze) blocks (Huang and Wu, 1992; Ames etal., 1993, 1996; Li et al., 1993; Yin and Nie, 1993;Li, 1994; Ernst and Liou, 1995) have generated agrowing interest in the possibility that the colli-sion zone may extend to the Korean peninsula.Despite the lack of definitive evidence for conti-nent collision and associated high-pressure meta-morphism such as diamond, coesite, and eclogite,several tectonic units of South Korea including

the Imjingang belt, the Gyeonggi massif, and theOgcheon belt (Fig. 1A) have been proposed aspossible candidates for the eastern continuation ofthe Chinese collision belt (Liu, 1993; Yin and Nie,1993; Ernst and Liou, 1995; Chang, 1996; Ree etal., 1996).

Although the characteristics of Korean base-ment rocks could be potentially important to thiskind of debate, there still remain many ambigui-ties regarding their ages, isotopic signatures, andtectono-metamorphic evolution processes. On thebasis of Paleozoic faunal differences, Kobayashi(1966) suggested that the Gyeonggi massif has an

Fig. 1. (A) Simplified tectonic map for northeastern Asia. The geology of the Korean peninsula is composed of seven tectonicprovinces (NM, Nangrim massif; PB, Pyeongnam basin; IB, Imjingang belt; GM, Gyeonggi massif; OB, Ogcheon belt; YM,Yeongnam massif; GB, Gyeongsang basin). The Yeongnam massif bounds with the Ogcheon belt by the dextral strike-slip ductileshear zone, called the Honam shear zone (HSZ). (B) The distribution of Precambrian basement rocks in South Korea. (C) Schematicgeologic map of the Pyeonghae area, northeastern Yeongnam massif (modified after Hwang et al., 1996).

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affinity to the South China block, and the Yeong-nam massif to the North China block. This ideaprovided a principal background for later tectonicinterpretations of the Korean peninsula by Cluzelet al. (1991) and Yin and Nie (1993). Cluzel et al.(1991) suggested that the Gyeonggi massif and theOgcheon belt of South China affinity have beenjuxtaposed with the Yeongnam massif of NorthChina affinity as a result of Triassic dextral dis-placement of the order of 200 km along theHonam shear zone. Yin and Nie (1993) adoptedCluzel et al. (1991)’s idea and further proposed anindentation model for explaining the diachronicnature of the Chinese collision belt and develop-ment of the Tan-lu and Honam fault systems. IfKobayashi (1966)’s scheme is valid, it is expectedthat the two massifs are different in terms ofisotopic signatures and ages of crustal formationand tectono-metamorphic events, considering apresumed distinction between the North andSouth China blocks (Ma and Wu, 1981; Jahn etal., 1990; Zhang et al., 1997; Chen and Jahn,1998).

In this study, we address this problem by Pband Nd isotope data. First, we present Pb–Pbages and Nd isotopic data of basement rocksfrom the Pyeonghae area, northeastern Yeong-nam massif (Fig. 1B). Using the Pb and Ndisotope data of this study and previous works, wecompare geochronology and isotopic characteris-tics between the Gyeonggi and Yeongnam mas-sifs. Second, we compare Nd isotopic signaturesof Korean basement rocks with those of Chineseblocks on the basis of compiled data set, anddiscuss their tectonic implications for the hypoth-esis of continuation of the Chinese collision beltto the Korean peninsula.

2. Geologic setting

The Korean peninsula can be divided into sevenmajor tectonic provinces: i.e. from northwest tosoutheast, the Precambrian Nangrim massif, thePaleozoic Pyeongnam basin, the Paleozoic Imjin-gang belt, the Precambrian Gyeonggi massif, thelate Precambrian to Paleozoic Ogcheon belt, thePrecambrian Yeongnam massif, and the Creta-

ceous Gyeongsang basin (Fig. 1A). The Gyeonggiand Yeongnam massifs constitute the Precam-brian basement in the southern Korean peninsula,and consist primarily of high-grade gneisses andschists. The Gyeonggi massif is bounded by nor-mal faults with the Imjingang belt to the north(Ree et al., 1996) and with the Ogcheon belt tothe south (Kwon et al., 1995; Ree et al., 1995).The boundary between the Yeongnam massif andthe Ogcheon belt is a dextral strike-slip ductileshear zone called the Honam shear zone (Yanai etal., 1985; Cluzel et al., 1991), which is overlainunconformably by the Gyeongsang basin. How-ever, many parts of the tectonic boundaries areobscured by extensive intrusions of Mesozoicgranites. The two belts comprise highly deformedmeta-volcanosedimentary sequences which experi-enced Barrovian metamorphism during Permian-Triassic time (Adachi et al., 1996; Ree et al.,1996). The Ogcheon belt is considered to havedeveloped in a failed intracontinental rift settingduring early Paleozoic time and therefore cannotbe a suture zone (Chough, 1981; Cluzel et al.,1991). Recently, Lee et al. (1998) reported a latePrecambrian age for a metavolcanic rock in theOgcheon belt. Ree et al. (1996) showed fromstructural, metamorphic and geochronologicalstudies that the Imjingang belt is a possible candi-date for the suture zone extending from theSulu belt in China. The Gyeongsang basin iscovered with volcano-sedimentary sequences(the Gyeongsang supergroup) and basement rocksare rarely exposed.

Previous age data for the formation and meta-morphism of basement rocks in the Gyeonggi andYeongnam massifs are mainly concentrated in theearly Proterozoic (ca. 2.2–1.8 Ga). However, anupper intercept age of U-Pb zircon (Turek andKim, 1996) and some Nd model ages (Lan et al.,1995) indicate the presence of Archean basementrocks in South Korea.

In the Pyeonghae area of northeastern Yeong-nam massif (Fig. 1C), the Precambrian rocks aredivided into the Wonnam group of metasedimen-tary rocks and the Pyeonghae gneiss of metaig-neous rocks on the basis of lithology and fieldoccurrence, with the latter intruding the former(Kim et al., 1963). They are unconformably over-

C.-S. Cheong et al. / Precambrian Research 102 (2000) 207–220210

lain by Phanerozoic sedimentary rocks, and arelocally intruded by Cretaceous granitic rocks inthe southern part of the area. The Wonnamgroup, the oldest unit in the study area, is mainlycomposed of mica schists, garnet-mica schists,biotite gneisses, quartzite, and aplitic gneisses to-gether with subordinate calcsilicates and amphi-bolites. The Pyeonghae gneiss comprises mainlywell-foliated biotite gneisses and aplitic gneisses,showing augen and banded structures. In the fel-sic interlayer, K-feldspar porphyroblasts about 2cm in length are commonly observed. Kim et al.(1991) suggested an upper amphibolite faciesmetamorphic condition for the Pyeonghae gneiss,but quantitative estimates of temperature andpressure are not available yet. No geochronologi-cal data have been reported for the Precambrianrocks in the Pyeonghae area.

3. Samples and experimental procedures

Pb and Nd whole rock isotopic compositionswere measured for selected samples of the Won-nam group and the Pyeonghae gneiss. The loca-tions of analyzed samples are shown in Fig. 1C.The rock chosen for the PbSL garnet dating(PH13) is a fresh specimen of garnet-biotite schistcollected from the central part of the Wonnamgroup (Fig. 1C). The garnet ranges from 1 to4 mm in diameter. The garnets are predomin-antly almandine-pyrope solid solutions withminor spessartine and grossular components(Alm66–71Pyr17–25Spe1.7–2.7Gro4.4–9.1). Pure garnetseparates were hand-picked under a binocular mi-croscope from rock fragments ranging from 20 to60 mesh in size. Garnet separates were repeatedlyrinsed with acetone and Millipore® water in anultrasonic cleaner for 30 min.

All the analyses including chemical separationand mass spectrometry were performed at theKorea Basic Science Institute. About 100 mg ofrock powder was mixed with a 150Nd–149Smmixed spike and then dissolved with a mixed acid(HF: HClO4: HNO3=4:1:1) in Teflon vessels.REE (rare earth element) fractions were collectedby the conventional cation column chemistry. Smand Nd fractions were separated from each other

by the second step cation column chemistry using0.2 M HIBA (alpha-hydroxy-iso butyric acid)(Makishima et al., 1993).

Three 120°C leaching steps were performed onthe garnet separate. The first step was treatmentwith a mixed acid of 12:1 1N HBr+2N HCl for30 min. The second and third steps were per-formed with 4.5N HBr for 3 h and 9N HBr for 18h, respectively. 30 ml Savillex® screw-top beakerswere used in the leaching experiment. The residuewas rinsed three times with purified water anddried between steps. Sm, Nd, Th, and U concen-trations of the leachates were measured using aVG PQ III® inductively coupled plasma massspectrometer (ICP-MS). For Pb isotope analysis,whole rock powders and PH 13 garnet were di-gested using the same method as above but with-out spikes. The Pb of the PbSL leachates,unleached garnet, and whole rock samples wasseparated by the anion exchange column chem-istry using an HBr medium.

Isotopic ratios were measured on a VG 54-30®

thermal ionization mass spectrometer (TIMS)equipped with nine Faraday buckets. The Nd andPb isotopic compositions were measured withdynamic and static modes, respectively. The143Nd/144Nd ratios were normalized to146Nd/144Nd=0.7219, and further corrected forNd contribution from added spikes. Replicateanalyses of La Jolla Nd gave 143Nd/144Nd=0.51183390.000005 (2sm, N=13). The Pb iso-tope ratios were corrected for instrumentalfractionation using average measured values ofthe NBS 981 standard. The measured isotopicratios of the NBS 981 showed mass fractionationof around 0.1% per atomic mass unit relative tothe recommended value. Total blank levels wereabout 10 pg for Sm and 50 pg for Nd. Pb blankswere about 0.3 ng for the PbSL and below 1 ngfor the whole rock experiment. Isochron parame-ters were calculated using the computer programof Ludwig (1994). In the isochron calculation, weassumed 2s error of 0.1% (=external reproduci-bility of NBS981 data, N=13) for most of 207Pb/204Pb and 206Pb/204Pb data because internal errorsfor individual data were smaller than 0.1%. Onlyfor the third leaching step data, internal errors of

C.-S. Cheong et al. / Precambrian Research 102 (2000) 207–220 211

Table 1Pb isotope data for Precambrian basement rocks from the Pyeonghae area, northeastern Yeongnam massif, South Korea

206Pb/204Pb 92s%a 207Pb/204Pb 92s%aRock types 208Pb/204PbSample 92s%a

Wonnam group30.303 17.215AmphiboliteYH01 40.00916.770 15.638PH03 Amphibolite 35.67316.833 15.599Biotite gneissbYH03 36.26522.995YH04 Biotite gneissb 16.347 42.95117.782 15.703Garnet-mica schistPH13 40.95517.177 15.563PH14 Garnet-mica schist 39.48820.565 16.056Garnet-mica schistPH25-1 41.777

Garnet-mica schistPH26 20.066 15.842 39.821

Pyeonghae gneissPH04 Augen gneiss 20.674 16.129 41.915

22.383 16.319 42.566PH09 Augen gneiss19.345 15.950Porphyroblastic gneissPH11 42.96919.585 15.970PH15 Porphyroblastic gneiss 42.31617.038 15.640Augen gneissPH19 37.30323.945 16.545PH20 49.406Augen gneiss

PbSL for garnet in PH1317.758 0.06 15.687Leaching step[1] 0.06 40.946 0.0620.636 0.07 15.979 0.07Leaching step[2] 49.740 0.06

Leaching step[3] 79.447 0.25 22.617 0.24 236.273 0.2426.094 0.09 16.582 0.09 50.851 0.09Unleached garnet bulk

a Internal errors (%SD, N=60). For whole rock data, within run errors are sufficiently smaller than 0.1%.b Gneissic part of schist-gneiss-quartzite interlayer.

ca. 0.25% were given. The residue after the PbSLgave a very poor signal during the mass spectro-metric run, probably indicating little Pb remainedafter the leaching. So no Pb isotopic data arereported for the residue. Errors of calculated ageswere reported at the 95% confidence level.

4. Results and discussion

Pb isotope data for the PbSL leachates, un-leached garnet, and whole rock samples are pre-sented in Table 1. Whole rock Sm–Nd isotopicdata are listed in Table 2. Chondritic uniformreservoir (DePaolo and Wasserburg, 1976) for thecalculation of oNd values is assumed to have thepresent values of 143Nd/144Nd=0.512638 and147Sm/144Nd=0.1967. The depleted mantle modelage (TDM) is calculated after Nagler and Kramers(1998).

4.1. Pb isotopes and geochronology

4.1.1. PbSL results of garnet from the Wonnamgroup (PH13)

The spread of Pb isotope ratios of the leachatesis considerable as shown in Table 1 and Fig. 2.The least radiogenic lead was released in the firststep. Increasingly radiogenic leads were recoveredfrom the second and third steps. A good linearityof data for the leachates, whole rock, and un-leached garnet in 207Pb/204Pb versus 206Pb/204Pbplot indicates an initial Pb isotopic equilibriumamong them, and yields a date of 1840926 Ma(MSWD=13.8) (Fig. 2). The Pb isotopic spreadin the leaching experiment of garnet can be at-tributed to the presence of heterochemical inclu-sions or different behavior of common andradiogenic lead in lattice siting and ionic charge(Frei and Kamber, 1995; Dewolf et al., 1996; Freiet al., 1997). In fact, microinclusions such as

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monazite, zircon, thorianite, rutile and ilmenitehave been identified in PH13 garnet by electronmicroprobe analyses. The 208Pb/206Pb trend of theleachates (Fig. 2) corresponds to a high Th/U of10.6. This trend appears to be dominated bymonazite (Th/U\3; Dewolf et al., 1996) andpossibly by thorianite for which no Th/U data areavailable. It seems that our leaching steps selec-tively dissolved inclusions with high Th/U ratios,because unleached garnet data plot below the208Pb/206Pb trend of the leachates (Fig. 2). Ele-mental ratios of the leachates confirm the pres-ence of high Th/U inclusions (i.e. monazite andthorianite) and their release during acid leaching.Monazite, garnet, and zircon have distinct fieldsin terms of Sm/Nd, U/Nd, and Th/U ratios (De-wolf et al., 1996). Sm/Nd, Th/U, and Nd/U ratiosof the leachates are listed in Table 3. Theleachates have a strong affinity with monazite inU/Nd versus Sm/Nd and Nd/U versus Th/U plots(Fig. 3). The effect of zircon dissolution is notvisible either in the 208Pb/206Pb trend or in elemen-tal ratios of the leachates, probably because wedid not use HF in the leaching step. Our 18409

26 Ma date is concordant with previously re-ported age data for the Yeongnam massif (seeTable 4), and could be correlated with the Lulian-gian orogeny in China (ca. 1850 Ma, Yang et al.,1986).

A blocking temperature for garnet U-Pb systemis considered to be higher than 800°C (Mezger etal., 1989, 1991). Although the leads from step-leaching are dominantly coming from microinclu-sions, we may use the blocking temperature ofgarnet because diffusion of the leads in microin-clusions would be ultimately governed by thegarnet structure. We obtained a peak metamor-phic temperature of 600–650°C for PH13 garnetfrom our preliminary microprobe work, whichagrees well with the qualitative estimate of Kim etal. (1991). Because the blocking temperature isconsidered to be higher than the metamorphictemperature, we think that the PbSL date repre-sents the time of garnet growth. We interpret thatour 1840926 Ma age represents the time ofamphibolite to upper amphibolite facies regionalmetamorphism and associated garnet growth ofthe Wonnam group.

Table 2Sm-Nd data for whole rock samples from the Pyeonghae area, northeastern Yeongnam massif

143Nd/144Nda Sm (ppm) oNd(2.1 Ga)Sample TDM (Ga)coNd(0)147Sm/144NdbNd (ppm)

Wonnam group0.512593 (7) 1.71 5.54YH01 0.1868 −0.9 2.55 1.78

(7) 1.56 4.68 0.2012 3.4 2.72PH03 2.220.512814(10) 3.93 17.78 0.1337 −18.3 2.56YH03 −1.370.511700

−1.032.47−21.90.119129.41YH04 5.79(6)0.5115160.511539 (6) 5.40 26.82 0.1217 −21.4 2.50 −1.29PH130.511310 (5) 6.51 34.73PH25-1 0.1134 −25.9 2.63 −3.52

−1.622.48−25.20.511349 0.1092PH26 28.335.11(19)

Pyeonghae gneissPH04 (5)0.511290 8.07 43.02 0.1134 −26.3 2.66 −3.93

0.511272 (6) 6.41 34.62PH09 0.1119 −26.6 2.65 −3.87PH11 0.511151 (5) 7.05 39.87 0.1070 −29.0 2.70 −4.91PH15 8.13(5) −3.532.71−22.30.12680.511495 38.77

0.511251PH17 6.78 38.20(5) 0.1073 −27.1 2.57 −3.040.511279 (5) 5.23PH20 28.28 0.1119 −26.5 2.64 −3.73

a Numbers in parenthesis refer to least significant digits and 92s mean.b Uncertainty is below 0.5%, checked by duplicate analysis.c Calculated after Nagler and Kramers (1998).

C.-S. Cheong et al. / Precambrian Research 102 (2000) 207–220 213

Fig. 2. Pb isotopic plots of garnet PbSL leachates, host wholerock, and unleached garnet from PH13 sample. The 207Pb/206Pb slope defined by them corresponds to 1840926 Ma(MSWD=13.8). The 208Pb/206Pb trend of the PbSL leachatesyields a Th/U ratio of 10.6. Note that data of unleached garnetplot below the leachates trend in 208Pb/204Pb versus 206Pb/204Pb plot. Abbreviations: WR; whole rock, Bulk; unleachedgarnet bulk, [1]; step 1 leachate, [2]; step 2 leachate, [3]; step 3leachate.

older than the metamorphic age of the Wonnamgroup. Because whole rock Pb–Pb dates can begenerally interpreted as representing intrusionages of granitic bodies (Moorbath and Taylor,1985), we consider the Pb–Pb date as an intrusionage of the Pyeonghae gneiss. As shown in Fig. 4,the data for whole rock samples of the Wonnamgroup are scattered around the 207Pb/206Pb trendof the Pyeonghae gneiss. The poor linear trend ofthe Wonnam group samples yields a slightlyyounger age (19839190 Ma, MSWD=92.5)than the Pyeonghae gneiss, probably suggestingthat the Pb isotopic system of the Wonnam group

Fig. 3. Nd/U versus Th/U and U/Nd versus Sm/Nd plotsshowing fields of zircon, monazite, and garnet (reviewed byDewolf et al., 1996). The PbSL leachates of PH13 garnet havesimilar chemical compositions to monazite.

Table 3Elemental ratios of the leachates from PH13 garnet

Th/U Nd/USm/Nd

9.860.319 16.82Step [1]12.93 22.97Step [2] 0.208

31.5810.710.374Step [3]

4.1.2. Pb-Pb whole rock age of the Pyeonghaegneiss

Pb isotopic compositions of whole rocks fromthe Pyeonghae gneiss (Table 1) yield a 207Pb/206Pbage of 2093986 Ma (MSWD=3.27) with modelm1 value of 8.69 (Fig. 4), which is about 250 Ma

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Table 4A summary of reported age data for the Gyeonggi and Yeongnam massifs

Methodology Age (Ma)Locality ReferencesLithology

Gyeonggi massifU-Pb zircon 2150920Granitic gneiss Gaudette and Hurley (1973)YoogooU-Pb zircon 1766926Seosan Turek and Kim (1996)Granitic gneissSm-Nd minerals 18979120Granulite Lee et al. (1997)Hwacheon

Middle Gyeonggi massif Sm-Nd garnet 1200–2100 Min et al. (1998)

Yeongnam massifSm-Nd mineralsJirisan 1678990Anorthosite Kwon and Jeong (1990)Sm-Nd whole rocks 1047969Biotite gneiss Lee et al. (1992)Kimcheon

Taebaegsan Granitic gneiss Pb-Pb whole rocks 1920956 Park et al. (1993)Pb-Pb whole rocks 1825986Granite Park et al. (1993)Taebaegsan

LeucogneissImwon Sm-Nd minerals 225094 Lee et al. (1994)Danyang Granitic gneiss Pb-Pb whole rocks 21609150 Kwon et al. (1995)

U-Pb zircon 2120920Granitic gneiss Turek and Kim (1996)KuryePorphyroblastic gneissKurye U-Pb zircon 194595 Turek and Kim (1996)

U-Pb zircon 1923914Chailbong Turek and Kim (1996)Granitic gneissSm-Nd garnet 1820911Charnockite Kim et al. (1998)Jirisan

CharnockiteJirisan Rb-Sr biotite 1123922 Kim et al. (1998)

was partially reset during the 1.84 Ga metamor-phism. Well-defined linearity of the Pb isotopedata for the Pyeonghae gneiss samples indicatesthat they were not disturbed during the metamor-phism. Therefore, we conclude from our Pb–Pbages and field relationship (Kim et al., 1963) thatthe Wonnam group was intruded by the Pyeong-hae gneiss at 2.09 Ga and was metamorphosedlater at 1.84 Ga.

4.1.3. Precambrian geochronology of SouthKorean basement rocks: a brief summary

Previous geochronological studies have revealedthree major episodes (i.e. ca. 2.1 Ga, 1.9 Ga, and1.0 Ga) of magmatic activity and one episode ofmetamorphism (ca. 1.8 Ga) in the other part ofthe Yeongnam massif (Table 4). The 2.1 Ga mag-matic episode, which could be correlated with theWutaian orogeny in China (ca. 2.2 Ga, Yang etal., 1986), is well constrained in the Yeongnammassif. Our Pb–Pb age of the Pyeonghae gneissconfirms the significance of the 2.1 Ga magma-tism in the Yeongnam massif.

As stated earlier, the comparison of magmaticand metamorphic ages between the Gyeonggi andYeongnam massifs is important in searching foran extension of the Chinese collision belt to the

Korean peninsula. It is interesting that the meta-morphic and magmatic ages of the Gyeonggi mas-sif are broadly coincident with important episodesof the Yeongnam massif (Table 4), although moredata are needed. The Pb–Pb isotope plots forsialic basement rocks in the Gyeonggi and Yeong-nam massifs are shown in Fig. 5. The data for

Fig. 4. Pb-Pb isochron diagram for whole rock samples of thePyeonghae gneiss and the Wonnam group. Data of thePyeonghae gneiss samples yield a 207Pb/206Pb age of 2093986Ma (MSWD=3.27) with model m1 value of 8.69. The data ofthe Wonnam group are scattered around the 207Pb/206Pb trendof the Pyeonghae gneiss.

C.-S. Cheong et al. / Precambrian Research 102 (2000) 207–220 215

Fig. 5. Compiled Pb isotope data for basement rocks from theGyeonggi and Yeongnam massifs show a good linearity (R2=0.996) corresponding to about 2.0 Ga in 207Pb/204Pb versus206Pb/204Pb plot. The evolution curve of an average crust (Sand K; Stacey and Kramers, 1975) is shown for references. Nocorrelation is found in 208Pb/204Pb versus 206Pb/204Pb plot.Data sources; Gyeonggi massif (Park et al., 1995; Cheong andChang, 1997), Yeongnam massif (Park et al., 1993; Kwon etal., 1995; this study).

lap in 208Pb/204Pb versus 206Pb/204Pb plot. Asshown in Fig. 5, the Pb line for South Koreanbasement rocks plots significantly above the aver-age crustal Pb growth curve of Stacey andKramers (1975), which is a common characteristicof old upper crust (Zartman and Doe, 1981;Tilton, 1983). Our compiled Pb isotope data indi-cate that the Gyeonggi and Yeongnam massifsshare a common precursor and have evolved to-gether for the past two billion years. This doesnot support the idea that they are geneticallyseparate blocks as suggested by Kobayashi (1966).

4.2. Nd isotopic signatures

4.2.1. Archean crust in South KoreaAs shown in Table 2, the Pyeonghae gneiss

samples have slightly lower oNd values than theWonnam group samples, but their Sm/Nd ratiosare indistinguishable from each other except twoamphibolite samples (YH01, PH03). Accordingly,TDM of the former is slightly but distinctly olderthan the latter (Table 2, Fig. 6). All the Pyeong-

Fig. 6. oNd — age evolution lines for the Wonnam group(open circles) and the Pyeonghae gneiss (closed circles).Metasedimentary rocks from the Wonnam group have higheroNd(2.1 Ga) values and younger TDM than the Pyeonghaegneiss samples. Two amphibolite samples of the Wonnamgroup have positive oNd(2.1 Ga) values. The evolution curvefor the depleted mantle (DM) (Nagler and Kramers, 1998) isshown relative to the chondritic uniform reservoir (CHUR).

supracrustal rocks and mafic intrusives are notincluded in the diagram, because our main inter-est lies in looking at the intrinsic characteristic ofthe basement. A good linearity (R2=0.996,slope=0.1207) is observed in 207Pb/204Pb versus206Pb/204Pb diagram for Korean basement rocks,which yields an apparent 207Pb/206Pb age of ca.2.0 Ga. The data from the two massifs also over-

C.-S. Cheong et al. / Precambrian Research 102 (2000) 207–220216

Fig. 7. Compiled Sm–Nd isotope data for basement rocksfrom the Gyeonggi and Yeongnam massifs. Reference lines forNd model ages are drawn by an approximation of the modelof Nagler and Kramers (1998). Chinese data are also shownfor references. Data sources; Gyeonggi massif (Lee et al., 1992;Lan et al., 1995; Cheong and Chang, 1997; Min et al., 1998),Yeongnam massif (Lee et al., 1992; Na, 1994; Lan et al., 1995;this study), North China block (Huang et al., 1986; Jahn et al.,1988; Sun et al., 1992), Cathaysian and South China blocks(Chen and Jahn, 1998).

amphibolite samples (Cheong, C.S., unpublisheddata) may represent a mantle component duringthe intrusion of the Pyeonghae gneiss. With thisinformation, we can qualitatively estimate that theresidence age of involved crustal materials shouldbe older than the Nd model age (2.71–2.57 Ga) ofthe Pyeonghae gneiss (Fig. 6). The role of crustalmaterials older than the Wonnam group is impor-tant even when the Pyeonghae gneiss is originatedfrom purely reworked crustal materials withoutany input from the mantle. Therefore it can beconcluded that the Nd isotopic signatures indicatethe presence and involvement of late Archeancrust during the 2.1 Ga magmatism in the north-eastern Yeongnam massif.

Lan et al. (1995) argued for a possible existenceof Archean crust in South Korea based upon Ndmodel ages. The inheritance of Archean crustalmaterials was confirmed by a U-Pb zircon upperintercept age (32949196 Ma) for a Proterozoicgneiss in the Gyeonggi massif (Turek and Kim,1996). Our compiled data also show commonArchean model ages of basement rocks from boththe Gyeonggi and Yeongnam massifs (Fig. 7).They are mostly Proterozoic in crystallization ormetamorphic ages (Table 4). It is not surprisingthat some Proterozoic basement rocks haveArchean model ages because they show, in manycases, negative oNd(t) values and thus are be-lieved to be originated from pre-existing crustalmaterials (for example, Hong et al., 1996). Thosemetasedimentary rocks intruded by early Protero-zoic granitic gneisses (Table 4) in the Gyeonggiand Yeongnam massifs may well be Archean inage.

4.2.2. Comparison with Chinese dataAvailable isotope data (Huang et al., 1986;

Jahn et al., 1988; Sun et al., 1992; Chen and Jahn,1998) confirm a conventional idea that the NorthChina block is older than the South China block(Fig. 7). Whereas Archean terranes are wide-spread in North China block (Jahn and Zhang,1984; Liu et al., 1985, 1990, 1992; Jahn et al.,1988; Song et al., 1996), they are limited in south-eastern China including the Yangtze andCathaysia blocks (Chen and Jahn, 1998). How-ever, Lan et al. (1995) showed that the North and

hae gneiss samples have rather tightly constrainedlate Archean model ages ranging from 2.71 Ga to2.57 Ga. Consistently negative oNd(2.1 Ga) valuesof the Pyeonghae gneiss (−4.91�−3.04) indi-cate important contributions of pre-existing conti-nental material to its sources. The possibility of adirect derivation of the Pyeonghae gneiss from theWonnam group can be easily excluded by higheroNd(2.1 Ga) values of the latter and age con-straints described earlier. The metasedimentaryrocks from the Wonnam group also have re-stricted TDM values ranging from 2.63 Ga to 2.47Ga. Generally the model ages of sedimentaryrocks are older than the depositional ages (Allegreand Rousseau, 1984), and thus the formation ageof the Wonnam group can be constrained between2.63 Ga to 2.47 Ga (Nd model age) and 2.1 Ga(intrusion age of the Pyeonghae gneiss), i.e. latestArchean to early Proterozoic. The range of oNd(2.1 Ga) of the Pyeonghae gneiss can be explainedby an important involvement of pre-existingcrustal materials older than the Wonnam group,either as a primary source material or crustalcontaminant. Positive oNd (2.1 Ga) values to-gether with low REE abundances, and flat chon-drite-normalized REE patterns of the two

C.-S. Cheong et al. / Precambrian Research 102 (2000) 207–220 217

South China blocks have overlapping Nd modelages, and Chen and Jahn (1998) confirmed theexistence of Archean rocks along the northernmargin of the South China block. Also shown inFig. 7 are compiled TDM values of the basementrocks from both the Gyeonggi and Yeongnammassifs. The Nd model ages, both Archean andProterozoic, of the two massifs agree with neitherthose of the North China block (predominantlyArchean) nor those of the South China block(predominantly Proterozoic). As a whole, theYeongnam massif is younger than the Gyeonggimassif in Nd model ages, but their considerableoverlap corroborates the similarity of crustal his-tories concluded from Pb isotope data describedpreviously.

4.3. Tectonic implications

Our compiled Pb and Nd data do not supporta previous idea that the Gyeonggi and Yeongnammassifs in South Korea are different continentalblocks. This observation contradicts a basicassumption of tectonic models suggested by Cluzelet al. (1991) and Yin and Nie (1993). Two basicassumptions of their models are: (1) different blockaffinity of the Gyeonggi and Yeongnam massifs;and (2) Triassic age of the dextral movement alongthe Honam shear zone. Our compiled isotope datado not support the first assumption. Cluzel et al.(1991) constrained the age of the dextralmovement along the Honam shear zone as Triassicby Choo and Kim (1986)’s Rb-Sr whole rock ageof 21193 Ma for the undeformed synkinematicgranite (the so-called Namweon granite).However, recent chronological data cast doubt onthe validity of Choo and Kim (1986)’s result. Kimand Turek (1996) confined the movement age ofthe Honam shear zone between 183 Ma to 176 Maon the basis of U-Pb zircon ages for deformed andundeformed granitic rocks. Their conclusion wascorroborated by monazite CHIME (chemicalTh-U-total Pb isochron method) data (Cho et al.,1999) for the same plutons. Thus, as summarizedby Kwon and Ree (1997), the movement age of theHonam shear zone is considered to be ca. 180 Ma,which is in conflict with the above mentionedsecond assumption.

Together with the geochronological resultsdescribed above, our compiled isotope data thatdenote a similar block affinity between theGyeonggi and Yeongnam massifs preclude apossibility for the presence of suture zone betweenthe two massifs. On the basis of a gross resemblancein TDM between the North China block and SouthKorea, especially the Gyeonggi massif, Chen andJahn (1998) considered the Ogcheon belt as aprobable extension of the Qinling-Dabie orogenicbelt in South Korea. Thus they implicitly supportedthe model of Li (1994) envisioning that thesubsurface position of the Chinese suture would befar south (about 32°N) of the surface boundary ofthe North and South China blocks. As shown inFig. 7, however, both the Gyeonggi and Yeongnammassifs do not show any particular affinity to eitherblock, because they have both Archean andProterozoic Nd model ages. The isotopicsimilarities of the Gyeonggi and Yeongnam massifsand intraplate rift setting of the Ogcheon belt(Chough, 1981; Cluzel et al., 1991) imply that thetwo massifs may belong to the same continentalblock. Therefore the Ogcheon belt located betweenthe two massifs cannot be the eastern continuationof the Chinese collision belt. This argument leavesthe Imjingang belt as the only option for the suturezone in the Korean peninsula.

However, there still remains the question‘‘Does South Korea belong to the North orSouth China block?’’ The vastness and geologiccomplexity of China make it difficult to compareKorean and Chinese basements directly. Forexample, the size of the Gyeonggi or Yeongnammassif is comparable with only a part of China,e.g., the Hubei province in the South Chinablock where Archean ages are well documented(Chen and Jahn, 1998). Our compiled isotopedata refute the presence of the suture betweenthe Gyeonggi and Yeongnam massifs but wecannot draw a definitive conclusion as to thequestion of the tectonic relationship betweenSouth Korea and China. Considering that boththe Archean and Proterozoic Nd model agesare reported in the South China block, wemight need much more work in comparing thegeology of South Korea and the South Chinablock. Alternatively, there is a possibility that

C.-S. Cheong et al. / Precambrian Research 102 (2000) 207–220218

South Korea is a separate microcontinent ac-creted to China as suggested by Lee and Cho(1995), because Nd isotope data of the two SouthKorean Precambrian massifs do not show anyparticular affinity with either Chinese block.

5. Concluding remarks

Our Pb isotope data yield relatively well-definedPaleoproterozoic ages for basement rocks fromthe Pyeonghae area, northeastern Yeongnam mas-sif, South Korea. The intrusion age of thePyeonghae gneiss is reported at 2093986 Mafrom whole rock Pb–Pb plot. The PbSL data ofmetamorphic garnet from the Wonnam groupyield a 207Pb/206Pb age of 1840926 Ma, which isconsidered to represent the timing of amphiboliteto upper amphibolite facies regional metamor-phism. Our Pb isotopic ages confirm the signifi-cance of the 2.1 Ga and 1.8 Ga episodes that havebeen broadly constrained in the Yeongnam mas-sif. The age constraints and Nd isotopic signa-tures clearly preclude a direct derivation of thePyeonghae gneiss from nearby Wonnam group,instead implying the presence and involvement ofthe older, probably late Archean crust during the2.1 Ga magmatism in the northeastern Yeongnammassif. Our compiled Pb isotope data for base-ment rocks from the Gyeonggi and Yeongnammassifs define ca. 2.0 Ga Pb-Pb age, indicatingtheir common crustal evolution process for thepast two billion years. This and compiled Ndisotope data do not support the traditional ideathat the Gyeonggi and Yeongnam massifs arerespectively correlated with the South and NorthChina blocks. The existence of Archean crusts inSouth Korea is highly probable based upon Ndmodel ages of basement rocks from both massifs.Together with recent geochronological result forthe Honam shear zone, our compiled isotope datapreclude the presence of a major suture zonebetween the two South Korean Precambrian mas-sifs. So, the Imjingang belt remains as the onlyoption for the suture zone. In order to answer thequestion if South Korea belongs to the South orNorth China block, we need much more work incomparing the geologic history of South Korea

with that of the South China block, because it isin the South China block that both Archean andProterozoic Nd model ages are reported.

Acknowledgements

This research is supported by Korea Basic Sci-ence Institute (KBSI) and Korea Institute of Nu-clear Safety (KINS) to C.-S. Cheong, and byKorea Science and Engineering Foundation grant97-07-03-01-01-3 and Basic Science Research In-stitute grant BSRI-97-5403 to S.-T. Kwon. Theauthors sincerely appreciate J.D. Kramers andC.Y. Lan for their careful and constructive re-views which improved the manuscript signifi-cantly. B.U. Chang, S.H. Lee, H. Sagong andS.R. Lee are acknowledged for their help in exper-imental works and field survey.

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