Directory UMM :Data Elmu:jurnal:P:Precambrian Research:Vol102.Issue3-4.2000:

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Pb and Nd isotopic constraints on Paleoproterozoic crustal

evolution of the northeastern Yeongnam massif, South

Korea

Chang-Sik Cheong

_{*, Sung-Tack Kwon}

_{, Kye-Hun Park}

a_{Isotope Research Team,}_{Korea Basic Science Institute,}₅₂_{Eoeun Dong,}_{Yusung Ku,}_Taejeon₃₀₅_-₃₃₃_,_{South Korea} b_{Department of Earth System Sciences,}_{Yonsei Uni}

6ersity,Seoul120-749,South Korea

c_{Department of Applied Geology,}_{Pukyong National Uni}

6ersity,Pusan608-737,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 Pyeonghae area, northeastern Yeongnam massif, South Korea. The PbSL (lead step-leaching) garnet data of the Wonnam group (Precambrian metasediments) yield a 207_Pb_/206_{Pb age of 1840}₉_{26 Ma, which can be regarded as the timing of}

amphibolite to upper amphibolite facies metamorphism and associated garnet growth. Whole rock data for the Pyeonghae gneiss intruding the Wonnam group give a207_Pb_/206_{Pb age of 2093}₉_{86 Ma, denying the possibility of a}

direct link between the intrusion of the Pyeonghae gneiss and the regional metamorphism of the Wonnam group. Our results confirm the significance of the 2.1 Ga and 1.8 Ga episodes that have been broadly constrained in the Yeongnam 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 isotopic signature also precludes a direct derivation of the Pyeonghae gneiss from the Wonnam Group, instead implying the presence and involvement of the older, probably late Archean crustal materials during the 2.1 Ga magmatism in the northeastern Yeongnam massif. Compiled Pb and Nd isotope data from the Yeongnam and Gyeonggi massifs suggest a similar geologic history for them, arguing against the conventional idea that the Gyeonggi and Yeongnam massifs are separate continental blocks respectively correlated to the South and North China blocks. The whole rock Pb isotope data of basement rocks from the two massifs form a well defined 207_Pb_/206_{Pb linearity of around 2.0 Ga,}

suggesting their common crustal evolution process for the past two billion years. A broad coincidence of major tectonic episodes in the two massifs is confirmed by reviewed geochronological data. The Nd model ages of basement rocks from the two massifs support a probable existence of Archean crusts in South Korea. The Nd model ages, both Archean 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 isotope data together with recent estimation for the age of the Honam shear zone appear to refute the presence of suture zone between 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:[email protected] (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 et al., 1993, 1996; Li et al., 1993; Yin and Nie, 1993; Li, 1994; Ernst and Liou, 1995) have generated a growing 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 the Ogcheon belt (Fig. 1A) have been proposed as possible candidates for the eastern continuation of the Chinese collision belt (Liu, 1993; Yin and Nie, 1993; Ernst and Liou, 1995; Chang, 1996; Ree et al., 1996).

Although the characteristics of Korean base-ment rocks could be potentially important to this kind of debate, there still remain many ambigui-ties regarding their ages, isotopic signatures, and tectono-metamorphic evolution processes. On the basis 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 tectonic provinces (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 ductile shear zone, called the Honam shear zone (HSZ). (B) The distribution of Precambrian basement rocks in South Korea. (C) Schematic geologic 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 idea provided a principal background for later tectonic interpretations of the Korean peninsula by Cluzel et al. (1991) and Yin and Nie (1993). Cluzel et al. (1991) suggested that the Gyeonggi massif and the Ogcheon belt of South China affinity have been juxtaposed with the Yeongnam massif of North China affinity as a result of Triassic dextral dis-placement of the order of 200 km along the Honam shear zone. Yin and Nie (1993) adopted Cluzel et al. (1991)’s idea and further proposed an indentation model for explaining the diachronic nature of the Chinese collision belt and develop-ment of the Tan-lu and Honam fault systems. If Kobayashi (1966)’s scheme is valid, it is expected that the two massifs are different in terms of isotopic signatures and ages of crustal formation and tectono-metamorphic events, considering a presumed distinction between the North and South China blocks (Ma and Wu, 1981; Jahn et al., 1990; Zhang et al., 1997; Chen and Jahn, 1998).

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

2. Geologic setting

The Korean peninsula can be divided into seven major tectonic provinces: i.e. from northwest to southeast, the Precambrian Nangrim massif, the Paleozoic Pyeongnam basin, the Paleozoic Imjin-gang belt, the Precambrian Gyeonggi massif, the late Precambrian to Paleozoic Ogcheon belt, the Precambrian Yeongnam massif, and the

Creta-ceous Gyeongsang basin (Fig. 1A). The Gyeonggi and Yeongnam massifs constitute the Precam-brian basement in the southern Korean peninsula, and consist primarily of high-grade gneisses and schists. 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 to the south (Kwon et al., 1995; Ree et al., 1995). The boundary between the Yeongnam massif and the Ogcheon belt is a dextral strike-slip ductile shear zone called the Honam shear zone (Yanai et al., 1985; Cluzel et al., 1991), which is overlain unconformably by the Gyeongsang basin. How-ever, many parts of the tectonic boundaries are obscured by extensive intrusions of Mesozoic granites. The two belts comprise highly deformed meta-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 have developed in a failed intracontinental rift setting during early Paleozoic time and therefore cannot be a suture zone (Chough, 1981; Cluzel et al., 1991). Recently, Lee et al. (1998) reported a late Precambrian age for a metavolcanic rock in the Ogcheon belt. Ree et al. (1996) showed from structural, metamorphic and geochronological studies that the Imjingang belt is a possible candi-date for the suture zone extending from the Sulu belt in China. The Gyeongsang basin is

covered with volcano-sedimentary sequences

(the Gyeongsang supergroup) and basement rocks are rarely exposed.

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

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

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

3. Samples and experimental procedures

Pb and Nd whole rock isotopic compositions were 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 schist collected from the central part of the Wonnam group (Fig. 1C). The garnet ranges from 1 to 4 mm in diameter. The garnets are

predomin-antly almandine-pyrope solid solutions with

minor spessartine and grossular components (Alm66 – 71Pyr17 – 25Spe1.7 – 2.7Gro4.4 – 9.1). Pure garnet

separates were hand-picked under a binocular mi-croscope from rock fragments ranging from 20 to 60 mesh in size. Garnet separates were repeatedly

rinsed with acetone and Millipore® _{water in an}

ultrasonic cleaner for 30 min.

All the analyses including chemical separation and mass spectrometry were performed at the Korea Basic Science Institute. About 100 mg of

rock powder was mixed with a 150_{Nd –}149_Sm

mixed spike and then dissolved with a mixed acid

(HF: HClO4: HNO3=4:1:1) in Teflon vessels.

REE (rare earth element) fractions were collected by the conventional cation column chemistry. Sm and Nd fractions were separated from each other

by the second step cation column chemistry using 0.2 M HIBA (alpha-hydroxy-iso butyric acid) (Makishima et al., 1993).

Three 120°C leaching steps were performed on the garnet separate. The first step was treatment

with a mixed acid of 12:1 1N HBr+2N HCl for

30 min. The second and third steps were per-formed with 4.5N HBr for 3 h and 9N HBr for 18

h, respectively. 30 ml Savillex®_{screw-top beakers}

were used in the leaching experiment. The residue was rinsed three times with purified water and dried between steps. Sm, Nd, Th, and U concen-trations of the leachates were measured using a

VG PQ III®

inductively coupled plasma mass spectrometer (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 was separated 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 and Pb isotopic compositions were measured with dynamic and static modes, respectively. The

143

Nd/144

Nd ratios were normalized to

146_Nd_/144_Nd₌_{0.7219, and further corrected for}

Nd contribution from added spikes. Replicate

analyses of La Jolla Nd gave 143

Nd/144

Nd=

0.51183390.000005 (2s_m, N=13). The Pb

iso-tope ratios were corrected for instrumental fractionation using average measured values of the NBS 981 standard. The measured isotopic ratios of the NBS 981 showed mass fractionation of around 0.1% per atomic mass unit relative to the recommended value. Total blank levels were about 10 pg for Sm and 50 pg for Nd. Pb blanks were about 0.3 ng for the PbSL and below 1 ng for the whole rock experiment. Isochron parame-ters were calculated using the computer program of Ludwig (1994). In the isochron calculation, we

assumed 2s error of 0.1% (=external

reproduci-bility of NBS981 data, N=13) for most of207_Pb_/

204_{Pb and}206_Pb

/204_{Pb data because internal errors}

for individual data were smaller than 0.1%. Only for the third leaching step data, internal errors of

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Table 1

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

206_Pb_/204_Pb ₉₂

s%a 207Pb/204Pb 92s%a

Rock types 208_Pb_/204_Pb

Sample 92s%a

Wonnam group

30.303 17.215

Amphibolite

YH01 40.009

16.770 15.638

PH03 Amphibolite 35.673

16.833 15.599

Biotite gneissb

YH03 36.265

22.995

YH04 Biotite gneissb _16.347 _42.951

17.782 15.703

Garnet-mica schist

PH13 40.955

17.177 15.563

PH14 Garnet-mica schist 39.488

20.565 16.056

Garnet-mica schist

PH25-1 41.777

Garnet-mica schist

PH26 20.066 15.842 39.821

Pyeonghae gneiss

PH04 Augen gneiss 20.674 16.129 41.915

22.383 16.319 42.566

PH09 Augen gneiss

19.345 15.950

Porphyroblastic gneiss

PH11 42.969

19.585 15.970

PH15 Porphyroblastic gneiss 42.316

17.038 15.640

Augen gneiss

PH19 37.303

23.945 16.545

PH20 Augen gneiss 49.406

PbSL for garnet in PH13

17.758 0.06 15.687

Leaching step[1] 0.06 40.946 0.06

20.636 0.07 15.979 0.07

Leaching step[2] 49.740 0.06

Leaching step[3] 79.447 0.25 22.617 0.24 236.273 0.24

26.094 0.09 16.582 0.09 50.851 0.09 Unleached 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 PbSL gave a very poor signal during the mass spectro-metric run, probably indicating little Pb remained after the leaching. So no Pb isotopic data are reported for the residue. Errors of calculated ages were 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 isotopic data are listed in Table 2. Chondritic uniform reservoir (DePaolo and Wasserburg, 1976) for the

calculation of oNd values is assumed to have the

present values of 143_Nd

/144_Nd

=0.512638 and

147_Sm_/144_Nd₌_{0.1967. The depleted mantle model}

age (TDM) is calculated after Na¨gler and Kramers

(1998).

4.1. Pb isotopes and geochronology

4.1.1. PbSL results of garnet from the Wonnam group (PH13)

The spread of Pb isotope ratios of the leachates is considerable as shown in Table 1 and Fig. 2. The least radiogenic lead was released in the first step. Increasingly radiogenic leads were recovered from the second and third steps. A good linearity of data for the leachates, whole rock, and

un-leached garnet in 207

Pb/204

Pb versus 206

Pb/204

Pb plot indicates an initial Pb isotopic equilibrium

among them, and yields a date of 1840926 Ma

(MSWD=13.8) (Fig. 2). The Pb isotopic spread

in the leaching experiment of garnet can be at-tributed to the presence of heterochemical inclu-sions or different behavior of common and radiogenic lead in lattice siting and ionic charge (Frei and Kamber, 1995; Dewolf et al., 1996; Frei et al., 1997). In fact, microinclusions such as

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monazite, zircon, thorianite, rutile and ilmenite have been identified in PH13 garnet by electron

microprobe analyses. The208_Pb

/206_{Pb trend of the}

leachates (Fig. 2) corresponds to a high Th/U of

10.6. This trend appears to be dominated by

monazite (Th/U\3; Dewolf et al., 1996) and

possibly by thorianite for which no Th/U data are

available. It seems that our leaching steps selec-tively dissolved inclusions with high Th/U ratios, because unleached garnet data plot below the

208

Pb/206

Pb trend of the leachates (Fig. 2). Ele-mental ratios of the leachates confirm the

pres-ence of high Th/U inclusions (i.e. monazite and

thorianite) and their release during acid leaching. Monazite, garnet, and zircon have distinct fields

in terms of Sm/Nd, U/Nd, and Th/U ratios

(De-wolf et al., 1996). Sm/Nd, Th/U, and Nd/U ratios of the leachates are listed in Table 3. The leachates have a strong affinity with monazite in U/Nd versus Sm/Nd and Nd/U versus Th/U plots (Fig. 3). The effect of zircon dissolution is not visible either in the208_Pb_/206_{Pb trend or in}

elemen-tal ratios of the leachates, probably because we

did not use HF in the leaching step. Our 18409

26 Ma date is concordant with previously re-ported age data for the Yeongnam massif (see Table 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 system is considered to be higher than 800°C (Mezger et al., 1989, 1991). Although the leads from step-leaching are dominantly coming from microinclu-sions, we may use the blocking temperature of garnet because diffusion of the leads in microin-clusions would be ultimately governed by the garnet structure. We obtained a peak metamor-phic temperature of 600 – 650°C for PH13 garnet from our preliminary microprobe work, which agrees well with the qualitative estimate of Kim et al. (1991). Because the blocking temperature is considered to be higher than the metamorphic temperature, we think that the PbSL date repre-sents the time of garnet growth. We interpret that

our 1840926 Ma age represents the time of

amphibolite to upper amphibolite facies regional metamorphism and associated garnet growth of the Wonnam group.

Table 2

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

143_Nd_/144_Nda _{Sm (ppm)} _o_{Nd(2.1 Ga)}

Sample Nd (ppm) 147Sm/144Ndb oNd(0) TDM(Ga)c

Wonnam group

0.512593 (7) 1.71 5.54

YH01 0.1868 −0.9 2.55 1.78

(7) 1.56 4.68 0.2012 3.4 2.72

PH03 0.512814 2.22

(10) 3.93 17.78 0.1337 −18.3 2.56

YH03 0.511700 −1.37

−1.03 2.47

−21.9 0.1191

29.41

YH04 0.511516 (6) 5.79

0.511539 (6) 5.40 26.82 0.1217 −21.4 2.50 −1.29 PH13

0.511310 (5) 6.51 34.73

PH25-1 0.1134 −25.9 2.63 −3.52

−1.62 2.48

−25.2

0.511349 0.1092

PH26 (19) 5.11 28.33

Pyeonghae gneiss

PH04 0.511290 (5) 8.07 43.02 0.1134 −26.3 2.66 −3.93

0.511272 (6) 6.41 34.62

PH09 0.1119 −26.6 2.65 −3.87

PH11 0.511151 (5) 7.05 39.87 0.1070 −29.0 2.70 −4.91

PH15 0.511495 (5) 8.13 38.77 0.1268 −22.3 2.71 −3.53

0.511251

PH17 (5) 6.78 38.20 0.1073 −27.1 2.57 −3.04

0.511279 (5) 5.23

PH20 28.28 0.1119 −26.5 2.64 −3.73

a_{Numbers in parenthesis refer to least significant digits and} ₉₂ smean. b_{Uncertainty is below 0.5%, checked by duplicate analysis.}

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Fig. 2. Pb isotopic plots of garnet PbSL leachates, host whole rock, and unleached garnet from PH13 sample. The 207_Pb_/ 206_{Pb slope defined by them corresponds to 1840}₉_{26 Ma}

(MSWD=13.8). The208_Pb_/206_{Pb trend of the PbSL leachates}

yields a Th/U ratio of 10.6. Note that data of unleached garnet plot below the leachates trend in 208_Pb_/204_{Pb versus} 206_Pb_/ 204_{Pb plot. Abbreviations: WR; whole rock, Bulk; unleached}

garnet bulk, [1]; step 1 leachate, [2]; step 2 leachate, [3]; step 3 leachate.

older than the metamorphic age of the Wonnam group. Because whole rock Pb – Pb dates can be generally interpreted as representing intrusion ages of granitic bodies (Moorbath and Taylor, 1985), we consider the Pb – Pb date as an intrusion age of the Pyeonghae gneiss. As shown in Fig. 4, the data for whole rock samples of the Wonnam

group are scattered around the 207_Pb

/206_{Pb trend}

of the Pyeonghae gneiss. The poor linear trend of the Wonnam group samples yields a slightly

younger age (19839190 Ma, MSWD=92.5)

than the Pyeonghae gneiss, probably suggesting that the Pb isotopic system of the Wonnam group

Fig. 3. Nd/U versus Th/U and U/Nd versus Sm/Nd plots showing fields of zircon, monazite, and garnet (reviewed by Dewolf et al., 1996). The PbSL leachates of PH13 garnet have similar chemical compositions to monazite.

Table 3

Elemental ratios of the leachates from PH13 garnet

Th/U Nd/U Sm/Nd

9.86

0.319 16.82

Step [1]

12.93 22.97 Step [2] 0.208

31.58 10.71

0.374 Step [3]

4.1.2. Pb-Pb whole rock age of the Pyeonghae gneiss

Pb isotopic compositions of whole rocks from the Pyeonghae gneiss (Table 1) yield a207_Pb_/206_Pb

age of 2093986 Ma (MSWD=3.27) with model

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Table 4

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

Methodology Age (Ma)

Locality Lithology References

Gyeonggi massif

U-Pb zircon 2150920

Granitic gneiss Gaudette and Hurley (1973)

Yoogoo

U-Pb zircon 1766926

Seosan Granitic gneiss Turek and Kim (1996)

Sm-Nd minerals 18979120

Granulite Lee et al. (1997)

Hwacheon

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

Yeongnam massif

Sm-Nd minerals

Jirisan Anorthosite 1678990 Kwon and Jeong (1990)

Sm-Nd whole rocks 1047969

Biotite gneiss Lee et al. (1992)

Kimcheon

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

Granite Park et al. (1993)

Taebaegsan

Leucogneiss

Imwon Sm-Nd minerals 225094 Lee et al. (1994)

Danyang Granitic gneiss Pb-Pb whole rocks 21609150 Kwon et al. (1995) U-Pb zircon 2120920

Granitic gneiss Turek and Kim (1996)

Kurye

Porphyroblastic gneiss

Kurye U-Pb zircon 194595 Turek and Kim (1996)

U-Pb zircon 1923914

Chailbong Granitic gneiss Turek and Kim (1996)

Sm-Nd garnet 1820911

Charnockite Kim et al. (1998)

Jirisan

Charnockite

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

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

4.1.3. Precambrian geochronology of South Korean basement rocks: a brief summary

Previous geochronological studies have revealed three major episodes (i.e. ca. 2.1 Ga, 1.9 Ga, and 1.0 Ga) of magmatic activity and one episode of metamorphism (ca. 1.8 Ga) in the other part of the Yeongnam massif (Table 4). The 2.1 Ga mag-matic episode, which could be correlated with the Wutaian orogeny in China (ca. 2.2 Ga, Yang et al., 1986), is well constrained in the Yeongnam massif. Our Pb – Pb age of the Pyeonghae gneiss confirms the significance of the 2.1 Ga magma-tism in the Yeongnam massif.

As stated earlier, the comparison of magmatic and metamorphic ages between the Gyeonggi and Yeongnam massifs is important in searching for an 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 episodes of the Yeongnam massif (Table 4), although more data are needed. The Pb – Pb isotope plots for sialic 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 the Pyeonghae gneiss and the Wonnam group. Data of the Pyeonghae gneiss samples yield a207_Pb_/206_{Pb age of 2093}₉₈₆

Ma (MSWD=3.27) with modelm1 value of 8.69. The data of the Wonnam group are scattered around the207_Pb_/206_{Pb trend}

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Fig. 5. Compiled Pb isotope data for basement rocks from the Gyeonggi and Yeongnam massifs show a good linearity (R2₌

0.996) corresponding to about 2.0 Ga in 207_Pb_/204_{Pb versus} 206_Pb_/204_{Pb plot. The evolution curve of an average crust (S}

and K; Stacey and Kramers, 1975) is shown for references. No correlation is found in 208_Pb_/204_{Pb versus} 206_Pb_/204_{Pb plot.}

Data sources; Gyeonggi massif (Park et al., 1995; Cheong and Chang, 1997), Yeongnam massif (Park et al., 1993; Kwon et al., 1995; this study).

lap in 208_Pb

/204_{Pb versus} 206_Pb

/204_{Pb plot. As}

shown in Fig. 5, the Pb line for South Korean basement rocks plots significantly above the aver-age crustal Pb growth curve of Stacey and Kramers (1975), which is a common characteristic of old upper crust (Zartman and Doe, 1981; Tilton, 1983). Our compiled Pb isotope data indi-cate that the Gyeonggi and Yeongnam massifs share a common precursor and have evolved to-gether for the past two billion years. This does not support the idea that they are genetically separate blocks as suggested by Kobayashi (1966). 4.2. Nd isotopic signatures

4.2.1. Archean crust in South Korea

As shown in Table 2, the Pyeonghae gneiss

samples have slightly lower oNd values than the

Wonnam group samples, but their Sm/Nd ratios

are indistinguishable from each other except two amphibolite samples (YH01, PH03). Accordingly,

TDM of the former is slightly but distinctly older

than 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 higher

oNd(2.1 Ga) values and younger TDM than the Pyeonghae

gneiss samples. Two amphibolite samples of the Wonnam group have positiveoNd(2.1 Ga) values. The evolution curve for the depleted mantle (DM) (Na¨gler and Kramers, 1998) is shown relative to the chondritic uniform reservoir (CHUR). supracrustal rocks and mafic intrusives are not

included in the diagram, because our main inter-est lies in looking at the intrinsic characteristic of

the basement. A good linearity (R2

=0.996,

slope=0.1207) is observed in 207_Pb

/204_{Pb versus}

206_Pb_/204_{Pb diagram for Korean basement rocks,}

which yields an apparent 207_Pb_/206_{Pb age of ca.}

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over-Fig. 7. Compiled Sm – Nd isotope data for basement rocks from the Gyeonggi and Yeongnam massifs. Reference lines for Nd model ages are drawn by an approximation of the model of Na¨gler and Kramers (1998). Chinese data are also shown for 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., unpublished data) may represent a mantle component during the intrusion of the Pyeonghae gneiss. With this information, we can qualitatively estimate that the residence age of involved crustal materials should be older than the Nd model age (2.71 – 2.57 Ga) of the Pyeonghae gneiss (Fig. 6). The role of crustal materials older than the Wonnam group is impor-tant even when the Pyeonghae gneiss is originated from purely reworked crustal materials without any input from the mantle. Therefore it can be concluded that the Nd isotopic signatures indicate the presence and involvement of late Archean crust during the 2.1 Ga magmatism in the north-eastern Yeongnam massif.

Lan et al. (1995) argued for a possible existence of Archean crust in South Korea based upon Nd model ages. The inheritance of Archean crustal materials was confirmed by a U-Pb zircon upper

intercept age (32949196 Ma) for a Proterozoic

gneiss in the Gyeonggi massif (Turek and Kim, 1996). Our compiled data also show common Archean model ages of basement rocks from both the Gyeonggi and Yeongnam massifs (Fig. 7). They are mostly Proterozoic in crystallization or metamorphic ages (Table 4). It is not surprising that some Proterozoic basement rocks have Archean model ages because they show, in many

cases, negative oNd(t) values and thus are

be-lieved to be originated from pre-existing crustal materials (for example, Hong et al., 1996). Those metasedimentary rocks intruded by early Protero-zoic granitic gneisses (Table 4) in the Gyeonggi and Yeongnam massifs may well be Archean in age.

4.2.2. Comparison with Chinese data

Available isotope data (Huang et al., 1986; Jahn et al., 1988; Sun et al., 1992; Chen and Jahn, 1998) confirm a conventional idea that the North China 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 and

Cathaysia blocks (Chen and Jahn, 1998). How-ever, Lan et al. (1995) showed that the North and hae gneiss samples have rather tightly constrained

late Archean model ages ranging from 2.71 Ga to

2.57 Ga. Consistently negativeoNd(2.1 Ga) values

of the Pyeonghae gneiss (−4.91−3.04)

indi-cate important contributions of pre-existing conti-nental material to its sources. The possibility of a direct derivation of the Pyeonghae gneiss from the Wonnam group can be easily excluded by higher

oNd(2.1 Ga) values of the latter and age

con-straints described earlier. The metasedimentary rocks from the Wonnam group also have

re-stricted TDMvalues ranging from 2.63 Ga to 2.47

Ga. Generally the model ages of sedimentary rocks are older than the depositional ages (Allegre and Rousseau, 1984), and thus the formation age of the Wonnam group can be constrained between 2.63 Ga to 2.47 Ga (Nd model age) and 2.1 Ga (intrusion age of the Pyeonghae gneiss), i.e. latest

Archean to early Proterozoic. The range of oNd

(2.1 Ga) of the Pyeonghae gneiss can be explained by an important involvement of pre-existing crustal materials older than the Wonnam group, either as a primary source material or crustal

contaminant. Positive oNd (2.1 Ga) values

to-gether with low REE abundances, and flat

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South China blocks have overlapping Nd model ages, and Chen and Jahn (1998) confirmed the existence of Archean rocks along the northern margin of the South China block. Also shown in

Fig. 7 are compiled TDM values of the basement

rocks from both the Gyeonggi and Yeongnam massifs. The Nd model ages, both Archean and Proterozoic, of the two massifs agree with neither those of the North China block (predominantly Archean) nor those of the South China block (predominantly Proterozoic). As a whole, the Yeongnam massif is younger than the Gyeonggi massif in Nd model ages, but their considerable overlap corroborates the similarity of crustal his-tories concluded from Pb isotope data described previously.

4.3. Tectonic implications

Our compiled Pb and Nd data do not support a previous idea that the Gyeonggi and Yeongnam massifs in South Korea are different continental blocks. This observation contradicts a basic assumption of tectonic models suggested by Cluzel et al. (1991) and Yin and Nie (1993). Two basic assumptions of their models are: (1) different block affinity of the Gyeonggi and Yeongnam massifs; and (2) Triassic age of the dextral movement along the Honam shear zone. Our compiled isotope data do not support the first assumption. Cluzel et al.

(1991) constrained the age of the dextral

movement along the Honam shear zone as Triassic by Choo and Kim (1986)’s Rb-Sr whole rock age

of 21193 Ma for the undeformed synkinematic

granite (the so-called Namweon granite).

However, recent chronological data cast doubt on the validity of Choo and Kim (1986)’s result. Kim and Turek (1996) confined the movement age of the Honam shear zone between 183 Ma to 176 Ma on the basis of U-Pb zircon ages for deformed and undeformed granitic rocks. Their conclusion was corroborated by monazite CHIME (chemical Th-U-total Pb isochron method) data (Cho et al., 1999) for the same plutons. Thus, as summarized by Kwon and Ree (1997), the movement age of the Honam shear zone is considered to be ca. 180 Ma, which is in conflict with the above mentioned second assumption.

Together with the geochronological results described above, our compiled isotope data that denote a similar block affinity between the Gyeonggi and Yeongnam massifs preclude a possibility for the presence of suture zone between the two massifs. On the basis of a gross resemblance

in TDMbetween the North China block and South

Korea, especially the Gyeonggi massif, Chen and Jahn (1998) considered the Ogcheon belt as a probable extension of the Qinling-Dabie orogenic belt in South Korea. Thus they implicitly supported the model of Li (1994) envisioning that the subsurface position of the Chinese suture would be far south (about 32°N) of the surface boundary of the North and South China blocks. As shown in Fig. 7, however, both the Gyeonggi and Yeongnam massifs do not show any particular affinity to either block, because they have both Archean and

Proterozoic Nd model ages. The isotopic

similarities of the Gyeonggi and Yeongnam massifs and intraplate rift setting of the Ogcheon belt (Chough, 1981; Cluzel et al., 1991) imply that the two massifs may belong to the same continental block. Therefore the Ogcheon belt located between the two massifs cannot be the eastern continuation of the Chinese collision belt. This argument leaves the Imjingang belt as the only option for the suture zone in the Korean peninsula.

However, there still remains the question ‘‘Does South Korea belong to the North or South China block?’’ The vastness and geologic complexity of China make it difficult to compare Korean and Chinese basements directly. For example, the size of the Gyeonggi or Yeongnam massif is comparable with only a part of China, e.g., the Hubei province in the South China block where Archean ages are well documented (Chen and Jahn, 1998). Our compiled isotope data refute the presence of the suture between the Gyeonggi and Yeongnam massifs but we cannot draw a definitive conclusion as to the question of the tectonic relationship between South Korea and China. Considering that both the Archean and Proterozoic Nd model ages are reported in the South China block, we might need much more work in comparing the geology of South Korea and the South China block. Alternatively, there is a possibility that

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

5. Concluding remarks

Our Pb isotope data yield relatively well-defined Paleoproterozoic ages for basement rocks from the Pyeonghae area, northeastern Yeongnam mas-sif, South Korea. The intrusion age of the

Pyeonghae gneiss is reported at 2093986 Ma

from whole rock Pb – Pb plot. The PbSL data of metamorphic garnet from the Wonnam group yield a207_Pb

/206_{Pb age of 1840}₉_{26 Ma, which is}

considered to represent the timing of amphibolite to 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 have been broadly constrained in the Yeongnam mas-sif. The age constraints and Nd isotopic signa-tures clearly preclude a direct derivation of the Pyeonghae gneiss from nearby Wonnam group, instead implying the presence and involvement of the older, probably late Archean crust during the 2.1 Ga magmatism in the northeastern Yeongnam massif. Our compiled Pb isotope data for base-ment rocks from the Gyeonggi and Yeongnam massifs define ca. 2.0 Ga Pb-Pb age, indicating their common crustal evolution process for the past two billion years. This and compiled Nd isotope data do not support the traditional idea that the Gyeonggi and Yeongnam massifs are respectively correlated with the South and North China blocks. The existence of Archean crusts in South Korea is highly probable based upon Nd model ages of basement rocks from both massifs. Together with recent geochronological result for the Honam shear zone, our compiled isotope data preclude the presence of a major suture zone between the two South Korean Precambrian mas-sifs. So, the Imjingang belt remains as the only option for the suture zone. In order to answer the question if South Korea belongs to the South or North China block, we need much more work in comparing the geologic history of South Korea

with that of the South China block, because it is in the South China block that both Archean and Proterozoic 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 by Korea Science and Engineering Foundation grant 97-07-03-01-01-3 and Basic Science Research In-stitute grant BSRI-97-5403 to S.-T. Kwon. The authors sincerely appreciate J.D. Kramers and C.Y. Lan for their careful and constructive re-views which improved the manuscript signifi-cantly. B.U. Chang, S.H. Lee, H. Sagong and S.R. Lee are acknowledged for their help in exper-imental works and field survey.

References

Adachi, M., Suzuki, K., Chwae, U.C., 1996. CHIME age determination of metamorphic rocks in the Okchon Belt. Korea. Geol. Soc. Japan Abstr. Prog. 103, 80 abstract. Allegre, C.J., Rousseau, D., 1984. The growth of the continent

through time studied by Nd isotope analyses of shales. Earth Planet Sci. Lett. 67, 19 – 34.

Ames, L., Tilton, G.R., Zhou, G., 1993. Timing of collision of the Sino-Korean and Yangtse cratons: U-Pb zircon dating of coesite-bearing eclogites. Geology 21, 339 – 342. Ames, L., Zhou, G., Xiong, B., 1996. Geochronology and

isotopic character of ultrahigh-pressure metamorphism with implications for collision of the Sino-Korean and Yangtze cratons, central China. Tectonics 15, 472 – 489. Chang, E.Z., 1996. Collision orogene between north and south

China and its eastern extension in the Korean Peninsula. J. SE. Asian Earth Sci. 13, 267 – 277.

Chen, J., Jahn, B.m., 1998. Crustal evolution of southeastern China: Nd and Sr isotopic evidence. Tectonophysics 284, 101 – 133.

Cheong, C.S., Chang, H.W., 1997. Sr, Nd, and Pb isotope systematics of granitic rocks in the central Ogcheon Belt, Korea. Geochem. J. 31, 17 – 36.

Cho, K.H., Takagi, H., Suzuki, K., 1999. CHIME monazite age of granitic rocks in the Sunchang shear zone, Korea: timing of dextral ductile shear. Geosci. J. 3, 1 – 15. Choo, S.H., Kim, S.J., 1986. Rb-Sr age determinations in the

Ryeongnam massif (II): granitic gneisses and gneissose granites in the south-western Jirisan region. Korea Inst. Energy Resoures Rep. 86 – 7, 7 – 33.

(13)

Chough, S.K., 1981. Submarine debris flow deposits in the Ogcheon Basin, Korean Peninsula, U.N. ESCAP, CCOP. Tech. Bull. 14, 17 – 29.

Cluzel, D., Lee, B.J., Cadet, J.P., 1991. Indosinian dextral ductile fault system and synkinematic plutonism in the southwest of the Ogcheon belt (South Korea). Tec-tonophysics 194, 131 – 151.

DePaolo, D.J., Wasserburg, G.J., 1976. Nd isotopic variations and petrogenetic models. Geophys. Res. Lett. 3, 249 – 252. Dewolf, C.P., Zeissler, C.J., Halliday, A.N., Mezger, K., Es-sene, E.J., 1996. The role of inclusions in U-Pb and Sm-Nd garnet geochronology: Stepwise dissolution experiments and trace uranium mapping by fission track analysis. Geochim. Cosmochim. Acta 60, 121 – 134.

Ernst, W.G., Liou, J.G., 1995. Contrasting plate-tectonic styles of the Qinling-Dabie-Sulu and Franciscan metamor-phic belts. Geology 23, 353 – 356.

Frei, R., Kamber, B.S., 1995. Single mineral Pb – Pb dating. Earth Planet. Sci. Lett. 129, 261 – 268.

Frei, R., Villa, I.M., Na¨gler, Th.F., et al., 1997. Single mineral dating by the Pb – Pb step-leaching method: assessing the mechanisms. Geochim. Cosmochim. Acta 61, 393 – 414. Gaudette, H.E., Hurley, P.M., 1973. U-Pb zircon age of

Precambrian basement gneiss of South Korea. Geol. Soc. Am. Bull. 84, 2305 – 2306.

Hong, Y.K., Lee, S.G., Lan, C.Y., 1996. Comparison between the Hongjesa granite and the Buncheon gneiss based on Sm-Nd age. Geol. Soc. Korea Abstr. Prog. 51, 100. Huang, W., Wu, Z.W., 1992. Evolution of the Qinling

oro-genic belt. Tectonics 11, 371 – 380.

Huang, X., Bi, Z., DePaolo, D.J., 1986. Sm-Nd isotope study of early Archean rocks, Qianan, Hebei Province, China. Geochim. Cosmochim. Acta 50, 625 – 631.

Hwang, J.H., Kim, D.H., Cho, D.L., Song, K.Y., 1996, Geo-logical report of the Andong sheet (1:250000). Korea Inst. Geol. Mining Materials (in Korean with English abstract). Jahn, B.m., Auvray, B., Shen, Q.H., et al., 1988. Archean crustal evolution in China: the Taishan complex, and evidence for juvenile crustal addition from long-term de-pleted mantle. Precambrian Res. 38, 381 – 403.

Jahn, B.m., Zhang, Z.Q., 1984. Archean granulite gneisses from eastern Hebei Province, China: rare earth geochem-istry and tectonic implications. Contrib. Mineral. Petrol. 85, 224 – 243.

Jahn, B.m., Zhou, X.H., Li, J.L., 1990. Formation and tec-tonic evolution of Southeastern China and Taiwan: iso-topic and geochemical constraints. Tectonophysics 183, 145 – 160.

Kim, C.B., Turek, A., 1996. Advances in U-Pb zircon geochronology of Mesozoic plutonism in the southwestern part of Ryeongnam massif, Korea. Geochem. J. 30, 323 – 338.

Kim, D.Y., Song, Y.S., Park, K.H., 1998. Petrology, geochem-istry, and geochronology of charnockite in the eastern Jirisan area. Geol. Soc. Korea Abstr. Prog. 53, 35 – 36 abstract (in Korean).

Kim, H.S., Lee, S.M., Kim, Y.K., Park, C.S., Kim, S.J., Chang, H.W., 1991. Proterozoic magmatism and metamor-phism in the north-eastern part of Korea – comparative studies between Buncheon and Pyeonghae granitic gneisses. J. Geol. Soc. Korea 27, 614 – 625 (in Korean with English abstract).

Kim, O.J., Hong, M.S., Won, J.K., Park, H.I., Park, Y.D., Kim, K.T., 1963, Geological Report of the Pyong Hae sheet (1:50000). Geol. Survey Korea.

Kobayashi, T., 1966. Stratigraphy of the chosen group in Korea and south Manchuria and its relation to the Cam-bro-Ordovician formations of other areas. J. Faculty Sci. Univ. Tokyo 2 (16), 209 – 311.

Kwon, S.T., Jeong, J.G., 1990. Preliminary Sr-Nd isotope study of the Hadong-Sanchung anorthositic rocks in Ko-rea: implications for their origin and for the Precambrian tectonics. J. Geol. Soc. Korea 26, 341 – 349.

Kwon, S.T., Ree, J.H., 1997. A note on the age of the Honam Shear Zone. J. Geol. Soc. Korea 33, 183 – 188 (in Korean with English abstract).

Kwon, S.T., Ree, J.H., Park, K.H., Jeon, E.Y., 1995. Nature of contact between the Ogcheon belt and Yeongnam mas-sif and the Pb-Pb age of granitic gneiss in Cheondong-ri, Danyang. J. Petrol. Soc. Korea 4, 144 – 152 (in Korean with English abstract).

Lan, C.Y., Lee, T., Zhou, X.H., Kwon, S.T., 1995. Nd iso-topic study of Precambrian basement of South Korea: evidence for early Archean crust? Geology 23, 249 – 252. Lee, K.S., Chang, H.W., Park, K.H., 1998. Neoproterozoic

bimodal volcanism in the central Ogcheon belt, Korea: age and tectonic implication. Precambrian Res. 89, 47 – 57. Lee, S.G., Masuda, A., Kim, H.S., 1994. An early Proterozoic

leuco-granitic gneiss with the REE tetrad phenomenon. Chem. Geol. 114, 59 – 67.

Lee, S.G., Shimizu, H., Masuda, A., Song, Y.S., 1992. Crustal evolution of the Precambrian basement in the Korean peninsula. J. Petrol. Soc. Korea 1, 124 – 131.

Lee, S.R., Cho, M., 1995. Tectonometamorphic evolution of the Chuncheon amphibolite, central Gyeonggi massif, South Korea. J. Metamorphic Geol. 13, 315 – 328. Lee, S.R., Cho, M., Cheong, C.S., Park, K.H., 1997. An early

Proterozoic Sm-Nd age of mafic granulite from the Hwacheon area, South Korea. Geosci. J. 1, 136 – 142. Li, S., Xiao, Y., Liou, D., et al., 1993. Collision of the North

China and Yangtse Blocks and formation of coesite-bear-ing eclogites: timcoesite-bear-ing and processes. Chem. Geol. 109, 89 – 111.

Li, Z.X., 1994. Collision between the North and South China blocks: a crustal-detachment model for suturing in the region east of the Tanlu fault. Geology 22, 739 – 742. Liu, D.Y., Nutman, A.P., Compston, W., Wu, J.S., Shen,

Q.H., 1992. Remnants of]3800 Ma crust in the Chinese part of the Sino – Korean craton. Geology 20, 339 – 342. Liu, D.Y., Page, R.W., Compston, W., Wu, J.S., 1985. U-Pb

zircon geochronology in the Taihangshan-Wutaishan area, North China. Precambrian Res. 27, 85 – 109.

(14)

Liu, D.Y., Shen, Q.H., Zhang, Z.Q., Jahn, B.m., Auvray, B., 1990. Archean crustal evolution in China: U-Pb geochronology of the Qianxi complex. Precambrian Res. 48, 223 – 244.

Liu, X., 1993. High-P metamorphic belt in central China and its possible eastward extension to Korea. J. Petrol. Soc. Korea 2, 9 – 18.

Ludwig, K.R., 1994. ISOPLOT — a plotting and regression program for radiogenic isotope data. version 2.71. USGS Open File Rep. 91, 445.

Ma, X.Y., Wu, Z.W., 1981. Early tectonic evolution of China. Precambrian Res. 14, 185 – 202.

Makishima, A., Nakamura, E., Akimoto, S., Campbell, I.H., Hill, R.I., 1993. New constraints on the 138_La _b_-decay

constant based on a geochronological study of the granites from the Yilgarn Block, Western Australia. Chem. Geol. 104, 293 – 300.

Mezger, K., Hanson, G.N., Bohlen, S.R., 1989. U-Pb system-atics of garnet in the late Archean Pikwitonei granulite domain at Cauchon and Natawahunan Lakes, Manitoba, Canada. Contrib. Mineral. Petrol. 101, 136 – 148. Mezger, K., Rawnsley, C.M., Bohlen, S.R., Hanson, G.N.,

1991. U-Pb garnet, sphene, monazite, and rutile ages: implications for the duration of high-grade metamorphism and cooling histories, Adirondack Mts., New York. J. Geol. 99, 415 – 428.

Min, J.H., Kwon, S.T., Cheong, C.S., 1998. Sm-Nd garnet ages reveal Proterozoic metamorphism of the Gyeonggi massif. Geol. Soc. Korea Abstr. Prog. 53, 91 – 92 abstract. Moorbath, S., Taylor, P.N., 1985. Precambrian geochronology and the geologic record. In: Snelling, N.J. (Ed.), The Chronology of the Geological Record. Blackwell Scientific Publishers, London, pp. 10 – 28.

Na, C.K., 1994, Genesis of granitoid batholiths of Okchon Zone, Korea and its implications for crustal evolution. Ph.D. Thesis, University Tsukuba, Japan.

Na¨gler, Th.F., Kramers, J.D., 1998. Nd isotopic evolution of the upper mantle during the Precambrian: models, data and the uncertainty of both. Precambrian Res. 91, 233 – 252.

Park, K.H., Cheong, C.S., Lee, K.S., Chang, H.W., 1993. Isotopic composition of lead in Precambrian granitic rocks of the Taebaeg area. J. Geol. Soc. Korea 29, 387 – 395 (in Korean with English abstract).

Park, K.H., Park, B.K., Kim, Y.J., Lee, I.S., Choi, M.S., Barg, E., Lee, K.S., Cheong, C.S., Han, J.H., Lee, S.H., Shin, H.S., Park, C.S., Kim, Y.J., 1995, A study of trace element composition and structural analysis of geologic and marine samples (I). Korea Basic Sci. Inst. Rep. 123 – 159 (in Korean with English abstract).

Ree, J.H., Park, Y.D., Kwon, S.T., 1995. The Bangdae shear zone: the boundary with the Gyeonggi massif and the Ogcheon belt. Geol. Soc. Korea Abstr. Prog. 50, 63 – 64 abstract (in Korean).

Ree, J.H., Cho, M., Kwon, S.T., Nakamura, E., 1996. Possible eastward extension of Chinese collision belt in South Ko-rea: the Imjingang belt. Geology 24, 1071 – 1074. Song, B., Nutman, A.P., Liu, D.Y., Wu, J.S., 1996. 3800 to

2500 Ma crustal evolution in the Anshan area of Liaoning Province, northeastern China. Precambrian Res. 78, 79 – 94.

Stacey, J.S., Kramers, J.D., 1975. Approximation of terrestrial lead isotope evolution by a two-stage model. Earth Planet. Sci. Lett. 26, 207 – 221.

Sun, M., Armstrong, R.L., Lambert, R.St.J., 1992. Petrochem-istry and Sr, Pb, and Nd isotopic geochemPetrochem-istry of early Precambrian rocks, Wutaishan and Taihangshan areas, China. Precambrian Res. 56, 1 – 31.

Tilton, G.R., 1983. Evolution of depleted mantle: the lead perspective. Geochim. Cosmochim. Acta 47, 1191 – 1197. Turek, A., Kim, C.B., 1996. U-Pb zircon ages for Precambrian

rocks in southwestern Ryeongnam and southwestern Gyeonggi massifs. Korea Geochem. J. 30, 231 – 249. Yanai, S., Park, B.S., Otoh, S., 1985. The Honam shear zone

(South Korea): deformation and tectonic implication in the Far East. Sci. Pap. Coll. Arts Sci. Univ. Tokyo 35, 181 – 210.

Yang, Z., Cheng, Y., Wang, H., 1986. The Geology of China. Clarendon Press, Oxford, p. 303.

Yin, A., Nie, S., 1993. An indentation model for the North and South China collision and the development of the Tan-Lu and Honam fault systems, eastern Asia. Tectonics 12, 801 – 813.

Zartman, R.E., Doe, B.R., 1981. Plumbotectonics — the model. Tectonophysics 75, 135 – 162.

Zhang, H.F., Gao, S., Zhang, B.R., Luo, T.C., Ling, W.L., 1997. Pb isotopes of granitoids suggest Devonian accretion of Yangtze (South China) craton to North China craton. Geology 25, 1015 – 1018.

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Fig. 5. Compiled Pb isotope data for basement rocks from the Gyeonggi and Yeongnam massifs show a good linearity (R2₌ 0.996) corresponding to about 2.0 Ga in 207_Pb/204_{Pb versus} 206_Pb/204_{Pb plot. The evolution curve of an average crust (S} and K; Stacey and Kramers, 1975) is shown for references. No correlation is found in 208_Pb/204_{Pb versus} 206_Pb/204_{Pb plot.} Data sources; Gyeonggi massif (Park et al., 1995; Cheong and Chang, 1997), Yeongnam massif (Park et al., 1993; Kwon et al., 1995; this study).

lap in

208

_Pb

/

204

_{Pb versus}

206

_Pb

/

204

_{Pb plot. As}

shown in Fig. 5, the Pb line for South Korean

basement rocks plots significantly above the

aver-age crustal Pb growth curve of Stacey and

Kramers (1975), which is a common characteristic

of old upper crust (Zartman and Doe, 1981;

Tilton, 1983). Our compiled Pb isotope data

indi-cate that the Gyeonggi and Yeongnam massifs

share a common precursor and have evolved

to-gether for the past two billion years. This does

not support the idea that they are genetically

separate blocks as suggested by Kobayashi (1966).

4 .

2 .

Nd isotopic signatures

4 .

2 .

1 .

Archean crust in South Korea

As shown in Table 2, the Pyeonghae gneiss

samples have slightly lower

o

Nd values than the

Wonnam group samples, but their Sm

/

Nd ratios

are indistinguishable from each other except two

amphibolite samples (YH01, PH03). Accordingly,

T

of the former is slightly but distinctly older

than 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 higher

oNd(2.1 Ga) values and younger TDM than the Pyeonghae gneiss samples. Two amphibolite samples of the Wonnam group have positiveoNd(2.1 Ga) values. The evolution curve for the depleted mantle (DM) (Na¨gler and Kramers, 1998) is shown relative to the chondritic uniform reservoir (CHUR).

supracrustal rocks and mafic intrusives are not

included in the diagram, because our main

inter-est lies in looking at the intrinsic characteristic of

the basement. A good linearity (

R

=

0.996,

slope

=

0.1207) is observed in

207

_Pb

/

204

_{Pb versus}

206

_Pb

_/

204

_{Pb diagram for Korean basement rocks,}

which yields an apparent

207

_Pb

_/

206

_{Pb age of ca.}

2.0 Ga. The data from the two massifs also

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amphibolite samples (Cheong, C.S., unpublished

data) may represent a mantle component during

the intrusion of the Pyeonghae gneiss. With this

information, we can qualitatively estimate that the

residence age of involved crustal materials should

be older than the Nd model age (2.71 – 2.57 Ga) of

the Pyeonghae gneiss (Fig. 6). The role of crustal

materials older than the Wonnam group is

impor-tant even when the Pyeonghae gneiss is originated

from purely reworked crustal materials without

any input from the mantle. Therefore it can be

concluded that the Nd isotopic signatures indicate

the presence and involvement of late Archean

crust during the 2.1 Ga magmatism in the

north-eastern Yeongnam massif.

Lan et al. (1995) argued for a possible existence

of Archean crust in South Korea based upon Nd

model ages. The inheritance of Archean crustal

materials was confirmed by a U-Pb zircon upper

intercept age (3294

9 196 Ma) for a Proterozoic

gneiss in the Gyeonggi massif (Turek and Kim,

1996). Our compiled data also show common

Archean model ages of basement rocks from both

the Gyeonggi and Yeongnam massifs (Fig. 7).

They are mostly Proterozoic in crystallization or

metamorphic ages (Table 4). It is not surprising

that some Proterozoic basement rocks have

Archean model ages because they show, in many

cases, negative

o

Nd(

t

) values and thus are

be-lieved to be originated from pre-existing crustal

materials (for example, Hong et al., 1996). Those

metasedimentary rocks intruded by early

Protero-zoic granitic gneisses (Table 4) in the Gyeonggi

and Yeongnam massifs may well be Archean in

age.

4 .

2 .

Comparison with Chinese data

Available isotope data (Huang et al., 1986;

Jahn et al., 1988; Sun et al., 1992; Chen and Jahn,

1998) confirm a conventional idea that the North

China 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

and

Cathaysia blocks (Chen and Jahn, 1998).

How-ever, Lan et al. (1995) showed that the North and

hae gneiss samples have rather tightly constrained

late Archean model ages ranging from 2.71 Ga to

2.57 Ga. Consistently negative

o

Nd(2.1 Ga) values

of the Pyeonghae gneiss (

−

4.91 −

3.04)

indi-cate important contributions of pre-existing

conti-nental material to its sources. The possibility of a

direct derivation of the Pyeonghae gneiss from the

Wonnam group can be easily excluded by higher

o

Nd(2.1 Ga) values of the latter and age

con-straints described earlier. The metasedimentary

rocks from the Wonnam group also have

re-stricted T

values ranging from 2.63 Ga to 2.47

Ga. Generally the model ages of sedimentary

rocks are older than the depositional ages (Allegre

and Rousseau, 1984), and thus the formation age

of the Wonnam group can be constrained between

2.63 Ga to 2.47 Ga (Nd model age) and 2.1 Ga

(intrusion age of the Pyeonghae gneiss), i.e. latest

Archean to early Proterozoic. The range of

o

Nd

(2.1 Ga) of the Pyeonghae gneiss can be explained

by an important involvement of pre-existing

crustal materials older than the Wonnam group,

either as a primary source material or crustal

contaminant. Positive

o

Nd (2.1 Ga) values

to-gether with low REE abundances, and flat

chon-drite-normalized

REE

patterns

of

the

two

(3)

South China blocks have overlapping Nd model

ages, and Chen and Jahn (1998) confirmed the

existence of Archean rocks along the northern

margin of the South China block. Also shown in

Fig. 7 are compiled T

values of the basement

rocks from both the Gyeonggi and Yeongnam

massifs. The Nd model ages, both Archean and

Proterozoic, of the two massifs agree with neither

those of the North China block (predominantly

Archean) nor those of the South China block

(predominantly Proterozoic). As a whole, the

Yeongnam massif is younger than the Gyeonggi

massif in Nd model ages, but their considerable

overlap corroborates the similarity of crustal

his-tories concluded from Pb isotope data described

previously.

4 .

3 .

Tectonic implications

Our compiled Pb and Nd data do not support

a previous idea that the Gyeonggi and Yeongnam

massifs in South Korea are different continental

blocks. This observation contradicts a basic

assumption of tectonic models suggested by Cluzel

et al. (1991) and Yin and Nie (1993). Two basic

assumptions of their models are: (1) different block

affinity of the Gyeonggi and Yeongnam massifs;

and (2) Triassic age of the dextral movement along

the Honam shear zone. Our compiled isotope data

do not support the first assumption. Cluzel et al.

(1991)

constrained

the

age

of

the

dextral

movement along the Honam shear zone as Triassic

by Choo and Kim (1986)’s Rb-Sr whole rock age

of 211

9 3 Ma for the undeformed synkinematic

granite

(the

so-called

Namweon

granite).

However, recent chronological data cast doubt on

the validity of Choo and Kim (1986)’s result. Kim

and Turek (1996) confined the movement age of

the Honam shear zone between 183 Ma to 176 Ma

on the basis of U-Pb zircon ages for deformed and

undeformed granitic rocks. Their conclusion was

corroborated by monazite CHIME (chemical

Th-U-total Pb isochron method) data (Cho et al.,

1999) for the same plutons. Thus, as summarized

by Kwon and Ree (1997), the movement age of the

Honam shear zone is considered to be ca. 180 Ma,

which is in conflict with the above mentioned

second assumption.

Together with the geochronological results

described above, our compiled isotope data that

denote a similar block affinity between the

Gyeonggi and Yeongnam massifs preclude a

possibility for the presence of suture zone between

the two massifs. On the basis of a gross resemblance

in T

between the North China block and South

Korea, especially the Gyeonggi massif, Chen and

Jahn (1998) considered the Ogcheon belt as a

probable extension of the Qinling-Dabie orogenic

belt in South Korea. Thus they implicitly supported

the model of Li (1994) envisioning that the

subsurface position of the Chinese suture would be

far south (about 32°N) of the surface boundary of

the North and South China blocks. As shown in

Fig. 7, however, both the Gyeonggi and Yeongnam

massifs do not show any particular affinity to either

block, because they have both Archean and

Proterozoic

Nd

model

ages.

The

isotopic

similarities of the Gyeonggi and Yeongnam massifs

and intraplate rift setting of the Ogcheon belt

(Chough, 1981; Cluzel et al., 1991) imply that the

two massifs may belong to the same continental

block. Therefore the Ogcheon belt located between

the two massifs cannot be the eastern continuation

of the Chinese collision belt. This argument leaves

the Imjingang belt as the only option for the suture

zone in the Korean peninsula.

However, there still remains the question

‘‘Does South Korea belong to the North or

South China block?’’ The vastness and geologic

complexity of China make it difficult to compare

Korean and Chinese basements directly. For

example, the size of the Gyeonggi or Yeongnam

massif is comparable with only a part of China,

e.g., the Hubei province in the South China

block where Archean ages are well documented

(Chen and Jahn, 1998). Our compiled isotope

data refute the presence of the suture between

the Gyeonggi and Yeongnam massifs but we

cannot draw a definitive conclusion as to the

question of the tectonic relationship between

South Korea and China. Considering that both

the Archean and Proterozoic Nd model ages

are reported in the South China block, we

might need much more work in comparing the

geology of South Korea and the South China

block. Alternatively, there is a possibility that

(4)

South Korea is a separate microcontinent

ac-creted to China as suggested by Lee and Cho

(1995), because Nd isotope data of the two South

Korean Precambrian massifs do not show any

particular affinity with either Chinese block.

5. Concluding remarks

Our Pb isotope data yield relatively well-defined

Paleoproterozoic ages for basement rocks from

the Pyeonghae area, northeastern Yeongnam

mas-sif, South Korea. The intrusion age of the

Pyeonghae gneiss is reported at 2093

9 86 Ma

from whole rock Pb – Pb plot. The PbSL data of

metamorphic garnet from the Wonnam group

yield a

207

_Pb

/

206

_{Pb age of 1840}

₉

_{26 Ma, which is}

considered to represent the timing of amphibolite

to 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 have

been broadly constrained in the Yeongnam

mas-sif. The age constraints and Nd isotopic

signa-tures clearly preclude a direct derivation of the

Pyeonghae gneiss from nearby Wonnam group,

instead implying the presence and involvement of

the older, probably late Archean crust during the

2.1 Ga magmatism in the northeastern Yeongnam

massif. Our compiled Pb isotope data for

base-ment rocks from the Gyeonggi and Yeongnam

massifs define ca. 2.0 Ga Pb-Pb age, indicating

their common crustal evolution process for the

past two billion years. This and compiled Nd

isotope data do not support the traditional idea

that the Gyeonggi and Yeongnam massifs are

respectively correlated with the South and North

China blocks. The existence of Archean crusts in

South Korea is highly probable based upon Nd

model ages of basement rocks from both massifs.

Together with recent geochronological result for

the Honam shear zone, our compiled isotope data

preclude the presence of a major suture zone

between the two South Korean Precambrian

mas-sifs. So, the Imjingang belt remains as the only

option for the suture zone. In order to answer the

question if South Korea belongs to the South or

North China block, we need much more work in

comparing the geologic history of South Korea

with that of the South China block, because it is

in the South China block that both Archean and

Proterozoic 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 by

Korea Science and Engineering Foundation grant

97-07-03-01-01-3 and Basic Science Research

In-stitute grant BSRI-97-5403 to S.-T. Kwon. The

authors sincerely appreciate J.D. Kramers and

C.Y. Lan for their careful and constructive

re-views which improved the manuscript

signifi-cantly. B.U. Chang, S.H. Lee, H. Sagong and

S.R. Lee are acknowledged for their help in

exper-imental works and field survey.

References