Pb and Nd isotopic constraints on Paleoproterozoic crustal
evolution of the northeastern Yeongnam massif, South
Korea
Chang-Sik Cheong
a,*, Sung-Tack Kwon
b, Kye-Hun Park
caIsotope Research Team,Korea Basic Science Institute,52Eoeun Dong,Yusung Ku,Taejeon305-333,South Korea bDepartment of Earth System Sciences,Yonsei Uni
6ersity,Seoul120-749,South Korea
cDepartment 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 207Pb/206Pb age of 1840926 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 a207Pb/206Pb age of 2093986 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 207Pb/206Pb 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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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
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.
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 150Nd –149Sm
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
146Nd/144Nd=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 (2sm, 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 of207Pb/
204Pb and206Pb
/204Pb data because internal errors
Table 1
Pb isotope data for Precambrian basement rocks from the Pyeonghae area, northeastern Yeongnam massif, South Korea
206Pb/204Pb 92
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
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
aInternal errors (%SD,N=60). For whole rock data, within run errors are sufficiently smaller than 0.1%. bGneissic 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 143Nd
/144Nd
=0.512638 and
147Sm/144Nd=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
monazite, zircon, thorianite, rutile and ilmenite have been identified in PH13 garnet by electron
microprobe analyses. The208Pb
/206Pb 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 the208Pb/206Pb 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
143Nd/144Nda Sm (ppm) oNd(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
aNumbers in parenthesis refer to least significant digits and 92 smean. bUncertainty is below 0.5%, checked by duplicate analysis.
Fig. 2. Pb isotopic plots of garnet PbSL leachates, host whole rock, and unleached garnet from PH13 sample. The 207Pb/ 206Pb slope defined by them corresponds to 1840926 Ma
(MSWD=13.8). The208Pb/206Pb 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 208Pb/204Pb versus 206Pb/ 204Pb 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 207Pb
/206Pb 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 a207Pb/206Pb
age of 2093986 Ma (MSWD=3.27) with model
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 a207Pb/206Pb age of 2093986
Ma (MSWD=3.27) with modelm1 value of 8.69. The data of the Wonnam group are scattered around the207Pb/206Pb trend
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 207Pb/204Pb versus 206Pb/204Pb plot. The evolution curve of an average crust (S
and K; Stacey and Kramers, 1975) is shown for references. No correlation is found in 208Pb/204Pb versus 206Pb/204Pb 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 208Pb
/204Pb versus 206Pb
/204Pb 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 207Pb
/204Pb versus
206Pb/204Pb diagram for Korean basement rocks,
which yields an apparent 207Pb/206Pb age of ca.
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
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.
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 a207Pb
/206Pb age of 1840926 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.
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