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

Table 1 Pb isotope data for Precambrian basement rocks from the Pyeonghae area, northeastern Yeongnam massif, South Korea 206 Pb 204 Pb 9 2s a 207 Pb 204 Pb 9 2s a Rock types 208 Pb 204 Pb Sample 9 2s a Wonnam group 30.303 17.215 Amphibolite YH01 40.009 16.770 15.638 PH03 Amphibolite 35.673 16.833 15.599 Biotite gneiss b YH03 36.265 22.995 YH04 Biotite gneiss b 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 49.406 Augen gneiss 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 T DM 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 PH 13 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 1840 9 26 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 monazite, zircon, thorianite, rutile and ilmenite have been identified in PH13 garnet by electron microprobe analyses. The 208 Pb 206 Pb trend of the leachates Fig. 2 corresponds to a high ThU of 10.6. This trend appears to be dominated by monazite ThU \ 3; Dewolf et al., 1996 and possibly by thorianite for which no ThU data are available. It seems that our leaching steps selec- tively dissolved inclusions with high ThU 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 ThU inclusions i.e. monazite and thorianite and their release during acid leaching. Monazite, garnet, and zircon have distinct fields in terms of SmNd, UNd, and ThU ratios De- wolf et al., 1996. SmNd, ThU, and NdU ratios of the leachates are listed in Table 3. The leachates have a strong affinity with monazite in UNd versus SmNd and NdU versus ThU plots Fig. 3. The effect of zircon dissolution is not visible either in the 208 Pb 206 Pb trend or in elemen- tal ratios of the leachates, probably because we did not use HF in the leaching step. Our 1840 9 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 1840 9 26 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 Nd a Sm ppm oNd2.1 Ga Sample T DM Ga c oNd0 147 Sm 144 Nd b Nd ppm 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 2.22 0.512814 10 3.93 17.78 0.1337 − 18.3 2.56 YH03 − 1.37 0.511700 − 1.03 2.47 − 21.9 0.1191 29.41 YH04 5.79 6 0.511516 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 28.33 5.11 19 Pyeonghae gneiss PH04 5 0.511290 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 8.13 5 − 3.53 2.71 − 22.3 0.1268 0.511495 38.77 0.511251 PH17 6.78 38.20 5 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 9 2s mean. b Uncertainty is below 0.5, checked by duplicate analysis. c Calculated after Na¨gler and Kramers 1998. 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. The 208 Pb 206 Pb trend of the PbSL leachates yields a ThU 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 1983 9 190 Ma, MSWD = 92.5 than the Pyeonghae gneiss, probably suggesting that the Pb isotopic system of the Wonnam group Fig. 3. NdU versus ThU and UNd versus SmNd 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 ThU NdU SmNd 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 a 207 Pb 206 Pb age of 2093 9 86 Ma MSWD = 3.27 with model m1 value of 8.69 Fig. 4, which is about 250 Ma Table 4 A summary of reported age data for the Gyeonggi and Yeongnam massifs Methodology Age Ma Locality References Lithology Gyeonggi massif U-Pb zircon 2150 9 20 Granitic gneiss Gaudette and Hurley 1973 Yoogoo U-Pb zircon 1766 9 26 Seosan Turek and Kim 1996 Granitic gneiss Sm-Nd minerals 1897 9 120 Granulite Lee et al. 1997 Hwacheon Middle Gyeonggi massif Sm-Nd garnet 1200–2100 Min et al. 1998 Yeongnam massif Sm-Nd minerals Jirisan 1678 9 90 Anorthosite Kwon and Jeong 1990 Sm-Nd whole rocks 1047 9 69 Biotite gneiss Lee et al. 1992 Kimcheon Taebaegsan Granitic gneiss Pb-Pb whole rocks 1920 9 56 Park et al. 1993 Pb-Pb whole rocks 1825 9 86 Granite Park et al. 1993 Taebaegsan Leucogneiss Imwon Sm-Nd minerals 2250 9 4 Lee et al. 1994 Danyang Granitic gneiss Pb-Pb whole rocks 2160 9 150 Kwon et al. 1995 U-Pb zircon 2120 9 20 Granitic gneiss Turek and Kim 1996 Kurye Porphyroblastic gneiss Kurye U-Pb zircon 1945 9 5 Turek and Kim 1996 U-Pb zircon 1923 9 14 Chailbong Turek and Kim 1996 Granitic gneiss Sm-Nd garnet 1820 9 11 Charnockite Kim et al. 1998 Jirisan Charnockite Jirisan Rb-Sr biotite 1123 9 22 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 a 207 Pb 206 Pb age of 2093 9 86 Ma MSWD = 3.27 with model m1 value of 8.69. The data of the Wonnam group are scattered around the 207 Pb 206 Pb trend of the Pyeonghae gneiss. Fig. 5. Compiled Pb isotope data for basement rocks from the Gyeonggi and Yeongnam massifs show a good linearity R 2 = 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 SmNd ratios are indistinguishable from each other except two amphibolite samples YH01, PH03. Accordingly, T DM 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 oNd2.1 Ga values and younger T DM than the Pyeonghae gneiss samples. Two amphibolite samples of the Wonnam group have positive oNd2.1 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 2 = 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 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 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 oNdt 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 negative oNd2.1 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 Nd2.1 Ga values of the latter and age con- straints described earlier. The metasedimentary rocks from the Wonnam group also have re- stricted T DM 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 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 chon- drite-normalized REE patterns of the two 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 DM 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 DM 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 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