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Khain, E.V., Bibikova, E.V., Kroner, A., Zhuravlev, D.Z., Sklyarov, E.V., Fcdotova, A.A. and Kravchenko-Berezhnoy, I.R. (2001) The most ancient ophiolite of the Central Asian fold belt: Sm-Nd and U-Pb evidence from the Dunzhugur complex, eastern Sayan. Earth and planet. Sci. Lett., (in press).

Khain, V.E., Gusev, G.S., Khain, E.V., Vernikovsky, V.A. and Volobuyev, M.I. (1997) Circum-Siberian Neoproterozoic ophiolite belt. Ofioliti,

Pfander, J., Jochum, K.P., Kroner, A., Kozakov, I., Oidup, C. and Todt, W. (1998) Age and geochemical evolution of an early Cambrian ophiolite-island arc system in Tuva, South Central Asia. In: generation and emplacement of ophiolites through time. Geol. Surv. Finland, Spl. Paper, No. 26, 42 p.

V. 22, pp. 195-200.

Ritsk, E.Yu., Amelin, Yu.V., Krymski, R.Sh., Shalaev, V.S. and Rizvanova, N.G. (1999) On the age of the Nyurundyukan sequence (Kichera zone, Baikal-Muya fold belt): new U-Pb and Sm-Nd isotope data. In: Geologicheskoe razvitie proterozoiskikh perikratonnykh paieookeanicheskikh struktur Severnoi Evrazii (Geologic evolution of Proterozoic marginal paleoceanic structures of northern Eurasia). St. Petersburg: Tema, pp. 130-132 (in Russian).

Sovetov, Yu.K. and Blagovidov, V.V. (1995) Evolution of the Riphean passive margin of the Siberian continent (Patom highland). In: obshchic problcmy stratigrafii i geologicheskoi istorii rifeya Sevemoy Azii (General problems of the stratigraphy and geologic history of the Riphean of northern Asia). Yekaterinburg, pp. 115-116 (in Russian).

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Paleoproterozoic Crustal Evolution of the Basement Rocks in the Northeastern Yeongnam Massif, Korea Jeongmin Kim1.* and Moonsup Cho2 'Korea Basic Science Institute, Yusung-ku, Taejon - 305-333, Korea, E-mail: j-mkirn@comp.kbsi.re.kr

'School of Earth and Environmental Sciences, Seoul National University, Seoul - 151-742, Korea " " " ,,,. "" .. .........I.. ". " ",.", ,,,,,,,,,,, "ll_ ,"__x_ ......................... "" ." ......lll

Precambrian evolution of the Korean Peninsula, situated between the Asian mainland and the Pacific active margin, is of great importance for understanding the continental growth and crustal evolution in East Asia. Proper interpretation on the Precambrian tectonics is prerequisite for any attempt to correlate various metamorphic terranes in China, Japan and Korea. Tectonic correlation of Precambrian rocks is also useful for investigating the Phanerozoic geodynamic processes: in particular, the current issue on continuation of the Dabie-Sulu continental collision belt in the Korean Peninsula. If the indentation and detachment models of Yin and Nie (1993) and Li (1994), respectively, are correct, the Yeongnam massif (YM) should belong to the Sino-Korean craton. Unfortunately, however, little tectono-magmatic and metamorphic studies on the YM are available yet except for a recent geochemical study of Cheong et al. (2000).

The Yeongnam Massif (YM), one of the Paleoproterozoic basements in Korean Peninsula, preserves important evidence for unraveling crustal evolution in the Far East Asia. We carried out an integrated study on metamorphism, magmatism and geochronology of medium to high-grade metamorphic rocks as well as Precambrian granitoids in northeastern YM, in order to understand their tectono-thermal evolution. The YM is one of three Precambrian basement massifs in Korea, and primarily consists of high-grade gneisses and schists constituting a metamorphic-magmatic complex. This massif is bordered to the north by the Paleozoic Taebaeksan sedimentary basin and to the south by the Cretaceous Gyeongsang basin. In the northeastern part of the YM, excellent exposures of medium to high-grade metamorphic rocks as well as Precambrian granitoids are available. As a part of our on-going project, we have studied the progressive metamorphic sequence reaching up to the anatectic P-T regime. The study area consists of ca. 5-km thick succession of pelitic-psammitic metasedimentary rocks and metagranodiorites. Peraluminous, garnet-bearing leucogranites

_ . . ~ " " l "" .__.̂,,,,,,,-I." , ,,

are widespread and intrude the high-temperature gneisses. Metasedimentary sequences record prograde mineral parageneses typical for the low-pressure-high-temperature metamorphism. In the study area, these sequences show progressive metamorphic zones: biotite-cordierite, sillimanite, garnet, and spinel zones with increasing grade. Peak metamorphic conditions are in the range of 600-625°C and 3.8- 4.1 kbar near the garnet isograd, and 730-750°C and 4.8-5.8 kbar near the spinel isograd. These P-T estimates define a metamorphic field gradient of ca. 25"C/km. Garnet-bearing leucogranites intruding the metasedimentary rocks are characterized by peraluminous chemistry with the E~~ values ranging from -8.5 to -4.9 (Table 1). The Sm-Nd whole-rock- garnet ages estimated from both leucogranite and metagranodiorite are identical within error range, and define the peak metamorphic age as 1.91-1.93 Ga. This result suggests that the heat source for the low-pressure/high-temperature metamorphism is attributed to the felsic magmatism widespread in the northeastern YM. Moreover, it is noted that Late Paleoproterozoic thermal event is prevalent not only in the Yeongnam massif but also the Gyeonggi massif. Thus, Paleoproterozoic tectonism appears to be more than regional in Korea, and could be related to the formation of the Paleoproterozoic supercontinent.

Our petrologic and geochronologic results suggest that multiple geologic events including two distinct stages of melt generation were operative during the syn-magmatic, low- pressure metamorphism. Although the age of the first-melt generation is unknown, the subsequent melt-forming event at 1.9 Ga is largely coeval with the intrusion of granitic batholiths in the northeastern YM (1920+ 56 Ma, Park et al., 1993). Moreover, the Paleoproterozoic age for a regional thermal event is well correlated with ca. 1.87 Ga reported for the granulite- facies metamorphism in the Gyeonggi Massif (Lee et al., 2000). Although Paleoproterozoic tectonic setting of YM is yet equivocal,

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Table 1. Sm-Nd isotopic compositions and model ages of the granitoid and metasedimentary rock in Yeongnam Massif

sample rock type Sm (ppm) Nd (ppm) 147Sm/'44Nd 143Nd/144Nd 20 T,, (Ga)* E~~ (T=1.9Ga)"

5522-1 lcucogranite garnet 0.2126 0.0731 1.7608 0.532064 0.000035

YJ13 lcucogranitc garnet 0.4829 0.7123 0.4101 0.514731 0.000021

YJ21 leucogranite garnet 0.5650 0.4088 0.8360 0.520328 0.000036

YP25 metagranodiorite garnet 1.8811 7.1121 0.1600 0.511888 0.000008

whole rock 1.6825 3.9490 0.2577 0.513147 0.000016 -4.98

whole rock 0.5376 1.5327 0.2122 0.512426 0.000007 -7.94

whole rock 2.7116 8.7294 0.1879 0.512094 0.000005 -8.51

whole rock 7.5021 40.2758 0.1127 0.511288 0.000003 2.65 -5.86 K31 metagranodiorite whole rock 9.1083 52.0294 0.1059 0.511182 0.000004 2.63 -6.28 NJlO metapelite whole rock 11.2307 61.7067 0.1101 0.511077 0.000003 2.88 -9.38 YP28-1 metapelite whole rock 10.1767 57.0717 0.1079 0.511185 0.000003 2.68 -6.71

* Nd model agcs following DePaolo (1981) **cNd values were calculated usng 147Sm/144Nd = 0.1967 and 143Nd/144Nd = 0.511847

tectono-thermal event at ca. 1.9 Ga in Proterozoic basements of Korean Peninsula appears to be widespread and distinct. Finally, T,, ages of the northeastern YM (2.6-2.9 Ga) concur with that of high-grade gneisses in the Gyeonggi Massif. These results suggest that ubiquitous magmatism and high-temperature metamorphism in the Korean Peninsula are part of global tectono-thermal event, following the episodic supercontinent growth (Condie, 2000).

Acknowledgments

We thank Dr. C.S. Cheong for his help with the Sm-Nd isotopic work. This study was supported by Korea Research Foundation Grant to M. Cho.

References Condie, K.C. (2000) Episodic continental growth models: afterthoughts

and extensions. Tectonophys., v. 322, pp. 153-162.

Cheong, C.S., Kwon, S.-T. and Park, K.H. (2000) Pb and Nd isotope constraints on Paleoproterozoic crustal evolution of the northern Yeongnam massif, South Korea. Precamb. Res., v. 102, pp. 207- 220.

DePaolo, D.J. (1981) Neodymium isotopes in the Colorado Front Range and crust-mantle in the Proterozoic. Nature, v. 291, pp. 193- 196.

Lee, S.R., Cho, M., Yi, K. and Stern, R. A. (2000) Early Proterozoic Granulites in Central Korea: tectonic correlation with Chinese cratons. J. Geol., v. 108, pp. 729-738.

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, v. 22, pp. 739-742.

Park, K.-H., Cheong, C-S., Lee, K-S. and Chang, H-W. (1993) Isotopic composition of lead in Precambrian granitic rocks of the Taebaeg area. J. Geol. SOC. Korea, v. 29, pp. 387-395.

Yin, A. and 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, v. 12, pp. 801-813.

Stratigraphy and Sedimentary Environment of the Alluvium in the Echigo Plain, Central Honshu, Japan Iwao Kobayashil, Yukihiko Kamoil, Satoshj Minagawal I Niigata University, Niigpta, Japan

Kyoto University of Education, Kyoto, Japan I"" .......... ". . "lll"... .. . -. . ........... . _" - .. . . ..

The Echigo Plain is one of the largest coastal plains in Japan, situated in the central part of Honshu along the Sea of Japan side. There are two great rivers, the Shinano (drainage area: 11,900 km2) and Agano (7,710 km2), and some sand dunes arranged along the coastal line. The maximum thickness of the alluvial strata is over 150 m, which is the most distinctive feature of' the Echigo Plain. The subsidence due to crustal movement has continued since Pliocene. The thickness of the alluvium is substantial in the central region of the plain. The deposition occurred under beach-shoreface and barrier-lagoon systems.

Stratigraphy, facies and sedimentary environments of alluvial sediments in the Echigo Plain were investigated with analyses

Yasuil, Satoshi Tanaka2 and Akihiko

of lithological and fossil facies, which enabled us to estimate the change of thickness, facies and sedimentary environment. In the northern region (the Iwafunegata Lagoon), the thickness of alluvial strata is ca. 20 m, which is mainly composed of clay including fine sand layers. This clay is mainly deposited under the influence of marine to brackish water (bay to lagoon). Sand dune and bar deposits are distributed in the seaside.

Alluvial strata in the Echigo Plain are different in thickness between the north and central regions. The boundary near the Agano River extends to NNE - SSW. In the central region, more than 30 m Uppermost Pleistocene and 100 m Holocene are distributed, while in the northern region, the uppermost

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