• The Journal of the Petrological Society of Korea
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• v.2 no.1
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• pp.1-8
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• 1993

We report on kyanite newly found in the Yongduri gneiss complex of the Chuncheon-Hongcheon area, central Gyeonggi massif. Major mineral assemblage of quartzofeldspathic gneisses in the study area consists of biotite+ garnet+ sillimanite + plagioclase+ quartz${\pm}$kyanite${\pm}$K-feldspar${\pm}$muscovite. Kyanite occurs in four samples, and coexists with sillimanite in three of these samples. In most cases, kyanite is anhedral to subhedral, ranges up to Imm in the maximum dimension, and occurs as metastable relict grains. These observations indicate that the Yongduri gneiss complex has experienced a medium-pressure type metamorphism, followed by low-pressure type one belonging to the sillimanite+K-feldspar zone. Average temperature and pressure of the peak metamorphism are $683{\pm}62^{\circ}C$ and 4.9-5.5 kbar, respectively, when the existing chemical data are re-interpreted. In conjunction with the finding of kyanite in the Cheongpyeong-Gapyeong area (Lee and Cho, 19921, this study demonstrates that kyanite may occur regionally in central Gyeonggi gneiss complex. Moreover, the persistence of kyanite even after the high-T metamorphism of the sillimanitetK-feldspar zone suggests that the central Gyeonggi massif has experienced a tectonometamorphic evolution characterized by a rapid uplift.

• The Journal of the Petrological Society of Korea
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• v.10 no.2
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• pp.95-105
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• 2001

U-Pb age determination was performed on the zircon fractions separated from the metamorphic rocks of three locations of the Gyeonggi Massif. The ages obtained from the upper and lower intersections between concordia curve and discordia lines made of the zircon fractions separated from the rocks of each locality we: $2168\pm$24 Ma and $1227\pm$40 Ma for the Yongduri Gneiss Complex, $1955\pm$22 Ma and $493\pm$32 Ma for the Euiam Group, and $3712\pm$244 Ma and $1613\pm$51 Ma for the Yongmunsan Group (2$\sigma$ errors). The upper intercept ages from the Yongduri Gneiss Complex and the Euiam Group of Gyeonggi massif are very similar to those obtained from the granitic gneisses and the porphyroblastic gneisses of Yeongnam massif respectively. Such similarities suggest that Gyeonggi and Yeongnam massifs might situate under the similar tectonic and geographic environment during ca. 2.2-1.9 Ga. The upper intercept age of Youngmunsan Formation (3.7 Ga) shows large error, because most of the zircon fractions are plotted very close to the lower intersection. It is necessary to investigate further to confirm this age. However, It may suggest the possibility of occurrence of the oldest crust of the northeast Asia similar to the one reported recently from the northeast China. The lower intercept age of the Yongmunsan Group is interpreted to indicate strong metamorphism. Such age postdates the 1.85-1.7 Ga metamorphism and igneous activities occurred in the Yeongnam massif, which might record the late Paleoproterozoic tectonic activities simultaneously occurred in both massifs.