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Imayama, T.,Takeshita, T.,Yi, K.,Cho, D.L.,Kitajima, K.,Tsutsumi, Y.,Kayama, M.,Nishido, H.,Okumura, T.,Yagi, K.,Itaya, T.,Sano, Y. Universitetsforlaget ; Elsevier Science Ltd 2012 Lithos Vol.134 No.-
The timing of partial melting and the pressure-temperature (P-T) paths in the High Himalayan Crystalline Sequence (HHCS) in far-eastern Nepal has been investigated using zircon chronology, rare earth element (REE) compositions, and P-T pseudosection analysis. Zircon from migmatites formed during Himalayan thermal events displays inherited magmatic core overgrown by two generations of metamorphic rims. The new rims are distinguished on the basis of their Tertiary ages, low MREE contents, and low Th/U ratios. The inner zircon rims from Sil+Grt+Bt+Kfs+Pl+Qtz and Ky+Sil+Grt+Bt+Ms+Pl+Qtz migmatites at different structural level of the HHCS display ages of c. 33-28Ma (Early Oligocene) and c. 21-18Ma (Early Miocene): these rims are characterized by flat MREE to HREE patterns and were overgrown by partial melt through muscovite dehydration melting under the stability of garnet, which occurred at P=c. 7-10kbar and T=c. 730-780<SUP>o</SUP>C, and at P=c. 8-14kbar and T=c. 720-770<SUP>o</SUP>C, respectively. The outer zircon rims are relatively enriched in HREE with respect to the inner rims and were overgrown at c. 27-23Ma (Late Oligocene) and at c. 18-16Ma (Early Miocene) during melt crystallization accompanying breakdown of garnet at P=c. 4-7kbar and T=c. 650-725<SUP>o</SUP>C. Early Miocene Ms-Bt leucogranites with two successively overgrown zircon rims at c. 18.3+/-0.3Ma and c. 16.3+/-0.2Ma were intruded into Early Oligocene migmatite hosts. Microstructural observations and the corresponding P-T conditions associated with the two generations of zircon rims indicate that the Early Oligocene and Early Miocene migmatites show relatively isobaric and nearly isothermal P-T paths during exhumation, respectively. The inferences are consistent with higher average cooling rates for the Early Miocene (c. 30-40<SUP>o</SUP>C/My) than the Early Oligocene (c. 15-25<SUP>o</SUP>C/My) migmatites, inferred from peak-T conditions and FT (c. 6Ma for both migmatites) and U-Pb zircon ages. The P-T-t paths of the two migmatites indicate that burial of the Early Miocene migmatites has been coeval with exhumation of the Early Oligocene migmatites, implying the formation of large-scale thrust within the HHCS.
Oh, C.W.,Imayama, T.,Lee, S.Y.,Yi, S.B.,Yi, K.,Lee, B.C. Universitetsforlaget ; Elsevier Science Ltd 2015 Lithos Vol.216 No.-
Gneisses and migmatites exposed in the Yangpyeong area in the northern Gyeonggi Massif provide insight into the Paleoproterozoic and Triassic metamorphic events in South Korea. Garnet-biotite gneiss and sillimanite-garnet-biotite gneiss in the western part of the area reveal Paleoproterozoic metamorphism (1888-1871Ma) at P-T conditions of 760-820<SUP>o</SUP>C and 8-10kbar and 710-750<SUP>o</SUP>C and 5-7kbar, respectively. These rocks were overprinted by low-P/T type metamorphism (590-650<SUP>o</SUP>C, 3-4kbar) during the Triassic (ca. 237Ma). In contrast, a cordierite-rich migmatite near the post-collisional Triassic igneous complex in the eastern part of the area was strongly metamorphosed during the Triassic (ca. 235Ma) at 750-790<SUP>o</SUP>C and 7-8kbar. The similar Triassic ages in the western and eastern areas suggest that low-P/T type metamorphism occurred as a second stage of regional metamorphism, which is characterized by the production of cordierite with an irregularly shaped garnet. The metamorphic grade of the Triassic metamorphism decreases spatially towards the west from granulite facies to amphibolite facies metamorphic conditions, and the Paleoproterozoic metamorphism is well preserved in the western part with low grade Triassic metamorphism. The new discovery of the Triassic metamorphic event in the Yangpyeong area, in addition to the previously reported Triassic post-collision igneous event, supports the idea that the continental collision belt between the North and South China blocks extends from the Hongseong area into the Odesan area through the Yangpyeong area in South Korea.
Oh, Chang Whan,Imayama, Takeshi,Jeon, Jimin,Yi, Keewook Elsevier 2017 Journal of Asian earth sciences Vol.145 No.2
<P><B>Abstract</B></P> <P>The Hongseong area in the southwestern Gyeonggi Massif in South Korea is considered to represent the eastward extension of the Qinling–Dabie–Sulu collision belt in China. We have carried out zircon U–Pb SHRIMP dating and P–T estimations of the gneisses and amphibolites in the eastern Wolhyeonri complex within the Hongseong area in order to constrain their metamorphic and tectonic evolutions. The protoliths of the migmatitic biotite gneisses formed during the Neoproterozoic and underwent granulite-facies metamorphism (750–880°C, 12–15kbar) at 442–413Ma. These rocks subsequently experienced amphibolite-facies retrograde metamorphism at 585–660°C and 7.5–10.3kbar. Mylonitic biotite gneiss, hornblende gneiss, and folded amphibolite in the study area yield metamorphic ages that range from 429 to 420Ma. The protoliths of some garnet amphibolites that formed at 470–456Ma are arc magmatic rocks; they experienced metamorphism at the boundary between amphibolite- and eclogite-facies (ca. 625–700°C and 13–15.5kbar) before 418Ma and underwent retrograde amphibolite-facies metamorphism (ca. 625–700°C and 8–9kbar) at 418–405Ma. These data suggest that a regional intermediate-P/T metamorphic event occurred during the Middle Paleozoic. In contrast, Paleoproterozoic augen gneiss blocks enclosed in the Deokjeongri gneiss complex preserve evidence of high-pressure (HP) metamorphism (840–960°C, 17–21.8kbar) at 234–230Ma, which are similar to the previously reported results from eclogite blocks in this area. The occurrence of Middle Paleozoic regional metamorphism before the Permo-Triassic HP metamorphism in the Hongseong area may be correlated with the Middle Paleozoic metamorphism in the Qinling belt in China; such regional metamorphic events were caused by the collision of microcontinents with the North or South China Cratons prior to the collision between the North and South China Cratons in the Permo-Triassic.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Regional Middle Paleozoic metamorphism occurred in the Gyeonggi massif, South Korea. </LI> <LI> Permo-Triassic high-pressure metamorphism is preserved in Paleoproterozoic augen gneisses. </LI> <LI> Regional metamorphism was possibly due to the microcontinental collision. </LI> </UL> </P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>