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Abstract: The Ogcheon metamorphic belt consists primarily of metasedimentary and metavolcanic rocks that have experienced polyphase tectonometamorphism since the Neoproterozoic. Peak metamorphism reaching up to lower-amphibolite facies produced ubiquitous garnet porphyroblasts in pelitic and mafic schists. To determine the timing of their formation, step-leaching experiments were undertaken for five garnet fractions separated from pelitic and quartz-hornblende-garnet schists. The U–Pb ages from three samples are identical within 2σ errors, ranging from 291±41 Ma to 276±29 Ma. The quasi-linearity of leachates in 238U–206Pb and 208Pb–206Pb diagrams suggests that U and Pb are released from a single mineral phase and that minor chemical fractionation between U and Pb may have occurred during the leaching experiment. Deviations of residues and bulk garnet fractions from the linear trend are attributed to partial dissolution of refractory inclusions of detrital zircon. Th/U ratios of leachates are in the range of 3.4–12, much higher than those of pure garnet, and suggest the contribution of allanite. Negative relationships in the Sm–Nd isochron diagram and similar 147Sm/144Nd ratios between whole rock and garnet corroborate the influence of light rare earth element (LREE)-rich allanite on the Sm–Nd isotopic system. Simple mass-balance calculations indicate that only a trace amount (0.35 modal%) of allanite inclusions should govern the U–Th–Pb systematics of garnet. Petrographic evidence together with the consistency in U–Pb ages suggests that allanite is a product of prograde metamorphism. Thus, peak metamorphism responsible for the growth of allanite-bearing garnet porphyroblasts in the Ogcheon metamorphic belt is best estimated to be Early Permian. [Copyright &y& Elsevier]

In order to elucidate metamorphic conditions of metasedimentary rocks of central Ogcheon metamorphic belt, we investigated the b0 parameter and the crystallinity index (CI) of K-white micas from low-grade metapelites. The CI values of muscovite from seventy-eight samples range from 0.08° to 0.19° Δ2θ with an average of 0.12±0.01° Δ2θ, and show no remarkable difference between structurally upper Pibanryeong and lower Poeun lithotectonic units. Slate and phyllite of the Poeun unit have the b0 parameters of 8.993–9.031 Å with an average of 9.010±0.007 Å (n=63), whereas those in the vicinity of Poeun granite have slightly lower values of 8.988–9.010 Å with an average of 8.998±0.006 Å (n=33). On the other hand, b0 parameters of pelitic schists in the Pibanryeong unit vary from 8.995 Å to 9.042 Å (n=29) with an average of 9.009±0.013 Å that is similar to 9.003±0.009 Å (n=26) in the Poeun unit. These results suggest that low-grade metapelites of central Ogcheon metamorphic belt have experienced a mediumpressure type metamorphism, reaching up to the upper greenschist facies (biotite zone) condition. Moreover, metamorphic pressures estimated from the b0 values show no apparent discontinuity across the unit boundary, implying an insignificant vertical displacement along this boundary after the peak metamorphism. [ABSTRACT FROM AUTHOR]

We report Pb–Pb whole rock and uraninite CHemical Th–U–total Pb Isochron MEthod (CHIME) ages of carbonaceous black slates from the Ogcheon metamorphic belt, South Korea. The Pb isotopic data of whole rock samples yield 207Pb/206Pb ages of 283±33 and 291±13 Ma for two outcrops from the northeastern part of the belt. The uraninite CHIME age is estimated at 283±26 and 281±27 Ma for the northeastern and the middle part of the belt, respectively. All of the above ages are identical within error ranges, and represent the timing of peak metamorphism after the late Precambrian intraplate rifting. On the other hand, the 207Pb/206Pb whole rock age for the southwestern part of the belt is estimated at 194±27 Ma, probably representing the timing of contact thermal metamorphism associated with the intrusion of Jurassic granitic plutons. Rb–Sr isotopic data of the black slates do not define any meaningful isochron. The early Permian metamorphic age of this study does not support any tectonic scheme in favor of major tectonometamorphism at either the Silurian–Devonian or the Triassic time. Instead, it corroborates the probability that the two zones in the Ogcheon belt, the Ogcheon metamorphic belt and the Taebaegsan zone, were separated from each other before the development of major structural framework in the former. Our data do not support an idea that the Ogcheon belt corresponds to the continuation of the Triassic collision belt in east central China. [Copyright &y& Elsevier]