Ogcheon Metamorphic Belt Research Articles

Metamorphic K-Feldspar in Low-grade Metasediments from the Ogcheon Metamorphic Belt in South Korea

Metamorphic K-feldspar was found in the low-grade pelitic rocks from the Ogcheon metamorphic belt in South Korea. It occurs as extremely fine-grained crystals (5-15 mm in width and 15-25 mm in length) closely associated with fine-grained muscovite and biotite. Their micro structural relations by X-ray mapping analyses using an electron-probe microanalyzer strongly suggest that K-feldspar has grown directly from the matrix phases as a stable phase coexisting with muscovite and biotite during the Ogcheon metamorphism. The phengite component in muscovite indicates about 4.2 kbar at 400°C, suggesting intermediate P/T type of metamorphism. Muscovite separates of two size fractions, 2-4 and 4-8 mm, give K-Ar ages of 153.4±3.3 and 156.7±3.4 Ma, respectively. The biotite separate is 156.5±3.3 Ma in age. Coarse-grained biotite crystal (ca. 0.5 mm) often occurs and it was analyzed by 40Ar/ 39Ar method using a laser probe step heating technique. It gives a plateau age of 158.2±0.5 Ma that is the same as the K-Ar muscovite and biotite ages. These data, including the previous works, suggest that intermediate P/T type of Ogcheon metamorphic belt exhumed in Middle Jurassic with the dextral strike-slip fault movement.

Early Permian peak metamorphism recorded in U–Pb system of black slates from the Ogcheon metamorphic belt, South Korea, and its tectonic implication

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.

A study on the b0 parameter and crystallinity index of K-white micas from low-grade metapelites in Deokpyeong and Miwon areas, central Ogcheon metamorphic belt, Korea

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 A with an average of 9.010±0.007 A (n=63), whereas those in the vicinity of Poeun granite have slightly lower values of 8.988–9.010 A with an average of 8.998±0.006 A (n=33). On the other hand, b0 parameters of pelitic schists in the Pibanryeong unit vary from 8.995 A to 9.042 A (n=29) with an average of 9.009±0.013 A that is similar to 9.003±0.009 A (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.

Polymetamorphism of Ogcheon Supergroup in the Miwon area, central Ogcheon metamorphic belt, South Korea

Metasedimentary rocks in the Miwon area of the central Ogcheon metamorphic belt (OMB) are composed of schist, phyllite, slate, quartzite, and marble, together with minor mafic and felsic layers, and are divided into Pibanryeong and Poeun units. Mineral assemblages of the Poeun unit are represented by low-grade assemblages including biotite and chlorite, whereas those of the Pibanryeong unit by kyanite, staurolite, garnet, and biotite. Retrograde minerals include chlorite and white mica. Subsequent to regional metamorphism, thermal flux during the Mesozoic magmatism produced sillimanite, andalusite, cordierite, staurolite, garnet, biotite, and white mica in the contact aureole. Contact metamorphism, however, did not broadly reset mineral compositions of the regional metamorphism. P-T conditions for regional metamorphism of the Pibanryeong unit are in the range of 4.3–8.0 kbar and 510–620°C, which belong to the kyanite stability field. Mineral assemblages and P-T conditions of the Pibanryeong unit in the Miwon area are identical to those reported in the Jeungpyeoung—Deokpyeong area. However, mineral assemblages of the Poeun unit in both areas are not compatible, and suggest a decrease in metamorphic grade towards the southwest along strike of the OMB.

Metamorphic evolution of the northwestern Ogcheon metamorphic belt, South Korea

The Ogcheon metamorphic belt (OMB) comprises Late Proterozoic to Paleozoic metasedimentary and metavolcanic sequences which are intruded by Mesozoic granitoid plutons. To delineate the metamorphic evolution of the OMB, we have investigated mineral parageneses, metamorphic reactions and P–T conditions of pelitic and mafic schists in the Chungju area, northwestern part of the OMB. The regional metamorphic grade increases north-westward from biotite to garnet zones, although low-pressure assemblages have developed in contact aureoles around Jurassic granites. Garnet crystals show chemical zoning, typical for prograde metamorphism, with decreasing Mn, and increasing Fe and Mg from core to rim. Compositions of amphiboles constituting the common assemblage in the mafic schist are consistent with those of biotite- and garnet-zones documented in the medium-pressure metabasites. P–T conditions of the garnet zone, estimated from garnet–biotite, garnet–chlorite and amphibole–plagioclase geothermometers together with garnet–plagioclase–biotite–muscovite/quartz geobarometers, are in the range of 5–8 kbar and 520–590°C. Retrograde P–T path based on fluid inclusion studies suggests that the exhumation of the OMB has passed through the P–T range of 1–3 kbar and 350–500°C, following the isochore curves of the CO 2 inclusions. In conjunction with structural and geochronologic data, we conclude that the OMB has experienced a polycyclic P–T evolution characterized by (1) crustal thickening during the Middle Paleozoic time and (2) regional retrograde metamorphism in the Triassic. Our result further suggests that the Triassic collision belt in east-central China does not pass through the OMB.

Geochemistry and genesis of hydrothermal Au-Ag-Pb-Zn deposits in the Hwanggangri mineralized district, Republic of Korea

The Hwanggangri mineralized district within the Ogcheon metamorphic belt of the southern Korean peninsula contains more than 60 W-Mo- and Cu-Pb-Zn(-Au-Ag)-bearing metallic ore occurrences and lesser fluorite and talc occurrences related to intrusion of Late Cretaceous (83-90 Ma), I-type and magnetite series granites. Fissure-filling veins in the district differ in mineralogy with distance from the granites: W-Mo vein deposits are located within or near (<500 m) the granites, whereas Cu-Pb-Zn vein deposits occur up to 5 km from the granites. The Geumsil and Daehyun Au-Ag-Pb-Zn mines are unique and represent the most distal deposits in the district. They consist of quartz-carbonate veins which contain economic concentrations of gold (3 g/t) and silver (250 g/t) in addition to abundant base metal sulfides (6 wt % Pb + Zn). Study of these Au-Ag-enriched mines provides a key to understanding ore-forming processes in the outer portions of a very large, batholith-driven hydrothermal circulation system.Phase equilibria, fluid inclusions, and stable isotopes indicate that ore deposition was the result of interaction of progressively less evolved meteoric waters with earlier, highly evolved meteoric waters. Early Fe sulfide mineralization occurred at temperatures of 380 degrees to 310 degrees C from fluids (delta 18 O = 10-12ppm) whose chemistry was controlled dominantly by reactions with granite and carbonate-bearing host rocks. Main base metal-Au-Ag mineralization occurred at temperatures of 250 degrees to 210 degrees C from fluids (delta 18 O = 6-7ppm) whose chemistry changed rapidly as a result of fluid unmixing and cooling. Late, postore carbonates were deposited at temperatures of 250 degrees to 165 degrees C from fluids whose stable isotope compositions (delta 18 O = 1-4ppm; delta D = -89ppm) and salinities (<3 wt % NaCl) reflect increasing meteoric water dominance.The most intriguing result of our studies is that gold deposition occurred from fluids whose chemistries were controlled dominantly by granites (and wall rocks) at temperatures well below magmatic conditions (250 degrees C) at great distances from the intrusion. The occurrence of increased gold and silver grades in these distal areas bodes well for increased exploration for Au-Ag-Pb-Zn deposits at relatively great distance from granite batholiths.