Moonsup Cho

Possible eastward extension of Chinese collision belt in South Korea: The Imjingang belt

Structural, petrological, and geochronological data from the middle Korean peninsula indicate that the Qinling-Dabie-Sulu collisional belt of east-central China crosses the Yellow Sea and extends into the Imjingang belt. The Yeoncheon complex, first identified as the western Imjingang belt, comprises primarily north-dipping metamorphic sequences: (1) the northern Jingok unit, consisting of Barrovian-type metapelites, and (2) the southern Samgot unit, consisting of calc-silicate and amphibolitic rocks. South-vergent structures with reverse-sense shearing are dominant in the Jingok unit, whereas late normal-sense shearing is pervasive in the Samgot unit and the deformed granitoid to the south. These structural patterns are interpreted to correspond to extensional deformation associated with uplift following compression in a collisional belt. Pressure-temperature ( P-T ) estimates from the amphibolites suggest a high-P amphibolite-facies metamorphism (8–13 kbar and 630–790 °C), possibly evolving from eclogite facies conditions along a clockwise P-T path. Sm-Nd and Rb-Sr geochronological data suggest that the amphibolites emplaced in Late Proterozoic time were metamorphosed during Permian-Triassic time.

Age, geochemistry, and tectonic significance of Neoproterozoic alkaline granitoids in the northwestern margin of the Gyeonggi massif, South Korea

Alkaline meta-granitoids, ranging in composition from syenite to alkali granite, occur in the northwestern Gyeonggi massif. Ion microprobe U–Pb zircon analyses indicate that the granitoids were emplaced at 742±13 Ma, and are corroborated by a Rb–Sr whole rock age of 770 ± 40 Ma. Major and trace element characteristics, together with Sr and Nd isotopic data, suggest that the granitoid magma was derived from ancient (TDM = 2.6–2.2 Ga) continental crust with addition of juvenile mantle-derived basaltic magma. The generation of the alkaline granitoid is attributed to crustal thinning induced by deep-seated thermal activity such as mantle upwelling or mafic magma influx. Alkaline igneous activity at 742 Ma is coeval with Neoproterozoic rift-related magmatism prevalent in South Korea and the South China Block but lacking in the North China Block. Thus, we suggest that the Gyeonggi massif is correlative with the South China Block and has experienced a rifting event during Rodinia breakup.

Early Proterozoic Granulites in Central Korea: Tectonic Correlation with Chinese Cratons

A coherent granulite complex has been discovered in the central part of the Korean Peninsula, formerly regarded as the eastern Imjingang belt. This granulite complex (Hwacheon granulite complex [HGC]) experienced two cycles of tectonometamorphic events. The first event corresponded to crustal thickening, followed by peak granulite‐facies metamorphism and associated partial melting. The quasi‐isobaric cooling path, suggested by the occurrences of secondary garnet mantling orthopyroxene and secondary kyanite in mafic and pelitic granulites, respectively, indicates substantial residence time at midcrustal levels prior to exhumation of the HGC. The timing of peak metamorphism was dated from the unzoned overgrowth rims on zircons in a migmatitic granulite at \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape

Metamorphic Evolution of the Ogcheon Belt, Korea: A Review and New Age Constraints

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

\end{document} (2&sgr;) Ma, using an ion microprobe. Detrital cores of zircon, however, give diachronous U‐Pb ages of ca. 2.30, 2.45, 2.65, and 2.90 Ga, attesting to the presence of Archean protoliths in South Korea. Sm‐Nd model ages of migmatitic granulites (ca. 2.8–2.6 Ga) further suggest that a significant addition of juvenile materials from the mantle took place prior to granulite‐facies metamorphism. The second tectonometamorphic event may be responsible for the final exhumation of the HGC toward the surface. The age of exhumation is interpreted to be Permo‐Triassic on the basis of available isotopic data. These results suggest correlation of the Gyeonggi massif with the Yangtze craton, and as a corollary, that both this massif and the Imjingang belt correspond to the eastward continuation of the Chinese collisional belt.

An experimental investigation of heulandite-laumontite equilibrium at 1000 to 2000 bar Pfluid

The Imjingang belt in the central Korean Peninsula is a strong candidate to represent the extension of the Dabie-Sulu ultrahigh-pressure (UHP) belt in China. This fold-thrust belt consists primarily of: (1) the northern, Jingok unit, characterized by Barrovian-type metapelites; and (2) the southern, Samgot unit, comprising calc-silicate rocks and amphibolites with or without garnet. The metamorphic grade of the Jingok unit increases southward from garnet through staurolite to kyanite zones. Peak metamorphic conditions estimated from the kyanite zone range up to ~11 kbar and 700°C. In contrast, no significant change in metamorphic assemblages is apparent in the Samgot unit. Characteristic minerals of the calc-silicate rocks include garnet, diopside, hornblende, scapolite, and plagioclase, and are compatible with upper amphibolite facies estimated from garnet amphibolite. P-T estimates from the Samgot amphibolites are generally in the range of 8.5-11.5 kbar and 660-780°C. These P-T estimates, together with near-isothermal decompression documented from the metapelitic unit, suggest that the Imjingang belt possibly evolved from eclogitefacies conditions along a clockwise P-T path. The timing of peak metamorphism was previously estimated at ~250 Ma, based on Sm-Nd and Rb-Sr mineral isochron ages and monazite chemical ages. This result is corroborated by an ion microprobe (SHRIMP) U-Pb age of metamorphic zircon dated at 253 ± 2 Ma. 40Ar/39Ar ages of hornblende from the amphibolite suggest regional cooling through ~500°C at 230-225 Ma in the Samgot unit. Two biotite separates in the kyanite zone suggest cooling through ~300°C at ~220 and 160 Ma, respectively, in the Jingok unit. These results, in conjunction with peak metamorphism at ~250 Ma, indicate that high-P metamorphism in the Imjingang belt occurred during the Permo-Triassic, and that subsequent rapid cooling followed during exhumation of a thickened orogen, producing a clockwise decompression. This P-T-t evolution is compatible with that of the basement gneisses in the Gyeonggi Massif. Thus, we conclude that both the Imjingang belt and Gyeonggi Massif in the Korean Peninsula are products of continental collision at ~250 Ma and subsequent exhumation at ~230-220 Ma.

Metamorphic evolution of the northwestern Ogcheon metamorphic belt, South Korea

The Ogcheon metamorphic belt (OMB), which is often correlated with the Dabie-Sulu ultrahighpressure (UHP) belt in China, comprises Neoproterozoic to Paleozoic metasedimentary and -volcanic sequences representing a stack of synmetamorphic nappes. Regional metamorphic grade increases northwestward to produce the assemblage, biotite + garnet ± staurolite ± kyanite + plagioclase + quartz. Garnet porphyroblasts show chemical zoning typical for prograde metamorphism, with decreasing Mn and increasing Fe and Mg from core to rim. P-T conditions were estimated to be 4.2-9.4 kbar and 490-630°C, corresponding to the medium-pressure type. In addition, the GIBBS calculation suggests a clockwise P-T-t path, corroborating a crust-thickening event associated with regional peak metamorphism. Using various isotopic techniques, the timings for intracontinental rift volcanism and syntectonic metamorphism were determined. New SHRIMP U-Pb zircon ages from a felsic tuff reflect the Neoproterozoic rifting at ˜;750 Ma. Regional metamorphism has been recently dated at ˜;285 Ma, based upon several independent sets of radiometric ages: U-Pb ages of monazite inclusions in garnet, Pb-Pb whole-rock ages of black slates, chemical ages of uraninite, and SHRIMP U-Pb zircon ages from a granitic gneiss pebble in metadiamictite. Thus, the Ogcheon orogeny is newly defined as an earliest Permian event, preceding a high-pressure metamorphic event in the Imjingang belt, central Korea, by ˜;30 Ma. During the subsequent Songrim orogeny, at ˜;250-220 Ma, the OMB experienced a second regional-thermal metamorphism under greenschist-to amphibolite-facies conditions. This Triassic event is interpreted to correspond to a major collisional orogeny in the Dabie-Sulu belt. These multiple metamorphic events in the Ogcheon belt are readily correlated with those reported from not only the UHP belt in east-central China, but also the Hida and Renge belts in Japan. Thus, eastward extension of the Dabie-Sulu belt through the Korean Peninsula to the Hida belt (Ernst and Liou, 1995) remains a valid hypothesis.

Parageneses and Th-U distributions among allanite, monazite, and xenotime in Barrovian-type metapelites, Imjingang belt, central Korea

U‐Pb zircon ages of tuffs and sandstones of the Daedong Supergroup (Bansong and Nampo groups) in the Korean Peninsula were determined using a sensitive high‐resolution ion microprobe (SHRIMP) in order to constrain their age of sedimentation and to unravel discrete geologic events as recorded in detrital zircons. The ages of four tuffaceous samples from the Bansong Group imply that the Daedong Supergroup formed at ca. 187–172 Ma in association with the Early‐Middle Jurassic orogeny. These data are in marked contrast with paleomagnetic arguments suggesting that the Bansong and Nampo groups are precollisional Early‐Middle Triassic deposits that are correlative with the North and South China blocks, respectively. Detrital zircons of the Daedong Supergroup define seven age components: (1) Early‐Middle Archean (3.64–2.97 Ga), (2) Late Archean–middle Early Proterozoic (2.63–2.33 Ga), (3) late Early Proterozoic (1.98–1.75 Ga), (4) Middle‐Late Proterozoic (1.2–0.6 Ga), (5) Devonian (400–355 Ma), (6) Early Permian (280–255 Ma), and (7) Middle Triassic–Early Jurassic (240–180 Ma). These age distributions, together with available geochronological data, suggest that crustal growth of the Korean Peninsula has continued since ca. 3.6 Ga and culminated at ca. 2.5 and 1.9–1.8 Ga. Major age populations of detrital zircons of the Bansong and Nampo groups are similar, except for the presence of Middle‐Late Proterozoic ages in the latter. Inasmuch as these ages are characteristic for the South China Block, the Gyeonggi massif, or at least the local source of the Nampo Group, is most likely a correlative of the South China Block.