Keewook Yi

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

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

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SHRIMP U-Pb Zircon Ages of the Yeongju and Andong Granites, Korea and their Implications

\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.

SHRIMP U-Pb ages and Hf isotopic composition of the detrital zircons from the Myogok Formation, SE Korea: development of terrestrial basin and igneous activity during the early Cretaceous

This study presents sensitive high-resolution ion microprobe (SHRIMP) U-Pb zircon ages, K-Ar ages, and whole-rock chemical and isotopic (Sr-Nd) compositions of the Early Cretaceous igneous activity in western South Korea. The volcanics and plutons yield zircon U-Pb ages of ca. 110 Ma, in the Albian of the late Early Cretaceous. K-Ar biotite and plagioclase ages are slightly younger, ca. 103 Ma. The geochemical signatures of the rocks are characterized by negative Nb, Ta, and Ti anomalies, relatively low εNd(T) values (between −15.91 and −9.36), and enrichment in light rare earth elements, suggesting the effects of subduction-related magmatism. Together with the Sr and Nd isotopic compositions of the intermediate to acid volcanics, geochemical characteristics reflect a lower-crustal magma source. This Albian event over the Korean Peninsula is related to Early Jurassic subduction and subsequent lithospheric delamination and/or slab rollback after the Early Cretaceous amalgamation between the North China Block (e.g., China, Korea, and Japan) and the paleo-Eurasia continent. This postorogenic event was accompanied by the initial formation of voluminous igneous activity and subsequent basin formation at about 130–110 Ma. Crustal relaxation during this stage should be related to this postorogenic event and is attributed to paleo–Pacific Plate subduction. This result provides important clues for the Cretaceous tectonic evolution of Northeast Asia in the context of plate convergence.

Medium Temperature and Lower Pressure Metamorphism and Tectonic Setting of the Pyeongan Supergroup in the Munkyeong Area

Abstract The paragenetic relationships and Th-U distributions among allanite, monazite, and xenotime were investigated in a progressive sequence of garnet- to kyanite-zone metapelites of the Imjingang belt, Korea. Allanite is predominant in the garnet and staurolite zones, whereas monazite and xenotime predominate in the kyanite zone. Epidote grains in the lower garnet zone are commonly zoned, from allanite (core) to relatively Y-rich, rare-earth-element (REE)-epidote to clinozoisite (rim), although both REE-epidote and clinozoisite disappear in higher-grade metapelites. Moreover, allanite and REEepidote often contain minute inclusions of thorium silicate. The isogradic distributions and similarity of REE patterns between allanite and monazite suggest that the latter has grown at the expense of the former. In addition, the discontinuous Th zoning in monazite is apparently inherited from heterogeneous Th distribution and thorium silicate inclusions in allanite. Thus, thorium silicate possibly provided the additional Th and U necessary for the monazite formation. Paragenetic relationships of allanite and monazite inclusions within various index minerals suggest that at amphibolite-facies conditions allanite is stable at higher pressures than monazite. Xenotime grains in the staurolite zone are rarely produced by the breakdown of clinozoisite and REE-epidote, whereas those in the kyanite zone are grown primarily at the expense of garnet. Incorporation of Th and U into monazite and xenotime is governed mainly by the brabantite and thorite substitutions, respectively. Taken together, our results suggest that the allanite-to-monazite transformation is primarily responsible for the distributions of REEs, Th, and U among metapelitic phases, and that the xenotime formation was facilitated by the contribution from major silicates, particularly garnet.

SHRIMP U-Pb Zircon Geochronology and Geochemistry of Drill Cores from the Pohang Basin

We investigated the various lithologies and zircon U-Pb ages of metasedimentary rocks from the Yeongheung-Seonjae-Daebu Islands, western Gyeonggi Massif, whose geologic and geochronologic features are poorly constrained in spite of their significance for tectonic interpretation. Major lithology consists of quartzites or meta-sandstones commonly alternating with semi-pelitic schists, together with lesser amounts of calcareous sandstones with matrix-supported quartzite clasts, calcareous schists, and pelitic schists. Pelitic schists uncommonly contain large porphyroblasts of garnet as well as quartz veins with large crystals of muscovite and andalusite or kyanite. SHRIMP U-Pb ages of detrital zircons from two analyzed metasandstones define four age populations: Neoarchean (~2.5 Ga), Paleoproterozoic (~2.0-1.5 Ga), Neoproterozoic (~1.1-0.7 Ga), and Early Paleozoic (~560-400 Ma). The youngest zircon ages are clustered at ~420 Ma. These results suggest that the deposition of meta-sandstones took place after the Silurian, possibly during the Devonian, and are analogous to those of the Taean Formation reported from the western part of the Gyeonggi Massif. Moreover, The age distribution patterns of detrital zircons and the Barrovian-type metamorphic facies of pelitic schists are similar to those reported from the Imjingang belt, suggesting that the Taean Formation likely corresponds to southwestward extension of the Imjingang Belt.

Mineral ages and zircon Hf isotopic composition of the Andong ultramafic complex: implications for the evolution of Mesozoic subduction system and subcontinental lithospheric mantle beneath SE Korea

SHRIMP zircon U-Pb age dating is carried out for the Yeongju and Andong granite batholiths intruding the Precambrian metamorphic complex and Paleozoic sedimentary formations within the NE Yeongnam Massif, Korea. Dating of zircons from a hornblende-biotite tonalite and an equigranular biotite granodiorite in the Yeongju granite has yielded ages of ca. 187 Ma and ca. 186 Ma, respectively. Also, dating of zircons from a biotite granodiorite and a very coarse-grained biotite granite in the Andong granite has yielded ages of ca. 182Ma and ca. 186Ma, respectively. These data indicate that the main intrusions of the Yeongju and Andong granite batholiths occur almost at the same age. The oldest age of ca. 194 Ma has been determined on zircons from a hornblende gabbro in the Andong granite, and the youngest age of 175 Ma is obtained from the Chunyang granite pluton, mainly consisting of fine-grained two-mica granite, of the Yeongju batholith. These results indicate that Jurassic Daebo magmatism in the Yeongju-Andong area, NE Yeongnam massif, started early at the Early Jurassic with an intrusion of mafic magma, and followed by an emplacement voluminous granite magma during the middle of the Early Jurassic, and was finalized with the emplacement of relatively small amount of much evolved granite magma at the end of Early Jurassic.

The Origin and Age of the Orbicular Granite Gneiss in Wangjungri, Muju

To constrain sedimentation age of the Myogok Formation and timing of the last episode of the Daebo orogeny, SHRIMP U-Pb age determination and Hf isotopic studies were carried out for the zircon separates. The detrital zircon U-Pb ages of the Myogok Formation show a wide range from Archean to Cretaceous and yield four main clusters at age intervals of 130–150 Ma, 170–220 Ma, 221–320 Ma and Paleoproterozoic. Such age clusters indicate that the main provenance rock types during the sedimentation were basement rocks of the Yeongnam massif and the Mesozoic granitoids distributed around the Myogok Formation. The concordia age of the youngest population is 138.6 ± 2.1 Ma (n = 7), indicating the possible maximum deposition age for the Myogok Formation. Considering the suggestion of probable deposition of Nakdong Formation during the Barremian (Lee et al., 2012), the Valanginian or Hauterivian seems to be the most appropriate timing for the deposition of the Myogok Formation. The last activity of the Daebo orogeny might be constrained to Hauterivian or Barremian considering some time lag needed for basin closure and deformation that followed. The measured εHf(t) values from the detrital zircons of the Myogok Formation vary from −25.7 to 4.8. However, most of the zircons, even those with Mesozoic ages, show negative values except a few, implying dominant influence of old continental crusts in the generation of their igneous protoliths.