Yoonsup Kim

The pattern of bone marrow oedema on MRI in osteonecrosis of the femoral head

It has been suggested that transient osteoporosis or the bone marrow oedema syndrome (BMOS) may be the initial phase of osteonecrosis of the femoral head (ONFH) and that there may be a common pathophysiology. In this study, we have assessed the MR images of 200 consecutive patients with ONFH in respect of the BMO pattern in order to test this hypothesis. This pattern was not observed in the early stage of ONFH. The initial abnormal finding detected on the MR images was an abnormal band of intensity at the junction between the necrotic area and the normal bone. Structural damage of the head seems to result in the appearance of the BMO pattern and the development of pain in ONFH. There was no finding to support the existence of a continuum between BMOS and ONFH.

Metamorphic Evolution of the Imjingang Belt, Korea: Implications for Permo-Triassic Collisional Orogeny

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.

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

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 Ages of Detrital Zircons from Metasedimentary Rocks in the Yeongheung-Seonjae-Daebu Islands, Northwestern Gyeonggi Massif

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.

An Efficient Method for Zircon Separation Using the Gold Pan

We report an easy and efficient method for separating zircon grains using a gold pan and disposable sieve together with the tap water. Samples powdered to less than in grain size are placed on the water-filled pan tilted at about . The outflowing water flooded with low-density particles is screened by a sieve. The most efficient rate of water outflow is estimated to be 0.1 L/sec, when the distance between the tap and the surface of water-filled pan is kept at 20-25 cm. After 20-30 minutes of water outflow, heavy mineral fractions are ready for zircon collection using a hand magnet (and heavy liquid when necessary) or simply hand-picking. This procedure is most likely applicable for the separation of other heavy minerals including monazite.

Jurassic (~170Ma) Zircon U-Pb Age of a "Granite Boulder" in the Geumgang Limestone, Ogcheon Metamorphic Belt, Korea: Reinterpretation of its Origin

A “granite boulder”, ~70 cm in size, was reported from the Geumgang Limestone, and has been considered as a glaciogenic dropstone. Since this interpretation has enormous implications for unraveling the evolution history of the Ogcheon Metamorphic Belt, we re-examined the contact relationship and structure of the “granite boulder”, and estimated its emplacement age based upon SHRIMP U-Pb zircon dating. The weighted mean Pb/U age pooled from 6 spot analyses of two specimens is 170±2 Ma (2σ, MSWD=2.2). This zircon age suggests that the “granite boulder” in the Geumgang Limestone is a part of Jurassic granite, rather than a glaciogenic dropstone.