Yong-Joo Jwa

Possible source rocks of Mesozoic granites in South Korea: implications for crustal evolution in NE Asia

The exposed Precambrian cratonic crust in South Korea is divided into two massifs – the Gyeonggi massif to the north and the Yeongnam massif to the south. Mesozoic granites intruded into both massifs and are mostly I-types. The Jurassic granites form extensive deep-seated batholiths, the Triassic granites are deep-seated stocks and the Cretaceous granites occur as volcanic-plutonic complexes. The systematic variation of e Nd and SrI in the Korean Mesozoic granites could result from the mixing of two components in different proportions to produce the source of the granites. Although most Mesozoic I-type granites were apparently derived from more juvenile crust, the old evolved crustal components seem to have been incorporated in the magmas in various proportions. Mantle–crust mixing can account for the generation of the source of the Triassic and Cretaceous granites in the Gyeongsang basin. On the other hand, crust–crust mixing can feasibly produce the source of the Triassic and Jurassic granites in the Yeongnam massif, the Jurassic granites in the Gyeonggi massif, and the Cretaceous granites in the Yeongdong–Gwangju basin and the Okcheon belt. However, some Jurassic granites in the Yeongnam massif and Cretaceous granites in the Yeongdong–Gwangju basin can be also explained by the mantle–crust mixing. Combined geochemical and isotopic signatures indicate that a simple binary mixing model is inadequate to explain both the geochemical and isotopic data. The chemistry of the granites is considered likely to reflect the composition of the igneous protolith that derived from depleted mantle, which explains why most Mesozoic granites in South Korea are represented by I-types, regardless of their temporal and spatial position. Nd-Sr isotopic signatures of the Mesozoic granites and basement rocks indicate that the continental crust beneath the Korean peninsular is vertically structured by the successive underplating of mantle-derived materials. It is postulated that the crust is vertically stratified from the surface to the lowermost crust with late Archean to early Proterozoic, early to middle Proterozoic (ca. 1·9 Ga), middle Proterozoic (ca. 1·5 Ga), and late Proterozioc (younger than 1·5 Ga) components.

A preliminary AMS (anisotropy of magnetic susceptibility) study of the Namwon granite, southwest Korea

Anisotropy of magnetic susceptibility (AMS) was measured in the Namwon granite body (217–176 Ma), southwest Korea, along with some microscopic observations. It was revealed that the Namwon granite has subvertical magnetic foliation and subhorizontal magnetic lineation, but only weak signs of solid-state deformation. The trend of the magnetic foliation and lineation dextrally converges into the trend of the Sunchang shear zone (NNE) along the western margin of the Namwon granite body. The patterns of the magnetic foliation and lineation, as well as the weak signs of solid-state deformation, suggest that the magnetic fabrics reflect the synplutonic flow pattern of the Namwon granite influenced by the dextral ductile shearing along the Sunchang shear zone. Although the Namwon granite was considered as post-tectonic pluton, i.e., a pluton that postdated the dextral shearing along the Sunchang shear zone, the dextral movement must have lasted after the full consolidation of the Namwon granite body.

Cretaceous to Early Tertiary Granites and Magma Mixing in South Korea : Their Spatio-temporal Variations and Tectonic Implications (Multiple Slab Window Model)

Based on the petrologic and age data of the Cretaceous to early Tertiary granites in south Korea, we propose a new tectonic model reflecting their temporal and spatial variations. A number of petrographic and geochemical studies on the granites suggest that they originated from the magma formed by subduction of oceanic crust in continental margin and were emplaced in epizone. The MMEs with various shapes and sizes, which were produced due to the magma mixing caused by the injection of mafic magma from mantle during the crystallization of the granitic magma, are observed in the granites. The distributions of the MMEs and ages of the granites show a distinctive spatio-temporal distribution pattern. The distribution pattern can be explained by a multiple slab window model related to the ridge subduction of Izanagi-Pacific plates during the Late Cretaceous.

Petrology and geochemistry of the Youngju and Andong granites in the northeastern Yeongnam Massif, Korea

Two late Paleozoic, partly deformed granitic batholiths, the Youngju and Andong granites, regionally occur in the northeastern Yeongnam Massif. The deformed granites generally occur along the shear zone trending northeast in the central part of the study area, and their foliations are subparallel to the trend of shear zone, indicating post-emplacement deformation. Both granites belong to tonalite-granodiorite-granite association, but have dominantly granodiorite composition. They have similar chemical features, corresponding to metaluminous, calc-alkaline, I-type, and volcanic arc granites. Most chemical variation trends suggest that mineral fractionation patterns in the two granites should be similar except for the plagioclase fractionation. Relative to the Andong granites, the Youngju granites contain significantly higher FeO, MnO, MgO, K2O, Rb, Th, U, Sc, and Pb, and lower Al2O3, Na2O, P2O5, TiO2, Sr, Zr, and Ga. The Youngju granites also have higher K2O/Na2O, Rb/Sr, and Th/Y ratios, and a lower K/Rb ratio than the Andong granites. The geochemical differences between the two granites are prominent, particularly, in mafic facies. The more incompatible nature of the Youngju granites may be attributed to higher degree of mixing or assimilation with old crustal materials during ascent or emplacement of magma. Taking into account that the tonalitic compositions are the most mafic facies of the two granites, the protoliths seem to have been mafic igneous rocks or their metamorphic equivalents distributed in the lowermost crust.

Petrological and geochemical evidences for magma mixing in the Palgongsan Pluton

Cretaceous Palgongsan pluton in the Gyeongsang basin is composed of coarse-grained porphyritic granite, medium-grained equigranular granite and aplite. The granites are designated to cale-alkaline I-type granites. Mafic enclaves are easily found in the pluton, and they are textually classified into fine-grained microgranular mafic enclaves and medium-grained mafic enclaves. The mafic enclaves in the granites are igneous in origin and comprise plagioclase, amphibole, biotite, and small amount of apatite, opaque, pyroxene and quartz. The mafic enclaves characteristically show petrographical features such as fine-grained texture, mafic clots, acicular amphibole, acicular apatite, plagioclase xenocryst, rapakivi texture and ocellar quartz. This observation demonstrates that the mafic enclaves are derived from mafic magma injection into crystallizing granitic magma chamber. There are good linear correlations in major oxide contents vs. SiO2 content in the Palgongsan pluton. Particularly the compositional variation between the granites and the mafic enclaves is possibly ascribed to two-component mixing model. Highly linear compositional variation trends between the Palgongsan porphyritic granite and the fine-grained mafic microgranular enclaves can be best explained by the compositional mixing of granitic magma with mafic magma which injected into the crystallizing granitic magma chamber. However the compositions of the equigranular granite and the aplite are characteristically of highly fractionated rock, and they are caused by later stage fractionation of granitic magma independent on the two-component mixing

Petrological Characteristics and Provenance Estimation on the Stone Artefacts from the Pocheon Neulgeori Prehistoric Site, Korea

본 연구에서는 경기도 포천시 관인면 중리 늘거리 유적지에서 출토된 일부 석기의 육안 및 현미경 관찰 그리고 모드분석 결과를 근거로 주변 지역에 대한 야외지질조사를 통해 석기의 산지를 추정하였다. 연구에 사용된 늘거리 석기 12점은 화산회 응회암, 화산력 응회암, 화강반암, 안산암, 섬록반암 등의 암석으로 동정되었으며, 석기의 용도와 암석의 종류를 살펴보면 격지는 다양한 암석으로, 돌날과 돌조각은 주로 응회암질 암석으로 제작되었다. 석기의 산지로 화산력 응회암은 철원분지에 분포하는 동막골 응회암, 화강반암은 옥산리 남동쪽, 연천 동쪽에 소규모 암주로 산출되는 백악기 관입암류인 화강반암, 안산암은 유적지 북쪽에 위치한 안산암체가 그 산지로 밝혀졌으며, 섬록반암은 유적지 남쪽에서 위치한 섬록암체가 산지로 추정된다. 산지해석 결과, 연구 대상 석기는 유적지로부터 최대 15 ㎞ 이내의 주변에서 산출되는 암석과 같거나 유사하다.

A Petrological Study of Stones Used in the Three Storied Stone Pagoda of Bulguksa Temple

본 연구에서는 현재 해체 복원으로 부재 접근성이 용이해진 불국사 삼층석탑의 전체 부재를 대상으로 암석기재학적 연구, 대자율과 감마스펙트로미터를 이용한 비파괴 분석을 실시하여 암석학적인 재질 특성을 밝히려 한다. 또한 주변 지역과의 비교 연구를 통해 삼층석탑의 보존처리에 활용 할 신석 후보지역을 선정하고자 한다. 육안관찰 결과, 삼층석탑에 사용된 암석부재의 특징은 경주시 주변에 분포하고 있는 남산화강암의 특징과 유사하고, 상륜부는 보개를 제외한 모든 부재는 남산화강암과 유사하지만 보개는 암색과 구성광물 및 입자크기가 주변의 토함산화강암과 유사하다. 비파괴 분석결과, 삼층석탑에 사용된 암석의 부재의 대자율 및 감마스펙트로미터 측정값은 보개를 제외하면 남산화강암과 유사하며, 반면에 보개는 토함산 화강암의 측정값과 유사하다. 삼층석탑에 사용된 부재가 훼손으로 인해 신석으로 교체가 불가피할 경우 남산화강암이 가장 적합하다. 한편 남산 화강암은 양산단층에 의해 분리되었으며, 이로 인해 경주 남산을 제외한 또 다른 야외에서 확인되는 지점은 경주시 동천동 일대와 포항시 흥곡리 부근이며 삼층석탑의 신석제작용 채석 후보지로 적절할 것으로 사료된다.In this study, the stone used for three storied pagoda of Bulguksa Temple, which is easy to access due to works for dismantle and restore will be analyzed on the basis of petrographic study, magnetic susceptibility, and -ray spectrometer, and identify petrographic characteristics. Also we will select candidate areas of fresh rock to change the stone used for three storied stone pagoda as from its provenance is presumed. According to the results of visual inspection, the stones used for the three storied stone pagoda are similar to the features of Namsan granite, which is distributed around Gyeongju-si, and when it comes to the section of the stupa finial, the features of all the parts from the stupa finial is similar to Namsan granite except for the section of Jeweled cover. On the other hand, the color, the composition, and the mineral size of the stones in the section from the part of jeweled cover are similar to Tohamsan granite. As a result of none-destruction inspection, it seems that the stones used for the three stories stone pagoda except for the section of jeweled cover are similar to the values for the magnetic susceptibility and the -ray spectrometer. On the other hand, Jeweled cover and the values of Tohamsan granite are alike. Namsan granite is appropriate in case that the stone used for three storied stone pagoda must be replaced to fresh rock because of damage by weathering. Meanwhile, Namsan granite is seperated from Yangsan fault. Due to this fact, Namsan granite appears to not only Mt. Namsan in Gyeongju, but also Dongcheon-dong, Gyeongju, and Huenggok-ri, Pohang. As a result, these two areas are suitable to the candidate area to change the stone used for three storied stone pagoda.

Determination of Rock Cleavages Using AMS (Anisotropy of Magnetic Susceptibility): a Case Study on the Geochang Granite Stone, Korea

In granite quarry, stones are generally quarried along easily separating planes called as ‘rock cleavage’. Because orientation and characteristics of the rock cleavage are directly involved with easy quarrying, it is the most important factor on selecting a direction of digging. Using AMS (anisotropy of magnetic susceptibility), we attempt to interpret rock fabrics in Geochang Granite Stone (JS, SD, AR, GD, BW, MD quarry) and discuss about determination of rock cleavages and correlation between the rock fabrics and cleavages. Based on mean susceptibility, thermo-susceptibility curves, and hysteresis parameters, Ti-poor MD and/or PSD magnetites are the main contributor to AMS of the granite stones. The systematic magnetic foliations with sub-vertical dip angle are developed in the whole granite quarries. In most of the granite quarries, the magnetic foliations are significantly consistent with grain plane. In the BW quarry, which has higher P J values than the others, the magnetic foliations coincide exceptionally with *Corresponding author Tel: +82-51-510-2248 E-mail: moonson@pusan.ac.kr 210 조형성·김종선·김건기·강무환·손영관·이윤수·좌용주·손 문 J. Petrol. Soc. Korea rift plane. These results suggest that rock cleavages in granite stone are related to rock fabrics meaning shape and spatial arrangement of crystals. Magnetic fabrics analysis using AMS method, therefore, can be a quantitative and effective tool for determination of rock cleavages in granite quarry.

A Preliminary Study of Korean Geostansdards Using Mesozoic Granites

In this study, we selected three representative granite samples and conducted petrological observation to establish the Korean geostandards. Samples were taken from the two Jurassic (KJG-1, KJG-2) and one Cretaceous (KCG1) granites in South Korea. The powder samples were prepared by the standard pulverization process, and glass beads were made for geochemical analysis using X-ray fluorescence (XRF) method, and finally, major element contents of the samples were acquired. The analytical data are shown with mean, standard deviation and relative standard deviation. The accuracy of the analysis was confirmed within an estimated error range of about 5% by comparing the recommended true values of the USGS and GSJ geostandards. Also, we checked the analytical precision by calculating a relative standard deviation of about 3% from the XRF analytical results for the three samples.