• The Journal of the Petrological Society of Korea
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• v.17 no.3
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• pp.144-153
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• 2008

Sr, Nd, Pb isotopic compositions of the Cenozoic basaltic rocks distributed in Pyeongtaek-Asan area display significantly enriched values compared with mid-ocean ridge basalts just like other Cenozoic basalts of Korea. The isotopic compositions of most of the Cenozoic basaltic rocks of Korea including those from Pyeongtaek-Asan area can be explained as mixing between enriched mantle component with relatively low $^{206}Pb/^{204}Pb$ ratios and depleted mantle component. In contrast, Jejudo basalts can be explained as mixing between enriched mantle component with realtively higher $^{206}Pb/^{204}Pb$ ratios and depleted mantle componsnt. Combined with that very similar division of enriched mantle components is applied to the Cenozoic basalts of northeast China and southeast China, it is suggested that subcontinental lithospheric mantle of central and southern parts of Korea represents eastern extension of North China Block and South China Block respectively. The indentation model for the late Paleozoic to early Mesozoic continental collision of China contradicts to such an interpretation, because it cannot explain occurrence of subcontinental lithospheric mantle component of South China Block-affinity under the Jejudo area. Instead, it is more probable that suture zone of the two continental blocks crosses between central and southern Korea and its location is further south from the Pyeongtaek-Asan area. Such distinct location compared with Imjingal belt, supposedly collisional boundary suggested before, suggests that mantle boundary may not be coincide with crustal boundary for the continental collision.

• Economic and Environmental Geology
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• v.31 no.3
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• pp.249-264
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• 1998

The Creataceous volcanic rocks distributed in the southeastern part (Kyeongsang basin) of Korea peninsula are composed of basalt, basaltic andesite, andesite, dacite and rhyolite. The variation of major elements show that contents of MgO, CaO, $FeO^T$, $Al_2O_3$, $TiO_2$ and $P_2O_5$ decrease with increasing of $SiO_2$, but $K_2O$ contents are increased slightly, $Na_2O$ widely dispersed. We can show slightly inflection point and low frequency of dacites in range between 63-65 wt.% $SiO_2$, while continuous trend exit in variation diagram. Variation trends in Harker diagrams for the major, minor, trace and REEs suggest that the BAV (basaltic to andesitic volcanics) and DRV (dacitic to rhyolitic volcanics) are not related to a simple crystal fractionation process. In the regime of under 65 wt. % in silica content, fractionation of olivine and clinopyroxene is predominant, while that of plagioclase happens strongly higher than 65 wt.% (e.g., $SiO_2$, vs. Eu and Sr, MgO vs. $Al_2O_3$ and CaO). The latter means low-pressure fractional crystallization for DRV. On the discriminant diagram, DRV are located in more mature environment than BAV. The $(Ce/Sm)_N$ vs. CeN digram shows that these two classes cannot be related to crystal fractionation. If they had been produced by fractionation, although they plotted in a slightly elongate cluster along the same horizontal trend, DRV should lie to the right of these primitive compositions. These diagrams clearly rule out a simple fractionation throughout from BAV to DRV. BAV had been influenced greatly subductiong slab as shown by K/Yb vs. Ta/Yb. We suggest that BAV primitive magma generated higher degree of partial melting than DRV primitive magma. LILE (K, Ba, $Rb{\pm}Th$) enriched characteristics as shown in BAV are inherited from subducting slab fluids and/or higher degree of partial melting of mantle material. However, lower degree of partial melting of mantle relative to BA V and contamination at high-level magma reservoir caused LILE enrichment to DRV.

• Journal of the Mineralogical Society of Korea
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• v.18 no.1
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• pp.33-43
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• 2005

Three types of clinopyroxenes in alkali basaltic rocks from Jeju Island can be identified on the basis of geochemical and textural data. Type Ⅰ is Cr-rich diopside in spinel peridotites from the upper mantle. Type Ⅱ is augite in fine-grained pyroxenites which are possibly either magmatic vein or metamorphic segregations owing to anatexis of the upper mantle. The augite of Type Ⅱ contains high Ca and Mg and relatively low Ti. Type Ⅲ is thought to be either cumulates or cognate phenocrysts and can be subdivided into Ⅲa, Ⅲb, and Ⅲc based on their occurrence mode. Clinopyroxenes of Type Ⅰ have the highest Mg# and Si and the lowest Ti, whereas those of Type Ⅲhave lower Mg#와 Si and higher Ti. These geochemical characteristics indicate that (Ti+Al/sup Ⅵ/)/Si and Al/sup Ⅵ//Al/sup Ⅵ/ increase from Type Ⅰ to Type Ⅲ. It is possibly interpreted that Type Ⅰ is of the highest pressure origin and Type Ⅲ of the lowest. Fractionation of high-pressure clinopyroxenes would result in evolved undersaturated alkali-enriched liquids, probably producing the alkali-enriched host basaltic rocks in Jeju Island.

• The Journal of the Petrological Society of Korea
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• v.1 no.1
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• pp.71-84
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• 1992

This paper reports the results about the petrography and mineral chemistry of 13 representative dredged basaltic rocks from the western Caroline Ridge (WCR) and Yap Trench-Arc system, and provides the chemical and tectonic informations based on the compositions of clinopyroxene phenocrysts. Compositions of olivine phenocrysts in some analyzed samples are Fo$_{86-80}$. Plagioclase phenocrysts have variable compositions ranging from An$_{90}$ to An$_{55}$. The compositions of clinopyroxene phenocrysts vary according to geological environments; titansalite in atoll and guyot of WCR, diopside-augite in trough and bank of WCR, and endiopside in Yap Trench-Arc system. Application of the discrimination schemes proposed by Leterrier et al. (1982) suggests: (1) the samples from atoll-guyot belong to within plate alkali basalt, implying that western CR could be the continuation of eastern CR formed by hot spot magmatism, (2) the samples from the Yap Trench-Arc system with no present-day magmatism clearly indicate the occurrence of orogenic tholeiites presumably related to early island arc magmatisms in this area, however, (3) the samples from the bank and trough do not provide definitive informations, which might indicate the complexity of their origins.

• Economic and Environmental Geology
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• v.55 no.3
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• pp.309-316
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• 2022

In order to minimize the human and time consumption required for rock classification, research on rock classification using artificial intelligence (AI) has recently developed. In this study, basic volcanic rocks were subdivided by using polarizing microscope thin section images. A convolutional neural network (CNN) model based on Tensorflow and Keras libraries was self-producted for rock classification. A total of 720 images of olivine basalt, basaltic andesite, olivine tholeiite, trachytic olivine basalt reference specimens were mounted with open nicol, cross nicol, and adding gypsum plates, and trained at the training : test = 7 : 3 ratio. As a result of machine learning, the classification accuracy was over 80-90%. When we confirmed the classification accuracy of each AI model, it is expected that the rock classification method of this model will not be much different from the rock classification process of a geologist. Furthermore, if not only this model but also models that subdivide more diverse rock types are produced and integrated, the AI model that satisfies both the speed of data classification and the accessibility of non-experts can be developed, thereby providing a new framework for basic petrology research.

• Korean Journal of Mineralogy and Petrology
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• v.35 no.3
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• pp.183-197
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• 2022

Geochemical data, including Sr-Nd-Pb-Mg-Zn isotopes, reported on the late Cenozoic intraplate basaltic rocks in the Korean Peninsula (Mt. Baekdu, Jeongok, Baengnyeong Island, Pyeongtaek, Asan, Ganseong, Ulleung Island, Dok Island, and Jeju Island) are summarized to constrain their mantle source lithologies, and the nature of mantle end-members required. In the Sr-Nd isotope correlation diagram, Jeju basalts plot in the field of EM2-type oceanic island basalts (OIB), while the other basalts fall in the EM1-type OIB field. In Pb-Pb isotope space, Jeju basalts show a mixing array between Indian MORB and EM2 component, whereas the other basalts display an array with EM1 component. The Korean basalts were derived from a hybrid source of garnet lherzolite and recycled stagnant slab materials (eclogite/pyroxenite, pelagic sediments, carbonates) in the mantle transition zone. The EM1 component could be ancient (~2.0 Ga) K-hollandite-bearing pelagic sediments that were isolated for a long period in the mantle transition zone due to their neutral buoyancy. The EM2 component might have been relatively young (probably Pacific slab) and recently recycled clay-rich pelagic sediments. Eclogite and carbonates are unlikely to account for the EM components, but they are common in the mantle source of the Korean basalts.

• Economic and Environmental Geology
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• v.8 no.4
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• pp.223-230
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• 1975

South Korea is divided tectonically into four segments. The Kyonggi-Ryongnam massif is composed of Precambrian schists and gneisses and consititutes a base for the succeeding formations. The Okcheon geosynclinal zone in the Kyonggi-Ryongnam massif strectches from southwest to northeast diagonally across the peninsula in a direction known as the Sinian direction. Its northeastern part is composed primarily of Paleozoic to early Mesozoic sedimentary formations and the southwestern part of the late Precambrian Okcheon metamorphic series. The Kyongsang basin occupies the southeast and southwest of the peninsula and is made up of a thick series of Cretaceous terrestrial sedimentary and andesitic rocks. A few small Tertiary basins are scattered in the eastern coastal area and in Cheju Island, and are composed of marine sedimentary and basaltic rocks. Jurassic Daebo granites intrude the Kyonggi-Ryongnam massif and the Okcheon zone in the Sinian direction, whereas late Cretaceous Bulkuksa granites are scattered randomly in the Kyongsang basin.

• The Journal of the Petrological Society of Korea
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• v.9 no.3
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• pp.142-168
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• 2000

The granites distributed in the Kyongsang basin contain the rocks which are different from the host rocks, and they are known as magic microgranular enclaves. The genesis of the magic micro-granular enclaves can be divided into four types: (1) rock fragments from country rocks; (2) cumulation of the early crystals in host magma or disruption of early chilled borders; (3) magma mingling; and (4) restite. These enclaves can be easily found in the granites around Mt. Wonhyo, Yangsan city. They are ellipsoidal in shape, and have phenocrysts might be originated from the host rocks and sharp contacts with the granites. Under the microscope, textures such as oscillation zoning, horn-blende-mantled quartz, rapakivi texture, and acicular apatite are observed, and these indicate that the enclaves were originated from magma and then produced by chilling. The evidences showing that the enclaves were formed by magma mingling are: (1) petrographical characteristics; (2) similarity of the compositions between the rim of plagioclase in the enclave and plagioclase in the granite; (3) linear trends of the major elements; (4) total REE content of the enclaves; and (5) Textural and compositional variations from rim to core in zoned enclaves. The magic end member of the enclave is regarded as the aphyric basaltic andesite in Mt. Sinbul-Youngchui area. The granites around Mt. Wonhyo experienced the magma mingling process which was produced by the injection of mafic magma at about 70 Ma, during the crystal differentiation, and then continued the crystallization. The equigranular granites and the micrographic granites in the study area are considered as the results after the magma mingling process.

• The Journal of the Petrological Society of Korea
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• v.17 no.2
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• pp.57-82
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• 2008

The volcanic sequence of the late Cretaceous Moonyu volcanic mass which distributed in the southwestern part of Ryeongnam massif, can be divided into felsic pyroclastic rocks, andesite and andesitic pyroclastic rocks, rhyolite in ascending order. The earliest volcanic activity might commence with intermittent eruptions of felsic magma during deposition of volcaniclastic sediments. Explosive eruptions of felsic pyroclastic rocks began with ash-falls, to progressed through pumice-falls and transmitted with dacitic to rhyolitic ash-flows. Subsequent andesite and andesitic pyroclastic rocks were erupted and finally rhyolite was intruded as lava domes along the fractures near the center of volcanic mass. Petrochemical data show that these rocks are calc-alkaline series and have close petrotectonic affinities with subduction-related continental margin arc volcanic province. Major element compositions range from medium-K to high-K. Petrochemical variation within the volcanic sequence can be largely accounted for tractional crystallization processes with subordinate mixing. The most mafic rocks are basaltic andesite, but low MgO and Ni contents indicate they are fractionated by fractional crystallization from earlier primary mafic magma, which derived from less than 20% partial melting of ultramafic rocks in upper mantle wedge. Based on the stratigraphy, the early volcanic rocks are zoned from lower felsic to upper andesitic in composition. The compositional zonation of magma chamber from upper felsic to lower andesitic, is interpreted to have resulted from fractionation within the chamber and replenishment by an influx of new mafic magma from depth. Replenishment and mixing is based on observations of disequilibrium phenocrysts in volcanic rocks. REE patterns show slight enrichment of LREE with differentiation from andesite to rhyolite. Rhyolite in the final stage can be derived from calc-alkaline andesite magma by fractional crystallization, but it might have underwent crustal contamination during the fractional crystallization.

• The Journal of the Petrological Society of Korea
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• v.19 no.3
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• pp.209-225
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• 2010

The Shinri area near the Yedang Lake, the eastern part of the Hongseong area in SW Gyeonggi Massif, consists of the Neoproterozoic Duckjeongri granodiorite-tonalite, mylonitized amphibole-bearing orthogneiss and impure marble with lens-shaped garnet-bearing metabasites. In this paper, we report mineralogical and geochemical data of Neoproterozoic lens-shaped garnet-bearing metabasites within marble of the Shinri area. The $SiO_2$ contents of garnet-bearing metabasites in marble vary between ~46.98 and 51.17 wt%, and the $Na_2O$ + $K_2O$ contents fall between ~1.95 and 2.85 wt%, similar to the tholeiitic sub-alkaline basaltic rocks. In the Zr/Y vs. Zr diagram, garnet-bearing metabasites also plot in the subalkaline basaltic rocks. The chondrite-normalized REE patterns for Shinri garnet-bearing metabasites show relatively flat patterns to that of chondrite. They show slight LREE-enriched and depleted patterns. The major and trace element data from lens-shaped garnet-bearing metabasites in marble of the Shinri area suggest that these rocks were formed in within plate. In contrast, previous major and trace element data of high pressure type garnet-bearing metabasites from the mafic-ultramafic complex in the Baekdong and Bibong areas suggest that these rocks were formed in a nascent arc to backarc spreading center within subduction zone setting. Based on mineral assemblage and mineral chemistry, P-T estimates for Shinri garnet-bearing metabasites are 9.6-12.7 kb, $695-840^{\circ}C$ for inclusions in the core, and 9.6-13.6 kb, $630-755^{\circ}C$ for those in the rim. These P-T estimates are distinct from those of the Baekdong and Bibong garnet-bearing metabasites with isothermal decompressional retrograde P-T path. In addition to Triassic tectonic activity previously reported in the Shinri area of Hongseong, the details of metamorphic history such as protolith age and Neo-Proterozoic metamorphic episode need to be solved.