• The Journal of the Petrological Society of Korea
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• v.9 no.1
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• pp.1-12
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• 2000

The Cretaceous basaltic rocks in Gyeongsang Basin are temporally and spatially dispersed widely in thick sedimentary piles: Chilgog basaltic rock (CGB) and Cheongyongsa basaltic rock (CSB) in the Shindong Group, and Hakbong basaltic rocks (HBB), Osibbong basalt (OSB), Secheondong basaltic rocks (SCB), Haman basaltic rocks (HAB), Hama basaltic rocks (HMB), and Chaeyaksan basaltic rocks (CYB) in the Hayang Group, upwardly in their stratigraphy. Chilgog basaltic rock is merely identified as pebbles in the Shilla Conglomerate and its provenance has not been found, and it is characteristics that the volcanics except Osibbong basalt and Chaeyaksan basaltic rocks are very small in both of their thickness and extension. Petrochemical diversity of the basaltic rocks are revealed; OSB and SCB distributed in the Yeongyang Minor Basin preserve the calc-alkaline natures in major and immobile minor element geochemistry, but CGB, HBB, HAB, and CYB reflect that they might be originated from calc-alkaline basaltic magma of volcanic arc in continental margin area by trace elements and altered to alkaline suites in the viewpoint of their major element geochemistry. Major and trace element geochemistry of CSB and HMB suggests that they may be derived from within -plate alkaline magma contaminated by the upper continental crust, especially in the case of the former.

• Proceedings of the Petrological Society of Korea Conference
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• 1999.06a
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• pp.12-12
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• 1999

• Economic and Environmental Geology
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• v.54 no.1
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• pp.1-19
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• 2021

This study reports the Ar-Ar dating results for the volcanic rocks from small volcanoes(oreum) of the Hallasan Nature Reserve. According to the age of 40Ar/39Ar, the volcanic activity of the Hallasan Natural Reserve was started from about 192 ka ago. The basaltic trachyandesite and trachyte located in the Y valley near the Eorimok in the western part of the Hallasan Natural Reserve represent an age of about 191~192 ka, showing the oldest record of volcanic activity in the Hallasan Natural Reserve. In the Hallasan Natural Reserve, the small volcanoes older than 100 ka are Y Valley in Eorimok area (192±5 and 191±5 ka), Dongsu-Ak (184±19 ka), Mansedongsan (153±5 ka), Janggumok-Orum (135±6 ka), Eoseungsaengak (123±9 ka), Samgagbong (105±2 ka). And the small volcanoes younger than 100 ka are Witbangae-Oreum, Seongneol-Oreum, Muljangol, Yeongsil, Bori-Ak, Witsenueun-Oreum, Witsejokeun-Oreum, Heugbuleun-Oreum, Bangae-Oreum, Albangae-Oreum, Witsebuleun-Oreum, Baengnokdam, Nongo-Ak. According to the eruption of trachytes, the Hallasan Natural Reserve can be interpreted as having about 8 volcanic activities. Among them, 4 volcanic activities are related with the formation of trachyte dome, such as Wanggwanneung, Samgakbong, Yeongsil, and Baengnokdam, and 4 volcanic activities are related with flow or dyke of trachyte. The volcanic activity at the Hallasan Natural Reserve was started from northwest area, to in the southern area, and in the eastern area, and finally volcanic activity related to the formation of Baengnokdam.

• The Journal of the Petrological Society of Korea
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• v.7 no.1
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• pp.27-36
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• 1998

The volcanic piles in the northern Yucheon Minor Basin area are the Hagbong basaltic rocks, the Chaeyaksan basaltic rocks, the Jusasan andesitic rocks, the Unmunsa rhyolitic rocks, and the Tertiary voicanics. Stratigraphically, from the lowermost, (1) the Hagbong basaltic rocks are composed mainly of basaltic tuff with two olivine basalt flows intercalated, (2) the Chaeyagsan basaltic rocks are predominantly in tuffs and agglomerate with 3 basaltic flow interlayers, (3) the Jusasan andesitic rocks consist of thick piles of alternated sequences of 4 andesite flows and 5 andesitic tuffs and tuffaceous sediments and (4) the Unmunsa rhyolitic rocks which embed some rhyolite and obsidian are dominant in tuffs such as ash flow and crystal welded tuff. These volcanics reveal distinguishable characteristics in petrochemistry. In discriminating by major elements, the Hagbong and the Chaeyagsan basaltic rocks are alkaline, whereas the latter is also spilitic. In comparison, the volcanic rocks of the Jusasan andesitic rocks and the Tertiary sequences are characteristically calc-alkaline although their distribution is spatially separated. On the other hand, the variations in immobile trace elements indicate that the Hagbong basaltic rocks range from alkaline to calc-alkaline and from WPB/VAB transition to VAB, whereas the Chaeyagsan basaltic rocks are calc-alkaline WPB/VAB transition type and the two others calc-alkaline VAB. In order to show such a variety in their rock series of the volcanic rocks, the environment during their magma generation, magma rising, and post-eruption alteration could be positively considered.

• The Journal of Engineering Geology
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• v.18 no.3
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• pp.321-330
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• 2008

This study, geological survey was carried out in order to study on the geology, geological structure and volcanic activity of the Ulleung-do volcano body. Ulleung-do is the volcano body of about 3,000m heights from the East Sea seabed. The geology of Ulleung-do is divided into basaltic agglomerate, trachytic agglomerate, trachyte, trachytic pumice and trachyandesite in ascending orders. The faults in caldera of Nari Basin came to make the reverse triangle style in compliance with sinking. The faults in circumference of Nari Basin are ranging with northeast-southwest direction and northwest-southeast direction. The Quaternary volcanic activities in the Ulleung-do are divided into 5 activity period. The engineering geologists and the applied geologists were not easy to apply because complicated geology of Ulleung-do. Therefore, this study supplied simple geology of Ulleung-do for them.

• The Journal of the Petrological Society of Korea
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• v.10 no.2
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• pp.121-131
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• 2001

The combination of geological, structural and satellite image studies is used to make an examination of the Miocene eruptive type in the Eoil Basin, SE Korea. The basin subsided by the NW-SE extension due to NNW dextral shearing during the East Sea opening. Based on geological structures as well as lithofacies and ages of the basin-fills, it is divided into the NE subbasin and the SW subbasin which were abundantly filled with basaltic volcanics and marine sediments without volcanic materials, respectively: Syndeposional synclines and anticlines are characteristically developed in the NE subbasin, which amplitudes decrease away from the adjacent normal faults to make them into a homoclinal structure. The thicker lavas as well as the younger agglomerates and lacustrine sediments, which show circular distributions, are distributed around the axial zones of major synclines. The satellite image shows four remarkable circular structures within the NE subbasin. They are located adjacent to and along the normal faults, and they are laid almost exactly on the axial zones of the synclines as well as on the distribution area of the agglomerates and lacustrine sediments. These facts indicate that the basaltic lava effusion were conducted by the normal faults like a kind of fissure-eruption and its activity was more predominant at the sites in where the synclines are developed. More active effusion of lava became a reason for deeper subsidence to make differential subsidence and syndepositional folding adjacent to and along the normal faults. Hence, we suggest that a nested cauldron structure was formed in the NE subbasin of the Eoil Basin, and that the volcanism made the subbasin to be a lava pond and controlled the process of filling and sedimentation in the subbasin.

• Economic and Environmental Geology
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• v.41 no.1
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• pp.93-113
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• 2008

This study presents petrochemistry and $^{40}Ar/^{39}Ar$ absolute ages of subsurface volcanic rock cores from twenty(20) boreholes in the eastern lowland (altitude loom below) of Jeju Island, Handeong-Jongdal-Udo-Susan-Samdal-Hacheon areas, and discusses topography and volcanism in the area. The subsurface volcanic rock cores are mainly basalts in composition with minor tholeiitic andesites and basaltic trachyandesites. Sequences of intercalated tholeiitic, transitional and alkalic lavas suggest that tholeiitic and transitional to alkalic lavas must have erupted contemporaneously. Especially, occurrences of trachybasalts and basaltic trachyandesites at the bases in the area imply that the volcanism in the area was initiated with slightly differentiated alkaline magma activity. The $^{40}Ar/^{39}Ar$ absolute ages of the subsurface volcanic rock cores range from $526{\pm}23ka\;to\;38{\pm}4Ka$. The lava-forming Hawaiian volcanic activities of the eastern lowland can be divided into five sequences on the basis of sediment distribution, whole rock geochemistry and $^{40}Ar/^{39}Ar$ absolute ages of the subsurface volcanic rock cores; stage I-U$(550{\sim}400Ka)$, stage II$(400{\sim}300Ka)$ and stage III$(300{\sim}200Ka)$ during syn-depositional stage of Seoguipo Formation, and stage IV$(200{\sim}100Ka)$ and stage V(younger than 100Ka) during post-depositional stage. In the eastern lowland of Jeju Island, compositional variations and local occurrences of the subsurface volcanic rocks as well as existences of various intercalated sediment layers (including hydrovolcanogenic clasts) suggest that the volcanism must have continued for long time intermittently and that the land has been progressively glowed from inland to coast by volcanic activities and sedimentation. It reveals that the subsurface volcanic rocks in the eastern lowland of Jeju Island must have erupted during relatively younger than 200Ka of stages IV and V. The results of this study are partly in contrast with those of previous studies. This study stresses the need that previous reported volcanic activities in Jeju Island based on K-Ar ages of volcanic rocks should be carefully reviewed, and that stratigraphic correlation from boreholes should be conducted by quantitative criteria combined with petrography and petrochemstry as well as radiometric studies of volcanic rock cores.

• The Journal of the Petrological Society of Korea
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• v.12 no.1
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• pp.32-52
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• 2003

The Mt. Baegyang in Busan, composed of sedimentary basement rocks (Icheonri Formation), andesite (lava), andesitic pyroclastic rocks, fallout tuff and tuffaceous sedimentary rocks, rhyolitic pyroclastic rocks, intrusive rocks (granite-porphyry, felsite, and biotite-granite) of Cretaceous age in ascending order. The volcanic rocks show a section of composite volcano which comprised alternation of andesitic lava and pyroclasitc rocks, rhyolitic pyrocalstic rocks (tuff breccia, lapilli tuff, fine tuff) from the lower to the upper strata. From the major element chemical analysis, the volcanic and intrusive rocks belong to calc-alkaline rock series. The trace element composition and REE patterns of volcanic and plutonic rocks, which are characterized by a high LILE/HFSE ratio and enrichments in LREE, suggest that they are typical of continental margin arc calc-alkaline rocks produced in the subduction environment. Primary basaltic magma might have been derived from partial melting of mantle wedge in the upper mantle under destructive plate margin. Crystallization differentiation of the basaltic magma would have produced the calc-alkaline andesitic magma. And the felsic rhyolitic magma seems to have been evolved from andesitic magma with crystallization differentiation of plagioclase, pyroxene, and hornblende.

• Journal of the Mineralogical Society of Korea
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• v.25 no.2
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• pp.63-76
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• 2012

Tachylite, black basaltic glass formed by the rapid cooling of molten basalt, locally occurs at the Basaltic Agglomerate Formation (BAF), the lowest formation of Ulleung Island. The purposes of this study are to characterize the occurrence and mineralogy of tachylite and to elucidate its formation condition, with emphasis on its fracture pattern, which can serve as implications for the early volcanic activity of Ulleung Island. To this end, we investigated the occurrence pattern of tachylite in the field and carried out mineralogical analyses using optical microscope, XRD, EPMA, and SEM. Tachylite occurs at the chilled margin of basic dikes which are distributed in Naesujeon, Dodong and Jeodong seasides, Turtle Rock, and Yaerimwon, whose widths vary from several cm to 10 cm. It is evident that the outer surface of tachylite is dense and smooth, whereas the inner surface, if fractured, is characterized by conchoidal fracture. The matrix of tachylite consists of amorphous, glass and some fine-grained phenocrysts present in tachylite include biotite, anorthoclase, sanidine, plagioclase, hornblende, and Fe-Ti oxides. The fracture patterns characteristic of tachylite are subrounded, oval, or less commonly polygonal, bounded by joints to form globule or lump. Taking into account texture and mineralogy, tachylite is interpreted to have undergone little subsequent alteration at low temperature via hydration or hydrolysis that could form clay minerals after it was formed. Because tachylite with peculiar fractures occurs as dikes in a close association with BAF, its presence is considered as reliable evidence that when tachylite formed, the most part of BAF was still under subaqueous conditions, or at least saturated with seawater.

• Journal of the Korean earth science society
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• v.30 no.1
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• pp.33-39
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• 2009

This paper presents the occurrence and characteristics of the basaltic andesite-siltstone peperite in the lower part of the Gyehwari Formation (Cretaceous), Buan-gun, Jeonbuk province, SW Korea. The peperite is associated with tabular basaltic andesite body, concordantly intercalated with red siltstone and silty sandstone interbeds of floodplain facies. Development of the peperite along the upper margin of the andesite and its textural transition from a dispersed blocky type inward into a closely packed type collectively indicate an intrusive origin (?sill) of the andesite. Magma intrusion and subsequent peperite formation suggest an active syndepositional volcanism since the early stage of evolution of the Gyehwa Basin. The andesite is dated at Late Cretaceous (Santonian) by K-Ar whole-rock radiometric method.