• Journal of the Geological Society of Korea
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• v.54 no.5
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• pp.513-527
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• 2018

We have carried out granulometry and component analysis on pyroclastic deposits of the Maljandeung Tuff, Ulleung Island, to interpret the eruption types and prime dynamic mechanisms. It is divided into three members in the extracaldera area, each of which comprises the lithofacies of coarse tuffs and lapillistones in the lower part, and pumice deposits in upper one. The lithofacies present quantitative evidences in the granularity and component distribution patterns. As compared to the pumice deposits, the coarse tuffs and lapillistones exhibit a relative increase in both the lithic/juvenile and the crystal/juvenile ratios, and a preferential fragmentation of the juvenile fraction. The abundance of lithics and crystals in the tuffs and lapillistones can be attributed to preferential fragmentation of the aquifer-hosting rocks due to explosive evaporation of ground water, and indirect enrichment in lithics and crystals due to removal of juvenile fines from eruptive cloud. The above data exhibit that early phreatopmagmatic phase was followed by purely magmatic fragmentation phases. The coarse tuffs and lapillistones suggest phreatoplinian eruption derived from explosive interaction of magma with ground water near the conduit, while pumice deposits indicate magmatic eruption by magmatic explosion from juvenile gas pressure. In early stage, phreatoplinian eruption occurred from explosive magma/water interaction in connecting confining water with drawdown of the magma column in the conduit; Later it shifted to plinian eruption by explosive expansion of only magmatic volatiles in intercepting water influx due to higher magmatic gas pressure than confining water pressure with rising of the magma column in the conduit.

• Journal of the Korean earth science society
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• v.43 no.1
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• pp.140-150
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• 2022

This paper focuses on introducing the concept of the multi-scale study on the Ulleungdo-Dokdo Volcanic Group in the East Sea and recent new findings from it. Multi-channel seismic reflection data reveals that the major volcanic activities of the Ulleungdo-Dokdo Volcanic Group took place between 5 and 2.5 Ma, which were propagated from Isabu Tablemount on the eastern end to the Ulleung Island on the western end. The terrestrial Ulleung Island was built via 5 stages, which eventually formed a 3 km-wide caldera, named Nari Caldera, and a volcanic dome, named Albong, within the caldera. The Albong and the unit N-1, the earliest phreatomagmatic explosive phase of the Albong volcano, were generated from a new magma injected into the existing phonolitic body. The generally trachyandesitic bulk rock composition of the pumice in unit N-1 and Albong is attributed to the contamination of the new magma by mafic cumulates at the base of the existing phonolitic chamber. The lines of evidence of a new magma injection point toward that Ulleung Island is an active volcano with a live subvolcanic magma plumbing system.

• Proceedings of the Mineralogical Society of Korea Conference
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• 2001.06a
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• pp.95-95
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• 2001

The Yeongdong basin is one of the pull-apart basins in the southwestern part of the Korean Peninsula that has developed during Cretaceous sinistal fault movement. The bimodal igneous activities (basalts and rhyolites) in the basin appear to be closely associated with the basin development. Here, we discuss the origin of the igneous rocks using chemical and radiogenic isotope data. Basaltic (48.4-52.7 wt% SiO$_2$) and rhyolitic (70.3-70.8 wt% SiO$_2$) rocks are slightly alkalic in a total alkali-silica diagram. The rhyolitic rocks with have unusually high K$_2$O contents (5.2-6.0 wt%). The basaltic rocks show an overall pattern of within-plate basalt in a MORB-normalized spider diagram, but have distinct negative anomaly of Nb, which indicates a significant amount of crustal component in the magma. The basaltic rocks plot within the calc-alkaline basalt field in the Hf/3-Th-Ta and Y/l5-La/10-Nb/8 discrimination diagrams. The eNd(T) values of the basaltic rocks (-13.6 to 14.3) are slightly higher than those of the rhyolitic rocks (-14.1 to 15.2), and the initial Sr isotopic ratios of the former (0.7085-0.7093) are much lower than those of the latter (0.7140-0.7149). However, the initial Nd and Sr isotope ratios of the igneous rocks in the Yeongdong basin are similar to those of the nearby Cretaceous igneous rocks in the Okcheon belt. The Pb isotope ratios plot within the field of Mesozoic granitoids outside of the Gyeongsang basin in Pb-Pb correlation diagrams. Since a basaltic magma requires the mantle source, the enriched isotopic signatures and negative Nb anomaly of the basaltic rocks suggest two possibilities for their origin: enriched mantle lithospheric source, or depleted mantle source with significant amount of crustal contamination. However, we prefer the first possibility since it would be difficult for a basaltic magma to maintain its bulk composition when it is significantly contaminated with granitic crustal material. The slightly more enriched isotopic signatures of rhyolitic rocks also suggest two possibilities: differentiate of the basaltlc magma with some crustal contamination, or direct partial melting of the lower crust. Much larger exposed volume of the rhyolitic rocks, compared with the basaltic rocks, indicates the latter possibility more favorable.

• Economic and Environmental Geology
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• v.9 no.3
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• pp.143-156
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• 1976

The Cretaceous metal ore deposits in the Gyeongsang basin of Gyeongsangbuk-do are characterized by the formation of metallogenic provinces which show zonal distribution pattern around Yeonil province where pneumatolytic type is dominated and hydrothermal type are distributed in the order of decreasing temperature type outward. Some Cretaceous granitic rocks include zoned alkali feldspars which reflect rapid variation of $H_2O$ during emplacement and crystallization of the water-saturated granitic magma. The ore deposits are considered to be originated from upward transportation of ore solution from the excess of water exhausted from uprising magma, which seems to be intimately related to the fact that the majority of the ore deposits in Daegu area are cummulated around the granites including zoned alkali feldspars. In order to collect geochemical data necessary for geochemical exploration in the study area, certain trace elements were chosen as pathfinders from monzonite and soil in the vicinity of Dalsung Tungsten Mine by studying the dispersion patterns of trace elements: Ba and Sr show trends to decrease toward ore deposit while Cu, Pb, and Mo increase. Around mining area there are distributed apparently Equisetum arvense Linne and Mentha sachinensis Kudo which may be used as index plants. In the viewpoint of geologic structure, the trends of the ore veins in contact aureole around the Palgongsan granite body correspond with the pre- and syn- plutonism joint pattern in hornfels in the area.

• Economic and Environmental Geology
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• v.31 no.4
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• pp.349-360
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• 1998

The alkali granite occurred as small stock and dyke is distributed in the Kyeomyeongsan Formation in the vicinity of the Chungju city. Geochemical characteristics in major and trace element of alkali granite in the Kyeomyeongsan Formation indicate that the alkali granites are peralkaline and have similar geochemical features to the A-type alkali granite. The rock enriched in HFSE such as Zr, Nb, Y, REE etc. According to the discrimination diagram the alkali granites mostly belong to the within-plate granite field, and to the $A_1$ group of A-type granite. This suggests that they might be emplaced in a extentional rift environment. The alkali granites are characterized by remarkably high total REE content, and enriched, relatively flat to somewhat HREE-depleted patterns with large negative Eu anomaly. The Sm-Nd age of the alkali granite is $338{\pm}30Ma$ with ${\varepsilon}_{Nd(t)}$ beings -7.3 to -8.5. On the basis of the geochemical studies the source magma was derived from a enriched mantle-like source and had a few or clearly interaction with sialic continental crust. In conclusion, the alkali granitic rock of the Kyeomyeongsan Formation might be formed from the high F peralkaline magma that was emplaced in continental rift environment, and generated at the early Carboniferous.

• Economic and Environmental Geology
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• v.10 no.4
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• pp.185-198
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• 1977

This paper is the result of a petrochemical study on the Jongam granite mass which intruded metasediments of Ogcheon zone, locating about 10km northwest from Miweon town, and the mass is $2.5km^2$ in area. In this study, the granite was petrographically analysed in such ways, modal analysis, myrmekite analysis, petrochemical study analysis and tectonic analysis of joint system. The findings are as follows: 1) The area of the granite distributed is a basin surrounded by metasediments. The granite is magma origin. 2) The major mineral composition of the granite are quartz, plagioclase, microcline orthoclase and biotite, and accessaries are muscovite, sericite, magnetite, zircon and sphene. 3) In triangles of $Qu-K{\cdot}fp-PI$, Bi-Or-Hb+Mt and Qu-Ab-Or, the data point areas are similar to those of the NW zone of D.S. Lee (1971) 4) The granite mass can be divided into three granites; quartz monzonite, granodiorite and biotite granite, by petrological aspect in detail. 5) It shows inverse ratio of content between quartz and plagioclase. 6) The myrmekite-texture of the granite are stem type and dominating in northern part. 7) In the tectonic analysis of joint, the strikes are almost constant in direction, N-S direction of dominant joints 8) The geological age of the granites estimated to be Jurassic in age. 9) According to the mineralogical distribution and joint systen, the intrusion direction of the granite magma was infered as from north to south.

• Economic and Environmental Geology
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• v.27 no.5
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• pp.459-467
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• 1994

The Gadaeri granite near Ulsan mine is an oval-shape isolated granitic body, and is genetically related to the iron-tungsten mineralization. The Gadaeri granite exhibits calc-alkaline and I-type characteristics, and generally shows the micrographic texture which indicates the shallow depth of emplacement. Consideration of the stratigraphic thickness of Ulsan formation and minimum-melt compositions suggests that the bulk magma crystallized at pressure of 0.5~2.0 kbar under water saturated condition. The evolutionary trend observed in the studied rocks represents that feldspar fractional crystallization has been a major magmatic process at the Gadaeri granite pluton. Different chemical characteristics between the Gadaeri and the Masan-Kimhae granites cannot be explained by fractional crystallization or different degrees of partial melting, and it reflects that the magma source for Gadaeri granite was different from that of the Masan and Kimhae granites.

• Economic and Environmental Geology
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• v.25 no.4
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• pp.463-477
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• 1992

The study area consists of sedimentary and volcanic rocks of Gyeongsang Supergroup, granitoid intrusives, and hornfelses around the granitoids. Granitoid intrusives occur in small stocks in Nijeon-ri, Uggogri, and Yul-ri area. The masses in Nijeon-ri and Uggog-ri are hornblende-biotite granodiorite, biotite granodiorite respectively, and Yul-ri mass is biotite granite. Surrounding sediments of these masses were thermally metamorphosed and contact aureoles were formed. The studied granitoids are considered to be formed by sequential crystallization-differentiation from calc-alkalic granitoid magma. Metamorphic minerals occurring in contact aureole are chlorite, actinolite, epidote, and biotite. Diopside and hornblende are observed in small amount in some lithology around contact aureole. The lithology of contact aureole is predominantly silty and sandy, and characteristic metamorphic minerals were poorly developed because of low temperature metamorphism. Low temperature in contact aureole could be deduced from the facts that the intrusions were small size, shallow depth, low temperature, and rare movement of volatiles from magma.

• Economic and Environmental Geology
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• v.25 no.4
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• pp.435-446
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• 1992

REE, major and trace elements analyses of the Jurassic Daebo granite and Cretaceous Bulguksa granite were carried out to interpet their petrogenesis and relationships between petrogenesis and tectonics. Analytical results are summarized as follows. (1) $SiO_2$ content of the Bulguksa granite (aver. 74.6%) are significantly higher than those of the Daebo granite (aver. 68.1%). Major elements of $TiO_2$, $Al_2O_3$, $P_2O_5$, CaO, MgO, Total FeO, and trace elements of Co, V and Sr are negatively correlated with $SiO_2$. Incompatible elements such as Ba, Sr, Y, Zr and HREE are contained differently in the Bulguksa granites distributed in between Okchon folded belt and Kyongsang sedimentary basin. (2) Trace element abundances show a good discrimination between two goups of granitic rocks. Ba, Sr and V are enriched in Daebo granites, while Zn and Cr are depleted in them. (3) Jurassic granites have quite different Eu anomalies and REE patterns from those of Cretaceous granites: Large negative Eu anomaly in the former and mild or absent Eu anomaly in the latter. The large Eu negative of Cretaceous granitic rocks are interpreted as a differentiated product of fractional crystallization of granitic magma from the upper mantle. Meanwhile, the Daebo plutonic rocks was resulted from the partial melting of subcrustal material or crustal contamination during ascending granitic magma from the mantle. Senario of igneous activities of Mesozoic age in South Korea was proposed based on Kula-Pacific ridge subduction model.

• Economic and Environmental Geology
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• v.28 no.5
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• pp.445-454
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• 1995

The petrochemical data of the porphyritic granites of Cretaceous age in the Yonghwa-Seolcheon area show the trend of subalkaline magma, calc-alkaline magma, I-type granitoid and magnetite series. This granite is the relevant igneous rock of gold-silver mineralization in this mining district Fluid inclusions have been studied in phenocryst quartz from the Cretaceous porphyritic granite. Three main types of fluid inclusion were found : liquid-rich inclusion(I type), gas-rich inclusion(II type) and solid-bearing inclusions(III-A, III-B). The solid-bearing inclusions(III-A,B) represent the earliest trapped fluids. They have salinities between 41.0 and 67.5 wt% equivalent to NaCl. These are high saline inclusions containing NaCl and KCl daughter crystals. Homogenization temperature inferred from the fluid inclusion study ranges from 650 to $75^{\circ}C$ Type I and II inclusions were observed within the same fracture. This cause for these differences in degree of filling is evidence of boiling. Salinities of type I and II inclusions range from 9.87 wt% to 15.29 wt%, from 8.40 wt% to 14.64 wt% NaCl equivalent, respectively.