• The Journal of the Petrological Society of Korea
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• v.9 no.2
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• pp.84-98
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• 2000

Daejeon-sa basalt in the Mt. Juwang area composed of 12 basalt flows alternate with 9 peperites and each basalt and peperite has the variety of thickness. Peperites yielded in Daejeon-sa basalt are mixed of basalt with reddish shale, of which textural type is globular peperite. Basalts yielded in Daejeon-sa basalt are massive basalt without vesicule, although sometimes vesicules are founded in upper within a flow unit. The basalt has mainly pseudomorph of olivine as phenocryst, and also plagioclase and clinopyroxene phenocryst. Matrix is mainly subophitic texture. The plotting result on the TAS diagram shows these basalts belong to the sub-alkaline, and it can be subdivided into calc-alkaline series on the basis of the diagram of Si02 vs. K20 and of alkali index vs. A1203 diagram. According to plots of wt.% oxides vs. wt.% MgO, abundances of A1203 and CaO increase with decreasing MgO while F ~ dOecre~ase . With decreasing MgO compatible elements decrease while incompatible elements increase. In spider diagram of MORB-normalized trace element patterns, HFS elements are nearly similiar with MORB, but LIL elements are enriched. Especially, contents of Ce, F: and Sm are enriched but Nb is depleted. In the chondrite-normalized REE patterns light REEs are enriched than heavy REEs. Tectomagmatic discrimination diagrams shows basalts in the study area are formed in the tectonomagmatic environment of subduction zone under continental margin. This result accord with characters of chemical composition mentioned above. Cr vs. Y diagram and CeM, vs. Ce diagram show that the primary magma of the basalts may formed by the about 15% partial melting of garnet-peridotite in the mantle wedge. After then, Daejeon-sa basalts may formed from evolved magma undergone mainly olivine fractional crystallization and contarnination of crustal materials before eruption.

• Economic and Environmental Geology
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• v.19 no.2
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• pp.97-107
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• 1986

The Cretaceous Imog granite is a calc·alkaline, subsolvus monzogranite and shows characteristics of "I-type" and "magnetite·series" granite by mineralogy and chemical composition. Many of the major and trace element characteristic of the Imog granite are consistent with a relationship by fractional crystallization of a basic magma. The primary magma of the granite derived from the subduction of oceanic crust at the destructive plate margin. The granite shows light REE enrichment with (Ce/Yb)N ratios of 7.77~12.55. All the REE patterns show Eu negative anomalies ($Eu/Eu^*=0.69$) in the pluton. The Imog granite at the southwestern part of the Hambaeg basin may be intruded along the tectonic intersections of the E-W and N-S lines such as deep faults and fractures. Radiometric age determination on the granite reveals as $96.7{\pm}2.0Ma$ by K-Ar dating on biotite.

• Economic and Environmental Geology
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• v.28 no.1
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• pp.25-31
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• 1995

In Dongnam area, Cretaceous igneous rocks, such as diorite, porphyritic granite, and quartz porphyry intruded Paleozoic sedimentary rocks, such as Myobong slate and Pungchon limestone. The Dongnam Fe-Mo skarn deposits were imposed on the diorite(endoskarn) and the Myobong slate(exoskarn). The ore deposits consist mainly of magnetite and molybdenite with small amounts of sulfides, such as galena, sphalerite, pyrite, chalcopyrite, and pyrrhotite. The igneous rocks show nearly constant $^{206}Pb/^{204}Pb(18.80{\sim}19.06)$ and $^{207}Pb/^{204}Pb(15.71{\sim}15.72)$ ratios. Their $^{207}Pb/^{204}Pb$ ratios higher than the typical ratios of orogene suggest that the igeneous rocks were formed from lower crust(or mantle) - derived magma excessively contaminated by upper crustal materials such as high radiogenic Precambrian basement rocks. The lead isotopic compositions of the igneous rocks, the Pungchon limestone, and the ore minerals show a well defined linear in $^{206}Pb/^{204}Pb$ - $^{207}Pb/^{204}Pb$ plot. The lead isotopic compositions of the igneous rocks are similar to those of magnetite and galena, which were formed at early skarn stage and significantly lower than those of altered quartz porphyry, molybdenites, and pyrite, which were formed at late epithermal alteration stage. Considering the systematic variation of the lead isotopic compositions in the ore minerals according to hydrothermal stages, the variation may be due to a relative variation in surrounding rock(Pungchon limestone) involvement in hydrothermal ore solution leaching the surrounding rock. Therefore, the variation of the lead isotopic compositions in ore minerals can be modeled in terms of the mixing of the leads derived from the igneous rocks as low radiogenic source and the surrounding rock(Pungchon limestone) as high radiogenic source.

• Journal of the Korean earth science society
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• v.21 no.3
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• pp.323-336
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• 2000

Ultramafic rocks in Choongnam area are mainly serpenitinites which are parent rock of talc and asbestos ore deposits. About 10 $^{\circ}$ NNE-trending parallel serpentinites masses occur as discontineous isolated lenticular intrusive bodies in Precambrian gneiss complex between Hongseong-Kwangcheon line and Onyang-Cheongyang line. The sizes of serpentinites vary from several centimeters to 1 kilometer in width and from several meters to 5 kilometers in length. The serpentinites show high SiO$_2$(39.99wt.% in average), MgO(38.46wt % in average), Cr(>1011ppm), Ni(>1660ppm), and Co(>80ppm). Most serpentinites contain serpentine more than 50%. Some serpentines contain original minerals such as olivine, pyroxene and chromite. Also, serpentinites body may contain a little serpentinized peridotite, and some talc and asbestos ore deposits. The original rocks of the serpentinites interpreted as Alpine type ultramafic rocks, and dunite and/or harzburgite which were originated from slightly depleted upper mantle(30${\sim}$40km deep), and emplaced in the crust through the large fault zones. It seems that main serpentinization from the original rocks was occurred during greenschist and/or amphibolite facies regional metamorphism in Choongnam area.

• The Journal of the Petrological Society of Korea
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• v.21 no.2
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• pp.89-112
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• 2012

The Korean Peninsula consists of three Precambrian blocks: Nangrim, Gyeonggi and Yeongnam massifs. Here we revisited previous stratigraphic relationships, largely based on new geochronologic data, and investigated the crustal evolution history of the Precambrian massifs. The Precambrian strata have been usually divided into lower crystalline basements and upper supracrustal rocks. The former has been considered as Archean or Paleoproterozoic in age, whereas the latter as Paleoproterozoic or later. However, both are revealed as the Paleoproterozoic (2.3-1.8 Ga) strata as a whole, and Archean strata are very limited in the Korean Peninsula. These make the previous stratigraphic system wrong and require reconsideration. The oldest age of the basement rocks can be dated as old as Paleoarchean, suggested by the occurrence of ~3.6 Ga inherited zircon. However, most of crust-forming materials were extracted from mantle around ~2.7 Ga, and produced major portions of crust materials at ~2.5 Ga, which make each massif a discrete continental mass. After that, all the massifs belonged to continental margin orogen during the Paleoproterozoic time, and experienced repeated intracrustal differentiation. After the final cratonization occurring at ~1.9-1.8 Ga, they were stabilized as continental platforms. The Nangrim and Gyeonggi massif included local sedimentary deposition as well as igneous activity during Meso-to Neoproterozoic, but the Yeongnam massif remained stable before the development of Paleozoic basin.

• The Journal of the Petrological Society of Korea
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• v.6 no.1
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• pp.1-18
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• 1997

Andesitic pyroclastics and lava flows are deposited as a part of composite volcanoes by Cretaceous volcanic activity in Geojae Island, off the coast of Korea. The andesitic pyroclastics are composed of tuff breccia and lapilli tuff minor intercalated tuff. Lava flows are divided into dense and porphyritic andesite containing phenocrysts of plagioclase, pyroxene, and/or hornblende. The andesitic rocks represent charactersitcs of carc-alkaline BAR association with basalt, basaltic andesite, andesite, and dacite to rhyolite. Major element variations of the volcanic rocks show that $Al_2O_3$, total FeO, CaO, MgO and $TiO_2$ decrease with increasing $SiO_2$ but $K_2O$ and total alkalis increase, and represent differntiation trend of calc-alkaline rock series. In spider diagram, contents of Sr, K, Rb, Ba, and Th are relatively high, but contents of Nb, P, Ti and Cr are low. These petrochemcial characteristics are similar to those of rocks from island arc or continental margein related to plate subduction. Chondrite-normalized REE patterns of volcanic rocks are paralle to subparallel, with LREE enriched than HREE, and show gradual increase of negative Eu anomaly from basalt to dacite and rhyolite, suggesting comagmatic fractional crystallization with minor effects of assimilation and magma mixing. Andesitic rocks are assumed medium-K orogenic andesites that formed in the tectomagmatic environment of subduction zone under normal continental margin arc.

• The Journal of Engineering Geology
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• v.13 no.3
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• pp.293-308
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• 2003

New results about the crustal structure down to a depth of 60 km beneath North Korea were obtained using the seismic tomography method. About 1013 P- and S-wave travel times from local earthquakes recorded by the Korean stations and the vicinity were used in the research. All earthquakes were relocated on the basis of an algorithm proposed in this study. Parameterization of the velocity structure is realized with a set of nodes distributed in the study volume according to the ray density. 120 nodes located at four depth levels were used to obtain the resulting P- and S-wave velocity structures. As a result, it is found that P- and S-wave velocity anomalies of the Rangnim Massif at depth of 8 km are high and low, respectively, whereas those of the Pyongnam Basin are low up to 24 km. It indicates that the Rangnim Massif contains Archean-early Lower Proterozoic Massif foldings with many faults and fractures which may be saturated with underground water and/or hot springs. On the other hand, the Pyongyang-Sariwon in the Pyongnam Basin is an intraplatform depression which was filled with sediments for the motion of the Upper Proterozoic, Silurian and Upper Paleozoic, and Lower Mesozoic origin. In particular, the high P- and S-wave velocity anomalies are observed at depth of 8, 16, and 24 km beneath Mt. Backdu, indicating that they may be the shallow conduits of the solidified magma bodies, while the low P-and S-wave velocity anomalies at depth of 38 km must be related with the magma chamber of low velocity bodies with partial melting. We also found the Moho discontinuities beneath the Origin Basin including Sari won to be about 55 km deep, whereas those of Mt. Backdu is found to be about 38 km. The high ratio of P-wave velocity/S-wave velocity at Moho suggests that there must be a partial melting body near the boundary of the crust and mantle. Consequently we may well consider Mt. Backdu as a dormant volcano which is holding the intermediate magma chamber near the Moho discontinuity. This study also brought interesting and important findings that there exist some materials with very high P- and S-wave velocity annomoalies at depth of about 40 km near Mt. Myohyang area at the edge of the Rangnim Massif shield.

• Economic and Environmental Geology
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• v.39 no.5 s.180
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• pp.555-566
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• 2006

Major, trace and rare earth elements data of the Cretaceous Jindong granitic rocks were investigated in order to constrain the magma source characteristics and to establish the paleotectonic environment of the southwestern part of the Gyeongsang Basin. Geochemical signatures of the granitic rocks from the study area indicate that all of the rocks have characteristics of calc-alkaline series in the subalkaline field, and progressively shift from metaluminous to peraluminous with differentiation. In the variation diagrams, the overall geochemical features of the granites show systematic variations in major and trace elements. Chondrite normalized REE patterns show generally enriched LREE((La/Yb)c=4.2-12.8) and slight negative to flat Eu anomalies. Rb-Sr isotope data of the Jindong granites show that the whole rock age and Sr initial ratio are $114.6{\pm}9.1Ma$ and 0.70457, respectively. The Sr initial ratio of the Jindong granites is very similar to those of the Creataceous granites from Masan, Kimhae and Busan area($^{87}Sr/^{86}Sr=0.7049-0.707$). These results suggest that the magma have the mantle signature and intruded into the area during the early Cretaceous age. The Jindong granites have higher $Al_{2}O_{3},\;Na_{2}O$, Sr and lower $K_{2}O$, Y concentrations compared with typical calc-alkaline granitic rocks. These adakitic signatures are similar to those of adakitic pluton on Kyushu Island, southwest Japan arc. On the ANK vs. ACNK and tectonic discrimination diagrams, parental magma type of the granites corresponds to I-type, VAG granite. Interpretations of the geochemical characteristics of the granitic rocks favor their emplacement at continental margin during the subduction of Izanagi plate.

• Economic and Environmental Geology
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• v.40 no.2 s.183
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• pp.223-236
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• 2007

The eastern extension of the Cordilleran-type orogenic belt continues from southeastern China to the Chukot Peninsula through the Korean Peninsula. The Gyeongsang basin, located in the southeastern part of the Korean Peninsula and the Inner Zone of southwest Japan are characterized by extensive distribution of Cretaceous to Tertiary I-type calc-alkaline series of intrusive rocks. These intrusive rocks are possibly the result of intensive magmatism which occurred in response to the subduction of the Izanagi Plate beneath the northeastern part of the Eurasian Plate. The Jindong granitoids within the Gyeongsang basin are reported to be adakites, whose signatures are high $SiO_2,\;Al_2O_3$, Sr, Sr/Y La/Yb and, low Y and Yb contents. The major and trace element contents of the Jindong granitoids fall well within the adakitic field, whereas other Cretaceous granites in the same basin are plotted in the island arc ADR area in discrimination diagrams. Chondrite normalized REE patterns show generally enriced LREEs (La/Yb)C = 3.6-13.8) and slight negative to flat Eu anomalies. The mean Rb-Sr whole rock isotopic age of the Jindong granitoids is $114.6{\pm}9.1$ Ma with an initial Sr isotope ratio of 0.70457. These values suggest that the magma has mantle signature and intruded into the area during Early Cretaceous. The Jindong granitoids have similar paleogeographical locations, paleotectonic environments and intrusion ages to those of the Shiraishino granodiorites of Kyushu Island and the Tamba granitoids of San'yo belt located on southwestern Japanese arc.

• Journal of the Korean earth science society
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• v.33 no.7
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• pp.627-636
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• 2012

In the Obongri-Goseong area of Gangwondo, South Korea, there are six densely distributed volcanic edifices i.e., Duibaejae, Oeumsan, Galmibong, 249 m height, 166 m height, and 102 m height, and two other volcanic edifices including Goseongsan and Unbongsan volcanic edifice that are separately located from a distance. A previously undiscovered 249m volcanic edifice in Obongri was found in this investigation, and the six volcanic edifices distributed in Obongri will be referred to as the Obongri volcanic edifice group. Volcanic edifices in this area were interpreted by other researchers as being volcanic plug, plug dome, and cylindrical volcanic pipe type edifices. The aim of this study is to investigate the aspect of volcanic activity in the Obongri-Goseong area and the formation of volcanic edifices by examining of the shape of volcanic edifices, stratigraphy, and characterization of volcanic products. All the volcanic edifices in the area are composed of basaltic rocks on the Mesozoic granite basement, and the prevalence of the dome shape increased towards the upper part of the mountain. Three volcanic edifices (Duibaejae, 166 m height, 102 m height) include intercalated pyroclastic deposits between the basaltic rocks and the basement. The pyroclastic deposit in the Duibaejae volcanic edifice is composed of quartz, feldspar, granite fragments originated from the basement, and scoria fragments originated from the volcanic eruption. In addition to angular olivine, plagioclase, and pyroxene xenocrysts, all the basaltic rocks contained mantle xenolith, gabbroic xenolith originated from the lower crust, and granitic xenolith originated from the basement. This fact indicates that magma rapidly rose to the surface and that the volcanic activity was explosive. It is also interpreted that, as the basaltic magma became highly viscous due to the large amount of xenocrysts, the erupted magma formed a dome structure on the surface. The original dome structure was then severely eroded out leaving a plug dome formation on the basement.