• The Journal of the Petrological Society of Korea
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• v.14 no.3 s.41
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• pp.169-179
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• 2005

Acidic metavolcanic rocks distributed around the Chungju iron deposit show significantly high abundances of rare earth elements and high field strength elements. Relatively high ${\epsilon}_{Nd}$(0) values and lack of negative Nb anomaly suggest that assimilation of crustal material is not involved in their generation. They are plotted within the within-plate environment according the tectonic discrimination diagrams. Such geochemical characteristics are very similar to the acidic metavolcanic rocks of Munjuri Formation. They also show geochemical characteristics of Al-type magma of Eby (1992). All such diagnostic characters indicate differentiation of mantle-derived magma produced from the rift environment, related to the breakup of continent. In contrast to the alkali granites and the rare metal deposit both having age of c. 330 Ma, Sm-Nd isotopic data of the acidic metavolcanic rocks do not form well defined isochron. However, the alkali granites reveal low ${\epsilon}_{Nd}$(0) values, while the acidic metavolcanic rocks and the rare metal deposit both have significantly higher ${\epsilon}_{Nd}$(0) values. Considering such differences, we propose following generation hypothesis: The acidic metavolcanic rocks around Chungju iron deposit was erupted at 750 Ma as rest of the acidic metavolcanic rocks of Gyemyeongsan and Munjuri Formations. About 330 Ma ago, partial melting of existing Al-type igneous materials and some old crustal materials produced alkali granite. The rare metal deposit was also produced by redistribution of related materials within the acidic volcanics due to hydrothermal activities occurred at the same time. Sm-Nd isotopic systematics of the acidic metavolcanic rocks were disturbed during the regional metamorphic event at ca. 280 Ma.

• The Journal of the Petrological Society of Korea
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• v.20 no.4
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• pp.231-241
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• 2011

Cheolwon Group in the Cheolwon Basin, which lies northwest of the Gyeonggi massif, has been correlated to the Yucheon Group in the Gyeongsang Basin, but its ages and volcanic times are defined to be considerately earlier than the other one. In this study, SHRIMP zircon U-Pb ages were determined from the igneous rocks in the Cheolwon Basin. The mean ages from zircons are $115.0{\pm}1.1Ma$ in rhyolite, and $111.24{\pm}0.85Ma$ and $109.1{\pm}1.1Ma$ in granite porphyry. The minimum age is 113 Ma in the Jijangbong Tuff. Such age in the rhyolite define the intrusion time of ring dykes, suggesting a caldera collapse following eruption of the Dongmakgol Tuff. Such age in the Jijangbong Tuff represent latest volcanism as postcaldera in the basin. The volcanic rocks in the basin were erupted during late Aptian, and are correlated to the Sindong Group in the Gyeongsang Basin. The plutonism in the basin occurred during $111.24{\pm}0.85Ma{\sim}109.1{\pm}1.1Ma$, following the volcanism. The age distribution of the analyzed zircons in the Jijangbong Tuff indicates the presence of foreign zircons derived from protoliths, regarding a wide span of zircon ages from Cretaceous to Jurassic, Triassic, early and late Protozoic, and Archean. The Archean age suggests the possible presence of the Archean protoliths with such age, which have not been exposed on the surface. The age distribution with wide span suggests that its vent is located in an area that several strata with different ages piled up and intercepted with some intrusives.

• The Journal of the Petrological Society of Korea
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• v.21 no.1
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• pp.31-45
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• 2012

We investigated the various lithologies and zircon U-Pb ages of metasedimentary rocks from the Yeongheung-Seonjae-Daebu Islands, western Gyeonggi Massif, whose geologic and geochronologic features are poorly constrained in spite of their significance for tectonic interpretation. Major lithology consists of quartzites or meta-sandstones commonly alternating with semi-pelitic schists, together with lesser amounts of calcareous sandstones with matrix-supported quartzite clasts, calcareous schists, and pelitic schists. Pelitic schists uncommonly contain large porphyroblasts of garnet as well as quartz veins with large crystals of muscovite and andalusite or kyanite. SHRIMP U-Pb ages of detrital zircons from two analyzed metasandstones define four age populations: Neoarchean (~2.5 Ga), Paleoproterozoic (~2.0-1.5 Ga), Neoproterozoic (~1.1-0.7 Ga), and Early Paleozoic (~560-400 Ma). The youngest zircon ages are clustered at ~420 Ma. These results suggest that the deposition of meta-sandstones took place after the Silurian, possibly during the Devonian, and are analogous to those of the Taean Formation reported from the western part of the Gyeonggi Massif. Moreover, The age distribution patterns of detrital zircons and the Barrovian-type metamorphic facies of pelitic schists are similar to those reported from the Imjingang belt, suggesting that the Taean Formation likely corresponds to southwestward extension of the Imjingang Belt.

• Economic and Environmental Geology
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• v.49 no.4
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• pp.315-323
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• 2016

This study reports a gneiss dome in the Hongseong area, southwestern margin of the Gyeonggi massif. This gneiss dome, named here as 'Oseosan dome' because it is located around the Oseosan, the highest peak along the western coastal area, is composed mainly of the Neoproterozoic to Paleozoic ortho- and paragneiss, mafic metavolcanic rock, and metadolerite. Migmatization affected these rock units, in which leucocratic(granitic) materials derived from anatexis frequently occur as patch and vein parallel to or cutting through internal foliation. The Oseosan dome shows overall concentric geometry and outward-dipping internal foliation, but also partly complicatedly changeable or inward-dipping foliation. Taking available petrological and geochronological data into account, the Oseosan dome is interpreted to be exhumed quickly into the upper crustal level during the Late Triassic, accompanied in part with anatexis and granite intrusion. In addition, extensional shear zone intruded by the Late Triassic synkinematic granite and sedimentary basin have been reported around the Oseosan dome. These evidences possibly suggest that the Oseosan dome formed in closely associated with the Late Triassic extensional movement and diapiric flow. Alternatively, 1) thrust- or reverse fault-related doming or 2) interference between independent folds during structural inversion of the Late Traissic to Middle Jurassic sedimentary basin can be also considered as dome-forming process. However, considering the northern limb of the Oseosan dome, cutting by the Late Traissic granite, and the southern limb, cutting by contractional fault reactivated after the Middle Jurassic, it is likely that the domal structure formed during or prior to the Late Triassic.

• The Journal of the Petrological Society of Korea
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• v.23 no.1
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• pp.17-30
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• 2014

In this study we carried out SHRIMP U-Pb age dating of detrital zircons from age-unknown meta-sedimentary formations distributed around the NNE-SSW trending Gwangju Shear Zone, a branch of Honam Shear Zone, in the southwestern region of the Korean Peninsula. The meta-sedimentary formations from the west (Yeonggwang) and east (Jangseong) areas of the Gwangju Shear Zone have different patterns of zircon age distributions. Zircons of quartzites from the Yeonggwang area yield clusters at Neoarchean (ca. 2,500 Ma), Paleoproterozoic (ca. 1,860 Ma), Neoproterozoic (ca. 960 Ma) and Paleozoic (ca. 380 Ma) ages, but those of the Jangseong area yield clusters at only Neoarchean (ca. 2,500Ma) and Paleoproterozoic (ca. 1,880 Ma) ages. The contrastive patterns in age indicate that the meta-sedimentary formations from the west and east areas correspond to the meta-sedimentary formations of the Okcheon Metamorphic Belt and the sedimentary formations overlying on the Yeongnam Massif, respectively. The results imply that the Gwangju Shear Zone is the tectonic boundary between the Okcheon Metamorphic Belt and the Yeongnam Massif.

• Economic and Environmental Geology
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• v.42 no.1
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• pp.9-25
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• 2009

The Northwestern Okcheon Metamorphic Belt in the Chungju area consists of the Munjuri Formation, the Daehyangsan Quartzite, the Hyangsanri Dolomite, and the Gyemyeongsan Formation, but the stratigraphy is still controversial. For a stratigraphic study, detailed stratigraphic sections were measured in two locations and mapping was carried out in the study area. The Munjuri Formation and the Daehyangsan Quartzite changed gradually in north and south section, but bedding parallel faults have developed in the boundary between two formations. The Daehyangsan Quartzite and the Hyangsanri Dolomite are conformable. Fault have developed in boundary between the Hyangsanri Dolomite and the Gyemyeongsan Formation. As a result of mapping in the study area, folding was recognized with $41^{\circ}/280^{\circ}$ plunging axis in the north part of the study area. Therefore, the bedding-parallel faults in the boundary might have occurred resulting from a layer parallel slip during the folding as well as the thrust. These results from this study and previous studies indicate that bedding-parallel faults in boundary between the Munjuri Formation and the Daehyangsan Quartzite are caused by a layer parallel slip during the folding. The fault between the Hyangsanri Dolomite and the Gyemyeongsan Formation is considered as a thrust fault, thereby the uppermost Gyemyeongsan Formation is placed under the Munjuri Formation. However the Gyemyeongsan Formation and the Munjuri Formation have similar age and rock composition. Hence, the Gyemyeongsan Formation is considered as an equivalent one with the Munjuri Formation. Therefore, the stratigraphy of Northwestern Okcheon Metamorphic Belt consists of the Gyemyeongsan/ Munjuri formations, the Daehyangsan Quartzite, and the Hyangsanri Dolomite in ascending order.

• The Journal of the Petrological Society of Korea
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• v.23 no.3
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• pp.197-208
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• 2014

Zircon separated from a biotite schist of the Guryong Group in Odesan area, eastern part of the Gyeonggi Massif in Korea were analysed for SHRIMP U-Pb ages. CL images display composite core-rim structures of the zircon, indicating an in-situ overgrowth of zircon through a high-grade metamorphism. The metamorphic zircon rims give a weighted mean age of $247{\pm}6Ma$. While the detrital zircon cores have zoning patterns and Th/U ratios indicative of a magmatic origin. Among 53 analyses from the cores, 46 data yield near concordant ages which are concentrated at $378{\pm}10Ma$ (n=9), $420{\pm}4Ma$ (n=6) and $1845{\pm}9Ma$ (n=18) with sporadic Neoproterozoic ($687{\pm}9Ma$) to late Archean ($2519{\pm}20Ma$) ages. The age data constraint sedimentation age of protolith of the Guryong Group, so far unknown, as late Paleozoic. The Guryong Group of this study is the first late Paleozoic strata reported from eastern Gyeonggi Massif, and its maximum depositional age (ca 378 Ma) is identical with those of the late Paleozoic strata in the southwestern Ogcheon Belt. The Triassic metamorphic age and abundant middle Paleozoic provenance (361~425 Ma) of the Guryong Group are similar with those reported from the Triassic collisional belt in central China. Thus this study indicates that the Odesan area would be an possible eastward extension of the Triassic collisional belt in central China.

• The Journal of the Petrological Society of Korea
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• v.28 no.2
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• pp.129-141
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• 2019

The Eoraesan area, Chungju, which is located in the northwestern part of Ogcheon Metamorphic Zone, Korea, mainly consists of the Neoproterozoic Gyemyeongsan Formation and the Mesozoic igneous rocks which intruded it. The metaacidic rocks (MAR) of the Gyemyeongsan Formation show a maximum radioactive value, and the Early Jurassic biotite granite is regionally distributed in this area. In this paper is researched the microstructure related to the growth of rare-earth mineral of allanite in the MAR, and is considered the source and occurrence time of rare-earth element (REE) mineralization. The MAR is mainly composed of alkalic feldspar (mainly microcline), quartz, iron-oxidizing mineral, biotite, muscovite, plagioclase, hornblende, allanite, zircon, epidote, fluorite, apatite, garnet, (clino)zoisite etc. The radioactive elements contained in the allanite cause a dark brown hale in the surrounding biotite, and the allinte also occurs as aggregate along the regional foliation. The deflection of regional foliation and the strain shadows, which are common to the pre-tectonic porphyroblast grown before the formation of regional foliation, can't be observed around most allanites (aggregates). The grain size and orientation of ironoxidizing mineral included in the allanite aggregate are the same as those in the matrix. It is recognized the hydrothermal conversion of hornblende to biotite due to the intrusion of igneous rock, and the secondary biotite occurs and contacts with allanite, zircon, epidote etc. These microstructures indicate that the rare-earth mineral of allanite (aggregate) grew by the hydrothermal alteration due to the intrusion of igneous rock after the formation of regional foliation. It is considered that the REE mineralization is closely related to the intrusion of Early Jurassic biotite granite which is regionally distributed in this area.

• The Journal of the Petrological Society of Korea
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• v.23 no.3
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• pp.167-185
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• 2014

SHRIMP zircon U-Pb ages and major element and Sr-Nd isotopic compositions were determined for drill cores (374-3390 m in depth) recovered from three boreholes in the Pohonag basin, southeastern Korea. Shallow-seated volcanic rocks and underlain plutonic rocks were geochemically classified as rhyolite and gabbro-granite, respectively. They showed high-K calc-alkaline trends on the $K_2O-SiO_2$ and AFM diagrams. Zircons from volcanic rocks of borehole PB-1 yielded concordia ages of $66.84{\pm}0.66Ma$ (n=12, MSWD=0.02) and $66.52{\pm}0.55Ma$ (n=12, MSWD=0.46). Zircons from volcanic rocks of borehole PB-2 gave a concordia age of $71.34{\pm}0.85Ma$ (n=11, MSWD=0.79) and a weighted mean $^{206}Pb/^{238}U$ ages of $49.40{\pm}0.37Ma$ (n=11, MSWD=1.9). On the other hand, zircons from plutonic rocks of borehole PB-3 yielded weighted mean $^{206}Pb/^{238}U$ ages of $262.4{\pm}3.6Ma$ (n=21, MSWD=4.5), $252.4{\pm}3.6Ma$ (n=8, MSWD=1.9) and $261.8{\pm}1.5Ma$ (n=31, MSWD=1.3). Detrital zircons from the sedimentary strata overlain the volcanic rocks showed a wide age span from Neoproterozoic to Cenozoic, with the youngest population corresponding to $21.89{\pm}1.1Ma$ (n=15, MSWD=0.04) and $21.68{\pm}1.2Ma$ (n=10, MSWD=19). These dating results indicate that the basement of the Pohang basin is composed of Late Permian plutonic rocks and overlain Late Cretaceous to Eocene volcanic sequences. Miocene sediments were deposited in the uppermost part of the basin, possibly associated with the opening of the East Sea. The Sr-Nd isotopic compositions of the Permian plutonic rocks were comparable with those reported from Permian-Triassic granitoids in the Yeongdeok area, northern Gyeongsang basin. They may have been recycled into parts of the Cretaceous-Paleogene magmatic rocks within the Gyeongsang basin.

• The Journal of the Petrological Society of Korea
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• v.28 no.1
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• pp.25-38
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• 2019

The Muju-Seolcheon area, which is known to be located in the boundary of Ogcheon Belt and Ryeongnam Massif (OB-RM), consists of age unknown or Precambrian metamorphic rocks (MRs) [banded biotite gneiss, metasedimentary rocks (black phyllite, mica schist, crystalline limestone, quartzite), granitic gneiss, hornblendite], Mesozoic sedimentary and igneous rocks. In this paper are researched the structural characteristics of each deformation phase from the geometric and kinematic features and the developing sequence of multi-deformed rock structures of the MRs, and is considered the boundary location of OB-RM with the previous geochemical, radiometric, structure geological data. The geological structure of this area is at least formed through four phases (Dn-1, Dn, Dn+1, Dn+2) of deformation. The Dn-1 is the deformation which took place before the formation of Sn regional foliation and formed Sn-1 foliation folded by Fn fold. The Dn is that which formed the Sn regional foliation. The predominant Sn foliation shows a NE direction which matches the zonal distribution of MRs. A-type or sheath folds, in which the Fn fold axis is parallel to the direction of stretching lineation, are often observed in the crystalline limestone. The Dn+1 deformation, which folded the Sn foliation, took place under compression of NNW~NS direction and formed Fn+1 fold of ENE~EW trend. The Sn foliation is mainly rearranged by Fn+1 folding, and the ${\pi}$-axis of Sn foliation, which is dispersed, shows the nearly same direction as the predominant Fn+1 fold axis. The Dn+2 deformation, which folded the Sn and Sn+1 foliations, took place under compression of E-W direction, and formed open folds of N-S trend. And the four phases of deformation are recognized in all domains of the OB-RM, and the structural characteristics and differences to divide these tectonic provinces can not be observed in this area. According to the previous geochemical and radiometric data, the formation or metamorphic ages of the MRs in and around this area were Middle~Late Paleproterozoic. It suggests that the crystalline limestone was at least deposited before Middle Paleproterozoic. This deposition age is different in the geologic age of Ogcheon Supergroup which was recently reported as Neoproterozoic~Late Paleozoic. Therefore, the division of OB-RM tectonic provinces in this area, which regards the metasedimentary rocks containing crystalline limestone as age unknown Ogcheon Group, is in need of reconsideration.