• 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.

• Economic and Environmental Geology
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• v.45 no.2
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• pp.181-188
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• 2012

The 1M and $2M_1$ stacking sequences are the most frequently encountered in the illite species among the possible 6 polytypes. The $1M_d$, derived from the 1M polytype which exhibits a variable degree of disorder in the stacking sequence, is also observed in illite samples. In this paper, the author introduces and reviews the theoretical background of the quantitative analysis method of illite polytypes, and considers the possibility to determine the fault age and its reactivation age using K/Ar age-dating based on the quantification of illite polytypes in the fault system. For the increase of the accuracy and precision of the illite age analysis method, the occurrence, identification, and mineralogical characterization of illite polytypes should be defined in detail. The broadening effect of (hkl) reflections, due to disordering of 1M polytype and the presence of I/S minerals with expandability, are also considered as the main parameters controlling the quantification of illite polytypes using the WILDFIRE(C)simulation.

• Economic and Environmental Geology
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• v.48 no.1
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• pp.41-49
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• 2015

Recently developed illite-age-analysis(IAA) approach was applied to determine the fault-reactivated events for the Inje fault that cut through Precambrian biotite granitic gneiss with NNE-SSW trend in the middle of Korean peninsula. Three distinct fault-reactivated events of shallow crustal regime were recognized using the combined approach of optimized illite-polytype quantification and K-Ar age-dating of clay fractions separated from 4 fault clay samples: $87.0{\pm}0.12Ma$, $65.5{\pm}0.05$ and $66.6{\pm}1.38Ma$, $45.6{\pm}0.15Ma$, respectively. As well, $2M_1$ illite ages of 193~196 Ma and $254.3{\pm}6.96Ma$ were discernible, which may be related to the fault-activated time in the relatively deep crust. The study results suggest that the Inje fault would be firstly formed at $254.3^{\circ}$ ${\ae}6.96Ma$ and sporadically reactivated in shallow regime since about 87 Ma. These reactivation events in shallow regime might be due to the Bulguksa orogeny that would be strongly influenced in Korean peninsula at that time.

• Proceedings of the KSEEG Conference
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• 2000.04b
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• pp.281-281
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• 2000

• Proceedings of the KSEEG Conference
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• 2000.04b
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• pp.283-286
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• 2000

• Proceedings of the Mineralogical Society of Korea Conference
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• 2003.05a
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• pp.53-53
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• 2003

남한에 분포하는 현생 화강암류는 트라이아스기-쥬라기의 소위 대보화강암과 백악기-제3기의 불국사화강암으로 분류되어 왔다. 대보조산운동은 대동누층군의 퇴적이후에 일어난 조구조운동을 지칭하므로 트라이아스기의 화강암을 포함하는 대보화강암이라는 명칭은 그와 사실상 직접적인 관계는 없다. 트라이아스기-쥬라기의 화강암은 영덕, 청송 암체 외에는 경상분지 밖에 위치하고 백악기-제3기의 화강암은 속리산, 월악산 암체 외에는 경상분지 안쪽에 주로 분포한다. 트라이아스기-쥬라기의 화강암 중 영광-대전-청주-충주-원주-강릉 방면에 걸쳐 북동-남서 방향으로 분포하는 화강암질 저반은 남한에서 가장 넓은 면적을 차지하는 화강암체지만 신뢰할만한 연대측정 자료가 매우 부족한 실정이다. 이 화강암질 저반에 대해서는 Rb-Sr, K-Ar법이 해답을 주기 어렵다. 예를 들어 청주-음성-증평 지역의 화강암류에 대한 Rb-Sr 전암 자료는 분산이 심하며 약 380 Ma에 해당되는 초시선을 보여 기원물질의 불균질성 내지 불완전한 혼합 효과를 반영하고 있다. 옥천대와 영남육괴에 분포하는 일부 화강암체에 대해 잘못 보고된 Rb-Sr 전암연대 역시 모두 중광물의 U-Pb 연대보다 오래된 값을 보이는 것으로 보아 이들은 생성 당시부터 일정한 $^{87}$ Sr/$^{86}$Sr 초기치를 가지지 않고 Rb/Sr 비에 따른 양(+)의 기울기를 가졌음이 확실하다. 과잉의 방사기원 Ar을 가지거나 폐쇄온도가 낮은 광물들을 대상으로 한 K-Ar 자료 역시 화강암체의 관입편대를 정확하게 지시할 수는 없다. 우리는 이에 대한 연구의 일환으로 충청남도 청원군의 물류센터에서 채취한 중립질의 흑운모화강암 한 시료에 대한 U-Pb 스핀연대측정 결과를 다음과 같이 보고한다. $^{206}$ Pb$^{*}$ /$^{238}$ U age = 174.6$\pm$2.7 Ma $^{207}$ Pb$^{*}$ /$^{235}$ U age = 170.3$\pm$14.6 Ma $^{207}$ Pb$^{*}$ /$^{206}$ Pb sup */ age = 111$\pm$187 Ma 위에서 볼 수 있듯이 청주화강암의 스핀에 대해 콘코던트(concordant)한 연대가 얻어졌으며 자료의 오차, 스핀의 U-Pb계에 대한 폐쇄온도 및 화강암의 솔리더스(solidus)를 고려할 때 $^{206}$ Pb$^{*}$ /$^{238}$ U 연대인 174.6$\pm$2.7 Ma를 관입정치시기로 해석한다. 동일 시료의 흑운모에 대해서는 145 Ma의 Rb-Sr 연대가 얻어졌으며 따라서 관입이후 약 35$0^{\circ}C$까지 대략 1$0^{\circ}C$/Ma의 냉각속도를 구할 수 있었다. 청주화강암의 쥬라기 중기 연대는 영광-대전-청주-충주-원주-강릉 지역의 화강암질 저반이 대동누층군 퇴적 이후에 일어난 지구조 사건과 연관되었을 가능성을 지시하지만 이를 확인하기 위해서는 더 많은 자료가 요구된다. 우리는 현재 충주, 괴산 지역의 화강암체에 대해서도 스핀 연대측정을 수행중에 있으며 이들 자료를 암상을 구분하여 해석한다면 우리나라 중생대 지구조운동에 대한 새로운 사실이 밝혀질 수 있을 것으로 믿는다.

• The Journal of the Petrological Society of Korea
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• v.12 no.2
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• pp.70-78
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• 2003

Sado, Chudo, Mokdo, Nangdo, and Jeokgeumdo are the islands which belong to Hwajeong-myeon, Yeosu-si, Jeollanam-do and there are various kinds of volcanic rocks, volcaniclastic sedimentary rocks, and dinosaur-fossil bearing sedimentary rocks on these islands. This study is designed to constrain geologic ages of these volcanic and sedimentary rocks. K-Ar ages of these rocks indicate that the volcanism of this area occurred mainly during the period of 91.8 ${\pm}$ 3.5∼65.5 ${\pm}$ 1.3(l$\sigma$) Ma. Deposition ages of the sedimentary rocks were bracketed based on the ages of the volcanic rocks and observed field relationship between sedimentary and volcanic rocks. The oldest sedimentary deposit of the area is the volcanic pebble bearing conglomerate of the Jeokgeumdo and its deposition age is ca. 81 Ma or less. The deposition age of the Chudo shale, which belongs to stratigraphically upper sequence and bears many dinosaur footprints, is at least ca. 77 Ma. Conglomerate of the Mokdo was deposited at ca. 72∼70 Ma. The deposition age of the dinosaur fossil deposit of the Sado is at least ca. 65 Ma. All the investigated volcanic and sedimentary rocks of the Yeosu islands were formed during the late Cretaceous and dinosaurs lived until the latest Cretaceous in this area.

• Economic and Environmental Geology
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• v.19 no.2
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• pp.147-151
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• 1986

K-Ar ages on biotites have been determined from the Proterozoic Buncheon and Hongjesa granitic rocks in comparison with the Rb-Sr whole-rock ages to investigate the ages of metamorphic events. The Rb-Sr whole-rock ages determinations on the Buncheon and Hongjesa granitoid rocks were previously reported as 2,100Ma and 1,700Ma, respectively. K-Ar ages on biotites separated from the studied rock have revealed three different age groups such as 1) 1,200~1,300Ma, 2) 600~700Ma and 3) 300~400Ma. The Rb-Sr whole-rock ages for the granitic rocks represent the time of emplacement, whereas the K-Ar ages on biotites generally indicate the time of metamorphism or alteration. The large discordance in the two age systems may not be explained as indicating the cooling period of the granitic batholiths. The K-Ar ages on biotites from the granitoid rocks might not be simply interpreted as the age of the last phase of metamorphism, since the granitic rocks had been undergone multistages of amphibolite facies-metamorphism in the Precambrian period. During the multistages of intermediate grade metamorphism, $^{40}Ar$-loss could be inevitably taken place as the metamorphic temperatures went up above the blocking temperature of biotite ($300{\pm}50^{\circ}C$). The results of the K-Ar dating on biotites from this study are probably minimum ages or hydrothermal alteration ages.

• The Journal of the Petrological Society of Korea
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• v.14 no.3 s.41
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• pp.149-156
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• 2005

Ar-Ar, carbon AMS and OSL dating was carried out to clarify the age of the human footmarks on the Hamori Formation, Jeju Island, Korea. $^{40}Ar/^{39}Ar$ ages of trachybasalt from the Songaksan Tuff, which is underlain by the Hamori Formation, range between $10.6{\pm}19.9\;Ka$ and $11.7{\pm}26.3\;Ka$. Radiocarbon AMS ages of humin fractions extracted from sediment samples yielded the maximum limit age of the Hamori Formation as $15,161{\pm}70\;yr$ B.P. The OSL dating of the top and bottom layers of the Hamori Formation gave $6.8{\pm}0.3\;ka$ and $7.6{\pm}0.5\;ka$, respectively, suggesting that timing of the human footmarks formation can be constrained as between ca 6,800 yr B.P. and 7,600 yr B.P.

• The Journal of the Petrological Society of Korea
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• v.14 no.2 s.40
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• pp.93-107
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• 2005

We determined $^{40}Ar/^{39}Ar$ ages of the Tertiary dike swarms and volcanic rocks distributed in the SE Korea where the most prevalent crustal-deformation and volcanism occurred during the period. In previous study, it was disclosed that the mafic dike swarms on both sides (east and west) of the Yeonil Tectonic Line (YTL) were originated from a same magma although they are consistently aligned with different intrusion directions of NS and NE, respectively. Ages of the mafic dike swarms of this study are $47.3\pm0.8Ma$ and $48.0\pm1.3Ma$, respectively and confirm such conclusion. These facts clarify that the YTL acted as a westernmost limit of the crustal deformation, especially clockwise crust-rotation, during the Miocene. Frequent occurrence of basic dikes indicate strongly that the southeastern part of the Korean Peninsula was under E-W extensional stress field at about 48 Ma, intimately related to the India-Asia collision and subsequent sudden change of the Pacific Plate motion. The ages of the uncommonly appearing intermediate and felsic dikes were determined as $55.9\pm1.5Ma$ and $53.0\pm1.0Ma$, respectively. Ages of the andesitic lava of the Hyodongri Volcanics, the dacitic lava of the Yongdongri Tuff, and dacitic rocks intruding and covering the Churyeong Breccia were determined as $24.0\pm0.5Ma,\;21.6\pm0.4Ma$, $21.8\pm0.1Ma,\;and\;22.0\pm0.5Ma$ respectively. The ages from the volcanics agrees well with the stratigraphy established by the latest field survey, which confirms that the $andesitic\~dacitic$ volcanism was followed by the basaltic volcanism during the Early Miocene.