• The Journal of the Petrological Society of Korea
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• v.26 no.1
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• pp.63-72
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• 2017

We report an Early Cretaceous zircon U-Pb age ($113.9{\pm}1.2Ma$) for the Heuksan-do granite located about 90km from Mokpo offcoast of the southwestern Korean peninsula. At this Aptian/Albian boundary, widespread igneous activities occurred not only in the Korean peninsula but also in the eastern China and Japan. We raise the possibility that the flat-slab subduction and delamination triggered such an episodic igneous activity over the large areas of East Asia.

• Journal of the Korean earth science society
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• v.45 no.1
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• pp.85-109
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• 2024

Detrital zircon U-Pb dating was performed to determine the depositional age and provenance of sediments in the Cretaceous Muju Basin in Muju-gun, Jeollabuk-do. Six samples were collected from the Seolcheon Tuff (SCT), Bangyiri Formation (BYR), Gobang Member of the Gilwangri Formation (GWR-G), Seochang Member of the Gilwangri Formation (GWR-S), Bukchang Member of the Gilwangri Formation (GWR-B), and Jeogsangsan Formation (JSS). Based on the dating results, the sedimentary strata of the Muju Basin were deposited for approximately 105.6-90.4 Ma corresponding to the Albian to Turonian. The youngest single zircon ages of about 94.4 Ma and 89.6 Ma were confirmed in the samples from the Bangyiri Formation and the Gobang Member of the Gilwangri Formation, respectively, distributed in the western part of the Muju Basin. The relative and numerical ages previously estimated based on the lithostratigraphic correlation of the Gilwangri conglomerate need to be revisited in further research. The results also suggest that most of the sediments filling the Muju Basin were supplied from a limited area adjacent to the basin.

• 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.

• Proceedings of the Mineralogical Society of Korea Conference
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• 2003.05a
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• pp.55-55
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• 2003

호남전단대는 옥천대 남서부지역에 북동 내지 북북동 방향으로 발달하는 일련의 우수향 연성전단대로 한반도를 포함하는 동북아 지역의 중생대 부가작용과 관련하여 매우 중요한 조구조적 요소이며, 특히 북중국 대륙과 남중국 대륙이 유라시아 대륙에 부가되는 과정과 관련하여 동북아 지역의 중생대 지체구조 발달사를 설정하는데 매우 중요하게 생각되고 있다. 그러나 이러한 조구조적 중요성에도 불구하고 호남전단대의 정확한 운동 시기는 아직 밝혀지지 않고 있다. 이번 연구는 전주전단대가 지나가는 김제 금산사 지역과 무안 지역에 분포하는 화강암류를 대상으로 SHRIMP U-Pb 저어콘 연대 측정을 실시하여 전단운동시기를 밝혔다. 금산사 지역은 엽리상 각섬석-흑운모 화강섬록암이 흑운모 화강암에 포획된 명확한 지질학적 증거를 보이고 있는 곳으로 화강섬록암의 U-Pb 저어콘 연대는 172.7 $\pm$ 1.4 Ma이며 화강암의 연대는 169.6 $\pm$ 1.8 Ma과 167.5 $\pm$ 2.4 Ma로 구해졌다. 따라서 전주전단대의 전단운동은 약 173 - 170 Ma 기간에 일어났다. 특히 화강암 내에 포획된 화강섬록암 내에는 전반적인 우수향 전단운동 후기에 관입한 다수의 석영질 맥이 좌수향의 전단운동을 받은 증거가 관찰되는데 이러한 사실은 우수향의 전단운동 이후 화강암의 관입 이전에 좌수향의 전단 운동이 있었음을 지시한다. 무안 지역은 전주전단대의 끝 부분에 해당하는 곳으로 각섬석화강섬록암과 이를 관입한 각섬석화강암이 모두 우수향의 전단운동을 받았다. 화강섬록암의 U-Pb 저어콘 연대는 176.3 $\pm$ 1.7 Ma이며 화강암의 연대는 165.8 $\pm$ 2.0 Ma로 구해졌으며, 따라서 최종 우수향 전단운동의 시기는 166 Ma 이후로 생각된다. 무안 지역에 분포하는 화강섬록암과 화강암의 관입시기는 금산사 지역의 화강섬록암과 화강암과 각각 조화적이다. 호남전단대의 운동 시기를 밝히기 위해 전주전단대에 해당하는 금산사 지역과 무안 지역에 분포하는 화강암류에 대한 U-Pb 저어콘 연대 측정을 실시한 결과 호남전단대의 특징적인 우수향 전단운동은 적어도 2회에 걸쳐 일어났음을 알 수 있다. 즉 첫 번째 광역적인 전단운동은 약 173 - 170 Ma 시기에 일어났으며, 두 번째 전단운동은 166 Ma 이후에 일어났음을 알 수 있다. 한편 전기의 우수향 전단운동은 후기 화강암 관입 이전에 좌수향 전단 운동에 의해 부분적으로 재활성 되었으며, 후기 화강암의 관입 이후에 재차 우수향 전단운동으로 활성화 되었음을 알 수 있다. 이상의 결과를 종합하면 호남전단대는 쥬라기 중기에 발생한 광역적인 우수향의 연성전단운동이나, 운동 특성은 연속적이기 보다는 단속적으로 일어난 것으로 생각된다.

• The Journal of the Petrological Society of Korea
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• v.10 no.3
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• pp.179-189
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• 2001

The determination of trace concentration of U, Th and Pb was carried out for chemical dating of zircon and monazite by electron microprobe. Detection limit and error range should be considered to measure characteristic X-rays of M-line from those minerals, which are low in the ionization of atom and low peak intensity in the spectrum. The element of U, Th and Pb were simultaneously measured with 3 spectrometers equipped with PET crystal to reduce a total counting time and error due to drift of instrumental operating condition. Detection limit could be improved from increase of the peak/background ratio through adjusting pulse height analyzer about 1000 mv baseline. Under permissible maximum analytical conditions, theoretical detection limit of U, Th and Pb is down to 30 ppm (99% confidence level). The analytical result was maintained at a relative error $\pm$10% ($2{\sigma}$) in 800 ppm Pb, $\pm$5% ($2{\sigma}$) in 2330 ppm U and $\pm$10% ($2{\sigma}$) in dating from a single measurement of zircon at 15 keV and 100 nA. However, for the precise dating of zircon and monazite, if it is considered a 3 $\mu\textrm{m}$ spatial resolution, <100 ppm ($3{\sigma}$) detection limit and <$\pm$10% ($2{\sigma}$) relative error, optimum analytical conditions are given as 15~20 keV accelerating voltage, 100~200 nA beam current and 300~1200 sec total counting time. To reduce material damage by high current, there is need to be up to 3~5 $\mu\textrm{m}$ of electron beam diameter, or to use arithmetic average of multiple measuring at a shorter counting time. A younger or relatively low concentration rocks can be dated chemically by lower detection limit and improved precision resulted from increase of current and measuring time.

• 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.26 no.1
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• pp.1-11
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• 2017

We have dated the K-Ar, Ar-Ar and U-Pb ages of the Masan hornblende-biotite granite in the southern Cretaceous Gyeongsang basin to constrain its emplacement age. The ~108 Ma hornblende K-Ar age obtained in the study is similar to the previously reported Rb-Sr age. However, the single grain total fusion $^{40}Ar/^{39}Ar$ dating on hornblende failed to yield statistically meaningful ages because the isotopic system was open during its alteration. Thus the hornblende K-Ar age in the study is also unlikely to be reliable. The single grain total fusion $^{40}Ar/^{39}Ar$ dating on biotite yielded an average age of $75.8{\pm}3.0Ma$. Apart from scattered data in the range of ~45-75 Ma, the average age increased to ~80 Ma. The SHRIMP and LA-MC-ICPMS U-Pb isotopic compositions of zircon from the Masan hornblende-biotite granite yielded its emplacement age as $87.6{\pm}2.7Ma$ and $86.8{\pm}0.4Ma$, respectively. It is thus likely that the ~80 Ma $^{40}Ar/^{39}Ar$ age of biotite might reflect the cooling age of Masan hornblende-biotite granite or the thermal influences from later intense igneous activities in the Gyeongsang basin.

• The Journal of the Petrological Society of Korea
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• v.22 no.4
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• pp.299-305
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• 2013

It is well known that the Hongcheon iron-rare earth deposit is composed of carbonatite-phoscorite complex. We conducted zircon U-Pb age determination for the gneissic country rocks of this deposit. As the result we obtained ca. 1830 Ma, which is somewhat younger than igneous and metamorphic ages of ca. 1870 Ma generally reported from the Gyeonggi massif, suggesting further investigations for the timing and evolution of the Paleoproterozoic activities of the Gyeonggi massif.

• The Journal of the Petrological Society of Korea
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• v.26 no.1
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• pp.73-82
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• 2017

We determined SHRIMP U-Pb ages of the detrital zircons separated from the Bangnim Group of the Pyeongchang area to constrain its depositional age. As the result, the minimum age group yielded $^{206}Pb/^{238}U$ age of $450.3{\pm}4.2Ma$ (n=3), suggesting depositional age younger than Late Ordovician. Therefore, the Bangnim Group cannot be a Precambrian sedimentary formation but is younger than Myobong Formation of the Early Paleozoic Joseon Supergroup of the Taebaeksan basin. Such a depositional age implies that the Bangnim Group and structurally overlying Jangsan Quartzite should be in fault contact, suggesting that the Jangsan Quartzite, Myobong Formation and Pungchon Limestone thrusted over the Bangnim Group. The zircon U-Pb age distribution pattern of the Bangnim Group resembles those of the Early Paleozoic Myobong and Sambangsan Formations of the Taebaeksan basin and seemingly Middle Paleozoic Daehyangsan Quartzite and the Taean Formation. However, detrital zircon U-Pb age patterns of the Late Paleozoic Pyeongan Supergroup are quite distinct from them, suggesting drastic change in provenance of the detrital zircon supply. Therefore, we suggest that the Bangnim Group was deposited before the Pyeongan Supergroup.

• Economic and Environmental Geology
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• v.48 no.4
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• pp.287-299
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• 2015

The metapsammite distributed in the Yeongam-Gangjin area had been classified into age-unknown Yongamsan Formation, Seologri Formation and age-unknown Seogisan Formation, and these formations are reported as each other different formations. These formations have been renamed Precambrian Galdu or Permian Songjong Formations. In this study, we present detrital zircon SHRIMP U-Pb age data from the metapsammite to examine deposition time and stratigraphy. The analyzed U-Pb zircon ages dominantly reveal Paleoproterozoic ages of ca. 1.87Ga and the youngest detrital grains are constrained by the age of 246-265 Ma. The youngest age indicates late Permian or early Triassic for the deposition time. Therefore, the metapsammite in the Yeongam-Gangjin area is considered to be the upper formation of the late Paleozoic Pyeongan Group which is correlated with the Gohan-Donggo Formations or Nokam Formation of the Samcheock coal field and the Cheonunsan Formation of the Hwasun coal field. The metapsammite of the study area is the late Paleozoic Pyeongan Group by the zircon age rather than Precambrian Galdu and Permian Songjeong Formations are no longer meaningful. Therefore, we propose the upper Paleozoic 'metapelite' and 'metaspammite', or original formation name defined by 1:50,000 geological maps, instead of Galdu and Songjeong Formations.