• The Journal of the Petrological Society of Korea
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• v.10 no.3
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• pp.172-178
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• 2001

K-Ar dating system of Korea Basic Science Institute (KBSI) was installed in 1997 and has been used since then. The system consists of high temperature graphite furnace, gas purification system, and mass spectrometer with data acquisition system. K-Ar age is determined by the measurement of the concentrations of Ar and K through isotope dilution method using $^{38}Ar$ as spike and flame spectroscopy, respectively. The accuracy and reliability for the K-Ar age are checked using the several K-Ar standard materials. Although the exact age determination for young samples of less than 1 Ma is hampered by small fluctuations of sensitivity and mass discrimination, the present system yields the reliable K-Ar age compared to the standard materials of Tertiary and Mesozoic age. The measurements for the SORI93 biotite with the recommended K-Ar age of $92.6\pm$0.6 Ma and Bern4M muscovite of $18.5\pm$0.6 Ma yield the reliable age of $92.1\pm$1.1 Ma and $17.8\pm$0.2 Ma, respectively.

• 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.

• Journal of the Korean earth science society
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• v.29 no.5
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• pp.386-395
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• 2008

The absolute age of lapilli tuff in sedimentary formation that contains dinosaur fossils in the Boseong area, Korea was determined radiometrically against volcanic rocks below and above the fossil-bearing horizons. The sanidine in the lapilli tuff below the fossil-bearing horizon (Seonso formation) has an $^{40}Ar-^{39}Ar$ age of $81.l{\pm}1.4Ma$. The Pilbong tuff above Seonso formation has an $^{40}Ar-^{39}Ar$ age of $81.0{\pm}2.4Ma$. An andesite dyke intruding all sedimentary units yields an $^{40}Ar-^{39}Ar$ age of $42.4{\pm}2.5Ma$. Thus 81 Ma age can be regarded as the best estimate for the age of the Seonso Formation and the associated the dinosaur eggs. This age correlates well with dinosaur fossil finds in the Haenam and Koseong regions of Korea. The occurrence of dinosaur eggs and clutches attests to the existence of dinosaurs in southern Korea at least inCampanian times.

• The Journal of the Petrological Society of Korea
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• v.7 no.2
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• pp.69-76
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• 1998

$^{40}Ar/^{39}Ar$ analytical data of hornblende and muscovite separates from granitic rocks in southwestern region of Ogcheon belt shows fellowing tectonic implication, $^{40}Ar/^{39}Ar$ data of 5 samples yield apparent age spectra and $^{37}Ar_{ca}/^{39}Ar_k$ and $^{38}Ar_{CI}/^{39}Ar_k$ plateaus for more than 60% of the $^{39}Ar$ release. Except for HN-100, the $^{36}Ar/^{40}Ar$ versus $^{39}Ar/^{40}Ar$ corelalation diagrams indicate the presence of one distint line. Muscovite of sample PKJ-44 yield flate apparent age plateau for > 60% of the $^{39}Ar_k$ release. In the high temperature steps, the $^{37}Ar_{ca}/^{39}Ar_k$ values are irregular with a correlative increase in $^{38}Ar_{CI}/^{39}Ar_k$, suggesting some Ca and CI rich phase, tapped between the silicate sheet is being argon degassed. The $^{40}Ar/^{39}Ar$ total gas age and the high temperature age of HN-100 is 918.2 Ma and 1360 Ma, respectively. The former affectted by recystallized age of Daebo Orogeny, and the latter indicated age of hornblende closure temperature for cooling stage of amphibole xenolith in granite gneiss. Three rock types of Kwangju granites show about 165 Ma hornblende and muscovite ages with some degassed argon at low temperature steps. These ages of 4 samples indicate also recrystallized age by Daebo Orogeny. In $^{40}Ar/^{39}Ar$ mineral age, Rb/Sr whole age and K/Ar mineral age, discordant ages of southwestern region of Ogcheon belt suggesting cooling rates approaching 3~4$^{\circ}C$/m. y. Such slow cooling rates can be produced by uplift rate of 100m/m.y. or slightly slower than isothem-migration rate derived from the hornblende samples. We conclude that the strongest Orogeny and igneous activity of southwestern region of Ogcheon belt are middle proterozoic era (about 1360 Ma) and middle Jurassic period (about 165 Ma).

• The Journal of the Petrological Society of Korea
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• v.14 no.1
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• pp.38-44
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• 2005

Hornblende $^{40}Ar/^{39}Ar$ age of $113.4{\pm}2.4(2{\sigma})$ Ma was determined from the volcanic pebble of the Silla Conglomerate which belongs to the Hayang Group of the Cretaceous Gyeongsang Supergroup. This age corresponds to the top of Aptian. Based on the reported age information, onset and duration of deposition of the constituting formations of the Hayang Group are constrained as follows; deposition of the Jindong Formation started from ca. 96~97 Ma and lasted for about 15 Ma. Therefore, Jindong Formation was deposited since Cenomanian to Santonian and it is likely to be extended to the early Campanian. We propose 81~80 Ma, which is in early Campanian, as the boundary between Hayang and Yucheon Groups. We suggest that the Silla Conglomerate was deposited during the early Albian and the Haman Formation was deposited during the rest of the Albian and also during the Cenomanian. The Chilgok Formation seems to be deposited during the late Aptian.

• The Journal of the Petrological Society of Korea
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• v.27 no.1
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• pp.67-72
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• 2018

The K-Ar ages of a sericite quartz schist in the lower Jangsan Formation along the Okdong fault zone reported by Yun (1983) have attracted attention again because of their potential to constrain the depositional timing of the Jangsan Formation. The oldest age of $562{\pm}2Ma$ among three reported K-Ar ages in the schist led to the claim that the depositional period of the lowermost Jangsan Formation in the Joseon Supergroup is late Neoproterozoic. Its depositional age is important for understanding the tectonic evolution of the Korean Peninsula including the formation and evolution histories of its sedimentary basins. Thus, the reliability and geological meaning of three K-Ar ages in the original paper (Yun, 1983) were revisited in the review. Quartz grains in the analyzed sample contain a considerable amount of excess Ar, and therefore it is inappropriate to use the ages as a basis for a depositional age constraint of the Jangsan Formation. The timing of mylonitization in the schist is recalculated as ~170 Ma.

• Ocean and Polar Research
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• v.23 no.1
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• pp.23-34
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• 2001

Geochemical and Sr-Nd isotopic compositions and K-Ar ages are analyzed in volcanic rocks from Weno Island, Caroline Islands. Seven Weno lava samples of alkali basalt and basaltic trachyandesite are aphyric or sparsely phyric comprising olivine, plagioclase, and clinopyroxene phenocrysts. Whole-rock geochemical variation of Weno lavas reflects main fractional crystallization of olivine and Cr-spinel phenocrysts. Newly determined K-Ar ages of Weno lavas range from 6.7 to 11.3 Ma (late Miocene), indicating their formation during primary volcanic stage of Chuuk Islands. Trace element compositions of Weno lavas are very similar to those of typical ocean island basalts (OIBs), suggesting their formation during intra-plate mantle plume activity. The plume composition is isotopically very similar to that of Hawaiian hot spot. However, the age span of Chuuk volcanism is longer than that of the other individual volcanoes in the Pacific.

• The Journal of the Petrological Society of Korea
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• v.1 no.1
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• pp.58-70
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• 1992

From the K-Ar age determinations for the clay deposits and their surrounded rocks in southwest Korea, the ages of the ore formation in all clay deposits fall in very narrow range from 78.1 to 81.4 Ma. K-Ar ages of clay deposits are slightly younger than those of the Cretaceous volcanic rocks (Hwangsan Formation, 81.4 to 86.4 Ma) and are slightly older than those of the Cretaceous granitic rocks (77.1 to 81.5 Ma). These results indicate that clay deposits were formed with genetical relation to late Cretaceous felsic magmatism. Weolgagsan granite, which has been previously considered to be Cretaceous, is proved to be formed its age in Jurassic (140.9 and 144.8 Ma). The close relationships of K-Ar ages between the clay deposits and Cretaceous granitic rocks suggest that the clay deposits were formed during the hydrothermal alterations caused by the thermal effects (hydrothermal circulation) of the granitic intrusions rather than by the hydrothermal activities associated with volcanic activities.

• The Journal of the Petrological Society of Korea
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• v.3 no.2
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• pp.128-137
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• 1994

The northeastern shore of the Anmyondo consists mainly of tuff, basalt, andesite and dacite. The K-Ar ages of the two mugearites are $89.4{\pm}2.4$ Ma and $91.9{\pm}2.3$ Ma which correspond to the middle Cretaceous age. Petrochemical reviews on the volanic rocks of the Cretaceous, Tertiary and Quaternary ages in the Korean Peninsula show that marked differences exist in chemical compositions according to the age of eruption.

• Journal of the speleological society of Korea
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• v.42 no.2
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• pp.17-28
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• 1995

THE K-AR METHOD of age determination is commonly used to date rocks from Pleistocene volcanoes in Japan (e.g. Kaneoka et al. 1980, Itaya et al. 1984, Shimizu et al. 1988, Itaya et al. 1989). However. there are still many problems with K-Ar dating of the young volcanic rocks, as reviewed by Itaya and Nagao (1988).(omitted)