• Economic and Environmental Geology
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• v.56 no.2
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• pp.115-123
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• 2023

We introduce a new 'image-mapping' in-situ U-Pb dating method using LA-ICP-MS, proposed by Drost et al. (2018), and show the characteristics and usability of this method through several examples of absolute age results determined by first applying it to samples from the Joseon Supergroup of the Early Paleozoic Era in Korea. Unlike the previous in-situ spot analysis, this in-situ U-Pb dating method for carbonate minerals can determine the absolute age with high reliability by applying the 'image-mapping' method of micro-sized domains based on micro-textural observation, as well as determine the absolute age of multiple geological 'events' that occurred after deposition. This was confirmed in the case of determining the syn-depositional age and the multiple post-depositional ages from carbonate minerals of the Makgol and the Daegi Formations. Therefore, if the 'image-mapping' in-situ U-Pb dating method is applied to determine the absolute age of various types of carbonate minerals that exist in various geological environments throughout the geologic era, it will be possible to secure new geological age information.

• The Journal of the Petrological Society of Korea
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• v.26 no.4
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• pp.341-352
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• 2017

The Jipum Volcanics, occurred in western Yeongdeok, are a stratigraphic unit that is composed of rhyolitic pyroclastic rocks, tuffites, andesitic hyaloclastites, rhyolite lavas, tuffaceous conglomerates and andesite lavas. The SHRIMP U-Pb zircon dating yielded eruption ages of $68.5{\pm}1.6Ma$ from the rhyolitic pyroclastic rocks. Around the time, the unit was generated by dominant rhyolitic volcanisms and locally added by concomitant andesitc volcanisms from another vents. The rhyolitic volcanisms first produced the pyroclastic rocks by phreatomagmatic explosions from rhyolitic magma, later made of the rhyolite lava dome by lava effusions from reopening of the rhyolitc magma at the existing vent. At the time between first and second rhyolitic volcanisms, the tuffites were deposited at a shallow depression in the distal volcanic edifice, and andesitic volcanisms first made of the hyaloclastites by quench fragmentation when hot andesite lavas flew into the depression to contact with cold water. and the Jipum volcano was finally covered with the thin andesitic lavas by lava effusions from another vent.

• The Journal of the Petrological Society of Korea
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• v.27 no.1
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• pp.25-36
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• 2018

The volcanic rocks around Mieukdo Island, Tongyeong, are classified as lower andesitic rocks (Jusasan Subgroup) and rhyolitic rocks (Unmunsa Subgroup), and upper andesitic rocks (Yokji Subgroup) and rhyolitic rocks (Saryang Subgroup). We confirmed their eruption timings and stratigraphic relationships, based on SHRIMP U-Pb zircon dating for zircons from major stratigraphic units of the subgroups. By the SHRIMP U-Pb dating, the samples yield the concordia ages of $88.95{\pm}0.44Ma$(n=11) in Punghwari Tuff and $82.56{\pm}0.95Ma$(n=10) in Chudo Tuff of the lower andesitic rocks, and $73.01{\pm}0.75Ma$(n=11) in Dara Andesite of the upper andesitic rocks. And then samples show a concordia age of $71.74{\pm}0.47Ma$(n=14) in Namsan rhyolite dyke of the upper rhyolitic rocks and an apparent age of $70.7{\pm}3.5Ma$ in granodiorite dyke, These data confirm the eruption or injection timings of the units and allow them to distinguish chronostratigraphy of Jusasan, Unmunsa, Yokji and Saryang Subgroups around the Mireukdo Island. In addition, the subgroups give a clue that can make a chronostratigraphical correlation with different volcanic units of the Late Cretaceous Yucheon Group in the Gyeongsang basin.

• Journal of the Korean earth science society
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• v.43 no.3
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• pp.405-429
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• 2022

Detrital zircon LA-MC-ICP-MS U-Pb dating of the Cretaceous Gurye Group, Gurye Basin, was carried out. Gurye Group consists of Supyeongri, Geumjeongri, Togeum, and Obongsan formations in ascending order, and five samples were collected for age dating. Based on the dating results, the lowermost Supyeongri and the uppermost Obongsan formations show narrow age ranges. Only Precambrian and Late Cretaceous zircons were found in the Supyeongri and Obongsan formations, respectively. However, the upper and lower Geumjeongri, and Togeum formations show wide age ranges from the Precambrian to Cretaceous. The youngest detrital zircon U-Pb ages of each formation except the Supyeongri Formation, which lacks Cretaceous zircon, were calculated to be ca. 107.4 Ma in the lower Geumjeongri Formation, ca. 104.6 Ma in the upper Geumjeongri Formation, ca. 97.7 Ma in the Togeum Formation, and ca. 88.5 Ma in the Obongsan Formation. Such results indicate that the depositional age of the Gurye Group can be constrained from the Lower Cretaceous Albian to the Upper Cretaceous Coniacian. Based on the distribution of the detrital zircon ages from each formation, the source area of the Gurye Group is interpreted to have been extended from the adjacent Youngnam Massif to the Okcheon Belt throughout the basin evolution. The increase of the Cretaceous zircon with time is thought to reflect the slab roll-back of the proto-Pacific plate during the Cretaceous.

• The Journal of the Petrological Society of Korea
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• v.25 no.1
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• pp.29-37
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• 2016

A "granite boulder", ~70 cm in size, was reported from the Geumgang Limestone, and has been considered as a glaciogenic dropstone. Since this interpretation has enormous implications for unraveling the evolution history of the Ogcheon Metamorphic Belt, we re-examined the contact relationship and structure of the "granite boulder", and estimated its emplacement age based upon SHRIMP U-Pb zircon dating. The weighted mean $^{206}Pb/^{238}U$ age pooled from 6 spot analyses of two specimens is $170{\pm}2Ma$ ($2{\sigma}$, MSWD=2.2). This zircon age suggests that the "granite boulder" in the Geumgang Limestone is a part of Jurassic granite, rather than a glaciogenic dropstone.

• Economic and Environmental Geology
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• v.50 no.6
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• pp.467-476
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• 2017

Volcanic rocks related to the Guamsan cadera, which find in the southeastern Cheongsong, are divided into Volcanic breccia, Guamsan Tuff and Post-collapse intrusions. We determined their eruption, intrusion and caldera-forming timings based on SHRIMP U-Pb zircon dating. The dating results yield earlier eruption age of $63.77{\pm}0.94Ma$ from the lower ash-flow tuff and an later eruption age of $60.1{\pm}1.8Ma$ from the upper ash-flow tuff of the Guamsam Tuff, and intrusion age of $60.65{\pm}0.95Ma$ from the rhyolite ring dyke of the Post-collapse intrusions. The age data suggest that the Guamsan caldera is formed in 60.65~60.1 Ma between eruption of the upper ash-flow tuff and intrusion of the rhyolite ring dyke. The Guamsan cadera exhibits the volcanic processes of a perfect igneous cycle passing from ash-flow eruptions through caldera collapse to ring intrusions during 63.77~60.1 Ma.

• The Journal of the Petrological Society of Korea
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• v.28 no.4
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• pp.237-249
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• 2019

The volcanic rocks around the Jayang caldera are classified in an order such as Jukjang Volcanics, Doil Rhyolite, Unjusan Tuff and Rhyolite intrusions. By the SHRIMP U-Pb zircon datings from zircons, eruption ages of the Unjusan Tuff are constrained as 66.65±0.96 Ma in the intracaldera, and 66.08±0.62 Ma in the extracaldera outflow, and intrusion age of the ring dike is investigated as 60.74±0.66 Ma. The age data indicate that the caldera was collapsed between 66.08 Ma and 60.74 Ma, just before the dike intruded after the explosive eruption of the Unjusan Tuff. The Jayang caldera shows the composite igneous process of a perfect volcanic cycle passing from ash-flow tuffs through caldera collapse into ring dikes in the Jayang area.

• 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.25 no.4
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• pp.401-407
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• 2016

The SHRIMP zircon U-Pb age dating was carried out for the age-unknown Haeinsa Granite located in the middle Yeongnam Massif. SHRIMP zircon U-Pb age determinations of 7 samples from the Haeinsa Granite in Geochang area show two age groups. Ages from 5 samples (M-3-1, H-1, 3, 5, 10) are $192.4{\pm}1.4{\sim}195.5{\pm}1.9Ma$, whereas ages from 2 samples (H-11 and 12) are $187.7{\pm}3.3Ma$ and $188.2{\pm}3.6Ma$, respectively.

• The Journal of the Petrological Society of Korea
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• v.23 no.3
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• pp.209-220
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• 2014

SHRIMP zircon U-Pb age dating is carried out for the Yeongju and Andong granite batholiths intruding the Precambrian metamorphic complex and Paleozoic sedimentary formations within the NE Yeongnam Massif, Korea. Dating of zircons from a hornblende-biotite tonalite and an equigranular biotite granodiorite in the Yeongju granite has yielded ages of ca. 187 Ma and ca. 186 Ma, respectively. Also, dating of zircons from a biotite granodiorite and a very coarse-grained biotite granite in the Andong granite has yielded ages of ca. 182Ma and ca. 186Ma, respectively. These data indicate that the main intrusions of the Yeongju and Andong granite batholiths occur almost at the same age. The oldest age of ca. 194 Ma has been determined on zircons from a hornblende gabbro in the Andong granite, and the youngest age of 175 Ma is obtained from the Chunyang granite pluton, mainly consisting of fine-grained two-mica granite, of the Yeongju batholith. These results indicate that Jurassic Daebo magmatism in the Yeongju-Andong area, NE Yeongnam massif, started early at the Early Jurassic with an intrusion of mafic magma, and followed by an emplacement voluminous granite magma during the middle of the Early Jurassic, and was finalized with the emplacement of relatively small amount of much evolved granite magma at the end of Early Jurassic.