• The Journal of the Petrological Society of Korea
• /
• v.28 no.4
• /
• pp.237-249
• /
• 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
• /
• v.26 no.4
• /
• pp.341-352
• /
• 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
• /
• v.20 no.4
• /
• pp.231-241
• /
• 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
• /
• v.21 no.1
• /
• pp.31-45
• /
• 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.

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

• The Journal of the Petrological Society of Korea
• /
• v.22 no.4
• /
• pp.299-305
• /
• 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
• /
• v.15 no.3 s.45
• /
• pp.119-127
• /
• 2006

Detrital zircons in iron-bearing quartzite of the Seosan Croup from southeastern part of the Cyeonggi Hassif were analysed for SHRIHP U-Pb ages. Among 42 analyses, 38 data yield concordant ages (less tan 10 % discordancy), and they concentrated at 1781~1898 Ma (n=19), $1781{\sim}1898\;Ma(n=19),\;1935{\sim}1941\;Ma(n=4),\;1996\;Ma,\;2120\;Ma\;2403{\sim}2459\;Ma(n=5)$, 2661 Ma and 3198 Ma. The data indicate that sedimentation of iron-bearing quartzite should be after ca 1.78 Ga (the youngest detrital zircon age), and argue against some of conventional idea that iron-bearing quartzite of the Seosan Group might be correlated with the Archean iron-bearing quartzite in the North China Craton.

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

• The Journal of the Petrological Society of Korea
• /
• v.23 no.3
• /
• pp.197-208
• /
• 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
• /
• v.23 no.3
• /
• pp.167-185
• /
• 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.