• The Journal of the Petrological Society of Korea
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• v.16 no.2 s.48
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• pp.82-99
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• 2007

The Paleogene dikes intruding into the late Cretaceous granodiorite are pervasively observed in the Irun-myeon, eastern Geoje Island. They are classified into three groups: NW-trending acidic dike swarm and WNW- (A-Group) and $NS{\sim}NNE-trending$ (B-Group) basic dike swarms. Based on their cross-cutting relationships, the earliest is the acidic dike group and fellowed by A- and B-Groups in succession. The acidic dikes seem to have intruded into tension gashes induced by the sinistral strike-slip faulting of the Yangsan fault system during the late $Cretaceous{\sim}early$ Paleogene. In terms of rock-type, orientation, age, and geochemistry, A-Group and B-Group are intimately correlated with the intermediate and basic dike swarms in the Gyeongju-Gampo area, respectively. These results significantly suggest that the corresponding dike swarms are genetically related. Based on the K-Ar and Ar-Ar age data, A- and B- Groups were intruded during $64{\sim}52\;Ma$ and $51{\sim}44\;Ma$, respectively. The result means that the direction of tensional stress in and around the SE Korean peninsula was changed abruptly from NNE-SSW to $EW{\sim}WNW-ESE$ at about 51 Ma. Considering the tectonic environments during the Paleogene, it is interpreted that A-Group was injected along the WNW-trending tensional fractures developed under an regional sinistral simple shear regime which was caused by the north-northwestward oblique subduction of the Pacific plate beneath the Eurasian plate. Meanwhile, the regional stress caused by the collision of India and Eurasia continents at about 55 Ma was likely propagated to the East Asia at about 51 Ma, and then the East Asia including the Korean peninsula was extruded eastwards as a trench-rollback and the dip of downgoing slab of the Pacific plate was abruptly steepened. As a result, the strong suction-force along the plate boundary produced a tensional stress field trending EW or WNW-ESE in and around the Korean peninsula, which resultantly induced B-Group to intrude passively into the study area.

• Korean Journal of Mineralogy and Petrology
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• v.36 no.1
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• pp.73-94
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• 2023

We carried out the sensitive high resolution ion microprobe (SHRIMP) zircon U-Pb age dating and whole-rock geochemical analysis of granitoids and felsic porphyries in the Ssangyong Valley, Yongchu Valley, and Mungyeong Saejae geosites in the Mungyeong Geopark. The igneous rocks crop out in the western, northwestern and central parts of the Mungyeong city area, respectively, and intruded (meta)sedimentary successions of the Ogcheon Metamorphic Belt, Cambro-Ordovician Mungyeong Group and Jurrasic Daedong Group. The U-Pb isotopic compositions of zircon from two felsic porphyries and one granite samples in the Ssanyeong Valley yielded the Cretaceous intrusion ages of 93.9±3.3 Ma (tσ), 95.1±4.0 Ma (tσ) and 94.4±2.0 Ma (tσ), respectively. On the other hand, a felsic dike sample and a granite in the Yongchu Valley and a porphyritic granite in the Mungyeong Saejae had intrusion ages of 90.2±2.0 Ma (tσ), 91.0±3.0 Ma (tσ) and 88.6±1.5 Ma (tσ), respectively. Based on the average standard error calculated in combination with results of previous studies in this area (Lee et al., 2010; Yi et al., 2014; Aum et al., 2019), the geochronological results show that spatial variation in intrusion age of ~5 Myr between the Ssangyong (94.5±0.2 Ma) and Yongchu Valleys (89.7±0.4 Ma) is apparent. The geochemical compositions of major and trace elements in the samples showed an affinity of typical post-orogenic granite, indicating their petrogenesis during the late stage of Early Cretaceous magmatic activity possibly in association with subduction events of the Izanagi Plate.

• Economic and Environmental Geology
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• v.44 no.5
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• pp.413-431
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• 2011

An Epithemal Au-Ag mineralized zone is developed in the Moisan area of Hwangsan-myeon, Haenam-gun, Jeol-lanam-do, Korea, which is located in the southwestern part of the Ogcheon metamorphic zone. It is hosted in the Hwangsan volcaniclastics of the Haenam Formation of the Late Cretaceous Yucheon Group. This research investigated the characteristics of bedding arrangement, fold, fault, fracture system, quartz vein and the time-relationship of the fracture system to understand the geological structure related to the formation of the mineralized zone. On the basis of this result, the tectonic environment at the time of the mineralization was considered. Beds mainly trend east-northeast and gently dip into north-northwest or south-southeast. Their poles have been rearranged by subhorizontal-upright open fold of (east)-northeast trend as well as dip-slip fault. Fracture system was formed through at least 6~7 different deformation events. D1 event; formation phase of the main fracture set of EW (D1-1) and NS (D1-2) trends with a good extensity, D2 event; that of the extension fracture of NW trend, and conjugate shear fracturing of the EW (dextral) and NS (sinistral) trends, D3 event; that of the extension fracture of NE trend, and conjugate shear refracturing of the EW (sinistral) and NS (dextral) trends, D4 event; that of the extension fracture of NS trend showing a poor extensity, D5 event; that of the extension fracture of NW trend, and conjugate shear refracturing of the EW (dextral) and NS (sinistral) trends, D6 event; that of the extension fracture of EW trend showing a poor extensity. Frequency distribution of fracture sets of each deformation event is D1-1 (19.73 %)> D1-2 (16.44 %)> D3=D5 (14.79 %)> D2 (13.70 %)> D4 (12.33 %)> D6 (8.22 %) in descending order. The average number of fracture sets within 1 meter at each deformation event is D6 (5.00)> D5 = D4 (4.67)> D2 (4.60)> D3 (4.13)> D1-1 (3.33)> D1-2 (2.83) in descending order. The average density of all fractures shows 4.20 fractures/1 m, that is, the average spacing of all fractures is more than 23.8 cm. The frequency distribution of quartz veins at each orientation is as follows: EW (52 %)> NW (28 %)> NS (12 %)> NE (8 %) trends in descending order. The average density of all quartz veins shows 4.14 veins/1 m, that is, the average spacing of all quartz veins is more than 24.2 cm. Microstructural data on the quartz veins indicate that the epithermal Au-Ag mineralization (ca. 77.9~73.1 Ma) in the Moisan area seems to occur mainly along the existing D1 fracture sets of EW and NS trends with a good extensity not under tectonic stress but non-deformational environment directly after epithermal rupture fracturing. The D1 fracturing is considered to occur under the unstable tectonic environment which alternates compression and tension of NS trend due to the oblique northward subduction of the Izanagi plate resulting in the igneous activity and deformation of the Yucheon Group and the Bulguksa igneous rocks during Late Cretaceous time.