• The Journal of the Petrological Society of Korea
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• v.20 no.2
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• pp.77-98
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• 2011

The igneous complex consisting of mangerite and gabbro in the Odaesan area, the eastem part of the Gyeonggi Massif, South Korea, intruded early Paleo-proterozoic migmatitic gneiss. The mangerite is composed of orthopyroxene, clinopyroxene, amphibole, biotite, plagioclase, pethitic K-feldspar, quartz. The gabbro has similar mineral assemblage but gabbro has minor amounts of amphibole and no perthitic K-feldspar. The gabbro occurs as enclave and irregular shaped body within the mangerite, and the boundary between the mangerite and gabbro is irregular. Leucocratic lenses with perthitic K-feldspar are included in the gabbro enclaves. These textures represent mixing of two different magmas in liquid state. SHRIMP U-Pb zircon age dating gave $234{\pm}1.2$ Ma and $231{\pm}1.3$ Ma for mangerite and gabbro, respectively. These ages are similar with the intrusion ages of post collision granitoids in the Hongseong (226~233 Ma) and Yangpyeong (227~231 Ma) areas in the Gyeonggi Massif. The mangerite and gabbro are high Ba-Sr granites, shoshonitic and formed in post collision tectonic setting. These rocks also show the characters of subduction-related igneous rock such as enrichment in LREE, LILE and negative Nb-Ta-P-Ti anomalies. These data represent that the mangerite and gabbro formed in the post collision tectonic setting by the partial melting of an enriched lithospheric mantle during subduction which occurred before collision. The heat for the partial melting was supplied by asthenospheric upwelling through the gab between continental and oceanic slabs formed by slab break-off after continental collision. The distribution of post-collisional igneous rocks (ca. 230 Ma) in the Gyeonggi Massif including Odaesan mangerite and gabbro strongly suggests that the tectonic boundary between the North and South China blocks in Korean peninsula passes the Hongseong area and futher exteneds into the area between the Yangpyeong-Odaesan line and Ogcheon metamorphic belt.

• The Journal of the Petrological Society of Korea
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• v.25 no.3
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• pp.169-193
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• 2016

In order to characterize the Neotectonic crustal deformation and current stress field in and around the Korean Peninsula and to interpret their tectonic implications, this paper synthetically analyzes the previous Quaternary fault and focal mechanism solution data and recent geotechnical in-situ stress data and examines the characteristics of crustal deformations and tectonic settings in and around East Asia after the Miocene. Most of the Quaternary fault outcrops in SE Korea occur along major inherited fault zones and show a NS-striking top-to-the-west thrust geometry, indicating that the faults were produced by local reactivation of appropriately oriented preexisting weaknesses under EW-trending pure compressional stress field. The focal mechanism solutions in and around the Korean Peninsula disclose that strike-slip faulting containing some reverse-slip component and reverse-slip faulting are significantly dominant on land and in sea area, respectively. The P-axes are horizontally clustered in ENE-WSW direction, whereas the T-axes are girdle-distributed in NNW direction. The geotechnical in-situ stress data in South Korea also indicate the ENE-trending maximum horizontal stress. The current crustal deformation in the Korean Peninsula is thus characterized by crustal contraction under regional ENE-WSW or E-W compression stress field. Based on the regional stress trajectories in and around East Asia, the current stress regime is interpreted to have resulted from the cooperation of westward shallow subduction of the Pacific Plate and collision of Indian and Eurasian continents, whereas the Philippine Sea plate have not a decisive effect on the stress-regime in the Korean Peninsula due to its high-angle subduction that resulted in dominant crust extension of the back-arc region. It is also interpreted that the Neotectonic crustal deformation and present-day tectonic setting of East Asia commenced with the change of the Pacific Plate motion during 5~3.2 Ma.

• Economic and Environmental Geology
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• v.52 no.2
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• pp.159-168
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• 2019

The western Gyeonggi Massif, where records evidence of Phanerozoic subduction/collision tectonics, is an important area to understand the crustal evolutionary history of the Korean Peninsula. This study presents geometric and kinematic characteristics of the geological structures of the Taean Formation in the Gomseom area, southwestern Gyeonggi Massif. We interpreted the geometric relationships between structural elements, and conducted stereographic and down-plunge projections for structural domains. As a result, at least three different deformational events ($D_1$, $D_2$ and $D_3$) are recognized in the study area. In the first deformational event ($D_1$), regional foliations being well defined by the preferred orientation of muscovite and biotite were formed. In the second deformational event ($D_2$), NNE-trending low-angle contractional faults and related crenulation lineations/cleavages were formed. The crenulation lineations shallowly plunge toward SSW~SSE or NNW~NNE. In the third deformational event ($D_3$), SE-plunging folds and NE-trending high-angle faults were formed as 'fault-related fold' and 'fold-accommodation fault', indicating that the $D_3$ folds and faults are genetically linked to each other. This contribution provides important insights into the structural evolution of the Taean Formation along western Gyeonggi Massif, where had evolved as subduction/collisional orogenic belts in the East Asia.

• Journal of the Korean earth science society
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• v.43 no.5
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• pp.618-638
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• 2022

The Middle Jurassic granite dike swarm intruding into the Paleoproterozoic banded gneiss is pervasively observed in Ueumdo, Hwaseong City, mid-western Gyeonggi Massif. Based on their cross-cutting relationships in a representative outcrop, there are four dikes (UE-A, UE-C, UE-D, UE-E), and depending on the direction, there are three granite dike groups, which are NW- (UE-A dike), NW to WNW- (UE-C dike), and NE-trending (UE-D and UE-E dikes). These granite dikes are massive, medium-to coarse-grained biotite granites, and their relative ages observed in outcrops are in the order of UE-A, UE-D (=UE-E), and UE-C. The geometric analysis of the dikes indicates that the UE-A and UE-C dikes intrude under approximately NE-SW trending horizontal minimum stress fields. The UE-A dike, which showed a relatively low average SiO2 content by major element analysis, is a product of early magma differentiation compared to other dikes; therefore, it is consistent with the relative age of each dike. The ²⁰⁶Pb/²³⁸U weighted mean ages for each dike obtained from SHRIMP zircon U-Pb dating were calculated to be 167 Ma (UE-A), 164 Ma (UE-C), 167 Ma (UE-D), and 167 Ma (UE-E), respectively. The samples of the UE-A, UE-D, and UE-E dikes showed very similar ages. The UE-C dike shows the youngest age, which is consistent with the results of the relative age in the outcrops and major element analysis. Therefore, the granite dikes intruded into the Middle Jurassic (approximately 167 and 164 Ma), coinciding with those of the Gyeonggi Massif, where the Middle Jurassic plutons are geographically widely distributed. This result indicates that the wide occurrence of the Middle Jurassic plutons on the Gyeonggi Massif was formed as a result of igneous activity moving in the northwest direction with the shallower subduction angle of the subducting oceanic plate during the Jurassic.

• Economic and Environmental Geology
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• v.55 no.3
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• pp.261-271
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• 2022

Six gas samples were collected from coal and coaly shale from core AA-1, which was acquired from the Asem-Asem Basin, southeast Kalimantan, Indonesia. These coalbed gas samples were analyzed for the molecular composition, carbon isotope (δ¹³C_CH4, δ¹³C_C2, and δ¹³C_CO2), hydrogen isotope (δD_CH4), hydrocarbon index (C_HC), and carbon dioxide-methane index (CDMI) to document their origin and methanogenic pathways. Core AA-1 successively consists of lower clastic sedimentary rocks (Sedimentary Unit-1, SU-1) containing coal and coaly shale, and upper limestone (Sedimentary Unit-2, SU-2), unconformably underlain by serpentinized basement interpreted as part of the Cretaceous Meratus subduction complex (MSC). The coal and coaly shale (SU-1) were deposited in a marshes nearby a small-scale river. Compositions of coalbed gases show that methane ranges from 87.35 to 95.29% and ethane ranges from 3.65 to 9.97%. Carbon isotope of coalbed methane (δ¹³C_CH4) ranges from -60.3 to -58.8‰, while hydrogen isotope (δD_CH4) ranges from -252.9 to -252.1‰. Carbon isotope of coalbed ethane (δ¹³C_C2) ranges from -32.8 to -31.2‰, carbon isotope of coalbed carbon dioxide (δ¹³C_CO2) ranges from -8.6 to -6.2‰. The coalbed CO₂ is interpreted to be an abiogenic origin based on a combination of δ¹³C_CO2 and CDMI and could have been transported from underlying CO₂ bearing MSC through faults. The methanogenic pathways of coalbed gases are interpreted to have originated from primary methyl-type fermentation and mixed with CO₂ reduction, affecting thermogenic non-marine coal-type gases based on analyses of isotopic ratios and various indexes.

• Journal of the Korean earth science society
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• v.43 no.1
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• pp.128-139
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• 2022

This study aimed to investigate the tectonic setting of the volcanic edifice at Mt. Baekdu by analyzing petrochemical characteristics of Holocene felsic volcanic rocks distributed in the Baekdusan stratovolcano edifice and summit of the Cheonji caldera rim, as well as Pleistocene mafic rocks of the Gaema lava plateau and Changbaishan shield volcano edifice. During the early eruption phases, mafic eruption materials, with composition ranging from alkali basalt to trachybasalt, or from subalkaline (tholeiitic) basalt to basaltic andesite formed the Gaema lava plateau and Changbaishan shield volcanic edifice, whereas the Baekdusan stratovolcano edifice and Holocene tephra deposits near the summit of the Cheonji caldera comprises trachytic and rhyolitic compositions. Analysis results revealed bimodal compositions with a lack of 54-62 SiO₂, between the felsic and mafic volcanic rocks. This suggested that magmatic processes occurred at the locations of extensional tectonic settings in the crust. Mafic volcanic rocks were plotted in the field of within-plate volcanic zones or between within-plate alkaline and tholeiite zones on the tectonic discrimination diagram, and it was in good agreement with the results of the TAS diagram. Felsic volcanic rocks were plotted in the field of within-plate granite tectonic settings on discrimination diagrams of granitic rocks. None of the results were plotted in the field of arc islands or continental margin arcs. The primitive mantle-normalized spider diagram did not show negative (-) anomalies of Nb and Ti, which are distinctive characteristics of subduction-related volcanic rocks, but exhibited similar patterns of ocean island basalt. Trace element compositions showed no evidence of, magmatic processes related to subduction zones, indicating that the magmatic processes forming the Baekdusan volcanic field occurred in an intraplate environment. The distribution of shallow earthquakes in this region supports the results. The volcanic rocks of the Baekdusan volcanic field are interpreted as the result of intraplate volcanism originating from the upwelling of mantle material during the Cenozoic era.

• Korean Journal of Mineralogy and Petrology
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• v.35 no.3
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• pp.367-376
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• 2022

This study is a preliminary step to understand the reaction between various liquids and zeolite in the subduction zone environment. Stilbite, NaCa₄(Al₉Si₂₇)O₇₂·28(H₂O), was selected and high pressure study was conducted on compressional behavior by the pressure-transmitting medium (PTM). Water and NaHCO₃ solution that can exist in the subduction zone was used as PTM, and samples were pressurized from ambient to a maximum of 2.5 GPa. Below 1.0 GPa, both experiments show a low linear compressibility in the range of 0.001 to 0.004 GPa^-1 and a high bulk modulus of 220(1) GPa. This is presumably because the structure of the stilbite becomes very dense due to insertion of water molecules or cations into the channel. On the other hand, at 1.0 GPa or higher, the trends of the two experiments are different. In the water run, the linear compressibility of the c-axis is increased to 0.006(1) GPa^-1. In the NaHCO₃ run, the linear compressibility of the b- and c-axis is increased to 0.006(1) GPa^-1. The bulk modulus after 1.0 GPa shows values of 40(1) and 52(7) GPa in water and NaHCO₃ run, respectively, confirming that stilbite becomes more compressible than that before 1.0 GPa. It is caused by the migration of cations and water molecules inside the channel, as the water molecules in the PTM start to freeze and stop to insert toward the channel at 1.0 GPa or more. In the NaHCO₃ run, it is assumed that the distribution of extra-framework species inside the structure is changed by substitution of the Na⁺ cation. It can be expected from tendency of the relative intensity ratio of the (001) and (020) peaks which show a different from that of the water run.

• Economic and Environmental Geology
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• v.57 no.1
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• pp.1-15
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• 2024

The Melanesian region in the western Pacific is dominated by complex plate tectonics, with the largest oceanic plateau, the OntongJava plateau, and a hotspot, the Caroline Islands. To better understand the complex geodynamics of the region, we estimate P- and S-velocity models and 𝛿 (V_P/V_S) model by using relative teleseismic travel times measured at seismometers on land and the seafloor. Our results show high-velocity anomalies in the subduction zones of the Melanesian region to a depth of about 400 km, which is thought to be subducting Solomon Sea, Bismarck, and Australian plates along plate boundaries. Along subduction zones, positive 𝛿 (V_P/V_S) anomalies are found, which may be caused by partial melting due to dehydration. A broad high-velocity anomaly is observed at 600 km depth below the Ontong-Java plateau, with a negative 𝛿 (V_P/V_S) anomaly. This is thought to be a viscous and dry remnant of the Pacific plate that subducted at 45-25 Ma, with a low volume of fluids due to dehydration for a long period in the mantle transition zone. Beneath the Caroline Islands, a strong low-velocity anomaly is obseved to a depth of 800 km and appears to be connected to the underside of the remnant Pacific plate in the mantle transition zone. This suggests that the mantle plume originating in the lower mantle has been redirected due to the interaction with the remnant Pacific plate and has reached its current location. The mantle plume also has a positive 𝛿 (V_P/V_S) anomaly, which is thought to be due to the influence of embedded fluids or partial melting. A high-velocity anomaly, interpreted as an effect of the thick lithosphere beneath the Ontong-Java plateau, is observed down to 300 km depth with a negative 𝛿 (V_P/V_S) anomaly, which likely indicate that little fluid remains in the melt residue accumulated in the lithosphere.

• Geophysics and Geophysical Exploration
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• v.11 no.3
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• pp.167-176
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• 2008

Lau basin of the south Pacific, as an active back arc basin, is promising area bearing seafloor massive hydrothermal deposit that is located in a subduction zone between the Pacific ocean plate and Indo-Australian continental plate. We performed multi-beam bathymetry survey in the Lau basin using EM120, to find out high hydrothermal activity Bone. Fonualei Rift and Spreading Center (FRSC) and Mangatolou Triple Junction (MTJ) area were selected for precise site survey through seafloor morphology investigation. The result of surface and deep-tow magnetometer survey showed that Central Anomaly Magnetization High (CAMH) recorded which is associated with active ridge in FRSC-2 and revealed very low magnetic anomalies that can be connected to past or present high hydrothermal activity in MTJ-1 seamount area. Moreover, the physical and chemical tracers of hydrothermal vent flume, i.e., transmission, hydrogen ion concentration (pH), adenosine triphosphate (ATP), methane (CH4) by use of CTD system, showed significant anomalies in those areas. From positive vent flume results, we could conclude that these areas were or are experiencing very active volcanic activities. The acquired chimney and hydrothermal altered bed rock samples gave us confidence of the existence of massive hydrothermal deposit. Even though not to use visual exploration equipment such as ROV, DTSSS, etc., traditional marine geophysical investigation approach might be a truly cost-effective tool for exploring seafloor hydrothermal massive deposit.

• The Journal of the Petrological Society of Korea
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• v.27 no.3
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• pp.121-138
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• 2018

We have examined the petrotectonic setting and magmatic evolution from petrochemical characteristics of major and trace elements for the Cretaceous volcanic rocks in and around the Mireukdo Island. The volcanic rocks, can be devided into Jusasan, Unmunsa, Yokji and Saryang subgroups on the ascending order, are classified as basalt, basaltic andesite, andesite, dacite and rhyolite on TAS diagram. Petrochemical data show that the rocks are calc-alkaline series, and suggest that erupted earlier medium-K series and later high-K series. The volcanic rocks provide a case in which the calc-alkaline magma are formed, not only from separate protoliths, but following separate paths from source to surface. Earlier and later subgroups take different paths to the surface respectively, and are emplaced in the shallow crust as a series of discrete magma chambers through the volcanic processes. After emplacement, each chamber evolves indepently through fractional crystallization with a little assimilation of wall rock. The volcanic rocks have close petrotectonic affinities with orogenic suite and subduction-related volcanic arc. The rhyolitic magma can be derived from calc-alkaline andesitic magma by fractional crystallization with crustal assimilation, which may be derived from a partial melt of peridotite in the upper mantle.