Okcheon Belt Research Articles

Middle Paleozoic metamorphic and sedimentary events in the central Korean Peninsula and their geological implications

The Middle Paleozoic tectonic evolution of the central Korean Peninsula (Gyeonggi Massif, Hongseong-Imjingang, and Okcheon zone of Okcheon Belts) remains controversial. Particularly, the occurrence of high-grade metamorphism and sedimentation need further examination. In this study, we conducted zircon U-Pb-rare earth element analyses from in the ultramafic-mafic complex in the central Korean Peninsula (Cheonan and Gapyeong areas) and the Paleozoic metasedimentary rocks in the Okcheon Zone of the Okcheon Belt to constrain the timing of maximum depositional and metamorphic ages. We also examined the metamorphic P-T-t path from garnet-bearing amphibolite in the central Korean Peninsula by pseudosection modeling and geothermobarometer. The results show that (i) some of the ultramafic-mafic complex and metasedimentary rocks in the central Korean Peninsula formed during the Middle Paleozoic (ca. 450–374 Ma); (ii) garnet-bearing amphibolite underwent successive metamorphism from amphibolite facies condition (7.5–8.0 kbar and 540–630 °C) at pre-peak stage to granulite facies condition (10.9–11.8 kbar and 740–820 °C) at peak stage, and then retrograded into amphibolite facies condition (5.7–7.7 kbar and 530–670 °C) along the clockwise P–T path during ca. 403–362 Ma. This and previous studies suggest that the central Korean Peninsula underwent subduction-related orogenic events during the Middle Paleozoic era, and it is well correlated to those of the orogenic events in the North Qinling belt.

Preliminary Structural Geometry Interpretation of the Pyeongchang Area in the Northwestern Taebaeksan Zone, Okcheon Belt: A Klippe Model

Preliminary Structural Geometry Interpretation of the Pyeongchang Area in the Northwestern Taebaeksan Zone, Okcheon Belt: A Klippe Model

Paleozoic tectonic evolution of the proto-Korean Peninsula along the East Asian continental margin from detrital zircon U-Pb geochronology and Hf isotope geochemistry

Detrital zircon geochronology and Hf isotope analysis can be used for inferring provenance characteristics, and to evaluate the tectonic evolution of sedimentary basins and their link with regional orogenesis. The Paleozoic sequences of the Okcheon Belt consist of the Lower Paleozoic Joseon and the Upper Paleozoic Pyeongan supergroups with Middle Paleozoic hiatus locally on top of the Neoproterozoic bimodal volcanic rocks, reflecting an intracontinental rift setting between the two basements (viz. Gyeonggi and Yeongnam massifs) at southern part of the Korean Peninsula. Our detrital zircon U-Pb ages and Lu-Hf isotope results show that all these Paleozoic strata commonly have Paleoproterozoic and Paleozoic zircon ages with rare Meso- to Neoproterozoic ages. The individual zircon populations display following features, allowing estimation of their sedimentary provenances: (i) The Paleoproterozoic zircons (ca. 1.85 Ga and 2.50 Ga) with similar ranges of εHf(t) values are most common in the basement rocks of the Korean Peninsula, and were sourced from both the Gyeonggi and Yeongnam massifs. (ii) The Meso- to Neoproterozoic zircons, preserved only in the Middle to Late Cambrian clastic sedimentary rocks within the carbonate sequences probably reflect proximal provenance. (iii) The youngest Paleozoic zircons of each formation, almost coincident with their deposition ages, suggest presence of syndepositional magmatism, indicating proximal magmatic sources during their deposition. (iv) The Cambrian–Ordovician zircons, from the Lower Paleozoic sequences, but rare in the successive Upper Paleozoic sequences, suggest a provenance change after the hiatus between the two sedimentary successions. (v) The Permian zircons showing different εHf(t) values indicate that detrital sources were varied at that time. The integrated results in our study suggest provenance variability linked to diverse tectonic environments, reflecting prolonged subduction-related crustal evolution of the proto-Korean Peninsula during the Paleozoic.

The Paleoproterozoic Evolution of Basement Rocks of the Taebaeksan Basin, Korean Peninsula, and Their Correlation to Those of the Paleoproterozoic Massifs in the Korean Peninsula

The Korean Peninsula mainly comprises the Paleoproterozoic Gwanmo, Nangnim, Gyeonggi, and Yeongnam massifs from north to south. The Paleoproterozoic basement is rarely exposed in the Paleozoic Taebaeksan basin, which is located in the northeastern part of the Okcheon belt between the Gyeonggi and Yeongnam massifs. One of the most important issues in the tectonic interpretation of the Korean Peninsula is whether Paleoproterozoic rocks in the Taebaeksan basin have an affinity with those in the Gyeonggi or Yeongnam massifs. To solve this problem, we focused on the petrogenesis of the Imgye gabbroic diorite, Jungbongsan granite, and Jangsan quartzite in the Imgye area of the Taebaeksan basin. The Imgye gabbroic diorite shows mafic to intermediate compositions with slightly enriched LREEs compared to HREEs, slightly positive Rb, K, and Pb anomalies, and negative Ta, Nb, and P anomalies. The Imgye gabbroic diorite formed in a volcanic arc tectonic setting. The geochemical compositions of the Jungbongsan granite show enriched LREEs compared to HREEs with negative Eu anomalies, and reveal strong positive Rb, Th, K, and Pb anomalies with negative Ba, Ta, Nb, Sr, P, Eu, and Ti anomalies. This Jungbongsan granite also formed in an arc tectonic setting like the Imgye gabbroic diorite. LA-ICP-MS zircon age dating of the Imgye gabbroic diorite gives an intrusion age of 1948 ± 21 Ma, whereas SHRIMP U–Pb zircon age dating on the Jungbongsan granite yields an emplacement age of 1873 ± 14 Ma. The εHf(t) values of the Imgye gabbroic diorite are from 3.5 to 9.7, whereas those of the Jungbongsan granite are from −2.9 to 0.6. These data imply that the Imgye gabbroic diorite formed from a depleted mantle in the arc tectonic environment, whereas the Jungbongsan granite formed by reworking pre-existing crust material in the arc environment. The detrital zircons in the Jangsan quartzite show ages ranging from 3.06 to 1.85 Ga, with a peak concentration of ca. 2.5 Ga. Previous studies have suggested that the northern Gyeonggi and Nangnim massifs underwent collision-related magmatism and metamorphism at ca. 1.93–1.90 Ga, and then post-collisional magmatism and metamorphism at ca. 1.89–1.83 Ga, whereas the southern Gyeonggi massif underwent subduction-related magmatism and metamorphism at ca. 1.94–1.92 Ga, and then post-collision-related magmatism and metamorphism at ca. 1.84–1.78 Ga. By contrast, subduction-related events were recognized in the northern Yeongnam massif at ca 2.02–1.96 Ga and 1.90–1.85 Ga. This work, combined with the previous studies, suggests that the Paleoproterozoic basement in the Imgye area of the Taebaeksan basin can be correlated with the Paleoproterozoic basement of the northern Yeongnam massif rather than with those of the Nangnim and Gyeonggi massifs.

Lithospheric thinning and ignition of a Cordilleran magmatic flare-up: Geochemical and O-Hf isotopic constraints from Cretaceous plutons in southern Korea

Northeast Asian continental margins contain the products of magma emplacement driven by prolonged subduction of the (paleo-)Pacific plate. As observed in many Cordilleran arcs, magmatic evolution in this area was punctuated by high-volume pulses amid background periods. The present study investigates the early evolution of the Cretaceous magmatic flare-up using new and published geochronological, geochemical, and O-Hf isotope data from plutonic rocks in the southern Korean Peninsula. After a long (∼50 m.y.) magmatic hiatus and the development of the Honam Shear Zone through flat-slab subduction, the Cretaceous flare-up began with the intrusion of monzonites, granodiorites, and granites in the inboard Gyeonggi Massif and the intervening Okcheon Belt. Compared to Jurassic granitoids formed during the former flare-up, Albian (∼111 Ma) monzonites found in the Eopyeong area of the Okcheon Belt have distinctly higher zircon εHf(t) (−7.5 ± 1.3) and δ18O (7.78‰ ± 0.25‰) values and lower whole-rock La/Yb and Sr/Y ratios. The voluminous coeval granodiorite and granite plutons in the Gyeonggi Massif are further reduced in Sr/Y and to a lesser extent, in La/Yb, and have higher zircon εHf(t) values (−13 to −19) than the Precambrian basement (ca. −30). These chemical and isotopic features indicate that Early Cretaceous lithospheric thinning, most likely resulting from delamination of tectonically and magmatically overthickened lithospheric keel that was metasomatized during prior subduction episodes, and consequent asthenospheric upwelling played vital roles in igniting the magmatic flare-up. The O-Hf isotopic ranges of synmagmatic zircons from the Albian plutons and their Paleoproterozoic and Jurassic inheritance attest to the involvement of lithospheric mantle and crustal basement in magma generation during this decratonization event. Arc magmatism then migrated trenchward and culminated in the Late Cretaceous, yielding widespread granitoid rocks emplaced at shallow crustal levels. The early Late Cretaceous (94–85 Ma) granites now prevalent in Seoraksan-Woraksan-Sokrisan National Parks are highly silicic and display flat chondrite-normalized rare earth element patterns with deep Eu anomalies. Synmagmatic zircons in these granites mimic their host rock’s chemistry. Delamination-related rejuvenation of crustal protoliths is indicated by zircon εHf(t) values of granites (−6 to −20) that are consistently higher than the Precambrian basement value. Concomitant core-to-rim variation in zircon O-Hf isotopic compositions reflects a typical sequence of crustal assimilation and fresh input into the magma chamber.

Detrital Zircon U-Pb Geochronology and Petrography Analysis of the Haenam Basin

The detrital zircons of the Cretaceous Haenam basin were examined to determine how they responded to the subduction of the oceanic plate to the eastern Asian continental margin during the Cretaceous. A sinistral strike-slip movement caused by the oblique subduction of the Paleo-Pacific plate created these two nonmarine subbasins in the northern marginal Okcheon Belt of the Korean Peninsula. In the late Cretaceous, due to the orthogonal subduction of the oceanic plate, sediments were deposited in terrestrial environments with associated volcanism. A total of 247 ages obtained from 300 zircon grains reveal that the maximum depositional ages of the Haenam basin is ca. 79.5 0.17 Ma, respectively. The detrital zircon age spectra indicate that their basin fills were mainly derived from the adjacent basement rocks comprising Paleoproterozoic metamorphic rocks and Jurassic granitoids with a minor supply from the Paleozoic metasedimentary rocks in the western Gyeonggi Massif and Okcheon Metamorphic Belt.

Jurassic Igneous Activity in the Yuseong Area on the Southern Margin of the Gyeonggi Massif, Korean Peninsula, and Its Implications for the Tectonic Evolution of Northeast Asia during the Jurassic

Jurassic dioritic to granitic igneous rocks extensively intrude into the southern Korean Peninsula, including the Yuseong area located at the boundary between the southern margin of the Gyeonggi Massif and the northern margin of the Okcheon Belt. In this study, the petrogenesis and sources of Jurassic igneous rocks in the Yuseong area were investigated. The U–Pb zircon age data from the Jurassic plutonic rocks in the Yuseong area give two igneous ages, ca. 178–177 Ma and 169–168 Ma, indicating that two stages of igneous activity occurred in the Yuseong area during the Jurassic. The geochemical characteristics of Jurassic diorites indicate that they originated from enriched mid-ocean ridge basalt (E-MORB; Nb/Yb = 5.63–7.27; Zr/Yb = 118–156). The enriched Th/Yb ratios (5.5–8.0) in the diorites imply that they experienced crustal contamination during magma ascent. The Jurassic granitoids in the Yuseong area are divided into I- and S-type granites. The Jurassic I-type granitoids may have formed via the partial melting of mafic rocks with mixtures of 10–40% pelite-derived melt, while the S-type granites originated from felsic pelite. The Jurassic diorites have low Nb/Th ratios with depletion of the Nb and Ta components, indicating that they formed in a volcanic arc tectonic environment. On the other hand, the Jurassic granitoids show two different tectonic environments: a volcanic arc, and a syncollisional environment. The granites with syncollisional character are S-type granites, and may give incorrect information about tectonic setting because of the changes in the trace elements of the S-type granite due to fractional crystallization. Early Jurassic (200–190 Ma) igneous rocks are distributed only in the southeastern Korean Peninsula, including the Yeongnam Massif; Jurassic igneous rocks formed at ca. 190–180 Ma occur mainly in the Okcheon Belt and southern Gyeonggi Massif, which includes the Yuseong area. Middle Jurassic igneous rocks widely intruded from the Okcheon Belt, through the Gyeonggi and Nangrim massifs in the Korean Peninsula, to the Liaoning area in the North China Craton at 180–160 Ma. This distribution pattern of the Jurassic granitoids suggests that flat subduction started after 180 Ma in Northeast Asia.

한반도 남부의 지질 유형별 지형 기복과 삭박 저항력

This study tried to reveal relative surface denudation resistance and ranking by geologic types in the Southern Korean Peninsula using an 1:250,000 digital geologic map and ASTER GDEM. Among rock types such as igneous, sedimentary and metamorphic rocks, metamorphic rock showed the greatest resistance to surface denudation. The most resistant rock to surface denudation by geologic periods (e.g., the Precambrian, Paleozoic, Mesozoic and Cenozoic) was found from the Precambrian. Among the major tectonic settings in the Southern Korean Peninsula such as the Gyeonggi massif, Okcheon belt, Yeongnam massif, Gyeongsang basin and Pohang basin, the Okcheon belt indicated the greatest resistance. The most and least resistant rocks from the representative nine rocks in the Southern Korean Peninsula were Paleozoic limestone, and Cretaceous sedimentary rock and Cenozoic sedimentary rock, respectively. This study suggests that Paleozoic limestone, Cretaceous volcanic rock, Paleozoic sedimentary rock and Precambrian gneiss can be regarded as hard rocks with high elevation, steep slope and complicated relief, while soft rocks with low elevation, gentle slope and simple relief are Jurassic granite, Cretaceous sedimentary rock and Cenozoic sedimentary rock.

Tectonomagmatic evolution of a Jurassic Cordilleran flare-up along the Korean Peninsula: Geochronological and geochemical constraints from granitoid rocks

This study used new and published U-Pb geochronological, chemical, and Sr-Nd-Hf-O isotopic data (n > 2500) from Jurassic granite-granodiorite-diorite-monzonite-gabbro plutons in the southern part of the Korean Peninsula to assess the spatiotemporal evolution of a flare-up magmatism, its tectonic connection, and specific contributions of mantle and crustal reservoirs to the magmas generated. After a ~15 m.y. magmatic gap in the Late Triassic, calc-alkaline granitoids intruded into the outboard Yeongnam Massif, then magmatic activity migrated systematically toward the inboard Gyeonggi Massif. The early phase of the Jurassic magmatism is represented by relatively sodic plutons showing distinctly primitive isotopic signatures. The crustal signature of the plutons became increasingly prominent with decreasing age. Voluminous inboard plutons in the Gyeonggi Massif and the intervening Okcheon Belt are dominated by Middle Jurassic peraluminous granites that show isotopic compositions conspicuously shifted toward old crustal values. The Nd-Hf isotopic compositions of the inboard plutons are distinctly less radiogenic than those of Jurassic plutons in Southwest Japan and southeastern China, which corroborates the North China affinity of the Yeongnam and Gyeonggi massifs. The geochronological and geochemical data compiled in this study suggest a tectonomagmatic model consisting sequentially of (1) an extension-dominated arc system in the margin of the Yeongnam Massif (ca. 200–190 Ma); (2) low-angle subduction and the development of an advancing arc system (ca. 190–180 Ma); (3) continued low-angle subduction, extensive underthrusting of fertile crustal materials to the arc root, and consequent magmatic flare-up (ca. 180–170 Ma); and (4) flat subduction and the development of the Honam Shear Zone (ca. 170–160 Ma). The subsequent magmatic lull and previous dating results for synkinematic rocks and minerals indicate that the compressional arc system was maintained until the Early Cretaceous.

부여 대조사 석조미륵보살입상 佛眼에 사용된 琉璃의 材質特性 및 原料 供給地 推定

In this study, we tried to unde rstand the construction background by analyzing the material characteristics and searching the raw material source site for glass used for eyes of the stone standing maitreya bodhisattva of Daeijosa Temple, Bueyeo.BR The result of component analysis of glass used for eyes of stone Buddha show that they are common lead glass system(PbO-SiO2). Especially, Cu, Fe and Zn were detected as a elements which are involved in the coloring agent, and the component content is higher than other analyzed lead glass. It is considered that the type and amount of additives were intentionally controlled in oder to express the black eyes in glass production. Estimated melting point of lead glass excavated from Mireuksaji site was reported to be 670℃. Compared with the composition and melting temperature of lead glasses from Mireuksaji site, the lead glass used for eyes of stone Buddha is estimated to have a higher melting point than mireuksaji site.BR Most of the lead glasses used for eyes of stone Buddha is deteriorated with pollutants. It is classified into the outside part which are always exposed to the outside environment, the boundary part and the inside part, and the weathering pattern is different. Inside, It is appeared the typical weathering patterns of glass. On the other hand, outer surface is high weathered due to discoloration and crystallization. Therefore, It is show different state from its original appearance. The result of the provenance of lead raw material used for eyes of stone Buddhas, It is estimated that the raw materials of lead were obtained from the northern part of China, or zone 3 of Jeolla province belonging to the Okcheon belt and Yongnam massif in Korea.BR Although the lead glass is now weathered and deteriorated from its original form, it can be seen that a high degree of glass manufacturing technology was used to express the eyes of the stone Buddha at that time. In this study, there is a lack of comparison with the results of the analysis of lead glass found or excavated around the same age. However, through this study, it is confirmed that the high degree of glass manufacturing technology was used in the production of the stone Buddhist, and it is a unique result reflecting the regional specificity.

Geochemical interpretation of magnesium and oxygen isotope systematics in granites with the REE tetrad effect

The rare earth element (REE) tetrad effect includes a large, strongly negative Eu anomaly, and appears in highly evolved, fractionated granite and/or A-type granite, which it is sometimes interpreted as evidence of fluid-rock interaction. Magnesium is a fluid-mobile element, and its isotopes show very little fractionation during igneous processes. In order to understand the formational processes that cause the REE tetrad effect in granitic magmas and the role of fluid-rock interaction in producing REE tetrad patterns, this study analyzed magnesium and oxygen isotope compositions along with REE patterns in three Cretaceous granites from the Okcheon belt, Korea. Samples exhibited a linear correlation between magnesium isotopic values (δ26Mg) and SiO2, MgO, TiO2, initial 87Sr/86Sr, 143Nd/144Nd values. Oxygen isotope values also showed some correlation with SiO2, TiO2, δ26Mg, initial 87Sr/86Sr, and 143Nd/144Nd values. Our data indicate that the three granites should be evolved from source magma derived from isotopically heterogeneous source materials within the crust. In addition, the relationship between initial 143Nd/144Nd and δ26Mg values strongly suggests that the REE tetrad effect may develop before the granitic magma evolved from crustal source material rather than during late stage of magma differentiation or hydrothermal alteration.

The tectonic boundary between the Okcheon and Taebaeksan basins, South Korea: A restraining bend of a continental transform fault between the South and North China Cratons

AbstractThe geological relationship between the Okcheon and Taebaeksan basins of the Okcheon belt on the Korean peninsula is a key issue in reconstructing the tectonic evolution of the peninsula. The boundary between the two basin sequences has been variously interpreted as a conformable, unconformable, or thrust contact, without clear evidence being provided for any of these hypotheses. Detailed examination of structures and microfabrics of deformed rocks adjacent to the contact in the Bonghwajae area suggests that the boundary between the two basin sequences is a thrust. Based on the U–Pb ages of detrital zircons from metasedimentary rocks and pre‐existing geologic data from the Okcheon belt, the thrust is a relay structure between two segments of a continental transform fault along which the Okcheon Basin was juxtaposed against the Taebaeksan Basin during the Permian–Triassic suturing of the North and South China Cratons.

Phanerozoic polyphase orogenies recorded in the northeastern Okcheon Belt, Korea from SHRIMP U-Pb detrital zircon and K-Ar illite geochronologies

We present the SHRIMP U-Pb detrital zircon and K-Ar illite 1Md/1M and 2M1 ages, suggesting new insight into the Phanerozoic polyphase orogenies preserved in the northeastern Okcheon Belt, Korea since the initial basin formation during Neoproterozoic rifting through several successive contractional orogens. The U-Pb detrital zircon ages from the Early Paleozoic strata of the Taebaeksan Zone suggest a Cambrian maximum deposition age, and are supported by trilobite and conodont biostratigraphy. Although the age spectra from two sedimentary groups, the Yeongwol and Taebaek Groups, show similar continuous distributions from the Late Paleoproterozoic to Early Paleozoic ages, a Grenville-age hiatus (1.3–0.9Ga) in the continuous stratigraphic sequence from the Taebaek Group suggests the existence of different peripheral clastic sources along rifted continental margin(s). In addition, we present the K-Ar illite 1Md/1M ages of the fault gouges, which confirm fault formation/reactivation during the Late Cretaceous to Early Paleogene (ca. 82–62Ma) and the Early Miocene (ca. 20–18Ma). The 2M1 illite ages, at least those younger than the host rock ages, provide episodes of deformation, metamorphism and hydrothermal effects related to the tectonic events during the Devonian (ca.410Ma) and Permo-Triassic (ca. 285–240Ma). These results indicate that the northeastern Okcheon Belt experienced polyphase orogenic events, namely the Okcheon (Middle Paleozoic), Songrim (Late Paleozoic to Early Mesozoic), Daebo (Middle Mesozoic) and Bulguksa (Late Mesozoic to Early Cenozoic) Orogenies, reflecting the Phanerozoic tectonic evolution of the Korean Peninsula along the East Asian continental margin.

Regional Seismic Intensity Anomalies in the Korean Peninsula and Its Implications for Seismic-Hazard Potentials

The strength of seismic ground motion is a consequence of seismic source strength and medium response. The dependence of seismic amplitudes and seismic intensity on regional geological structures and crustal properties in the stable intraplate region around the Korean Peninsula is investigated. An instrumental seismic intensity scale based on spectral accelerations are proposed after calibrating with the reported macroseismic intensities. A representative seismic intensity attenuation curve for the Korean Peninsula is given by $$I(M_{\text {L}},l,h) = -0.998 (\pm 0.222) + 1.72 (\pm 0.04) M_{\text {L}} - 0.322 (\pm 0.027) \, \text {ln}(l^2 + h^2) - 0.00608 (\pm 0.00049) \, \sqrt{l^2 + h^2}$$ , where $$I(M_{\text {L}},l,h)$$ is the seismic intensity at an epicentral distance l in km for an earthquake with local magnitude $$M_{\text {L}}$$ and focal depth h in km. Seismic intensities decay slowly with distance in the Korean Peninsula. The observed decay rate for the Korean Peninsula is comparable with those for other stable intraplate regions, while are lower than those for active regions. The regional seismic intensity anomalies present a characteristic correlation with geological structures. Positive seismic intensity anomalies appear in the Yeongnam massif, Okcheon belt and Gyeongsang basin, while negative seismic intensity anomalies in the Gyeonggi massif. The regional seismic intensity anomalies display positive correlations with crustal thicknesses, crustal amplifications, and seismicity density and negative correlations with heat flows. Positive seismic intensity anomalies are observed in the Yeongnam massif and Gyeongsang basin, suggesting high seismic-hazard potentials in the regions. The regional crustal properties may provide useful information on potential seismic hazards.

Magnesium and Oxygen isotope composition of the Muamsa and Weolaksan granites, Okcheon belt, Korea

Magnesium and Oxygen isotope composition of the Muamsa and Weolaksan granites, Okcheon belt, Korea

Contrasting source domains for the Phanerozoic granitoids in South Korea revealed by zircon Hf isotopic signatures

In situ zircon Hf isotopic analysis was conducted to investigate the characteristics of source materials responsible for generations of the Phanerozoic granitoids in South Korea. The Hf isotopic data compiled from the literature together with this study revealed two principal types of source materials governing the generation of the Permo-Triassic granitoids. One type corresponds to long-lasting, ancient lithospheric materials having highly negative eHf(t) values of –24.3 to –18.6 and is temporally equivalent to the Paleoproterozoic to Neoarchean basement rocks prevalent in South Korea. This type is a primary source material for the Triassic postorogenic granitoids in the Gyeonggi massif. The other type corresponds to juvenile materials accreted by Late Paleozoic continental arc magmatism. This relatively young crust occurs largely in and around the Gyeongsang basin. Systematic southeastward increase in eHf(t) values of both Permo-Triassic and Jurassic granitoids indicates enhanced contributions of juvenile materials toward the paleo-trench. Such a trend is best explained by the chemical mixing of two principal types of source. Northern limit of chemical interactions between ancient and juvenile components is inferred to lie along the central Okcheon belt during the Permo-Triassic. However, this boundary has shifted southeastward in Jurassic to be defined by the Sunchang–Yecheon shear zone, a part of the Honam Shear Zone. The arc-related juvenile crust was also a principal source for the Cretaceous-Paleogene granitoids in the Gyeongsang basin.

Geochemical and Sr–Nd isotopic constraints on the petrogenesis of the Goesan monzodiorite pluton in the central Okcheon belt, Korea

AbstractThe Permian–Triassic high pressure metamorphism and potassic magmatism in central Korea attest to the extension of the Dabie‐Sulu collision belt in central‐eastern China towards the Korean Peninsula and possibly the Japanese Islands. We present major and trace element and Sr–Nd isotope data for a ca. 230 Ma monzodiorite pluton emplaced in the Goesan area, central Okcheon belt, Korea. This pluton shows geochemical features comparable with those of the coeval monzonite–syenite–gabbro–mangerite suite documented recently in the Gyeonggi massif. The metaluminous and alkali–calcic signatures of the Goesan intrusives correspond to the Caledonian‐type post‐orogenic granitoids. The K2O/Na2O ratios of all analyzed samples are greater than 1, and are not correlative with their SiO2 contents. The enrichment of both large‐ion‐lithophile elements and highly compatible elements in the Goesan pluton is probably indicative of metasomatized mantle origin. The elemental fractionation in the source region must have occurred in the distant past, possibly the Paleoproterozoic, to generate significantly negative εNd(t) values (< –16). Chondrite‐normalized rare earth element patterns as well as Rb/Sr and Ba/Rb ranges suggest that the source consists of amphibole‐bearing rocks. Progressive decreases in negative Eu anomaly and Ba, Sr, Ni, Cr and V contents with increasing SiO2 contents reflect an important role of plagioclase, biotite and hornblende for the fractionation process. Zr is undersaturated in the potassic, metaluminous melt. The initial Sr–Nd isotopic compositions of the samples are correlated with their SiO2 contents, substantiating a role of crustal assimilation during the magmatic differentiation. The Sr–Nd elemental and isotopic modeling suggests that the Goesan pluton was initially slightly heterogeneous in its isotopic composition, and underwent concurrent assimilation and fractional crystallization. The occurrence of the Goesan pluton provides further evidence corroborating the amalgamation of allochthonous terranes within the Okcheon belt during the Permian–Triassic collisional orogeny.

Geochronological and geochemical implications of Early to Middle Jurassic continental adakitic arc magmatism in the Korean Peninsula

Sensitive high-resolution ion microprobe (SHRIMP) zircon U–Pb ages and whole-rock chemical compositions of Early to Middle Jurassic plutons from the central to southern Korean Peninsula are reported to investigate the effect of paleo-Pacific plate subduction preserved along the continental margin. Twenty-one plutonic rocks from the Yeongnam massif (i.e., Sunchang and Namwon plutons), the Okcheon belt (Jeongup, Boeun, and Mungyeong plutons), the northeast (Gangreung pluton), and the Gyeonggi massif (Gonam, Anheung, and Ganghwa plutons) have age ranges from ca. 189–186Ma to 177Ma, 177–166Ma, and 177–173Ma, respectively. Most plutonic rocks have chemical compositions similar to adakites, showing high SiO2 (45.62–74.96wt.%), low MgO (0.01–2.84wt.%), high Na2O (2.65–4.83wt.%), high Sr/Y and La/Yb, low Y and Yb, as well as low HFSEs (Nb and Ta), but initial Sr ratios (0.7048–0.7262), K2O (0.50–5.88wt.%), and K2O/Na2O (0.34–2.1) were unlikely to have been formed by melting of either a thickened and/or delaminated lower continental crust, or an altered oceanic crust. These suggest that the “adakitic” plutonic rocks in this region resulted from partial melting of an enriched mantle source metasomatized by dewatering from a delaminated flat-slab. The spatial distributions of this continental adakitic plutonic belt, based on the present study, together with previously reported geochronological results, indicate that magmatic pulses gradually migrated toward the continent across the Korean Peninsula as a result of slab shallowing caused by periodic slab buckling. The similar geochronological and geochemical characteristics, petrogenesis and tectonic setting of the plutonic belt spanning the Korean Peninsula, Japan, eastern China, and eastern Russia indicate a possible link to an active subduction system that existed during the Early to Middle Jurassic, although the products of the plate subduction might differ in different locations along the East Asian continental margin.

Structural style of the Okcheon fold-thrust belt in the Taebaeksan Zone, Korea

Structural interpretation of the distinct map pattern defined by highly connected thrust traces in the map view of the Taebaeksan Zone provides insight into the structural style of the northeastern Okcheon Belt. The map pattern can generally be explained by either a folded imbricate fan or a hinterland dipping duplex. The same geometry could also be formed by a complex combination of major imbricate thrusts and their connecting splays, having the structural architecture of a typical fold-thrust belt. However, a folded imbricate fan model is not adequate to explain the absence of late Paleozoic to early Mesozoic strata (i.e. the Pyeongan Supergroup) between two major thrusts (viz. the Pyeongchang and Machari thrusts) in this area. This result further suggests that the Yeongwol area, the western part of the Taebaeksan Zone, is a duplex that corresponds to a more internal and deeper hinterland part of the fold-thrust belt, while the imbricate thrusts with low connectivity in the Taebaek area, the eastern part of the Taebaeksan Zone, indicate a more external and shallower foreland portion of the belt. In addition, cross-cutting relations and newly obtained sensitive high-resolution ion microprobe (SHRIMP) U–Pb zircon ages from a cross-cutting dike and syntectonic sedimentary rocks suggest limited thrust reactivation after the early Jurassic and early Tertiary, respectively. In spite of this, the original geometry of the fold-thrust wedge in the Taebaeksan Zone remained well preserved.

Detrital zircon U–Pb geochronology and tectonic implications of the Paleozoic sequences in western South Korea

The Middle Paleozoic metasedimentary sequences of the western Gyeonggi Massif (Taean Formation and Wolhyeonri Complex), Imjingang Belt, and southwestern Okcheon Belt, provide clues to the successive tectonic events that occurred during the evolution of the Korean Peninsula. In this study, we present SHRIMP U–Pb ages of detrital zircon grains from the metasandstones of the Taean Formation and Yeoncheon Group of the Imjingang Belt and from quartzites in three Paleozoic formations of the southwestern Okcheon Belt. The zircon grains from all of the samples yielded similar U–Pb age spectra of Paleoarchean/Neoarchean to Middle Paleozoic, with a dominant age population within the Paleozoic. The maximum ages of deposition estimated from the youngest dominant concordant detrital zircons were 431–457Ma for the western Gyeonggi Massif and Imjingang Belt, and 380–355Ma for the southwestern Okcheon Belt. Permian metamorphic zircon overgrowths correlated with subduction prior to the Triassic collision were also found. Middle Paleozoic metamorphic zircon overgrowths that can be compared to those of the Yunkai–Wuishan Orogenic Belt of the Cathaysia Block were indentified in the Wolhyeonri Complex. The dominant zircon populations of this study and the available data for the Korean Peninsula compare well with those from the Yangtze Block, Cathaysia Block, and Qilian–Qinling Orogenic Belt, which were located at the northeastern margin of eastern Gondwana during the Early Paleozoic.