Okcheon Metamorphic Belt Research Articles

Water-rock interactions of uranium deposits: A field investigation and laboratory batch experiment

Interactions between bedrock and groundwater in the far-field of a deep geological repository (DGR) for high-level radioactive waste significantly affect the transport of uranium, which is the major radionuclide in a DGR. Therefore, the present study evaluated the geochemical behavior of uranium by assessing the hydrogeochemical characteristics of groundwater, surface water, mine water, and quarry runoff water in the Okcheon metamorphic belt, which represents a natural analogue study site in Korea. A batch experiment was also conducted to further elucidate the interactions between the uranium-containing rock and groundwater in the study area. The results of the field investigation indicated that uranium in groundwater formed aqueous complexes in the form of calcium uranyl tricarbonate. The concentrations of uranium in groundwater were low, at less than 1 μg L−1, which seems to be attributable to the dissolutions of uraninite, coffinite, UO2(am), and U4O9(c) under the moderately reduced condition and subsequent control by co-precipitation with iron (hydr)oxides. The results of the batch experiment showed that uranium concentrations may be controlled by adsorption or surface complexation with iron oxides. The field study demonstrates the equilibrium state maintained in the current geochemical conditions at the study site, while the batch experiment provides implications regarding the reaction processes over time. Overall, this study showed that uranium forms complexes with carbonate and sulfate under the geochemical conditions of the study site, and that its mobility could be restricted through co-precipitation or adsorption/surface complexation. This study is expected to contribute to a comprehensive understanding of how the behavior of uranium is affected by the evolution of geochemical conditions around the DGR.

Zircon geochronology, Hf isotope, and rare earth element geochemistry of the Apdong Nb-Ta deposit in North Korea

The Apdong Nb-Ta deposit is hosted within the syenite and in the Hoamsan area in the northern part of the Imjingang Belt in North Korea. Our new sensitive high-resolution microprobe zircon U-Pb results reveal two stages of Neoproterozoic to Ordovician inherited cores (887 Ma to 465 Ma) and Silurian emplacement and mineralization (425.5 ± 7.4 Ma) events. The zircon grains from the Apdong syenite show typical magmatic or textureless and sponge-like hydrothermal domains. These textural analyses supported the rare earth element characteristics obtained from the laser ablation-induced coupled plasma mass-spectrometry analyses (LA-ICP-MS). Neoproterozoic magmatic activities reportedly occur in the Pyeongnam Basin, Imjingang Belt [present study], Gyeonggi Massif, and Okcheon metamorphic belt in the central part of the Korean Peninsula and China (North China Craton, Qinling Orogenic Belt, and South China Craton). Notably, the Caledonian mineralization events in the southeastern part of the South China Craton coincided with emplacement and mineralization ages from the Apdong syenite (425.5 ± 7.4 Ma). The LA multi-collector ICP-MS Hf isotope of zircons from the Apdong syenite yielded positive initial εHf values plotted along the Neoproterozoic crustal evolution path, indicating that the host rock of Nb-Ta ore was formed from the pre-existing Neoproterozoic source. The source materials of Neoproterozoic intrusives in the central part of the Korean Peninsula were generally derived from Mesoproterozoic depleted mantle sources. Considering the previously reported Hf isotopic results, the involvement of pre-existing crustal materials is more prominent in the Neoproterozoic intrusives in the southern part of the central Korean Peninsula, i.e., the Gyeonggi Massif, than in the northern part of the central Korean Peninsula, such as the Imjingang Belt and Pyeongnam Basin in the northern part of the central Korean Peninsula.

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.

Different characteristics of barren and W–Mo–productive Cretaceous granites in the Hwanggangri mineralized district, South Korea

The Hwanggangri mineralized district (HMD) in South Korea contains a variety of ore deposits such as porphyry or skarn (Mo, Cu–Pb–Zn), carbonate replacement, and fissure–filling vein types (W–Mo, Au–Ag, fluorite). The HMD is located on the boundary of the Okcheon metamorphic belt and the Taebaeksan Basin. Most of the deposits are hosted in limestones/dolomitic limestones of the Joseon Supergroup, and the age of mineralization is Late Cretaceous (96.5–76.2 Ma) including Geumseong, Jungbo, Kwangsin, and Geumsil deposits. These granites are mainly magnetite-series biotite granites, and they can be categorized into early- and late-stage on the basis of mineral assemblage, texture, grain size, and magnetic susceptibility. Magnetite is present in some of the granites, and some of the early-stage granites retain the high magnetic susceptibility values typical of the magnetite series. However, the late-stage granites show a wide range of magnetic susceptibility values, including low ones due to fractionation. The emplacement ages of the Sokrisan, Weolaksan, Muamsa, and Susan granite bodies are 94–88 Ma, similar to the ages of mineralization in the study area. The Cretaceous granites reported to be related to W–Mo mineralization in the HMD show miarolitic, porphyritic, and micrographic textures indicating considerable hydrothermal fluid activity in a shallow environment. The rocks of the Sokrisan granite are mostly early stage, those of the Weolaksan and Muamsa granites include both early and late stages, and the studied samples of the Susan granite are entirely late stage. These four Cretaceous granites have more than 70 wt% SiO2, high Rb/Sr ratios, depletions in compatible elements, enrichments in incompatible elements, and tetrad patterns with depleted LREEs, enriched HREEs, and strong negative Eu anomalies. They all belong to the high–K calc–alkaline series, are dominantly peraluminous, and can be classified as strongly fractionated I–type granites. The Sokrisan, Weolaksan, Muamsa, and Susan granites have similar emplacement ages and geochemical characteristics that indicate the same magma source. However, the Sokrisan granite, with its mainly early-stage rocks, consistently higher magnetic susceptibility values, and relatively high Nb/Ta values, differs from the other Cretaceous granites in the HMD. This suggests that the widely exposed Sokrisan granite of batholith type has been eroded substantially, removing the late-stage rocks and ore deposits. In contrast, the stock–type Susan granite has undergone relatively little erosion, and it represents well the late-stage parts of the Cretaceous granites in the HMD. The Cretaceous granites in the HMD have mostly highly-fractionated geochemical characteristics, and the late-stage rocks that are more associated with W–Mo mineralization are more fractionated than the early-stage rocks. The respective productivities of the Susan, Muamsa, Weolaksan, and Sokrisan granites range from high to low, and the more-productive late-stage granites are still present, as they have undergone less erosion.

U-Pb ages and Hf isotopic compositions of detrital zircons from the Seochangni Formation of the northeastern Okcheon Metamorphic Belt: Implications to the crustal evolution of the Sino-Korean craton since Paleoproterozoic

The age-unknown Seochangni Formation is distributed in the northeastern part of the Okcheon Metamorphic Belt in central South Korea. It unconformably overlies the Paleoproterozoic Busan gneiss complex exposed as a small dome and is in contact with the Cambrian-Ordovician Joseon Supergroup of the Taebaeksan Basin to the east. We separated detrital zircons from the quartzite units of the Seochangni Formation and used SHRIMP and LA-MC-ICPMS to determine their UPb ages and Hf isotopic compositions. The UPb age distribution of the detrital zircons is divided into two types. In the main type, the ages of Late Paleoproterozoic to Mesoproterozoic are dominant, whereas they have little or no older age components of about 1.87Ga and 2.5Ga, which are characteristic of the Korean peninsula basement rocks. However, the minor type has age components of 0.75 Ga and 1.87 Ga. Age patterns similar to the former type have recently been reported in the western region of the Gyeonggi Massif on the Korean peninsula, as well as in the Sangwon System in North Korea and the southern margin of North China Craton. It suggests a correlation between them and a common crustal evolution within a single craton linked since the Paleoproterozoic. The high positive εHf(t) values of detrital zircons in the Late Paleoproterozoic to Mesoproterozoic period indicate an enhanced input of juvenile materials from the depleted mantle, reflecting the igneous activity associated with the breakup of the supercontinent Columbia.

Carbon, oxygen and strontium isotope geochemistry of the late Neoproterozoic carbonate platform deposit Hyangsanni Dolomite of the Okcheon metamorphic belt, Korea

The age-unknown Hyangsanni Dolomite is distributed in the northeastern part of the Okcheon metamorphic belt in central South Korea. It has been claimed as Early to Middle Cambrian because of the misidentified Archaeocyatha fossil in the past, but the time of sedimentation has not yet been confirmed. We performed elemental and isotopic analyses for the Hyangsanni Dolomite. The REE + Y patterns of the samples of the Hyangsanni Dolomite are generally flat and Ce negative anomaly is generally negligible to the extent that it appears only weakly in some samples. Y/Ho values are in the range of 29–43, which is fairly lower than typical Y/Ho ratio of seawater, indicating the influence of terrigenous materials. REE and trace element geochemical discrimination diagrams indicate that the Hyangsanni Dolomite was deposited in a continental margin environment rather than an open ocean. We suggest that the Hyangsanni Dolomite was deposited in a rift basin connected to the ocean rather than in the passive continental margin, considering occurrence of magmatism related to continental rifting within the Okcheon metamorphic belt. Carbon and oxygen isotope analysis was performed on dolomite powders scraped off from polished surfaces unstained with Alizarin-Red-S in order to avoid the recrystallized calcite. The Hyangsanni Dolomite has δ13C(V-PDB) values between +2.9‰ and + 6.2‰, and δ18O(V-SMOW) values between +11.3‰ and + 24.5‰. These δ13C values are markedly higher than the Cambro-Ordovician values, and are consistent with the Neoproterozoic values. The minimum value of 87Sr/86Sr ratio obtained from the Hyangsanni Dolomite also refers to Neoproterozoic, not Phanerozoic. We suggest that the Hyangsanni Dolomite belongs to the sedimentary sequence of the Neoproterozoic, along with other sediments of the northeastern Okcheon metamorphic belt.

Regional variations of sulfur isotope compositions for metallic deposits in South Korea

AbstractSouthern Korean peninsula comprises five major geotectonic provinces, throughout which various metallic deposits are distributed. We reviewed sulfur isotope data (n = 1,574) of sulfide minerals collected from previous works for 177 metallic deposits in the provinces to interpret the sulfur isotope characteristics of each province, comprising different wall rocks and geologic settings. The averaged δ34S values of each metallic deposit associated with Precambrian metamorphic rocks and Jurassic granitoids in the Gyeonggi massif and Yeongnam massif range from −7.1 to +10.3‰ (av. +4.5‰) and from −3.6 to +7.8‰ (av. +3.5‰), respectively. The Taebaeksan basin produced the highest δ34S value among the five, −0.4 to +13.2‰ (av. +6.1‰). This was influenced by sulfate sulfur derived from marine carbonate host rock. The Okcheon metamorphic belt, comprising metasedimentary and metavolcanics rocks, shows an isotope range from +1.9 to +8.3‰ (av. +5.7‰). The sulfur isotope distribution of the Gyeongsang basin with a range from −1.2 to +11.7‰ (av. +5.2‰) can be divided into two zones: higher δ34S values from the inner zone related to the volcanic rocks and magnetite‐series Cretaceous granitoids, and lower δ34S values from the outer zone related to the organic‐rich sedimentary rocks. Sulfur isotope variations of metallic deposits in each geotectonic province were mainly influenced by igneous sulfur and inherent wall rock sulfur sources, 32S‐enriched sedimentary sulfur (e.g., Precambrian metasedimentary rocks and biogenic sulfur‐rich sedimentary rocks), and 34S‐enriched seawater sulfur (e.g., carbonates and acid to intermediate volcanic rocks). These wall rocks also contributed to the changes in δ34S values for granitoid rocks and metallic deposits by time, ore genetic type, and ore species in South Korea.

The Geochronology and Geochemistry of Zircon as Evidence for the Reconcentration of REE in the Triassic Period in the Chungju Area, South Korea

The Chungju rare-earth element (REE) deposit is located in the central part of the Okcheon Metamorphic Belt (OMB) in the Southern Korean Peninsula and research on REE mineralization in the Gyemyeongsan Formation has been continuous since the first report in 1989. The genesis of the REE mineralization that occurred in the Gyemyeongsan Formation has been reported by previous researchers; theories include the fractional crystallization of alkali magma, magmatic hydrothermal alteration, and recurrent mineralization during metamorphism. In the Gyemyeongsan Formation, we discovered an allanite-rich vein that displays the paragenetic relationship of quartz, allanite, and zircon, and we investigated the chemistry and chronology of zircon obtained from this vein. We analyzed the zircon’s chemistry with an electron probe X-ray micro analyzer (EPMA) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). The grain size of the zircon is as large as 50 µm and has an inherited core (up to 15 µm) and micrometer-sized sector zoning (up to several micrometers in size). In a previous study, the zircon ages were not obtained because the grain size was too small to analyze. In this study, we analyzed the zircon with laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS) for dating purposes. The REE patterns and occurrence of zircon in the quartz–allanite vein match well with previous reported recrystallized zircon, while the behavior of the trace elements shows differences with magmatic and hydrothermal zircon. The 206Pb/238U ages obtained from the zircon in the quartz–allanite vein are from 240.1 ± 2.9 to 257.1 ± 3.5 Ma and this age is included in the tectonic evolution period of the study area. Therefore, we suggest that the quartz–allanite veins in the Gyemyeongsan Formation were formed during the late Permian to early Triassic metamorphic period and the zircon was recrystallized at that time. The Triassic age is the first reported age with zircon dating in the Gyemyeongsan Formation and will be an important data-point for the study of the tectonic evolution of the OMB.

Retrieval of P–T–t–d paths in a syn-metamorphic shear zone: Implications for P-T calculations and the Permian–Triassic orogeny on the Korean Peninsula

Crustal-scale shear zones developed on the boundaries of the Precambrian Gyeonggi Massif (GM) can play key role on understanding collisional orogeny during the Late Paleozoic to Early Mesozoic in the Korean Peninsula. Garnet schist formed on the boundary between the GM and Okcheon Metamorphic Belt (OMB) experienced contractional (Dn) and extensional (Dn+1) shearing combined with a clockwise pressure–temperature–deformation (P–T–d) path. Two distinct mineral assemblages were found in the schists, (1) garnet (pre- and/or syn-Dn) + staurolite (syn-Dn and/or inter Dn and Dn+1, and syn-Dn+1) + andalusite (syn-Dn+1)-bearing and (2) garnet (pre- and/or syn-Dn) + epidote-bearing assemblages, depending on bulk-rock compositions. Various compositions (An10–40 in the former assemblage and An45–70 in the latter one) and texture of plagioclase resulted from bulk-rock compositions, heterogeneous strain and dissolution and precipitation during the multiple shearing. The P–T conditions estimated for the early contractional shearing (top-to-the-north reverse shear) are 560 °C–600 °C and 5.5–7.0 kbar based on the intersections between compositional isopleths for pre- and/or syn-Dn garnet and plagioclase (An30–40 & An45–55) calculated in the MnO–Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O and MnO–Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–Fe2O3 systems. Subsequent extensional shearing (top-to-the-south normal shear) was characterized by recrystallization of fine-grained syn-Dn+1 staurolite with growth of syn-Dn+1 andalusite porphyroblasts during cooling and decompression to 560 °C and 3.5 kbar. The contractional and extensional shearing probably occurred at the Permian and the Late Triassic to Early Jurassic, respectively, based on the previous geochronological data. Consequently, inferred P–T–t–d paths on the GM–OMB boundary probably suggest not only crustal thickening followed by thinning, but also multiple amalgamations of several terranes.

Depositional environments of redox-sensitive trace elements in the metalliferous black slates of the Okcheon Metamorphic Belt, South Korea

Metalliferous black slates consisting of black slates and interbedded coaly slates in the Okcheon Metamorphic Belt of South Korea were analyzed for redox-sensitive trace elements and rare earth elements (REEs) to examine their depositional conditions. Our data show that the coaly slates have elevated concentrations of redox-sensitive trace metals (U, V, Mo, and Cr), low Mn contents, high V/(V + Ni), V/Cr, and U/Th ratios, and high total organic carbon (TOC) contents. A general tendency of positive correlation between TOC and trace metals was established. The results suggest that the coaly slates were developed under a strongly reducing environment, while the black slates were deposited under a suboxic-oxic condition. The REE patterns of the coaly slates typically show positive Eu and negative Ce anomalies compared to the North American Shale Composite reference and they are essentially inherited from submarine hydrothermal fluids under high temperature reducing condition. The enrichment of the redox-sensitive elements including V (up to 3,564 ppm) and Mo (up to 358 ppm) may have been controlled by direct hydrothermal input of metals into the rift basin from hydrothermal vents, proximal from the vent sites, as supported by textural evidences, and the metals could have been fixed from seawater by means of scavenging process via exhalative oxide particles in a hydrothermal plume. The sorption of metals from seawater under anoxic conditions into organic-rich sediments occurred as well. The metal enrichments in the black slates seem to have been achieved in somewhat similar way to black shales in South China.

Lead isotope mapping of galena from base metal deposits in the southern Korean Peninsula

The purpose of this study is to investigate the differences in sources of discrete four zones that are represented by the lead isotope map of metal mines using galena samples. The lead isotope map has been constructed based on 119 galena samples from 38 metal mines in the southern Korean Peninsula, together with previously published data, using spatial and zoning distribution analysis. The spatial pattern of 206Pb/204Pb ratio shows relatively dense distribution with highest value at the northeastern region, while 207Pb/204Pb and 208Pb/204Pb ratios are the lowest at the southeastern region. These results indicate possible subdivision of the lead isotope maps based on geotectonic subdivisions and geographical locations of the Korean Peninsula. This subdivision has been checked by linear discriminant analysis (LDA). The range of lead isotopic composition for zone 1 has fairly homogeneous values (206Pb/204Pb = 18.156–18.591, 207Pb/204Pb = 15.482–15.644, and 208Pb/204Pb = 37.870–38.768), and are geographically similar to the Gyeongsang Basin. Lead isotopic compositions of zone 2 show the highest ratios (206Pb/204Pb = 18.625–20.483, 207Pb/204Pb = 15.708–16.068, 208Pb/204Pb = 37.734–40.463) in the southern Korean Peninsula, that are geographically corresponding to the Taebaeksan Basin and the northeastern Yeongnam Massif. Relatively lower values (206Pb/204Pb = 17.149–17.817, 207Pb /204Pb = 15.497–15.731, 208Pb /204Pb = 37.847–39.773) along the western Gyeonggi Massif are grouped as zone 4, while the rest of the areas, including the Okcheon metamorphic belt, most of the Gyeonggi Massif, and the Yeongnam Massif, might represent zone 3. The results clearly indicate the possible differences in the source reservoirs that are similar to the geotectonic provinces of the Korean Peninsula.

Nature and evolution of the Cretaceous basins in the eastern margin of Eurasia: A case study of the Gyeongsang Basin, SE Korea

The Gyeongsang Basin is the largest among the non-marine Cretaceous sedimentary basins in Korea and Japan, formed by sinistral strike-slip movement induced by the oblique subduction of the paleo-Pacific plate under the eastern Asian continental margin, and has an evolutionary history of about sixty million years. SHRIMP and LA-MC-ICPMS U-Pb analyses were performed on the detrital zircons separated from the constituent formations of the Gyeongsang Basin. We determined the maximum depositional ages of the four formations based on their ages of the youngest groups as follows; 118.0 ± 2.6 Ma (n = 4) for the Hasandong Formation, 108.7 ± 0.8 Ma (n = 11) for the Chilgok Formation, 106.0 ± 0.2 Ma (n = 63) for the Silla Conglomerate, 105.4 ± 0.4 Ma (n = 14) for the Haman Formation, and 99.9 ± 0.7 Ma (n = 5) for the Jindong Formation, belonging to the period from Aptian to Cenomanian. The pre-Cretaceous detrital zircons of the lower to middle Gyeongsang Supergroup indicate their derivation from the Yeongnam Massif and the Okcheon Metamorphic Belt surrounding the Gyeongsang Basin. At around 115 Ma, as reflected as enhanced proportion of such ages in the detrital zircons and occurrence of farther inland igneous activities, the tectonic environment of Gyeongsang Basin changed to intra-arc due to the more active reestablishment of subduction-related igneous activity. However, the upper Gyeongsang Supergroup has a new age component of the mid-Permian, along with changes in the direction of paleocurrent flowing from the northeast and east, indicating sediment supply from SW Japan with rocks of that age. The changes in the direction of the paleocurrent flow seem to indicate that the center of igneous activity has moved in the direction of the subduction zone to the east. As the intensity of igneous activity continued to increase, the Gyeongsang Basin was closed by the filling of the basin by volcanic activity. The Cretaceous basins of the Korean Peninsula and southwest Japan show similar evolutionary histories, suggesting a type model of the creation and the evolution of basin around the continental margin subduction zone; creation of sedimentary basin by pull-apart or rifting in the backarc region, evolution to intra-arc basin by the landward progress of subduction-related igneous activity, and finally the closure of the basin due to the advent of highly enhanced igneous activity.

SHRIMP U–Pb ages of detrital zircons from the Early Cretaceous Nakdong Formation, South East Korea: Timing of initiation of the Gyeongsang Basin and its provenance

AbstractTo constrain the depositional age of the lowermost Nakdong Formation in the Early Cretaceous Gyeongsang Basin, SHRIMP U–Pb age determination was carried out on zircon separates. The U–Pb compositions of detrital zircons from the Nakdong Formation yield a wide range of ages from the Archean to the Cretaceous but show a marked contrast in age distribution according to the geographical locations within the basin. The provenance of the southern Nakdong Formation is dominantly the surrounding Yeongnam Massif, which is composed of Paleoproterozoic metamorphic rocks and Triassic to Jurassic plutonic rocks, whereas the central to northern Nakdong Formation records significant sediment derivation from the Okcheon Metamorphic Belt, which is distributed to the northwest, in addition to the contribution from the Yeongnam Massif. It is suggested that the maximum depositional age of the Nakdong Formation is ca 127 Ma, based on its youngest detrital zircon age population. The onset of its deposition at 127 Ma coincided with the tectonic inversion in East Asia from a compressional to an extensional geodynamic setting, probably due to the contemporaneous change in the drift direction of the Izanagi Plate and its subsequent oblique subduction.

Juxtaposition of allochthonous terranes in the central Korean Peninsula: Evidence from zircon U-Pb ages and O-Hf isotopes in Jurassic granitoids

The eventual docking of an exotic continental mass over the course of a Wilson cycle is marked by intracontinental sites of prograde metamorphism, contractional deformation, and syn- or post-collisional magmatism. This study shows that inherited premagmatic zircon in subsequently emplaced granitoid rocks provides another line of evidence for the collisional orogeny. Following the continental collision between the North and South China Cratons, Jurassic granitoids intruded widely throughout the central Korean Peninsula in association with the subduction of the Paleo-Pacific plate. The latest Early to Middle Jurassic (177–167Ma) granitoids occur throughout the cratonic Gyeonggi Massif, whereas Early Jurassic (194–184Ma) granitoids occur locally along the Okcheon Metamorphic Belt to the southeast of the massif. These two spatiotemporal groups differ profoundly in their age patterns and O-Hf isotopic signatures of inherited zircon. The younger group yields a typically North China-like Archean-Paleoproterozoic (mostly 2.5–1.8Ga) zircon inheritance. In contrast, inherited zircons from the older group exhibit exclusively Neoproterozoic (ca. 0.9–0.7Ga) and Paleozoic (ca. 0.4Ga) cluster along with Archean-Paleoproterozoic populations. The exotic feature of the protolith of the older group is further indicated by significantly lower δ18O (−0.9 to +4.9‰) and positive εHf (+3 to +15) values of the Cryogenian zircon cores. The results of zircon analysis demonstrate that South and North China-like terranes were juxtaposed infracrustally along the southeastern margin of the Gyeonggi Massif during the pre-Jurassic collisional orogeny, and then selectively provided major source material to the older and younger Jurassic granitoids, respectively.

PGE distribution in the metalliferous black slates of the Okcheon Metamorphic Belt, South Korea

Chinese Ni-Mo sulfide ores. Chondrite-normalized PGE pattern for the coaly slates displays a strong positive Ru anomaly and slightly negative Pt anomaly. The Zn-rich shale of Czech and the black shale of South China show similar patterns to the coaly slates except its slightly enriched Au and positive Pt anomaly, respectively. Positive correlations of organic carbon and the redox-sensitive trace elements such as U, V and Mo with PGEs concentrations in the coaly slates suggest that the PGEs were most likely introduced by submarine hydrothermal activity and precipitated with organic matters under anoxic environment. In general, the metalliferous black slates in the OMB seem to be metamorphosed analogue of the Early Cambrian black shales of the South China Block in the former continental back-arc basin setting, which host metalliferous Ni-Mo-PGE sulfide ores.

Re-examination of a supposed ‘archaeocyath’ specimen from the Hyangsanni Formation, Okcheon Basin, Korea

A supposed ‘archaeocyath’ specimen from the Hyangsanni Formation of the Okcheon Basin (Okcheon Metamorphic Belt), Korea, reported about 50 years ago, has previously been used to constrain the depositional age of the metasedimentary succession in the basin. However, a re-examination of the specimen reveals that it lacks the typical morphology of archaeocyaths. A detailed petrographic and microfabric analysis of the specimen, combined with a review of outcrop structures and microstructures of the dolomitic marble in the Hyangsanni Formation indicates that the supposed archaeocyath- like shape of the specimen was caused by deformations and metamorphic reactions. The Cambrian age of the Hyangsanni Formation should therefore be re-examined on the basis of unequivocal palaeontological evidence and/or radiometric dating.

Recurrent rare earth element mineralization in the northwestern Okcheon Metamorphic Belt, Korea: SHRIMP U–Th–Pb geochronology, Nd isotope geochemistry, and tectonic implications

Rare earth element (REE) mineralization is hosted within Neoproterozoic alkaline metaigneous rocks in the northwestern part of the Okcheon Metamorphic Belt (OMB), a polymetamorphosed fold-and-thrust belt transecting the Paleoproterozoic Gyeonggi and Yeongnam Massifs in the southern Korean Peninsula. The principal carrier phase of REEs is allanite. Allanite grains can be subdivided into several types based on the texture and mineral assemblage including quartz, K-feldspar, biotite, britholite, apatite, fergusonite, andradite, magnetite, zircon, titanite and fluorite. Electron microprobe analysis of allanite clearly distinguishes sample-to-sample variations in total REEs, Ca, Al, Fe and Y but the textural varieties from each rock sample do not show marked differences in those elements. Sensitive high-resolution ion microprobe dating of allanite and zircon reveals a complex history of multistage mineralization. Allanite grains from REE ores yielded Late Ordovician (444.6±8.0Ma), Permian to Triassic (ca. 300–220Ma) and Early Jurassic (199–183Ma) 208Pb/232Th ages. These multiple age components often coexist in single grains showing slight differences in backscattered electron brightness. The Ordovician components have distinctly higher Th/U than the younger domains in the same rock sample. The cores and rims of zircon from a syenite hosting REE ore bodies yielded Neoproterozoic (858.2±6.3Ma) and Early Jurassic (ca. 190Ma) 206Pb/238U ages, respectively. The Early Jurassic ages (194–187Ma) also obtained from zircon grains from granites taken from dykes occurring close to the ores and a drill core indicate the correspondence between granitic magmatism and REE mineralization. The Neoproterozoic zircon inheritance (weighted mean=853.9±3.8Ma) in these granites is in sharp contrast to the dominant Paleoproterozoic inherited zircon from the widespread earliest Middle Jurassic granites enclosing the mineralized zone. The geotectonic significance of the Late Ordovician event recorded in the allanite, as well as in detrital zircon from the OMB, is still unclear but its temporal coincidence with intraplate volcanism and arc-related igneous activity, respectively, reported from the southwestern edge of the adjacent Taebaeksan Basin and the southwestern Gyeonggi Massif is noteworthy. The following Permian–Triassic and Early Jurassic mineralization events are probably linked to the continental suturing between the North and South China blocks and subsequent post-orogenic magmatism, and arc magmatism resulting from the paleo-Pacific plate subduction, respectively. Sub-grain Sm–Nd isotopic analyses of allanite by laser ablation multiple collector ICPMS yielded initial εNd values plotting along the Nd isotopic evolution path of the Neoproterozoic metaigneous rocks, indicating that REEs originating from the host rock have been recycled during multistage mineralization events. The profound differences in inherited zircon ages and Nd isotopic compositions between the Early and Middle Jurassic granites may reflect the presence of a major thrust-bounded crustal structure beneath the OMB.

Scientific Analysis of the Gilt-bronze Incense Burner of Baekje from the Neungsalli Temple Site in Buyeo, South Korea

The Gilt-bronze incense burner, excavated at the Buyeo Neungsalli temple site,was made in the 6th century. It consists of a knob, lid, body and support, and features various patterns and ornaments representing Buddhism and Taoism. Following the assessment of its overall design and aesthetic senses, the incense burner has been recognized as a cultural masterpiece in East Asia. The present study analyzes the alloy composition, micro structure and plating layer of the incense burner. Atomic absorption (AAS) and micro beam X-ray fluorescence (micro-XRF) methods applied to the incense burner base. Analytical results revealed it to a binary alloy of copper and tin with compositions of 82 - 86% and 13 - 14%, respectively. The melting temperature and strength of the burner were estimated based on the tin concentration. The micro structure and the nonmetallic inclusions were analyzed by optical microscopy and scanning electron microscopy coupled with energy dispersive spectrometry (SEM-EDS). The analysis revealed an α+δ eutectoid with the α phase featuring a high copper content, along with a small amount of 4 - 5um sized nonmetallic inclusions. This composition revealed the use of sophisticated technologies in the fabrication and refinement of the burner. The lead isotope ratios from the Thermal Ionization Mass Spectrometry (TIMS) suggested that the raw material of the incense burner originated from Zone 3 (Yeongnam Massif, Okcheon Metamorphic Belt) of the southern part of Korea. The chemical composition of the plating layer cross-section was revealed to comprise gold and copper at 60% and 37%, respectively. The plating layer included both copper and gold owing to diffusion effect between the gold and the base copper during the gold amalgam plating process. The surface color of the incense burner was, therefore most likely reddish brown, not gold.

Geochemistry and uranium mineralogy of the black slate in the Okcheon Metamorphic Belt, South Korea

Geochemistry and uranium mineralogy of the black slate in the Okcheon Metamorphic Belt, South Korea

광주전단대 : 영남육괴와 옥천변성대의 지구조적 경계?

이 연구에선 한반도 남서부에 북북동-남남서 주향으로 발달된 호남전단대의 지류 중의 하나인 광주 전단대 주변에 전단대를 따라 분포된 변성퇴적암층에 대해 고분해능이온마이크로프로브를 이용하여 쇄설성 저어콘의 우라늄-납 연대측정을 수행하였다. 광주전단대를 경계로 각각 서쪽과 동쪽에 위치한 영광지역과 장성지역의 변성퇴적암층들은 저어콘 연대분포가 구별되는 특징을 보인다. 영광지역 규암층의 저어콘들은 신시생대(약 2,500 Ma), 고원생대(약 1,860 Ma), 신원생대(약 960Ma) 및 고생대(약 380 Ma) 연령에 집중점을 이루는데 반해, 장성지역 규암층의 저어콘들은 신시생대(약 2,500 Ma)와 고원생대(약 1,880 Ma) 연령에 집중점을 이루고 보다 젊은 연대는 산출되지 않는다. 두 지역의 대조적인 연대분포 특성은 서쪽의 영광지역 변성퇴적암층은 옥천변성대 변성퇴적암층에, 동쪽의 장성지역 변성퇴적암층은 영남육괴의 퇴적암층에 각각 대비됨을 지시한다. 이러한 결과는 광주전단대가 옥천변성대와 영남육괴의 지구조적 경계일 가능성을 암시한다. In this study we carried out SHRIMP U-Pb age dating of detrital zircons from age-unknown meta-sedimentary formations distributed around the NNE-SSW trending Gwangju Shear Zone, a branch of Honam Shear Zone, in the southwestern region of the Korean Peninsula. The meta-sedimentary formations from the west (Yeonggwang) and east (Jangseong) areas of the Gwangju Shear Zone have different patterns of zircon age distributions. Zircons of quartzites from the Yeonggwang area yield clusters at Neoarchean (ca. 2,500 Ma), Paleoproterozoic (ca. 1,860 Ma), Neoproterozoic (ca. 960 Ma) and Paleozoic (ca. 380 Ma) ages, but those of the Jangseong area yield clusters at only Neoarchean (ca. 2,500Ma) and Paleoproterozoic (ca. 1,880 Ma) ages. The contrastive patterns in age indicate that the meta-sedimentary formations from the west and east areas correspond to the meta-sedimentary formations of the Okcheon Metamorphic Belt and the sedimentary formations overlying on the Yeongnam Massif, respectively. The results imply that the Gwangju Shear Zone is the tectonic boundary between the Okcheon Metamorphic Belt and the Yeongnam Massif.