Western Gyeonggi Massif Research Articles

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.

Development and timing of two orthogonal fold systems in the western Gyeonggi Massif, Korea

Development and timing of two orthogonal fold systems in the western Gyeonggi Massif, Korea

Fluid Infiltration and Mass Transfer along a Lamprophyre Dyke–Marble Contact: An Example from the South-Western Korean Peninsula

In this contribution, we report the metasomatic characteristics of a lamprophyre dyke–marble contact zone from the Hongseong–Imjingang belt along the western Gyeonggi Massif, South Korea. The lamprophyre dyke intruded into the dolomitic marble, forming a serpentinized contact zone. The zone consists of olivine, serpentine, calcite, dolomite, biotite, spinel, and hematite. Minor F and Cl contents in the serpentine and biotite indicate the composition of the infiltrating H2O-CO2 fluid. SiO2 (12.42 wt %), FeO (1.83 wt %), K2O (0.03 wt %), Sr (89 ppm), U (0.7 ppm), Th (1.44 ppm), and rare earth elements (REEs) are highly mobile, while Zr, Cr, and Ba are moderately mobile in the fluid. Phase equilibria modelling suggests that the olivine, spinel, biotite, and calcite assemblage might be formed by the dissolution of dolomite at ~700 °C, 130 MPa. Such modelling requires stable diopside in the observed conditions in the presence of silica-saturated fluid. The lack of diopside in the metasomatized region is due to the high K activity of the fluid. Our log activity K2O (aK2O)–temperature pseudosection shows that at aK2O~−40, the olivine, spinel, biotite, and calcite assemblage is stable without diopside. Subsequently, at ~450 °C, 130 MPa, serpentine is formed due to the infiltration of H2O during the cooling of the lamprophyre dyke. This suggests that hot H2O-CO2 fluids with dissolved major and trace elements infiltrated through fractures, grain boundaries, and micron-scale porosity, which dissolved dolomite in the marble and precipitated the observed olivine-bearing peak metasomatic assemblage. During cooling, exsolved CO2 could increase the water activity to stabilize the serpentine. Our example implies that dissolution-reprecipitation is an important process, locally and regionally, that could impart important textural and geochemical variations in metasomatized rocks.

Microstructural and Geochronological Analyses of Mesozoic Ductile Shear Zones in the Western Gyeonggi Massif, Korea: Implications for an Orogenic Cycle in the East Asian Continental Margin

In response to orogenic cycles, the ductile shear zone records a complex crustal deformation history. In this study, we conducted a microstructural analysis of two NW–SE trending ductile shear zones (Deokjeok Shear Zone (DSZ) and Soya Shear Zone (SSZ)) in the Late Triassic post-collisional granites along the western Gyeonggi Massif in the Korean Peninsula. The DSZ, overlain by the Late Triassic to the Early Jurassic post-collisional basin fill (Deokjeok Formation), has asymmetric microstructures indicative of a top-down-to-the-northeast shear. Depending on the structural position, the SSZ, which structurally overlies the Deokjeok Formation, exhibits two contrasting styles of deformation. The lower portion of the SSZ preserves evidence of top-up-to-the-southwest shearing after top-down-to-the-northeast shearing; on the other hand, the upper portion only indicates a top-up movement. Given the primary deformation mechanisms of both quartz and feldspar, the deformation temperatures of DSZ and SSZ were estimated at ~300–350 °C and ~350–400 °C, respectively, indicative of the mid-crustal condition. New zircon U-Pb isotopic ages from mylonitic granite in the SSZ and volcanic rocks in the Deokjeok Formation, combined with previously published geochronological data, indicate that the post-collisional granites and volcano-sedimentary sequence were nearly contemporaneous (ca. 223–217 Ma) and juxtaposed because of the Late Triassic orogenic collapse and subsequent new orogenic event. In this study, we highlight the role of the extensional DSZ as a detachment propagated into the middle crust during the Late Triassic orogenic collapse. Our results report a deformational response to a transition from the collisional Songrim Orogeny to the subduction-related Daebo Orogeny in the western Gyeonggi Massif. This, in turn, provides essential insight into cyclic mountain building/collapse in the East Asian continental margin during the Mesozoic time.

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 of Late Mesoproterozoic to Early Neoproterozoic magmatism in the western Gyeonggi massif, Korean Peninsula and its tectonic significance

The western margin of the Gyeonggi massif, southern Korean Peninsula, has preserved N–S trending Neoproterozoic and sporadic Late Mesoproterozoic metaigneous rocks. Here we present the results from systematic field mapping, sensitive high-resolution ion microprobe (SHRIMP) zircon U–Pb dating, and whole-rock geochemical analyses of the Mesoproterozoic and Early Neoproterozoic metaplutonic rocks in the Hongseong area, together with previously published data from the western Gyeonggi massif. The SHRIMP ages of these rocks are categorized into three groups: (1) Late Mesoproterozoic (ca. 1.25–1.15Ga), (2) Early Neoproterozoic (ca. 900–770Ma), and (3) late Early Neoproterozoic (ca. 762–730Ma). The geochronological and geochemical features of the Late Mesoproterozoic rocks suggest that they were possibly formed in association with convergent plate motion. The Early and late Early Neoproterozoic rocks are interpreted to arc-related orogenic and rift-related post-orogenic environments, respectively. These age results and the tectonic signatures provide insight into the convergence process along the margins of the Rodinia supercontinent.

Decoding Neoarchaean to Palaeoproterozoic tectonothermal events in the Rangnim Massif, North Korea: regional correlation and broader implications

ABSTRACTAny knowledge about Archaean to Palaeoproterozoic magmatic and metamorphic events in North Korea has the potential to make a significant difference to our understanding of the early tectonic configuration and evolution of East Asia. This zircon U–Pb dating and Hf isotopic study documents multiple Neoarchaean to Palaeoproterozoic tectonothermal events from the meta-igneous complex in the Machollyong ‘Group’ of the Rangnim Massif. Two tonalitic-trondjemitic gneiss samples record a crystallization age of meta-igneous protoliths at ca. 2.56 Ga and multiple migmatization and metamorphism from 2.52 to 1.85 Ga. A meta-dolerite sample yields a magmatic emplacement age of ca. 1.83 Ga. In situ zircon Hf isotopic data indicate that most zircons from the gneiss samples have εHf(t) values from –16.9 to + 3.1 and crustal model ages from 2.84 to 3.73 Ga, whereas magmatic zircons from the meta-dolerite dike record εHf(t) values from –5.2 to + 5.2 and model ages of 2.05–2.44 Ga. The first-recognized Neoarchaean tonalitic-trondjemitic migmatite complex in the Rangnim Massif, together with previously identified tonalitic-trondhjemitic-granodioritic (TTG) rocks in the Rimjingang Belt and the coeval counterparts in western Gyeonggi massif, represents the oldest crustal nuclei in the Korean Peninsula. The multiple tectonothermal events in this study present reliable evidence not only for attesting to consanguinity of the basement between the Korean Peninsula and the North China Craton but also for defining the influence scope of the late Palaeoproterozoic orogeny in the Korean Peninsula.

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.

An occurrence of the post-orogenic Triassic strata on Deokjeok Island, western Gyeonggi massif, Korea

The U-Pb isotopic compositions of zircon from a metasandstone cobble and a granitic dyke in Deokjeok Island, western Gyeonggi massif, Korea, were measured using a SHRIMP-II ion microprobe to constrain the depositional age of cobble-bearing metasedimentary strata. The metasandstone cobble, collected from a metaconglomerate of the slighltly-metamorphosed sedimentary unit (SMU), yielded 50 to 200 μm long zircon crystals showing variable internal structures together with rounded corners. Sixty spots were analysed from 60 detrital crystals, including two spots of Permian metamorphic overgrowth dated at 255 ± 4 Ma (1σ) and 263 ± 4 Ma (1σ), respectively. Except for the Permian ages, their U-Pb age distribution pattern is characterized by a few major populations at ~400 Ma, ~1.0 Ga and ~2.5 Ga, and several minor peaks between 400 Ma and 2.5 Ga. This pattern is identical to that of the Devonian Taean Formation widespread along the western Gyeonggi massif. On the other hand, the ~260 Ma overgrowth age in the cobble could represent the zircon fraction formed in the sedimentary protolith of the cobble during the Permo-Triassic collisional orogeny. Zircon crystals from a granitic dyke intruding the SMU generally showed the oscillatory zonation, and yielded the weighted mean 206Pb/238U age of 225 ± 3 (tσ), corresponding to its intrusion age. It is thus likely that the cobble-bearing strata might have deposited during the Triassic prior to 225 Ma. Moreover, the SMU unit may represent post-orogenic sediementary strata filling the extensional basin developing along the suture at the western margin of the Gyeonggi massif.

서부 경기육괴에 위치한 덕적도의 암상과 지질

서부 경기육괴에 위치한 덕적도의 암상과 지질

Arc magmatism in the Yeongnam massif, Korean Peninsula: Imprints of Columbia and Rodinia supercontinents

Sensitive high-resolution ion microprobe (SHRIMP) U–Pb zircon ages and whole-rock chemical compositions are reported here from charnockite, anorthosite and metagranite (orthogneisses) from the Jirishan complex in the southwestern part of the Yeongnam massif, Korea. The charnockite and the Kfs-megacryst metagranite yielded zircon U–Pb ages of ca. 1871–1863Ma. In contrast, the Neoproterozoic porphyritic metagranite and anorthosite yielded zircon U–Pb ages of ca. 908–904Ma. The geochemical signatures of both Paleoproterozoic and Neoproterozoic rocks display enrichment in LREEs and negative Nb, Ta and Ti anomalies. The geochronological and geochemical data suggest that the Proterozoic Jirishan complex preserves the imprints of arc-related magmatism at different periods associated with the formation of the Paleoproterozoic Columbia and the Neoproterozoic Rodinia supercontinents. The ca. 1871–1863Ma ages from the charnockites and metagranites from the Jirishan complex are coeval with similar events in the North China Craton, which also represent arc magmatism in a convergent margin setting related to the amalgamation of the Columbia supercontinent. In contrast, the Neoproterozoic porphyritic metagranite and anorthosite, together with the Neoproterozoic metagranites reported from the western Gyeonggi massif, provide a temporal link to the Neoproterozoic tonalite–granodiorite plutons recently identified in and around the South China Craton. They represent orogenic-type magmas that can be attributed to the amalgamation of the Rodinia supercontinent. However, the foliated granites in the central to southwestern Yeongnam massif are related to an arc setting associated with the Permo–Triassic Paleo-Pacific subduction.

Fluid-present disequilibrium melting in Neoarchean arc-related migmatites of Daeijak Island, western Gyeonggi Massif, Korea

The melting process of meta-igneous rocks was investigated via field, petrographic and geochemical analyses of the Neoarchean (~2.51Ga) migmatite complex in Daeijak Island, western Gyeonggi Massif. This complex consists primarily of garnet-free amphibolites and tonalitic migmatites, both of which contain hornblende, plagioclase and quartz as major constituents. Neosomes and leucosomes in the migmatite have dioritic–tonalitic and tonalitic–trondhjemitic compositions, respectively. Compositions of hornblende (XFe=0.39–0.42) and plagioclase (An24–27) vary little between the neosomes and leucosomes. The amphibolites show distinct depletions in Nb, Ta, Zr, and Ti relative to large ion lithophile elements, suggesting an arc-related origin for their basaltic protolith. Leucosomes have lower contents of K2O, MgO, FeO⁎, TiO2, Zr, Rb, and rare earth elements (REE) than amphibolites and neosomes, but are higher in SiO2, Na2O, and Sr contents. Leucosomes and neosomes have positive [(Eu/Eu⁎)N=1.32–7.26] and negative (0.71–0.97) Eu anomalies, respectively, which are attributed to the variable degree of plagioclase fractionation during the partial melting. The P–T condition for the migmatite formation was estimated to be ~700–730°C and 4.7–5.5kbar, primarily based on the hornblende–plagioclase thermobarometry and phase equilibria. Various lines of textural evidence, such as the channel flow of melt along migmatitic layers and the segregation of melt into shear bands or boudin necks suggest a syn-deformation crystallization of melt. Chemical disequilibrium in migmatites is documented not only by petrographic and geochemical data but also by the REE modeling between melt product and source rock. Disequilibrium process is most likely attributed to the rapidity of melt extraction or migration, compared to chemical diffusion rate. In summary, the fluid-present disequilibrium melting of dioritic–tonalitic protoliths has produced tonalitic–trondhjemitic leucosomes in a dynamic arc environment prevalent in the Neoarchean crust of the Korean Peninsula as well as the Shandong Peninsula, North China Craton.

Paleozoic tectonics of the southwestern Gyeonggi massif, South Korea: Insights from geochemistry, chromian-spinel chemistry and SHRIMP U–Pb geochronology

Arc magmas, metavolcanics and hydrated mantle peridotite from the Hongseong area in western Gyeonggi massif provide evidence for Paleozoic subduction tectonics in the Korean Peninsula. Zircons in a tonalitic orthogneiss from the Wolhyeonri complex dated in this study through SHRIMP U–Pb technique yield ages of ca. 437Ma and ca. 414Ma. In addition, SHRIMP U–Pb zircon dating of a garnet-bearing paragneiss in the Wolhyeonri complex records evidence for Middle Paleozoic (ca. 427Ma) thermal metamorphism. Petrological and geochemical data from the Paleozoic suite provide robust evidence for mid Paleozoic arc-related magmatism.Petrological and geochemical data from the Paleozoic suite, including the composition of chromian spinel from the serpentinite bodies enclosing high-pressure mafic and felsic blocks also suggest rifted arc/forearc tectonic setting in a supra-subduction zone environment. The serpentinite bodies preserve the imprints of forearc magmas derived through significant mantle/melt interaction during the opening and subsequent evolution of the forearc. These results indicate that the Paleozoic Hongseong area might have been located in an arc–forearc tectonic environment coeval with the plutonism and volcanism preserved in the Wolhyeonri complex. The Hongseong area thus preserves important clues to the Paleozoic subduction prior to the subsequent Triassic “Alpine-type” collision, suggesting a common tectonic linkage among Paleozoic to Triassic East Asian continents before the final assembly of the Pangaea supercontinent.

Regional variations in the lead isotopic composition of galena from southern Korea with implications for the discrimination of lead provenance

This study presents a comprehensive database (n=215) of lead isotopes in galena from the southern Korean peninsula using new and published data. Of the 69 metal mines examined, predominantly skarn- and hydrothermal-type Pb–Zn–Au–Ag–Cu deposits were observed and were associated with Mesozoic magmatic activities. Galena samples from each geotectonic unit showed discrete lead isotopic signatures. The Gyeongsang basin samples were characteristically unradiogenic and had restricted variations in lead isotopic composition (206Pb/204Pb=18.16–18.59, 207Pb/204Pb=15.48–15.64, 208Pb/204Pb=37.87–38.77). Their 208Pb/204Pb range indicated an involvement of source materials less thorogenic than the associated granites. The galena samples from Cambro–Ordovician carbonate rocks of the northeastern Yeongnam massif and eastern Taebaeksan basin had the most radiogenic 206Pb/204Pb (19.28±0.14) and 207Pb/204Pb (15.833±0.027) ratios. Their lead isotopic trend indicated a combined contribution of ore lead from granitic magmas, Precambrian basements, and overlain host rocks. Less radiogenic galena samples from the middle to southwestern parts of the Yeongnam massif and Okcheon belt showed limited lead isotopic variations (206Pb/204Pb=18.332±0.065, 207Pb/204Pb=15.693±0.012, 208Pb/204Pb=38.93±0.07 on average), probably resulted from mixing with a common crustal basement. The differences in lead isotopes between the radiogenic and unradiogenic groups from the Yeongnam massif and Okcheon belt may reflect the spatial dissimilarity of involved crustal rocks. The old crust appears to have significantly contributed ore lead to galenas from the western Gyeonggi massif, but the geochronological meaning of their steep 207Pb/204Pb–206Pb/204Pb trend is not clear. The comprehensive database constructed by the present study suggests that lead province in the southern Korean peninsula may be subdivided into four discrete zones. Linear discriminant analysis showed that more than 90% of the compiled data were correctly reclassified within their a priori zones. Such discrimination may be useful for a wide range of applications, including mineral exploration, archaeological correlation, and environmental characterization.

경기육괴 북서부 영흥도-선재도-대부도에 분포하는 변성퇴적암 내 쇄설성 저어콘의 SHRIMP U-Pb 연대

경기육괴의 북서부에 위치한 영흥도-선재도-대부도는 지구조적 중요성에도 불구하고 지질 및 지질연대 자료가 거의 없는 지역이다. 우리는 이 지역에 산출하는 변성퇴적암의 암상과 쇄설성 저어콘의 U-Pb 연대를 밝히는 연구를 수행하였다. 연구 지역의 변성퇴적암은 대부분 규암-변성사암 및 규암과 편암의 호층대로 이루어져 있으며, 규암 역을 포함하는 석회질 역질 사암, 석회질 편암 그리고 변성이질암이 소규모로 산출한다. 변성이질암에는 석류석 반상변정이 두드러지게 산출하며, 변성이질암 내의 석영맥에서는 남정석 또는 홍주석이 백운모와 함께 산출한다. 두개의 변성사암 시료에서 분석한 쇄설성 저어콘들은 신시생대(~2.5 Ga), 고원생대(~2.0-1.5 Ga), 신원생대(~1.1-0.7 Ga), 초기 고생대(~560-400Ma) 연대가 우세하게 분포하며, 가장 젊은 연령의 군집은 ~420 Ma에서 나타난다. 이러한 결과는 분석된 변성사암이 실루리아기 이후, 아마도 데본기에 퇴적되었음을 시사하며, 태안층에서 보고된 쇄설성 저어콘의 연령분포와 유사하다. 이와 함께 임진강대 변성퇴적암에서도 유사한 쇄설성 저어콘의 연령분포와 바로비안형 변성상이 보고되었다. 따라서, 경기육괴 서부에 산출하는 태안층은 임진강대의 서남 연장부에 해당될 것으로 판단된다. We investigated the various lithologies and zircon U-Pb ages of metasedimentary rocks from the Yeongheung-Seonjae-Daebu Islands, western Gyeonggi Massif, whose geologic and geochronologic features are poorly constrained in spite of their significance for tectonic interpretation. Major lithology consists of quartzites or meta-sandstones commonly alternating with semi-pelitic schists, together with lesser amounts of calcareous sandstones with matrix-supported quartzite clasts, calcareous schists, and pelitic schists. Pelitic schists uncommonly contain large porphyroblasts of garnet as well as quartz veins with large crystals of muscovite and andalusite or kyanite. SHRIMP U-Pb ages of detrital zircons from two analyzed metasandstones define four age populations: Neoarchean (~2.5 Ga), Paleoproterozoic (~2.0-1.5 Ga), Neoproterozoic (~1.1-0.7 Ga), and Early Paleozoic (~560-400 Ma). The youngest zircon ages are clustered at ~420 Ma. These results suggest that the deposition of meta-sandstones took place after the Silurian, possibly during the Devonian, and are analogous to those of the Taean Formation reported from the western part of the Gyeonggi Massif. Moreover, The age distribution patterns of detrital zircons and the Barrovian-type metamorphic facies of pelitic schists are similar to those reported from the Imjingang belt, suggesting that the Taean Formation likely corresponds to southwestward extension of the Imjingang Belt.

Neoproterozoic plutonic rocks from the western Gyeonggi massif, South Korea: Implications for the amalgamation and break-up of the Rodinia supercontinent

We report sensitive high-resolution ion microprobe (SHRIMP) U–Pb zircon ages, and whole-rock chemical and isotopic (Sr–Nd) compositions from tonalite–trondhjemite–granodiorite (TTG) and alkaline plutons in and around the “Hongseong-Imjingang suture” along the western margin of the Gyeonggi massif, South Korea. The Neoproterozoic TTG and the alkali plutons from the Hongseong area of the Gyeonggi massif yield zircon U–Pb ages of ca. 841–822 Ma and ca. 751–746 Ma, respectively. Based on the geochemical and Nd–Sr isotope data on these plutons, together with the previously reported SHRIMP age of ca. 900 Ma from a tonalite, TTG plutons of ca. 900–820 Ma are interpreted as the products of arc-related magmatism. Their geochemical signatures include enrichment in LREE, negative Nb, Ta and Ti anomalies and relatively low ɛNd(T) values between −0.5 and +5.0. The TTG plutons are coeval with those recently identified in and around the Yangtze block in China. In contrast, the alkali plutons from the Hongseong area, together with the previously reported ones around the Imjingang belt, represent anorogenic-type magmas that can be attributed to the disruption of Rodinia. They display significant negative Eu anomalies, and are strongly depleted in Sr, P and Ti. This transition from arc setting at ca. 900–820 Ma to continental rift setting at ca. 750–740 Ma suggests a regional tectonic link for the amalgamation and disruption of Rodinia in Northeast Asia.

U–Pb SHRIMP zircon geochronology, petrogenesis, and tectonic setting of the Neoproterozoic Baekdong ultramafic rocks in the Hongseong Collision Belt, South Korea

The Hongseong area, located in the western Gyeonggi Massif, South Korea, can be correlated with the northern margin of the South China block (Yangtze Craton). This area experienced Neoproterozoic igneous activity related to subduction before the amalgamation of Rodinia. Several isolated, lenticular, and serpentinized ultramafic–mafic bodies occur in the Hongseong area. The Baekdong body, one of the largest ultramafic bodies, has been highly deformed and metamorphosed to eclogite- and granulite-facies. The petrogenesis and tectonic environment of the Baekdong rocks are assessed using the composition of unaltered cores of spinel and olivine grains, and show that these rocks represent the mantle section of a suprasubduction ophiolite. The rocks originated from oceanic lithosphere that formed during the transition from nascent back-arc to mature island arc, related to subduction roll-back. During the back-arc stage, Al-rich spinel harzburgite formed through melt–rock interaction caused by the intrusion of magma. This magma was produced in small amounts, by less than 10% of partial melting of the wedge mantle. Subsequently, during the mature island arc stage, Cr-rich spinel dunite formed through melt–rock interaction caused by the intrusion of relatively evolved magma that formed by 30–35% partial melting due to a high input of volatiles from the subducted slab and sediments. The Baekdong ultramafic rocks, together with the Bibong ultramafic rocks, indicate that a suprasubduction tectonic setting prevailed before the amalgamation of Rodinia (at 860–890 Ma) in the Hongseong area, which may be an extension of the northern margin of the Yangtze Craton.

SHRIMP U-Pb ages of detrital zircons in metasandstones of the Taean Formation, western Gyeonggi massif, Korea: Tectonic implications

Detrital zircons in three metasandstones of the Taean Formation, western Gyeonggi massif, Korea, were analyzed using a sensitive high-resolution ion microprobe (SHRIMP), in order to provide geochronologic constraints for the depositional age as well as the protolith ages. This formation mainly comprises the sandstone-mudstone units which experienced regional metamorphism at the transitional greenschist-amphibolite facies condition. The majority of detrital zircons yielded concordant ages defining four major age components: (1) Late Archean-Early Proterozoic (2.5–2.4 Ga); (2) Middle to Earliest Late Proterozoic (1.0–0.9 Ga); (3) Middle Late Proterozoic (820–750 Ma); and (4) Silurian (∼430 Ma). The youngest group gives a weighted mean 206Pb/238U age of 431.3 ± 4.1 Ma (n = 17), confirming the Upper Paleozoic deposition of the Taean Formation. Together with this Silurian peak, the presence of Middle to Late Proterozoic ages is characteristic for the source rock, suggesting that the western Gyeonggi massif could be correlative with the South China craton. On the other hand, the lack of Paleoproterozoic age of ∼1.87 Ga is in marked contrast with its predominance in the basement rocks of central to eastern Gyeonggi massif. The overall distribution pattern of detrital zircon ages is consistent with that of the Devonian metasedimentary rock in the Imjingang belt, suggesting an extension of this collisional belt towards the south.

Spinel compositions and tectonic relevance of the Bibong ultramafic bodies in the Hongseong collision belt, South Korea

The Hongseong area in the western Gyeonggi Massif, South Korea, is characterized by the occurrence of several isolated ultramafic lenses in close association with metabasites within the granitic gneiss. Most of these bodies suffered various degrees of serpentinization and also have been highly deformed and metamorphosed. The Bibong ultramafic rock is assumed to be a mantle section of a probable Neoproterozoic supra-subduction zone (SSZ) and its evolution involved two igneous stages overprinted by later stage metamorphism. The compositions of the chromian spinel cores and olivines representative of the igneous stages were used to deduce the petrogenesis and tectonic environments for the formation of the Bibong ultramafic rocks. Spinel Cr# (100 ⁎ Cr/(Cr + Al)) for the igneous stages in lherzolite, harzburgite and chromitite are 24.5–33.3, 37.4–61.2 and 37.3–50.9, respectively. During the first stage, magma formed in the nascent back-arc basin spreading center by a small degree of partial melting of wedge mantle intruded the preexisting lithospheric mantle causing the melt/rock interaction that resulted in lherzolite with Al-rich chromian spinel. During the second stage more evolved magma formed in the mature back-arc basin by high degrees of partial melting due to the increased supply of fluid or melt from the subducting oceanic crust and intruded the lherzolitic mantle forming harzburgite with chromian spinel (with high Cr#) through the coupled process of olivine precipitation and dissolution of pyroxene. The Bibong ultramafic rocks have later undergone three metamorphic episodes. The first stage is evidenced by development of medium to fine grained olivine, orthopyroxene, clinopyroxene and amphibole around orthopyroxene porphyroblasts. The second stage is serpentinization which is followed by the third stage amphibolite facies metamorphism as evidenced by Fe-rich rims around Al-rich chromian spinel due to the diffusional exchange of Al, Mg, Cr, and Fe between chromian spinel and adjacent silicate minerals. The spinel composition shows that the transition from the nascent back-arc basin to mature back-arc basin tectonic setting may have been caused by trench roll back during the Neoproterozoic in the Hongseong area, and the serpentinization and amphibolite facies metamorphism could have occurred during uplift after Triassic collision between the North and South China blocks.

Three-dimensional surface wave tomography for the upper crustal velocity structure of southern Korea using seismic noise correlations

The cross-correlation technique was applied to ambient seismic noises to investigate the upper crustal velocity structure of the southern Korean Peninsula based on surface wave group velocities. We used seismograms recorded continuously during August 2005 at 91 accelerograph stations in the southern Korean Peninsula. From the correlation results of 4095 data pairs, the arrival times of Green’s functions along paths between stations of pairs were measured in discrete bins of frequency bands with central frequencies of 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, and 0.45 Hz. The spatial distribution of arrival times for each frequency band bin was inverted to obtain a two-dimensional tomographic map of Rayleigh wave group velocity. For each tomographic cell of the maps, we derived dispersion curves by extracting group velocities at center frequencies of the 7 frequency band bins. Shear-wave velocity obtained by depth inversion of the dispersion curve at each cell produced a one-dimensional velocity profile at depths. After aligning the profiles corresponding to tomographic cells, we obtained a three-dimensional uppermost crustal velocity model at depths down to approximately 8 km in the southern Korean Peninsula. These results show low-velocity distributions in the Gyeongsang Basin, western Gyeonggi Massif, and the area of Jeju Island and its northeastern coast. The Okcheon Fold Belt and Yeongnam Massif have relatively high velocity distributions. The low-velocity distribution in the Gyeongsang Basin shrinks in northsouth direction at depths around 4 km and widens again with a tendency to migrate to the east as the depth is increased. However, the linear trend of low velocities in NNE-SSW direction at deeper depths (6.25–8.25 km) in the western area distinctively coincides with the western boundary of the Gyeongsang Basin. The background noise analysis in this study provided the constraining information for determination of depth variation in the shear-wave velocity distribution.