Southwestern Gyeonggi Massif Research Articles

Petrogenesis and tectonic implications of the late Paleoproterozoic (ca. 1.7 Ga) post-collisional magmatism in the southwestern Gyeonggi Massif at Garorim Bay, South Korea

The Precambrian basement of the Gyeonggi Massif in the central Korean Peninsula consists mainly of Paleoproterozoic (2.0–1.8 Ga) gneisses. In this study, we present petrological, zircon U–Pb age and Hf isotope, and whole-rock geochemical and Sr–Nd–Pb isotope data for late Paleoproterozoic (ca. 1.7 Ga) granitic gneisses and anatectic granites from the southwestern Gyeonggi Massif. Zircon U–Pb dating of two granitic gneisses and three anatectic granites yielded ages of 1685 ± 14, 1693 ± 8, 1683 ± 18, 1690 ± 6, and 1706 ± 17 Ma, respectively. The samples exhibit LILE enrichment, negative Nb–Ta anomalies, high Ga/Al ratios and HFSE contents, similar to A-type granites. Compared with typical anorogenic intra-continental rift-related granites (A1-type), Y/Nb and Ce/Nb ratios of the samples are much higher, and similar to those of post-collisional granites (A2-type). The εHf(t) values of late Paleoproterozoic zircons from the granitic gneisses and anatectic granites range from –17.8 to –7.0 and –15.3 to –9.7, respectively, indicating derivation from ancient crustal sources. The zircon U–Pb ages of the anatectic granites show that some of the late Paleoproterozoic intrusions underwent re-melting (i.e., anatexis) in the Late Triassic (ca. 230 Ma). Whole-rock initial εNd (230 Ma) values of the anatectic granites (–29.4 to –28.1) and granitic gneisses (–26.3 to –24.3) are similar, indicating that the anatectic granites were sourced from the granitic gneisses. Late Paleoproterozoic igneous activity in the southwestern Gyeonggi Massif can be correlated with post-collisional activity in North China Craton, and was distinct from the coeval rift-related magmatism in South China Craton.

The relationships among faults, geology and geophysical data in the southwestern Korean Peninsula, including the Haenam area, and their application for the interpretation of earthquakes

The southwestern Gyeonggi Massif features many faults. The main faults are NE-striking faults, including the Jeonju and Kwangju faults, and they are cut by N-S- and NNE-striking faults. Additionally, E-W-, WNW-, and NW-striking faults occur as minor faults. Historical earthquakes with magnitudes greater than 5.0 have occurred on or near these faults. Earthquakes with focal depths shallower than 12 km have generally occurred on the NE-, NNE-, and N-S-striking faults, while those with hypocenters deeper than 12 km have occurred on the NW-, WNW-, and NE-striking faults or do not show a relationship with any faults. The instrumental earthquakes with magnitudes greater than 5.0 in the Korean Peninsula mainly have hypocenters deeper than 12 km, and their epicenters form a NW-oriented trend. These data effectively reflect two seismogenic layers with a boundary at ca. 12 km, as suggested in a previous study. The recent earthquake in the Haenam area occurred in the lower seismogenic layer. The shallower earthquakes seem to be the result of the reactivation of pre-existing faults. In comparison, the deeper earthquakes tend to occur on recently formed or newly formed faults related to the present regional stress of the Korean Peninsula. The shear-wave velocity tomography of the southwestern Gyeonggi Massif at 1 km corresponds well to the geology. In contrast, the shear-wave velocity variations at 5–9 km generally correspond to the locations of brittle faults. The depth of the ~3.5 km/s isovelocity line is quite irregular and is deeper in areas with active fault movement. In the Gyeongsang basin, the relationship between the shear-wave velocity and faults is somewhat unclear because the thickness of low-density sedimentary rocks is up to 8 km. Shear-wave velocity tomography at 13 km can be used to identify areas with high shear stress. These data indicate that shear-wave velocity tomography may provide different information depending on the depth and that the brittle faults in this region may not extend to a depth of 13 km.

Structural Analysis of a Fault-related Anticline in the Southwestern Gyeonggi Massif, Korea Using an Unmanned Aerial Vehicle and Field Surveys: The Role of Rejoining Splays in a Duplex-like Structure

Kim, I.; Park, S.-I., and Lee, H.-J., 2020. Structural analysis of a fault-related anticline in the southwestern Gyeonggi Massif, Korea using an unmanned aerial vehicle and field surveys: The role of rejoining splays in a duplex-like structure. In: Jung, H.-S.; Lee, S.; Ryu, J.-H., and Cui, T. (eds.), Advances in Geospatial Research of Coastal Environments. Journal of Coastal Research, Special Issue No. 102, pp. 261-270. Coconut Creek (Florida), ISSN 0749-0208.In fold and thrust belts, various contractional structures accommodate horizontal shortening and vertical thickening in response to orogenic events. In this study, it was conducted a structural analysis of the Gomseom anticline, exposed along the coast in a part of a Phanerozoic orogenic belt in the southwestern Gyeonggi Massif, on the southern Korean Peninsula. To combat difficulties related to accessing the outcrop under tidal effects, high-resolution unmanned aerial vehicle (UAV) images alongside field data was used. The UAV photography technique allowed easy macro and mesoscale structural mapping and down-plunge projection, thereby enabling the efficient interpretation of the geometric and kinematic features of the Gomseom anticline. In the core of the Gomseom anticline, fault-bounded sheets are vertically stacked and folded, giving rise to a unique structural geometry analogous to duplexes. This duplex-like structure is bounded by a submerged thrust at the base and tightly folded thrust imbrication at the top and defined by intervening shortcut rejoining splays branching from the latter. Taking the kinematics of the sheet-bounding faults and intra-sheet minor faults into account, it was suggested an evolutionary model of the Gomseom anticline in which repeated fault-related folding and sequential propagation of rejoining splays give rise to the duplex-like structure. In the rejoining splay model, displacement on the basal thrust is transferred to a passive-roof thrust without hard links between them. Our result can be evaluated as a possible alternative duplex model that contrasts pre-existing models such as “Boyer-type”, “connecting splay”, and “fold” duplexes. However, further testing of the model is required.

Neoproterozoic and middle Paleozoic geological events in the eastern Wolhyeonri complex of the southwestern Gyeonggi Massif, South Korea, and their tectonic correlations in northeastern Asia

The eastern Wolhyeonri complex in the southwestern Gyeonggi Massif, South Korea, which has a close affinity with the northern margin of the South China Craton, primarily consists of Neoproterozoic igneous and sedimentary rocks that were metamorphosed by the middle Paleozoic thermal event. In this study, we present the results from thermobarometry, whole-rock chemistry, and zircon geochronology of rocks in the eastern Wolhyeonri complex near Gwangcheon. The main rock type, such as the migmatitic biotite gneisses, was deposited after ca. 722–680 Ma, and the associated foliated amphibolites exhibit a rift-related Neoproterozoic origin. The ca. 845–820 Ma arc-related metadolerite and mafic layer in the metavolcanic-sedimentary unit are also exposed. Two Neoproterozoic magmatic events support the idea that the southwestern Gyeonggi Massif developed along the northern margin of the South China Craton. These Neoproterozoic rocks experienced upper amphibolite-facies (7–10.5 kbar and 710–735 °C) to low-pressure granulite-facies metamorphism (10.5–12.0 kbar and 810–830 °C) during 424–412 Ma, were associated with ca. 418 Ma leucogranite formation, and then underwent retrograde amphibolite-facies metamorphism. Combined with the results from previous studies, the early-middle Paleozoic orogeny in the southern Gyeonggi Massif is characterized by 470–456 Ma subduction associated with a high P/T metamorphism, and the initiation of intermediate P/T metamorphism from ca. 445 Ma followed by low-pressure granulite-facies metamorphism at 432–403 Ma, and high-pressure granulite-facies metamorphism at 392–381 Ma, indicating that the tectonic setting transitioned from a continental collision to a post-collisional setting between 432 Ma and 392 Ma. These events and the absence of Paleozoic ultrahigh-pressure metamorphism strongly correlated with the Wuyi-Yunkai orogen in the South China Craton, rather than the Qinling belt in the North China Craton. Therefore, the early-middle Paleozoic orogeny in the southern Gyeonggi Massif was caused by the subduction and subsequent collision within the South China Craton.

Ages and tectonic settings of the Neoproterozoic igneous rocks in the Gyeonggi Massif of the southern Korean Peninsula and the correlation with the Neoproterozoic igneous rocks in China

The Neoproterozoic igneous rocks in the northern and southern Gyeonggi Massifs in the southern Korean Peninsula are important for the interpretation of the Neoproterozoic tectonic evolution of the Korean Peninsula and northeastern Asia. The Gonamsan and Chuncheon metabasites in the northern Gyeonggi Massif intruded in a within-plate tectonic setting at ca. 851–873 Ma and 888 Ma, respectively. The Nb/Yb ratios (7–23) of these metabasites represent enriched mid-ocean ridge basalt (E-MORB) to ocean island basalt (OIB) characteristics. Most zircons found within the Gonamsan metabasites have positive εHf(t) values and Paleoproterozoic to Mesoproterozoic isotopic model ages (TDM2). These metabasites have initial 87Sr/86Sr ratios that range between 0.7033 and 0.7058 and εNd(t) values of −0.68 to 5.02. These geochemical characteristics of these metabasites indicate that these rocks formed in a rift-related environment.The Neoproterozoic metabasites from the Dangjin area in the southwestern Gyeonggi Massif can be subdivided into arc and rift types. The Dangjin arc-type metabasites intruded at ca. 820–833 Ma. These metabasites have Nb/Yb ratios of 4–7, Nb/U ratios of 3.58–27.5 and show an increase in Th/Nb ratios with increasing La/Sm ratios. Most zircons from these metabasites have negative εHf(t) values and Paleoproterozoic-Archean TDM2 ages, except one that has positive εHf(t) values and a Mesoproterozoic TDM2 age. Most these metabasites have initial 87Sr/86Sr ratios of 0.7048–0.7105 and negative εNd(t) values from −11.67 to −5.25. These geochemical characteristics indicate that the arc type Dangjin metabasites experienced crustal contamination during differentiation in an arc tectonic environment.The rift-related Dangjin metabasites intruded at ca. 793 Ma in a similar manner to the Gonamsan and Chuncheon metabasites. The Nb/Yb ratios (8–11) of these metabasites are higher than those of the arc-type metabasites. These metabasites have lower initial 87Sr/86Sr ratios of 0.704 and higher εNd(t) values of 1.96 than the Dangjin arc-type metabasites. Together with the results of previous work, this study indicates that the tectonic setting of the Dangjin-Hongseong area was an arc during the period 820–900 Ma and then changed into a rift during the period 703–793 Ma.The Neoproterozoic igneous rocks in the northern Gyeonggi Massif can be correlated with those formed in the within-plate tectonic setting in the southern and southeastern margins of the North China Craton during the period 830–930 Ma. On the other hand, the Neoproterozoic igneous rocks in the southwestern Gyeonggi Massif can be correlated to the Neoproterozoic arc- and rift-related igneous rocks in the northern margin of South China Craton, which intruded during 850–871 and 637–820 Ma, respectively.

경기육괴 서남부 가로림만의 지곡리층 혼성편마암 저어콘에 대한 SHRIMP U-Pb 연대

경기육괴 서남부 가로림만의 지곡리층 혼성편마암 저어콘에 대한 SHRIMP U-Pb 연대

Neoproterozoic igneous activity and Permo-Triassic metamorphism inthe Gapyeong area within the Gyeonggi Massif, South Korea, and theirimplication for the tectonics of northeastern Asia

Although Neoproterozoic rocks were recently found in the western margin of the Gyeonggi Massif, no Neoproterozoic rocks were believed to be present within the interior of this massif. However, in this study, Neoproterozoic rocks are recognized for the first time within the interior of the Gyeonggi Massif through the discovery of the Neoproterozoic volcano-sedimentary rocks of the Gapyeong Formation (GF). The GF is located in the Gapyeong area within the northern Gyeonggi Massif to the north of the Permo-TriassicHongseong-Odesan continental collision boundary in South Korea. The GF mainly consists of mafic granulite interlayered with metacalcsilicate, marble, biotite gneiss, and metatrachyandesite and contains a serpentinized ultramafic block and hornblende biotite gneiss slab. The ages and whole-rock geochemistry indicate that the interlayered metatrachyandesite and mafic granulite in the GF were trachyandesite and basalt that formed through bimodal volcanism in a continental rift tectonic setting at ca. 854 Ma, which matches the timing of the Rodinia supercontinent breakup. The GF can be correlated to the 900–850 Ma Neoproterozoic rifting sequences in the southern Nangrim and northern Gyeonggi Massifs in the Korean Peninsula, which can be correlated to the 930–830 Ma Neoproterozoic rifting sequence along the southern margin of the North China Craton. The GF is difficult to correlate with the ca. 890–790 Ma subduction-related Neoproterozoic rocks in the Hongseong area in the southwestern Gyeonggi Massif, which are considered equivalent to rocks in the northern margin of the South China Craton. The GF underwent upper granulite-facies metamorphism (816–882 °C, 9.0–10.9 kbar) at ca. 250 Ma during the collision between the North and South China Cratons. The serpentinite block in the GF formed in a suprasubduction zone, and the hornblende biotite gneiss slab in the GF was metamorphosed under lower granulite-facies conditions (725–767 °C, 7.0–8.4 kbar) at ca. 414 Ma, indicating that the block and slab were tectonically incorporated into the GF. A strong mylonitic texture is observed in the GF but not in the basement Paleoproterozoic gneiss except at its boundary contact with the GF. These data indicate that the GF, together with the serpentinite block and hornblende biotite gneiss slab, was tectonically transported to the present area after the Triassic metamorphism.

Petrogenesis of Mesozoic granites at Garorim Bay, South Korea: evidence for an exotic block within the southwestern Gyeonggi massif?

We present data from the Mesozoic Keumkang, Palbong, and Baekhwa granites in Garorim Bay, in the southwestern part of the Gyeonggi massif, South Korea. Using major and trace element concentrations, Sr-Nd-Pb isotopic compositions, and sensitive high-resolution ion microprobe (SHRIMP) zircon U-Pb ages, we aim to constrain the petrogenesis of the granites and explain their origin within a broader regional geological context. SHRIMP U-Pb zircon ages of 232.8 ± 3.2, 175.9 ± 1.2, and 176.8 ± 9.8 Ma were obtained from the Keumkang, Palbong and Baekhwa granites, respectively. The Late Triassic Keumkang granites belong to the shoshonite series and show an overall enrichment in large ion lithophile elements (LILE), a depletion in high field strength elements (HFSE) relative to primitive mantle, compared with neighboring elements in the primitive mantle-normalized incompatible trace element diagram with notable high Ba and Sr contents, and negligible Eu anomalies. The Keumkang granites are typified by highly radiogenic Sr and unradiogenic Nd and Pb isotopic compositions: (87Sr/86Sr)i = 0.70931–0.70959, (143Nd/144Nd)i = 0.511472–0.511484 [(eNd)i =–17.0 to–16.7], and (206Pb/204Pb) = 17.26–17.27. The Middle Jurassic Palbong and Baekhwa granites belong to the medium- to high-K calc-alkaline series, and show LILE enrichment and HFSE depletion similar to the Keumkang granites, but exhibit significant negative anomalies in Ba, Sr, and Eu. Furthermore, they have elevated Y and Yb contents at any given SiO2 content compared with other Jurassic granitoids from the Gyeonggi massif. The Palbong and Baekhwa granites have slightly less radiogenic Sr and more radiogenic Nd and Pb isotopic compositions [(87Sr/86Sr)i = 0.70396–0.70908, (143Nd/144Nd)i = 0.511622–0.511660, (eNd)i =–15.4 to–14.7, (206Pb/204Pb) = 17.56–17.76] relative to the Keumkang granites. The Keumkang granites are considered to have formed in a post-collisional environment following the Permo-Triassic Songrim orogeny that records continent-continent collision between the North and South China blocks, and may have formed by fractional crystallization of metasomatized lithospheric mantle-derived mafic melts. The Palbong and Baekhwa granites may have been produced from a gabbroic assemblage at pressures of less than ~15 kbar, associated with subduction of the paleo-Pacific (Izanagi) plate at the Eurasian continental margin. Elevated eNd(t) values in the granitoids from the southwestern part of the Gyeonggi massif relative to those of the central and northern parts, together with the comparatively shallow depth of origin, imply the presence of an exotic block in the Korean lithosphere.

Regional Middle Paleozoic metamorphism in the southwestern Gyeonggi Massif, South Korea: Its implications for tectonics in Northeast Asia

The Hongseong area in the southwestern Gyeonggi Massif in South Korea is considered to represent the eastward extension of the Qinling–Dabie–Sulu collision belt in China. We have carried out zircon U–Pb SHRIMP dating and P–T estimations of the gneisses and amphibolites in the eastern Wolhyeonri complex within the Hongseong area in order to constrain their metamorphic and tectonic evolutions. The protoliths of the migmatitic biotite gneisses formed during the Neoproterozoic and underwent granulite-facies metamorphism (750–880°C, 12–15kbar) at 442–413Ma. These rocks subsequently experienced amphibolite-facies retrograde metamorphism at 585–660°C and 7.5–10.3kbar. Mylonitic biotite gneiss, hornblende gneiss, and folded amphibolite in the study area yield metamorphic ages that range from 429 to 420Ma. The protoliths of some garnet amphibolites that formed at 470–456Ma are arc magmatic rocks; they experienced metamorphism at the boundary between amphibolite- and eclogite-facies (ca. 625–700°C and 13–15.5kbar) before 418Ma and underwent retrograde amphibolite-facies metamorphism (ca. 625–700°C and 8–9kbar) at 418–405Ma. These data suggest that a regional intermediate-P/T metamorphic event occurred during the Middle Paleozoic. In contrast, Paleoproterozoic augen gneiss blocks enclosed in the Deokjeongri gneiss complex preserve evidence of high-pressure (HP) metamorphism (840–960°C, 17–21.8kbar) at 234–230Ma, which are similar to the previously reported results from eclogite blocks in this area. The occurrence of Middle Paleozoic regional metamorphism before the Permo-Triassic HP metamorphism in the Hongseong area may be correlated with the Middle Paleozoic metamorphism in the Qinling belt in China; such regional metamorphic events were caused by the collision of microcontinents with the North or South China Cratons prior to the collision between the North and South China Cratons in the Permo-Triassic.

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.

Continental origin of the Bibong eclogite, southwestern Gyeonggi massif, South Korea

Eclogite is a high-pressure (HP) metamorphic rock that provides important information about the subduction of both continental and oceanic crusts. In this study we present SHRIMP zircon U–Pb isotopic data for a suite of the basement gneisses to investigate the origin of the Proterozoic Bibong eclogite in the Hongseong area, South Korea. Zircon grains from the basement felsic gneisses yielded Paleoproterozoic protolith ages ranging from ca. 2197 to 1880Ma, and were intruded by syenite at ca. 750Ma. A HP regional metamorphic event of Triassic age (ca. 255–227Ma) is recorded in the zircon rims of the country rocks, which is also observed in the zircons from the eclogite. The contacts between the Bibong eclogite and its host rocks support an origin for the Proterozoic protoliths, indicating continental intrusions. The Hongseong area thus preserves evidence for the Triassic collision, indicating a tectonic linkage among the northeast Asian continents.

홍성지역 월현리 복합체 내에 분포하는 트라이아스기 심성암류의 지질연대학 및 지구화학적 연구

한국 경기육괴의 서남부 홍성지역은 지구조적으로 중국의 친링-다비에-수루대와 비교 연장되는 봉합대의 하나로 고려된다. 홍성지역의 중앙부에 위치하는 월현리복합체는 주로 신원생대의 편마암과 고생대 중기의 중-고변성의 편암, 정편마암과 변성염기성화산암으로 주로 구성된다. 이 지역은 트라이아스기 중기에서 말기까지의 암맥상 혹은 암주상의 다양한 관입암이 분포한다. 주 암상은 몬조니암과 반화강암으로 구성되며 부수적으로 소규모 암주상의 몬조섬록암, 섬장암과 섬록암 등이 산출된다. 연구지역 심성암에 대한 SHRIMP U-Pb 저어콘 연대결과는 237 Ma부터 222 Ma의 범위를 보여준다. 연구대상 트라이아스기 심성암은 TaNb의 골, P와 Ti의 결핍과 저장력 원소들이 부화된 경향을 가지는 섭입형 혹은 호상형과 유사한 지화학적 특징을 보여준다. 연구지역 트라이아스기 심성암과 홍성지역의 주 트라이아스기 심성암인 화강암-섬장암은 대부분 칼륨 함량이 높은 고칼륨 캘크-알칼라인에서 쇼쇼나이트까지의 지구조 특징을 가진다. 이들 결과들은 중국의 북중국판과 남중국판의 트라이아스기 충돌이후 생성된 후충돌형 마그마 사건의 지구조적 연결을 가능하게 한다. 결과적으로 홍성지역의 트라이아스기 심성암은 동북아시아의 지구조동력 역사 내 트라이아스기 봉합의 중요한 이해를 제공하여준다. The Hongseong area of the southwestern Gyeonggi massif is considered to be part of suture zone that is tectonically correlated with the Qinling-Dabie-Sulu belt of China in terms of the preservation of collisional evidences during Triassic in age. The Wolhyeonri complex, preserved at the center of the Hongseong area, consists mainly of Neoproterozoic orthogneisses and Middle Paleozoic intermediate- to high-grade metamorphic schists, orthogneisses and mafic metavolcanics. The area includes various Middle to Late Triassic intrusives (e.g. dyke or stock). They are mainly monzonite and aplite with small intrusions of monzodiorit, syenite and diorite in composition. The SHRIMP U-Pb zircon ages yield 237 Ma to 222 Ma. The geochemistry of the studied Triassic intrusives show similar subuction- or arc-type signatures having Ta-Nb troughs, depletion of P and Ti, and enrichment of LILEs (large ion lithophile elements). In addition, the Triassic plutons in the Hongseong area, including those from this study, mostly possess high-K calc-alkaline to shoshonitic tectonic affinity. These results could be tectonically correlated to the post-collisional magmatic event following the Triassic collision between the North and South China blocks in China. Therefore, the Triassic plutons in the Hongseong area offer an important insight into the Triassic geodynamic history of the NE Asian region.

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.

남한 지역 현생 화강암류의 연대측정 결과 정리

남한 지역에 분포하는 현생 화강암류의 연대측정 결과를 정리하였다. 경기육괴 29개, 옥천대 36개, 영남육괴 13개, 경상분지 20개 등 총 98개 지점의 연대에 대하여 보고된 자료를 검토하여 신뢰도를 평가하고 가장 대표성 있는 자료를 선별하였다. 고생대 화강암으로는 경상분지 북부에 장사리와 영덕 암체가 분포하고 있으며 영덕 암체 내에 포획암으로 잡혀 있는 섬록암의 최고연대가 265 Ma까지 보고되므로 시추코어 자료와 함께 고려할 때 페름기 초부터 한반도 남동부에서 섭입환경과 연관되어 활발한 화성활동이 있었음을 알 수 있다. 트라이아스기 초기에는 영남육괴 상주-김천-안동-청송 지역에서 역시 섭입 환경과 관련된 토날라이트-트론제마이트-화강섬록암-몬조나이트질의 화성활동이 일어났다. 트라이아스기 후기에는 경기육괴 남서부와 옥천대 중부를 중심으로 후충돌형 쇼쇼나이트 내지 high-K 계열 화강암과 A-형 알칼리 화강암의 관입이 있었다. 쥬라기 초기 화강암류는 영남육괴 중부와 경기육괴 북동부 설악산 지역, 경상분지 북부에 분포하며 경기육괴 나머지 지역과 옥천대 지역에는 쥬라기 중기 화강암류가 주를 이룬다. 이러한 차이는 쥬라기 초기와 중기에 해구의 위치, 해양판의 침강각 또는 섭입각도가 달랐기 때문으로 생각된다. 쥬라기 후기부터 백악기 초까지의 화성활동 휴지기를 거쳐 백악기 후기부터 경상분지 내에서 집중적인 화강암의 관입이 있었다. 경상분지 밖에서는 110 Ma 내외에 경기육괴 남서부와 중부에서, 또 90 Ma 내외에 설악산, 무암사, 월악산, 속리산 지역 등 경기육괴 북동부와 옥천대 중부에서 화강암의 관입이 일어났는데, 선백악기 화강암류에 비해 지하 천부에 정치하였다. 고제3기 화강암은 경주 지역을 중심으로 동해안 지역에 제한적으로 분포한다. Previous age data were reviewed for 98 sites of Phanerozoic granitoids in the southern part of the Korean Peninsula. Subduction-related granitic magmatism has occurred in southeastern Korea since Early Permian. In the middle part of the Yeongnam massif, arc-related tonalites, trondhjemites, granodiorites, and monzonites were emplaced during Early Triassic. After Middle Triassic continental collision in central Korean Peninsula, post-collisional shoshonitic and high-K series and A-type granitoids were emplaced in the southwestern Gyeonggi massif and central Okcheon belt during Late Triassic. Early Jurassic calc-alkaline granitoids are mostly distributed in the middle part of the Yeongnam massif and Mt. Seorak area, northeastern Gyeonggi massif. On the other hand, Middle Jurassic calc-alkaline granitoids pervasively occur in the Okcheon belt and central Gyeonggi massif. This selective distribution could be attributed to the change in the position of trench, subduction angle, or the direction of subduction. Most Cretaceous and Paleogene granitoids are distributed in the Gyeongsang basin, with the latter emplaced exclusively along the eastern coastline. Outside the Gyeongsang basin, Cretaceous granitoids emplaced in relatively shallow depth occur in the Gyeonggi massif and central Okcheon belt.

Neoproterozoic tectonic evolution of the Hongseong area, southwestern Gyeonggi Massif, South Korea; implication for the tectonic evolution of Northeast Asia

Two types of Neoproterozoic metabasites occur together with regionally intruded arc-related Neoproterozoic granitoids (ca. 850–830 Ma) in the Hongseong area, southwestern Gyeonggi Massif, South Korea, which is the extension of the Dabie–Sulu collision belt in China. The first type of metabasite (the Bibong and Baekdong metabasites) is a MORB-like back-arc basin basalt or gabbro formed at ca. 890–860 Ma. The Bibong and Baekdong metabasites may have formed during back-arc opening by diapiric upwelling of deep asthenospheric mantle which was metasomatized by large ion lithophile element (LILE) enriched melt or fluid derived from the subducted slab and/or subducted sediment beneath the arc axis. The second type of metabasite (the Gwangcheon metabasite) formed in a plume-related intra-continental rift setting at 763.5 ± 18.3 Ma and is geochemically similar to oceanic island basalt (OIB). These data indicate a transition in tectonic setting in the Hongseong area from arc to intra-continental rift between ca. 830 and 760 Ma. This transition is well correlated to the Neoproterozoic transition from arc to intra-continental rift tectonic setting at the margin of the Yangtze Craton and corresponds to the amalgamation and breakup of Rodinia Supercontinent.

Genetic Implications of Two Different Ultramafic Rocks from Hongseong Area in the Southwestern Gyeonggi Massif, South Korea

Two distinct ultramafic bodies occur in Baekdong and Bibong in the Hongseong area within Gyeonggi massif of South Korea. The Hongseong area is now extensively documented as an extension of the Dabie-Sulu collision belt in China. The Baekdong ultramafic body has a NWW elongation direction. This elongation trend is similar to the general trend of the Dabie-Sulu collision belt. The Bibong ultramafic body is elongated in a NNE direction and runs parallel to the direction of the main fault in the study area. The Baekdong ultramafic bodies show porphyroclastic and mylonitic textures while those at Bibong exhibit a mosaic texture. Both were grouped into peridotite and serpentinite based on their modal abundance of serpentine. In the olivine (Fo) vs. spinel [Cr# = Cr/ (Cr+Al)] diagram, both ultramafic rocks fall with in olivine spinel mantle array. The compositions of olivine, orthopyroxene and spinel indicate that the Baekdong ultramafic rock formed in deeper parts of the upper-mantle under passive margin tectonic setting. The SREE content of Baekdong ultramafic rock vary from 0.19 to 5.7, exhibits a flat REE pattern in the chondrite-normalized diagram, and underwent 5% partial melting. Conversely, large variation in SREE (0.5 21.53) was observed for Bibong ultramafic rocks with an enrichment of LREE with a negative slope and underwent 17 24% partial melting. The Baekdong ultramafic rocks experienced three stages of metamorphism after a high pressure residual mantle stage. The first stage of metamorphism occurred under the eclogite-granulite transitional facies (1123 911°C, >16.3 kb) the second under the granulite facies (825 740°C, 16.3 11.8 kb) and the third is the retrogressive metamorphism under amphibolite facies (782 718°C, 8.2 8.7 kb) metamorphism. The Baekdong ultramafic rocks had undergone high-P/T metamorphism during subduction of the South China Block, and experienced a fast isothermal uplift, and finally cooled down isobarically. Evidences for metamorphism were not identified in Bibong ultramafic rocks. Hence, the Baekdong ultramafic rocks with in the Hongseong area may indicate a link on the Korean counterpart of Dabie-Sulu collision belt between North and South China Blocks.