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Isotopic dating has established that the middle Paleozoic turbidites of the Taean Formation on Anmyeondo in the West Sea were affected by metamorphism the Late Triassic. We obtained a 206Pb/238U lower intercept age of 232.5 ± 3.0 Ma (95 % confidence, MSWD = 1.2) of metamorphic titanite from a calc-silicate rock by Multi Collector Sensitive High-Res...
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... Gyeonggi Massif (Fig. 1) is a poly-metamorphic terrane that mainly comprises middle Paleoproterozoic (∼1.93-1.83 Ga) in part high-grade gneiss and variably metamorphic metasediments (e.g., Lee and Cho, 2012;Lee et al., 2014) and minor Neoproterozoic (0.9-0.75 Ga) magmatic and sedimentary rocks in its western and central parts ( Oh et al., 2009) and, at least partly Paleozoic orthogneiss, metasediments, including marble, as well as metabasites, felsic rocks, lens-shaped bodies of highly serpentinized ultramafic rocks (Weolhyeonri complex;Kim and Kee, 2010;Kim et al., 2011b, c). Some serpentinites are associated with rare bodies of strongly retrogressed mafic granulite with exceptional omphacite relics in some garnet porphyroblasts, recording pressures and temperatures of 1.65-2.1 GPa and 775-850°C ( Oh et al., 2005; S.W. Kim et al., 2006;Zhai et al., 2007), acquired during the Triassic ( ; S.W. Kim et al., ...
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... Imjingang Belt (Fig. 1), recrystallized under syn-tectonic medium-pressure, medium-to high-temperature "Barrovian type" conditions (T= 500-800 °C; P= <1.2 GPa: Cho et al., 2007), during the Triassic, as revealed by 263-250 Ma isotopic ages (e.g. Ree et al., 1996;Kee, 2011). Rocks of the Ogcheon Metamorphic Belt (Fig. 1) are also affected by several phases of superimposed deformation and metamorphism, but at lower maximum temperature and pressure (T= 500-650°C; P= 0.4-0.8 GPa: ...
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... Imjingang Belt (Fig. 1), recrystallized under syn-tectonic medium-pressure, medium-to high-temperature "Barrovian type" conditions (T= 500-800 °C; P= <1.2 GPa: Cho et al., 2007), during the Triassic, as revealed by 263-250 Ma isotopic ages (e.g. Ree et al., 1996;Kee, 2011). Rocks of the Ogcheon Metamorphic Belt (Fig. 1) are also affected by several phases of superimposed deformation and metamorphism, but at lower maximum temperature and pressure (T= 500-650°C; P= 0.4-0.8 GPa: ...
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... plutonic rocks ( Fig. 1; Kee, 2011;Kim et al., 2011a;Park et al., 2010;Sagong et al., 2005). Late Triassic (Carnian to early Norian) magmatism is widespread and affects all major tectonic terranes (Fig. 1). This gabbro-monzonite and syenite-granite suite has yielded 237 to 219 Ma isotopic ages, with part of this medium-and high-K calc-alkaline magmatic suite having shoshonitic affinity ( Oh et al., 2006b;Seo et al., 2010;Kee, 2011;Kim et al., 2011a). This type of Mg-rich potassic magmatism has its source in the mantle and typically evolves over a short time in an extensional tectonic setting during plate convergence, amongst others in a post-collisional setting Liégeois and Black, 1987;Turner et al., 1996;Liégeois et al., 1998;von Raumer et al., 2014). Although not limited to continental collision belts and the architecture of the Korean tectonic system being yet far from clear, the Late Triassic magmatism is usually interpreted as due to a change of tectonic regime subsequent to plate collision from compressional to tensional Kim et al., 2011a), often linked to asthenospheric upwelling induced by lithospheric delamination ( , or oceanic slab break-off ( Seo et al., 2010;Oh, ...
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... plutonic rocks ( Fig. 1; Kee, 2011;Kim et al., 2011a;Park et al., 2010;Sagong et al., 2005). Late Triassic (Carnian to early Norian) magmatism is widespread and affects all major tectonic terranes (Fig. 1). This gabbro-monzonite and syenite-granite suite has yielded 237 to 219 Ma isotopic ages, with part of this medium-and high-K calc-alkaline magmatic suite having shoshonitic affinity ( Oh et al., 2006b;Seo et al., 2010;Kee, 2011;Kim et al., 2011a). This type of Mg-rich potassic magmatism has its source in the mantle and typically evolves over a short time in an extensional tectonic setting during plate convergence, amongst others in a post-collisional setting Liégeois and Black, 1987;Turner et al., 1996;Liégeois et al., 1998;von Raumer et al., 2014). Although not limited to continental collision belts and the architecture of the Korean tectonic system being yet far from clear, the Late Triassic magmatism is usually interpreted as due to a change of tectonic regime subsequent to plate collision from compressional to tensional Kim et al., 2011a), often linked to asthenospheric upwelling induced by lithospheric delamination ( , or oceanic slab break-off ( Seo et al., 2010;Oh, ...
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... 231 and 229 Ma old muscovites and the 233 Ma old metamorphic titanite are concordant to the SHRIMP 206 Pb/ 238 U age of 229.6 ± 3.5 Ma that Han (2014) obtained on zircon from the syenite that intruded the Taean Formation at Mongsanpo (Fig. 2) after the second folding phase. Consequently, metamorphism of the Taean Formation and syenitic magmatism in and around Anmyeon Island are coeval. Mica and titanite are from rocks located at 6.5 to 20 km from the dated syenite pluton, which does not show contact metamorphism. The mineral ages are therefore probably not due to the heating by the relative small intrusion itself. Thus, metamorphism and magmatism probably have a common tectonic cause. The Late Triassic (237-219 Ma) gabbro-monzonite and syenite-granite suite in Korea forms relatively small, compositionally zoned, isolated plutons (Fig. 1). This spatial pattern and short time span points to a focused heat source, limited in space and timing. SHRIMP dating of zircons in (migmatitic) gneisses along the western Gyeonggi Massif has revealed that a number of rims have 237-228 Ma ages (errors: 3-5 Ma), pointing to a regional metamorphic overprint Kee, 2011). Metamorphic conditions during this event are not well known but in order to enable formation of such rims rocks must have been at least in the upper amphibolite facies (Williams, 2001;Parrish and Noble, 2003), in agreement with the moderate but widespread anatexis observed in them. Rare spinel granulites occur in the eastern Gyeonggi Massif (Odesan area) within 1-2 km of a hypersthene-bearing monzonite intrusion, dated at 228.7 ± 0.9 Ma (U-Pb on zircon; , record even higher temperatures (T= >900°C; P = 0.75 GPa, Oh et al., 2006a). This underscores that regional metamorphism and magmatism in the Gyeonggi Massif, the Deokjeongri gneisses and Weolhyeonri complex, as well as the Taean Formation at higher crustal level essentially took place during the same, well-defined, short period in the early Late Triassic, and thus by the same tectonic ...
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... of Korea consists of Precambrian continental crust that is subdivided into three terranes, viz. the Nangrim, Gyeonggi and Yeongnam Massifs, from North to South (Fig. 1). These terranes mainly comprise Palaeoproterozoic high-grade gneiss with minor Meso-and Neoproterozoic additions and rare Paleozoic rocks (e.g., Lee and Cho, 2012;Oh, 2012;). The Precambrian terranes are separated by two belts of multiple- deformed and metamorphosed sedimentary and volcanic rocks of late Neoproterozoic to middle and late Palaeozoic age (e.g., Lim et al., 2005;: the Imjingang Belt and the Ogcheon Metamorphic Belt (Fig. 1). Additional multiple-deformed meta-sedimentary rocks, forming the Taean Formation, crop out discontinuously along the western margin and structurally uppermost part of the Gyeonggi Massif ( Kee, 2011;Na et al., 2012;So et al., 2013) in the Taean-Seosan-Dangjin, Anmyeondo-Boryeong areas, and the Yeongheung- Seonjae-Daebu Islands (Fig. 1). In Anmyeondo at least four generations of more or less undeformed and partly metamorphosed intrusive rocks occur in the Taean Formation. Sensitive High-Resolution Ion Micro-Probe (SHRIMP) dating of zircon has shown that one magmatic system is Jurassic and another Late Triassic ) in age. Until quite recently, the Taean Formation has been regarded as Precambrian in age because of absence of fossils (Na, 1992), but are now known to have been deposited after the late Silurian on the basis of the youngest concordant U-Pb SHRIMP spot ages in (rims of) detrital zircons Kee, 2011;Na et al., 2012;So et al., 2013;). The zircon age distribution in the these rocks is similar to that for sandstones of the Imjingang Belt ( ) and some parts of the Ogcheon Metamorphic Belt ( . However, SHRIMP analysis has not been able to provide precise estimates of the age of metamorphism of these rocks, because rims of newly formed zircon around older cores are too thin to be analyzed with high accuracy. This probably indicates that metamorphic recrystallization did not occur at sufficiently high temperatures, viz. the upper amphibolite-facies or higher grade (Parrish and Noble, 2003;Williams, 2001) needed to form significant overgrowths around older ...
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... of Korea consists of Precambrian continental crust that is subdivided into three terranes, viz. the Nangrim, Gyeonggi and Yeongnam Massifs, from North to South (Fig. 1). These terranes mainly comprise Palaeoproterozoic high-grade gneiss with minor Meso-and Neoproterozoic additions and rare Paleozoic rocks (e.g., Lee and Cho, 2012;Oh, 2012;). The Precambrian terranes are separated by two belts of multiple- deformed and metamorphosed sedimentary and volcanic rocks of late Neoproterozoic to middle and late Palaeozoic age (e.g., Lim et al., 2005;: the Imjingang Belt and the Ogcheon Metamorphic Belt (Fig. 1). Additional multiple-deformed meta-sedimentary rocks, forming the Taean Formation, crop out discontinuously along the western margin and structurally uppermost part of the Gyeonggi Massif ( Kee, 2011;Na et al., 2012;So et al., 2013) in the Taean-Seosan-Dangjin, Anmyeondo-Boryeong areas, and the Yeongheung- Seonjae-Daebu Islands (Fig. 1). In Anmyeondo at least four generations of more or less undeformed and partly metamorphosed intrusive rocks occur in the Taean Formation. Sensitive High-Resolution Ion Micro-Probe (SHRIMP) dating of zircon has shown that one magmatic system is Jurassic and another Late Triassic ) in age. Until quite recently, the Taean Formation has been regarded as Precambrian in age because of absence of fossils (Na, 1992), but are now known to have been deposited after the late Silurian on the basis of the youngest concordant U-Pb SHRIMP spot ages in (rims of) detrital zircons Kee, 2011;Na et al., 2012;So et al., 2013;). The zircon age distribution in the these rocks is similar to that for sandstones of the Imjingang Belt ( ) and some parts of the Ogcheon Metamorphic Belt ( . However, SHRIMP analysis has not been able to provide precise estimates of the age of metamorphism of these rocks, because rims of newly formed zircon around older cores are too thin to be analyzed with high accuracy. This probably indicates that metamorphic recrystallization did not occur at sufficiently high temperatures, viz. the upper amphibolite-facies or higher grade (Parrish and Noble, 2003;Williams, 2001) needed to form significant overgrowths around older ...
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... of Korea consists of Precambrian continental crust that is subdivided into three terranes, viz. the Nangrim, Gyeonggi and Yeongnam Massifs, from North to South (Fig. 1). These terranes mainly comprise Palaeoproterozoic high-grade gneiss with minor Meso-and Neoproterozoic additions and rare Paleozoic rocks (e.g., Lee and Cho, 2012;Oh, 2012;). The Precambrian terranes are separated by two belts of multiple- deformed and metamorphosed sedimentary and volcanic rocks of late Neoproterozoic to middle and late Palaeozoic age (e.g., Lim et al., 2005;: the Imjingang Belt and the Ogcheon Metamorphic Belt (Fig. 1). Additional multiple-deformed meta-sedimentary rocks, forming the Taean Formation, crop out discontinuously along the western margin and structurally uppermost part of the Gyeonggi Massif ( Kee, 2011;Na et al., 2012;So et al., 2013) in the Taean-Seosan-Dangjin, Anmyeondo-Boryeong areas, and the Yeongheung- Seonjae-Daebu Islands (Fig. 1). In Anmyeondo at least four generations of more or less undeformed and partly metamorphosed intrusive rocks occur in the Taean Formation. Sensitive High-Resolution Ion Micro-Probe (SHRIMP) dating of zircon has shown that one magmatic system is Jurassic and another Late Triassic ) in age. Until quite recently, the Taean Formation has been regarded as Precambrian in age because of absence of fossils (Na, 1992), but are now known to have been deposited after the late Silurian on the basis of the youngest concordant U-Pb SHRIMP spot ages in (rims of) detrital zircons Kee, 2011;Na et al., 2012;So et al., 2013;). The zircon age distribution in the these rocks is similar to that for sandstones of the Imjingang Belt ( ) and some parts of the Ogcheon Metamorphic Belt ( . However, SHRIMP analysis has not been able to provide precise estimates of the age of metamorphism of these rocks, because rims of newly formed zircon around older cores are too thin to be analyzed with high accuracy. This probably indicates that metamorphic recrystallization did not occur at sufficiently high temperatures, viz. the upper amphibolite-facies or higher grade (Parrish and Noble, 2003;Williams, 2001) needed to form significant overgrowths around older ...
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Within deforming continental regions where metamorphic core complexes (MCCs) and synextensional granitoids are closely associated, deciphering the link between detachment faulting and magmatism often remains complex as (1) the rheological weakness of magma may stimulate mechanisms of strain localization, and conversely, (2) tectonic processes may o...
Citations
... The Korean Peninsula includes three Precambrian basement blocks (the Nangnim, Gyeonggi, and Yeongnam massifs from north to south), which are mainly composed of Paleoproterozoic granitoids and gneisses (Fig. 1a). These Precambrian basement blocks are separated by two Phanerozoic orogenic belts, the Hongseong-Imjingang Belt and the Okcheon Belt (Cho and Kim, 2005;Kim et al., 2006Kim et al., , 2014aKim et al., , 2018Kim et al., , 2019bCho et al., 2007Cho et al., , 2017Kwon et al., 2009;de Jong et al., 2014de Jong et al., , 2015Park et al., 2014aPark et al., , 2017Kee et al., 2019a). ...
... The Hongseong-Imjingang Belt, recently refined by Kim et al. (2018Kim et al. ( , 2019b and Kee et al. (2019a), is located along the central-western part of the Korean Peninsula ( Fig. 1a and b). This orogenic belt preserves petrographic, geochemical, geochronological, and structural evidence of multiple orogenic events from Proterozoic to Phanerozoic age (Oh et al., 2005;Kim et al., 2006Kim et al., , 2017Kwon et al., 2009;de Jong et al., 2014de Jong et al., , 2015Park et al., 2014aPark et al., , b, 2017Park et al., , 2018Park et al., , 2019Han et al., 2017;Park et al., 2020;Lee et al., 2021). Based on the high-pressure metamorphic rocks reflecting a Permo-Triassic crustal thickening event, likely related to the closure of Paleo-Tethys ocean, the area has been tectonically Kee et al., 2019b). ...
Crustal shortening in an elastico-frictional regime is mainly accommodated by contractional fault–fold systems with fracture networks. According to recent research, fracture networks in fold–thrust belts express complex internal strain states in response to thrusting and related folding. Furthermore, their connectivity and fluid flow characteristics likely depend on the structural positions and mechanical stratigraphy that control heterogeneous deformation processes. This study provides characteristics of fold-related fracture networks in the Sinon Anticline, which was formed by fault–bend folding in the southwestern Hongseong–Imjingang Belt, Korea. The fracture networks in the metamorphosed turbidites characterized by interbedded competent metasandstone layers and relatively thin incompetent schist layers have evolved through pre-, syn-, and post-folding fracturing events. Their complexity reflects the spatiotemporal variation in the strain pattern related to early layer-parallel shortening and subsequent fault–bend folding. Based on insights from detailed mapping and topological analysis of the fracture network, we conclude that strain partitioning that occurs during flexural folding results in a superposed tangential longitudinal strain expressed by fractures with a high (hydraulic) connectivity in the hinge zones. Strain partitioning is caused by flexural interlayer slip along incompetent schist layers in the fold limbs. Bed-parallel slip localization zones probably have low porosity and permeability and may act as barriers to fluid migration across beds. We suggest that heterogeneous vertical axis rotation, which occurred as the system's hanging wall slid over the footwall ramp, increased the complexity of fracture networks within the Sinon Anticline. Our findings indicate that the evolution, connectivity, and fluid flow properties of fracture networks can be characterized through careful interpretation of folding mechanisms and related strain states during formation of fault–bend fold systems.
... 250 Ma) for monazite from mica schist on Daebu Island (Han, Cheng, Kim, Yi, & Jeong, 2016). The timing of the retrograde metamorphism can be constrained by earlier published mineral ages for titanite and muscovite from calcareous and mica schists of the Taean Formation on Anmyeon Island (De Jong, Han, Ruffet, & Yi, 2014). For titanite, the concordant U-Pb ages are c. ...
... For titanite, the concordant U-Pb ages are c. 233 Ma, and for muscovite, the 40 Ar-39 Ar ages are 229-231 Ma, and although the muscovite ages could be cooling ages, they were interpreted by De Jong, Han, Ruffet, and Yi (2014) to be metamorphic. This retrograde metamorphism was nearly coeval with the intrusion of syenite (232-220 Ma; Kim, Song, Kwon, Lee, & Choi, 2018;Kim, Kang, Lim, & Lee, 2021) and granitoids (242-227 Ma;Kim, Song, Kwon, Lee, & Choi, 2018) into the Taean-Hongseong Complex, and these igneous rocks have been interpreted as products of post-collisional magmatism (Cheong, Jo, Jeong, & Li, 2019;De Jong, Han, & Ruffet, 2015). ...
The Taean Formation on the central western margin of the Korean Peninsula contains layered metapelite, quartzo‐feldspathic schist, amphibolite, calcsilicate, and impure marble. The layering was formed by tectonic transposition of the original bedding. The formation has lithologic and isotopic age signatures similar to those of metasedimentary rocks of the Imjingang Belt, which is the eastern extension of the Qinling–Dabie–Sulu Belt, and the depositional age of the formation can be constrained to the Late Devonian. The metamorphic pressure–temperature (P–T) path for the formation is characterized by a high ΔP/ΔT prograde part and subsequent near‐isothermal decompression with peak P–T conditions of 600–620 °C and 9.0–10.5 kbar, based on observations of mineral assemblages and phase equilibria modeling. Strong compressional deformation (Dn) produced a N–S‐trending Sn foliation that transposed the earlier Sn–1 and isoclinal Fn folds and was coeval with an episode of medium‐ to high‐pressure amphibolite‐facies metamorphism. The NNE–SSW‐trending left‐lateral Ryeseonggang Fault played a key role in producing the N–S distribution of the metamorphosed fore‐arc deposits along the western coast of the Korean Peninsula. The peak metamorphism and deformation occurred during the early Permian to Early Triassic (276–250 Ma), with retrogression taking place during the Late Triassic (c. 230 Ma), as inferred from new U–Pb zircon age data and previous titanite and muscovite ages for the formation. The clockwise P–T–t (time)–d (deformation) path for the Taean Formation indicate rapid burial during Dn followed by rapid exhumation during the Permian–Triassic collision between the Qinling microcontinent and the North China Craton.
... 280 Ma) to Camian (ca. 230 Ma) based on titanite SHRIMP U-Pb and muscoAr ages from the Taean Formation(De Jong et al., 2014 ...
... 420-430 Ma (Cho et al., 2010;Na et al., 2012), based on the youngest Early Devonian detrital zircon ages and the Triassic intrusion ages cross-cutting the formation. On the other hand, titanite SHRIMP U-Pb and muscovite 40 Ar/ 39 Ar ages from the Taean Formation indicate subsequent metamorphism at ca. 240-230 Ma (de Jong et al., 2014(de Jong et al., , 2015. ...
... 280 Ma, suggesting a possible Permian metamorphism . Recently, de Jong et al. (2014) reported a well-defined regression line with a lower intercept age of 232.5 ± 3.0 Ma (MSWD = 1.2) from metamorphic titanite at the same locality using SHRIMP. They suggested that the ca. ...
... They suggested that the ca. 280 Ma age from a zircon rim might be mixing age during that dates the thin outermost metamorphic rim (de Jong et al., 2014). To verify this, based on detailed zircon and titanite morphology, we analyzed thin zoned zircon overgrowths from sample TKJ-1B using LA-MC-ICPMS U-Pb method because of its small beam size. ...
The southern part of the Korean Peninsula preserves important records of the Paleozoic evolutionary history of East Asia. Here we present SHRIMP U–Pb ages of detrital zircon grains from Paleozoic metasedimentary successions (Okcheon and Joseon Supergroups, Yeoncheon Group, Taean Formation, and Pyeongan Supergroup) that are incorporated into the major Phanerozoic mountain belts (Okcheon and Hongseong-Imjingang Belts) in South Korea, providing new insights for provenances and paleotectonic evolution of the South Korean Peninsula during Paleozoic time. The zircon ages from our samples display two distinct spectra patterns in their presence/absence of Neoproterozoic and/or Paleozoic populations. Our results, together with the available data from the Korean Peninsula, suggest that: (1) the Early to Middle Paleozoic successions in the Okcheon Belt were deposited in continental margin setting(s) formed by Neoproterozoic intracratonic rifting, (2) the Middle Paleozoic metasedimentary rocks in the Imjingang belt can be interpreted as molasse and flysch sediments along an active continental margin, (3) the Late Paleozoic to Early Triassic Taean Formation along the western Gyeonggi Massif represents a syn- to post-collision deltaic complex of a remnant oceanic basin, and (4) the Late Paleozoic to possibly Early Triassic Pyeongan Supergroup in the Okcheon Belt might represent a wedge-top and/or foreland basin. The spatial and temporal discrepancy between the South Korean Peninsula and the Central China Orogenic Belt during Paleozoic might reflect lateral variations in crustal evolution history along the East Asian continental margin during the Paleo-Tethyan Ocean closure.
... 255 Ma) and the Late Triassic (ca. 230 Ma) ( de Jong et al., 2014de Jong et al., , 2015de Jong et al., , 2016. ...
... To transfer isotopic dates to chronostratigraphical ages we use the international chronostratigraphic chart of the International Commission on Stratigraphy (Cohen et al., 2013). ment terranes was extracted from the mantle around ∼2.7 Ga, with major additions at ∼2.5 Ga, influenced by metamorphism in the amphibolite-to granulite-facies and magmatism in the Paleoproterozoic, and together with the bordering belts variably affected by deformation and/or metamorphism during the Permo-Triassic Songrim orogeny, followed by a tectonically induced, thermal and magmatic pulse in early Late Triassic time due to post-collisional delamination and/or oceanic slab break-off and finally by renewed subduction-related processes in the Jurassic (e.g., Cho and Kim, 2005;Cho et al., 2007;Cheong et al., 2015a;Chough et al., 2000;de Jong and Ruffet, 2014a,b;de Jong et al., 2014de Jong et al., , 2015Kee, 2011;Kim et al., 2011a;Lee and Cho, 2012;Lee et al., 2014;Oh et al., 2005Oh et al., , 2015Seo et al., 2010;Williams et al., 2009;Yengkhom et al., 2014). Early-Middle Jurassic non-marine sedimentary series of the Daedong Supergroup occur folded and imbricated in small isolated fault-bounded outcrops on top of this basement ensemble (Egawa and Lee, 2009;Jeon et al., 2007). ...
... 3a-e and 4a-d) and sampling. Metamorphic isotopic ages, U-Pb on zircon (Zr) from Kee (2011); 40 Ar/ 39 Ar on muscovite (Ms) and SHRIMP U-Pb on titanite (Tt) from de Jong et al. (2014). Ages magmatic intrusions (SHRIMP U-Pb, zircon) from Han (2014) and Kee (2011). ...
... We put these new dates into perspective with recently published isotopic ages in the ca. 243-220 Ma range obtained by 40 Ar/ 39 Ar laser-probe and SHRIMP of metamorphic silicates and accessory minerals (e.g., de Jong and Ruffet, 2014a,b;de Jong et al., 2014;Han, 2014;Oh et al., 2014;Park et al., 2014b;Kim et al., 2014a;Kim et al., 2014b) from different key areas along the northern and western margins of the Gyeonggi Massif (Figs. 2,4,7), combined with structural and other field data. Especially, information offered by low-grade metamorphic middle Paleozoic sediments (Taean Formation) on Anmyeon Island (Fig. 7), place important constraints on the tectonic evolution, as in contrast to the other key locations these rocks have only experienced early Mesozoic deformation and metamorphism. ...
... 1,7;Taean Formation), which are comparable to similar series in the Imjingang Belt and part of the southwestern Ogcheon Belt (Cho et al., 2013a;Choi et al., 2008;Kee, 2011;Kim et al., 2014a;So et al., 2013), crop out discontinuously along the western margin, and structurally uppermost part, of the Gyeonggi Massif (Fig. 1). These middle Paleozoic meta-sedimentary terranes, draped around the Gyeonggi Massif, have been variously deformed and metamorphosed starting from the latest Paleozoic to early Mesozoic Cho et al., 2005; D.L. Cho et al., 1996;de Jong and Ruffet, 2014a,b;de Jong et al., 2014;Han, 2014;Kee, 2011;Kim, 2005;Kim et al., 2007;Oh et al., 2004;Kim et al., 2014a). Paleozoic meta-sedimentary rocks surrounded by gneisses also occur as small isolated outcrops in the eastern part of the Gyeonggi Massif (Kee, 2011;Kim et al., 2014a). ...
... De Jong et al. (2014) argued that metamorphism of meta-pelites (main metamorphic minerals: biotite and muscovite) in the Taean Formation, with garnet being extremely rare, and aluminum-silicates absent, occurred below ca. 450°C. ...
... This approach helps to meet a major geochronological challenge of obtaining age estimates for the duration and speed of tectonic and metamorphic processes in the Korean orogenic system -information that is currently essentially lacking. In a companion paper, de Jong et al. (2014) ...
... Interestingly, the Late Triassic M2 metamorphic event in the Gyeonggi Massif is also recorded as a metamorphic event in the tectonically overlying middle Paleozoic greenschist facies Taean meta-sediments on Anmyeon Island. Han (2014) and de Jong et al. (2014) reported a concordant titanite U-Pb age of ∼233 and a muscovite 40 Ar/ 39 Ar age of ∼230 Ma in greenschist facies metamorphic rocks (T< 450˚C), on this island located about 50 km to the west of the Hongseong area. Syenitic magmatism on Anmyeondo is of the same age (Han, 2014). ...
We obtained identical 40Ar/39Ar (pseudo)plateau ages of 230.1±1.0 and 229.8±1.0 Ma (1σ on two hornblendes from garnet-bearing corona-textured amphibolites in the Hongseong area. These ages are concordant with the 228.1±1.0 Ma plateau age of biotite in the slightly older amphibolite. The concordant ages of hornblende and biotite, minerals with very different closure temperatures, show that the samples cooled very rapidly, probably in the order of 100-150°C/Ma. The efficiency of cooling is further underlined by the near-coincidence of these 40Ar/39Ar ages with 243-229 Ma (error 2-4%, average: 234.5 Ma) zircon U-Pb ages in the Gyeonggi Massif and the Hongseong belt, reported in the literature.
Very fast cooling rates require a fundamental tectonic control. Consequently, we discuss our data in the context of a relatively short-lived, tectonically induced, magmatic and metamorphic pulse that affected the crust in Korea in the Late Triassic. This could have been post-collisional delamination of the lower crust and uppermost mantle, and/or oceanic slab break-off to which the 237-219 Ma mantle-sourced potassic Mg-rich magmatic rocks that are widespread in Korea, also points.
The Korean Collision Belt originally constituted a single coherent southwards-thinning tectonic wedge formed by accretion of the Qinling-type Barrovian metamorphic Jingok and Samgot units (Yeoncheon Complex) and Taean Formation onto the Sino–Korean Craton’s southern extension (Precambrian Gyeonggi Massif) while being underthrust by the South China Block. Detailed structural geological study and Ar/Ar laser-probe dating of 47 mineral single-grain and 2 whole-rock samples with both syn-collisional peak metamorphic and retrograde mineral assemblages reveal a prolonged tectonic evolution from the Devono–Carboniferous pre-collisional stage (375–370 Ma; ~315 Ma) to the Midde–Late Jurassic reactivation of the Permian–Triassic wedge during the second major exhumation phase of the Gyeonggi Massif (194–165 Ma). The latest Permian to Late Triassic main orogeny comprises three distinct correlated tectono-metamorphic phases. Essentially concordant (pseudo-)plateau ages (255.2–249.9 ± 0.4–0.9 Ma, 1σ) for main-phase-fabric-forming muscovite and biotite in the garnet, staurolite and kyanite zones and discordant late-stage andalusite-quartz veins (Jingok Unit) show fast cooling during exhumation from ~30 km to ≤12.5 km. Hornblendes (248.8–247.0 ± 0.6–1.6 Ma, 1σ) in the underlying higher-grade metamorphic, deeper underthrust Samgot Unit show later cooling. Fabric asymmetry implies cooling and age differences stem from (S)SE-ward exhumation along low-angle ductile normal faults. Incipient exhumation of mid-Triassic eclogites (Hongseong Belt) induced strong tectono-metamorphic reworking of the overriding plate (242–237 Ma; Yeoncheon Complex, Taean Formation, Gyeonggi Massif). Structurally downwards increasing resetting of mica (225–220 Ma), retrogression and overprinting by top-to the-north post-main-phase-shear constrains the Gyeonggi Massif’s further exhumation and cooling.
Key Points:
Korean Collision Belt: a southwards-tapering tectonic wedge with discrete tectono-metamorphic phases at 255–247, 245–237, 225–220 Ma
255–247 Ma: fast cooling, rapid exhumation, top-to-SE normal shear; 225–220 Ma: downwards increase retrogression, top-to-N normal shear
245–237 Ma: strong reworking; exhumation mid-Triassic eclogite; 194–165 Ma: final metamorphic reactivation and top-to-N normal shear
The geodynamic framework of the south China Craton in the Early Paleozoic and Early Mesozoic has been modeled as developing through either oceanic convergence or intracontinental settings. On the basis of an integrated structural, geochemical, zircon U-Pb and Hf isotopic, and mica 40 Ar/ 39 Ar geochronologic study we establish that an intracontinental setting is currently the best fit for the available data. Our results suggest that widespread tectonomagmatic activity involving granite emplacement and mylonitic deformation occurred during two distinct stages: ~435–415 Ma and ~230–210 Ma. The coeval nature of emplacement of the plutons and their ductile deformation is corroborated by the subparallel orientation of the mylonitic foliation along the pluton margins, gneissose foliation in the middle part of pluton, the magmatic foliation within the plutons, and the schistosity in the surrounding metamorphosed country rocks. The 435–415 Ma granitoids exhibit peraluminous, high-K characteristics, and zircons show negative εHf(t) values (average À6.2, n = 66), and Paleoproterozoic two-stage model ages of circa 2.21–1.64 Ga (average 1.84 Ga). The data suggest that the Early Paleozoic plutons were derived from the partial melting of the Paleoproterozoic basement of the Cathaysia Block. The 230–210 Ma granites are potassic and have zircons with εHf(t) values of À2.8–À8.7 (average À5.4, n = 62), corresponding to T DM2 ages ranging from 2.0 to 1.44 Ga (average 1.64 Ga), suggesting that the Early Mesozoic partial melts in Cathaysia were also derived from basement. The geochemical distinction between the two phases of granites traces continental crustal evolution with time, with the Early Mesozoic crust enriched in potassium, silicon, and aluminum, but deficient in calcium, relative to the Paleozoic crust. Kinematical investigations provide evidence for an early-stage ductile deformation with a doubly vergent thrusting pattern dated at 433 ± 1 to 428 ± 1 Ma (40 Ar/ 39 Ar furnace step-heating pseudoplateau ages obtained on muscovite and biotite from mylonite and deformed granite) and a late-stage strike-slip movement with sinistral sense of ductile shearing at 232 ± 1 to 234 ± 1 Ma (40 Ar/ 39 Ar furnace step-heating pseudoplateau ages) along an E-W direction. The geological, geochemical, and isotopic signatures likely reflect far-field effects in response to continental assembly events at these times.
Laser step-heating of muscovites from strongly retrogressed and ductilely deformed rocks in the top of the northern Gyeonggi massif (Juksung area) yielded 1σ 40Ar/39Ar (pseudo)plateau ages of: 242.8 ± 1.0 Ma and 240.3 ± 1.0 Ma (mica schists) and 219.7 ± 0.9 Ma (mylonitic quartzite). A biotite single grain yielded a hump-shaped age spectrum with ∼245-250 Ma step ages pointing to 39Ar recoil, an irradiation artifact. It is possible that the ∼243-240 Ma muscovite ages record an early phase of exhumation following or during collision, and that the much younger muscovite age from the mylonitic quartzite implies extended or renewed recrystallization. These ages are ∼10 million years older and younger, respectively, than 40Ar/39Ar ages from amphibolites in anatectic gneisses (Hongseong area) and low-grade metasediments on Anmyeondo, linked to post-collisional tectonic and magmatic processes. Our study thus shows that cooling, exhumation and recrystallization in the Triassic occurred in distinct phases that were not coeval in all areas. This underscores that the younger age sets in metamorphic terranes cannot always be simply interpreted as due to passive post-tectonic cooling, but rather reflects distinct tectonic phases.
