Magmatic Response to the Interplay of Collisional And

Geochronology and geochemistry of Triassic plutons in Korea Magmatic response to the interplay of collisional and accretionary orogenies in the Korean Peninsula: Geochronological, geochemical, and O-Hf isotopic perspectives from Triassic plutons

Albert Chang-sik Cheong1,†, Hui Je Jo1,§, Youn-Joong Jeong1,§, and Xian-Hua Li2,§ 1Division of Earth and Environmental Sciences, Korea Basic Science Institute, Chungcheongbukdo 28119, Republic of Korea 2Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100027, China

ABSTRACT geochemically arc-like Andong ultramafic Ernst, 2005, 2010; Cawood et al., 2009; Collins complex (222 Ma). Zircon O-Hf isotopic et al., 2011). Because orogenesis basically re- Phanerozoic internal and peripheral oro- compositions of the older plutons reflect the flects the resistance of a buoyant lithosphere to gens in Northeast Asia converge toward the mixing of metasomatized lithospheric man- plate underflow, the driving mechanism is more Korean Peninsula situated between cratonic tle, young (probably Paleozoic) arc crust, obvious in the case of continental subduction. Asia and the outboard magmatic arc. Wide- and Precambrian basement crust, whereas Continental collisional [also referred to as spread Mesozoic plutons in the peninsula those of the younger plutons reflect input of “Alpine” or Bally’s (1981) “type-A”] orogens, provide first-hand information about the the asthenospheric/lithospheric mantle and represented by wrinkled mountain belts in the magmatic response to the continental and mafic lower crust. Meanwhile, the Late Tri- internal part of the assembled (super)continent, oceanic plate subduction. The present study assic (233–224 Ma) potassic plutons in and mark the termination of a Wilson cycle of ocean addresses this issue using comprehensive (n around the Gyeonggi Massif represent post- opening and closing. The collisional boundaries >1100) whole-rock geochemical, zircon U-Pb collisional magmatism most likely induced are characterized by distinct geologic features, geochronological, and O-Hf isotopic data by slab breakoff, which may also have been such as oceanward verging thrust sheets neigh- obtained from Triassic gabbro-pyroxenite- responsible for the shoshonitic magmatism bored by elongated foreland basins, regional mangerite-monzonite-syenite-granodiorite- in the Yeongnam Massif. Spatial differences Barrovian metamorphism linked to crustal granite plutons in the central and southern in the age pattern and O-Hf isotopic signa- thickening, and exhumation of (ultra)high- parts of the peninsula. The intrusion of ture of inherited and synmagmatic zircons pressure rocks containing diamond or coesite ca. 265–250 Ma calc-alkaline granitoids, from the potassic plutons indicate a selective (e.g., Ernst et al., 1997; Rumble et al., 2003; including the high-silica adakite, along the contribution from an ancient metasoma- Song et al., 2014). On the other hand, accretion- outboard (in present coordinates) Yeongnam tized lithospheric mantle beneath the North ary [also termed as “Pacific” or Bally’s (1981) Massif is coeval with or slightly younger than China-like craton and an allochthonous “type-B”] plate convergence at sites of oceanic the Barrovian metamorphism recognized South China-like lithosphere. The formation subduction has produced subparallel belts con- in fold-and-thrust belts surrounding the of the Triassic plutons could be explained sisting of an outboard trench complex, a medial inboard (northward, present coordinates) by a series of tectonomagmatic events con- forearc basin, and an inboard arc, each of which Gyeonggi Massif, suggesting a close link sisting sequentially of the ridge subduction, has undergone contrasting pressure-temperature between the collisional orogenesis and sub- low-angle subduction, slab breakoff beneath conditions (Miyashiro, 1961). The negative duction initiation as commonly documented the collisional orogen, tectonic switch to an buoyancy of the sufficiently aged and cooled in Phanerozoic supercontinents. Subse- extension-dominated arc system, and delami- oceanic lithosphere provides most of the force quent Late Triassic plutons emplaced in the nation of an overthickened arc lithosphere. required for its sinking and rollback in sub- Yeongnam Massif are subdivided into the duction zones and consequent mantle convec- older (232–224 Ma) magnesian and alkali- INTRODUCTION tion (Stern, 2004, and references therein). This calcic to calc-alkalic group and the younger “Western Pacific” type [or “Mariana” (Uyeda (220–217 Ma) ferroan and alkalic to alkali- Orogenic belts have formed in response to and Kanamori, 1979) type] of subduction leads calcic group temporally intervened by the interior collisional and peripheral accretionary to the development of a retreating arc system, plate convergence since the onset of plate tec- where upper plate extension promotes the for- tonics on Earth. These two end-member types mation of arc-flanking basins. Conversely, the of subduction, with a broad continuum between advancing or “Andean” [or “Chilean” (Uyeda †Corresponding author: [email protected]. §Hui Je Jo—[email protected], Youn-Joong them, have left their own characteristic geologi- and Kanamori, 1979) type] arc system induced Jeong—[email protected], Xian-Hua Li—lixh@ cal and geochemical features among the oro- by the subduction of buoyant oceanic litho- gig.ac.cn. gens and associated igneous rocks (Bally, 1981; sphere, terrane accretion, or plate reorganization

GSA Bulletin; March/April 2019; v. 131; no. 3/4; p. 609–634; https://doi.org/10.1130/B32021.1; 12 figures; 2 tables; Data Repository item 2018374; published online 29 November 2018.

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results in the development of retroarc fold-and- The Paleozoic–Mesozoic history of the Europe to the Cenozoic Himalayan orogen be- thrust belts (Cawood et al., 2009). The accre- northwestern edge of the circum-Pacific is char- tween India and Asia. The Qinling-Dabie-Sulu tionary orogenic system may have switched acterized by the interplay of continental and orogen in central-eastern China is in the middle between phases of advance and retreat, possibly oceanic plate convergence (Engebretson et al., of such a spatiotemporal sequence. Triassic caused by transient flat subduction (Collins, 1985; Ernst et al., 2007). Phanerozoic internal (ca. 240–220 Ma) high-pressure and ultrahigh- 2002; Beltrando et al., 2007; Li and Li, 2007). and peripheral orogens in Northeast Asia con- pressure mineral parageneses have been re- Magmatic responses to collisional and ac- verge toward the Korean Peninsula, situated in ported from the leading edge of the Yangtze cretionary orogenies differ in terms of their a tectonic link between cratonic Asia and an Block in South China (Hacker et al., 2000, driving mechanism and general geochemical outboard magmatic arc. Widespread Mesozoic 2004). Before the Triassic continental colli- characteristics. Collisional orogeny is typi- plutons exposed in the peninsula provide first- sion, the Qinling-Tongbai-Hong’an areas to the cally associated with peraluminous, S-type hand information about the individual or super- west of the Dabie-Sulu Mountains had experi- leucogranites generated by partial melting of imposed roles of collisional and accretionary enced a series of oceanic subduction and arc- the overthickened crust virtually without man- orogenies in making the magma. The present continent collision from the latest Cambrian to tle input (Sylvester, 1998; Barbarin, 1999). It study addresses this issue using new and pub- the Carboniferous (Wu and Zheng, 2013, and has also been recognized that alkaline igne- lished whole-rock geochemical, in situ zircon references therein). The tectonic evolution of ous rocks and carbonatites are concentrated in U-Pb geochronological, and O-Hf isotopic data the curvilinear orogen extending eastward from some suture zones in Africa, the Kola Penin- obtained from gabbro-pyroxenite-mangerite- the Qinling-Dabie-Sulu belt is complicated by sula in Russia, the Himalayan Mountains, and monzonite-syenite-granodiorite-granite plutons severe tectonic and magmatic overprinting and in India (Burke et al., 2003; Burke and Khan, in the central and southern parts of the peninsula transverse fault activities (Ernst et al., 2007), 2006; Hou et al., 2006; Leelanandam et al., that intruded approximately coevally during making it difficult to trace the continuation of 2006). The generation of these rocks is consid- the Triassic, a period marking the culmination this belt into the Korean Peninsula. ered to have been facilitated by density- and of the collisional and accretionary orogenies. Widespread Phanerozoic plutons in North- thermally-driven vertical processes such as This comprehensive data set (n >1100) bears east Asia are believed to have been chiefly pro- delamination and slab breakoff. However, col- important implications for deciphering key is- duced in association with the (paleo-)Pacific lisional convergence does not generally gener- sues concerning the magmatic response to the plate subduction and, to a lesser extent, with ate a large volume of magma, probably due to continental and oceanic plate subduction, such continental collision between the North and the limited supply of volatile components from as the driving mechanism of collision- and arc- South China cratons (Sagong et al., 2005; Yang the descending sialic plate (Ernst, 1999, 2010; related magmatism, the geodynamic evolution et al., 2005, 2007a, 2007b; Jahn, 2010; Zhang Davidson et al., 2007). In contrast, continued of the arc system, and the specific contributions et al., 2014; Zhu et al., 2014; Cheong and Jo, subduction of oceanic plate commonly pro- of mantle and crustal reservoirs to the gener- 2017). Such a presumed connection between duces substantial amounts of juvenile and re- ated magmas. plate convergence and magma generation is cycled crust composed mainly of calc-alkaline corroborated by the distribution of the plu- granitoids. Zircons’ Hf isotopes reflect such GEOLOGICAL BACKGROUND tons concentrated along the plate boundaries differences. According to Collins et al. (2011), (Fig. 1). The initiation of Phanerozoic magma- the range of Hf isotope signatures for the ac- The Pacific (and the former Panthalassic) tism in the Northeast Asian margin is marked cretionary orogens in the circum-Pacific nar- Ocean, created at the breakup of Rodinia, has by the occurrence of latest Paleozoic (267– rows and trends toward more radiogenic com- never been closed during its life (Dalziel, 1991; 256 Ma) calc-alkaline granitoids in southeast- positions since 550 Ma, whereas the range of Hoffman, 1991). Prolonged subduction of the ern China, southeastern Korea, and southwest signatures from the collisional Phanerozoic (paleo-)Pacific plate, initiated in the latest Neo- Japan (Li et al., 2006; Horie et al., 2010; Yi et orogens in China and the Himalayas becomes proterozoic and still ongoing, has left quasi- al., 2012a, 2012b; Cheong et al., 2013, 2014). broader. These contrasting patterns demon- continuous accretionary orogens along the mar- The magmatism then culminated in the Trias- strate the fundamental difference between gin of the circum-Pacific. Opposed accretionary sic, Jurassic, and Cretaceous periods, with mag- oceanic and continental plate subduction in the orogens existing on either side of the Pacific matic lulls of ~20–50 m.y. between the flare- productivity of juvenile crust. Ocean differ substantially in the kinematic ups (Sagong et al., 2005; Cheong and Kim, The generation of arc magma is explained framework of the overriding plate, as exempli- 2012; Zhang et al., 2014; Zhu et al., 2014). The by the melting of the mantle wedge (or of the fied in the classic classification scheme of oce- tectonic significance of such cyclic igneous ac- downgoing slab itself) in response to the addi- anic subduction zones (Uyeda and Kanamori, tivities has been discussed mainly in view of arc tion of fluids from the subducting oceanic litho- 1979; Uyeda, 1982). The present-day western system evolution (Sagong et al., 2005; Li and sphere (Gill, 1981; Peacock et al., 1994; Tat- Pacific margin has undergone several episodes Li, 2007, Choi et al., 2012; Li et al., 2012). sumi and Eggins, 1995; Tatsumi, 2005; Ernst, of tectonic switch between retreating and ad- The basement of the Korean Peninsula is 2010; Grove et al., 2012), or in the case of the vancing arc systems since the late Paleozoic (Li composed of three Precambrian (mostly Paleo continental arc, by the influx of underthrusting et al., 2012). proterozoic) massifs (Nangrim—or Rangnim, fertile materials from the retroarc area to the arc Eurasia is a tectonic collage of Gondwanan Gyeonggi, and Yeongnam) intervened by root (Ducea, 2001; Ducea and Barton, 2007; landmasses that have been transported north- Neoproterozoic to Paleozoic fold-and-thrust DeCelles et al., 2009). The retreating arc sys- ward since the middle Paleozoic (Sengör et belts (Imjingang and Okcheon—or Ogcheon) tem typically displays the trenchward younging al., 1988; Li and Powell, 2001; Collins, 2003). (Figs. 1 and 2). The southeastern margin of trend of igneous activity, whereas magmatism The successive collisional orogens developed the peninsula forms the Cretaceous Gyeong- tends to migrate inboard or temporally shuts off between continental blocks in the interior of sang arc system comprising an arc platform under the advancing arc system (Cawood et al., Eurasia tend to be progressively younger south- and a backarc basin (Chough and Sohn, 2010). 2009; Ducea et al., 2015). eastward from the Paleozoic Caledonides in The Korean Peninsula has traditionally been

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considered to be a coherent part of the North (ca. 2.0–1.85 Ga) arc system and a collisional events related to the internal continental colli- China Craton, as reflected by its alternative orogen probably stretching from the eastern sion and external oceanic plate subduction. name, the Sino-Korean Craton. The shared li- North China Craton (Cho et al., 2017b). After Many tectonic models have proposed that the thologies and metamorphic evolution histories a long magmatic quiescence from the Paleopro- Qinling-Dabie-Sulu belt extends into the Ko- of the three Precambrian massifs suggest that terozoic to the middle Paleozoic, the peninsula rean Peninsula. The tectonic schemes of Cluzel they collectively comprise the Paleoproterozoic experienced another series of tectonothermal et al. (1991) and Yin and Nie (1993) led many

E110° E115° E120° E130° E135°

N40° N40° Gyeonggi East Sea Nangrim Massif (Sea of Japan) North China Craton Massif

N35°

Ye ongnam Sulu -DabieQinling Belt Belt Massif

Yangtze Block N30° N30°

k N25° N25° N

ysia Bloc

Catha 0500km

South China Sea N20° N20°

E115° E120° E125° E130° E135°

Figure 1. Generalized tectonic map of Northeast Asia showing the distribution of Phanerozoic plutons (in black) (after KIGAM, 2001; Jahn, 2010; Cheong and Kim, 2012; Li et al., 2012; Zhang et al., 2014).

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Nangrim Massif Sample location City Cretaceous to Paleogene pluton Jurassic pluton Permian to Tr iassic pluton Fault & thrust

GyGyyeeongeonggiggg N38° MassifMMaassssifss f E129°

Odaesan Hongcheon Imjingang Belt Yangpyeong

Namyang

N37° Seosan Taean Goesan Haemi Andong Hongseong Ian Gwangcheon Baegnok Sangju Cheongsong Yeongdeok Jangsari Cheongsan

Gimcheon Pohang N36° Yeongnaeonongnaamm MassifMaMasM ssif Okcheon Hamyang Hapcheon GyGyeongsaeongeeooonngngsasangaannngg Belt ArcArc SySysSSysteysteemem Sancheong Macheon Daegang

N35° N35° E126° N E129°

0 100 km

Figure 2. Distribution of Phanerozoic plutonic rocks in the central and southern Korean Peninsula and sampling sites (modified from Cheong and Kim, 2012).

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to suggest that the Imjingang and Okcheon belts Approximately comparable SHRIMP zircon central Yeongnam Massif (Daegang, Macheon, correspond to the geographic position of the ages (262–252 Ma) were reported from gab- Hamyang, Hapcheon, Sancheong, and Sangju). suture(s) (Ree et al., 1996; Chough et al., 2000; bros and granites recovered from a drill core The other ten samples were collected previ- Ernst et al., 2007). Alternatively, the schemati- (~3400–2700 m in depth) in Pohang (Lee et al., ously for geochemical and isotopic studies. cally proposed Hongseong-Imjingang or Hong- 2014) (see Fig. 2), indicating that Permian rocks Three (monzonite and syenite), two (gabbro seong-Odaesan (for locations, see Fig. 2) belts comprise the upper part of the basement of and monzonite), and two (mangerite) samples were considered to be the extended collision the Gyeongsang arc. The Paleozoic–Mesozoic were collected from the western (Gwangcheon zone (Oh, 2006, 2016; Kwon et al., 2009; Oh transitional arc magmatism in the Yeongdeok- and Haemi), central (Yangpyeong), and eastern et al., 2015). More recently, Cho et al. (2017a) Andong-Cheongsong-Gimcheon area was fol- (Odaesan) Gyeonggi Massif, respectively, by suggested that the Imjingang, Okcheon, and lowed by the intrusion of Late Triassic plutons Cheong et al. (2015a). Two orthopyroxenites Taean-Hongseong belts, collectively referred in the central and southwestern parts of the from the Andong ultramafic complex (Jeong to as the Gyeonggi Marginal Belt, represent the Yeongnam Massif (Park et al., 2006; Kim et al., et al., 2014) and one granodiorite from the Neoproterozoic to Paleozoic rift- and arc-related 2011b). The latest stage of Triassic magmatism Yeongdeok pluton (Yi et al., 2012a) were also tectonic slivers built upon the North China-like in the Yeongnam Massif is represented by the analyzed in this study. Gyeonggi Massif. occurrence of geochemically arc-like (i.e., en- Zircon U-Th-Pb isotopic analyses were car- The compilation of sensitive high-resolution riched in large-ion lithophile elements [LILEs], ried out using a SHRIMP IIe/MC at the Korea ion microprobe (SHRIMP) data has confirmed but depleted in high-field-strength elements Basic Science Institute (KBSI) (Ochang, Ko- that the Gyeonggi Massif experienced two [HFSEs]) Andong peridotite-pyroxenite com- rea). Prior to the SHRIMP analysis, cathodo- distinct tectonothermal events, in Paleopro- plex (Whattam et al., 2011) and the Daegang A- luminescence (CL) and backscattered electron terozoic and Triassic times (Cho et al., 2017a). type granite (for locations, see Fig. 2), of which (BSE) images of separated zircon grains were The Triassic crustal thickening event resulted emplacement ages were constrained at 222 Ma examined using a scanning electron microscope in high-pressure metamorphism highlighted and 220 Ma, respectively (Cho et al., 2008; (JEOL JSM-6610LV) at the KBSI. Zircon U-Pb by the occurrence of eclogitic amphibolite in Jeong et al., 2014). ages were newly determined for four samples the western Gyeonggi Massif (Oh et al., 2005). After the Permian–Triassic tectonomagmatic from Taean in the Gyeonggi Massif, and Hap- The SHRIMP zircon dates (ca. 230 Ma) of the events, the Korean Peninsula was subjected to cheon and Sancheong in the Yeongnam Massif. eclogite facies metamorphism, reported by Kim an accretionary orogenic system. The Jurassic In addition, seven samples from the Gyeonggi et al. (2006), are mostly discordant, and thus and Cretaceous magmatism migrated inland Massif and four samples from the Yeongnam may not constrain the timing of peak metamor- and trenchward, respectively (Kee et al., 2010; Massif were dated again in this study consid- phism exactly. The post-collisional stage is rep- Cheong and Kim, 2012; Choi et al., 2012), per- ering the relatively high scatter of previous resented by the Late Triassic gabbro-mangerite- haps resulting from the tectonic switch between SHRIMP zircon ages (i.e., relative error >1%, monzonite-syenite-granite suite, occurring the advancing and retreating arc systems. The mean square weighted deviation [MSWD] commonly as small stocks in the internal and Phanerozoic plutons comprise around one-third >5). Whole-rock major element analyses for marginal parts of the Gyeonggi Massif (Fig. 2) of the total landmass of the southern Korean eleven samples (three from the Gyeonggi Mas- (Cheong et al., 2015a, 2016, and references Peninsula (Fig. 2). They are subdivided into sif and eight from the Yeongnam Massif) were therein). This suite is approximately coeval with four spatiotemporal groups that intruded epi- performed at Pukyong National University rare earth element (REE)-enriched carbonatite sodically in the Permian–Triassic, the Early Ju- (Busan, Korea) and Activation Laboratories in Hongcheon, in the central Gyeonggi Mas- rassic, the Middle Jurassic, and the Cretaceous Ltd. (Canada) using an X-ray fluorescence sif (for location, see Fig. 2) (Kim et al., 2016), to Paleogene (Cheong and Kim, 2012). Triassic, spectrometer. Trace element compositions of and silica-undersaturated alkaline silicate rocks Jurassic, and Cretaceous igneous activities were sodium peroxide-fused whole-rock powders occurring to the north of the Imjingang Belt also identified by detrital and magmatic zircons (ten samples from the Gyeonggi Massif and (Peng et al., 2008; Yang et al., 2010). The Trias- collected from the North Korean territory (Wu eight samples from the Yeongnam Massif) were sic crustal thickening event was followed by a et al., 2007a, 2007b). determined at Activation Laboratories Ltd. with gravitational collapse at ca. 225 Ma to form the a quadrupole inductively coupled plasma–mass Gyeonggi Shear Zone (Kim et al., 2000). MATERIALS AND METHODS spectrometer (ICP-MS). Zircon oxygen iso- The initiation of extensive arc magmatism topes were measured for seventeen samples in southeastern Korea is marked by the oc- This study used new and published whole- (eight from the Gyeonggi Massif and nine from currence of late Permian (257 Ma) Jangsari rock geochemical, zircon U-Pb geochrono- the Yeongnam Massif) using the Cameca IMS and immediately subsequent (253–247 Ma) logical, and O-Hf isotopic data for 21 gabbro- 1280 ion probe at the secondary ion mass spec- Yeongdeok plutons (Yi et al., 2012a) (Fig. 2). pyroxenite-mangerite-monzonite-syenite- trometry laboratory of the Institute of Geology The high Sr/Y ratios (>140), intermediate felsic granodiorite-granite samples collected from and Geophysics, Chinese Academy of Sciences compositions, and light REE (LREE)-enriched Triassic plutons in the Gyeonggi (10 samples) in Beijing, China. Zircon Lu-Yb-Hf isotopes patterns of the Yeongdeok samples (Cheong et and Yeongnam (11 samples) massifs. The sam- al., 2002) are typical for the high-silica adakite ple locations are shown in Figure 2, and their 1 derived by slab melting (Martin et al., 2005). GPS coordinates are presented in Table DR1 , 1GSA Data Repository item 2018374, Ta- Sodic metagranitoids (tonalitic-trondhjemitic- with petrographic and mineralogic summa- bles DR1–DR4: GPS coordinates and petrographic granodioritic gneisses) occurring in the adja- ries. Eleven new samples were collected for and mineralogic summaries of whole-rock samples, cent Andong-Cheongsong area, and the “young this study. They were three granites from the SHRIMP U-Th-Pb results for zircon, whole-rock major and trace element compositions, and zircon gneisses” near Gimcheon (for locations, see western Gyeonggi Massif (Taean, Seosan, and O and Lu-Yb-Hf isotopic compositions, is available Fig. 2) yielded coeval (ca. 250 Ma) zircon core Namyang) and eight monzonite-mangerite- at http://www.geosociety.org/datarepository/2018 or ages (Cheong et al., 2014; Song et al., 2015). syenite-granodiorite-granite samples from the by request to [email protected].

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were measured for nineteen samples (eight respectively, which were marginally consistent at a fine scale. The grains typically contained from the Gyeonggi Massif and eleven from the with previous SHRIMP zircon age estimates corroded cores (Fig. 3) yielding Paleopro- Yeongnam Massif) using a Plasma II multicol- (227.0 ± 2.4 and 228.4 ± 1.2 Ma; Williams et al., terozoic 207Pb/206Pb dates (1.91–1.77 Ga). The lector ICP-MS (Nu Instruments) equipped with 2009; Kim et al., 2011b). presence of Paleoproterozoic inherited cores an NWR193-nm ArF Excimer laser ablation Zircon grains from the Gwangcheon monzo- was also reported by Kim et al. (2011b). Our system at the KBSI. Laser ablation was targeted nite (sample 140403-03), Haemi syenite (sam- dating result for the magmatic oscillatory CL on the points selected for the determination of ple 140404-01), Yangpyeong gabbro (sample domains of zircons from the Hamyang grano- U-Th-Pb-O isotopes or new points within the 140402-03), monzonite (sample 140402-02), diorite (226.8 ± 1.3 Ma, n = 21, MSWD = 2.3) same CL domains. Details of the zircon analy- and Odaesan mangerite (sample 070809-03) was younger than the previous estimate by Kim ses are available in Appendix 1. were dated by Cheong et al. (2015a). In this study, et al. (2011b) (232.2 ± 2.9 Ma, MSWD = 9.1). we obtained similar but more tightly constrained The reason for this discrepancy is unknown, RESULTS 206Pb/238U ages for these samples: 229.5 ± but the relatively high scatter of the age data 1.7 Ma (n = 20, MSWD = 1.9), 229.6 ± 2.2 Ma of Kim et al. (2011b) is noteworthy. Zircon Zircon Texture and U-Pb Age (n = 20, MSWD = 3.4), 232.1 ± 1.5 Ma (n = 12, grains from the monzonite (sample 140423-02) MSWD = 0.9), 228.0 ± 1.5 Ma (n = 16, MSWD = and granite (sample 140423-04), collectively Representative CL and BSE images of the zir- 1.4), and 231.2 ± 2.4 Ma (n = 16, MSWD = 3.9), constituting the Sangju pluton, were character- con grains are shown in Figure 3. The SHRIMP respectively (Fig. 4). From the Yangpyeong ized by banded and oscillatory CL zoning, re- zircon U-Th-Pb data are listed in Table DR2 monzonite, we found Neoarchean and Paleopro- spectively (Fig. 3). Some recrystallized grains (see footnote 1). terozoic (2.65 and 1.88–1.81 Ga) zircon cores. from the former had a relatively high U content (>1000 ppm) and low Th/U ratios (≤0.1). Gyeonggi Massif Plutons Yeongnam Massif Plutons These grains were not considered in the age Figure 4 shows the zircon U-Pb isotopic com- Zircon U-Pb isotopic compositions of seven calculation. In the latter sample, zircon grains position of eight samples from the Gyeonggi samples from the Yeongnam Massif are graphi- commonly contained cores with truncated CL Massif in concordia diagrams. cally displayed in Figure 5. bands (Fig. 3). These cores yielded consis- Zircon grains from the Taean granite (sample The Hapcheon and Sancheong samples were tently Paleoproterozoic 207Pb/206Pb dates (1.90– 170329-04) were generally euhedral and pris- newly dated here. Zircon grains from the Hap- 1.85 Ga). The U-Pb isotope data for the syn- matic. Some grains showed clear oscillatory CL cheon syenite (sample 150312-03) and man- magmatic zircons in the monzonite and granite zoning, indicating their synmagmatic growth. gerite (sample 150312-04A) had banded, sec- yielded identical weighted mean 206Pb/238U ages Their magmatic origin was also corroborated tor, or oscillatory CL zoning. The Th/U ratios (224.3 ± 1.9 Ma, n = 17, MSWD = 2.2; 225.9 ± by the mostly high (>0.7) Th/U ratios (i.e., Ru- were lower in the former (~0.5) than in the lat- 1.4 Ma, n = 16, MSWD = 1.0), which were batto, 2002). Some grains containing high lev- ter (~0.9), but were still higher than the gen- younger than the previous results (238.8 ± 4.4 els of uranium (~23,000–9700 ppm) were dark erally accepted boundary value between mag- and 230.6 ± 2.5 Ma; Kim et al., 2011b) reported in CL images, and typically had a sponge-like matic and metamorphic domains (Th/U = 0.1; with relatively high MSWD va1ues (>5). BSE texture, with abundant micro-inclusions Rubatto, 2002). The U-Pb isotope data yielded (Fig. 3). These domains were avoided during the a weighted mean 206Pb/238U age of 216.9 ± Whole-Rock Chemical Composition SHRIMP analysis due to their high common Pb 0.9 Ma for the syenite (n = 31, MSWD = content (9.7%–0.7%, in f206Pb) and suspected 1.6) and 227.4 ± 1.5 Ma for the mangerite (n Whole-rock major and trace element com- metamictization. The U-Pb isotope data for the = 29, MSWD = 2.6). Zircons from the San- positions of the plutons are listed in Table DR3 oscillatory-zoned domains yielded a weighted cheong syenite (sample 140424-04) typically (see footnote 1). Some major element classifi- mean 206Pb/238U age of 229.8 ± 2.2 Ma (n = 12, showed fine-scale oscillatory zoning in CL cation diagrams are presented in Figure 6 using MSWD = 2.5). Zircon grains from granites in images. Some grains contained corroded early new data obtained in this study and previous Seosan (sample 170330-06) and Namyang magmatic (i.e., not xenocrystic) cores. The results from the literature (Cheong et al., 2002, (sample 140403-02) were mostly euhedral and U-Pb data yielded a weighted mean 206Pb/238U 2015a; Oh et al., 2006b; Choi et al., 2009; Wil- prismatic crystals with distinct oscillatory zon- age of 218.5 ± 1.0 Ma (n = 20, MSWD = 1.2). liams et al., 2009; Kim et al., 2011b, 2011d; Yi ing. Dark-CL grains were frequently observed The Hapcheon and Sancheong syenites had the et al., 2012a). in the Seosan granite and, to a lesser extent, in same age, within error ranges. In the classification scheme of Frost et al.

the Namyang granite. Some grains from both Zircon grains from the Daegang alkali gran- (2001), mafic to intermediate (SiO2 ≤ 60 wt%) samples contained rounded or resorbed cores ite (sample DGA1-2) were typically euhedral samples from the Gyeonggi Massif were invari- that were texturally discordant to the oscilla- and prismatic crystals that frequently showed ably magnesian (Fig. 6A). Granite samples from tory rims (Fig. 3). These cores mostly yielded dark-CL emissions. The relatively bright-CL the western parts of the massif (Taean, Seosan, Neoproterozoic ages (0.83–0.68 Ga), with domains were targeted for the analysis. Zircons Hongseong, Haemi, and Namyang) were scat- subordinate Paleoproterozoic (2.3–2.1 Ga) and from the Macheon monzonite (sample 140424- tered between magnesian and ferroan fields. Of Mesoproterozoic (1.4–1.1 Ga) populations. 02) had banded, sector, or oscillatory CL zon- the Yeongnam Massif samples analyzed in this The Neoproterozoic inherited zircon ages ing. They yielded a weighted mean 206Pb/238U study, only alkali granite and syenite samples (0.82–0.65 Ga) were reported previously from age of 232.4 ± 1.4 Ma (n = 21, MSWD = 1.6), from Daegang, Hapcheon, and Sancheong were the Seosan granite (Williams et al., 2009). The which was marginally consistent with a previ- ferroan. These ferroan samples were relatively oscillatory rims from the Seosan and Namy- ously published result (236.8 ± 3.4 Ma; Kim young (220–217 Ma) compared with the other ang granites yielded weighted mean 206Pb/238U et al., 2011b). Zircon grains from a K-feldspar Yeongnam samples (232–224 Ma) mostly be- ages of 224.2 ± 2.3 Ma (n = 16, MSWD = 3.9) megacryst-bearing granodiorite from Hamyang longing to the magnesian group. The Gyeonggi and 232.5 ± 1.5 Ma (n = 24, MSWD = 2.6), (sample 140423-06) had oscillatory CL zoning Massif samples were plotted predominantly

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Taean Taean Seosan Seosan Namyang granite granite granite granite granite (BSE) -14.9 ± 1.4 8.278.27 ± 0.240.24 ‰ -10.8 ± 2.9 -20.2-202 .2 ± 1.1.0

7.59 ± 0.25 ‰ -11.9 ± 1.7 7.647.64 ± 00.29.29 ‰ 3.39 ± 0.30 ‰ -15.1-15.1 ± 1.0 -17.7 ± 0.088 767.633±0 ± 0.2525 ‰ 758 ± 16 Ma 7.98 ± 0.29 ‰ -11.7 ± 1.5 -14.7 ± 0.9 8.09 ± 0.16 ‰ -17.5 ± 1.3 0.5 ± 1.4

Namyang Gwangcheon Haemi Ya ngpyeong Ya ngpyeong granite monzonite syenite gabbro monzonite

7.87 ± 0.34 ‰ -13.5 ± 1.3

832 ± 5 MaMa 6.43 ± 0.32 ‰ 7.3 ± 1.1.55 -24.5 ± 1.4

8.39 ± 0.24 ‰ 7.98 ± 0.25 ‰ 7.75 ± 0.311 ‰ 7.347.34 ± 0.300.30 ‰ 7.16 ± 0.28 ‰ -12.1 ± 1.1 -14.6 ± 1.1 13.1 ± 0.8 -23.8 ± 1.0 -22.9 ± 1.2 Figure 3. Representative back- Yangpyeong Odaesan Hapcheon Hapcheon Sancheong scattered electron (marked monzonite mangerite mangerite syenite syenite with a “BSE” label) and cath- odoluminescence images of 5.30 ± 0.13 ‰ zircon grains, from the Triassic 6.81 ± 0.22 ‰ 8.1 ± 1.7 7.987.98 ± 0.21 ‰ plutons in the Korean Penin- -22.8 ± 1.0 3.83.8 ± 1.7 sula, recording results for U-Pb 6.56 ± 0.25 ‰ dating (for inherited cores only) 6.2 ± 1.5 and O-Hf isotopic measure- 1876 ± 17 Ma ments. The scale is referenced 5.26 ± 0.17 ‰ 6.94 ± 0.22 ‰ 7.48 ± 0.20 ‰ 7.377.37 ± 0.19 ‰ by Hf isotope spot (large dotted 8.3 ± 1.7 -4.7 ± 0.9 -18.7 ± 1.1 3.434 ± 1.3 circle, diameter = 50 µm).

Daegang Macheon Hamyang Hamyang Sangju alkali granite monzonite granodiorite granodiorite monzonite

6.26 ± 0.23 ‰ 7.55 ± 0.29 ‰ 1849 ± 33 Ma -1.7 ± 1.41.4 -4.1 ± 1.6 7.56 ± 00.24.24 ‰ 9.32 ± 0.18 ‰ -4.1 ± 1.41.4 3.0 ± 1.2

6.49 ± 0.23 ‰ 99.36 ± 0.28 ‰ -10.2 ± 1.1 -1.66 ± 1.61.6 7.76 ± 00.17.17 ‰ -4.4 ± 11.1.1 7.76 ± 0.23 ‰ -7.1 ± 1.1 7.59 ± 0.30 ‰ -4.5 ± 1.0 -3.2 ± 1.6

Sangju Sangju Yeongdeok Andong Andong granite granite granodiorite pyroxenite pyroxenite

6.00 ± 0.20 ‰ 3.7 ± 1.1 -8.88 ± 0.0.99 11.3 ± 1.0 6.97 ± 0.22 ‰ 2.1 ± 1.0 1864 ± 17 Ma 6.99 ± 0.22 ‰ -3.7 ± 1.1 6.02 ± 0.26 ‰ 4.7 ± 0.8 -9.11 ± 1.01.0 10.5 ± 0.8

0.9 ± 1.1

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0.044 0.4 Taean granite Seosan granite 229.8 ± 2.2 Ma 224.2 ± 2.3 Ma (n = 12, MSWD = 2.5) (n = 16, MSWD = 3.9) 0.040 0.3 0.042 U 0.040

238 0.036 0.2 0.038 Pb/

206 0.036 0.032 0.1 0.034

0.032 0.18 0.20 0.22 0.24 0.26 0.28 0.30 0.32 0.028 0 0 0.1 0.2 0.3 0.4 0.5 02468 0.5 0.040 Namyang granite 232.5 ± 1.5 Ma 0.4 (n = 24, MSWD = 2.6) 0.038 0.041

U 0.3 0.039

238 0.036

Pb/ 0.2 0.037 206 0.035 0.034 0.1 Gwangcheon monzonite 229.5 ± 1.7 Ma 0.033 0 0.1 0.2 0.3 0.4 0.5 (n = 20, MSWD = 1.9) 0 0.032 024681012 0 0.2 0.4 0.6 0.040 0.0395

0.0385 0.038 0.0375 U

238 0.036 0.0365 Pb/

206 0.0355 0.034 Haemi syenite Yangpyeong gabbro 229.6 ± 2.2 Ma 0.0345 232.1 ± 1.5 Ma (n = 20, MSWD = 3.4) (n = 12, MSWD = 0.9) 0.032 0.0335 0 0.2 0.4 0.6 0.19 0.21 0.23 0.25 0.27 0.29 0.31 0.5 0.039 Yangpyeong monzonite 228.0 ± 1.5 Ma 0.4 (n = 16, MSWD = 1.4)

0.041 0.037

U 0.3 0.039 238

Pb/ 0.2 0.037 0.035 20 6 0.035 Odaesan mangerite 0.1 231.2 ± 2.4 Ma 0.033 (n = 16, MSWD = 3.9) 0.14 0.18 0.22 0.26 0.30 0.34 0.38 0.0 0.033 02468101214 0.12 0.16 0.20 0.24 0.28 0.32 0.36 0.40 207Pb/235U 207Pb/235U Figure 4. Concordia diagrams of sensitive high-resolution ion microprobe zircon U-Pb data from the Gyeonggi Massif, central Korean Peninsula, with weighted mean 206Pb/238U ages, uncertainties at the 95% confidence level, and statistical parameters. Error ellipses are at the 1-sigma level. Gray and dashed ellipses represent the inherited cores and outliers determined by t-tests in the calculation of the weighted mean ages, respectively. Data points represent 208Pb-corrected ratios. MSWD—mean square weighted deviation.

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0.039 0.039 Hapcheon syenite 216.9 ± 0.9 Ma (n = 31, MSWD = 1.6) 0.037 0.037 U

238 0.035 0.035 Pb/

206 Hapcheon mangerite 0.033 0.033 227.4 ± 1.5 Ma (n = 29, MSWD = 2.6)

0.031 0.031 0.16 0.20 0.24 0.28 0.32 0.36 0.14 0.18 0.22 0.26 0.30 0.34 0.38 0.041 0.045

0.043 0.039 Sancheong syenite 0.041 0.037 218.5 ± 1.0 Ma 0.039

U (n = 20, MSWD = 1.2)

238 0.035 0.037

Pb/ 0.035

206 0.033 0.033 Macheon monzonite 0.031 0.031 232.4 ± 1.4 Ma (n = 21, MSWD = 1.6) 0.029 0.029 0 0.1 0.2 0.3 0.4 0.14 0.18 0.22 0.26 0.30 0.34 0.38 0.5 0.044 Hamyang granodiorite 0.042 0.4 226.8 ± 1.3 Ma (n = 21, MSWD = 2.3) 0.040 0.039

U 0.3 0.038 23 8 0.037 0.036

Pb/ 0.2 Sangju monzonite 20 6 0.034 0.035 224.3 ± 1.9 Ma 0.1 (n = 17, MSWD = 2.2) 0.032

0.033 0 0.16 0.20 0.24 0.28 0.32 0.36 0.030 02468 00.2 0.40.6 0.81.0 0.4 207Pb/235U Sangju granite 225.9 ± 1.4 Ma 0.3 (n = 16, MSWD = 1.0)

0.041 U 0.039 23 8 0.2 Figure 5. Concordia diagrams of sensitive high- 0.037

Pb/ resolution ion microprobe zircon U-Pb data from

20 6 0.035 the Yeongnam Massif, southern Korean Penin- 0.1 sula. Statistical treatments and symbols are the 0.033 same as in Figure 4.

0.031 00.1 0.20.3 0.40.5 0 02468 207Pb/235U

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1.1 15 AB 1.0 Hapcheon Hapcheon Daegang 10 0.9 Sancheong Sancheong alkali-calcic 0.8 ferroan 5 +MgO) (wt%) 0.7 magnesian c total O-CaO (wt%) alkali 2 0 0.6 calc-alkalic O+K /(FeO 0.5 2

total -5 calcic Na 0.4 Gyeonggi Massif Yeongnam Massif FeO 0.3 -10 45 50 55 60 65 70 75 80 45 50 55 60 65 70 75 80

SiO2 (wt%)SiO2 (wt%) 8 3.0 CD

Sancheong 2.5 6 Hapcheon 2.0 Daegang

shoshonite

4 O (wt%) 1.5 Macheon 2 O (wt%) 2 K

O/Na 1.0 2 K 2 high-K 0.5 medium-K low-K 0 0 45 50 55 60 65 70 75 80 45 50 55 60 65 70 75 80

SiO2 (wt%) SiO2 (wt%)

Figure 6. Major element classification diagrams for the Triassic plutons from the Gyeonggi and Yeongnam massifs in the central and southern Korean Peninsula, respectively, analyzed in this study and previous works (Cheong et al., 2002, 2015a; Oh et al., 2006b; Choi et al., 2009; Williams et al., 2009; Kim et al., 2011b, 2011d; total total Yi et al., 2012a). (A) A plot of FeO /(FeO + MgO) versus SiO2 showing the ranges of ferroan and magnesian

fields (Frost et al., 2001). (B) A plot of (Na2O + K2O – CaO) versus SiO2 showing the ranges of alkalic, alkali-

calcic, calc-alkalic, and calcic rock fields (Frost et al., 2001). (C) A plot of 2K O versus SiO2 showing the shoshon-

ite, high-K, medium-K, and low-K series fields (Rickwood, 1989). (D) A plot of 2K O/Na2O versus SiO2. Data for

some high-K2O/Na2O (>3.5) samples from the Gyeonggi Massif are not shown in (D) for clarity.

in alkalic and alkali-calcic fields, whereas the were mostly plotted in the shoshonite series K2O/Na2O ratio (Fig. 6D). The dominantly po-

Yeongnam Massif samples straddled the re- field, whereas felsic (SiO2 ≥70 wt%) samples tassic (K2O > Na2O, in wt%) and sodic (Na2O >

gion between alkali-calcic to calc-alkalic fields, were dispersed from the shoshonite to high-K K2O) signatures of the Gyeonggi and Yeongnam

except for two syenites from Hapcheon and series fields. The Yeongnam Massif samples samples were not correlated with their SiO2 con- Sancheong that were plotted distinctly in the straddled the region between the high-K and tents. Among the Yeongnam samples analyzed

alkalic field (Fig. 6B). At the given SiO2 con- medium-K series fields, except for the ferroan in this work, only the Daegang alkali granite

tent, the Gyeonggi samples tended to be more samples and the Macheon monzonite plotted in was higher than unity in K2O/Na2O ratio. The

enriched in K than the Yeongnam samples the shoshonite series field. The geochemical dif- A/CNK (molar Al2O3/[CaO + Na2O + K2O]) ra-

(Fig. 6C). Mafic to intermediate (SiO2 = 60– ference between the Gyeonggi and Yeongnam tios of the samples from the two massifs were 45 wt%) samples from the Gyeonggi Massif samples was most sharply distinguished by the mostly below 1.1, which is the conventionally

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10000 A (Gyeonggi Massif) A (Yeongnam Massif) 1000

100

Sample/Chondrite 1 Hamyang Taean ferroan magnesian Yeongdeok 0.1 La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Yb Lu La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Yb Lu

10000 B (Gyeonggi Massif) B (Yeongnam Massif) 1000

100

1 Sample/N-MOR B 0.1 Taean Yeongdeok 0.01 Cs Ba U Ta La Pb Sr Nd Hf Eu Gd Y Lu Cs Ba U Ta La Pb Sr Nd Hf Eu Gd Y Lu Rb Th Nb K Ce Pr P Zr Sm Ti Dy Yb Rb Th Nb K Ce Pr P Zr Sm Ti Dy Yb

Figure 7. (A) Chondrite-normalized rare earth element distribution patterns and (B) normal-mid oceanic ridge basalt (N-MORB)-normalized spidergrams of the Triassic plutons from the Gyeonggi and Yeongnam massifs in the central and southern Korean Peninsula, respectively. The chondrite and N-MORB values are from Mc- Donough and Sun (1995) and Sun and McDonough (1989), respectively.

accepted boundary value between I- and S-type syenite from the western Gyeonggi Massif was liquid, the Gyeonggi and Yeongnam samples granites (Chappell and White, 1992). exceptionally high in Th (53 ppm). exhibited apparent arc affinities characterized Of the samples analyzed in this study, the Figure 7 shows the normalized geochemical by an enrichment in LILEs and relative deple- mafic to intermediate rocks from the Gyeonggi patterns of the plutons analyzed in this study. tion in HFSEs (Fig. 7B). Peaks for K and Pb, Massif had distinctly higher Ba contents (3040– The average composition of the Yeongdeok ada- and troughs for Nb, P, and Ti were prominent 1950 ppm) than the other samples (≤1300 ppm). kite pluton (Cheong et al., 2002) is also shown in most samples. The troughs were particularly

At given SiO2 contents, the Gyeonggi samples for reference. Most Gyeonggi samples displayed distinct in the Taean granite. tended to have higher Cs, Rb, and Pb content LREE-enriched chondrite-normalized patterns than the Yeongnam samples. Five syenite- (Fig. 7A). The exception was the Taean gran- Zircon O-Hf Isotopes granodiorite-granite samples from Hapcheon, ite, which displayed the “seagull” chondrite- Sancheong, Hamyang, and Daegang in the normalized pattern (see Glazner et al., 2008) The new zircon O and Lu-Yb-Hf isotope Yeongnam Massif, and Taean in the Gyeonggi with a deep Eu anomaly (Eu/Eu* = 0.02). The data are listed in Table DR4 (see footnote 1). Massif were relatively abundant in Y (62– older (232–224 Ma) Yeongnam Massif samples The Gyeonggi Massif plutons are hereafter 44 ppm) compared with the other samples had LREE-enriched normalized patterns, with- subdivided into the western and central-eastern (<30 ppm). The Zr and Hf concentrations of out conspicuous Eu anomalies, except for the groups, considering their substantial differences all samples were strongly and positively cor- Hamyang granodiorite, which had a high heavy in inherited zircon age patterns. Cheong et al. related (R2 > 0.95). The Sancheong syenite had REE content and stronger negative Eu anomaly (2015a) revealed that these two spatial groups the largest abundance of these elements (Zr = (Eu/Eu* = 0.39). The younger (220–217 Ma) differ profoundly in their whole-rock Nd and 1020 ppm, Hf = 20 ppm). The Nb concentration ferroan Yeongnam Massif samples mimicked zircon Hf isotopic compositions. It is also noted was the highest in the Taean granite (49 ppm). the REE pattern of the Hamyang granodiorite. In that the western group occurs within the Taean- Overall, both the U and Th contents increased the normal mid-oceanic ridge basalt-normalized Hongseong Complex composing the Gyeonggi but the V, Cr, and Ni contents decreased progres- spider diagram, which arranges the elements ac- Marginal Belt proposed by Cho et al. (2017a).

sively with increasing SiO2 content. The Haemi cording to their compatibility with the basaltic The zircon ages and whole-rock geochemical

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compositions described above allowed us to subdivide the Yeongnam Massif plutons into x

the older (232–224 Ma) magnesian group and , and

the younger (220–217 Ma) ferroan group. Zir- 1.1 to –5.3 6.49 to 5.71 4.4 to 2.2 6.2 to –0.6 11.2 to 6.4 8.41 to 6.50 c comple 7.58 to 6.68 5.42 to 4.76 con U-Pb ages and O-Hf isotope data are sum- enite: –

marized in Table 1 for the four subgroups using enite: enite: enite: enite: enite: x erroan group of x 222–217 ro ro (3.0 ± 0.6) (2.6 ± 1.4) data from the present analyses and the literature (8.1 ± 1.1) (–1.9 ± 1.3) (5.10 ± 0.17 ) (6.04 ± 0.19 ) (7.43 ± 0.41 ) (Jeong et al., 2014; Cheong et al., 2015a). (7.08 ± 0.26 ) ounger f Y Andong ultramaﬁ eongnam Massif plutons Sancheong sy Hapcheon sy Andong py Y Hapcheon sy

Western Group of Gyeonggi Massif Plutons Andong py Sancheong sy Daegang alkali granite: In zircons from the Gwangcheon monzonite Daegang alkali granite:

(sample 140403-03), there was little variation in OREA 18 δ O (8.05 ± 0.27‰) and εHf(t) (–12.2 ± 0.8) values (t = zircon crystallization age, ± 1 standard deviation [SD], statistical treatment of Hf isotope

data from the Gyeonggi samples included data 13.1 to 9.6 6.27 to 5.23 8.0 to 4.8 8.44 to 7.24 –3.8 to –9.1 2.8 to –6.7 7.04 to 6.18 7.77 to 7.42 ite: , Sangju granite) ite: from Cheong et al., 2015a, same hereafter un- ite: –7.9 to –10.3 ite: 18 –5.6 to –9.9

less otherwise stated). Comparable δ O (7.96 ± onite: onite: onite:

0.17‰) and slightly lower εHf(t) (–14.5 ± 0.7) 250–224 1.91–1.77 (6.4 ± 0.7) (–5.8 ± 1.5) (–3.4 ± 1.5) (–9.1 ± 0.7) (–8.4 ± 1.2) (11.1 ± 0.8) (5.69 ± 0.26 ) (7.74 ± 0.32 ) (6.65 ± 0.22 ) values were obtained from zircons in the Haemi (7.58 ± 0.11 ) syenite. Zircons from the Taean granite were eongnam Massif plutons ang granodiorite: ang granodiorite: Y y y Older magnesian group of ang granodiorite OM CENTRAL AND SOUTHERN K Sangju granite: divided into lower (<6‰) and higher (>6‰) y Hapcheon manger Macheon monz Sangju monz eongdeok granodior Macheon monz Hapcheon manger 18 eongdeok granodior Ham Ham Y

groups with respect to δ O. Except for one Y (Ham

grain, the lower group zircons were significantly ONS FR high in U (>9700 ppm), with a decreasing trend in δ18O with increasing U concentrations. The ite) high-δ18O group yielded an average δ18O value of

7.53 ± 0.26‰, with lower εHf(t) values (–14.2 ±

18 TRIASSIC PLUT 1.6) than those of the low-δ O group (εHf(t) = 18 –11.1 ± 1.1). The εHf(t) values of the high-δ O 7.40 to 6.94 group of –17.7 to –20.7 , Odaesan manger zircons tended to decrease with decreasing ite : 7.58 to 7.07 onite : 7.22 to 6.43 ite : Lu/Hf ratios, reaching –16.1 for a rim spot of onite : –18.7 to –25.8 onite 176 177 238–227 (7.26 ± 0.14 ) (6.85 ± 0.24 ) (7.36 ± 0.17 ) (–22.5 ± 0.8) (–23.1 ± 1.4) (–19.5 ± 0.8) grain 2–8, which had the lowest Lu/ Hf OT OPES FOR eong gabbro : 3.2, 2.6, 1.88–1.8 1 (0.00033). Synmagmatic zircons from the Seosan eonggi Massif plutons eong gabbro : –20.6 to –23.9 eong monz eong monz Central-easte rn Gy

granite were divided into two groups: one with ong monz ng py ng py ng py

18 18 ye Odaesan manger Ya relatively low O and high values ( O = ng py Odaesan manger δ εHf δ Ya Ya Ya

8.4–7.0‰, εHf(t) = –6.8 to –11.9) and the other angp in the opposite side ( 18O = 9.8–8.3‰, (t) = ~ (Y

δ εHf GES AND O-Hf IS –20). The Paleoproterozoic (2107 Ma) inherited core had the highest δ18O value (8.79‰) and

the lowest εHf(t) (–4.7). Conversely, two Meso- proterozoic (1385 and 1304 Ma) cores had the 18 lowest δ O values (5.47 and 5.87‰), with εHf(t) 8.05 to 7.28 9.77 to 7.01 –12.8 to –18.8 ang granites) –5.8 to –20. 7 8.04 to 6.95 ranging from 5.4 to –0.2. The most abundant onite : 8.46 to 7.58 –9.7 to –16. 3 y group of onite : –10.7 to –14.0 Neoproterozoic (0.82–0.68 Ga) cores had mod- Y OF ZIRCON U-Pb A enite : 8.30 to 7.55 18 233–224 ste rn enite : –13.1 to –16.0 (7.53 ± 0.26 ) (8.07 ± 0.99 ) (7.66 ± 0.22 ) (8.05 ± 0.27 ) (7.96 ± 0.17 ) (–12.2 ± 0.8) (–13.2 ± 5.4) (–15.4 ± 1.3) est variation in δ O (8.4–6.9‰) and εHf(t) (5.0 (–14.5 ± 0.7) (–14.2 ± 1.6) We ang granite: to –2.0). Synmagmatic zircons from the Namy- eonggi Massif plutons ang granite: SUMMAR aean granite: Gy aean granite:

ang granite had narrow ranges of O and Hf iso- T Haemi sy 2.3–2.1, 1.4–1.1, 0.83–0.65 T (Seosan-Nam Seosan granite: Seosan granite: Haemi sy Na my 18 angcheon monz Na my topic compositions (δ O = 7.66 ± 0.22‰, εHf(t) angcheon monz Gw ABLE 1. Gw

= –15.4 ± 1.3). Their εHf(t) values were weakly T and positively correlated with their Lu/Hf ratios, reaching –18.8 in the lower Lu/Hf side. The Paleoproterozoic (2346 Ma) inherited core from the Namyang granite had an inner part viation ) 18 O (‰) with moderate δ O (6.31‰) and negative εHf(t) (t) , in Ma ) 18 Hf ) δ (–5.2) values and an outer part yielding slightly ε

18 mean ± standard higher δ O (6.88‰) and lower εHf(t) (–8.3) val- U age ues. The Mesoproterozoic (1076 Ma) core had ng ro ck 23 8 ited core age (Ga) viation ) much lower 18O (5.04‰) and positive (t) xcluding data from high-U δ εHf Pb / (bea ri (e domain s, de (mean ± standard de (synmagmatic Synmagmatic Synmagmatic Inher (3.9) values. The outer part of this core had a Subgroup 20 6

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comparatively low εHf(t) (–0.2). The highest with previous data for the Yeongdeok pluton U-Pb dating of zircon overgrowth rims from a

εHf(t) value (7.3) was obtained from the Neopro- (11.7 ± 1.3; Cheong et al., 2013). garnet-biotite gneiss (252.9 ± 1.9 Ma; Cho et terozoic (832 Ma) core. al., 2005). Recently, Late Triassic magmatic Younger Group of Yeongnam Massif Plutons ages were reported from the adjacent Nangrim Central-Eastern Group of Gyeonggi Except for dark-CL spots, the zircon δ18O Massif in the North Korean territory. Peng et al. Massif Plutons values of the Daegang alkali granite were quite (2008) presented a zircon crystallization age of Zircons from the Yangpyeong gabbro in the consistent (6.04 ± 0.19‰). As shown by zir- 224 ± 4 Ma for a biotite syenite that comprises central Gyeonggi Massif showed little varia- con grains from the Taean granite in the west- a gabbro-pyroxenite-syenite complex (the so- tion in O and Hf isotopic compositions (δ18O = ern Gyeonggi Massif, the uraniferous dark-CL called Tokdal Complex) probably stretching 18 7.26 ± 0.14‰, εHf(t) = –22.5 ± 0.8). Synmag- spots had comparatively low δ O values (5.63 from eastern China. A comparable Rb-Sr age

matic zircons from the Yangpyeong monzonite and 5.89‰). The εHf(t) values varied moderately (223.3 ± 6.6 Ma) was obtained from phlogo- had slightly lower δ18O (6.85 ± 0.24‰) and from 1.1 to –5.3. Zircons from the Hapcheon pite in a kimberlite associated with the Tokdal 18 comparable εHf(t) (–23.1 ± 1.4) values. These syenite had the lowest δ O (5.10 ± 0.17‰) Complex (Yang et al., 2010). These two age

two Yangpyeong samples had the lowest zir- and the second highest εHf(t) (8.1 ± 1.1) values data, together with a monazite Th-Pb age of the

con εHf(t) among the samples analyzed in this among the synmagmatic grains from the plu- Hongcheon carbonatite in the Gyeonggi Mas- study. Three grains (#2-7, 2-9, and 2-10) from tons analyzed in this study. Zircon δ18O values sif (232.9 ± 1.6 Ma; Kim et al., 2016), confirm the monzonite showed core-to-rim increases of the Sancheong syenite varied from 8.41‰ to the occurrence of Late Triassic mantle-derived in δ18O (~6.4 → ≥7.0‰) that were marginally 6.50‰. But this sample yielded consistent zir- magmatism in central Korea.

outside the limit of analytical uncertainty. Their con εHf(t) values (3.0 ± 0.6). Combined with data Cho et al. (2017a) constrained the tim- Lu/Hf ratios decreased toward the rims, but the from Jeong et al. (2014), the zircon Hf isotopic ing of Triassic crustal thickening event in the

core and rim εHf(t) values did not vary consis- compositions of the two pyroxenites from An- Gyeonggi Massif to 245–230 Ma based on a

tently. The Neoarchean and Paleoproterozoic in- dong differed slightly from each other (εHf(t) = compilation of the SHRIMP U-Pb and Th-Pb herited cores from the monzonite had moderate 1.7 ± 1.0 and 3.6 ± 1.2). However, their zircon O ages of accessory minerals. Their new SHRIMP to high δ18O values (8.4–6.9‰) and consistently isotopic compositions were comparable, yield- monazite (235–231 Ma) and zircon ages 18 negative εHf(t) (–3.6 to –8.9). Zircons from the ing an average δ O value of 7.08 ± 0.26‰. (ca. 226 Ma) from the Mount Cheonggye area Odaesan mangerite (sample 070809-03) had near Seoul, Korea, may indicate that the Triassic 18 narrow ranges of δ O (7.36 ± 0.17‰) and εHf(t) DISCUSSION collisional orogeny was accompanied by rapid (–19.5 ± 0.8). cooling and exhumation. However, we note that Temporal Correspondence of Collisional at least part of zircon rim ages reported previ- Older Group of Yeongnam Massif Plutons Orogeny and Arc Magmatism ously from the Gyeonggi Massif should be in- Zircon grains from the Macheon monzonite terpreted with caution. Importantly, it is quite 18 had little variation in δ O and εHf(t). Two spots After Paleoproterozoic (ca. 2.0–1.85 Ga) ac- difficult to distinguish the age of a regional from grain 2-2 had relatively high δ18O (7.77 cretionary and collisional events, the cratonic metamorphic event from that of a local thermal

and 7.72‰) and εHf(t) (–1.2 and 0.1) values. part of the Korean Peninsula remained tectoni- overprint. The influence of the magmatic over- One grain (#2-64) showed a distinct core-to- cally and magmatically calm until the continen- print on the zircon age was typically exempli-

rim increase in εHf(t) (–2.2 → 2.8) outside tal collision between the North and South China fied in the Yangpyeong area, where Paleopro- analytical uncertainties. Except for these spots, cratons, although it should be noted that Neo- terozoic basement gneisses yielded a zircon 18 δ O and εHf(t) values were highly consistent proterozoic–Paleozoic magmatic and metamor- U-Pb age of ca. 235 Ma from overgrowth rims 18 (δ O = 7.55 ± 0.08‰, εHf(t) = –3.8 ± 0.8). Syn- phic events have occurred along the fold-and- or newly grown grains (Oh et al., 2015). This magmatic zircons from the Hamyang granodi- thrust belts surrounding the Gyeonggi Massif age is indistinguishable from the crystallization orite also had little variation in δ18O (7.63 ± (Gyeonggi Marginal Belt; Cho et al., 2017a, ages of synmagmatic zircons from gabbros and 0.17‰), except for two spots in grain 2-1 2018). The Phanerozoic orogenies left a strong monzonites in this area (232–228 Ma; Cheong that had substantially higher values (8.44 and imprint on pre-existing rocks, and directly or et al., 2015a; this study), considering the error

8.33‰). The εHf(t) values of synmagmatic zir- indirectly caused the formation of magma. In ranges. This is especially true when the pooled cons varied from –3.8 to –9.1 and had a weak the following, the temporal relation between age of zircon cores from the monzonite (238.4 positive correlation with the Lu/Hf ratio. The the Phanerozoic orogenic event(s) that affected ± 5.0 Ma; Cheong et al., 2015a) is taken to rep- Paleoproterozoic cores had significantly high the Gyeonggi Massif and the surrounding Im- resent the crystallization timing of “antecryst” 18 δ O (9.4–8.7‰) values and variable εHf(t) (4.1 jingang and Okcheon belts, and arc magmatism (see Miller et al., 2007). Moreover, among the to –2.3). Zircons from the Hapcheon mangerite along the Yeongnam Massif is discussed based samples analyzed by Oh et al. (2015), the cor- had relatively low δ18O values (6.65 ± 0.22‰) on our dating results combined with available dierite-sillimanite-garnet-biotite gneiss (sample

and consistently positive εHf(t) (6.4 ± 0.7). The age data from the literature. The age data for the no. YP182F), located closest to the gabbro Sangju monzonite and granite were compa- Permian–Triassic metamorphic and magmatic (<1 km), contained an abundance of newly

rable in their zircon εHf(t) values (–9.1 ± 0.7 events are compiled in Table 2. grown zircon grains. The maximum age of Tri-

and –8.4 ± 1.2). The εHf(t) values of the Pa- Metamorphic ages reported from the Imjin- assic metamorphism was determined by Cho et leoproterozoic inherited cores from the gran- gang Belt are rare. Ree et al. (1996) reported al. (2017a) (245 Ma) on the basis of zircon ages ite ranged from 3.7 to –3.7. Zircons from the an earliest Triassic Sm-Nd age (249 ± 31 Ma) from basement rocks in the Odaesan area (Oh et Yeongdeok granodiorite showed little varia- using hornblende, garnet, and plagioclase sepa- al., 2006a; Cho, 2014). In fact, however, the Pa- tion in O and Hf isotopic compositions (δ18O = rated from a Neoproterozoic amphibolite com- leoproterozoic spinel granulite and migmatitic

5.69 ± 0.26‰, εHf(t) = 11.1 ± 0.8). Their sig- prising the southern part of the belt (the Samgot gneiss in this area yielded highly scattered zir-

nificantly high εHf(t) values are consistent unit). This age was later confirmed by SHRIMP con rim dates ranging from 265 Ma to 230 Ma

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Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/131/3-4/609/4651432/609.pdf by guest on 01 October 2021 Chang-sik Cheong et al. TABLE 2. SUMMARY OF GEOCHRONOLOGICAL DATA FOR THE PERMIAN–TRIASSIC METAMORPHIC AND MAGMATIC EVENTS REPORTED FROM CENTRAL AND SOUTHERN KOREA Locality Rock type Method Result Source (Ma) Imjingang Belt and Nangrim Massif Metamorphic age Yeoncheon Amphibolite Mineral Sm-Nd 249 ± 31 Ree et al. (1996) Yeoncheon Garnet-biotite gneiss Zircon U-Pb 252.9 ± 1.9Cho et al. (2005) Magmatic age Tokdal Biotite syenite Zircon U-Pb 224 ± 4Peng et al. (2008) SW Nangrim Massif Kimberlite Phlogopite Rb-Sr 223.3 ± 6.6Yang et al. (2010) Gyeonggi Massif Metamorphic age Daeijak IslandTonalitic gneiss Allanite U-Th-Pb 229 ± 2Kim et al. (2009) Daeijak IslandTonalitic gneiss Allanite U-Th-Pb 215 ± 4Kim et al. (2009) Daeijak IslandTonalitic gneiss Allanite U-Th-Pb 227 ± 7Kim et al. (2009) Daeijak IslandTonalitic gneiss Allanite U-Th-Pb 213 ± 4Kim et al. (2009) Hongseong Amphibole-bearing granitic gneiss Zircon U-Pb 229 ± 10 Kim et al. (2008b) Hongseong Biotite granitic gneiss Zircon U-Pb 235 ± 8Kim et al. (2008b) Hongseong Eclogitic amphibolite Zircon U-Pb 231 ± 3Kim et al. (2006) Hongseong Maﬁ c granulite Zircon U-Pb 236 ± 5Kim et al. (2011c) Hongseong Maﬁ c dyke Zircon U-Pb 233 ± 4Kim et al. (2008b) Hongseong Porphyroblastic orthogneiss Zircon U-Pb 237 ± 5Kim et al. (2008b) Hongseong Porphyroblastic orthogneiss Zircon U-Pb 236 ± 6Kim et al. (2008b) Mount CheonggyeBiotite quartzofeldspathic gneiss Zircon U-Pb 232 ± 7Cho et al. (2017a) Mount CheonggyeBiotite quartzofeldspathic gneiss Zircon U-Pb 241 ± 5Cho et al. (2017a) Mount CheonggyeBiotite quartzofeldspathic gneiss Zircon U-Pb 228 ± 15 Cho et al. (2017a) Mount CheonggyeCordierite-garnet-biotite gneiss Monazite U-Pb 232 ± 2Cho et al. (2017a) Mount CheonggyeBiotite quartzofeldspathic gneiss Monazite U-Pb 235 ± 2Cho et al. (2017a) Mount CheonggyeBiotite quartzofeldspathic gneiss Monazite U-Pb 231 ± 2Cho et al. (2017a) Yangpyeong Cordierite-sillimanite-garnet-biotite gneiss Zircon U-Pb 235 ± 6Oh et al. (2015) Yangpyeong Sillimanite-garnet-biotite gneiss Zircon U-Pb 237 ± 4Oh et al. (2015) Hongcheon Augen gneiss Zircon U-Pb 226 ± 7Yengkhom et al. (2014) Chuncheon Amphibolite Titanite U-Pb 224 ± 14 Kim et al. (2008a) Hwacheon Amphibolitized granuliteMonazite U-Pb 223 ± 3Yi and Cho (2009) Hwacheon Amphibolitized granulite Allanite U-Th-Pb 232 ± 8Yi and Cho (2009) Hwacheon Amphibolitized granulite Allanite U-Th-Pb 229 ± 11 Yi and Cho (2009) Odaesan Biotite schist Zircon U-Pb 247 ± 6Cho (2014) Odaesan Spinel granulite Zircon U-Pb 245 ± 10 Oh et al. (2006a) Magmatic age Gwangcheon Monzonite Zircon U-Pb 229.5 ± 1.7This study Gwangcheon Monzonite Zircon U-Pb 227.3 ± 3.7Cheong et al. (2015a) HaemiSyenite Zircon U-Pb 229.6 ± 2.2This study HaemiGranite Zircon U-Pb 233 ± 2Choi et al. (2009) Taean Granite Zircon U-Pb 229.8 ± 2.2This study Seosan Granite Zircon U-Pb 224.2 ± 2.3This study Namyang Granite Zircon U-Pb 232.5 ± 1.5This study YangpyeongHornblende gabbro Zircon U-Pb 232.1 ± 1.5This study Yangpyeong Monzonite Zircon core U-Pb 238.4 ± 5.0Cheong et al. (2015a) Yangpyeong Monzonite Zircon U-Pb 228.0 ± 1.5This study YangpyeongSyenodiorite Zircon U-Pb 233.3 ± 1.3Yi et al. (2016) Hongcheon Carbonatite Monazite Th-Pb 232.9 ± 1.6 Kim et al. (2016) Yangyang Syenite Zircon U-Pb 233 ± 1Seo et al. (2015) Yangyang Syenodiorite Zircon U-Pb 229.9 ± 1.0Yi et al. (2016) Odaesan Pyroxene-mica gabbro Zircon core U-Pb 231.3 ± 1.3 Kim et al. (2011d) Odaesan Mangerite Zircon core U-Pb 231.9 ± 4.6Cheong et al. (2015a) Odaesan Mangerite Zircon rim U-Pb 227.1 ± 5.2Cheong et al. (2015a) Odaesan Mangerite Zircon U-Pb 231.2 ± 2.4This study Okcheon Belt Metamorphic age Turungsan unit Muscovite-chlorite-quartz schist Allanite CHIME251 ± 10 Adachi et al. (1996) Dukpyeongri Black slate Uraninite CHIME283 ± 26 Cheong et al. (2003) Dukpyeongri Black slate Whole-rock Pb-Pb 283 ± 33 Cheong et al. (2003) Dukpyeongri Black slate Whole-rock Pb-Pb 291 ± 13 Cheong et al. (2003) Poeun Black slate Uraninite CHIME281 ± 27 Cheong et al. (2003) Hwanggangri Formation Granitic gneiss pebble Zircon U-Pb 300 ± 40 Cho and Kim (2005) Miwon Pelitic schist Garnet step-leaching U-Pb 285 ± 12 Kim et al. (2007) Miwon Pelitic schist Garnet step-leaching U-Pb 276 ± 29 Kim et al. (2007) Miwon Quartz-hornblende-garnet schist Garnet step-leaching U-Pb 291 ± 41 Kim et al. (2007) Kyemyeongsan Metavolcanic rock Zircon U-Pb 259.7 ± 3.3 Kim et al. (2013b) Kyemyeongsan REE-rich gneiss Allanite Th-Pb 445–183 Cheong et al. (2015c) Magmatic age Ian Alkali granite Zircon U-Pb 219.3 ± 3.3Cho et al. (2008) Goesan Monzodiorite Zircon U-Pb 231.0 ± 1.3 Kim et al. (2011a) Cheongsan K-feldspar megacryst-bearing granodiorite Zircon U-Pb 224.7 ± 1.8Yi et al. (2014) Baegnok Granodiorite Zircon U-Pb 225.6 ± 1.8Yi et al. (2014) Yeongnam Massif Magmatic age Yeongdeok Granodiorite Zircon U-Pb 250 ± 3Yi et al. (2012a) Yeongdeok Diorite xenolith Zircon U-Pb 261.3 ± 2.4Yi et al. (2012b) Yeongdeok Granodiorite xenolith Zircon U-Pb 265.5 ± 2.2Cheong et al. (2013) JangsariGabbro Zircon U-Pb 255.7 ± 1.4Yi et al. (2012a) JangsariGranite Zircon U-Pb 257.3 ± 2.0Yi et al. (2012a) Andong Granodioritic gneiss Zircon core U-Pb 262.4 ± 2.6Cheong et al. (2014) Andong Granodioritic gneiss Zircon core U-Pb 252.1 ± 1.1Cheong et al. (2014) Andong Orthopyroxenite Zircon U-Pb 222.1 ± 1.0Jeong et al. (2014) Cheongsong Trondhjemitic gneiss Zircon core U-Pb 251.2 ± 1.7Cheong et al. (2014) Sangju Monzonite Zircon U-Pb 224.3 ± 1.9This study Sangju Granite Zircon U-Pb 225.9 ± 1.4This study Daegang Alkali granite Zircon U-Pb 219.6 ± 1.9Cho et al. (2008) Macheon Monzonite Zircon U-Pb 232.4 ± 1.4This study Hamyang K-feldspar megacryst-bearing granodiorite Zircon U-Pb 226.8 ± 1.3This study Sancheong Syenite Zircon U-Pb 218.5 ± 1.0This study Hapcheon Mangerite Zircon U-Pb 227.4 ± 1.5This study HapcheonSyenite Zircon U-Pb 216.9 ± 0.9This study Kimcheon Granite gneiss Zircon U-Pb 252.2 ± 2.9Song et al. (2015) Kimcheon Granite gneiss Zircon U-Pb 241.7 ± 1.2Song et al. (2015) Note: REE—rare earth element; CHIME—chemical Th-U-total Pb isochron method.

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(Oh et al., 2006a). The zircon crystallization age oldest age is not clear yet, but the latter two in the Gyeonggi Massif and the Imjingang and of the Odaesan mangerite (232–227 Ma; Jeong ages—albeit scattered—may broadly represent Okcheon belts (ca. 285–250 Ma). This coin- et al., 2008; Cheong et al., 2015a; this study) the timing of crustal thickening orogeny and cidence may suggest that internal orogenic is close to the younger boundary of these rim subsequent paleo-Pacific plate subduction. Kim event(s) transferred shortening to the margin of dates. Oh et al. (2006a, page 570) stated that “it et al. (2013b) also reported a Permian metamor- the Yeongnam Massif, although there remains is not possible to distinguish undisturbed analy- phic age (259.7 ± 3.3 Ma), based on SHRIMP uncertainty about what specific orogeny was ses from those possibly affected by Pb loss or U-Pb analyses of zircon rims from a Neopro- primarily responsible for subduction initiation. diffusion of older Pb from the cores into the terozoic volcanic rock in the Kyemyeongsan The temporal correspondence between interior overgrowths.” Although the median values of area. Cho et al. (2018) attributed the thrusting collisional and external accretionary orogenesis the zircon rim dates adopted by Oh et al. (2006a) event between the Pibanryeong and Poeun units was well established by a previous study of (245 ± 10 and 248 ± 18 Ma) are consistent with to the middle-late Permian (ca. 270–250 Ma) Phanerozoic supercontinents (Cawood and Bu- the later zircon rim age from unconformably “Ogcheon Orogeny.” They distinguished this chan, 2007). overlain biotite schist (247 ± 6 Ma; Cho, 2014), event from the successive Middle-Late Trias- it is noted that this age was based on data from sic “Songrim Orogeny” related to the North Source Characterization by Zircon Data only three spots, and thus the thermal effect can- and South China collision. Further geochrono- not be sufficiently evaluated. The Yangpyeong logical, petrological, and structural studies are The age pattern and isotopic signature of in- and Odaesan cases suggest that at least part of required to confirm this hypothesis. herited zircons provide first-hand information the Triassic ages reported from the Gyeonggi The Paleozoic–Mesozoic metamorphism in regarding the magma source deep in the crust Massif represent local, post-collisional thermal central Korea is broadly synchronous with mag- (Hawkesworth and Kemp, 2006; Roberts and events. Alternatively, the magmatic overprint matic and thermal events that occurred in the Spencer, 2015). In this study, we found inher- may have induced a partial loss of radiogenic Pb. Yeongnam Massif. The arc magmatism occurred ited zircon cores from five granitoid samples If this is the case, the zircon rim dates from the extensively along the margin of the Yeongnam (Seosan and Namyang granites from the west- Odaesan area (265–230 Ma; Oh et al., 2006a) Massif in the earliest Triassic (ca. 250 Ma), as ern Gyeonggi Massif, Yangpyeong monzonite leave the possibility that the crustal thickening represented by the Yeongdeok adakite (Yi et al., from the central Gyeonggi Massif, and Hamy- event commenced in the Gyeonggi Massif dia- 2012a), and sodic metagranitoids and ortho ang granodiorite and Sangju granite from the chronously (i.e., in the late Permian and in the gneisses in the adjacent area (Cheong et al., Yeongnam Massif). Their age patterns are Middle-Late Triassic). 2014; Song et al., 2015). The initiation of arc shown in Figure 9, together with those of the Regional structures in the metamorphic rocks magmatism may be traced back to the middle inherited zircon cores from the Jurassic granit- of the Okcheon Belt are considered to have been Permian, considering the SHRIMP zircon ages oids in the Gyeonggi Massif and the Okcheon governed by large-scale thrust faults. Cluzel et (266–261 Ma) of diorite-granodiorite xenoliths Belt (Jo et al., 2018). As shown in this figure, the al. (1990) divided the metamorphosed south- found in the Yeongdeok pluton and Andong to- age pattern of the zircon inheritance differs pro- western part of the belt (Okcheon Metamorphic nalitic gneiss (Yi et al., 2012b; Cheong et al., foundly between the western Gyeonggi Massif Belt [OMB]) into five thrust-bounded lithotec- 2013, 2014). An even older, middle Paleozoic and the Yeongnam Massif. The Neoproterozoic tonic units: the structurally overlain units of arc magmatism was suggested from the margin (0.83–0.65 Ga) component is predominant Pibanryeong and Chungju and the lower ones of the Yeongnam Massif on the basis of single in the former with subordinate Paleoprotero- of Turungsan, Poeun, and Iwharyeong. The age populations of detrital zircons (ca. 330– zoic (2.3–2.1 Ga) and Mesoproterozoic (1.4– isotopic imprint of metamorphism is relatively 310 Ma, Kim et al., 2012; ca. 430–370 Ma, 1.1 Ga) populations. In contrast, the Orosirian weak in the OMB, probably due to the moderate Cheong et al., 2015b) as typically displayed by (ca. 1.85 Ga) component is overwhelming in the peak temperature conditions (490–630 °C; Cho zircons from arc-flanking basins (Cawood et al., latter, which is also the case for the Yangpyeong and Kim, 2005). Adachi et al. (1996) reported 2012). It is noted that many detrital and inher- monzonite and Middle Jurassic (177–167 Ma) an earliest Triassic chemical Th-U-total Pb iso- ited zircons from the Paleoproterozoic basement granitoids in the interior Gyeonggi Massif. chron method (CHIME) age of 251 ± 10 Ma gneisses in the northeastern Yeongnam Massif Meanwhile, the Early Jurassic (194–184 Ma) for allanite in muscovite-chlorite-quartz schist and overlying Cambrian supracrustal rocks (the granitoids in the central Okcheon Belt have a in the Turungsan unit (traditionally referred to Myeonsan Formation) have experienced severe comparable inherited zircon age pattern with the as the Munjuri Formation). On the other hand, Pb loss events (Kim et al., 2013a, 2014). Some Seosan and Namyang granites analyzed in this early Permian ages (ca. 290–280 Ma) were ob- of them yielded a cluster of lower intercept study. To summarize, whereas the ca. 1.85 Ga tained by whole-rock Pb isotopic and uraninite U-Pb ages at ca. 380 Ma (Kim et al., 2014), sug- zircon inheritance is predominant in the interior CHIME data of black slates in the Poeun unit gesting a possible link between the Pb loss event Gyeonggi and Yeongnam massifs, the Neopro- (Cheong et al., 2003). The early Permian ages and the middle Paleozoic arc magmatism. These terozoic inheritance is prominent in the western were later confirmed by U-Pb isotope data of zircon data collectively indicate that the margin Gyeonggi Massif and the Okcheon Belt. The acid step-leaching experiments conducted on of the Yeongnam Massif has been activated re- former is typically observed in North China, pelitic and quartz-hornblende-garnet schists of currently during the Paleozoic. Further studies in association with the collision between the the Pibanryeong unit (Kim et al., 2007). Alla- are necessary to understand the linkage between eastern and western blocks (Zhao et al., 2002), nite Th-Pb dating for REE ores distributed in the the orogenic event and arc magmatism that oc- while the latter is more popular in South China Chungju unit (traditionally referred to as the Ky- curred in the Paleozoic. related to the assembly and breakup of the Ro- emyeongsan Formation) revealed the presence In conclusion, as graphically summarized in dinia supercontinent (Li et al., 2003). of multiple age components in the Late Ordovi- Figure 8, the middle Permian to earliest Trias- The exotic feature of the protolith of the Early cian (445 ± 8 Ma), Permian to Triassic (ca. 300– sic (ca. 265–250 Ma) arc magmatism in the Jurassic granitoids was further highlighted by 220 Ma), and Early Jurassic (199–183 Ma) Yeongnam Massif is coeval with or slightly the significantly lowδ 18O values 4.9 to –0.9‰) (Cheong et al., 2015c). The significance of the younger than the metamorphic events recorded of Cryogenian (ca. 750–700 Ma) zircon cores

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Tectonic Imjingang Belt & Western Central-eastern Okcheon Belt Yeongnam Massif Time unit scale (Ma) Nangrim Massif Gyeonggi Massif Gyeonggi Massif 200

L 220 TRIASSIC 240 M E L

MESOZOIC 260 Metamorphic age

G Allanite U-Th-Pb Allanite CHIME Garnet step-leaching U-Pb Mineral Sm-Nd 280 Monazite U-Th-Pb Titanite U-Pb Uraninite CHIME

PERMIAN Whole-rock Pb-Pb Zircon U-Pb C Magmatic age W E 300 Monazite U-Th-Pb Phlogopite Rb-Sr Zircon U-Pb C, Cisuralian; G, Guadalupian; L, Lopingian; E, EARLY; M, MIDDLE; L, LATE.

Figure 8. Age compilation of Permian–Triassic metamorphic and magmatic events reported from central and southern Korea. Open and closed symbols represent metamorphic and magmatic events, respectively. Note that the zircon age from the Odaesan area (Oh et al., 2006a) is represented by 265–230 Ma, instead of the median value (see text). Data sources are given in Table 2. The geological time scale on the y-axis is after Walker et al. (2012). CHIME—chemical Th-U-total Pb isochron method.

(Jo et al., 2018). Neoproterozoic rocks and cores in the former granitoid group have chon- ous zircon domains in the Taean and Daegang

zircons from South China, especially from the dritic or positive εHf(t) values. This range can- granites were not considered here because the northern margin of the Yangtze Block, com- not be explained by the evolution of the North correlation between U contents and δ18O val- monly have δ18O values substantially lower than China crust, which has a zircon Hf model age ues indicates the post-crystallization interac- the normal mantle range (5.3 ± 0.3‰; Valley typically between 3.4 and 2.7 Ga (Geng et al., tion of metamictized parts with hydrothermal et al., 1998), possibly resulting from the high- 2012; Kim et al., 2014), but is most strongly fluids (see Gao et al., 2014). The significantly

temperature reaction of their source rocks with correlated with εHf(t) values of the Neoprotero- negative zircon εHf(t) values of the Yangpyeong glacial water (Zheng et al., 2008, and references zoic (780–750 Ma) rocks in the northern and gabbro and the Odaesan mangerite (–18 to –24) therein). Jo et al. (2018) therefore interpreted western margins of the Yangtze Block in South make it evident that the chemical fractionation that the geochronological and oxygen isotopic China (~11–2; Liu et al., 2009; Zheng et al., of the lithospheric mantle beneath the interior contrasts of inherited zircon cores observed 2007). Jo et al. (2018) suggested that mixing Gyeonggi Massif occurred in the distant past. between the Early and Middle Jurassic granit- between this Neoproterozoic component with This antiquity is also evidenced by a substan-

oids resulted from selective contributions from near chondritic εHf (t = Jurassic), and the Ar- tially negative εNd(t) (~ –26) value of monazite South and North China-like terranes that had chean–Paleoproterozoic component with sig- from the Hongcheon carbonatite (Kim et al.,

been juxtaposed infracrustally along the margin nificantly negative εHf (t = Jurassic) resulted 2016). The exact mechanism and timing of this of the Gyeonggi Massif during the prior colli- in a diverse range of Hf isotopic compositions event are difficult to ascertain, but broad geo- sional orogeny. of the magmatic zircons in the Early Jurassic chemical arc affinities of the gabbro and man- 18 The zircon εHf(t) and δ O values of the Trias- granitoids. In summary, the inherited zircon gerite (Fig. 7B) and the Re-Os isotopic study of sic plutons are plotted against their correspond- data of the Triassic and Jurassic granitoids Lee and Walker (2006) imply that it occurred in ing crystallization ages in Figure 10. In this collectively indicate that the infracrustal base- the Paleoproterozoic time under an arc environ- figure, O and Hf isotope data for the inherited ments of the interior Gyeonggi and Yeongnam ment. Probably, the lithospheric mantle beneath zircon cores from the Jurassic granitoids (Jo massifs and the marginal parts of the Gyeonggi the Nangrim Massif also formed in geologi- et al., 2018) are also shown for reference. Al- Massif (the western Gyeonggi Massif and the cally ancient time, considering the significantly

though the Cryogenian oxygen isotopic feature Okcheon Belt) were composed of North China- negative whole-rock εNd(t) values of the Tokdal was not observed in the Neoproterozoic zircon and Yangtze-like terranes, respectively. The Complex (–14 to –20; Peng et al., 2008). The cores from the Triassic Seosan and Namyang conspicuous lack of a ca. 1.85 Ga zircon in- metasomatized lithospheric mantle origin of the granites (δ18O = 8.4–6.9‰), zircon Hf isotopes heritance in the Seosan and Namyang granites Yangpyeong gabbro and Odaesan mangerite is reveal the fundamental difference in the evolu- suggests that the basement crust beneath the consistent with their zircon δ18O values (7.49– tionary history of the crustal source between western Gyeonggi Massif consisted of solely 6.94‰) being far from the normal mantle range. granitoid rocks in the western Gyeonggi Mas- Yangtze-like rocks during the Late Triassic. The Neoarchean to Paleoproterozoic zircon in- sif and the Okcheon Belt, and those in the in- Oxygen and hafnium isotope data of syn- heritance and core-to-rim increase in δ18O ob- terior Gyeonggi and Yeongnam massifs. As magmatic zircons from the Triassic plutons are served from some synmagmatic zircons in the shown in Figure 10B, Neoproterozoic zircon shown in Figure 11. The data from uranifer- Yangpyeong monzonite suggest the interaction

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10 between the lithospheric mantle and basement Triassic granitoids in western Gyeonggi Massif (n=21) 9 crust, of which Hf isotopic coupling is indicated Triassic granitoids in central-eastern Gyeonggi Massif (n=5) by comparable εHf(t) values of the zircon cores 8 and rims. As seen in Figure 11, the Gwangcheon mon- 7 zonite and the Haemi syenite in the western Gyeonggi Massif have distinctly higher zir- 6 18 con εHf(t) (–11 to –16) and δ O (8.5–7.6‰) 5 values than those of mafic-intermediate rocks in Yangpyeong and Odaesan, suggesting the

Number 4 fundamental difference of mantle lithosphere between the two areas. Synmagmatic zircons 3 from the granites in the western area (Seosan, 2 Namyang, and Taean) have wide variations in 18 εHf(t) (–6 to –21) and δ O (9.8–7.0‰) val- 1 ues, encompassing the ranges of zircon values from the adjacent Gwangcheon monzonite 0 and Haemi syenite. The Hf isotopic range of 12 the source crust could be estimated using the Triassic granitoids in Yeongnam Massif (n=17) inherited zircon data in the granites. For example, as could be assumed in Figure 10B, the 10 age and Hf isotope data of the Neoproterozoic

zircon cores yield moderately negative εHf(t 8 = 230 Ma) values (–6.3 ± 2.2) for the Neo- proterozoic component in the source crust if the average crustal 176Lu/177Hf ratio (0.0116; 6 Rudnick and Gao, 2003) is employed. When the typical 176Lu/177Hf ratio of the mafic crust Number (0.024) is adopted, the εHf(t = 230 Ma) of the 4 source crust increases to –1.7 ± 2.5. These

values are comparable with the highest εHf(t) of synmagmatic zircon in the Seosan granite

2 (–5.8). Much lower εHf(t = 230 Ma) values are calculated for the Mesoproterozoic (–8 to –17 or –1 to –8, depending on the assumed Lu/Hf 0 ratio) and Paleoproterozoic (–32 to –38 or –16 12 to –21) components. This calculation shows Early Jurassic granitoids (n=40) that the Hf isotopic variation of synmagmatic Middle Jurassic granitoids (n=21) zircons in the Seosan and Namyang granites 10 is attributable to the contribution from diverse age components in the source crust. The cor- 8 relation between εHf(t) and Lu/Hf observed in synmagmatic zircons from the Taean and Na- myang granites suggests that the mixing of 6 the crustal components was accompanied with magmatic differentiation (i.e., DePaolo, 1981).

Numbe r The oxygen isotopic evolution of the source 4 crust is more difficult to trace, because low- and high-temperature surface processes could modify the original value significantly (Valley, 2 2003; Bindeman, 2008; Jo et al., 2016). On the other hand, oxygen and hafnium isotope data of synmagmatic zircons from the 0 0 500 1000 1500 2000 2500 3000 3500 4000 older magnesian group in the Yeongnam Mas- Age (Ma) sif reflect the contribution of diverse mantle and crustal reservoirs under arc setting. The litho- Figure 9. Histograms of concordant (<10% discordancy) dates for inherited spheric mantle component could be best repre- zircon cores from the Triassic and Jurassic plutons in the Korean Penin- sented by zircons from the Andong pyroxenites sula, using data from this study and previous works (Williams et al., 2009; that formed magmatically in a suprasubduction Kim et al., 2011b; Cheong et al., 2015a; Jo et al., 2018). zone (Whattam et al., 2011). Their δ18O values

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12 (~7.1‰) are in the range of zircon values from the central-eastern Gyeonggi Massif. However,

10 the near chondritic εHf(t) values of the zircons indicate that the chemical fractionation in the lithospheric mantle source beneath the Yeong- 8 nam Massif occurred much more recently than in the case of the interior Gyeonggi Massif, Mantle range 6 possibly during the Paleozoic in association with continued subduction as suggested by the zircon data presented by Kim et al. (2012, 4 10 2013a, 2014) and Cheong et al. (2015b). Zir- 8 con data for the Yeongdeok adakite represent 2 O (‰ V-SMOW) 6 the isotopic composition of arc crust derived

18 4 from slab melting. As most adakite samples δ 0 2 show (Bindeman et al., 2005), the Yeongdeok A 0 220 230 240 zircons have oxygen isotopic compositions close to the normal mantle range. Their εHf(t) –2 values are plotted around the evolutionary path of the “arc mantle” projecting from the present- 20 Mafic crust day Hf isotopic composition of island arc rocks Average continental crust (Dhuime et al., 2011) (Fig. 10B). The magmatic reworking of this arc mantle-like Yeongdeok 10 Depleted mantle adakite was indicated by the time-εHf trend of Arc mantle the zircons from the Late Cretaceous granitoids 0 composing the platform of the Gyeongsang arc Chondritic Uniform system (Cheong and Jo, 2017). The contribu- Reservoir Mafic crust tion of the Paleoproterozoic crust is evidenced –10 by the presence of coeval inherited zircon cores ε Hf in the Hamyang granodiorite and the Sangju

10 granite. The negative array of zircon εHf(t) and –20 0 δ18O values from the older magnesian plutons (Fig. 11) could be explained by the mixing of Average continental crust-10 these three mantle and crustal components: -20 –30 the metasomatized lithospheric mantle as rep- -30 B 220 230 240 resented by the Andong pyroxenite (zircon δ18O = 7.1 ± 0.3‰, ε = 6.2 to –0.6), young –40 Hf arc crust as represented by the Yeongdeok ada- 0 500 1000 1500 2000 2500 3000 3500 4000 18 kite (zircon δ O = 5.7 ± 0.3‰, εHf > 10), and Age (Ma) the Paleoproterozoic basement crust (δ18O > Western Gyeonggi Massif 7.7‰, εHf < –10) contributing most intensively Gwangcheon Haemi Taean Seosan Namyang to the Hamyang granodiorite and the Sangju Central-eastern Gyeonggi Massif granite. The addition of Precambrian basement Yangpyeong (gabbro) Yangpyeong (monzonite) Odaesan rocks into the magma source of the Hamyang Yeongnam Massif granodiorite was probably accompanied with Macheon Hamyang Hapcheon (mangerite) Sangju (monzonite) Sangju (granite) magmatic differentiation, considering the cor- Yeongdeok Andong Daegang Hapcheon (syenite) Sancheong relation of synmagmatic zircons between ε (t) Jurassic granitoids (Jo et al., 2018) Hf Early Jurassic granitoids Middle Jurassic granitoids and Lu/Hf. Zircons from the younger ferroan plutons in 18 Figure 10. Plots of zircon (A) δ O and (B) εHf versus crystallization ages for the Yeongnam Massif have their own distinct the Triassic and Jurassic plutons in the Korean Peninsula, using data from this O and Hf isotopic compositions. The low δ18O

study and Jo et al. (2018). Synmagmatic zircon data from the Triassic plutons (~5.1‰) and highly positive εHf(t) (~8) of zir- are plotted in the enlarged inset figures. (A) The dashed lines indicate theδ 18O cons from the Hapcheon syenite, higher range range for the mantle zircon (5.3 ± 0.3‰; Valley et al., 1998). (B) The depleted of δ18O (8.4–6.5‰) and lower but consistently

mantle evolutionary path was extrapolated from the average modern-day val- positive εHf(t) (~3) of zircons from the San- ues of 176Hf/177Hf = 0.28325 and 176Lu/177Hf = 0.0384 (Griffin et al., 2000). Also cheong syenite, and intermediate δ18O (~6‰)

shown is the evolutionary path of “arc mantle” suggested by Dhuime et al. and slightly positive or negative εHf(t) (1.2 to (2011). The lines from the bottom upward indicate the modeled evolutionary –5.2) of zircons from the Daegang alkali gran- arrays of the average crustal reservoir (176Lu/177Hf = 0.0116; Rudnick and Gao, ite may reflect the dominance of asthenospheric 2003) and a typical mafic crust (176Lu/177Hf = 0.024) extracted from 2.5 Ga and mantle, lithospheric mantle, and lower crustal 1.5 Ga. V-SMOW—Vienna standard mean ocean water. sources, respectively.

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20 due. The coeval occurrence of carbonatite and Normal mantle range silica-undersaturated potassic and ultrapotassic rocks (Peng et al., 2008; Yang et al., 2010; Kim 10 et al., 2016) manifests that mantle materials were intensively involved in the post-collisional magma. Detachment models to explain the asthenospheric upwelling and its impingement 0 on the lithospheric mantle surface include the convective thinning and removal of the litho-

ε Hf spheric mantle (Houseman et al., 1981; Turner –10 et al., 1992) and slab breakoff (Davies and von Blanckenburg, 1995). Of the two models, the slab breakoff may better explain the limited range in time and space for the occurrence of –20 post-collisional plutons in central Korea, as previous works have suggested (Oh et al., 2006b; Seo et al., 2010). In this model, the breakoff –30 of subducted oceanic slab results in upwelling of the hot asthenosphere through the breakage 45678910 11 and consequent melting of the metasomatized 18O (‰ V-SMOW) part of the overriding lithospheric mantle which δ has relatively low solidus temperatures. As il- lustrated in Figure 12B, it is suggested that the 18 Figure 11. Plot of εHf and δ O values for synmagmatic zircon domains from the slab breakoff not only triggered the generation Triassic plutons in the Korean Peninsula. Note that the data for high-U zircon of potassic/ultrapotassic magmas in and around spots in the Taean and Daegang granites are not considered here. The typical the Gyeonggi Massif but also facilitated the range of the normal mantle value (t = 230 Ma) is shown for reference. Symbols reinitiation of magmatism in the Yeongnam are the same as in Figure 10. V-SMOW—Vienna standard mean ocean water. Massif. It is notable that the Macheon monzonite is uniquely shoshonitic in the magnesian Yeongnam plutons (Fig. 6C). Calc-alkaline arc magma then intruded further inland in parts of Triassic Tectonomagmatic Evolution reinitiation of magmatism was synchronous the Yeongnam Massif and the Okcheon Belt with the commencement of potassic and car- at ca. 225 Ma (Sangju monzonite-granite and The geochronological, geochemical, and bonatite magmatism (ca. 233 Ma) in and around Cheongsan-Baegnok granodiorite; see Fig. 2 for O-Hf isotopic data described above allow us the Gyeonggi Massif (Haemi, Namyang, Yang- locations). to propose a tectonomagmatic model depicted pyeong, Hongcheon, Yangyang, Odaesan, and The geochemical composition of the 222 Ma schematically in Figure 12. This model starts Goesan; see Fig. 2 for locations) (Table 2). This Andong ultramafic rocks is consistent with their with the generation of adakite and sodic mag- correspondence may suggest that the Late Tri- formation in a suprasubduction zone (Whattam mas along the Yeongnam Massif at ca. 250 Ma. assic magmatism along the Yeongnam Massif et al., 2011), possibly in response to the sinking The Yeongdeok adakite pluton is believed was related to the post-collisional event on the of the old, cold, and dense oceanic lithosphere, to have been produced under a hot subduction other side. which left a gap fed by melts flowing upwards regime possibly triggered by ridge subduction The Late Triassic plutons in and around the from the asthenosphere shortly after the initia- (Yi et al., 2012a) (Fig. 12A). The subduction of Gyeonggi Massif intruded in a relatively short tion of subduction (Stern and Bloomer, 1992). the ridge and consequent development of a slab time interval (<10 Ma). Mafic to intermedi- If this hinge rollback model is correct, the oc- window may have facilitated partial melting of ate silicate plutons among them are consider- currence of the Andong ultramafic complex is lower crustal rocks that produced the protolith ably enriched in K and Ba, and are predomi- indicative of the initiation of a new extension- magma of sodic metagranitoids in the adjacent nantly metaluminous, alkalic to alkali-calcic, dominated arc system. The narrow age range Andong-Cheongsong area. Their REE pattern and magnesian (Figs. 6 and 7). This rock type, (220–217 Ma) and consistent geochemical pe-

and zircon εHf(t) range (–0.3 ± 2.4) support this classified as “Caledonian type” (Pitcher 1983), culiarities of the ferroan granitoids in the Yeong- interpretation (Cheong et al., 2014). The subduc- “post-orogenic granitoids” (Maniar and Piccoli nam Massif and Okcheon Belt (Ian-Daegang tion of the young and hot ridge likely increased 1989), “shoshonitic granitoids” (Duchesne et alkali granites and Sancheong-Hapcheon sy- the coupling between the subducting and over- al., 1998), “K-feldspar porphyritic calc-alkaline enites) suggest their common tectonic setting. riding plates (i.e., Murphy et al., 1998). Under granitoids” (Barbarin, 1999), or “appinite” suite Ferroan granitoids reflect a close affinity to dry, this advancing arc system, magmatism may (Murphy, 2013), is commonly associated with reduced magmas that mainly, albeit not exclu- have ceased or migrated inboard. Based on our the post-collisional relaxation (Bonin et al., sively, form in extensional environments (Frost new SHRIMP zircon results, after the emplace- 1998). As shown in Figure 7A, they invariably et al., 2001; Bonin, 2007). The arc root that had ment of the Yeongdeok adakite at ca. 250 Ma, display LREE-enriched normalized patterns evolved during the continued magmatism at arc magmatism was temporally shut off in the without prominent Eu anomalies, indicating >250–224 Ma likely became negatively buoy- Yeongnam Massif until the intrusion of the Ma- the presence of garnet or amphibole, but the ab- ant as the mafic-ultramafic cumulate piles at the cheon monzonite at 232 Ma. Interestingly, this sence of plagioclase feldspar in the source resi- base of the crust attained mineral equilibration

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A ~ 250 Ma

GM YDA THC OMB ACT YM 18 εHf: –0.3 ± 2.4 δ O: 5.7 ± 0.3 ‰ εHf: > +10 Oceanic crust

Lithospheric mantle

Asthenosphere

tonalite-trondhjemite-granodiorite-granite gabbro-pyroxenite-mangerite-monzonite-syenite B 233-224 Ma

δ18O: 9.8 ~ 7.0 ‰ ε : –6 ~ –21 Hf 18 δ O: > 7.7 ‰, εHf: < –10 MCM Arc crust

δ18O: 8.5 ~ 7.6 ‰ εHf: –11 ~ –16 δ18O: 7.6 ~ 6.4 ‰ δ18O: 7.1 ± 0.3 ‰ ε : –18 ~ –26 Hf εHf: +6 ~ –1

C 222-217 Ma

18 δ O: 6.5 ~ 5.7 ‰ AUC εHf: +1 ~ –5

Figure 12.

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Figure 12. Schematic cartoons summarizing under eclogite facies. We postulate that the for- cally North China-like and allochthonous South the Triassic tectonomagmatic evolution of the mation of the ferroan granitoids resulted from China-like terranes, respectively. Korean Peninsula, with oxygen and hafnium the eventual foundering of such an eclogitized (4) The Late Triassic K- and Ba-rich, metalu- isotopic compositions of mantle and crustal crustal base at 220–217 Ma (Fig. 12C). Consid- minous, alkalic to alkali-calcic, and magnesian reservoirs. (A) In this model, the Yeongdeok ering the limited occurrence of the ferroan gran- plutons in the Gyeonggi Massif were derived adakite (ca. 250 Ma) was produced by slab itoids, the size of the drips must have been quite from lithospheric mantle and basement crust. melting in association with the ridge sub- small, which is the case in many areas suspected Zircon O-Hf data indicate ancient (most prob- duction. The development of a slab window to have undergone recent delamination (Ducea, ably Paleoproterozoic) chemical fractionation facilitated the formation of sodic magmas 2011, and references therein). This presumed of the lithospheric mantle beneath the interior (i.e., the Andong-Cheongsong tonalite and lithospheric delamination and consequent in- Gyeonggi Massif. The fundamental difference trondhjemite) from the lower crust. (B) The trusion of the Daegang and Ian A-type granites of lithospheric mantle between the western and subduction of the hot ridge increased the cou- were virtually coeval with the onset of an ex- central-eastern Gyeonggi Massif is evidenced pling between the subducting and overriding tensional arc system indicated by the occurrence by contrasting Hf isotopic compositions of syn- plates. Consequently, after the emplacement of the Andong ultramafic complex, suggesting magmatic zircons from the mafic-intermediate of the Yeongdeok adakite, the arc magma- a close link between the two tectonic events. It plutons. The Late Triassic granites in the west- tism was shut off in the Yeongnam Massif is also notable that the emplacement of the An- ern Gyeonggi Massif were derived from the until the intrusion of the Macheon monzonite dong complex was only slightly younger than Yangtze-like basement crust composed of di- at 232 Ma. This reinitiation of magmatism the development of the Gyeonggi Shear Zone verse age components with significantly differ- was synchronous with the commencement (ca. 225 Ma; Kim et al., 2000) that may have ent Hf isotopic compositions. of post-collisional magmatism in and around resulted from a gravitational collapse of thick- (5) Geochronological, geochemical, and the Gyeonggi Massif. During or after the con- ened crust. This temporal correspondence indi- O-Hf isotopic data collectively indicate that tinental collision, the subducted oceanic slab cates that the Gyeonggi and Yeongnam massifs during the Triassic, the Korean Peninsula ex- broke off and the asthenosphere upwelled shared an extensional environment in the late perienced complex tectonomagmatic events through the breakage. The consequent melt- stage of Triassic tectonomagmatic evolution. consisting of: (1) ridge subduction and gen- ing of the lithospheric mantle produced potas- eration of high-silica adakite and sodic granit- sic and shoshonitic magmas. Calc-alkaline arc CONCLUSIONS oids along the margin of the Yeongnam Massif magma then intruded further inland in parts (ca. 250 Ma), (2) development of an advancing of the Yeongnam Massif and the Okcheon This geochronological, geochemical, and arc system and magmatic quiescence, (3) post- Belt. Three mantle and crustal sources are O-Hf isotopic study of the Triassic plutons in collisional slab breakoff and consequent potas- recognized for the older magnesian plutons in central and southern Korea reached the follow- sic/ultrapotassic magmatism in and around the the Yeongnam Massif: metasomatized litho- ing conclusions. Gyeonggi Massif and shoshonitic magmatism spheric mantle as represented by the Andong (1) The emplacement of ca. 265–250 Ma in the Yeongnam Massif (ca. 230 Ma), (4) inland pyroxenite, young (probably Paleozoic) arc calc-alkaline granitoids in southeastern Ko- migration of arc magmatism until 224 Ma, and crust as represented by the Yeongdeok adakite, rea is coeval with or slightly younger than the (5) tectonic switch to the extension-dominated and Paleoproterozoic basement crust. The late Barrovian metamorphism (ca. 285–250 Ma) arc system and delamination of an overthick- Triassic post-collisional plutons in the interior recorded in the Neoproterozoic–Paleozoic ened arc lithosphere that resulted in the forma- Gyeonggi Massif and in the Taean-Hongseong fold-and-thrust belts in central Korea. This tion of the ferroan plutons in the Yeongnam Complex (present-day western Gyeonggi Mas- temporal correspondence suggests a close link Massif (222–217 Ma). sif) formed by the selective melting of an an- between the collisional orogenesis and subduc- cient metasomatized lithospheric mantle and tion initiation. ACKNOWLEDGMENTS an allochthonous South China-like lithosphere, (2) The Late Triassic mangerite-monzonite- We are grateful for the company of Yong-Sun Song respectively. (C) At 222 Ma, the arc system syenite-granodiorite-granite plutons emplaced and Kye-Hun Park during the sample collection in switched to an extension-dominated environ- in the Yeongnam Massif could be divided into the Yeongnam Massif. Sook Ju Kim and Guo-Qiang ment, along with the formation of the Andong two geochemically distinct age groups: the Tang helped in the laboratory work. Insightful reviews by Moonsup Cho and Jacob Mulder improved the ultramafic complex. The ferroan plutons in the older (232–224 Ma) magnesian and alkali- manuscript substantially. We also appreciate the care- Yeongnam Massif formed in association with calcic to calc-alkalic group and the younger ful editorial handling from Rob Strachan. This work the eventual foundering of an eclogitized crustal (220–217 Ma) ferroan and alkalic to alkali- was supported by a Korea Basic Science Institute base at 220–217 Ma. Zircon O-Hf isotope data calcic group, which were temporally intervened grant (C38709) and a National Research Foundation of Korea grant funded by the government of Korea’s for the ferroan plutons reflect the contribu- by a geochemically arc-like ultramafic complex Ministry of Science and ICT (2016R1A2B4007283), tions from asthenospheric mantle, lithospheric (222 Ma). Zircon O-Hf isotope data indicate awarded to Albert Chang-sik Cheong. mantle, and mafic lower crust. The Gyeonggi that the former was generated by the mixing and Yeongnam massifs may have shared an of the relatively young (most likely Paleozoic) APPENDIX 1. ANALYTICAL METHODS extensional environment in the late stage of metasomatized lithospheric mantle and arc crust SHRIMP Zircon U-Pb Dating Triassic tectonomagmatic evolution. GM– and Paleoproterozoic crust, whereas the latter – Gyeonggi Massif; THC–Taean-Hongseong formed through the melting of asthenospheric/ The primary O2 beam was focused into a ~25-µm- Complex; OMB–Okcheon Metamorphic Belt; lithospheric mantle and lower crust. diameter spot at an accelerating voltage of 10 kV. The YM–Yeongnam Massif; YDA–Yeongdeok (3) The inherited zircon age patterns indi- collector slit width was fixed at 100 µm, achieving a mass resolution of ~5000 at 1% peak height. FC1 adakite; ACT–Andong-Cheongsong tonalite cate that the infracrustal basements of the inte- (1099 Ma; Paces and Miller, 1993) and SL13 (U = and trondhjemite; MCM–Macheon monzo- rior Gyeonggi and Yeongnam massifs, and the 238 ppm) standard zircons were used for Pb/U cali- nite; AUC–Andong ultramafic complex. western Gyeonggi Massif are composed of typi- bration and to determine U abundance, respectively.

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Pb/U ratios were calibrated against FC1 according to 6.3.5.5 software (Paton et al., 2011), and corrected Chappell, B.W., and White, A.J.R., 1992, I- and S-type the power law relationship between Pb+/U+ and UO+/ for the background, laser-induced elemental frac- granites in the Lachlan Fold Belt, in Brown, P.E., and U+. Th/U ratios were estimated using a fractionation tionation, and mass discrimination. All ratios were Chappell, B.W., eds., The Second Hutton Symposium on the Origin of Granites and Related Rocks: Geologi- factor derived from the measured 232Th16O+/238U16O+ calculated with 2σ errors. During the sample analy- 208 206 cal Society of America Special Papers, v. 272, p. 1–26, versus Pb/ Pb of the SL13 standard. The common sis, 91500 and FC1 zircons were repeatedly analyzed https://doi.org/10.1130/SPE272. 207 Pb was removed by the Pb (for spots <1000 Ma) or at the beginning and end of each analytical session, Cheong, A.C.S., and Jo, H.J., 2017, Crustal evolution in 204Pb (for spots >1000 Ma) correction method (Wil- and at regular intervals during each session. The run- the Gyeongsang Arc, southeastern Korea: Geochrono- liams, 1998) using the model of Stacey and Kram- ning results of zircon standards (91500 and FC1, see logical, geochemical and Sr-Nd-Hf isotopic constraints ers (1975). Data processing was conducted using the Table DR4) were consistent with the recommended from granitoid rocks: American Journal of Science, SQUID 2.50 and Isoplot 3.75 programs (Ludwig, values in the literature (Griffin et al., 2000; Kemp et v. 317, p. 369–410, https://doi.org/10.2475/03.2017.03. 2008, 2009). Weighted mean ages were calculated af- al., 2010) within uncertainties. 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