Jurassic Igneous Activity in the Yuseong Area on the Southern Margin of the Gyeonggi Massif, Korean Peninsula, and its Implications for the Tectonic Evolution of Northeast Asia during the Jurassic

: Jurassic dioritic to granitic igneous rocks extensively intrude into the southern Korean Peninsula, including the Yuseong area located at the boundary between the southern margin of the Gyeonggi Massif and the northern margin of the Okcheon Belt. In this study, the petrogenesis and sources of Jurassic igneous rocks in the Yuseong area were investigated. The U–Pb zircon age data from the Jurassic plutonic rocks in the Yuseong area give two igneous ages, ca. 178–177 Ma and 169– 168 Ma, indicating that two stages of igneous activity occurred in the Yuseong area during the Ju ‐ rassic. The geochemical characteristics of Jurassic diorites indicate that they originated from en ‐ riched mid ‐ ocean ridge basalt (E ‐ MORB; Nb/Yb = 5.63–7.27; Zr/Yb = 118–156). The enriched Th/Yb ratios (5.5–8.0) in the diorites imply that they experienced crustal contamination during magma ascent. The Jurassic granitoids in the Yuseong area are divided into I ‐ and S ‐ type granites. The Ju ‐ rassic I ‐ type granitoids may have formed via the partial melting of mafic rocks with mixtures of 10– 40% pelite ‐ derived melt, while the S ‐ type granites originated from felsic pelite. The Jurassic diorites have low Nb/Th ratios with depletion of the Nb and Ta components, indicating that they formed in a volcanic arc tectonic environment. On the other hand, the Jurassic granitoids show two different tectonic environments: a volcanic arc, and a syncollisional environment. The granites with syncol ‐ lisional character are S ‐ type granites, and may give incorrect information about tectonic setting be ‐ cause of the changes in the trace elements of the S ‐ type granite due to fractional crystallization. Early Jurassic (200–190 Ma) igneous rocks are distributed only in the southeastern Korean Penin ‐ sula, including the Yeongnam Massif; Jurassic igneous rocks formed at ca. 190–180 Ma occur mainly in the Okcheon Belt and southern Gyeonggi Massif, which includes the Yuseong area. Middle Ju ‐ rassic igneous rocks widely intruded from the Okcheon Belt, through the Gyeonggi and Nangrim massifs in the Korean Peninsula, to the Liaoning area in the North China Craton at 180–160 Ma. This distribution pattern of the Jurassic granitoids suggests that flat subduction started after 180 Ma in Northeast Asia.


Introduction
In Northeast Asia, Mesozoic igneous rocks, including Jurassic granitoids, are widely distributed. Therefore, it is important to study Jurassic igneous activities in the Korean Peninsula in order to understand the tectonic evolution of the Korean Peninsula and Northeast Asia during the Jurassic Period. According to previous studies on the Jurassic

General Geology and Petrography
The basement of the Korean Peninsula is composed of four main Precambrian massifs: the Kwanmo, Nangrim, Gyeonggi, and Yeongnam massifs, from north to south. The Macheonryeong system and Imjingang Belt are located between the Kwanmo and Nangrim massifs and between the Nangrim and Gyeonggi massifs, respectively [14]. The Okcheon Belt is located between the Gyeonggi Massif and the YM, and divided into the Okcheon Metamorphic Belt and the Taebaeksan Basin. The Okcheon Metamorphic Belt is a fold and thrust belt, with a northeastern trend mainly composed of Neoproterozoic metamorphosed volcano-sedimentary rocks and Paleozoic metasedimentary rocks, while the Taebaeksan Basin consists of Paleozoic sedimentary rocks [1,2,15]. The Permo-Triassic collision event between the NCC and the South China Craton may have triggered the beginning of the subduction zone in which the Paleo-Pacific plate subducted beneath the Northeast Asian plate along the southeastern margin of Northeast Asia, which includes the southern margin of the South China Craton and the YM in the Korean Peninsula [16,17]. According to previous studies, the Jurassic igneous rocks in the South China Craton formed due to subduction of the Paleo-Pacific plate beneath the South China Cratons [18][19][20][21]. In particular, Zhou and Li (2000) [20] suggested that the wide distribution of Mesozoic igneous rocks in southeastern China was caused by low-angle subduction of the Paleo-Pacific plate. The Jurassic igneous rocks in northeast China are also considered to have formed via flat subduction on the Paleo-Pacific plate [13,22]. The Jurassic plutons in southwestern Japan also intruded in a subduction tectonic setting [23,24], and Japan was connected to the Korean Peninsula during the Jurassic. Egawa (2013) [23] noted that the several Jurassic basins in the Korean Peninsula were developed via thrust movement with crustal shortening, caused by flat subduction of the Paleo-Pacific plate beneath the Northeast Asian continent. Thus, these previous studies indicate that the subduction zone was developed along the eastern margin of Northeast Asia, including the Korean Peninsula. These subduction-related arc igneous activities also occurred in the Korean Peninsula, and continued until the Middle Jurassic, ca. 160 Ma [3,12]. The Late Triassic and Early Jurassic igneous rocks mainly intruded in the YM and Okcheon Belt, whereas Middle Jurassic igneous rocks are widely distributed not only in the Okcheon Belt and the Gyeonggi Massif in the southern Korean Peninsula, but also in the northern Korean Peninsula and northeastern NCC [1,3,[10][11][12]25,26].
The Triassic igneous rocks in the YM include alkali granite, hornblende-biotite granodiorite, and biotite granite, and they are considered to have intruded in an arc tectonic environment [1,11]. The Jurassic igneous rocks are mainly diorite, granodiorite, two-mica granite, and biotite granite with small diorite enclaves [2,3,12]. Some igneous rocks intruded during the Triassic and Early Jurassic have foliated textures formed by right-lateral strike-slip movement along the Honam shear zone [2,27,28]. The Honam shear zone is considered a dextral ductile shear zone forming several mylonite zones [1,29]. The rightlateral movement of the Honam shear zone may have started from 180-178 Ma, based on zircon overgrowth ages of 180-178 Ma using sensitive high-resolution ion microprobe (SHRIMP) U-Pb analysis [30].
The study area in Yuseong is located at the boundary between the Gyeonggi Massif and the Okcheon Belt in the southern Korean Peninsula (Figure 1a). The basement rocks in the Yuseong area mainly consist of Paleoproterozoic biotite gneiss and schist. The Paleozoic metasedimentary rocks unconformably overlie the basement, and are composed of mica schist and quartzite. The study area was intruded by Mesozoic igneous rocks such as diorite, biotite granite, two-mica granite, and granophyre ( Figure 1b). Most Mesozoic granitoids are Jurassic granitoids that are medium-to coarse-grained, and consist of quartz, plagioclase, alkali feldspar, biotite, and muscovite, with fewer opaque minerals. Some granitoids include alkali feldspar megacrysts or small-scale mafic enclaves. The diorite is almost medium-grained, and is composed mainly of plagioclase, amphibole, biotite, and alkali feldspar.

Analytical Methods
The Jurassic granitoids and diorite collected from the Yuseong area were crushed to the point at which 90% passed through a 10-mesh sieve, and were then split to yield ~250 g samples. Each split sample was pulverized to the point at which 95% passed through a 200-mesh sieve, in order to provide a homogeneous and representative sample for analysis. The whole-rock major and trace element chemistries of the Jurassic granitoids and diorites were analyzed using inductively coupled plasma atomic emission spectroscopy (ICP-AES; Termo Jarrel Ash ENVIRO II) and inductively coupled plasma mass spectrometry (ICP-MS; Perkin Elmer Optima 3000) at Activation Laboratories Ltd., Ancaster, Canada. The results are shown in Table 1. The zircon grains were separated using standard crushing, water-based panning, magnetic separation, and heavy liquid techniques, in order to analyze the U-Pb isotopic composition of zircon in the Jurassic granite and diorite. The separated zircon grains were handpicked and mounted with standard zircons of 91,500 (1062.6 ± 1.2 Ma) and Plesovice (337.6 ± 0.69 Ma) in an epoxy disk. The internal textures of the zircon grains were examined by cathodoluminescence (CL) images obtained using scanning electron microscopy with a JEOL JSM-6610 LV instrument. The in situ zircon ages were analyzed by LA-MC-ICP-MS at the Ohchang Center, Korea Basic Science Institute (KBSI), with a Nu Plasma II MC-ICP-MS and a NWR193UC (193 nm) laser ablation system. Mean 238 U/ 206 Pb ages of the Jurassic zircons were calculated by assuming that the analyses were concordant. Common Pb was removed via the 207 Pb correction method, using the model used by Stacy & Kramers (1975) [32]. The zircon U-Pb age data are summarized in Tables S1 and S2.

Geochronology
The zircons in diorite YS170616-1E are 100-200 μm in grain size, with length to width ratios of approximately 1:1 to 3:1. These zircons show mostly rounded and elongated shapes with banded and concentric zoning patterns, representing their igneous origin.

Whole-Rock Geochemistry
The Jurassic diorites in the Yuseong area in the boundary between the Okcheon Belt and the Gyeonggi Massif have geochemical characteristics comparable to typical diorite compositions ( Figure 3a; SiO2 = 58.7-60.9 wt%, Na2O + K2O = 6.1-6.8 wt%). The granitoid that formed at 178 Ma plots in the granodiorite field area (SiO2 = 69.3 wt%, Na2O + K2O = 6.3 wt%), and the other granitoids that formed at 169 Ma plot in the quartz monzonite and granite field areas (SiO2 = 67.7-74.7 wt%, Na2O + K2O = 8.0-9.8 wt%) (Figure 3a). On the other hand, the Early Jurassic (200-190 Ma) igneous rocks in the YM reported by Kee et al. [1] and Kim et al. [30] plot in the fields of gabbro, gabbroic diorite, diorite, granodiorite, syenite, and granite. All Jurassic igneous rocks in the Yuseong area are calc-alkaline series in the AFM diagram, and they plot in the high-K calc-alkaline series-except for the 178 Ma granodiorite, which shows a medium-K calc-alkaline series in the SiO2 vs. K2O diagram (Figure 3b,c). The Early Jurassic igneous rocks in the YM plot in the medium-K to high-K calc-alkaline series, and one of them plots in the shoshonitic series field. All Jurassic granitoids in the Yuseong area show peraluminous character, whereas Early Jurassic granitoids plot in the metaluminous and peraluminous fields in the A/CNK vs. A/NK diagram (Figure 3d). The Jurassic igneous rocks in the Yuseong area are plotted in several tectonic discrimination diagrams, together with Early Jurassic igneous rocks in the YM, to investigate the tectonic setting of Jurassic igneous activities. The Jurassic diorite in the Yuseong area has Nb/Th and Zr/Nb ratios comparable to those of typical arc-related basalt, implying a volcanic arc tectonic setting, and plots in the arc-basalt and calc-alkaline basalt (CAB) fields in the Hf/3-Th-Ta diagram, together with the Early Jurassic mafic igneous rocks (Figure 4a  The Jurassic granitoids in the Yuseong area plot in the volcanic arc granite (VAG) or syncollisional granite (Syn-COLG) fields in the Rb vs. Y + Nb diagram (Figure 5a). On the other hand, all Jurassic granitoids in the Yuseong area correspond to the typical igneous rocks formed in an arc-related tectonic setting, based on the Zr vs. Y diagram (Figure 5b). Two different tectonic settings for the granites in the Rb vs. Y + Nb diagram will be further discussed in the discussion in Section 5.2. The Early Jurassic granitoids in the YM plot mostly in an arc tectonic setting in the above two diagrams, but some of them plot in the syncollisional granite field in the Rb vs. Y + Nb diagram, and in the within-plate field in the Zr vs. Y diagram. The Middle Jurassic granitoids in the southern Korean Peninsula also show mostly typical arc signatures, but some of them plot in the syncollisional and within-plate granite fields in the Rb vs. Y + Nb diagram. The Middle Jurassic granitoids, including the Yuseong Jurassic granitoids, plot in the I-, S-, M-, and A-type granite fields in the K2O + Na2O vs. 10,000Ga/Al and Nb vs. 10,000Ga/Al diagrams, and they plot in the fractionated I-and S-type granite fields in the Zr vs. 10,000Ga/Al diagram (Figure 5c-e). The Early Jurassic granitoids in the YM plot mostly in the normal I-, S-, and M-type fields, and some of them plot in the A-type field (Figure 5c-e). The granodiorite that formed at 178 Ma, along with one of the 168 Ma granitoids which plots in the quartz monzonite field, plot in the I-type granite field area in the A/CNK vs. SiO2 diagram, and two granites that formed at 168 Ma plot in the S-type granite field (Figure 5f). All of the Early Jurassic granitoids are distributed in the I-type granite field area, whereas the Middle Jurassic granitoids in the southern Korean Peninsula plot both in the I-type and S-type granite fields as the Yuseong Jurassic granitoids (Figure 5e,f).  (Figure 6a,c). The Jurassic diorites in the Yuseong area and the gabbro and intermediate igneous rocks in the YM show negative Nb and Ta anomalies, with moderate negative Ti anomalies in the primitive mantle-normalized spider diagram (Figure 6b). The early Jurassic gabbro in the YM shows more depleted character than the diorite (Figure 6a,b). In the primitive mantle-normalized diagram, the I-type granitoids in the Yuseong area show weaker negative anomalies of Nb than do the Early Jurassic granites in the YM (Figure 6d). Compared with the I-type granitoids in the Yuseong area and the YM, granites with S-type characteristics in the Yuseong area have strong negative Eu anomalies in the chondrite-normalized REE diagram, and stronger negative Ba and Ti anomalies in the primitive mantle-normalized diagram. The Middle Jurassic granitoids show stronger negative anomalies of Eu and Ti, and weaker negative anomalies of Nb and Ta components, compared with the Early Jurassic granitoids (Figure 6c,d)

Petrogenesis of the Jurassic Diorite in the Yuseong Area and the YM
The Jurassic diorites in the Yuseong area display high-K calc-alkaline characteristics (Figure 3b,c). Their Nb/Th and Zr/Nb characteristics are comparable to those of igneous rocks formed in an arc tectonic setting (Figure 4a), and enrichments in large-ion lithophile elements (LILEs; e.g., Rb and Ba), along with significant depletions in high-field-strength elements (HFSEs)-such as Nb and Ta-also indicate an arc tectonic environment [1,44]. These Jurassic diorites have higher Mg# (Mg# = molar Mg 2+ /(Mg 2+ + Fe 2+ ) × 100) (47-50) and MgO (4.0-5.9 wt%) contents than those in the typical melt from mafic lower crust, implying their origin from the mantle instead of the crust [45,46] (Figure 7a,b). In the Zr/Yb vs. Nb/Yb and Y/Yb vs. Nb/Yb diagrams, including the mid-ocean ridge basalt (MORB) array, the Jurassic diorite shows enriched characteristics, with an E-MORB-like signature instead of normal mid-ocean ridge basalt (N-MORB) or ocean island basalt (OIB) characteristics (Figure 4c,d). In particular, the higher Nb/Yb (5.6-7.3) and Zr/Yb (118-156) ratios in the diorite, compared with those of depleted asthenosphere-derived magma (N-MORB) (Nb/Yb = 0.8, Zr/Yb = 24.3; McDonough and Sun [43]), suggest that the diorite did not originate from depleted asthenospheric mantle. The early Jurassic gabbro and intermediate igneous rocks in the YM show trace element characteristics similar to those in the Yuseong area, such as Zr/Nb, Nb/Th, Zr/Yb, Y/Yb, and Nb/Yb ratios, and they are also considered to have formed via the melting of enriched mantle sources [1,3].  (Figure 7c), the Jurassic diorite plots above the MORB array, with high Th/Yb ratios of 5.5-8.0, which also supports the significant crustal contamination of source magma. Additionally, the existence of inherited zircons showing Paleoproterozoic ages in the diorite also indicates crustal assimilation. The early Jurassic gabbro and intermediate igneous rocks in the YM also show elevated Th/Yb and (Th/Yb)PM ratios, implying crustal contamination, but they do not seem to have experienced a high degree of contamination, because their (Th/Yb)PM values are lower than 20. To calculate the degree of crustal contamination for the diorites in the Yuseong area, mass balance modeling was carried out using the trace element ratios of possible contaminants and primary magma sources. Most enriched Paleoproterozoic gneisses in the southern Gyeonggi Massif reported by Lee et al. [49] were chosen as possible contaminants because they were expected to be basement rocks in the Yuseong area, and the E-MORB component suggested by McDonough and Sun [43] was chosen as the source material. The results show that the Jurassic diorite in the Yuseong area formed from an E-MORB-like enriched mantle source, with 10-20% crustal assimilation by Paleoproterozoic gneiss (Figure 7e,f). The Jurassic diorite in the Yuseong area also shows relatively high Ce/Th (6.9-7.9) and Ba/Th (107-130) ratios, which are similar to those of subducted sediment-derived melts (Ce/Th = ~8, Ba/Th = ~111; Plank and Langmuir [50]), suggesting that the source magma of Jurassic diorite underwent metasomatism caused by subducted sediment-derived melt or fluid. Therefore, it is difficult to exclude a minor effect of mantle metasomatism during the formation of the diorite.

Petrogenesis of the Jurassic Granite in the Yuseong Area and YM
In the 10,000Ga/Al vs. Zr diagram, Jurassic granitoids in the Yuseong area have geochemical characteristics of I-and S-type granites with different degrees of fractionation. They are further classified in the A/CNK vs. SiO2 diagram (Figure 3d). The granitoids in the Yuseong area with A/CNK values of 1.22-1.25 are classified as S-type granites, but other granitoids with A/CNK values of 1.03-1.07 can be classified as I-type granites.
The difference in trace element patterns between the I-and S-type granitoids in the Yuseong area also supports their different origins. Strong negative anomalies of Ba, Eu, Sr, and Ti components in the trace element patterns are observed in the S-type granites, suggesting that the magma for S-type granites underwent fractional crystallization of plagioclase-, K-feldspar-, and Ti-bearing oxide minerals (Figure 6c,d). On the other hand, Itype granitoids in the Yuseong area do not show depletions in Eu or Sr, and show weak negative anomalies of P and Ti. These features suggest that the I-type granitoids in the Yuseong area experienced no fractional crystallization of feldspars, and weak fractional crystallization of P-and Ti-bearing oxides, such as amphibole and garnet. In the C1 chondrite-normalized REE diagrams, the I-type granitoids show more enrichment of LREEs and depletion of HREEs than the S-type granites.
The chemical differences between I-and S-type granitoids in the Yuseong area may be attributed not only to fractional crystallization, but also to different sources. To identify the source material of the Jurassic granitoids in the Yuseong area, they were plotted in the CaO/Al2O3 vs. CaO + Al2O3 and Rb/Ba vs. Rb/Sr diagrams. The S-type granites in the Yuseong area plot in the felsic pelite source field, and their high Rb/Sr and Rb/Ba ratios also suggest that they formed via the melting of plagioclase-poor and clay-rich pelites (Figure 8a,b). Generally, I-type granitoids can be formed via the partial melting of both mantle and lower crust [55][56][57], and fractional crystallization of mantle-derived magmas during the underplating process [58,59]. The Mg# values of I-type granitoids (16)(17) in the Yuseong area show crustal-like signatures, because these values are significantly lower than the mantle composition (40; Rapp and Watson [60]). In the CaO/Al2O3 vs. CaO + Al2O3 diagram, the 179 Ma granodiorite-which is one of the I-type granitoids in the Yuseong area-plots in the amphibolite and basalt source field, while the 168 Ma quartz monzonite showing I-type characteristics plots in the areas between the amphibolite and basalt source field and the felsic pelite source field (Figure 8a). In the Rb/Sr vs. Rb/Ba diagram, the I-type granitoids in the Yuseong area can be considered to have formed via mixing between basaltic-derived melt and pelite-derived melt; the mixing percentages of melt from basalts are approximately 90% and 60%, for the 179 Ma granodiorite and the 168 Ma quartz monzonite, respectively (Figure 8b). These data indicate that the I-type Jurassic granitoids in the Yuseong area formed from basalt-derived melt that was mixed with 10-40% magma derived from pelites. The crystallization temperature of the I-type granitoids was estimated to be 922-924 °C, based on the calculation of zircon saturation temperature suggested by Watson and Harrison [31]. These calculated crystallization temperatures would also support the melting of mafic rocks. The Early Jurassic granitoids in the YM mostly originated from basalt-derived melt with 10-40% mixing of pelite-derived melt, and only a few granitoids originated from pelite or metagraywacke compositions, whereas the Middle Jurassic granitoids, including the Yuseong Jurassic granitoids in the southern Korean Peninsula, originated from both basaltic-derived melt and felsic peliteor clay-rich-material-derived melt (Figure 8a,b). The changes in specific trace elements that indicate fractional crystallization, such as Rb and Ba, are not as noticeable in the Early Jurassic granitoids in the YM, compared with the Jurassic granitoids in the southern Korean Peninsula, including the Yuseong area (Figure 8c,d). The tendency of C1 chondrite and primitive mantle-normalized trace element patterns is also different between the Early Jurassic granitoids and the Middle Jurassic granitoids ( Figure 6). Furthermore, the Early Jurassic granitoids do not include S-type granites, but the Middle Jurassic granitoids do (Figure 5f). These geochemical characteristics of the Early Jurassic granitoids in the YM indicate that the Early Jurassic magmatism mainly formed via the melting of mafic lower crust instead of upper crust. On the other hand, the Middle Jurassic granitoids can be considered to have formed not only via the melting of mafic lower crust, but also via the melting of upper crust.
Based on the results in Sections 5.1 and 5.2, the following interpretations can be made: During the Early Jurassic, magma formed via the partial melting of mantle and of lower mafic crust, whereas during the Middle Jurassic, magma formed via the partial melting of mantle and of upper and lower crusts that had pelitic and mafic compositions, respectively. This difference indicates that there was more prolonged arc magmatism in the Yuseong area than in the YM due to flat subduction, which will be explained in the next section.

The Regional Evolution of the Subduction Process in Northeast Asia during the Jurassic
According to previous studies, it was reported that, along the margin of the Northeast Asia, including the Korean Peninsula, there was a subduction of the Paleo-Pacific plate during the Jurassic era. Park et al. (2019) [61] suggested that the Daebo Orogeny occurred via low-angle subduction of the Paleo-Pacific plate in the Korean Peninsula during the Jurassic, based on study of the structural and deformation age from the Mesozoic basin in the Korean Peninsula. The Early to Middle Jurassic igneous rocks which are widely distributed in the Korean Peninsula are also considered to have formed due to northwestern subduction of the Paleo-Pacific plate beneath Northeast Asia, based on their geochemical signature [1,[3][4][5][6]12]. These Jurassic igneous rocks were divided into two groups based on their emplacement age and spatial distribution: Early Jurassic (ca. 200-180 Ma) igneous rocks are mainly distributed in the YM, the Okcheon Belt, and the eastern margin of the Korean Peninsula, while Early to Middle Jurassic (ca. 180-160 Ma) igneous rocks are widely distributed in the Okcheon Belt, the Gyeonggi and Nangrim massifs in the Korean Peninsula, and the northeastern part of the NCC [1,[3][4][5][9][10][11][12]62]. Park et al. [4] suggested that the granite that formed at 190-180 Ma is concentrated in the Okcheon Belt and the southern Gyeonggi Massif, based on zircon U-Pb SHRIMP analysis and sphene U-Pb thermal ionization mass spectrometry (TIMS) analysis. Thus, the Jurassic igneous activities in the Korean Peninsula can be divided into three subgroups: (1) (Figure 9). These spatial distributions of Jurassic igneous rocks represent a northwestward trend of becoming younger on the Korean Peninsula, and the distribution area of Jurassic igneous rocks increased with time. This pattern can be interpreted as a result of the decreasing subduction angle of the subducting Paleo-Pacific plate from the Early to Middle Jurassic, resulting in flat subduction during the Middle Jurassic [1,6,10,23]. Although various petrological studies of the Jurassic igneous rocks in the Korean Peninsula have been conducted as discussed above, several problems remain in interpreting the tectonic evolution of Northeast Asia during the Jurassic period. Kee et al. [1] suggested inland propagation of the edge of the subducted plate, but they did not consider the Jurassic igneous rocks in the northern Korean Peninsula, NCC, or Japan. Cheong and Jo [6] reported that low-angle subduction of the Paleo-Pacific plate occurred during 190-170 Ma, and that the subduction angle became flat with the termination of igneous activity in the Korean Peninsula during 170-160 Ma. However, the Jurassic igneous rocks that formed at 170-160 Ma occur widely in the Korean Peninsula and the northeastern part of the NCC [3,4,9,10,12,13]. Therefore, it is necessary to interpret the evolution of Jurassic subduction in the Korean Peninsula more clearly, by considering Jurassic igneous rocks in the northern Korean Peninsula and Northeast Asia, including the northeastern NCC and Japan.
In Japan, igneous activity occurred during the Early Jurassic period of ca. 197-180 Ma, forming granodiorite and biotite granite [7,8,63], and Jurassic igneous activity stopped after ca. 180 Ma. However, Middle to Late Jurassic (177-148 Ma) igneous rocks are reported in the Liaoning area on the northern margin of the NCC [11,64], and in the Beijing area, which is located west of the Liaoning area [25,62]. The presence of Middle Jurassic age data in the eastern NCC indicates that ca. 177-160 Ma igneous activity occurred not only in the Korean Peninsula, but also in the eastern NCC. The Jurassic igneous activities in the Korean Peninsula finished at approximately 160 Ma, whereas they continued until ca. 148 Ma in the Liaoning area [25,62], and until ca. 140 Ma in the area near Beijing.
These data indicate that: (1)   Wu et al. [10] also reported that the northwestern younging trend of Jurassic magmatism finished around the Beijing area at 140 Ma, and that a reverse younging trend toward the southeast started from the Beijing area during the Cretaceous. Their interpretation suggests that the Jurassic flat subduction of the Paleo-Pacific plate continued until the edge of the flat subduction reached the Beijing area, and then the reverse younging trend of Cretaceous magmatism was caused by slab rollback of the flat subducted Paleo-Pacific plate. As a result, the magmatic gap increases southeastwards as the distance from the Beijing area increases; the magmatic gap in the Liaoning area is 148-126 Ma [11], and that in the Korean Peninsula is 160-110 Ma.
The Yuseong area is located at the boundary between the Okcheon Belt and the Gyeonggi Massif, which can be considered an arc front during flat subduction. As a result, igneous activity continued for a long time from 178 Ma to 168 Ma. During the early igneous stage, diorite and I-type granitoids intruded, whereas during the later igneous stage, diorite, I-type granitoids, and S-type granite intruded. The S-type granite can be interpreted as having formed due to prolonged arc activity, which finally caused melting of the upper crust.

Conclusions
(1) The Jurassic igneous rocks in the Yuseong area consist of diorite and granitoids, and they formed at 177-178 Ma and 168-169 Ma, indicating that there were two stages of igneous activity in the Yuseong area during the Jurassic. (2) The Jurassic diorite has geochemical characteristics that are representative of a volcanic arc tectonic setting. The geochemical characteristics of the Jurassic granitoids indicate two tectonic environments: a volcanic arc, and a syncollisional environment. The syncollisional character is not correct because it resulted from strong fractionation of plagioclase and alkali feldspar from the parental magma, which formed in a volcanic arc tectonic setting. These data indicate that the Jurassic igneous rocks in the study area intruded in an arc tectonic setting. (3) The Jurassic diorites originated from an E-MORB-type mantle source, and have relatively high Th/Yb ratios of 8.0-5.5, indicating that they underwent 10-20% crustal contamination during magma ascent. The Jurassic I-type granodiorite that formed at 178 Ma originated from basalt-derived melt, with 10% mixing of pelite-derived melt, and the I-type quartz monzonite that intruded at 168 Ma formed from basalt-derived melt that mixed with approximately 40% pelite-derived melt. The Jurassic S-type granite in the Yuseong area was formed from a pelite-derived melt source. (4) Flat subduction began to occur at 175 Ma under the northern Asian continent, and continued until 140 Ma.
Supplementary Materials: The following are available online at www.mdpi.com/2075-163X/11/5/466/s1, Table S1: LA-MC-ICPMS zircon ages of the Jurassic diorite in the Yuseong area, Table S2: LA-MC-ICPMS zircon ages of the Jurassic granitoids in the Yuseong area.  Data Availability Statement: All data analyzed during this study are included in this published article [ Table 1, Supplementary Tables S1 and S2]