Geochronology, magma source, and geochemistry of igneous rocks in the northern Taebaeksan metallogenic region, South Korea

Cretaceous igneous rocks are concentrated in the northern Taebaeksan metallogenic region of South Korea, some of which are related to Fe skarns and/or hydrothermal vein Au–Ag deposits. However, detailed studies on the emplacement age and magma source of igneous rocks supplying this metallogenic area are lacking. In this study, we investigated the emplacement age, magma sources, and geochemical characteristics of seven Cretaceous igneous rocks around the ore deposit, comparing them with previous studies. Zircon U–Pb and mica K–Ar age dating indicated that two magmatism events occurred in the Early Cretaceous (~113.7 ± 0.2 to 104.7 ± 0.5 Ma) and the Late Cretaceous (~85.8 ± 1.1 to 77.6 ± 0.4 Ma). Negative ɛHf(t) values (−2.57 to −22.05 approx.) and the calculated TDMC$$ {T}_{\mathrm{DMC}} $$ (2.55–1.33 Ga) suggested that the magma source of these igneous rocks was derived from the Proterozoic crust. Whole‐rock geochemical data indicated that the northern Taebaeksan igneous rocks are mostly high‐K calc‐alkaline series, enriched in light rare‐earth elements (LREEs) and large‐ion lithophilic (LIL) elements but depleted in high‐field‐strength (HFS) elements. Although igneous rocks in the northern Taebaeksan metallogenic region have broadly similar magma source characteristics, they can be classified into two groups: one comprises Early Cretaceous intermediate rocks, some of which are related to Au–Ag mineralization, whereas the other includes Late Cretaceous intermediate to felsic rocks, related to Au–Ag or Fe mineralization.


| INTRODUCTION
The Taebaeksan metallogenic region is one of the main metal mineralized zones in South Korea, hosting many Fe-polymetallic skarn and hydrothermal vein Au-Ag deposits.These ore deposits are located in and around Jurassic to Cretaceous igneous rocks that intrude into Paleozoic sedimentary rocks.Cretaceous igneous rocks are concentrated in the northern Taebaeksan metallogenic region (study area), some of which have been suggested to be Fe skarn and/or Au-Ag vein-related igneous rocks (Chang & Park, 1988;Chang & Chang, 1992;Lee & Park, 1994;Park & Lee, 1990;Park & Park, 1990).However, there is a lack of detailed studies on the emplacement age and magma source of the igneous rocks supplying this area with metallic elements.To understand the ore-forming processes in this area, the timing of magmatism and the source of magma must first be investigated.
In the past, studies on the emplacement age were conducted only on ore-related igneous rocks using K-Ar or U-Pb age dating methods in the study area (Lee & Park, 1994;Oh et al., 2022;Park et al., 2013;Park & Lee, 1990;Park & Park, 1990); however, no age dating analysis has been conducted on igneous rocks of unknown mineralization and/or infertility.Moreover, most K-Ar age dating results were retrieved from only a few samples (one or two).As K-Ar age dating techniques occasionally yield anomalous age dating results attributed to excess argon or lower closure temperature of the isotopic system, several measurements are required to enhance accuracy (Cao, Wu, et al., 2018;Kelly, 2002;Li et al., 2017).
This study focused on the emplacement age and magma source of igneous rocks in the northern Taebaeksan metallogenic region.We investigated ore-related igneous rocks, as well as igneous rocks of unknown mineralization and/or infertility, using zircon U-Pb age dating, Hf isotopic analysis, and whole-rock geochemical data.

| Regional geology
The Korean Peninsula is located at the northeastern margin of the Eurasian Plate, and the southern Korean Peninsula contains the Gyeonggi massif (GM), Ogcheon belt (OB), Yeongnam massif, and Gyeongsang basin (GB).The OB is subdivided into two zones: the Ogcheon (OZ) and Taebaeksan zones (TZ; Figure 1).The TZ consists mainly of Paleozoic sedimentary rocks.Lower Paleozoic sedimentary rocks are mainly shallow marine in origin and predominantly contain carbonates with smaller amounts of sandstones and shales, whereas upper Paleozoic sedimentary rocks include thick clastic successions deposited in marginal marine to non-marine environments (Cheong, 1969).The Yuli Group forms the basement of the TZ and comprises the Paleoproterozoic Buncheon granite gneiss, Weonnam Group (metasedimentary rocks), Hongjesa granite, Nonggeori granite, and Naedeokri granite (Lee et al., 2011).In this area (TZ), igneous rocks can be classified into Jurassic Daebo granites, Cretaceous Bulgugsa granites, and/or Cretaceous volcanic rocks (Chang et al., 1990;Jin et al., 1989;Pak et al., 2004).
The major tectonic evolution of the Korean Peninsula was initiated in the Triassic when the South China Block collided with the Sino-Korean Block (Cho et al., 1995;Ree et al., 1996).In the Jurassic, orthogonal subduction of the Pacific Plate caused compressional deformation with an extensive emplacement of granite batholiths (Chough et al., 2018).In the Cretaceous, the eastern margin of the Asian continent experienced left-lateral wrench tectonics induced by oblique subduction of the Pacific Plate, forming a number of continental back-arc basins (Chough et al., 2018;Chough & Sohn, 2010).Magmatism also occurred intensively during the Cretaceous period.
The northern Taebaeksan metallogenic region is located within the TZ and comprises the Choseon supergroup (lower Paleozoic sedimentary rocks; Figure 1).The Choseon supergroup comprises nine formations: the at a late stage.The igneous rock consists of quartz monzodiorite (DN-1), granite (DN-2), granodiorite, and dikes (Oh et al., 2022).Quartz monzodiorite (DN-1) intrudes into the Pungchon Formation and Myobong slate Formation and is lenticular-shaped.This igneous rock is exposed at the surface for approximately 0.12 km 2 and is divided by an NNE-trending fault from north to south.Granite (DN-2) intruded into the Pungchon Formation and quartz monzodiorite (DN-1).Granite (DN-2), suggested to be a Fe skarnrelated igneous rock, formed Fe-(Mo) skarn deposits (Oh et al., 2022).This igneous rock is exposed at the surface for about 0.2 km 2 and is lenticular-or vein-shaped.Some granodiorite and dikes intrude into the Pungchon and Hwajeol formations in the southeastern Dongnam area.The boundary between these granodiorites and surrounding igneous rocks is unclear, and the temporal and intrusive relationships are unknown.Zircon U-Pb dating of quartz monzodiorite (DN-1), granodiorite, and granite (DN-2) yield 206 Pb/ 238 U ages of 113.73 ± 0.21 Ma, 109.36 ± 0.36 Ma, and 80.33 ± 0.16 Ma, respectively (Oh et al., 2022).The mineralization age of 75.86 ± 1.7 Ma was obtained from phlogopite that had intergrown with magnetite using K-Ar age dating (Chang & Park, 1988).Based on the temporal and spatial relationships between the Fe-(Mo) skarn and igneous rocks, granite has been suggested to be an Fe-(Mo) skarnrelated igneous rock (Oh et al., 2022).

| Wondong (WD-1) area
The Wondong area is divided by a NE-trending thrust fault.The northern part of the area consists of the Cambrian Pungchon and Hwajeol formations.The southern part consists of the Ordovician quartzite Formation, Dumudong shale Formation, and Makdong Limestone Formation.The granite (WD-1) intruded the Makdong Limestone Formation.This igneous rock is exposed at the surface for about 0.7 km 2 and is sill-or vein-shaped.Based on the temporal and spatial relationships between the Fe-(W) skarn and igneous rocks, granite has been suggested as an Fe-(W) skarn-related igneous rock (Kim et al., 2015).Zircon U-Pb age dating using SHRIMP yielded 79.37 ± 0.94 Ma from this igneous (Park et al., 2013).

| Molwoon (MW-1, MW-2) area
The Molwoon area consists of the Cambrian-Ordovician Jangsan quartzite Formation, Myobong slate Formation, Pungchon Formation, Hwajeol Formation, Dongjeom quartzite Formation, Dumudong shale Formation, and Makdong limestone Formation, as well as igneous rocks that intruded at a late stage.These igneous rocks consist of quartzdiorite-quartz monzodiorite (MW-1), quartz monzonite, granodiorite (MW-2), porphyritic granite, and dikes (Park & Park, 1990).These igneous rocks intruded into the Myobong and Pungchon formations and formed barren skarn and hornfels near the contact zone with the wall rocks.These igneous rocks were exposed at the surface for approximately 0.37 km 2 .Biotite K-A rage dating yielded an age of 85.8 ± 1.12 Ma for the granodiorite in the Molwoon area (Park & Park, 1990).The mineralization age of 86.16 ± 2.37 Ma was obtained from K-Ar age dating (Park et al., 1988).This mineralization age is similar to the emplacement age of the igneous rock in this area.Park (2009) conducted an Sr-Nd isotope study on igneous rocks in the Molwoon area.These values were plotted for the continental crust field on the 87 Sr/ 86 Sr initial ratios versus the ϵNd(t) diagram.Based on the magma sources, it is inferred that the Molwoon igneous rocks were probably derived from the continental crust.
The igneous rocks of Yegdun B are located 2.5 km away from Yegdun A. These igneous rocks were exposed at the surface for approximately 0.07 km 2 .The Yegdun B area consists of the Cambrian Hwajeol Formation and igneous rocks that intruded later.These late igneous rocks consisted of diorite (YD-3) and granodiorite (YD-4) and formed a barren skarn around the igneous rocks.

| Jotan A (JT-1) and Jotan B (JT-2) area
The area of Jotan A consists of the Cambrian Myobong slate Formation, Pungchon Formation, and Hwajeol Formation, as well as igneous rocks that intruded at a later stage.These late igneous rocks consist of diorite, granodiorite (JT-1), granite, and rhyolitic rocks (Lee & Park, 1994).These igneous rocks intruded into the Pungchon and Hwajeol formations and created a very narrow barren skarn zone (<20 cm) within the igneous rocks.These igneous rocks were exposed at the surface for approximately 0.2 km 2 .K-Ar age dating yielded an age of 108 ± 2 Ma for the granodiorite in Jotan A (Lee & Park, 1994).
The igneous rocks of Jotan B are located 2 km southwest of Jotan A. These igneous rocks were exposed at the surface for approximately 0.04 km 2 .The area of Jotan B consists of the Cambrian-Ordovician Hwajeol Formation, Dongjeom quartzite Formation, and the Dumudong shale Formation, as well as granodiorite (JT-2) that intruded at a later stage, and they formed a barren skarn zone around the igneous rocks.

| Igneous rock samples
Eleven fresh and representative igneous rock samples were collected from the northern Taebaeksan metallogenic region (Figure 2, Table 1).For igneous rocks collected around the ore deposit, we analyzed the orerelated igneous rock and representative infertile igneous rock; other infertile igneous rocks suggested to be representative rocks as a result of the geological survey were also analyzed.These igneous rock samples were examined using microscopic observations to avoid the use of altered samples (Figure 3).Based on the mineral assemblage, these igneous rocks were classified into four groups: quartz monzodiorite (DN-1, MW-1), granite (DN-2, WD-1), diorite (YD-1, YD-3), and granodiorite (YD-2, YD-4, JT-1, JT-2, and MW-2; Figure 4).

| Whole-rock geochemistry
Nine fresh rock samples were collected for whole-rock major and trace element (including rare-earth elements) analyses (Table 2).Samples of Dongnam (DN-1, DN-2) referred to the previous analysis results (Oh et al., 2022).Major element compositions were determined using an x-ray fluorescence spectrometer (XRF) at the Activation Laboratories Ltd., Canada, and the Korea Institute of Geoscience and Mineral Resources (KIGAM), South Korea.The compositions of the trace elements, including REEs, were determined using inductively coupled plasma mass spectrometry (ICP-MS) at the Activation Laboratories Ltd., Canada.The powdered rock samples (approximately 0.5 g) were dissolved in an acid mixture at 190 C (on a hotplate) for 48 h prior to analysis.The analytical error was ±5%.

| Zircon U-Pb dating and Hf isotopic analyses
Except for the igneous rocks of the Dongnam (DN-1, DN-2) and Molwoon (MW-1, MW-2) area, which have a magma source, as indicated by Sr-Nd and Hf isotopic analyses (Oh et al., 2022;Park, 2009), we conducted zircon U-Pb dating and Hf isotopic analyses on seven igneous rocks (Tables 3 and 4).The rock samples were powdered, and the zircons were separated using heavyliquid and magmatic techniques.The separated zircon grains were mounted on epoxy resin cylinders, which were subsequently polished and coated with gold.In addition, cathodoluminescence (CL) images were obtained to check the internal textures of the zircons and to select the analysis spot using a scanning electron microscope at the Korea Basic Science Institute (KBSI), South Korea.Zircon ranges from approximately 30 to 400 μm in length.Most zircons were euhedral-subhedral shaped (Figure 5).The elongation (length-to-width) ratios, which reflect the crystallization velocity (Corfu et al., 2003), range from 1.0 to 2.5.In addition, the zircons exhibited well-developed magmatic zonation (e.g., oscillatory zonation, sector zonation).Some zircons exhibited inherited cores.In situ analyses were conducted at the rim of zircon that exhibited magmatic zonation to avoid inherited cores.
U-Pb age dating and Hf isotopic analyses were carried out using the New Wave NWR 193UC ArF excimer 193 nm laser, attached to a Nu Plasma II MC-ICP-MS (laser ablation multi-collector inductively coupled plasma mass spectrometry, LA-MC-ICP-MS) at KBSI, South Korea.Detailed analytical methods and isotope calculation methods are described by Li et al. (2021).In our experiment, U-Pb age dating of zircon was performed with a beam size of 15-20 μm, laser frequency  et al., 2002).DN-2 and WD-1 data are from Chang (1998) and Kim et al. (1983), respectively.
T A B L E 2 Major, trace element, and rare earth element compositions of the igneous rocks from the northern Taebaeksan metallogenic region.    of 5 Hz, dwell time of 30 s, and energy fluence of approximately 3.5 J/cm 2 .Hf isotopic analyses were performed with a beam size of 50 μm, laser pulse frequency of 10 Hz, dwell time of 60 s, and energy fluence of approximately 6.7 J/cm 2 .Both analyses were performed at the same time points.The standard material 91,500, with a reference value of 1065 Ma (Wiedenbeck et al., 1995) and 0.282306 ± 0.000008 (Woodhead & Hergt, 2005), was applied for quality control.The Plešovice standard, with a reference value of 337 Ma (Sl ama et al., 2008) and 0.282484 ± 0.000008 (Sl ama et al., 2008), was used for matrix-matched calibration.

| Whole-rock geochemistry
The whole-rock analysis results for the igneous rocks from the northern Taebaeksan metallogenic region are shown in Table 2.The SiO 2 content of the igneous rocks ranges from 54.4 to 77.1 wt%, and the K 2 O content varies from 2.6 to 6.5 wt%.In the SiO 2 -K 2 O diagram, the igneous rocks in the study area are mostly plotted in the high-K calc-alkaline, and Shoshonite field (Figure 6a).On the A/CNK-A/NK diagram, the igneous rocks are plotted in the metaluminous field, except for the highly fractionated granodiorites and granites (Figure 6b).In this diagram, some granodiorites with a high SiO 2 content, as well as granites, are plotted in the peraluminous field.The diorite and granodiorite are mostly plotted in the I-type granite field (A/CNK <1.1), whereas some granites are plotted in the I-and S-type granite fields (A/CNK = 1.0-1.4).On the Yb versus Ta tectonic diagram, igneous rocks are plotted in the volcanic arc granite (VAG) field (Figure 6c).Igneous rocks are characterized by a gabbroic diorite-monzodiorite-diorite- monzonite-granodiorite-quartz monzonite-granite composition and are mainly plotted on the subalkaline field (Figure 6d).The chondrite-normalized REEs and primitive mantle-normalized trace element patterns are shown in Figure 7.The igneous rocks showed similar chondritenormalized REEs and primitive mantle-normalized trace element patterns, except for Eu and Ba anomalies in the granites (Figure 7).

| Zircon U-Pb-Hf isotopes
The zircon U-Pb dating results are shown in Table 3. Except for the YD-3 igneous rock, which was separated into only six zircon grains, U-Pb analyses were conducted on all samples at 9-15 points.The U-Pb concordia diagrams (Figure 8) show zircon U-Pb ages reflecting the timing of the crystallization of the igneous rocks.For   4. Except for the YD-3 igneous rock, which was separated into only six zircon grains, Hf isotopic analyses were conducted on all samples at 7-15 points.We calculated the ϵHf(t) values and model ages (T DM , T DMC ) using zircon U-Pb ages.The 176 Hf/ 177 Hf ratios varied from 0.282086 to 0.282636, and the ϵHf (t) values varied from À2.57 to À22.05.Zircon model ages (T DM ) were calculated at 0.89-1.65Ga.

| DISCUSSION
Zircon, a prevalent accessory mineral in igneous rocks, is one of the most widely used minerals for U-Pb dating of magmatic events because of its resilience to weathering and ability to record geochronological, isotopic, and geochemical changes (Hoskin, 2005;Valley et al., 2010;Vincent et al., 2023).Recently, the geochronology and geochemistry of zircons have been increasingly used to investigate the origin of hydrothermal fluids and the genesis of polymetallic ore bodies (Jiang et al., 2019;Li, Cao, et al., 2019;Li et al., 2014;Li, Sun, et al., 2019;Pettke et al., 2005;Wang et al., 2017).Hf isotopic analysis using zircon provides us with the timing of separation from the depleted mantle (depleted mantle (DM) model age, T DM ), degree of mixing between the mantle and crust, degree of partial melting, and degree of fractional crystallization (Faure, 1986).
In this study, we obtained zircon U-Pb ages using LA-MC-ICP-MS.Previous studies on emplacement ages were conducted using the K-Ar dating method only in and around some deposits, except for the Dongnam and Wondong areas (DN-1, DN-2, WD-1).Oh et al. (2022) obtained a U-Pb age of 113.73 ± 0.21 Ma from quartz monzodiorite, 109.36 ± 0.36 Ma from granodiorite, and 80.33 ± 0.16 Ma from granite in the Dongnam area using LA-MC-ICP-MS.Park et al. (2013) obtained a U-Pb age of 79.37 ± 0.94 Ma from granite in the Wondong area using SHRIMP.Moreover, most of the previous K-Ar ages matched our zircon U-Pb age results well.Therefore, we could identify the overall emplacement age of the igneous rocks in the northern Taebaeksan metallogenic region.
Two emplacement age groups were identified from zircon U-Pb and mica K-Ar dating, which correspond to two periods of magmatism (Early Cretaceous and Late Cretaceous).Early Cretaceous igneous rocks contain diorite groups, such as diorite, quartz monzodiorite, and granodiorite.These igneous rocks yielded zircon U-Pb ages of 113.7 ± 0.2 to approximately 104.7 ± 0.5 Ma.Late Cretaceous igneous rocks contain granite groups, such as granite (quartz porphyry, rhyolite).These igneous rocks yielded zircon U-Pb ages of 80.3 ± 0.2 to approximately 77.6 ± 0.4 Ma.However, in a previous study, a K-Ar age of 85.8 ± 1.1 Ma was obtained from biotite in Molwoon granodiorite (MW-2; Park & Park, 1990).As we did not conduct zircon U-Pb dating on the Molwoon igneous rocks in this study, we could not verify this K-Ar age.These K-Ar age dating results were obtained from a single sample (Park & Park, 1990).Considering that most Late Cretaceous igneous rocks contain granite groups in the northern Taebaeksan metallogenic region, we suggest this K-Ar age dating result must be carefully reconsidered.However, if this result is correct, we can infer that the Late Cretaceous igneous rocks in the northern Taebaeksan metallogenic region are generally intermediate to felsic in composition.
In addition, the geotectonic discrimination diagrams suggest that igneous rocks in the northern Taebaeksan metallogenic region are related to the VAG.The chondrite-normalized REE patterns of these igneous rocks show slight enrichment of light REEs (LREEs) and depletion of heavy REEs (HREEs), having a similarity with the REE pattern of Ocean Island Basalts (OIB).In general, intermediate and felsic rock types have slopes declining from LREEs to HREEs, similar to the OIB curve (Turner, 2014).Diorite, quartz monzodiorite, and granodiorite are enriched with large-ion lithophilic elements (LILs), such as K, Rb, Th, and U, whereas highfield-strength (HFS) elements, such as Nb, Ti, P, and HREEs, are significantly depleted (Figure 7b).Granites also showed similar primitive mantle-normalized trace element patterns.These characteristics, together with their geotectonic discrimination, indicate that the source of these igneous rocks was mainly continental crust in a subduction environment (Almeida et al., 2007;Cao, Ye, et al., 2018;Karsli et al., 2017;Tchameni et al., 2006;Wilson, 1989).
The Sr/Y ratio is generally used to classify the adakitic characteristics of magmas (Richards & Kerrich, 2007).Figure 9 shows the whole-rock Sr/Y ratio versus the Y diagram (Figure 9a) and the whole-rock Sr/Y ratio versus wt% SiO 2 diagram (Figure 9b) for the igneous rock samples from the northern Taebaeksan metallogenic region.Most igneous rocks are plotted on the adakitic field except granites (DN-2, WD-1).According to the prevailing genetic models, adakitic magma is derived from partial melting of the lower continental crust, partial melting of subducted oceanic crust, or crustal assimilation and fractional crystallization of parental basaltic magmas (Castillo et al., 1999;Defant et al., 2002;Richards & Kerrich, 2007;Wang et al., 2017).The Sr/Y ratio in whole-rock analyses can also be used to assume the initial hydrous state of magma (Chiaradia et al., 2012;Loucks, 2014).During magma differentiation, elements such as Al and Sr partition strongly into plagioclase, whereas elements such as Y, Ti, and Sc partition strongly into hornblendes.The hydrous state of magma is an important factor in ore deposits.In more hydrous mafic-felsic magmatic differentiation series, Al 2 O 3 /TiO 2 and Sr/Y in whole-rock analyses may be expected to increase more strongly with increasing SiO 2 , whereas they may be expected to remain stable or decline with increasing SiO 2 in weakly hydrous or dry mafic-tofelsic differentiation series (Loucks, 2000).Thus, igneous rocks showing a high Sr/Y ratio with increasing SiO 2 are considered to be more hydrous.In Figure 9b, most igneous rocks are plotted on the relatively high Sr/Y ratio field, whereas the granites related to the Fe skarn (DN-2, WD-1) are plotted on the relatively low Sr/Y ratio field.
Furthermore, we conducted zircon Hf isotopic analyses and calculated ϵHf(t) values to study the magma source of the igneous rocks in the northern Taebaeksan metallogenic region.If the ϵHf(t) values are negative, then we can infer that the igneous rocks are derived from a source with a Lu/Hf ratio lower than that of the chondritic uniform reservoir (CHUR), as is the case for old crustal rocks.In contrast, if the ϵHf(t) values are positive, then the magma source has a high Lu/Hf ratio, similar to a depleted mantle (DM).In addition, if the magma is derived from the juvenile crust, it would still have positive ϵHf(t) values because there is not enough time for it to deviate from the mantle value (Ji et al., 2009).Most zircons from igneous rocks in the northern Taebaeksan metallogenic region showed negative ϵHf(t) values ranging from À2.57 to À22.05.These features indicate that the source magmas of the igneous rocks in the study area were mainly derived from the continental crust.This result is similar to that of a previous study of the Molwoon (MW-1, MW-2) area using Nd isotopic analyses (Park, 2009).For the Molwoon igneous rocks, the 87 Sr/ 86 Sr initial ratios ranged from 0.709977 to 0.711179, and the calculated ϵNd(t) values ranged from À8.29 to À10.11.These results were plotted inside the continental crust field on the 87 Sr/ 86 Sr initial ratios versus ϵNd (t) values.Therefore, we can infer that the magmatic source of the Molwoon igneous rocks is the continental crust.
The calculated crustal model age (T DMC ) ranged from 2.55 to 1.33 Ga, indicating that the magma source is mainly Proterozoic continental crust.A previous study reported that the Yuli Group was formed around 2.18-2.01Ga (Lee et al., 2011), and the Hongjesa granites were formed around 2.03-1.71Ga (Kim et al., 1978;Lee et al., 2010).The Yuli Group and Hongjesa granites are the basement rocks in the study area.Consequently, we can infer that the magma sources of the northern Taebaeksan metallogenic region Cretaceous igneous rocks are mainly Proterozoic continental crust, such as the Yuli Group (basement rocks in the study area) and Hongjesa granites (basement rocks in the study area).Compared to the results of the Hf isotopic analysis of Phanerozoic granitoids in South Korea (Cheong et al., 2013), most of the magma at the Ogcheon belt (OB), which includes the study area, was derived from the Proterozoic continental crust (Figure 10a).Meanwhile, the zircons in the igneous rocks showed variable ϵHf(t) values, indicating that these zircons do not have a uniform Hf composition.A non-uniform Hf isotopic composition within magma can develop in several ways, including progressive fractional crystallization within an evolving magma or the mixing of two or more magmas that possess different initial isotopic compositions (Reid et al., 2007).
In the northern Taebaeksan metallogenic region, the igneous rocks show similar ϵHf(t) values regardless of rock type and emplacement age (Figure 10b).The Fe skarn deposits were related to granite series (relatively low Sr/Y ratio, peraluminous), whereas the Au-Ag vein deposits were not related with rock type and their geochemical characteristics.Most of the diorite series were infertile.Although the difference between fertile (ore-related) and infertile igneous rocks was not classified in this study, these zircon U-Pb-Hf isotopes and whole-rock chemistry data will be helpful for future studies in the northern Taebaeksan metallogenic region.

| CONCLUSION
The zircon U-Pb dating, Hf isotopic analyses, and wholerock geochemical data of not only ore-related igneous rocks in and around deposits but also igneous rocks of undiscovered mineralization and/or infertile rocks in the northern Taebaeksan metallogenic region allowed us to reach the following conclusions.
There are two periods of magmatism: the Early Cretaceous ($113.7 ± 0.2 to 104.7 ± 0.5 Ma) and the Late Cretaceous ($85.8 ± 1.1 to 77.6 ± 0.4 Ma).Most Early Cretaceous igneous rocks consist of intermediate rocks, whereas Late Cretaceous igneous rocks vary in type from intermediate to felsic rock.
Negative ϵHf(t) values (À2.57 to approx.À22.05) and whole-rock geochemical data suggest that the igneous rocks were mainly derived from the continental crust.These igneous rocks are enriched in LILs, such as K, Rb, Th, and U, but depleted in HFS elements, such as Nb, Ti, and P, which are indicative of subduction-related magmas.These characteristics also indicate that the magma source was mainly crust-derived.
F I G U R E 1 0 (a) Hf isotope evolution diagram (after Zhang et al., 2018).The DM line means the evolution of DM with present-day 176 Hf/ 177 Hf = 0.28325 and 176 Lu/ 177 Hf = 0.0384 (Griffin et al., 2000).The crustal line is calculated by assuming 176 Lu/ 177 Hf = 0.015 for the average continental crust (Wong et al., 2009), and the lower crustal line is calculated by assuming 176 Lu/ 177 Hf = 0.022 for the average lower crust (Amelin et al., 1999).The white and gray rectangles are Hf isotopic data of the Gyeongsang basin (GB) and the Ogcheon zone in South Korea (after Cheong et al., 2013).The black rectangle is Hf isotopic data of the Taebaeksan zone (TZ).(b) U-Pb age versus ϵHf (t) diagram of igneous rocks in the northern Taebaeksan metallogenic region.GB, Gyeongsang basin, OB, Ogcheon belt.

F
I G U R E 4 QAP modal classification diagram of igneous rocks in the northern Taebaeksan metallogenic region (after Le Maitre the detection limit.a DN-1, DN-2 data from Oh et al. (2022).T A B L E 3 Zircon U-Pb ages of the igneous rocks from the northern Taebaeksan metallogenic region.

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I G U R E 5 Representative CL images of zircons from igneous rocks in the northern Taebaeksan metallogenic region, showing the location of LA-MC-ICP-MS analysis points.The rose-red circles show the location of U-Pb age dating and Hf isotopic analyses.DN-1 and DN-2 data are from Oh et al. (2022).

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I G U R E 9 (a) Whole-rock Sr/Y ratio versus Y and (b) whole-rock Sr/Y ratio versus SiO 2 wt% diagrams for the igneous rock samples in the northern Taebaeksan metallogenic region (afterDefant et al., 1991).
Zircon Hf isotopic data of the igneous rocks from the northern Taebaeksan metallogenic region.
JT-2, zircon grains can be divided into a group of younger grains, with an age value concentrated around 105 Ma, and a group of older grains, with age values ranging from 129 to 114 Ma.The younger age group showed a weighted-mean age of 105.4 ± 1.6 Ma (n = 7).Zircons from both groups showed similar magmatic zonation.Therefore, the younger zircon U-Pb age of the JT-2 sample represents the emplacement age, whereas the older zircon U-Pb ages are influenced by zircon captured from earlier rocks during magmatism.The Hf isotopic analyses and U-Pb age dating results are presented in Table