Tracking and Evaluating the Concentrations of Natural

minerals

Article Tracking and Evaluating the Concentrations of Natural Radioactivity According to Chemical Composition in the Precambrian and Mesozoic Granitic Rocks in the Jangsu-gun Area, Central Southwestern South Korea

Sung Won Kim 1, Weon-Seo Kee 1,*, Saro Lee 2 , Byung Choon Lee 1 and Uk Hwan Byun 1

1 Geology Division, Korea Institute of Geoscience and Mineral Resources, 124, Gwahak-ro Yuseong-gu, Daejeon 34132, Korea; [email protected] (S.W.K.); [email protected] (B.C.L.); [email protected] (U.H.B.) 2 Geoscience Platform Research Division, Korea Institute of Geoscience and Mineral Resources, 124, Gwahak-ro Yuseong-gu, Daejeon 34132, Korea; [email protected] * Correspondence: [email protected]

Abstract: The Jangsu-gun area in the central Southwestern South Korea consists of a well-preserved Middle Paleoproterozoic gneissic basement, as well as the Late Triassic and Early Jurassic granitic rocks. Here, we present the detailed zircon U-Pb age data and whole-rock chemical compositions, including radioactive elements (e.g., U and Th) and activity concentrations of 226Ra, 232Th and 40K for the Middle Paleoproterozoic gneisses, and Late Triassic and Early Jurassic granitic rocks of the Jangsu-gun area. The Middle Paleoproterozoic gneissic basement, and the Late Triassic and Early Jurassic granitic rocks have ages of ca. 1988 Ma and 1824 Ma, 230 Ma and 187–189 Ma, respectively. Citation: Kim, S.W.; Kee, W.-S.; Lee, Geochemically, the Middle Paleoproterozoic orthogneiss, Late Triassic granites and Early Jurassic S.; Lee, B.C.; Byun, U.H. Tracking and granitic rocks show typical arc-related metaluminous to weakly peraluminous fractionated granite Evaluating the Concentrations of Natural Radioactivity According to features with ASI (aluminum saturation index) values of 0.92 to 1.40. The mean values of U (ppm) Chemical Composition in the and Th (ppm) of the Middle Paleoproterozoic orthogneisses (6.4 and 20.5, respectively), Late Triassic Precambrian and Mesozoic Granitic granites (1.5 and 10.9), and Early Jurassic granites (3.5 and 16.5) were similar to those (5 and 15) 226 232 40 Rocks in the Jangsu-gun Area, of the granitic rocks in the Earth’s crust. The mean Ra (Bq/kg), Th (Bq/kg), and K (Bq/kg) Central Southwestern South Korea. activity concentrations and radioactivity concentration index (RCI) are 62, 71, 1,214 and 0.96 for Minerals 2021, 11, 684. https:// the Middle Paleoproterozoic orthogneisses; 16, 39, 1,614 and 0.78 for the Late Triassic granites; and doi.org/10.3390/min11070684 56, 70, 1031 and 0.88 for the Early Jurassic granitic rocks, respectively. The U, Th, 226Ra, 232Th, 40K, and RCI of the Middle Paleoproterozoic biotite paragneisses are similar to those of the Middle Academic Editor: Jean-Michel Lafon Paleoproterozoic orthogneisses. The trend of 226Ra, 232Th, and 40K activity concentrations, and the composition of U and Th from the Precambrian and Mesozoic rocks in the Jangsu-gun area indicates Received: 25 April 2021 that monazite is the main accessory mineral controlling the concentration of natural radioactivity. Accepted: 22 June 2021 Based on a detailed examination of the natural radioactivity in the rocks of the Jangsu-gun area, Published: 25 June 2021 the Middle Paleoproterozoic orthogneisses and paragneisses, and Late Triassic and Early Jurassic − Publisher’s Note: MDPI stays neutral granitic rocks show average high mean RCI values of 0.88 0.96, such that 32% of the rocks exceeded with regard to jurisdictional claims in the recommended value of one in the guidelines for the RCI in South Korea. Especially, the RCI is published maps and institutional afﬁl- closely related to the radon levels in the rocks. As a result, the Jangsu-gun area in South Korea is a iations. relatively high radiological risk area, which exhibits higher indoor radon levels in the residences, compared with residences in the other areas in South Korea.

Keywords: Middle Paleoproterozoic; Late Triassic; Early Jurassic; granitic rock; central Southwestern

Copyright: © 2021 by the authors. South Korea; natural radioactivity Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons 1. Introduction Attribution (CC BY) license (https:// All minerals and raw materials in the rocks present on Earth commonly contain creativecommons.org/licenses/by/ natural radioactive elements with radionuclides, but natural radioactivity exposures to 4.0/).

Minerals 2021, 11, 684. https://doi.org/10.3390/min11070684 https://www.mdpi.com/journal/minerals Minerals 2021, 11, 684 2 of 17

humans are substantially lower than the normal background levels in human activity; thus, there are no concerns regarding protection from radiation [1]. However, the natural radioactivity in some rocks can also cause high radiation exposure [1–7]. These rocks have been referred to as naturally occurring radioactive materials (NORMs). Among the NORMs, radionuclides of the 238U and 232Th decay series, and 40K, which potentially occur in the environment, are of the greatest interest. In addition, various social problems have arisen because of long-lived radioactive elements (e.g., uranium [U], thorium [Th] and potassium [K]) and their decay products (e.g., radium [Ra] and radon [Rn]) in household goods and construction materials. In South Korea, NORMs, such as U and Rn in soil or groundwater, have been investigated by the government for more than 20 years, and their origins have been closely connected with geological factors, such as rock chemical composition, deformation characteristics, and others [2–7]. In general, South Korea, in the southeastern part of the Asian continent (Figure1A,B ), is likely to have been located near a boundary of the continental plates during the Pre- cambrian to Phanerozoic, creating an environment in which various rocks (igneous rock, metamorphic rock, and sedimentary rock) could form easily and be disrupted by igneous activities [6,8–15]. The Precambrian to Phanerozoic granitic rocks generated by this igneous activity occupy more than 40% of the land area in South Korea (Figure1C) [ 11]. Consistent with NORMs, U and Th are enriched from the mantle to the lower continental crust and upper continental crust. Among the igneous rocks in the continental crust, U and Th are more enriched in the granitic rocks. This enrichment is caused by the processes of partial melting and fractional crystallization pertaining to granitic magma, which concentrate U and Th in the liquid phase and then into more silica-rich granitic rocks [5,16–19]. In South Korea, the granitic rocks comprise more than 90% of the domestic stone resources. Recently, natural rocks, in addition to building materials, have been presumed to affect overall radioactivity. Therefore, in South Korea, a standard guideline for reducing and managing radon in building materials has been applied since June of 2020, by a group of government ministries (Ministry of Environment, Ministry of Land, Infrastructure and Transport, and Nuclear Safety Committee) [20]. The essence of this guideline is to control radon contamination from building materials, by introducing a standard value according to the ‘radiation concentration index’, which is based on the radiological protection principles pertaining to the natural radioactivity of building materials, established by the European Union [21]. The target of management is natural stone-based materials that used as interior materials for buildings and apartment houses. The radioactivity concentration index is the ratio of the measured value to the combined radioactivity reference values of radium (226Ra), thorium (232Th), and potassium (40K), which are natural radioactive nuclides included in building materials. When the three ratios are summed, the value is one or less. The recommended standards for 226Ra, 232Th, and 40K, which are natural radionuclides, are 300 Bq/kg for radium, 200 Bq/kg for thorium, and 3000 Bq/kg for potassium [20]. Recently, the Jangsu-gun area in central Southwestern South Korea has higher indoor radon levels in residential areas compared with other areas of South Korea. To understand natural radioactivity from the granitic rocks in the Jangsu-gun area, we report the concentrations of radioactive elements, such as U, Th, and K, with the activity concentrations of 226Ra, 232Th and 40K for different Middle Paleoproterozoic, Late Triassic and Early Jurassic granitic rock samples from the Jangsu-gun area. We also report the U-Pb zircon age and geochemical data of the Middle Paleoproterozoic, Late Triassic and Early Jurassic granitic rocks, to clarify the relationships between the concentration trends of natural radioactivity and rock chemical composition. Minerals 2021, 11, 684 3 of 17 Minerals 2021, 11, x FOR PEER REVIEW 3 of 17

FigureFigure 1. 1. (A(A)) LocationLocation mapmap in the East East Asian Asian countries countries including including South South Korea, Korea, (B ()B t)ectonic tectonic map map of South Korea, and (C) geological map of South Korea showing the distribution of the Middle Paleo- of South Korea, and (C) geological map of South Korea showing the distribution of the Middle proterozoic gneisses and Phanerozoic granitic rocks. HIB, Hongseong–Imjingang belt; GM, Paleoproterozoic gneisses and Phanerozoic granitic rocks. HIB, Hongseong–Imjingang belt; GM, Gyeonggi massif; OB, Okcheon belt; YM, Yeongnam massif; GB, Gyeongsang basin; YeB, Yeonil Gyeonggibasin; JVT, massif; Jeju v OB,olcanic Okcheon terrain. belt; YM, Yeongnam massif; GB, Gyeongsang basin; YeB, Yeonil basin; JVT, Jeju volcanic terrain.

2.2. Geological Geological Setting Setting SouthSouth Korea Korea is is located located at at the the margin margin of of the the East East Asian Asian continent continent (Figure (Figure1A). 1A The). The tectonictectonic provinces provinces of of SouthSouth KoreaKorea largelylargely consistconsist ofof PrecambrianPrecambrian (mainly Middle Paleo- Pale- oproterozoic)proterozoic) basement basement rocks rocks,, such such as as those those of ofthe the Gyeonggi Gyeonggi and and Yeongnam Yeongnam massif massifs.s. The ThePrecambrian Precambrian basement basement rocks rocks are separated are separated by two by narrow two narrow supracrustal supracrustal units ( unitsthe Hongs- (the Hongseong–Imjingangeong–Imjingang and and Okcheon Okcheon belts belts).). These These provinces provinces also also include the the Cretaceous Cretaceous GyeongsangGyeongsang basin, basin, Cenozoic Cenozoic Yeonil Yeonil basin, basin, and and Jeju Jeju volcanic volcanic terrain, terrain, which which formed formed after after EastEast Asian Asian continent–continent continent–continent collision collision during during the the Mesozoic Mesozoic (Figure (Figure1B). 1B). TheThe study study area area is is located located in in the the Jangsu-gun Jangsu-gun area area of of central central Southwestern Southwestern South South Korea; Korea; itit is is distributed distributed withinwithin 3535°28'◦280 toto 3535°49'N◦490 N latitude latitude and and 127°22' 127◦22 to0 to 127°42'E 127◦42 0longitudeE longitude,, and andhas hasa total a total area area of approximately of approximately 533.43 533.43 km2 km. It 2is. Itgeologically is geologically located located in the in central the central--west- westernern area area of ofthe the Precambrian Precambrian Yeongnam Yeongnam m massif.assif. In In the the Jangsu Jangsu-gun-gun area, therethere isis a a wide wide distributiondistribution of of the the Middle Middle Paleoproterozoic Paleoproterozoic (2.02–1.96 (2.02−1.96 Ga) Ga) basement basement gneisses, gneisses with, with minor minor gneissesgneisses of of ca. ca. 1.82 1.82 Ga Ga (Figure (Figure2)[ 2) 14[14].]. The The Middle Middle Paleoproterozoic Paleoproterozoic gneisses gneisses are are mainly mainly intrudedintruded by by the the Late Late Triassic Triassic (ca. (ca. 230 230 Ma) Ma) and and the the Early Early Jurassic Jurassic ( 187–180(187–180 Ma Ma)) granitic granitic rocks (Figure 2) [6,8−10,13,15,22−24]. The Middle Paleoproterozoic gneisses consist mainly

Minerals 2021, 11, 684 4 of 17 Minerals 2021, 11, x FOR PEER REVIEW 4 of 17

rocks (Figure2)[ 6,8–10,13,15,22–24]. The Middle Paleoproterozoic gneisses consist mainly ofof paragneisses paragneisses (biotite (biotite paragneiss)paragneiss) andand orthogneissesorthogneisses (g (graniteranite orthogneiss, orthogneiss, leucogranite leucogranite orthogneiss,orthogneiss, and and porphyritic porphyritic orthogneiss),orthogneiss), withwith metasedimentary rocks rocks (schist) (schist) and and minor minor pegmatiticpegmatitic dikesdikes andand amphibolitesamphibolites (Figure 33AA–F).–F). TheThe MiddleMiddle Paleoproterozoic para parag-g- neissesneisses also also include include granite granite orthogneiss, orthogneiss and, and minor minor pegmatitic pegmatitic dikes dikes and and amphibolites. amphibolites The. MiddleThe Middle Paleoproterozoic Paleoproterozoic orthogneisses orthogneisses have well-developedhave well-developed gneissosity, gneissosity with a, with K-feldspar a K- megacryst-feldspar megacryst or augen-bearing- or augen banded-bearing magmaticbanded magmatic structure. structure. They include They include mainly mainly quartz, quartz, plagioclase, biotite, and K-feldspar (microcline, and orthoclase), with or without plagioclase, biotite, and K-feldspar (microcline, and orthoclase), with or without horn- hornblende and garnet. The Late Triassic granitic rocks are mainly porphyritic biotite blende and garnet. The Late Triassic granitic rocks are mainly porphyritic biotite granite, granite, with K-feldspar megacrysts (Figure 3G,H). The Early Jurassic granitic rocks are with K-feldspar megacrysts (Figure3G,H). The Early Jurassic granitic rocks are mainly mainly composed of hornblende-biotite granite, biotite granite, and two-mica granite with composed of hornblende-biotite granite, biotite granite, and two-mica granite with minor minor diorite and gabbro (Figure 3I–L). The Late Triassic porphyritic biotite and the Early diorite and gabbro (Figure3I–L). The Late Triassic porphyritic biotite and the Early Jurassic Jurassic biotite, hornblende-biotite, and two-mica granites commonly include K-feldspar, biotite, hornblende-biotite, and two-mica granites commonly include K-feldspar, plagio- plagioclase, quartz and biotite. However, the Early Jurassic hornblende-biotite and two- clase, quartz and biotite. However, the Early Jurassic hornblende-biotite and two-mica mica granite are characterized by the occurrence of hornblende and muscovite, respec- granite are characterized by the occurrence of hornblende and muscovite, respectively. tively.

FigureFigure 2. Sketch2. Sketch geologic geologic maps maps of of the the Jangsu-gun Jangsu-gun area area from from the the Southwestern Southwestern Yeongnam Yeongnam massif, massif South, South Korea, Korea, showing showing the the locations of samples analyzed in the present study. locations of samples analyzed in the present study.

3. Analytical Methods Zircon Pb-Th-U isotopes were analyzed using a Nu Plasma II multi-collector inductively coupled plasma mass spectrometer (LA-MC-ICPMS), equipped with a New Wave Research 193-nm ArF excimer laser ablation system, at the Korea Basic Science Institute

Minerals 2021, 11, x FOR PEER REVIEW 5 of 17

(KBSI). For some samples, U-Pb dating was also performed using a sensitive high-resolu- tion ion microprobe (SHRIMP-IIe/MC) at the KBSI. Zircon grains were hand-picked from heavy mineral concentrates and mounted in epoxy. Before analysis, the grains were photographed under an optical microscope, and their internal zoning was imaged by cathodoluminescence (CL) using a JEOL 6610LV scanning electron microscope at KBSI (Figure 4). The conditions and data acquisition procedures were similar to those described by Kim Minerals 2021, 11, 684 et al. [15]. All ages were calculated with 2σ error, and data reduction was conducted using5 of 17 Iolite 2.5 [25,26], SQUID 2.50 and Isoplot 3.71 [27,28]. The U-Pb results are listed in Table S1, and are illustrated on concordia plots in Figure 5.

FigureFigure 3.3. Outcrop photographs photographs showing showing (A (A,B,B) Middle) Middle Paleoproterozoic Paleoproterozoic granite granite orthogneiss orthogneiss with with magmatic magmatic flow ﬂow structure, struc- (C) Middle Paleoproterozoic leucogranite orthogneiss, (D) Middle Paleoproterozoic K-feldspar megacryst-bearing por- ture, (C) Middle Paleoproterozoic leucogranite orthogneiss, (D) Middle Paleoproterozoic K-feldspar megacryst-bearing phyritic orthogneiss, (E,F) Middle Paleoproterozoic banded biotite paragneiss having alternation of biotite paragneiss and porphyritic orthogneiss, (E,F) Middle Paleoproterozoic banded biotite paragneiss having alternation of biotite paragneiss granite orthogneiss, (G,H) Late Triassic biotite granite with textures commonly characterized by K-feldspar megacrysts, and(I,J) graniteEarly Jurassic orthogneiss, porphyritic (G,H) Latebiotite Triassic granite biotite, (K) granitehornblende with- texturesbiotite granite commonly with characterizedor without K- byfeldspar K-feldspar megacrysts, megacrysts, and ((IL,J)) Early Early Jurassic Jurassic two porphyritic-mica granite biotite in granite, the Jangsu (K) hornblende-biotite-gun area. granite with or without K-feldspar megacrysts, and (L) Early Jurassic two-mica granite in the Jangsu-gun area. Seventy-seven whole-rock samples of the Middle Paleoproterozoic orthogneisses 3.and Analytical paragneisses, Methods and Mesozoic granitic rocks were analyzed for major, trace, and rare earthZircon element Pb-Th-U (REE) abundances isotopes were, using analyzed inductively using coupled a Nu plasma Plasma atomic II multi-collector emission spec- in- ductivelytrometry (ICP coupled-AES) plasma (ENVIRO mass II; spectrometer Thermo Jarrel (LA-MC-ICPMS),-Ash) and inductively equipped coupled with plasmaa New Wavemass spectrometry Research 193-nm (ICP-MS) ArF (Optima excimer 3000; laser Perkin ablation-Elmer) system, at Activation at the Korea Laboratories, Basic Science Ltd. Institute(Ancaster, (KBSI). ON, Canada) For some (Table samples, S2). The U-Pb analytical dating wasuncertainties also performed ranged usingfrom 1% a sensitiveto 3%. high-resolutionPrecise determinati ion microprobeon of the (SHRIMP-IIe/MC) activity of naturally at occurring the KBSI. radionuclides Zircon grains were(226Ra, hand- 232Th pickedand 40K) from was heavy performed mineral on concentratesseventy-seven and whole mounted-rock in samples epoxy. Beforefrom the analysis, Jangsu the-gun grains area, wereand the photographed radioactivity under concentration an optical index microscope, was calculated. and their Most internal of the zoning nuclide was analyses imaged bywere cathodoluminescence performed using a (CL)p-type using high a-purity JEOL 6610LV germanium scanning radiation electron detector microscope (HPGe at radia- KBSI (Figuretion detector)4). The at conditions the Hanil andNuclear data Power acquisition Co., Ltd. procedures A multichannel were similar spectrum to those analyzer described (BSI byHybrid, Kim et Latvia) al. [15]. was All used ages werefor the calculated measurement. with 2 σTheerror, detector and data was reduction calibrated was through conducted self- usingabsorption Iolite correction 2.5 [25,26],, SQUIDaccording 2.50 to and the Isoplotenergy 3.71 and [ 27efficiency,28]. The calibration U-Pb results density are listed of the in Tabledetector, S1, andusing are a illustratedgeometrical on standard concordia source plots identical in Figure to5. the source used for the analysis sample.Seventy-seven Each analysis whole-rock sample was samples subjected of the to Middlesample Paleoproterozoicpretreatment processes orthogneisses, such as andgrinding, paragneisses, sieving, drying, and Mesozoic mixing, granitic and filling rocks, using were an analyzed aluminum for major,mixing trace, container; and rare the earth element (REE) abundances, using inductively coupled plasma atomic emission spectrometry (ICP-AES) (ENVIRO II; Thermo Jarrel-Ash) and inductively coupled plasma mass spectrometry (ICP-MS) (Optima 3000; Perkin-Elmer) at Activation Laboratories, Ltd. (Ancaster, ON, Canada) (Table S2). The analytical uncertainties ranged from 1% to 3%. Minerals 2021, 11, x FOR PEER REVIEW 6 of 17

amount of the sealed sample was generally 300 g. In particular, the sample was sealed and stored with consideration of the sample's radial equilibrium time. The 226Ra in the sample was measured after sealing and storing for more than three weeks, to prevent 222Rn from escaping out of the measuring container. For 232Th, 228Ac, a progeny of Th, was measured immediately after sealing; 40K was also measured at that time. However, because it is inefficient to perform several measurements per sample, the three nuclides were measured concurrently using the p-type high-purity germanium (HPGe) radiation detector on the radiation-equilibrated sample, after it had been sealed for 25 days. The 226Ra, 232Th, and 40K radioactivity concentrations obtained for each of the measured samples, together with their corresponding total uncertainties, are summarized in Table S3. Importantly, no ra- Minerals 2021, 11, 684 dionuclides other than naturally occurring radionuclides were detected in the samples,6 of 17 and the small contribution of the environmental γ-ray background at the laboratory site was subtracted from the spectra of the measured samples.

FigureFigure 4. 4.Scanning Scanning electron electron microscope microscope (SEM) (SEM) cathodoluminescence cathodoluminescence images images of of sectioned sectioned zircon zircon grainsgrains from from ((AA,,BB)) Middle Paleoproterozoic Paleoproterozoic granite granite orthogneisses, orthogneisses, (C ()C Late) Late Triassic Triassic porphyritic porphyritic bio- biotitetite granite granite,, (D (D) )Early Early Jurassic Jurassic hornblende hornblende-biotite-biotite granite, (EE,F,F)) EarlyEarly JurassicJurassic biotite biotite granites, granites, and and (G(G) Early) Early Jurassic Jurassic two two mica mica granite granite in in the the Jangsu-gun Jangsu-gun area. area.

4. ResultsPrecise determination of the activity of naturally occurring radionuclides (226Ra, 232Th and4.1.40 UK)-Pb was Geochronology performed on seventy-seven whole-rock samples from the Jangsu-gun area, and theGranite radioactivity orthogneisses concentration (KJS23-1index and JS was-46) calculated.from the Middle Most Paleoproterozoic of the nuclide analyses gneissic wererocks, performed one K-feldspar using a megacryst p-type high-purity-bearing porphyritic germanium biotite radiation granite detector (JS- (HPGe92) from radiation the Late detector)Triassic granite, at the Hanil two hornblende Nuclear Power-biotite Co., granite Ltd. A(JS multichannel-53 and KJS-04), spectrum one biotite analyzer porphyritic (BSI Hybrid,granite (KJS Latvia)-24) was, and usedone two for- themica measurement. granite (KJS-28) The from detector the Early was Jurassic calibrated granite through in the self-absorption correction, according to the energy and efficiency calibration density of the Jangsu-gun area were selected for dating (Figure 2). Zircons commonly had euhedral detector, using a geometrical standard source identical to the source used for the analysis shapes up to 50–350 μm in diameter, with aspect ratios ranging from 1 to 4. All the grains sample. Each analysis sample was subjected to sample pretreatment processes, such as had strong oscillatory growth zoning, which is typical of magmatic origin (Figure 4). grinding, sieving, drying, mixing, and filling, using an aluminum mixing container; the Granite orthogneisses with biotite paragneiss are regionally exposed in the Jangsu- amount of the sealed sample was generally 300 g. In particular, the sample was sealed and gun area. Zircons from the granite orthogneisses in the biotite gneiss give dispersed ages stored with consideration of the sample’s radial equilibrium time. The 226Ra in the sample along the discordance line, which projected to a Middle Paleoproterozoic upper concordia was measured after sealing and storing for more than three weeks, to prevent 222Rn from escaping out of the measuring container. For 232Th, 228Ac, a progeny of Th, was measured immediately after sealing; 40K was also measured at that time. However, because it is inefficient to perform several measurements per sample, the three nuclides were measured concurrently using the p-type high-purity germanium (HPGe) radiation detector on the radiation-equilibrated sample, after it had been sealed for 25 days. The 226Ra, 232Th, and 40K radioactivity concentrations obtained for each of the measured samples, together with their corresponding total uncertainties, are summarized in Table S3. Importantly, no radionuclides other than naturally occurring radionuclides were detected in the samples, and the small contribution of the environmental γ-ray background at the laboratory site was subtracted from the spectra of the measured samples. Minerals 2021, 11, x FOR PEER REVIEW 7 of 17

intersection (Figure 5A, B). The precise estimate of the upper intersection age is obtained if all of the data are combined in a single regression, yielding 1988.2 ± 3.0 Ma (n = 66 of 100, MSWD = 2.8). On the other hand, zircons from small stock-type granite orthogneiss in granite orthogneiss and biotite paragneiss show younger Middle Paleoproterozoic age, giving an upper intercept 207Pb/206Pb age of 1824.0 ± 2.8 Ma (n = 29, MSWD = 2.2) (Figure 5C). Late Triassic granite plutons are exposed as a dike type in the southern part of the Jangsu-gun area. Zircons from the Late Triassic granite sample (KJS-24) give a weighted mean 206Pb/238U age of 230.4 ± 1.2 Ma (Figure 5D). Early Jurassic granite plutons are mainly exposed in the central part of the Jangsu-gun area. Zircons from the hornblende-biotite granite samples (JS-53 and KJS-04) give weighted mean 206Pb/238U ages of 187.2 ± 2.3 Ma and 181.8 ± 0.4 Ma, respectively (Figure 5E,F). Zircons from the biotite granite sample 206 238 Minerals 2021, 11, 684 (KJS-24) give a weighted mean Pb/ U age of 184.8 ± 0.4 Ma (Figure 5G).7 ofMost 17 of the analyzed zircons with oscillatory zoning from two-mica granite give a weighted mean 206Pb/238U age of 180.3 ± 1.6 Ma (Figure 5H).

FigureFigure 5. 5Tera–Wasserburg. Tera–Wasserburg concordia concordia plots plots of LA-MC-ICPMS of LA-MC-ICPMS and SHRIMP and SHRIMP U-Pb isotopic U-Pb analysesisotopic analyses ofof zirconszircons from from the the Middle Middle Paleoproterozoic Paleoproterozoic granite granite orthogneisses, orthogneisses, Late Triassic Late Triassic and Early and Jurassic Early Jurassic granitesgranites in in the the Jangsun-gun Jangsun-gun area. area KJS-23-1. KJS-23 Middle-1 Middle Paleoproterozoic Paleoproterozoic granite orthogneissgranite orthogneiss (A,B), JS-46 (A,B), JS- Middle46 Middle Paleoproterozoic Paleoproterozoic granite granite orthogneiss orthogneiss (C), JS-92 (C), Late JS-92 Triassic Late biotiteTriassic granite biotite (D granite), JS-53 ( andD), JS-53 KJS-04and KJS Early-04 JurassicEarly Jurassic hornblende-biotite hornblende granites-biotite ( Egranites,F), KJS-24 (E Early,F), KJS Jurassic-24 Early biotite Jurassic granite biotite (G), and granite KJS-28(G), and Early KJS Jurassic-28 Earl two-micay Jurassic granite two- (micaH). granite (H). 4. Results 4.1. U-Pb Geochronology Granite orthogneisses (KJS23-1 and JS-46) from the Middle Paleoproterozoic gneissic rocks, one K-feldspar megacryst-bearing porphyritic biotite granite (JS-92) from the Late Triassic granite, two hornblende-biotite granite (JS-53 and KJS-04), one biotite porphyritic granite (KJS-24), and one two-mica granite (KJS-28) from the Early Jurassic granite in the Jangsu-gun area were selected for dating (Figure2). Zircons commonly had euhedral shapes up to 50–350 µm in diameter, with aspect ratios ranging from 1 to 4. All the grains had strong oscillatory growth zoning, which is typical of magmatic origin (Figure4). Granite orthogneisses with biotite paragneiss are regionally exposed in the Jangsu- gun area. Zircons from the granite orthogneisses in the biotite gneiss give dispersed ages Minerals 2021, 11, 684 8 of 17

along the discordance line, which projected to a Middle Paleoproterozoic upper concordia intersection (Figure5A,B). The precise estimate of the upper intersection age is obtained if all of the data are combined in a single regression, yielding 1988.2 ± 3.0 Ma (n = 66 of 100, MSWD = 2.8). On the other hand, zircons from small stock-type granite orthogneiss in granite orthogneiss and biotite paragneiss show younger Middle Paleoproterozoic age, giving an upper intercept 207Pb/206Pb age of 1824.0 ± 2.8 Ma (n = 29, MSWD = 2.2) (Figure5C ). Late Triassic granite plutons are exposed as a dike type in the southern part of the Jangsu-gun area. Zircons from the Late Triassic granite sample (KJS-24) give a weighted mean 206Pb/238U age of 230.4 ± 1.2 Ma (Figure5D). Early Jurassic granite plutons are mainly exposed in the central part of the Jangsu-gun area. Zircons from the hornblende-biotite granite samples (JS-53 and KJS-04) give weighted mean 206Pb/238U Minerals 2021, 11, x FOR PEER REVIEW ages of 187.2 ± 2.3 Ma and 181.8 ± 0.4 Ma, respectively (Figure5E,F). Zircons from the 8 of 17

biotite granite sample (KJS-24) give a weighted mean 206Pb/238U age of 184.8 ± 0.4 Ma (Figure5G ). Most of the analyzed zircons with oscillatory zoning from two-mica granite give a weighted mean 206Pb/238U age of 180.3 ± 1.6 Ma (Figure5H). 4.2. Geochemistry 4.2.The Geochemistry Middle Paleoproterozoic granite orthogneisses, and Early Jurassic granitic rocks from theThe Jangsu Middle-gun Paleoproterozoic area were mainly granite analyzed orthogneisses, to determine and Early Jurassictheir geochemica granitic rocksl signa- turesfrom (Table the Jangsu-gun S2). The Middle area were Paleoproterozoic mainly analyzed tobiotite determine paragneisses their geochemical and Late signatures Triassic granites(Table were S2).also The analyzed Middle to Paleoproterozoic interpret geochemical biotite paragneisses characteristics and in Late the TriassicJangsu- granitesgun area. On were also analyzed to interpret geochemical characteristics in the Jangsu-gun area. On the the SiO2 versus Na2O + K2O diagram, with fields defined by [29] (Figure 6), the ca. 1.99 Ga SiO2 versus Na2O + K2O diagram, with fields defined by [29] (Figure6), the ca. 1.99 Ga MiddleMiddle Paleoproterozoic Paleoproterozoic samples mainlymainly plotted plotted in in the the diorite, diorite, quartz quartz monzonite, monzonite, quartz quartz dioritediorite and and granite granite fields. fields. The ca.ca. 1.821.82 Ga Ga Middle Middle Paleoproterozoic Paleoproterozoic orthogneiss orthogneiss samples samples areare plotted plotted in in the the quartz quartz diorite andand granite granite fields. fields. The The Late Late Triassic Triassic porphyritic porphyritic granite granite andand Early Early Jurassic Jurassic biotite biotite granite granite samplessamples are are also also plotted plotted in thein the quartz quartz diorite diorite andgranite and granite fields.fields. The The Early Early Jurassic Jurassic hornblende hornblende-biotite-biotite granite granite samples samples are are mainly mainly plotted plotted in quartz in quartz dioritediorite,, and and the the E Earlyarly Jurassic Jurassic two-micatwo-mica granite granite samples samples are are plotted plotted in the in granite the granite field. field.

FigureFigure 6. SiO 6.2 SiOwt.%2 wt.% versus versus Na2 NaO+K2O+K2O 2wt.%O wt.% diagram diagram for for the the Middle Middle PaleoproterozoicPaleoproterozoic orthogneisses orthogneisses (A), ( andA), Lateand TriassicLate Triassic granitegranite and andEarly Early Jurassic Jurassic granitic granitic rocks rocks (B (B) )in in the the Jangsu Jangsu-gun-gun area. area.

The major element abundances plotted against SiO2 of the orthogneisses (leucogranite The major element abundances plotted against SiO2 of the orthogneisses (leucogran- orthogneiss, porphyritic orthogneiss, and granite orthogneiss) with biotite paragneisses, ite orthogneiss, porphyritic orthogneiss, and granite orthogneiss) with biotite parag- Late Triassic porphyritic granite and Early Jurassic granitic rocks show negative corre- neisses, Late Triassic porphyritic granite and Early Jurassic granitic rocks show negative lations with Al2O3, Fe2O3*, MgO, CaO, P2O5 and TiO2; they show no correlations with 2 3 2 3∗ 2 5 2 correlationsK2O or Na with2O (Figure Al O7,). Fe TheO Middle, MgO, Paleoproterozoic CaO, P O and TiO orthogneisses; they show have no most correlations weakly with K2O or Na2O (Figure 7). The Middle Paleoproterozoic orthogneisses have most weakly peraluminous in an aluminum saturation index (ASI) defined by Shand [30], which ranged from 0.95 to 1.20 (Figure 8). The Late Triassic porphyritic biotite granite and Early Jurassic biotite, and two-mica granites are also weakly peraluminous (ASI = 1.00 to 1.39), with the exception of one low ASI (0.7). The Early Jurassic hornblende-biotite granites ranged from metaluminous to weakly peraluminous (ASI = 0.93 to 1.13).

Minerals 2021, 11, 684 9 of 17

peraluminous in an aluminum saturation index (ASI) deﬁned by Shand [30], which ranged from 0.95 to 1.20 (Figure8). The Late Triassic porphyritic biotite granite and Early Jurassic biotite, and two-mica granites are also weakly peraluminous (ASI = 1.00 to 1.39), with the Minerals 2021, 11, x FOR PEER REVIEWexception of one low ASI (0.7). The Early Jurassic hornblende-biotite granites ranged from9 of 17

metaluminous to weakly peraluminous (ASI = 0.93 to 1.13).

Figure 7. Plots of major elements (A) Al2O3, (B) Fe2O3*, (C) MgO, (D) CaO, (E) Na2O, (F) K2O, (G) Figure 7. Plots of major elements (A) Al2O3,(B) Fe2O3*, (C) MgO, (D) CaO, (E) Na2O, (F)K2O, P2O5, and (H) TiO2 versus SiO2 for the Middle Paleoproterozoic gneisses, Late Triassic granite, and (G)P2O5, and (H) TiO2 versus SiO2 for the Middle Paleoproterozoic gneisses, Late Triassic granite, Early Jurassic granitic rocks in the Jangsu-gun area. Symbols are as in Figure 6. and Early Jurassic granitic rocks in the Jangsu-gun area. Symbols are as in Figure6. The chondrite-normalized rare earth elements (REE) distribution patterns of the Mid- dle Paleoproterozoic leucogranite orthogneiss, porphyritic orthogneiss, granite orthogneisses with biotite paragneisses, Late Triassic porphyritic granite and Early Jurassic biotite granite, hornblende-biotite granite and two-mica granite have commonly light REE enrichments (Figure 9). Most rock samples have moderately negative to strongly positive Eu anomalies (0.4–2.86), but many Middle Paleoproterozoic granite orthogneiss samples show strongly negative Eu anomalies (0.02–0.38). On the primitive mantle normalized diagrams (Figure 9), the Middle Paleoproterozoic and Early Jurassic rock samples from the Jangsu-gun area display large ion lithophile element (LILE) enrichment with Ta–Nb troughs, as well as P and Ti depletions. The Middle Paleoproterozoic samples also show

Minerals 2021, 11, x FOR PEER REVIEW 10 of 17

Minerals 2021, 11, 684 10 of 17 Sr depletion, in comparison with the Late Triassic and Early Jurassic granite samples (Fig- ure 9).

Minerals 2021, 11, x FOR PEER REVIEW 10 of 17

Sr depletion, in comparison with the Late Triassic and Early Jurassic granite samples (Fig- ure 9).

FigureFigure 8.8. SiOSiO2 wt.%2 wt.% versus versus ASI ASIdiagram diagram [30] for [30 the] for Middle the Middle Paleoproterozoic Paleoproterozoic orthogneisses orthogneisses (A), (Aand), andLate LateTriassic Triassic granite granite and Early and Jurassic Early Jurassic granitic granitic rocks (B rocks) in the (B Jangsu) in the-gun Jangsu-gun area. ASI: area. ASI: Al/(CaAl/(Ca−1.67P+Na+K−1.67P+Na+K).). Symbols Symbols are are as as in inFigure Figure 6.6 .

The chondrite-normalized rare earth elements (REE) distribution patterns of the Middle Paleoproterozoic leucogranite orthogneiss, porphyritic orthogneiss, granite orthogneisses with biotite paragneisses, Late Triassic porphyritic granite and Early Jurassic biotite granite, hornblende-biotite granite and two-mica granite have commonly light REE enrichments (Figure9). Most rock samples have moderately negative to strongly positive Eu anomalies (0.4–2.86), but many Middle Paleoproterozoic granite orthogneiss samples show strongly negative Eu anomalies (0.02–0.38). On the primitive mantle normalized diagrams (Figure9), the Middle Paleoproterozoic and Early Jurassic rock samples from the Jangsu-gun area display large ion lithophile element (LILE) enrichment with Ta–Nb

troughs, as well as P and Ti depletions. The Middle Paleoproterozoic samples also show Figure 8. SiO2 wt.% versus ASI diagram [30] for the Middle Paleoproterozoic orthogneisses (A), Sr depletion, in comparison with the Late Triassic and Early Jurassic granite samples and Late Triassic granite and Early Jurassic granitic rocks (B) in the Jangsu-gun area. ASI: (FigureAl/(Ca−1.67P+Na+K9). ). Symbols are as in Figure 6.

Figure 9. (A–D) Chondrite-normalized rare earth element (REE) patterns, and (E–H) primitive mantle-normalized trace element distribution diagrams [31] for the Middle Paleoproterozoic orthogneisses, biotite gneisses, Late Triassic granite and Early Jurassic granitic rocks in the Jangsu-gun area. Symbols are as in Figures 6 and 7.

4.3. Gamma Nuclide Analysis The activity concentrations of 226Ra, 232Th and 40K from the ca. 1.99 Ga orthogneisses in the Jangsu-gun area are 19–42, 46–67, and 1,148–1,621 Bq/kg for leucogranitic orthogneiss; 9–12, 3–86, and 929–1,530 Bq/kg for porphyritic orthogneiss; and 21–241, 29– 140, and 473–1,857 Bq/kg for the granite orthogneiss, respectively. All the radiation concentration index (RCI) values are within the range of 0.48 to 1.98 (Figure 10A–C). Among the 21 samples, 10 samples showed a high RCI of 1.00 to 1.98 (Figure 10A–C). The concentrations of 226Ra, 232Th, and 40K for the 2.02−1.96 Ga biotite gneisses with granite orthogneisses were 4 –81, 4–94, and 421–1,603 Bq/kg, respectively, and the RCI values are

within an intermediate range of 0.39 to 1.02 (Figure 10D). Two of the 21 samples show a Figure 9. (A–D) Chondrite-normalized rare earth element (REE) patterns, and (E–H) primitive mantle-normalized trace Figure 9. (A–D) Chondrite-normalizedvalue higher rare than earth 1.00. element In contrast, (REE) patterns, the concentrations and (E–H) primitive of 226Ra, mantle-normalized 232Th, and 40K for trace the ca. element distribution diagrams [31] for the Middle Paleoproterozoic orthogneisses, biotite gneisses, Late Triassic granite element distribution diagrams [31] for the Middle Paleoproterozoic orthogneisses, biotite gneisses, Late Triassic granite and and Early Jurassic granitic rocks1.82 inGa the small Jangsu-stock-gun-type area. granit Symbolse orthogneiss are as in Figure in thes 6 and Jangsu 7. -gun area are 6–16, 42–61, and Early Jurassic granitic rocks in the Jangsu-gun area. Symbols are as in Figures6 and7. 4.3. Gamma Nuclide Analysis The activity concentrations of 226Ra, 232Th and 40K from the ca. 1.99 Ga orthogneisses in the Jangsu-gun area are 19–42, 46–67, and 1,148–1,621 Bq/kg for leucogranitic orthogneiss; 9–12, 3–86, and 929–1,530 Bq/kg for porphyritic orthogneiss; and 21–241, 29– 140, and 473–1,857 Bq/kg for the granite orthogneiss, respectively. All the radiation concentration index (RCI) values are within the range of 0.48 to 1.98 (Figure 10A–C). Among the 21 samples, 10 samples showed a high RCI of 1.00 to 1.98 (Figure 10A–C). The concentrations of 226Ra, 232Th, and 40K for the 2.02−1.96 Ga biotite gneisses with granite orthogneisses were 4 –81, 4–94, and 421–1,603 Bq/kg, respectively, and the RCI values are within an intermediate range of 0.39 to 1.02 (Figure 10D). Two of the 21 samples show a value higher than 1.00. In contrast, the concentrations of 226Ra, 232Th, and 40K for the ca. 1.82 Ga small-stock-type granite orthogneiss in the Jangsu-gun area are 6–16, 42–61, and

Minerals 2021, 11, 684 11 of 17

4.3. Gamma Nuclide Analysis The activity concentrations of 226Ra, 232Th and 40K from the ca. 1.99 Ga orthogneisses in the Jangsu-gun area are 19–42, 46–67, and 1148–1621 Bq/kg for leucogranitic orthogneiss; 9–12, 3–86, and 929–1530 Bq/kg for porphyritic orthogneiss; and 21–241, 29–140, and 473–1857 Bq/kg for the granite orthogneiss, respectively. All the radiation concentration index (RCI) values are within the range of 0.48 to 1.98 (Figure 10A–C). Among the 21 samples, 10 samples showed a high RCI of 1.00 to 1.98 (Figure 10A–C). The concentrations of 226Ra, 232Th, and 40K for the 2.02–1.96 Ga biotite gneisses with granite orthogneisses were 4–81, Minerals 2021, 11, x FOR PEER REVIEW4–94, and 421–1603 Bq/kg, respectively, and the RCI values are within an intermediate11 of 17

range of 0.39 to 1.02 (Figure 10D). Two of the 21 samples show a value higher than 1.00. In contrast, the concentrations of 226Ra, 232Th, and 40K for the ca. 1.82 Ga small-stock- type1,083 granite–1,604 Bq/kg, orthogneiss respectively; in the the Jangsu-gun RCI values area are within are 6–16, the narrow 42–61, andrange 1083–1604 of 0.72 to 0.7 Bq/kg,6 respectively;(Table S3). the RCI values are within the narrow range of 0.72 to 0.76 (Table S3).

226 232 40 FigureFigure 10. 10.Plots Plots of of U U andand ThTh concentrations of of geochemical geochemical analysis analysis and and the theactivity activity concentrations concentrations of Ra, of 226ThRa,, 232K Th,and 40K RCI values for (A–D) Middle Paleoproterozoic granite orthogneisses, (E) Late Triassic porphyritic biotite granite, (F) Early and RCI values for (A–D) Middle Paleoproterozoic granite orthogneisses, (E) Late Triassic porphyritic biotite granite, Jurassic biotite granite, (G) Early Jurassic hornblende biotite granite, and (H) Early Jurassic two mica granite in the Jangsu- (F)gun Early area.Jurassic biotite granite, (G) Early Jurassic hornblende biotite granite, and (H) Early Jurassic two mica granite in the Jangsu-gun area. The concentrations of 226Ra, 232Th and 40K for the Late Triassic (ca. 230 Ma) porphyritic 226 232 40 biotiteThe granite concentrations are 5 to 19 of, 1 to Ra,80, andTh 62 and5 to 2,57K for0 Bq/kg, the Late respectively, Triassic (ca. and 230 the Ma) RCI porphyritic values biotiteare within granite the intermediate are 5 to 19, 1 range to 80, of and 0.32 625 to to1.21 2,570 (Figure Bq/kg, 10E respectively,). The concentrations and the of RCI 226Ra, values 226 are232Th, within and 40 theK for intermediate the Early Jurassic range of (187 0.32–180 to 1.21Ma) (Figurebiotite granite10E). The are concentrations13–422, 37–154, of and Ra, 232 40 80Th,8–1,53 and1 Bq/kg,K for respectively, the Early Jurassic and the (187–180 RCI ranged Ma) from biotite Middle granite to high are 13–422,values of 37–154, 0.57 to and 808–1,5312.10 (Figure Bq/kg, 10F). respectively,Six of the 13 andsamples the RCIhave ranged an RCI from value Middle higher tothan high 1.00. values The ofEarly 0.57 to 2.10Jurassic (Figure (187 –10180F). Ma) Six ofhornblende the 13 samples-biotite granite have an ha RCIs concentrations value higher, for than 226Ra 1.00., 232Th The and Early 226 232 Jurassic40K, of 12 (187–180–76, 43–17 Ma)3, and hornblende-biotite 630–960 Bq/kg, respectively; granite has the concentrations, RCI has a wide for rangeRa, of 0.56Th toand 401.1K,2 of(Figure 12–76, 10 43–173,G). Two and among 630–960 the eleven Bq/kg, samples respectively; show a thehigh RCI activity has aconcentration wide range ofin- 0.56 todex 1.12 (1.0 (Figure2 and 1.1 102G)., respectively). Two among The the concentrations eleven samples of show226Ra, a232 highTh, and activity 40K of concentration the Early indexJurassic (1.02 (187 and–180 1.12,Ma) two respectively).-mica granite The are concentrations 18–23, 16–46, and of 1,00226Ra,2–1,123291Th, Bq/kg, and respec-40K of the Earlytively; Jurassic the RCI had (187–180 intermediate Ma) two-mica values of granite 0.54 to 0.69 are 18–23,(Figure 16–46,10H). and 1002–1191 Bq/kg, respectively; the RCI had intermediate values of 0.54 to 0.69 (Figure 10H). 5. Discussion 5.1. Petrogeneses of the Middle Paleoproterozoic Orthogneiss, Late Triassic Granite and Early Jurassic Granitic Rocks of the Jangsu-gun Area From the LA-MC-ICPMS and SHRIMP U-Pb zircon ages presented in this study, there is the possibility to characterize the Middle Paleoproterozoic (ca. 1.99 Ga and ca. 1.82 Ga) magmatic events, and Late Triassic (ca. 230 Ma) and Early Jurassic (187–180 Ma) magmatic events in the Jangsu-gun area (Figure 8). The trace elements and REE geochemical characteristics of them are similar to those of subduction-related granitic rocks, with a subalkalic chemical trend. In the classification established by Pearce et al. [32], they occupy the volcanic arc granitoid field (Figure 11).

Minerals 2021, 11, 684 12 of 17

5. Discussion 5.1. Petrogeneses of the Middle Paleoproterozoic Orthogneiss, Late Triassic Granite and Early Jurassic Granitic Rocks of the Jangsu-gun Area From the LA-MC-ICPMS and SHRIMP U-Pb zircon ages presented in this study, there is the possibility to characterize the Middle Paleoproterozoic (ca. 1.99 Ga and ca. 1.82 Ga) magmatic events, and Late Triassic (ca. 230 Ma) and Early Jurassic (187–180 Ma) magmatic events in the Jangsu-gun area (Figure8). The trace elements and REE geochemical characteristics of them are similar to those of Minerals 2021, 11, x FOR PEER REVIEW 12 of 17 subduction-related granitic rocks, with a subalkalic chemical trend. In the classiﬁcation established by Pearce et al. [32], they occupy the volcanic arc granitoid ﬁeld (Figure 11).

FigureFigure 11. Rb 11. versusRb versus (Y + (YNb) + Nb)tecton tectonicic discrimination discrimination diagram diagram for for (A (A) the) the Middle Middle Paleoproterozoic Paleoproterozoic orthogneisses,orthogneisses and, and (B) Late Triassic(B) Late graniteTriassic and granite Early and Jurassic Early Jurassic granitic granitic rocks rocksin the in Jangsu the Jangsu-gun-gun area. area. VAG, VAG, volcanic volcanic arc arc granites: granites: ORG, ORG, ocean ocean ridge granites;ridge WPG, granites; within WPG,-plate within-plate granites; granites; syn-COLG syn-COLG,, syn-collision syn-collision granites. granites. Symbols Symbols are areas in as Figure in Figure 6.6 .

TheThe ASI ASI values values of of the the Middle PaleoproterozoicPaleoproterozoic orthogneisses orthogneisses (1.0 (1.0 to 1.1, to except1.1, except for for some samples with values 1.1 to 1.2) show the characteristics of weakly peraluminous some samples with values 1.1 to 1.2) show the characteristics of weakly peraluminous granites. Moreover, in many parts of the Jangsu-gun area, biotite gneiss (i.e., metasedimen- granites.tary rock) Moreover, is widely in distributed. many parts The of Late the Triassic Jangsu and-gun Early area, Jurassic biotite granitic gneiss rocks (i.e., inmetasedi- the mentaryJangsu-gun rock) areais widely showed distributed. metaluminous The to Late weakly Triassic peraluminous and Early granite Jurassic characteristics, granitic rocks in thewith Jangsu an ASI-gun of area 0.9 to showed 1.1. In some metaluminous rocks, the ASI to values weakly range peraluminous from 1.1 to 1.4. granite An ASI characteris- value ticschange, with an of 0.9ASI to of 1.4 0.9 in to the 1.1. Middle In some Paleoproterozoic rocks, the ASI orthogneisses, values range and from Late 1.1 Triassic to 1.4. and An ASI valueEarly change Jurassic of granitic0.9 to 1.4 rocks in the in the Middle Jangsu-gun Paleoproterozoic area discriminates orthogneisses amphibole,and and pyroxene Late Triassic andformation Early Jurassic during granitic magma rocks crystallization, in the Jangsu which-gun increases area discriminates the ASI value amphibole of the residual and py- roxenemelt. formation This reveals during the evolution magma process crystallization, to peraluminous which granitesincreases containing the ASI biotitevalue of and the re- muscovite. Therefore, tracking the degree of fractional crystallization or partial melting of sidual melt. This reveals the evolution process to peraluminous granites containing biotite granitic rocks provides important information that is useful for explaining the petrogeneses andof muscovite. granitic rocks Therefore, distributed tracking in the Jangsu-gun the degree area of fractional [33]. crystallization or partial melting of graniticThe degree rocks of fractional provides crystallization important information in the peraluminous that is useful granitic for rocks explaining can be the petrogenesesdetermined of from granitic the Nb/Ta rocks ratiodistributed in the whole-rock in the Jangsu chemistry-gun area (Figure [33]. 12 )[34]. In general,The a degree decrease of in fractional the Nb/Ta crystallization ratio in evolved in the melts peraluminous suggests effects granitic of both rocks fractional can be de- termincrystallizationed from the and Nb/Ta sub-solidus ratio hydrothermalin the whole- alteration.rock chemistry The extensively (Figure 12 surveyed) [34]. In Middle general, a decreasePaleoproterozoic in the Nb/Ta orthogneiss ratio in andevolved Early melts Jurassic suggests granitic rockseffects in of the both Jangsu-gun fractional area crystalliza- show tiona and degree sub of-solidus fractional hydrothermal crystallization alteration. of approximately The extensively 50%, with surveyed a maximum Middle Nb/Ta Pa ofleopro- five. However, most of these rocks in the Jangsu-gun area do not show Nb/Ta values much terozoic orthogneiss and Early Jurassic granitic rocks in the Jangsu-gun area show a de- lower than five, and therefore the role of sub-solidus hydrothermal alteration in their late greestages of fractional of evolution crystallization is considered toof beapproximately insufficient. 50%, with a maximum Nb/Ta of five. However, most of these rocks in the Jangsu-gun area do not show Nb/Ta values much lower than five, and therefore the role of sub-solidus hydrothermal alteration in their late stages of evolution is considered to be insufficient. Nevertheless, zircon Hf data of the Middle Paleoproterozoic (ca. 1.99 Ga) gneisses in the Jangsu-gun area have been suggested to reflect ancient crustal material that may have been extracted from the depleted mantle at ca. 3022–3670 Ma [14]. These subduction-related Middle Paleoproterozoic (ca. 1.99 Ga) gneisses occur widely along the northern margin of the Yeongnam massif [14]. The zircon Hf data of the Late Triassic (ca. 230 Ma) and Early Jurassic (187–180 Ma) granitic rocks in the Jangsu-gun area, together with the whole- rock Sr and Nd isotopic data [8−10], strongly reflect the mixing between primitive melts originated from the lithospheric mantle and the arc-related Middle Paleoproterozoic basement [13].

Minerals 2021, 11, 684 13 of 17 Minerals 2021, 11, x FOR PEER REVIEW 13 of 17

FigureFigure 12.12. NbNb versus versus Nb/Ta Nb/Ta diagram diagram for for the the Middle Middle Paleoproterozoic Paleoproterozoic orthogneisses orthogneisses and andEarly Early JurassicJurassic graniticgranitic rocksrocks inin thethe Jangsu-gunJangsu-gun area.area. Grey-coloredGrey-colored curvescurves representrepresent modelmodel ofof evolutionevolution of of NbNb andand TaTa in in liquid liquid L L00(Nb (Nb == 1212 ppm,ppm, TaTa == 1.51.5 ppm,ppm, Nb/TaNb/Ta = 8) 8) during during fractionation of assemblage mademade ofof 10 wt.%wt.% b biotiteiotite + + 10 10 wt wt.%.% muscovite muscovite + +80 80wt wt.%.% (quartz (quartz and and feldspar). feldspar). Numbers Numbers above above curvescurves indicate indicate amount amount of of fractional fractional crystallization. crystallization. Black Black dashed dashed line line represents represents same same model model during dur- fractionationing fractionation of assemblage of assemblage composed composed of 10 of wt.% 10 wt biotite.% biotite + 10 + wt.% 10 wt muscovite.% muscovite + 0.5 + wt.% 0.5 wt ilmenite.% il- + menite + 79.5 wt.% (quartz and feldspar). Symbols are as in Figure 4. 79.5 wt.% (quartz and feldspar). Symbols are as in Figure4.

5.2. TrackingNevertheless, of NORMs zircon from Hf the data Granitic of the R Middleocks in the Paleoproterozoic Jangsu-gun Area (ca. 1.99 Ga) gneisses in theThe Jangsu-gun natural radioactivity area have been levels suggested from the toMiddle reﬂect Paleoproterozoic ancient crustal material orthogneisses that may and haveparagneisses, been extracted and Late from Triassic the depleted and Early mantle Jurassic at ca. granitic 3022–3670 samples Ma [ 14in]. the These Jangsu subduction--gun area relatedmay be Middlerelated to Paleoproterozoic the occurrence characteristics (ca. 1.99 Ga) gneisses of radioactivity occur widely-related along accessory the northern minerals margin(i.e., with of thehigh Yeongnam concentrations massif of [ 14radioactive]. The zircon elements), Hf data and of the feldspar Late Triassic in the (ca.intermediate 230 Ma) andto fe Earlylsic rocks. Jurassic The (187–180Middle Paleoproterozoic Ma) granitic rocks orthogneisses in the Jangsu-gun and paragneisses, area, together and with Late theTri- whole-rockassic and Early Sr and Jurassic Nd isotopic granitic data samples [8–10 in], the strongly Jangsu reﬂect-gun area the mixing common betweenly include primitive zircon, meltsuraninite originated and monazite from the of lithospheric the accessory mantle minerals and the, and arc-related K-feldspar Middle of the Paleoproterozoic main minerals. basementThe variations [13]. between 226Ra, 232Th, and 40K activity concentrations, and U, Th and K2O contents in the whole-rock chemistry from the Middle Paleoproterozoic orthogneisses, 5.2.and Tracking Late Triassic of NORMs and Early from theJurassic Granitic granitic Rocks samples in the Jangsu-gun with continuously Area evolving smooth linearThe positive natural trends radioactivity well support levels fromthe presences the Middle of Paleoproterozoicthem (Figure 13). orthogneisses On variation anddia- paragneisses,grams in between and Late 226Ra, Triassic 232Th, and 40 EarlyK activity Jurassic concentrations granitic samples and inRCI the (radiation Jangsu-gun concen- area maytration be relatedindex) value to the occurrencefrom the Middle characteristics Paleoproterozoic of radioactivity-related orthogneiss and accessory paragneiss, minerals and (i.e.,Early with Jurassic high concentrationsgranitic samples of (Figure radioactive 14), elements),the RCI values and feldsparof the peraluminous in the intermediate Early Ju- to felsicrassic rocks. granitic The rocks Middle show Paleoproterozoic a good positive orthogneisses correlation with and 226 paragneisses,Ra and 232Th and concentration. Late Trias- sicIn andcontrast, Early the Jurassic Middle granitic Paleoproterozoic samples in theparagneisses Jangsu-gun show area rough commonly positive include linear zircon, trends uraninitebetween 226 andRa, monazite 232Th, and of 40 theK activity accessory concentrations minerals, and, and K-feldspar U, Th and of K the2O contents main minerals. (Figure 226 232 40 The13). variationsThey also display between a roughRa, positiveTh, and correlationK activity between concentrations, RCI values and, and U, 226 ThRa andand K2322ThO contentsconcentration in the whole-rock (Figure 14). chemistry The metaluminous from the Middle Early Paleoproterozoic Jurassic hornblende orthogneisses,-biotite granite and Late Triassic and Early Jurassic granitic samples with continuously evolving smooth linear samples show no correlation between 40K activity concentration and K2O content, because positivethey have trends an insufficient well support amount the presences of K-feldspar of them in (Figure the rocks 13). (Figure On variation 13). diagrams in between 226Ra, 232Th, and 40K activity concentrations and RCI (radiation concentration index) value from the Middle Paleoproterozoic orthogneiss and paragneiss, and Early Jurassic granitic samples (Figure 14), the RCI values of the peraluminous Early Jurassic granitic rocks show a good positive correlation with 226Ra and 232Th concentration. In contrast, the Middle Paleoproterozoic paragneisses show rough positive linear trends between 226Ra, 232 40 Th, and K activity concentrations, and U, Th and K2O contents (Figure 13). They also

Minerals 2021, 11, 684 14 of 17

display a rough positive correlation between RCI values, and 226Ra and 232Th concentration (Figure 14). The metaluminous Early Jurassic hornblende-biotite granite samples show no correlation between 40K activity concentration and K O content, because they have an Minerals 2021, 11, x FOR PEER REVIEW 2 14 of 17 insufﬁcient amount of K-feldspar in the rocks (Figure 13).

Figure 13. 226Ra versus U, 232Th versus Th, and 40K versus KO2 diagrams for the Middle Paleoproterozoic orthogneisses Figure 13. 226Ra versus U, 232Th versus Th, and 40K versus KO diagrams for the Middle Paleoproterozoic orthogneisses (A,E,I), Middle Paleoproterozoic paragneisses (B,F,J), Late Triassic2 granite (C,G,K), and Early Jurassic granites (D,H,L) in (A,theE,I), Jangsu Middle-gun Paleoproterozoic area. R2 is value paragneissesusing linear regression (B,F,J), Late for the Triassic number granite of analyzed (C,G,K sample), and Earlys in each Jurassic rock unit. granites Symbols (D,H are,L) in 2 theas Jangsu-gun in Figures 6 area. and R7. is value using linear regression for the number of analyzed samples in each rock unit. Symbols are as in Figures6 and7. In the present study, the Middle Paleoproterozoic granite orthogneisses and parag- In the present study, the Middle Paleoproterozoic granite orthogneisses and paragneisses, and Early Jurassic biotite and hornblende-biotite granites, with many analyzed neisses, and Early Jurassic biotite and hornblende-biotite232 granites, with many analyzed samples, are a good positive correlation between Th232 concentration and RCI values, com- samples,pared with are 226 aRa good and positive40K (Figure correlation 14). This reason between can beTh assumed concentration to be that and monazite RCI values, is 226 40 comparedthe main accessory with Ra mineral and controllingK (Figure 14 the). concentrations This reason can of be NORMs assumed in Middle to be that Paleopro- monazite isterozoic the main and accessory Early Jurassic mineral rocks controlling distributed the in concentrations the Jangsu-gun ofarea NORMs; although in Middlezircon, ap- Paleo- proterozoicatite, and titanite and Early can generally Jurassic be rocks produced distributed as accessory in the Jangsu-gunminerals containing area; although U and Th zircon, in apatite,the rocks and. Monazite titanite canexhibit generallys strong be compositional produced as accessorydifferences minerals at a variety containing of scales U in and the Th ingranitic the rocks. rocks Monazite [35]. In addition, exhibits strongmonazite compositional in peraluminous differences granites at is a reported variety ofto scalesbe of in thehigher granitic importance rocks [35, of]. all In the addition, investigated monazite elements in peraluminous, for the rock granitesbudget (e is.g. reported U, Th and to Y) be of higherin peraluminous importance, granites of all thethan investigated zircon, as an elements, important for carrier the rock of Y budget and U [36]. (e.g., U, Th and Y) in peraluminousThe RCI values granites for the than Middle zircon, Paleoproterozoic as an important gneisses carrier and of Y the and Early U [ 36Jurassic]. granitic rocks in the Jangsu area indicate that 32% of the rocks exceed the RCI guideline value of one for natural stone construction materials in South Korea, and that 58% of the rocks have a high RCI value of 0.8 or more (Table S3). Recently, in a preliminary study of natural radioactivity levels of 72 samples of the representative Permian, Triassic, Jurassic and Cre- taceous granitic rocks in South Korea, 29% of the rocks have been reported to have a high RCI value of 0.8 or more [6]. The 226Ra, 232Th, and 40K activity concentrations with the high RCI values in the rocks are being reported, which can affect the lungs or organs of the human body within an enclosed indoor space [21]. The RCI value must be reported in the use of finishing materials for natural stone in South Korea's apartment houses, and the use of natural stone is beginning to be regulated [20]. Therefore, regional and systematical

Minerals 2021, 11, x FOR PEER REVIEW 15 of 17

Minerals 2021, 11, 684 15 of 17 studies on 226Ra, 232Th, and 40K activity concentrations, and the RCI value of various rocks in South Korea, are required.

226 232 40 FigureFigure 14. 14.Plots Plots of theof the concentrations concentrations of of226 Ra,Ra,232 Th,Th, 40K, and RCI RCI values values for for the the Middle Middle Paleoproterozoic Paleoproterozoic orthogneisses orthogneisses (A,E,I), Middle Paleoproterozoic paragneisses (B,F,J), Late Triassic granite (C,G,K), and Early Jurassic granites (D,H,L) in (A,E,I), Middle Paleoproterozoic paragneisses (B,F,J), Late Triassic granite (C,G,K), and Early Jurassic granites (D,H,L) in the Jangsu-gun area. RCI, radiation concentration index. Symbols are as in Figures 6 and 7. the Jangsu-gun area. RCI, radiation concentration index. Symbols are as in Figures6 and7. 6. Conclusion The RCI values for the Middle Paleoproterozoic gneisses and the Early Jurassic granitic rocks inThe the Middle Jangsu Paleoproterozoic area indicate that (ca. 32%1.99 Ga of theand rocksca. 1.82 exceed Ga) gneisses the RCI form guideline the basement value of onein forthe naturalJangsu- stonegun area, construction while the materials Late Triassic in South (ca. 230 Korea, Ma) and and that Early 58% Jurassic of the rocksgranitic have rocks (187–180 Ma) mainly intrude the Middle Paleoproterozoic gneisses. The Middle a high RCI value of 0.8 or more (Table S3). Recently, in a preliminary study of natural Paleoproterozoic orthogneiss, and Late Triassic and Early Jurassic granitic rocks show arc- radioactivity levels of 72 samples of the representative Permian, Triassic, Jurassic and related granite features and have a weak to medium degree of fractional crystallization, Cretaceous granitic rocks in South Korea, 29% of the rocks have been reported to have a from metaluminous to weakly peraluminous granite, with ASI values of 0.92 to 1.40. The high RCI value of 0.8 or more [6]. The 226Ra, 232Th, and 40K activity concentrations with Middle Paleoproterozoic orthogneisses and paragneisses, and Late Triassic and Early Ju- the high RCI values in the rocks are being reported, which can affect the lungs or organs of rassic granitic rocks commonly have high concentrations of natural radioactivity com- the human body within an enclosed indoor space [21]. The RCI value must be reported in pared with the other gneiss and granitic rocks in South Korea. The trend of 226Ra, 232Th, theand use 40K of activity ﬁnishing concentrations materials for, naturaland the stonecomposition in South of Korea’s trace elements apartment (e.g., houses, U and and Th) the usefrom of naturalthe Middle stone Paleoproterozoic is beginning to orthogneisses be regulated, [and20]. Late Therefore, Triassic regional and Early and Jurassic systematical gra- 226 232 40 studiesnitic rocks on inRa, the JangsuTh, and-gun Karea activity, indicate concentrations, that monazite and is thethe RCImain value accessory of various mineral rocks incontrolling South Korea, the areconcentration required. of NORMs. Based on a detailed examination of the concentration changes in natural radioactivity in the granitic rocks of the Jangsu-gun area, 32% 6. Conclusions of the rocks exceed the recommended value of one of the guidelines for the RCI in South Korea.The Therefore, Middle Paleoproterozoic the Jangsu-gun (ca.area 1.99 presumably Ga and ca. has 1.82 a high Ga) concentration gneisses form of the NORMs, basement inwhi thech Jangsu-gun results in higher area, whileindoor the radon Late levels Triassic in residences, (ca. 230 Ma) compared and Early with Jurassic residences granitic in rocks (187–180 Ma) mainly intrude the Middle Paleoproterozoic gneisses. The Middle Paleoproterozoic orthogneiss, and Late Triassic and Early Jurassic granitic rocks show arc-related granite features and have a weak to medium degree of fractional crystallization, from metaluminous to weakly peraluminous granite, with ASI values of 0.92 to 1.40. The Middle Paleoproterozoic orthogneisses and paragneisses, and Late Triassic and Early Juras- sic granitic rocks commonly have high concentrations of natural radioactivity compared with the other gneiss and granitic rocks in South Korea. The trend of 226Ra, 232Th, and 40K Minerals 2021, 11, 684 16 of 17

activity concentrations, and the composition of trace elements (e.g., U and Th) from the Middle Paleoproterozoic orthogneisses, and Late Triassic and Early Jurassic granitic rocks in the Jangsu-gun area, indicate that monazite is the main accessory mineral controlling the concentration of NORMs. Based on a detailed examination of the concentration changes in natural radioactivity in the granitic rocks of the Jangsu-gun area, 32% of the rocks exceed the recommended value of one of the guidelines for the RCI in South Korea. Therefore, the Jangsu-gun area presumably has a high concentration of NORMs, which results in higher indoor radon levels in residences, compared with residences in the parts of South Korea. Accordingly, careful tracking and management of natural radioactive substances from rocks is required in the Jangsu area of central Southwestern South Korea.

Supplementary Materials: The following are available online at https://www.mdpi.com/article/ 10.3390/min11070684/s1, Table S1: U-Pb zircon data of the Middle Paleoproterozoic, Late Triassic and Early Jurassic rocks in the Jangsu-gun areas in South Korea, Table S2: major and trace element analyses of the Middle Paleoproterozoic, Late Triassic and Early Jurassic rocks in the Jangsu-gun areas in South Korea, Table S3: natural radioactivity concentrations of 226Ra (238U), 232Th and 40K in the Middle Paleoproterozoic, Late Triassic and Early Jurassic rocks in the Jangsu-gun areas in South Korea. Author Contributions: Data curation, S.W.K., W.-S.K. and S.L.; formal analysis, S.W.K., W.-S.K., B.C.L. and U.H.B.; funding acquisition, S.W.K.; investigation, S.W.K., W.-S.K., B.C.L. and U.H.B.; methodology, S.W.K. and S.L.; project administration, S.W.K.; supervision, S.W.K.; writing—original draft, S.W.K. and W.-S.K.; writing—review and editing, S.W.K. and S.L. All authors have read and agreed to the published version of the manuscript. Funding: This work was financially supported by grants from the Korea Institute of Geoscience and Mineral Resources (GP2021-004), funded by the Ministry of Science, Information, Communication and Technology. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Not applicable. Acknowledgments: We thank D.H. Kim at the Hanil Nuclear Power Co., Ltd. for assistance with gamma nuclide analysis. Three anonymous reviewers are thanked for their insightful suggestions and comments, which improved the quality of this manuscript. Conflicts of Interest: The authors declare no conflict of interest.

References 1. Mason, B.; Moore, C.B. Principles of Geochemistry, 4th ed.; John Wiley: New York, NY, USA, 1982. 2. Hwang, J. Occurrence of U-minerals and source of U in Groundwater in Daebo granite, Daejeon area. J. Eng. Geol. 2013, 23, 399–407, (In Korean with English abstract). [CrossRef] 3. Kim, M.S.; Kim, T.S.; Kim, H.K.; Kim, D.S.; Jeong, D.H.; Ju, B.K.; Hong, J.K.; Kim, H.J.; Park, S.H.; Jeong, C.H.; et al. Study on temporal decay characteristics of naturally occurring radionuclides in groudwater in two mica granite area. J. Soil Groundw. Environ. 2013, 18, 19–31, (In Korean with English abstract). [CrossRef] 4. Yun, S.W.; Lee, J.; Park, Y. Occurrence of radionuclides in groundwater of Korea according to geologic condition. J. Eng. Geol. 2016, 26, 71–78, (In Korean with English abstract). [CrossRef] 5. Hwang, J. Geological Review on the distribution and source of uraniferous groundwater in South Korea. J. Eng. Geol. 2018, 28, 593–603, (In Korean with English abstract). 6. Kim, S.W. Concentration of Radioactive Materials for the Phanerozoic Plutonic Rocks in Korea and Its Implication. Econ. Environ. Geol. 2020, 53, 565–583, (In Korean with English abstract). 7. Hwang, J.; Moon, S.-H. Geochemistry of U and Th of Mesozoic granites in South Korea: Implications of occurrences of different U-host minerals and dissolved U and Rn between Jurassic and Cretaceous granite aquifers. Geosci. J. 2021, 25, 183–195. [CrossRef] 8. Kee, W.S.; Kim, S.W.; Jeong, Y.J.; Kwon, S. Characteristics of Jurassic continental arc magmatism in South Korea: Tectonic implications. J. Geol. 2010, 118, 305–323. [CrossRef] 9. Kim, S.W.; Kwon, S.; Koh, H.J.; Yi, K.; Jeong, Y.; Santosh, M. Geotectonic framework of Permo-Triassic magmatism within the Korean Peninsula. Gondwana Res. 2011, 20, 865–889. [CrossRef] Minerals 2021, 11, 684 17 of 17

10. Kim, S.W.; Kwon, S.; Koh, K.; Yi, K.; Cho, D.-L.; Kee, W.-S.; Kim, B.C. Geochronological and geochemical implications of Early to Middle Jurassic continental adakitic arc magmatism in the Korean Peninsula. Lithos 2015, 227, 225–240. [CrossRef] 11. Kee, W.S.; Kim, S.W.; Hong, P.S.; Lee, B.C.; Cho, D.R.; Byun, U.H.; Ko, K.; Kwon, C.W.; Kim, H.C.; Jang, Y.; et al. Geologic Map of Korea (1:1,000,000); Korea Institute of Geoscience and Mineral Resources: Daejeon, Korea, 2019. 12. Oh, C.W.; Lee, B.C.; Yi, S.; Ryu, H.I. Correlation of Paleoproterozoic igneous and metamorphic events of the Korean Peninsula and China; Its implication to the tectonics of Northeast Asia. Precambrian Res. 2019, 326, 344–362. [CrossRef] 13. Cheong, A.C.; Jo, H.J. Tectonomagmatic evolution of a Jurassic Cordilleran flare-up along the Korean Peninsula: Geochronological and geochemical constraints from granitoid rocks. Gondwana Res. 2020, 88, 21–44. [CrossRef] 14. Cho, D.L.; Lee, B.C.; Oh, C.W. Petrogenesis of Paleoproterozoic (2.02–1.96 Ga) metagranitoids in the southwestern Yeongnam Massif, Korean Peninsula, and their significance for the tectonic history of northeast Asia: Insights from zircon U–Pb–Hf isotope and whole-rock geochemical compositions. Precambrian Res. 2020, 340, 105631. 15. Kim, S.W.; Kwon, S.; Jeong, Y.-J.; Kee, W.-S.; Lee, B.C.; Byun, U.H.; Ko, K.; Cho, D.-L.; Hong, P.S.; Park, S.-I.; et al. The Middle Permian to Triassic tectono-magmatic systems in the southern Korean Peninsula. Gondwana Res. 2020, in press. [CrossRef] 16. Wilson, M. Magmatic differentiation. J. Geol. Soc. 1993, 150, 611–624. [CrossRef] 17. Cuney, M. Uranium and thorium: The extreme diversity of the resources of the world’s energy minerals. In Non-Renewable Resource Issues; Springer: Dordrecht, The Netherlands, 2012; pp. 91–129. 18. Cuney, M. Felsic magmatism and uranium deposits. Bull. Soc. Geol. Fr. 2014, 185, 75–92. [CrossRef] 19. Gelman, S.E.; Deering, C.D.; Bachmann, O.; Huber, C.; Gutiérrez, F.J. Identifying the crystal graveyards remaining after large silicic eruptions. Earth Planet. Sci. Lett. 2014, 403, 299–306. [CrossRef] 20. Guidelines on the Radon Reduction and Management of Construction Material; Joint Ministries (Ministry of Environment, Ministry of Land, Infrastructure and Transport, and Nuclear Safety Committee): Sejong City, Korea, 2019; p. 21. (In Korean) 21. EC (European Comission). Radiation Protection 112—Radiological Protection Principles Concerning the Natural Radioactivity of Building Materials; European Commission: Luxembourg, 1999; ISBN 92-828-8376-0. 22. Kim, K.-B.; Chwae, U.; Hwang, J.H.; Kim, J.H. Geological Report of the Osu Sheet (1:50,000); Korea Institute of Geology, Mining and Materials: Daejeon, Korea, 1984; p. 38, (In Korean with English summary). 23. Hong, S.H.; Yun, W. Geological Report of the Changkye Sheet (1:50,000); Korea Institute of Geology, Mining and Materials: Daejeon, Korea, 1993; p. 17, (In Korean with English summary). 24. Kim, K.-B.; Chwae, U. Geological Report of the Hamyang Sheet (1:50,000); Korea Institute of Geology, Mining and Materials: Daejeon, Korea, 1994; p. 16, (In Korean with English summary). 25. Paton, C.; Woodhead, J.D.; Hellstrom, J.C.; Hergt, J.M.; Greig, A.; Maas, R. Improved laser ablation U-Pb zircon geochronology through robust downhole fractionation correction. Geochem. Geophys. Geosyst. 2010, 11, Q0AA06. [CrossRef] 26. Paton, C.; Hellstrom, J.; Paul, B.; Woodhead, J.; Hergt, J. Iolite: Freeware for the visualization and processing of mass spectrometric data. J. Anal. At. Spectrom. 2011, 26, 2508–2518. [CrossRef] 27. Ludwig, K.R. User’s Manual for Isoplot 3.6: A Geochronological Toolkit for Microsoft Excel; Berkeley Geochronology Center Special Publication 2008; Berkeley Geochronology Center: Berkeley, CA, USA, 2008. 28. Ludwig, K.R. User’s Manual for SQUID 2; Berkeley Geochronology Center Special Publication 2009; Berkeley Geochronology Center: Berkeley, CA, USA, 2009. 29. Middlemost, E.A.K. Naming materials in the magma/igneous rock system. Earth Sci. Rev. 1994, 37, 215–224. [CrossRef] 30. Shand, S.J. The Erutive Rocks: Their Genesis, Composition, and Classification, with a Chapter on Meteorites, 2nd ed.; John Wiley & Sons: New York, NY, USA, 1947; 444p. 31. Sun, S.; McDonough, W.F. Chemical and isotopic systematics of oceanic basalts: Implications for mantle composition and processes Basins. In Magmatism in the Ocean Basins; Saunders, A.D., Norry, M.J., Eds.; Special Publications 42; Geological Society of London: London, UK, 1989; pp. 313–345. 32. Pearce, J.A.; Harris, N.B.W.; Tindle, A.G. Trace element discrimination diagrams for the tectonic interpretation of granitic rocks. J. Petrol. 1984, 25, 956–983. [CrossRef] 33. Bateman, P.C.; Chappell, B.W. Crystallization, fractionation, and solidification of the Tuolumne intrusive series, Yosemite National Park, California. Geol. Soc. Am. Bull. 1979, 90, 465–482. [CrossRef] 34. Ballouard, C.; Poujol, M.; Boulvais, P.; Branquet, Y.; Tartèse, R.; Vigneresse, J.L. Nb-Ta fractionation in peraluminous granites: A marker of the magmatic-hydrothermal transition. Geology 2016, 44, 231–234. [CrossRef] 35. David, A.W.; Calvin, F.M. Accessory mineral behavior during differentiation of a granite suite: Monazite, xenotime and zircon in the Sweetwater Wash pluton, southeastern California, USA. Chem. Geol. 1993, 110, 49–67. 36. Karel, B. Monazite and zircon as major carriers of Th, U, and Y in peraluminous granites: Examples from the Bohemian Massif. Miner. Petrol. 2016, 110, 767–785.