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

: 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 226 Ra, 232 Th and 40 K 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. Geochemically, the Middle Paleoproterozoic orthogneiss, Late Triassic granites and Early Jurassic granitic rocks show typical arc-related metaluminous to weakly peraluminous fractionated granite features with ASI (aluminum saturation index) values of 0.92 to 1.40. The mean values of U (ppm) and Th (ppm) of the Middle Paleoproterozoic orthogneisses (6.4 and 20.5, respectively), Late Triassic granites (1.5 and 10.9), and Early Jurassic granites (3.5 and 16.5) were similar to those (5 and 15) of the granitic rocks in the Earth’s crust. The mean 226 Ra (Bq/kg), 232 Th (Bq/kg), and 40 K (Bq/kg) activity concentrations and radioactivity concentration index (RCI) are 62, 71, 1,214 and 0.96 for the Middle Paleoproterozoic orthogneisses; 16, 39, 1,614 and 0.78 for the Late Triassic granites; and 56, 70, 1031 and 0.88 for the Early Jurassic granitic rocks, respectively. The U, Th, 226 Ra, 232 Th, 40 K, and RCI of the Middle Paleoproterozoic biotite paragneisses are similar to those of the Middle Paleoproterozoic orthogneisses. The trend of 226 Ra, 232 Th, and 40 K activity concentrations, and the composition of U and Th from the Precambrian and Mesozoic rocks in the Jangsu-gun area indicates that monazite is the main accessory mineral controlling the concentration of natural radioactivity. Based on a detailed examination of the natural radioactivity in the rocks of the Jangsu-gun area, the Middle Paleoproterozoic orthogneisses and paragneisses, and Late Triassic and Early Jurassic granitic rocks show average high mean RCI values of 0.88 − 0.96, such that 32% of the rocks exceeded the recommended value of one in the guidelines for the RCI in South Korea. Especially, the RCI is closely related to the radon levels in the rocks. As a result, the Jangsu-gun area in South Korea is a relatively high radiological risk area, which exhibits higher indoor radon levels in the residences, compared with residences in the other areas in South Korea.


Introduction
All minerals and raw materials in the rocks present on Earth commonly contain natural radioactive elements with radionuclides, but natural radioactivity exposures to

Geological Setting
South Korea is located at the margin of the East Asian continent ( Figure 1A). The tectonic provinces of South Korea largely consist of Precambrian (mainly Middle Paleoproterozoic) basement rocks, such as those of the Gyeonggi and Yeongnam massifs. The Precambrian basement rocks are separated by two narrow supracrustal units (the Hongseong-Imjingang and Okcheon belts). These provinces also include the Cretaceous Gyeongsang basin, Cenozoic Yeonil basin, and Jeju volcanic terrain, which formed after East Asian continent-continent collision during the Mesozoic ( Figure 1B).
The study area is located in the Jangsu-gun area of central Southwestern South Korea; it is distributed within 35°28' to 35°49'N latitude and 127°22' to 127°42'E longitude, and has a total area of approximately 533.43 km 2 . It is geologically located in the central-western area of the Precambrian Yeongnam massif. In the Jangsu-gun area, there is a wide distribution of the Middle Paleoproterozoic

Geological Setting
South Korea is located at the margin of the East Asian continent ( Figure 1A). The tectonic provinces of South Korea largely consist of Precambrian (mainly Middle Paleoproterozoic) basement rocks, such as those of the Gyeonggi and Yeongnam massifs. The Precambrian basement rocks are separated by two narrow supracrustal units (the Hongseong-Imjingang and Okcheon belts). These provinces also include the Cretaceous Gyeongsang basin, Cenozoic Yeonil basin, and Jeju volcanic terrain, which formed after East Asian continent-continent collision during the Mesozoic ( Figure 1B).

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 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 et al. [15]. All ages were calculated with 2σ error, and data reduction was conducted using 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. Seventy-seven whole-rock samples of the Middle Paleoproterozoic orthogneisses and paragneisses, and Mesozoic granitic rocks were analyzed for major, trace, and rare 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%.
Precise determination of the activity of naturally occurring radionuclides ( 226 Ra, 232 Th and 40 K) was performed on seventy-seven whole-rock samples from the Jangsu-gun area, and the radioactivity concentration index was calculated. Most of the nuclide analyses were performed using a p-type high-purity germanium radiation detector (HPGe radiation detector) at the Hanil Nuclear Power Co., Ltd. A multichannel spectrum analyzer (BSI Hybrid, Latvia) was used for the measurement. The detector was calibrated through selfabsorption correction, according to the energy and efficiency calibration density of the detector, using a geometrical standard source identical to the source used for the analysis sample. Each analysis sample was subjected to sample pretreatment processes, such as grinding, sieving, drying, mixing, and filling, using an aluminum mixing container; the

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 (KBSI). For some samples, U-Pb dating was also performed using a sensitive high-resolution ion microprobe (SHRIMP-IIe/MC) at the KBSI. Zircon grains were handpicked 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 et al. [15]. All ages were calculated with 2σ error, and data reduction was conducted using 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.
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 226 Ra, 232 Th, and 40 K 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.
Granite orthogneisses with biotite paragneiss are regionally exposed in the Jangsugun area. Zircons from the granite orthogneisses in the biotite gneiss give dispersed ages along the discordance line, which projected to a Middle Paleoproterozoic upper concordia Precise determination of the activity of naturally occurring radionuclides ( 226 Ra, 232 Th and 40 K) was performed on seventy-seven whole-rock samples from the Jangsu-gun area, and the radioactivity concentration index was calculated. Most of the nuclide analyses were performed using a p-type high-purity germanium radiation detector (HPGe radiation detector) at the Hanil Nuclear Power Co., Ltd. A multichannel spectrum analyzer (BSI Hybrid, Latvia) was used for the measurement. The detector was calibrated through self-absorption correction, according to the energy and efficiency calibration density of the detector, using a geometrical standard source identical to the source used for the analysis sample. Each analysis sample was subjected to sample pretreatment processes, such as grinding, sieving, drying, mixing, and filling, using an aluminum mixing container; the 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 226 Ra in the sample was measured after sealing and storing for more than three weeks, to prevent 222 Rn from escaping out of the measuring container. For 232 Th, 228 Ac, a progeny of Th, was measured immediately after sealing; 40 K 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 226 Ra, 232 Th, and 40 K 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. exposed in the central part of the Jangsu-gun area. Zircons from the hornblende-biotit granite samples (JS-53 and KJS-04) give weighted mean 206 Pb/ 238 U ages of 187.2 ± 2.3 M and 181.8 ± 0.4 Ma, respectively ( Figure 5E,F). Zircons from the biotite granite sampl (KJS-24) give a weighted mean 206 Pb/ 238 U age of 184.8 ± 0.4 Ma ( Figure 5G). Most of th analyzed zircons with oscillatory zoning from two-mica granite give a weighted mean 206 Pb/ 238 U age of 180.3 ± 1.6 Ma ( Figure 5H).
Granite orthogneisses with biotite paragneiss are regionally exposed in the Jangsugun area. Zircons from the granite orthogneisses in the biotite gneiss give dispersed ages along the discordance line, which projected to a Middle Paleoproterozoic upper concordia 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 207 Pb/ 206 Pb 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 206 Pb/ 238 U 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 206 Pb/ 238 U ages of 187.2 ± 2.3 Ma and 181.8 ± 0.4 Ma, respectively ( Figure 5E,F). Zircons from the biotite granite sample (KJS-24) give a weighted mean 206 Pb/ 238 U age of 184.8 ± 0.4 Ma ( Figure 5G). Most of the analyzed zircons with oscillatory zoning from two-mica granite give a weighted mean 206 Pb/ 238 U age of 180.3 ± 1.6 Ma ( Figure 5H).

Geochemistry
The Middle Paleoproterozoic granite orthogneisses, and Early Jurassic granitic rocks from the Jangsu-gun area were mainly analyzed to determine their geochemical signatures (Table S2). The Middle Paleoproterozoic biotite paragneisses and Late Triassic granites were also analyzed to interpret geochemical characteristics in the Jangsu-gun area. On the SiO 2 versus Na 2 O + K 2 O diagram, with fields defined by [29] (Figure 6

Geochemistry
The Middle Paleoproterozoic granite orthogneisses, and Early Jurassic granitic rocks from the Jangsu-gun area were mainly analyzed to determine their geochemical signatures (Table S2). The Middle Paleoproterozoic biotite paragneisses and Late Triassic granites were also analyzed to interpret geochemical characteristics in the Jangsu-gun area. On the SiO2 versus Na2O + K2O diagram, with fields defined by [29] (Figure 6  The major element abundances plotted against SiO2 of the orthogneisses (leucogranite orthogneiss, porphyritic orthogneiss, and granite orthogneiss) with biotite paragneisses, Late Triassic porphyritic granite and Early Jurassic granitic rocks show negative correlations with Al2O3, Fe2O3 * , MgO, CaO, P2O5 and TiO2; they show no correlations 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), 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).

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). The ASI values of the Middle Paleoproterozoic orthogneisses (1.0 to 1.1, except for 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., metasedimentary rock) is widely distributed. The Late Triassic and Early Jurassic granitic rocks in the Jangsu-gun area showed metaluminous to weakly peraluminous granite characteristics, with an ASI of 0.9 to 1.1. In some rocks, the ASI values range from 1.1 to 1.4. An ASI value change of 0.9 to 1.4 in the Middle Paleoproterozoic orthogneisses, and Late Triassic and Early Jurassic granitic rocks in the Jangsu-gun area discriminates amphibole and pyroxene formation during magma crystallization, which increases the ASI value of the residual melt. This reveals the evolution process to peraluminous granites containing biotite and muscovite. Therefore, tracking the degree of fractional crystallization or partial melting of granitic rocks provides important information that is useful for explaining the petrogeneses of granitic rocks distributed in the Jangsu-gun area [33].
The degree of fractional crystallization in the peraluminous granitic rocks can be determined from the Nb/Ta ratio in the whole-rock chemistry ( Figure 12) [34]. In general, a decrease in the Nb/Ta ratio in evolved melts suggests effects of both fractional crystallization and sub-solidus hydrothermal alteration. The extensively surveyed Middle Paleoproterozoic orthogneiss and Early Jurassic granitic rocks in the Jangsu-gun area show a degree of fractional crystallization of approximately 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 mar- The ASI values of the Middle Paleoproterozoic orthogneisses (1.0 to 1.1, except for 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., metasedimentary rock) is widely distributed. The Late Triassic and Early Jurassic granitic rocks in the Jangsu-gun area showed metaluminous to weakly peraluminous granite characteristics, with an ASI of 0.9 to 1.1. In some rocks, the ASI values range from 1.1 to 1.4. An ASI value change of 0.9 to 1.4 in the Middle Paleoproterozoic orthogneisses, and Late Triassic and Early Jurassic granitic rocks in the Jangsu-gun area discriminates amphibole and pyroxene formation during magma crystallization, which increases the ASI value of the residual melt. This reveals the evolution process to peraluminous granites containing biotite and muscovite. Therefore, tracking the degree of fractional crystallization or partial melting of granitic rocks provides important information that is useful for explaining the petrogeneses of granitic rocks distributed in the Jangsu-gun area [33].
The degree of fractional crystallization in the peraluminous granitic rocks can be determined from the Nb/Ta ratio in the whole-rock chemistry ( Figure 12) [34]. In general, a decrease in the Nb/Ta ratio in evolved melts suggests effects of both fractional crystallization and sub-solidus hydrothermal alteration. The extensively surveyed Middle Paleoproterozoic orthogneiss and Early Jurassic granitic rocks in the Jangsu-gun area show a degree of fractional crystallization of approximately 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.

Tracking of NORMs from the Granitic Rocks in the Jangsu-gun Area
The natural radioactivity levels from the Middle Paleoproterozoic orthogneisses and paragneisses, and Late Triassic and Early Jurassic granitic samples in the Jangsu-gun area may be related to the occurrence characteristics of radioactivity-related accessory minerals (i.e., with high concentrations of radioactive elements), and feldspar in the intermediate to felsic rocks. The Middle Paleoproterozoic orthogneisses and paragneisses, and Late Triassic and Early Jurassic granitic samples in the Jangsu-gun area commonly include zircon, uraninite and monazite of the accessory minerals, and K-feldspar of the main minerals. The variations between 226 Ra, 232 Th, and 40 K activity concentrations, and U, Th and K2O contents in the whole-rock chemistry from the Middle Paleoproterozoic orthogneisses, and Late Triassic and Early Jurassic granitic samples with continuously evolving smooth linear positive trends well support the presences of them ( Figure 13). On variation diagrams in between 226 Ra, 232 Th, and 40 K 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 226 Ra and 232 Th concentration. In contrast, the Middle Paleoproterozoic paragneisses show rough positive linear trends between 226 Ra, 232 Th, and 40 K activity concentrations, and U, Th and K2O contents ( Figure  13). They also display a rough positive correlation between RCI values, and 226 Ra and 232 Th concentration ( Figure 14). The metaluminous Early Jurassic hornblende-biotite granite samples show no correlation between 40 K activity concentration and K2O content, because they have an insufficient amount of K-feldspar in the rocks ( Figure 13). 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 subductionrelated 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][9][10], strongly reflect the mixing between primitive melts originated from the lithospheric mantle and the arc-related Middle Paleoproterozoic basement [13].

Tracking of NORMs from the Granitic Rocks in the Jangsu-gun Area
The natural radioactivity levels from the Middle Paleoproterozoic orthogneisses and paragneisses, and Late Triassic and Early Jurassic granitic samples in the Jangsu-gun area may be related to the occurrence characteristics of radioactivity-related accessory minerals (i.e., with high concentrations of radioactive elements), and feldspar in the intermediate to felsic rocks. The Middle Paleoproterozoic orthogneisses and paragneisses, and Late Triassic and Early Jurassic granitic samples in the Jangsu-gun area commonly include zircon, uraninite and monazite of the accessory minerals, and K-feldspar of the main minerals. The variations between 226 Ra, 232 Th, and 40 K activity concentrations, and U, Th and K 2 O contents in the whole-rock chemistry from the Middle Paleoproterozoic orthogneisses, and Late Triassic and Early Jurassic granitic samples with continuously evolving smooth linear positive trends well support the presences of them ( Figure 13). On variation diagrams in between 226 Ra, 232 Th, and 40 K 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 226 Ra and 232 Th concentration. In contrast, the Middle Paleoproterozoic paragneisses show rough positive linear trends between 226 Ra, 232 Th, and 40 K activity concentrations, and U, Th and K 2 O contents ( Figure 13). They also display a rough positive correlation between RCI values, and 226 Ra and 232 Th concentration ( Figure 14). The metaluminous Early Jurassic hornblende-biotite granite samples show no correlation between 40 K activity concentration and K 2 O content, because they have an insufficient amount of K-feldspar in the rocks ( Figure 13). In the present study, the Middle Paleoproterozoic granite orthogneisses and paragneisses, and Early Jurassic biotite and hornblende-biotite granites, with many analyzed samples, are a good positive correlation between 232 Th concentration and RCI values, compared with 226 Ra and 40 K ( Figure 14). This reason can be assumed to be that monazite is the main accessory mineral controlling the concentrations of NORMs in Middle Paleoproterozoic and Early Jurassic rocks distributed in the Jangsu-gun area; although zircon, apatite, and titanite can generally be produced as accessory minerals containing U and Th in the rocks. Monazite exhibits strong compositional differences at a variety of scales in the granitic rocks [35]. In addition, monazite in peraluminous granites is reported to be of higher importance, of all the investigated elements, for the rock budget (e.g. U, Th and Y) in peraluminous granites than zircon, as an important carrier of Y and U [36].
The RCI values for the Middle Paleoproterozoic gneisses and the Early Jurassic 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 Cretaceous 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 226 Ra, 232 Th, and 40 K 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 In the present study, the Middle Paleoproterozoic granite orthogneisses and paragneisses, and Early Jurassic biotite and hornblende-biotite granites, with many analyzed samples, are a good positive correlation between 232 Th concentration and RCI values, compared with 226 Ra and 40 K ( Figure 14). This reason can be assumed to be that monazite is the main accessory mineral controlling the concentrations of NORMs in Middle Paleoproterozoic and Early Jurassic rocks distributed in the Jangsu-gun area; although zircon, apatite, and titanite can generally be produced as accessory minerals containing U and Th in the rocks. Monazite exhibits strong compositional differences at a variety of scales in the granitic rocks [35]. In addition, monazite in peraluminous granites is reported to be of higher importance, of all the investigated elements, for the rock budget (e.g., U, Th and Y) in peraluminous granites than zircon, as an important carrier of Y and U [36].

Conclusion
The Middle Paleoproterozoic (ca. 1.99 Ga and ca. 1.82 Ga) gneisses form the basement in the Jangsu-gun area, while the Late Triassic (ca. 230 Ma) and Early Jurassic granitic rocks (187-180 Ma) mainly intrude the Middle Paleoproterozoic gneisses. The Middle Paleoproterozoic orthogneiss, and Late Triassic and Early Jurassic granitic rocks show arcrelated 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 Jurassic granitic rocks commonly have high concentrations of natural radioactivity compared with the other gneiss and granitic rocks in South Korea. The trend of 226 Ra, 232 Th, and 40 K 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 RCI values for the Middle Paleoproterozoic gneisses and the Early Jurassic 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 Cretaceous 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 226 Ra, 232 Th, and 40 K 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 studies on 226 Ra, 232 Th, and 40 K activity concentrations, and the RCI value of various rocks in South Korea, are required.

Conclusions
The Middle Paleoproterozoic (ca. 1.99 Ga and ca. 1.82 Ga) gneisses form the basement in the Jangsu-gun area, while the Late Triassic (ca. 230 Ma) and Early Jurassic granitic 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 Jurassic granitic rocks commonly have high concentrations of natural radioactivity compared with the other gneiss and granitic rocks in South Korea. The trend of 226 Ra, 232 Th, and 40 K 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