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Article The and of as Evidence for the Reconcentration of REE in the Triassic Period in the Chungju Area, South Korea

Sang-Gun No 1,* and Maeng-Eon Park 2 1 Resources Development Research Center, Korea Institute of Geoscience and Mineral Resources, Daejeon 34132, Korea 2 Department of Environmental Science, Pukyong National University, Busan 48513, Korea; [email protected] * Correspondence: [email protected]; Tel.: +82-10-9348-7807

 Received: 1 November 2019; Accepted: 2 January 2020; Published: 5 January 2020 

Abstract: The Chungju rare-earth element (REE) deposit is located in the central part of the Okcheon Metamorphic Belt (OMB) in the Southern Korean Peninsula and research on REE mineralization in the Gyemyeongsan Formation has been continuous since the first report in 1989. The genesis of the REE mineralization that occurred in the Gyemyeongsan Formation has been reported by previous researchers; theories include the fractional crystallization of alkali , magmatic hydrothermal alteration, and recurrent mineralization during . In the Gyemyeongsan Formation, we discovered an allanite-rich vein that displays the paragenetic relationship of , allanite, and zircon, and we investigated the chemistry and chronology of zircon obtained from this vein. We analyzed the zircon’s chemistry with an electron probe X-ray micro analyzer (EPMA) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). The grain size of the zircon is as large as 50 µm and has an inherited core (up to 15 µm) and micrometer-sized sector zoning (up to several micrometers in size). In a previous study, the zircon ages were not obtained because the grain size was too small to analyze. In this study, we analyzed the zircon with laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS) for dating purposes. The REE patterns and occurrence of zircon in the quartz–allanite vein match well with previous reported recrystallized zircon, while the behavior of the trace elements shows differences with magmatic and hydrothermal zircon. The 206Pb/238U ages obtained from the zircon in the quartz–allanite vein are from 240.1 2.9 to 257.1 3.5 Ma and this age is included in the tectonic evolution period of the study ± ± area. Therefore, we suggest that the quartz–allanite veins in the Gyemyeongsan Formation were formed during the late Permian to early Triassic metamorphic period and the zircon was recrystallized at that time. The Triassic age is the first reported age with zircon dating in the Gyemyeongsan Formation and will be an important data-point for the study of the tectonic evolution of the OMB.

Keywords: allanite-rich vein; LA-MC-ICP-MS; recrystallized zircon; Triassic metamorphism; tectonic evolution of the OMB

1. Introduction Rare-earth elements (REE) are of great importance and are used in a wide range of applications, especially in high-tech consumer products, such as cellular telephones, computer hard drives, lasers, magnets, batteries, radar and sonar systems, electric and hybrid vehicles, and flat-screen monitors and televisions. However, global REE production has been dominated by China. The United States of America (USA) was a significant producer through the 1990s, but the sale of low-priced materials by China forced mines in the USA and other countries out of operation. As a result of China limiting

Minerals 2020, 10, 49; doi:10.3390/min10010049 www.mdpi.com/journal/minerals Minerals 2020, 10, 49 2 of 14 exports and the dramatic increase in prices in 2009 and 2010, mines in worldwide exploration became active again. Since REE and high-field-strength element (HFSE) mineralization was first reported in 1989 in the Chungju area in the central part of the Southern Korean Peninsula [1], a steady stream of studies has been carried out [2–10]. The genesis of REE and HFSE mineralization [2,3], the chemical compositions of alkali [4], the chemistry of REE-bearing minerals [5,6], and the associated and mineralization [7] have been studied. Constraining the age of the REE and HFSE mineralization at the Chungju area is important to understand the genesis and complicated history of mineralization. Recently, dating of zircon that is too small (10–20 µm) for sensitive high-resolution ion microprobe (SHRIMP) analysis was conducted with laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS) [10]. This analysis revealed that Paleozoic granite (331 1.5 Ma), ± which intruded into the Gyemyeongsan Formation, is involved in REE and HFSE mineralization [10]. We discovered a quartz–allanite vein containing REE minerals and aggregates of zircon in the Gyemyeongsan Formation. In this study, we focus on zircon geochronology and chemistry with an electron probe X-ray micro analyzer (EPMA), laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), and LA-MC-ICP-MS in the quartz–allanite vein to clarify the mineralization history and the origin of the REE and HFSE mineralization in the Gyemyeongsan Formation.

2. Geological Setting The Korean Peninsula comprises three major massifs: the Nangrim, Gyeonggi, and Yeongnam massifs (Figure1). The Gyeonggi and Yeongnam massifs are separated by the Okcheon Belt, which comprises the weakly metamorphosed Paleozoic Taebaeksan Basin and the Okcheon Metamorphic Belt (OMB). The OMB is a notoriously complicated polymetamorphic terrane in the Korean Peninsula. The OMB mainly consists of metavolcanic and metasedimentary sequences that range in metamorphic grade up to amphibolite facies conditions [11]. Oh (2006) [12] suggests that the OMB can be correlated with the suture zone between the Yangtze and Cathaysia blocks because the suture zone in the South China Block was reactivated as a rift at 820–750 Ma during the breakup of the Rodinia [13], which caused rift-related bimodal volcanism (756 Ma) in the OMB [14]. The OMB has been interpreted to represent a stack of syn-metamorphic nappes that comprise, from bottom to top, the Chungju, Turungsan, Pinbanryeong, Iwharyeong, and Poeun structural units [15,16] (Figure1). The Gyemyeongsan Formation in the Chungju unit (Figure1) is mainly composed of metavolcanic rocks [17] and iron-bearing metaquartzites [2]. Schistose rocks are the dominant rock type of the Gyemyeongsan Formation and are derived from lava and tuffs [17]. The quartz–feldspar schist, which is one of the schistose rocks, contains substantial amounts of REE-bearing minerals, such as allanite, sphene, , and zircon [2]. and bearing is commonly intercalated with schistose rocks [2]. The granitic rocks that intruded into the Gyemyeongsan Formation occur as plutons, stocks, and dikes (Figure1). These granitic rocks include Paleozoic alkali granite [ 10] and Mesozoic granite [9]. Minerals 2020, 10, 49 3 of 14 Minerals 2019, 9, x FOR PEER REVIEW 3 of 14

Figure 1. TheThe location location of of the the Chungju Chungju deposit deposit and its geological geological setting. ( (aa)) A A simplified simplified map map showing showing the tectonic provinces of East Asia,Asia, including the KoreanKorean Peninsula.Peninsula. ( b) A geological map showing the stratigraphic/lithotectonic stratigraphic/lithotectonic un unitsits of of the the Okcheon Okcheon Metamorphic Metamorphic Belt Belt and and the theTaebaeksan Taebaeksan Basin. Basin. (c) (c) A geological map of the study area. The figures are modified from reference [9]. A geological map of the study area. The figures are modified from reference [9].

3. Methods

Minerals 2020, 10, 49 4 of 14

3. Methods

3.1. EPMA The chemical compositions of the collected from the hydrothermal vein were analyzed by wavelength dispersive spectrometry using an electron microprobe (EPMA; JEOL JXA-8100) at Gyeongsang National University (Jinju, Korea). Quantitative analyses were carried out using instrumental settings, including an accelerating voltage of 15 kV, a beam diameter of 5 µm, and a counting time of 20–40 s for peak measurements. In-house standards were used for the analysis.

3.2. LA-ICP-MS Analyses were carried out at the Korea Basic Science Institute, Ochang (Cheongju, Korea), using a NewWave UP 213 Nd:YAG laser coupled to a Thermo X2 series ICP-MS. The samples were then analyzed for about 30 s using a spot size of 30 µm for zircon at a 10 Hz repetition rate and with a fluence of 15 J/cm2. Data reduction was carried out with the Glitter program [18]. The ablated aerosol was carried to the ICP source with He gas. The plasma was operated at 1300 W and the Ar carrier gas flow rate was 0.73 L/min. The certified reference glass NIST 612 was used to tune the instrument, to monitor drift, and as the primary standard for calibration. 29Si was used as the internal standard, with the Si concentrations having been previously determined by EMPA.

3.3. Laser Ablation Multi-Collector (LA-MC)-ICP-MS Zircons collected for U–Pb isotopic analyses were mounted as rock chips in epoxy resin. We analyzed the zircons with a Nu Plasma II Multi-collector ICP-MS coupled to a NewWave Research 193 nm ArF excimer Laser Ablation system at the Korea Basic Science Institute, Ochang (Cheongju, Korea). This technique is very similar to that published by Paton et al. [19]. Samples were ablated in helium (He) gas (with a flow rate of 970 mL/min). Prior to entering the plasma, the He aerosol was mixed with Ar (flow rate = 600 mL/min). All analyses were completed in the static ablation mode under normal conditions, including a beam diameter of 7 to 15 µm, a pulse frequency of 5 Hz, and a beam energy density of 2.81 J/cm2. A single U–Pb measurement included 30 s of on-mass background measurement, followed by 30 s of ablation with a stationary beam. The masses of Pb 202, 204, 206, 207, and 208 were measured in secondary electron multipliers, and masses of 232 and 238 were measured in an extra-high-mass Faraday collector. 235U was calculated from the signal measured at mass 238, with 238U/235U = 137.88. Mass 204 was used to monitor common 204Pb after discarding the 204Hg background. In ICP-MS analyses, 204Hg mainly originates from the He supply. The background counting rate for a mass of 204 was calculated on the basis of 202Hg. Age-related common lead correction [20] was used when the analysis revealed common lead contents above the detection limit. Two calibration standards were run in duplicate at the beginning and end of each analytical session, as well as at regular intervals during each session. Raw data were corrected for background, laser-induced elemental fractionation, mass discrimination, and drift in ion counter gains. They were then reduced to U–Pb ratios by their calibration to concordant reference zircons of known ages, with protocols adapted from Andersen et al. [21]. The reference materials zircons 91500 (1065 Ma; [22]) and Plešovice (337 Ma; [23]) were used for this calibration. Data processing and age calculations were performed off-line with the Iolite 2.5 [19] and Isoplot 3.71 [24] programs. To minimize the effects of laser-induced elemental fractionation, the depth-to-diameter ratio of the ablation pit was kept low, and isotopically homogeneous segments of time-resolved traces were calibrated against the corresponding time interval for each mass in the reference zircon. To compensate for drift in instrument sensitivity and Faraday versus electron multiplier gain within an analytical session, a correlation of signal versus time was assumed for the reference zircons. All ages were calculated with 2σ uncertainties and without decay constant uncertainty. Data point uncertainty ellipses in all figures are presented at the 2σ level. Minerals 2020, 10, 49 5 of 14

4. Results

4.1.Minerals Quartz–Allanite 2019, 9, x FOR VeinPEER andREVIEW Zircon Occurrence in the Gyemyeongsan Formation 5 of 14 The quartz–allanite (allanite has not yet been defined according to an official nomenclature) veins The quartz–allanite (allanite has not yet been defined according to an official nomenclature) wereveins emplaced were emplaced along the along foliation the foliation in the metavolcanic in the metavolcanic rocks (Gyemyeongsan rocks (Gyemyeongsan Formation) Formation) and crosscut and thecrosscut Paleozoic the alkali Paleozoic granite alkali [10 ]granite (Figure 2[10]). In (Figure some areas,2). In thesesome veinsareas, appear these veins similar appear to coarse-grained similar to pegmatitecoarse-grained (Figure pegmatite2). It has sharp (Figure contacts 2). It has with sharp metavolcanic contacts with rocks metavolcanic and alkali graniterocks and (Figure alkali2b). granite These veins(Figure are mainly2b). These composed veins are of allanitemainly composed (up to 80%), of quartz,allanite fluorite,(up to 80%), microcline, quartz, minor , chlorite, microcline, sericite, andminor microcrystalline chlorite, sericite, (up and to 50 microcrystallineµm in length) zircon(up to (Figure50 µm in3). length) Coarse-grained zircon (Figure euhedral 3). Coarse-grained allanite grains showeuhedral various allanite sizes andgrains shapes show (Figure various3). sizes Aggregates and shapes of microcrystalline (Figure 3). Aggregates zircon (eachof microcrystalline grain size from 5 tozircon 50 µ m(each in length) grain size have from irregular 5 to 50 shapes,µm in length) coprecipitated have irregular with allaniteshapes, coprecipitated and unknown with REE-minerals allanite (Figureand unknown3d). The resultsREE-minerals of EDS analysis(Figure 3d). (3 points) The results unknown of EDS minerals analysis are (3 composedpoints) unknown of O, F, minerals Ca, V, Ag, Cs,are Ba, composed and La. of O, F, Ca, V, Ag, Cs, Ba, and La.

FigureFigure 2. 2.Allanite Allanite vein vein intrusion intrusionin in thethe GyemyeongsanGyemyeongsan Formation Formation and and in in alkali alkali granite. granite. (a) ( aThe) The allanite allanite veinvein consists consists of allaniteof allanite aggregates aggregates and and felsic minerals. minerals. (b) The(b) The sharp sharp contact contact between between the alkali the alkali granite andgranite allanite and vein allanite composed vein composed of allanite of aggregates, allanite aggr fragmentsegates, fragments of alkali granite, of alkali and granite, the latest and quartzthe latest vein. quartz vein.

Minerals 2020, 10, 49 6 of 14 Minerals 2019, 9, x FOR PEER REVIEW 6 of 14

Figure 3. Photomicrograph showing the paragenesis of thethe quartz–allanitequartz–allanite vein.vein. (a) Coarse-grained allanite. ( b)) Coarse-grained allanite, quartz, and aggregates of microcrystalline zircon. ( c) Irregular shape ofof thethe aggregatesaggregates ofof microcrystalline microcrystalline zircon zircon (Back-scattered (Back-scattered electron electron image). image). (d ()d Enlarged) Enlarged part part of figureof figure c. Zirconc. Zircon present present varied vari ined grain in grain size (from size 5(from to 50 5µ m)to and50 µm shapes.) and Abbreviations:shapes. Abbreviations: aln (allanite), aln qz(allanite), (quartz), qz and (quartz), zrn (zircon). and zrn (zircon). 4.2. Geochronology 4.2. Geochronology In previous studies, the zircon ages associated with REE mineralization from metavolcanic In previous studies, the zircon ages associated with REE mineralization from metavolcanic rock rock (Gyemyeongsan Formation) were not obtained because the grain size was too small to analyze. (Gyemyeongsan Formation) were not obtained because the grain size was too small to analyze. In In this study, we analyzed U–Th–Pb isotope systematics (Table1) in zircon from the quartz–allanite this study, we analyzed U–Th–Pb isotope systematics (Table 1) in zircon from the quartz–allanite vein that occurred in metavolcanic rock via LA-MC-ICP-MS. Analysis of the cathodoluminescence vein that occurred in metavolcanic rock via LA-MC-ICP-MS. Analysis of the cathodoluminescence images (Figure4) revealed that the zircons from the quartz–allanite veins comprised aggregated images (Figure 4) revealed that the zircons from the quartz–allanite veins comprised aggregated microcrystalline grains. The zircon grains were euhedral and short-prismatic. Most of grains were less microcrystalline grains. The zircon grains were euhedral and short-prismatic. Most of grains were than 30 µm in length and several exceeded 50 µm. In the CL (cathodoluminescence) images (Figure4b), less than 30 µm in length and several exceeded 50 µm. In the CL (cathodoluminescence) images the zircon grains showed overgrowth rim (up to several micrometers in size) or weak zoning. Some (Figure 4b), the zircon grains showed overgrowth rim (up to several micrometers in size) or weak zircon grains (50 µm size in length) have inherited cores with very small sizes (<20 µm) (Figure4b). zoning. Some zircon grains (50 µm size in length) have inherited cores with very small sizes (<20 µm) We tried to analyze of parts of the zircon grains except the inherited cores. (Figure 4b). We tried to analyze of parts of the zircon grains except the inherited cores. Forty U–Th–Pb isotopic analyses of zircon in quartz–allanite vein are reported in Table1. 15 spots Forty U–Th–Pb isotopic analyses of zircon in quartz–allanite vein are reported in Table 1. 15 on zircon yield concordant to near-concordant 206Pb/238U ages ranging from 240.1 2.9 to 257.1 3.5 Ma, spots on zircon yield concordant to near-concordant 206Pb/238U ages ranging from± 240.1 ± 2.9± to 257.1 with a weighted mean age of 247.9 2.8 Ma (2σ, MSWD = 15, Figure4e,f). The zircons had Th /U ratios ± 3.5 Ma, with a weighted mean age± of 247.9 ± 2.8 Ma (2σ, MSWD = 15, Figure 4e,f). The zircons had of 0.21 to 0.68. The other twenty-five spots yielded discordant ages. The age versus Th/U ratio was Th/U ratios of 0.21 to 0.68. The other twenty-five spots yielded discordant ages. The age versus Th/U presented with Paleozoic magmatic zircon from alkali granite [10] in Figure4d. ratio was presented with Paleozoic magmatic zircon from alkali granite [10] in Figure 4d.

Minerals 2020, 10, 49 7 of 14 Minerals 2019, 9, x FOR PEER REVIEW 7 of 14

Figure 4. U–Pb isotope analyses. (a) Back-scattered electron (BSE) image of zircon from the Figure 4. U–Pb isotope analyses. (a) Back-scattered electron (BSE) image of zircon from the zircon– zircon–allanite vien. (b) Cathodoluminescence (CL) image of the euhedral microcrystalline zircon. allanite vien. (b) Cathodoluminescence (CL) image of the euhedral microcrystalline zircon. Note that Note that some zircons have an inherited core and oscillatory zoning. (c) Tera–Wasserburg Concordia some zircons have an inherited core and oscillatory zoning. (c) Tera–Wasserburg Concordia diagram diagram (all data). (d) Age versus Th/U ratio scatter plot (all data; forty). (e) Tera–Wasserburg Concordia (all data). (d) Age versus Th/U ratio scatter plot (all data; forty). (e) Tera–Wasserburg Concordia diagram (selected data; fifteen). (f) Weighted average zircon U–Pb ages of the quartz–allanite vein. diagram (selected data; fifteen). (f) Weighted average zircon U–Pb ages of the quartz–allanite vein.

Table 1. Laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICP- MS) U–Pb–Th isotopic analytical data for zircon from the quartz–allanite vein.

Anderson Correction Non Correction Ratio Age [25] U Th Pb No. Th/U 238Pb/206U 2σ 207Pb/206Pb 2σ 206Pb/238U 2σ (ppm) (ppm) (ppm)

Minerals 2020, 10, 49 8 of 14

Table 1. Laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS) U–Pb–Th isotopic analytical data for zircon from the quartz–allanite vein.

Anderson Correction Non Correction Ratio Age [25] U Th Pb No. Th/U 238Pb/206U 2σ 207Pb/206Pb 2σ 206Pb/238U 2σ (ppm) (ppm) (ppm) 1 462 715 232 1.43 14.938750 0.071413 0.062570 0.000810 412.6 2.3 2 407 145 14 0.36 32.000000 0.399360 0.052200 0.001400 196.4 2.7 3 434 238 66 0.53 20.833330 0.520833 0.065000 0.001700 299.2 8.0 4 116 80 9 0.68 24.740230 0.232590 0.050900 0.002500 254.1 2.4 5 113 57 6 0.51 25.214320 0.292451 0.050600 0.003100 250.9 2.9 6 286 126 29 0.42 22.558090 0.432537 0.059300 0.001200 275.1 5.4 7 411 596 223 1.40 16.528930 0.273205 0.089000 0.001200 362.2 6.3 8 229 103 17 0.45 24.119630 0.378142 0.060700 0.002300 256.9 3.5 9 175 250 12 0.88 26.917900 0.688345 0.053600 0.002500 235.2 6.5 10 266 74 9 0.26 26.518160 0.246125 0.052200 0.001500 236.9 2.4 11 168 65 8 0.40 26.371310 0.271224 0.054700 0.002800 236.1 3.1 12 276 60 8 0.22 25.634450 0.197138 0.052700 0.002000 244.5 2.1 13 197 76 12 0.38 23.148150 0.492970 0.060600 0.001300 267.8 5.7 14 488 104 12 0.21 25.246150 0.216705 0.053200 0.001700 248.6 2.2 15 305 158 19 0.51 25.634450 0.210280 0.055500 0.001500 244.1 1.9 16 323 103 13 0.32 25.258900 0.421088 0.052300 0.001800 248.2 4.2 17 233 80 11 0.35 25.581990 0.242142 0.053800 0.001900 245.4 2.7 18 636 696 177 1.13 17.793590 0.379934 0.072900 0.004000 343.6 6.3 19 349 188 48 0.51 20.755500 0.392020 0.055060 0.000690 301.2 5.8 20 201 54 9 0.27 24.503800 0.300218 0.050400 0.002500 257.1 3.5 21 229 69 9 0.31 24.721880 0.415597 0.050600 0.001600 254.6 3.9 22 311 154 35 0.48 20.618560 0.552662 0.058600 0.002300 300.7 8.5 23 283 59 7 0.21 25.799790 0.226314 0.051400 0.001400 244.1 2.2 24 162 84 11 0.52 24.461840 0.365013 0.053200 0.002000 256.0 3.7 25 438 121 13 0.28 26.191720 0.315563 0.052200 0.001100 240.1 2.9 26 398 307 82 0.77 20.374900 0.319655 0.067800 0.001600 303.0 4.8 27 366 203 36 0.55 25.406500 0.342110 0.054700 0.001500 245.4 3.0 28 337 108 24 0.32 20.820320 0.177729 0.057700 0.001700 297.9 2.9 29 342 79 9 0.22 24.863250 0.185454 0.052270 0.000790 252.6 1.8 30 303 128 15 0.41 25.947070 0.309695 0.055800 0.001400 241.1 3.0 31 343 126 23 0.36 22.794620 0.233818 0.061200 0.002300 271.0 2.3 32 575 285 54 0.51 23.629490 0.256842 0.058600 0.001900 263.1 3.0 33 713 675 172 0.95 17.304030 0.254515 0.060800 0.001300 358.3 5.4 34 409 123 17 0.30 24.061600 0.237374 0.054900 0.001700 260.5 3.1 35 284 90 14 0.31 25.627880 0.229876 0.058200 0.001500 242.7 2.3 36 292 362 111 1.22 17.921150 0.321168 0.061200 0.001300 346.0 6.3 37 444 345 91 0.77 20.416500 0.337635 0.058700 0.001500 305.7 5.4 38 478 652 109 1.33 20.833330 0.998264 0.072700 0.003600 292.0 13.0 39 166 53 10 0.31 29.163020 0.263649 0.060600 0.002300 213.3 1.8 40 240 66 8 0.28 26.874500 0.180560 0.053800 0.001400 233.7 1.4

4.3. Zircon Composition The chemical compositions of the zircon grains from the quartz–allanite vein have 32.3 to 33.1 wt% SiO2, 63.7 to 66.4 wt% ZrO2, and 1.6 to 1.9 wt% HfO2 (Table2). They have similar chondrite-normalized REE patterns (Figure5). Light REEs vary from 62 to 165 ppm, and total REEs range from 564 to 808 ppm, with light REE (LREE)/heavy REE (HREE) ratios of 0.11 to 0.26 (Table3). They display positive Ce anomalies and negative Eu anomalies with Ce/Ce* ratios ranging from 1.53 to 4.89 and an Eu/Eu* ratio from 0.14 to 0.17, respectively (Figure5). The zircon grains have La concentrations ranging from 5.2 to 20.5 ppm, with (Sm/La)N ratios ranging from 0.18 to 0.89. Minerals 2020, 10, 49 9 of 14

Table 2. Electron microprobe analyses (wt%) of the zircon from the quartz–allanite vein.

Spots SiO2 HfO2 UO2 Al2O3 Ce2O3 ThO2 La2O3 PbO TiO2 ZrO2 Y2O3 Total 1 32.5 1.8 <0.1 <0.1 0.3 <0.1 <0.1 <0.1 <0.1 63.7 <0.1 98.6 2 32.6 1.7 <0.1 0.2 0.2 <0.1 <0.1 <0.1 <0.1 64.4 <0.1 99.3 3 33.1 1.8 <0.1 <0.1 <0.1 <0.1 <0.1 <0.1 <0.1 65.0 <0.1 100.1 4 32.6 1.7 <0.1 <0.1 <0.1 <0.1 <0.1 <0.1 <0.1 66.4 <0.1 100.9 5 32.3 1.6 <0.1 <0.1 <0.1 <0.1 <0.1 <0.1 <0.1 64.3 <0.1 98.4 6 32.5 1.9 <0.1 <0.1 <0.1 <0.1 <0.1 <0.1 <0.1 66.2 <0.1 100.9 7 32.4 1.8 <0.1 <0.1 0.1 <0.1 <0.1 <0.1 <0.1 65.6 <0.1 100.0 8 32.6 1.6 <0.1 <0.1 <0.1 0.1 <0.1 <0.1 <0.1 65.5 <0.1 99.8 9 32.7 1.6 <0.1 <0.1 <0.1 <0.1 <0.1 <0.1 <0.1 65.5 <0.1 99.9 10 32.5 1.6 <0.1 <0.1 <0.1 <0.1 <0.1 <0.1 <0.1 65.6 <0.1 99.9 11 32.6 1.8 <0.1 <0.1 <0.1 <0.1 <0.1 <0.1 <0.1 65.5 <0.1 100.2 12 32.8 1.9 <0.1 <0.1 <0.1 <0.1 <0.1 <0.1 <0.1 65.0 <0.1 99.8

Table 3. Laser ablation ICP-MS analyses (in parts per million) of zircon from the quartz–allanite vein.

Elements 1 2 3 4 5 6 7 P 167 155 149 113 117 74.6 81.1 Y 651 577 766 581 885 718 697 Nb 118 105 152 122 112 109 107 La 11.8 10.3 16.3 11.1 15.4 14.3 20.5 Ce 51.2 45.3 109.4 31.7 75.2 55.9 71.1 Pr 2.6 2.1 4.2 2.3 3.3 3.4 4.1 Nd 9.4 8.2 17.9 8.6 17.0 11.8 11.9 Sm 2.6 2.1 5.3 1.9 4.1 3.5 2.3 Eu 0.2 0.2 0.4 0.2 0.3 0.3 0.2 Gd 7.4 6.1 11.9 5.9 12.5 9.6 7.3 Tb 2.9 2.4 4.2 2.2 4.1 3.2 2.6 Dy 42.7 35.1 57.2 34.7 57.1 44.7 39.7 Ho 16.7 14.7 20.4 14.5 22.4 18.1 15.9 Er 93.4 85.0 110.9 85.0 117.9 98.2 93.3 Tm 27.1 25.1 31.6 24.2 33.0 28.0 27.5 Yb 316 282 351 287 367 316 315 Lu 62.7 55.9 67.9 55.1 72.7 63.4 64.2 Hf 8406 7785 10086 7228 11441 9380 11780 Ta 116 102 134 107 116 109 140 Pb 4.6 2.8 6.1 2.8 6.7 4.8 4.0 Th 149 114 190 121 184 170 157 U 180 154 203 161 194 154 165 Ce/Ce* 2.24 2.33 3.21 1.53 2.56 1.95 1.87 Eu/Eu* 0.15 0.14 0.14 0.17 0.14 0.16 0.17 (Sm/La)N 0.35 0.32 0.52 0.27 0.43 0.40 0.18 LREE 85 74 165 62 128 99 118 HREE 562 500 643 502 674 572 558 TREE 647 574 808 564 802 671 676 Minerals 2020, 10, 49 10 of 14 Minerals 2019, 9, x FOR PEER REVIEW 10 of 14

FigureFigure 5. 5.Chondrite-normalized Chondrite-normalized rare rareearth earth elementelement (REE) patterns patterns for for zircon zircon from from the the quartz–allanite quartz–allanite veinvein from from study study area area and and the the partially partially to to fullyfully recrystallizedrecrystallized zircon from from high-grade high-grade metagranitoids metagranitoids fromfrom Queensland, Queensland, Australia [26 [26].].

5. Discussion5. Discussion and and Conclusions Conclusions TheThe results results of datingof dating have have been reportedbeen reported for REE for mineralization REE mineralization in the Gyemyeongsan in the Gyemyeongsan Formation, locatedFormation, in the Chungjulocated in area: the Chungju Park and area: Kim Park [2,3 ]and and Kim No and[2,3] Parkand No [10 ]and reported Park [10] the REEreported mineralization the REE agesmineralization associated withages Paleozoicassociated alkaliwith Paleozoic granite, andalkali Cheong granite,et and al. Cheong [9] reported et al. [9] data reported for allanite data for from metavolcanicsallanite from metavolcanics (Gyemyeongsan (Gyemyeongsan Formation) Formation) and suggested and suggested that REE that was REE reconcentrated was reconcentrated during metamorphismduring metamorphism (446 8.0, (446 300–220, ± 8.0, 300–220, 199–183 199–183 Ma). Ma). ± InIn this this study, study, we we investigated investigated the thezircon zircon fromfrom thethe quartz–allanite vein vein which which emplaced emplaced in inthe the metavolcanicsmetavolcanics (Gyemyeongsan (Gyemyeongsan Formation), Formation), andand the ages ranging from from 240.1 240.1 ± 2.92.9 to to 257.1 257.1 ± 3.53.5 Ma, Ma, ± ± withwith a weighted a weighted mean mean age age of of 247.9 247.9 ±2.8 2.8 MaMa was obtained. This This age age has has not not been been reported reported for for intrusive intrusive ± rocksrocks distributed distributed within within the the metavolcanics metavolcanics (Gyemyeongsan(Gyemyeongsan Formation). However, However, it itis isincluded included withinwithin the the metamorphism metamorphism age age of of 300–220 300–220 Ma,Ma, asas reportedreported by Cheong Cheong et et al. al. [9]. [9]. And, And, the the most most similar similar zirconzircon age age (257 (257 ±3.3 3.3 Ma) Ma) was was reported reported byby KimKim etet al.al. [[27].27]. The The zircon zircon grains grains separated separated from from the the ± metavolcanicsmetavolcanics (Gyemyeongsan (Gyemyeongsan Formation)Formation) and conducted SHRIMP SHRIMP U–Pb U–Pb spot spot analyses analyses from from the the overgrowth rims [27]. It means that the zircon from the quartz–allanite vein was not related with overgrowth rims [27]. It means that the zircon from the quartz–allanite vein was not related with igneous activity, but related with metamorphism. igneous activity, but related with metamorphism. In addition, the partially to fully crystallization of zircon during metamorphism has been In addition, the partially to fully crystallization of zircon during metamorphism has been reported reported over a wide range of temperatures and pressures during prograde, retrograde, and peak over a wide range of temperatures and pressures during prograde, retrograde, and peak metamorphic metamorphic conditions [28–36]. The Permian–Triassic allanite ages linked with the collision in the conditionsOMB have [28 been–36 ].reported The Permian–Triassic [9,37]. Observations allanite of cathodoluminescence ages linked withthe images collision show inthat the most OMB of the have beenzircons reported are [euhedral9,37]. Observations and short-prismatic, of cathodoluminescence some of which imageshave inherited show that cores most and of theovergrowth zircons are euhedralzoning. and This short-prismatic, feature is similar some to that of which metamorphi have inheritedc zircon coresformed and in overgrowthveins (quartz zoning. vein or This complex feature is similarvein) that to thatcan be metamorphic formed during zircon the formedprograde, in veinspeak, (quartzand retrograde vein or stages complex [38,39]. vein) that can be formed duringThe the prograde,Th/U ratio peak, in zircon and retrograde has been stagesreported [38 ,to39 ].decrease during recrystallization [38]. The shapes—especiallyThe Th/U ratio small in zircon sizes has(~50 beenµm)—and reported the vari toation decrease of the during Th/U ratio recrystallization (fifteen concordant [38]. to The shapes—especiallynear–concordant grains) small sizesfrom (~500.21 toµm)—and 0.68 for the the zircon variation from of the the quartz–allanite Th/U ratio (fifteen vein, suggest concordant that to near–concordantthe zircon recrystallized grains) from from 0.21 microcrystalline to 0.68 for the Paleozoic zircon from magmatic the quartz–allanite zircon from vein, alkali suggest granite that [10], the zirconwhich recrystallized has a Th/U ratio from from microcrystalline 0.69 to 0.75. Paleozoic magmatic zircon from alkali granite [10], which has a ThZircon/U ratio is from 0.69 toorthosilicate, 0.75. ZrSiO4 and has a stoichiometric composition (ideally) of 67.2 2 2 wtZircon % ZrO is and zirconium 32.8 wt % orthosilicate, SiO . Zircon ZrSiO from 4theand quartz–allanite has a stoichiometric vein in compositionthe study area, (ideally) however, of 67.2ranges wt % from 63.7 to 66.4 wt % ZrO2 and from 32.3 to 33.1 wt % SiO2 (Table 2). In addition, the zircon from the ZrO2 and 32.8 wt % SiO2. Zircon from the quartz–allanite vein in the study area, however, ranges from quartz–allanite vein has non-formula elements Hf (1.6–1.9 wt % HfO2; Table 2). enrichment 63.7 to 66.4 wt % ZrO2 and from 32.3 to 33.1 wt % SiO2 (Table2). In addition, the zircon from the quartz–allanite vein has non-formula elements Hf (1.6–1.9 wt % HfO2; Table2). Hafnium enrichment Minerals 2020, 10, 49 11 of 14

Minerals 2019, 9, x FOR PEER REVIEW 11 of 14 is a feature of recrystallized zircon [38], however metamorphic zircons that have formed under low temperatureis a feature have of recrystallized low Ti (<2 ppm;zircon [ 36[38],]). however metamorphic zircons that have formed under low temperatureAs can be have seen fromlow Ti Figure (<2 ppm;6, the [36]). Ce /Ce* vs. (Sm/La)N and (Sm/La)N vs. La (ppm) for the zircons from theAs quartz–allanitecan be seen from vein Figure are 6, not the magmaticCe/Ce* vs. or(Sm/La) hydrothermalN and (Sm/La) zirconsN vs. [La40 (ppm)]. While for thethe zircons values of from the quartz–allanite vein are not magmatic or hydrothermal zircons [40]. While the values of Ce/Ce* and (Sm/La)N are slightly higher than those of the Baerzhe hydrothermal zircon, the values Ce/Ce* and (Sm/La)N are slightly higher than those of the Baerzhe hydrothermal zircon, the values of of (Sm/La)N and La (ppm) are slightly lower than those of the Baerzhe hydrothermal zircon due to the(Sm/La) medium-REEsN and La (MREE)(ppm) are that slightly have lower relatively than lowthose values of the comparedBaerzhe hydrothermal to LREE and zircon HREE. due Moreover, to the themedium-REEs steep REE patterns (MREE) from that Lahave to relatively Lu with positivelow values Ce compared and Negative to LREE Eu anomaliesand HREE. ofMoreover, metamorphic the steep REE patterns from La to Lu with positive Ce and Negative Eu anomalies of metamorphic zircon zircon was reported [26]. They show clear differences with magmatic and hydrothermal zircon was reported [26]. They show clear differences with magmatic and hydrothermal zircon (Figure 6), (Figure6), while the REE patterns are well matched with the recrystallization front of the zircon while the REE patterns are well matched with the recrystallization front of the zircon (Figure 5), as (Figure5), as reported by Hoskin and Black [26]. reported by Hoskin and Black [26].

Figure 6. Discriminant diagrams for zircons. (a) Ce/Ce* vs. (Sm/La)N (b) (Sm/La)N vs. La (ppm) for Figure 6. Discriminant diagrams for zircons. (a) Ce/Ce* vs. (Sm/La)N (b) (Sm/La)N vs. La (ppm) for zirconszircons from from the the quartz–allanite quartz–allanite vein vein compared compared to the magmatic magmatic and and hydrothe hydrothermalrmal zircons zircons from from other other areasareas such such as as the the Boggy Boggy Plain Plain Zoned Zoned PlutonPluton (BPZP)(BPZP) and and Jack Hills zircon data data are are taken taken fromfrom Hoskin, Hoskin, 2005 2005 [40 [40];]; the the Baerzhe Baerzhe alkalialkali granite,granite, Inner Mongolia, Mongolia, NE NE China, China, data data are are taken taken from from YangYang et al.,et al., 2014 2014 [41 [41].].

Minerals 2020, 10, 49 12 of 14

In conclusion, the Gyemyeongsan Formation in the OMB was metamorphosed during the late Permian to early Triassic period (240.1 2.9 to 257.1 3.5 Ma). As reported by previous studies [9,10], ± ± the REEs that existed in the Gyemyeongsan Formation were mobilized and reconcentrated during metamorphism. The previously reported Pb–Pb whole-rock and U–Pb mineral ages indicate that the OMB experienced peak metamorphism during the Permian (290–260 Ma) age [27,42]. It means the quartz–allanite vein emplaced in the metavolcanics (Gyemyeongsan Formation) after the peak metamorphism. The Triassic age is the first reported through zircon dating from the Gyemyeongsan Formation. This first reported Triassic age is considered to offer important data to reveal the metamorphic evolution of the OMB.

Author Contributions: S.-G.N. designed and initiated the research. S.-G.N. and M.-E.P. were responsible for geochronological and geochemical analyses. S.-G.N. prepared the manuscript. All authors have read and agreed to the published version of the manuscript. Funding: This research was supported by the Basic Research Project of the Korea Institute of Geoscience and Mineral Resources (KIGAM; GP2017-022, Development of mineral potential targeting and efficient mining technologies based on 3D geological modeling platform) funded by the Ministry of Science and ICT. Conflicts of Interest: The authors declare no conflict of interest.

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