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[SGL23-P06] Zircon U–Pb age of Paleoproterozoic granite gneiss from the Ogcheon Belt, Geumsan area and Paleoproterozoic leucogranite and quartzite from the Yeongnam Massif, Seolcheon area, South Korea
Keywords:Korea, Ogcheon Belt, Yeongnam Massif, zircon, granite, quartzite
Jurassic Daebo granite is widespread in the Geumsan area, Ogcheon Belt, South Korea, and Precambrian rocks are present on a small scale in it (Hong and Choi, 1978). Precambrian rocks are mainly composed of biotite gneiss and granite gneiss. Iwamizu et al. (2021, in press) reported that the granite gneiss (Gn) has an igneous age of ca. 2.5 Ga and two metamorphic ages of ca. 1.89 Ga and 174 ± 74 Ma by LA-ICP-MS zircon U–Pb analysis. Gn is one of the second oldest rocks in south Korea, along with the granite gneisses which have the same igneous ages reported by Lee et al. (2016) in the southern Gyeonggi Massif.
The zircon U–Pb ages of Gn that Iwamizu et al. (2021, in press) reported are mostly the result of analysis of the zircon core and mantle data. The cores have various internal structures and show a wide range of ages of 2.5–1.9 Ga. The older the age, the higher the Th/U ratio and the smaller the discordance. Therefore, the igneous age is expected to be ca. 2.5 Ga. The 1.9–1.8 Ga mantle data show vague and round oscillatory zoning and all the Th/U ratios are lower than 0.04. A discordia line can be defined by 1.9–1.8 Ga mantle data in the Tera-Wasserburg concordia diagram. The upper intercept age and the lower intercept age are ca. 1.89 Ga and 174 ± 74 Ma, respectively, which are interpreted to be metamorphic ages related to Paleoproterozoic and Jurassic magmatism, respectively. The upper intercept age is consistent with metamorphic ages of the Gyeonggi and Yeongnam Massif (Lee and Cho, 2012). The lower intercept age is consistent with surrounding Jurassic granite. All the zircon U–Pb data are discordant because of the Jurassic metamorphism. Since the detailed location of SKTL (South Korean Tectonic Line) in the Geumsan area is unknown, it is unknown whether the Gn is derived from the Gyeonggi or Yeongnam Massif.
In addition to the above, we newly report Paleoproterozoic leucogranite and quartzite in the Seolcheon area, Yeongnam Massif, which is about 25 km east of the Geumsan area.
The leucogranite looks white because it does not have biotite. It contains approximately 73 wt% SiO2. It exhibits metaluminous whole rock chemical composition, which is different from the leucogranitoids that have peraluminous whole rock chemical composition reported by Lee et al. (2010) and Lee et al. (2019) in the Yeongnam Massif. Its zircons exhibit two layered structures, core and mantle. The cores (5 points) have homogeneous or banded zoning, concordant U–Pb ages of ca. 2.5–2.4 Ga and high Th/U ratios higher than 0.1. The mantle data (12 points) have oscillatory or banded zoning, concordant U–Pb ages concentrating near 2 Ga and Th/U ratios higher than 0.1. The mean age of the concordant mantle data concentrating near 2 Ga is 1997 ± 10 Ma, which is interpreted to be an igneous age. This age is close to an igneous age of 1985 ± 10 Ma of the leucogranite gneiss in the Muju area (Lee et al., 2019). The 2.5–2.4 Ga cores are considered to be inherited zircons. Metamorphic age has not been obtained.
The quartzite is mainly composed of coarse quartz and contains a small amount of muscovite. It contains approximately 98 wt% SiO2. Its detrital zircons mainly show oscillatory zoning. The zircon U–Pb ages (63 points) show a wide range of ages of ca. 3.5–1.9 Ga. Most data are concordant, and all the discordances are smaller than ca. 7.7 %. The oldest age is 3505 ± 28 Ma. The youngest age is 1902 ± 31 Ma, which is an upper limit of deposition. The detrital zircons of the quartzite have the biggest age peak at ca. 2.5 Ga, which is the same as the Paleoproterozoic Yuli Group, Yeongnam Massif (Lee et al., 2011) and the Cambrian Jangsan Formation, Taebaeksan zone (Kim et al., 2019).
From the above, ca. 2.5 Ga granite gneiss exists in the Geumsan area, Ogcheon Belt (Iwamizu et al., 2021, in press) and ca. 2.5 Ga inherited or detrital zircons are dominant in the Seolcheon area, Yeongnam Massif (this study). Therefore, extensive magmatism at ca. 2.5 Ga can be suggested in the Geumsan–Seolcheon area if Gn is located southeast of SKTL. If Gn is located northwest of SKTL, Gn can be correlated with the same age granite gneiss in the southern Gyeonggi Massif (Lee et al., 2016), which indicates ca. 2.5 Ga extensive magmatism there.
[References]
Hong, S.-H. and Choi, W.-C. (1978) Geumsan Sheet. Korea Research Inst. of Geoscience and Mineral Resources.
Iwamizu, K. et al. (2021) J. Geol. Soc. Japan, 127, in press, doi: 10.5575/geosoc.2020.0057
Kim, H.-S. et al. (2019) Int. J. Earth Sci., 108, 1509–1526.
Lee, B.-C. et al. (2016) Precambrian Res., 283, 169–189.
Lee, B.-C. et al. (2019) Gondwana Res., 72, 34–53.
Lee, S.-G. et al. (2010) Chem. Geol., 276, 360–373.
Lee, S.-R. and Cho, K. (2012) J. Petrol. Soc. Korea, 21, 89–112.
Lee, Y.-I. et al. (2011) J. Geol. Soc. Korea, 47, 81–87.
The zircon U–Pb ages of Gn that Iwamizu et al. (2021, in press) reported are mostly the result of analysis of the zircon core and mantle data. The cores have various internal structures and show a wide range of ages of 2.5–1.9 Ga. The older the age, the higher the Th/U ratio and the smaller the discordance. Therefore, the igneous age is expected to be ca. 2.5 Ga. The 1.9–1.8 Ga mantle data show vague and round oscillatory zoning and all the Th/U ratios are lower than 0.04. A discordia line can be defined by 1.9–1.8 Ga mantle data in the Tera-Wasserburg concordia diagram. The upper intercept age and the lower intercept age are ca. 1.89 Ga and 174 ± 74 Ma, respectively, which are interpreted to be metamorphic ages related to Paleoproterozoic and Jurassic magmatism, respectively. The upper intercept age is consistent with metamorphic ages of the Gyeonggi and Yeongnam Massif (Lee and Cho, 2012). The lower intercept age is consistent with surrounding Jurassic granite. All the zircon U–Pb data are discordant because of the Jurassic metamorphism. Since the detailed location of SKTL (South Korean Tectonic Line) in the Geumsan area is unknown, it is unknown whether the Gn is derived from the Gyeonggi or Yeongnam Massif.
In addition to the above, we newly report Paleoproterozoic leucogranite and quartzite in the Seolcheon area, Yeongnam Massif, which is about 25 km east of the Geumsan area.
The leucogranite looks white because it does not have biotite. It contains approximately 73 wt% SiO2. It exhibits metaluminous whole rock chemical composition, which is different from the leucogranitoids that have peraluminous whole rock chemical composition reported by Lee et al. (2010) and Lee et al. (2019) in the Yeongnam Massif. Its zircons exhibit two layered structures, core and mantle. The cores (5 points) have homogeneous or banded zoning, concordant U–Pb ages of ca. 2.5–2.4 Ga and high Th/U ratios higher than 0.1. The mantle data (12 points) have oscillatory or banded zoning, concordant U–Pb ages concentrating near 2 Ga and Th/U ratios higher than 0.1. The mean age of the concordant mantle data concentrating near 2 Ga is 1997 ± 10 Ma, which is interpreted to be an igneous age. This age is close to an igneous age of 1985 ± 10 Ma of the leucogranite gneiss in the Muju area (Lee et al., 2019). The 2.5–2.4 Ga cores are considered to be inherited zircons. Metamorphic age has not been obtained.
The quartzite is mainly composed of coarse quartz and contains a small amount of muscovite. It contains approximately 98 wt% SiO2. Its detrital zircons mainly show oscillatory zoning. The zircon U–Pb ages (63 points) show a wide range of ages of ca. 3.5–1.9 Ga. Most data are concordant, and all the discordances are smaller than ca. 7.7 %. The oldest age is 3505 ± 28 Ma. The youngest age is 1902 ± 31 Ma, which is an upper limit of deposition. The detrital zircons of the quartzite have the biggest age peak at ca. 2.5 Ga, which is the same as the Paleoproterozoic Yuli Group, Yeongnam Massif (Lee et al., 2011) and the Cambrian Jangsan Formation, Taebaeksan zone (Kim et al., 2019).
From the above, ca. 2.5 Ga granite gneiss exists in the Geumsan area, Ogcheon Belt (Iwamizu et al., 2021, in press) and ca. 2.5 Ga inherited or detrital zircons are dominant in the Seolcheon area, Yeongnam Massif (this study). Therefore, extensive magmatism at ca. 2.5 Ga can be suggested in the Geumsan–Seolcheon area if Gn is located southeast of SKTL. If Gn is located northwest of SKTL, Gn can be correlated with the same age granite gneiss in the southern Gyeonggi Massif (Lee et al., 2016), which indicates ca. 2.5 Ga extensive magmatism there.
[References]
Hong, S.-H. and Choi, W.-C. (1978) Geumsan Sheet. Korea Research Inst. of Geoscience and Mineral Resources.
Iwamizu, K. et al. (2021) J. Geol. Soc. Japan, 127, in press, doi: 10.5575/geosoc.2020.0057
Kim, H.-S. et al. (2019) Int. J. Earth Sci., 108, 1509–1526.
Lee, B.-C. et al. (2016) Precambrian Res., 283, 169–189.
Lee, B.-C. et al. (2019) Gondwana Res., 72, 34–53.
Lee, S.-G. et al. (2010) Chem. Geol., 276, 360–373.
Lee, S.-R. and Cho, K. (2012) J. Petrol. Soc. Korea, 21, 89–112.
Lee, Y.-I. et al. (2011) J. Geol. Soc. Korea, 47, 81–87.