Yong-Sun Song

SHRIMP Zircon Ages of the Basement Gneiss Complex in the Pyeongchang-Wonju Area, Gyeonggi Massif, Korea

Precambrian gneiss complex in the Pyeongchang-Wonju area, which lies west of the Paleozoic sedimentary basin of the Yeongwol-Taebaek area, is being considered as a part of the Gyeonggi massif, but its ages of formation and metamorphic events are not well defined yet. In this study, SHRIMP zircon U-Pb ages were determined from the gneiss complex in the area, We obtained the discrete ages of magmatic (ca. 1960 Ma) and metamorphic (ca. 1860 Ma) events through the interpretation of the SHRIMP data based on the internal structures of zircons. These are almost the same to the ages of main intrusion and metamorphism reported from the Precambrian basements of Gyeonggi, Yeongnam and Nangnim massifs of the Korean Peninsula, Ages of 3200~3300 Ma, 2900 Ma, 2660 Ma, 2430 Ma, 2260 Ma, and 2080~2070 Ma obtained from inherited cores of studied zircons are also very similar to the frequently reported ages from the basement rocks of the Gyeonggi and Yeongnam massifs, Lower intercept age of about 270 Ma calculated from the rim data seems to indicate that the study area suffered from a late Paleozoic metamorphism (Okcheon Orogeny), but we need more reasonable and sufficient data to confirm it. According to the results of this study, it is suggested that the Bangnim group unconformably overlying the gneiss complex was deposited after the Paleoproterozoic granitic magmatism (ca. 1960 Ma) and metamorphism (ca. 1860 Ma).

SHRIMP U-Pb Zircon Ages of the Yeongju and Andong Granites, Korea and their Implications

SHRIMP zircon U-Pb age dating is carried out for the Yeongju and Andong granite batholiths intruding the Precambrian metamorphic complex and Paleozoic sedimentary formations within the NE Yeongnam Massif, Korea. Dating of zircons from a hornblende-biotite tonalite and an equigranular biotite granodiorite in the Yeongju granite has yielded ages of ca. 187 Ma and ca. 186 Ma, respectively. Also, dating of zircons from a biotite granodiorite and a very coarse-grained biotite granite in the Andong granite has yielded ages of ca. 182Ma and ca. 186Ma, respectively. These data indicate that the main intrusions of the Yeongju and Andong granite batholiths occur almost at the same age. The oldest age of ca. 194 Ma has been determined on zircons from a hornblende gabbro in the Andong granite, and the youngest age of 175 Ma is obtained from the Chunyang granite pluton, mainly consisting of fine-grained two-mica granite, of the Yeongju batholith. These results indicate that Jurassic Daebo magmatism in the Yeongju-Andong area, NE Yeongnam massif, started early at the Early Jurassic with an intrusion of mafic magma, and followed by an emplacement voluminous granite magma during the middle of the Early Jurassic, and was finalized with the emplacement of relatively small amount of much evolved granite magma at the end of Early Jurassic.

SHRIMP U-Pb Age of the Early Jurassic Deformed Granites in the Aneui Quadrangle, SW Yeongnam Massif

Department of Earth Environmental Sciences, Pukyong National University, Busan 48513, Korea(Received: 18 March 2016 / Revised: 7 April 2016 / Accepted: 8 April 2016)SHRIMP U-Pb age determination was carried out for deformed granites in the Aneui quadrangle, SW YeongnamMassif. Dating of zircons from a highly deformed mylonitic granite with banded structure and a relatively lessdeformed porphyritic to augenic granites, that were known as Precambrian gneisses, yielded the same age of ca.195 Ma. On the basis of this result and previous age data, Early to Middle Mesozoic igneous activity around theAneui area was interpreted as follows; Subduction-related granitic magmatism started with the intrusion of theHamyang Granite in the middle Triassic (ca. 225−219 Ma) mainly in the west of the area and ended with syeniticintrusion at the end of Triassic period (ca, 220−210 Ma). After a relatively short period of quiescency, graniticmagmatism restarted with the intrusion of magma forming deformed granites dated in this study at the Early Juras-sic of ca. 195 Ma and continued to ca. 189 Ma and dioritic intrusion was associated around the late stage of gra-nitic magmatism.