Kye-Hun Park

Neoproterozoic bimodal volcanism in the central Ogcheon belt, Korea: age and tectonic implication

Abstract UPb zircon ages were determined for a metavolcanic rock from the Munjuri Formation in the central Ogcheon belt, Korea. Four zircon fractions were analyzed of which three are nearly concordant; the data define a discordia with an upper intercept at 755.8 ± 1.3 Ma and a lower intercept at 160 ± 19 Ma (MSWD = 0.72). The upper intercept age is interpreted to indicate the timing of bimodal volcanism, and the lower intercept age is interpreted to reflect Jurassic thermal metamorphism. This is the first definite evidence supporting the existence of a formation in the Ogcheon belt that is at least in part late Precambrian. Nd isotope and trace element geochemistry of the amphibolites and felsic metavolcanics suggests that they originated from a mantle source in a continental rift environment. The rift does not appear to have matured enough to develop oceanic crust, since rocks indicating deep sea environments are absent. The possibility that the Ogcheon belt is an extension of the east Chinese collision belt, as suggested by several authors, is not supported by the available evidence.

Pb and Nd isotopic constraints on Paleoproterozoic crustal evolution of the northeastern Yeongnam massif, South Korea

Abstract We report Pb isotopic ages and Nd isotopic signatures of Paleoproterozoic basement rocks from the Pyeonghae area, northeastern Yeongnam massif, South Korea. The PbSL (lead step-leaching) garnet data of the Wonnam group (Precambrian metasediments) yield a 207Pb/206Pb age of 1840±26 Ma, which can be regarded as the timing of amphibolite to upper amphibolite facies metamorphism and associated garnet growth. Whole rock data for the Pyeonghae gneiss intruding the Wonnam group give a 207Pb/206Pb age of 2093±86 Ma, denying the possibility of a direct link between the intrusion of the Pyeonghae gneiss and the regional metamorphism of the Wonnam group. Our results confirm the significance of the 2.1 Ga and 1.8 Ga episodes that have been broadly constrained in the Yeongnam massif. The depleted mantle Nd model ages of metasedimentary rocks from the Wonnam group (2.63–2.47 Ga) are slightly younger than those of the Pyeonghae gneiss samples (2.71–2.57 Ga). This Nd isotopic signature also precludes a direct derivation of the Pyeonghae gneiss from the Wonnam Group, instead implying the presence and involvement of the older, probably late Archean crustal materials during the 2.1 Ga magmatism in the northeastern Yeongnam massif. Compiled Pb and Nd isotope data from the Yeongnam and Gyeonggi massifs suggest a similar geologic history for them, arguing against the conventional idea that the Gyeonggi and Yeongnam massifs are separate continental blocks respectively correlated to the South and North China blocks. The whole rock Pb isotope data of basement rocks from the two massifs form a well defined 207Pb/206Pb linearity of around 2.0 Ga, suggesting their common crustal evolution process for the past two billion years. A broad coincidence of major tectonic episodes in the two massifs is confirmed by reviewed geochronological data. The Nd model ages of basement rocks from the two massifs support a probable existence of Archean crusts in South Korea. The Nd model ages, both Archean and Proterozoic, of the Gyeonggi and Yeongnam massifs agree with neither those of the North China block (predominantly Archean) nor those of the South China block (predominantly Proterozoic). Our compiled isotope data together with recent estimation for the age of the Honam shear zone appear to refute the presence of suture zone between the two South Korean Precambrian massifs, which leaves the Imjingang belt as the possible suture zone.

Sr, Nd and Pb Isotopic Systematics of the Cenozoic Basalts of the Korean Peninsula and Their Implications for the Permo-Triassic Continental Collision Boundary

Abstract Sr, Nd and Pb isotopic compositions of the Cenozoic basalts were analyzed from Baengnyeongdo Island, Jeongok, Ganseong, and Jejudo Island of Korea. They reveal relatively enriched Sr and Nd isotopic compositions (87Sr/86Sr = 0.70330∼0.70555, 143Nd/144Nd = 0.51298∼0.51256) compared with MORB.207Pb/204Pb and 208Pb/204Pb values of all the analyzed Korean basalts lie above the Northern Hemisphere Reference Line (NHRL) defined by Hart (1984). Pb isotopic compositions of basalts from Jejudo Islands (206Pb/204Pb = 18.61∼19.12, 207Pb/204Pb = 15.54∼15.69, 208Pb/204Pb = 38.98∼39.72) are significantly more radiogenic than the rest (206Pb/204Pb = 17.72∼18.03, 207Pb/204Pb = 15.44∼15.58, 208Pb/204Pb = 37.77∼38.64). The Cenozoic Korean basalts thus can be divided into two groups based on their Sr, Nd and Pb isotopic compositions. The north group reveals mixing between DMM and EM1 while the south group displays DMM-EM2 mixing. Such a distribution is the same as Chinese Cenozoic basalts and it can be interpreted that the subcontinental lithospheric mantle under Korea represents simple lateral continuation of the South and North China Blocks. We suggest that Korean continental collision zone cross the Korean Peninsula through the region between the north and south basalt groups of Korea.

An early Proterozoic Sm-Nd age of mafic granulite from the Hwacheon area, South Korea

The Sm−Nd mineral isochron ages of 1897±120 and 1742±140 Ma were obtained from two mafic granulites of the Hwacheon granulite complex, east-central Korean Peninsula. These ages are interpreted to date the granulite-facies metamorphism and/or subsequent cooling to ca. 700°C. Our result apparently contradicts the Permo-Triassic age for the regional metamorphism that was reported in the Imjingang belt, and instead suggests an early Proterozoic tectonometamorphic event that may be related to the crustal formation of the Gyeonggi massif. Thus, the Hwacheon granulite complex is likely to represent Proterozoic crystalline basement that was reactivated during the Permo-Triassic orogeny, corresponding to the collision between the Sino-Korean and Yangtze cratons.

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 ages and Hf isotopic composition of the detrital zircons from the Myogok Formation, SE Korea: development of terrestrial basin and igneous activity during the early Cretaceous

To constrain sedimentation age of the Myogok Formation and timing of the last episode of the Daebo orogeny, SHRIMP U-Pb age determination and Hf isotopic studies were carried out for the zircon separates. The detrital zircon U-Pb ages of the Myogok Formation show a wide range from Archean to Cretaceous and yield four main clusters at age intervals of 130–150 Ma, 170–220 Ma, 221–320 Ma and Paleoproterozoic. Such age clusters indicate that the main provenance rock types during the sedimentation were basement rocks of the Yeongnam massif and the Mesozoic granitoids distributed around the Myogok Formation. The concordia age of the youngest population is 138.6 ± 2.1 Ma (n = 7), indicating the possible maximum deposition age for the Myogok Formation. Considering the suggestion of probable deposition of Nakdong Formation during the Barremian (Lee et al., 2012), the Valanginian or Hauterivian seems to be the most appropriate timing for the deposition of the Myogok Formation. The last activity of the Daebo orogeny might be constrained to Hauterivian or Barremian considering some time lag needed for basin closure and deformation that followed. The measured εHf(t) values from the detrital zircons of the Myogok Formation vary from −25.7 to 4.8. However, most of the zircons, even those with Mesozoic ages, show negative values except a few, implying dominant influence of old continental crusts in the generation of their igneous protoliths.

Cyclic Igneous Activities During the Late Paleozoic to Early Cenozoic Period Over the Korean Peninsula

There were three cycles of igneous activities from the late Paleozoic to early Cenozoic; Permian to Triassic, Jurassic, and Cretaceous to Paleogene. After the beginning of each igneous activity cycle, igneous activity became more frequent until its climax. It is noteworthy that A-type magmatisms are reported from near the ends of the all three igneous activity cycles. In addition, adakitic magmatisms occurred at the beginning of both the Permian-Triassic and the Cretaceous-Paleogene cycles. Most of the igneous activities during the late Paleozoic to early Cenozoic period were subduction-related. Therefore, transitions among beginning, proceeding, and closing of the igneous activity cycles would be intimately related with changes in directions of plate movements. In this context, I suggest following hypotheses. The closing of the Permian-Triassic igneous cycle was possibly a consequence of radical adjustment of plate motion occurred due to continental collision between north and south China blocks. Considering that no appreciable tectonic activities were recognized from the east Asian continent at the closing of the Jurassic igneous cycle, it seems that one of the strong events related with Gondwanaland-breakup and subsequent birth of the new oceans, which might cause sudden adjustments of plate motions. The closing of the Cretaceous-Paleogene igneous cycle seems to be caused as a consequence of the collision between India and Asia continents. Meanwhile, adakitic igneous bodies emplaced at the beginnings of the Permian-

Characteristics of Nd Isotopic Compositions of the Phanerozoic Granitoids of Korea and Their Genetic Significance

Nd isotopic compositions analyzed from the Phanerozoic granitoids of Korea are integrated and discussed. Variations in Nd isotopic compositions can be explained either by temporal trend or by regional differences. Among the three active periods, first two periods during the Permian-Triassic and Jurassic seem to show variations from rather high values at the beginning to lower values during the later stages. Such trends probably reflect melting of the subducting oceanic crust and producing magma with higher proportion of depleted mantle derived materials during the early stage of subduction process, and subsequent magmas with greater proportion of old continental crust with progress of subduction. However, the Cretaceous-Paleogene period of active magmatism displays higher values during the advanced stage of the igneous activities, which is opposite to the previous active periods. The other explanation is that such differences in reflect regional differences, based on the observations that such high- granitoids distribute in the northeastern Gyeongbuk Province and Gyeongsang Basin. If this is the case, the regions with highr values may have distinct crustal evolution histories, e.g. younger average age. The choice between the two hypothesis could be made through further studies.

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.

The Late Cretaceous Emplacement Age of Masan Hornblende-Biotite Granite

백악기 경상분지 남부의 마산 각섬석-흑운모 화강암의 정치연령을 규명하기 위하여 K-Ar, Ar-Ar 및 U-Pb 연대측정을 수행하였다. 각섬석 K-Ar 연대측정으로 구한 약 108 Ma의 연령은 이전에 보고된 Rb-Sr 연령과 비슷하다. 그러나 단입자 전용융으로 구한 각섬석