Western Gyeonggi Massif Research Articles

High P–T granulite relicts from the Imjingang belt, South Korea: Tectonic significance

This study presents a detailed description on crustal metamorphic signatures of garnet–clinopyroxene–quartz–rutile-bearing high P– T granulites, Samgot unit, Imjingang belt, northwestern Korean Peninsula that formed during Permo-Triassic regional metamorphism related to the amalgamation of East Asian continental fragments. Lenses and blocks of high P– T granulites and garnet-bearing leucosomes occur within mafic metamorphic rocks (mainly amphibolites). The mafic blocks comprise relicts of granoblastic garnet and clinopyroxene with medium-grained quartz and rutile. These relict mineral assemblages are confined to local micro-domains and constitute remnants of peak metamorphism. Plagioclase and amphibole form only as retrograde phases in medium to coarse-grained moats that rim grain boundaries between relict peak mineral assemblages. This microstructure represents the reaction between garnet, clinopyroxene, quartz and rutile in the presence of melt to form amphibole, plagioclase and titanite with minor biotite. The leucosome domains consist of euhedral garnets within the quartz-K feldspar-plagioclase (granitic) matrix, probably representing peritectic garnet growth along with melting. The rare earth element (REE) composition of minerals also support the peritectic garnet growth with a positive Eu/Eu⁎ (positive Eu anomaly), while the relict garnet shows a slight negative anomaly typical for high-grade granulites. The peak-metamorphic conditions calculated from thermodynamic modeling and compositional isopleths indicate a temperature around c. 900 °C at a pressure around c. 20 kbar. The present P–T path indicates a clear multi-stage decompression history with initial decompression and cooling followed by a stage of decompression during hydration possibly during Late Triassic exhumation. The results from this study together with the presence of eclogites from the Hongsung area suggest that the Imjingang area and the western Gyeonggi massif likely resided at crustal levels deeper than those of the eastern and southern part of the Gyeonggi massif.

The oldest (ca. 2.51 Ga) rock in South Korea: U-Pb zircon age of a tonalitic migmatite, Daeijak Island, western Gyeonggi massif

We discovered the oldest rock in South Korea, and dated this migmatite using a Sensitive High-Resolution Ion Micro-probe (SHRIMP). Zircon grains from a tonalitic migmatite, Daeijak Island, western Gyeonggi massif, are distinctly zoned and commonly characterized by oscillatory-zoned cores and structureless rims. The U-Pb zircon ages estimated from six spot analyses of the rims in five grains define a concordant age group with the weighted mean 207Pb/206Pb age of 2508 ± 18 Ma, interpreted as the age of migmatization. On the other hand, the cores yield an older 207Pb/206Pb age of ca. 2.58 Ga, interpreted as the crystallization age of tonalitic protolith. These results demonstrate not only the presence of Neoarchean granitoids but also high-grade migmatitic metamorphism at ca. 2.51 Ga in the Gyeonggi massif, which are correlative with those reported from the North China craton.

Nanhua Rift: A Story of Continental Rift Related to Rodinia Break-up

The southern part of the Korean Peninsula preserves important records of the Paleozoic evolutionary history of East Asia. Here we present SHRIMP U–Pb ages of detrital zircon grains from Paleozoic metasedimentary successions (Okcheon and Joseon Supergroups, Yeoncheon Group, Taean Formation, and Pyeongan Supergroup) that are incorporated into the major Phanerozoic mountain belts (Okcheon and Hongseong-Imjingang Belts) in South Korea, providing new insights for provenances and paleotectonic evolution of the South Korean Peninsula during Paleozoic time. The zircon ages from our samples display two distinct spectra patterns in their presence/absence of Neoproterozoic and/or Paleozoic populations. Our results, together with the available data from the Korean Peninsula, suggest that: (1) the Early to Middle Paleozoic successions in the Okcheon Belt were deposited in continental margin setting(s) formed by Neoproterozoic intracratonic rifting, (2) the Middle Paleozoic metasedimentary rocks in the Imjingang belt can be interpreted as molasse and flysch sediments along an active continental margin, (3) the Late Paleozoic to Early Triassic Taean Formation along the western Gyeonggi Massif represents a syn- to post-collision deltaic complex of a remnant oceanic basin, and (4) the Late Paleozoic to possibly Early Triassic Pyeongan Supergroup in the Okcheon Belt might represent a wedge-top and/or foreland basin. The spatial and temporal discrepancy between the South Korean Peninsula and the Central China Orogenic Belt during Paleozoic might reflect lateral variations in crustal evolution history along the East Asian continental margin during the Paleo-Tethyan Ocean closure.