Jurassic Igneous Activity in the Yuseong Area on the Southern Margin of the Gyeonggi Massif, Korean Peninsula, and Its Implications for the Tectonic Evolution of Northeast Asia during the Jurassic

Jurassic dioritic to granitic igneous rocks extensively intrude into the southern Korean Peninsula, including the Yuseong area located at the boundary between the southern margin of the Gyeonggi Massif and the northern margin of the Okcheon Belt. In this study, the petrogenesis and sources of Jurassic igneous rocks in the Yuseong area were investigated. The U–Pb zircon age data from the Jurassic plutonic rocks in the Yuseong area give two igneous ages, ca. 178–177 Ma and 169–168 Ma, indicating that two stages of igneous activity occurred in the Yuseong area during the Jurassic. The geochemical characteristics of Jurassic diorites indicate that they originated from enriched mid-ocean ridge basalt (E-MORB; Nb/Yb = 5.63–7.27; Zr/Yb = 118–156). The enriched Th/Yb ratios (5.5–8.0) in the diorites imply that they experienced crustal contamination during magma ascent. The Jurassic granitoids in the Yuseong area are divided into I- and S-type granites. The Jurassic I-type granitoids may have formed via the partial melting of mafic rocks with mixtures of 10–40% pelite-derived melt, while the S-type granites originated from felsic pelite. The Jurassic diorites have low Nb/Th ratios with depletion of the Nb and Ta components, indicating that they formed in a volcanic arc tectonic environment. On the other hand, the Jurassic granitoids show two different tectonic environments: a volcanic arc, and a syncollisional environment. The granites with syncollisional character are S-type granites, and may give incorrect information about tectonic setting because of the changes in the trace elements of the S-type granite due to fractional crystallization. Early Jurassic (200–190 Ma) igneous rocks are distributed only in the southeastern Korean Peninsula, including the Yeongnam Massif; Jurassic igneous rocks formed at ca. 190–180 Ma occur mainly in the Okcheon Belt and southern Gyeonggi Massif, which includes the Yuseong area. Middle Jurassic igneous rocks widely intruded from the Okcheon Belt, through the Gyeonggi and Nangrim massifs in the Korean Peninsula, to the Liaoning area in the North China Craton at 180–160 Ma. This distribution pattern of the Jurassic granitoids suggests that flat subduction started after 180 Ma in Northeast Asia.

Detrital zircon U-Pb and Hf isotope studies of the Paleozoic successions in the Korean Peninsula: Implications for the provenances and tectonic evolution of the Phanerozoic orogenic belts

<p>Paleozoic orogenic belts developed between the basement rocks in the southern Korean Peninsula records important information to reconstruct the tectonic evolution of East Asia. Here we present SHRIMP and LA–(MC)–ICP MS U-Pb ages and Hf isotopes of detrital zircon grains from the Paleozoic metasedimentary successions that are incorporated into the major Phanerozoic orogenic belts (Okcheon and Hongseong-Imjingang Belts) in South Korea, providing new insights into provenances and tectonic evolution during the Paleozoic period. Based on the internal structures of the zircons from all the samples, they are mostly derived from igneous source rocks, showing two distinct spectra patterns in their presence/absence of Neoproterozoic ages. Our results suggest that (1) the presence/absence of the Grenville-age (ca. 1.3–0.9 Ga) detrital zircons and Hf data from the Early Paleozoic Joseon Supergroup in the Okcheon Belt suggest their derivations from different peripheral clastic provenances at least after the Early Cambrian, (2) ages and Hf isotope signatures of dominant Early Neoproterozoic and Silurian-Devonian detrital zircon populations from the Middle Paleozoic metasedimentary rocks in the Hongseong-Imjingang Belt reflect magmatic history involving juvenile input and crustal reworking, and (3) zircons from the Late Paleozoic Pyeongan Supergroup in the Okcheon Belt display dominant Paleoproterozoic and Carboniferous-Permian ages with Hf patterns showing vertical mixing trends between juvenile and recycled crustal material. These results, integrated with U-Pb and Hf isotope data from other parts of the Korean Peninsula and the Chinese cratons, will eventually help to understand the spatial and temporal relations of basins and orogenic belts in the Korean Peninsula, and will further provide important clues about Paleozoic evolution of the Korean Peninsula in relation to the tectonic history of East Asia.</p>