Yirang Jang

Detrital zircon geochronology and isotope analysis can be used for inferring the provenance of detrital sedimentary sources in general, and can provide useful information to establish the tectonic evolution of a sedimentary basin. In this respect, analyzed detrital zircon SHRIMP U-Pb ages and LA–(MC)–ICP MS Lu-Hf isotopes from the Paleozoic metasedimentary successions in the Okcheon fold-thrust belt (FTB), providing new insights into their provenance and evolution during the Phanerozoic orogenesis.The Okcheon FTB has been considered a prominent belt between the two basements (viz. Gyeonggi and Yeongnam massifs) in the southern part of the Korean Peninsula, and subdivided the Okcheon Zone to the west and the Taebaeksan Zone to the east. The Taebaeksan Zone consists predominantly of the Paleozoic metasedimentary rocks comprising Early Paleozoic Joseon and the Late Paleozoic Pyeongan supergroups with Middle Paleozoic hiatus locally. On the contrary, the Okcheon Zone exposed the post-Devonian to Permian clastic wedge on top of the Neoproterozoic bimodal volcanic rocks related to the intracontinental rifted settings.Our results show that all Paleozoic strata commonly have Paleoproterozoic and Paleozoic zircon ages with rare Meso- to Neoproterozoic ones. Each zircon population shows the following notable results, allowing estimation of their sedimentary sources: (1) The Paleoproterozoic zircons (ca. 1.85 and 2.50 Ga) with similar ranges of εHf(t) values are most common in the basement rocks of the Korean Peninsula, thus can be sourced from both the Gyeonggi and Yeongnam massifs. (2) The Meso- to Neoproterozoic zircons, only preserved in the late Middle to Late Cambrian strata, probably reflected abrupt changes in source areas. (3) The youngest Paleozoic zircons of each formation, almost coincident with its deposition ages, suggest the presence of syndepositional magmatism. This means that detritus was supplied from proximal magmatic sources during the deposition (4) The Cambrian-Ordovician zircons from the Lower Paleozoic sequences, but rarely not included in the successive Upper Paleozoic sequences, suggest a provenance change after the hiatus between the two sedimentary successions. (5) The Permian zircons showing different εHf(t) values in each locality within the study area indicate that detrital sources were varied and localized. These integrated results of the detrital zircon U-Pb ages and Hf isotope data from the Paleozoic successions together with the existence of a post-Devonian to Permian clastic wedge in the Okcheon Zone might possibly be related to the subduction in relation to Okcheon Orogeny similar to the Allegany orogenic time span in Appalachian. The entire sedimentary sequences within the Okcheon Belt experienced deformation and metamorphism related to the subsequent (Permo-) Triassic collisional orogeny (viz. Songrim Orogeny) along the Korean collision belt, forming regional fold-dominated mountains of the Okcheon FTB. This suggests that the Okcheon FTB records important information for Paleozoic provenance changes linked to the tectonic evolution of the Korean Peninsula, which will further help to understand the spatial and temporal evolution of orogenic belts during the Phanerozoic along the East Asian continental margin.

In this contribution, we report the metasomatic characteristics of a lamprophyre dyke–marble contact zone from the Hongseong–Imjingang belt along the western Gyeonggi Massif, South Korea. The lamprophyre dyke intruded into the dolomitic marble, forming a serpentinized contact zone. The zone consists of olivine, serpentine, calcite, dolomite, biotite, spinel, and hematite. Minor F and Cl contents in the serpentine and biotite indicate the composition of the infiltrating H₂O-CO₂ fluid. SiO₂ (12.42 wt %), FeO (1.83 wt %), K₂O (0.03 wt %), Sr (89 ppm), U (0.7 ppm), Th (1.44 ppm), and rare earth elements (REEs) are highly mobile, while Zr, Cr, and Ba are moderately mobile in the fluid. Phase equilibria modelling suggests that the olivine, spinel, biotite, and calcite assemblage might be formed by the dissolution of dolomite at ~700 °C, 130 MPa. Such modelling requires stable diopside in the observed conditions in the presence of silica-saturated fluid. The lack of diopside in the metasomatized region is due to the high K activity of the fluid. Our log activity K₂O (a_K2O)–temperature pseudosection shows that at a_K2O~−40, the olivine, spinel, biotite, and calcite assemblage is stable without diopside. Subsequently, at ~450 °C, 130 MPa, serpentine is formed due to the infiltration of H₂O during the cooling of the lamprophyre dyke. This suggests that hot H₂O-CO₂ fluids with dissolved major and trace elements infiltrated through fractures, grain boundaries, and micron-scale porosity, which dissolved dolomite in the marble and precipitated the observed olivine-bearing peak metasomatic assemblage. During cooling, exsolved CO₂ could increase the water activity to stabilize the serpentine. Our example implies that dissolution-reprecipitation is an important process, locally and regionally, that could impart important textural and geochemical variations in metasomatized rocks.