TIMING OF ERUPTION AND ALTERATION OF RHYOLITIC VOLCANICS IN CHUJA ISLAND (SW KOREA) REVEALED FROM ZIRCON U/PB AND (U-TH)/HE GEOCHRONOLOGY

Chuja Island, located ~40 km from the Korean Peninsula’s SW coastline, is a relatively small (~7 km²) island primarily composed of Cretaceous volcanic rocks. To uncover formation and post-eruptional history, we performed in-situ U/Pb dating and single-grain (U-Th)/He analysis of zircons from five rhyolitic tuff units. Among >180 in-situ analyses for U/Pb dating, we selected only high quality ²³⁸U/²⁰⁶Pb ages with less than 5% discordance. The weighted means of the resulting ²³⁸U /²⁰⁶Pb ages were: 77.36 ± 0.37 Ma (2 sigma error, n=11, MSWD = 2.31, sample CJ20); 75.16 ± 0.68 Ma (n=7, MSWD = 0.47, sample CJ01); 73.85 ± 1.01 Ma (n=5, MSWD = 0.02, sample CJ14); and 75.00 ± 1.08 Ma (n=5, MSWD = 0.62, sample CJ37). The remaining sample (CJ24) did not yield any reliable ages (<5% discordance), even after a significant number of analyses (n=64). We suggest the weighted means of the zircon ²³⁸U/²⁰⁶Pb ages (~77 – 74 Ma) represent the timing of eruption. The identified Late Cretaceous volcanism on Chuja Island is significantly younger than numerous eruption ages reported on the SW Korean Peninsula but is coeval to widespread back-arc volcanism on the SE Korean Peninsula and NE Japanese Island. Therefore, the new zircon U/Pb ages from Chuja Island illustrate the extension of the NE–SW trending back-arc volcanic zone on the Korean Peninsula and Japanese Island.

For single-grain (U-Th)/He dating, we analyzed two or three grains per sample and report average ages and their standard deviations. The resulting zircon (U-Th)/He ages were: 49.85 ± 2.51 Ma (average ± standard deviation, n=2, CJ01); 49.59 ± 1.85 Ma (n=3, CJ14); 50.22 ± 4.70 Ma (n=3, CJ20); 47.32 ± 10.72 Ma (n=2, CJ24); and 61.15 ± 11.79 Ma (n=3, CJ37). Although one sample (CJ37) yielded a relatively large age spread, all five samples yielded indistinguishable average ages clustered at ~50 Ma. The zircon (U-Th)/He ages were significantly younger than the U/Pb ages, suggesting the zircon (U-Th)/He system was completely reset after eruption. Which geological process caused the zircon He system’s complete resetting is unclear but is most likely post-eruptional hydrothermal alteration. This is supported by (1) pervasive devitrification of the volcanics observed in thin sections, (2) hydrothermal veins identified throughout the island, and (3) occurrence of pyrites near hydrothermal veins.