An occurrence of the post-orogenic Triassic strata on Deokjeok Island, western Gyeonggi massif, Korea

Abstract

The U-Pb isotopic compositions of zircon from a metasandstone cobble and a granitic dyke in Deokjeok Island, western Gyeonggi massif, Korea, were measured using a SHRIMP-II ion microprobe to constrain the depositional age of cobble-bearing metasedimentary strata. The metasandstone cobble, collected from a metaconglomerate of the slighltly-metamorphosed sedimentary unit (SMU), yielded 50 to 200 μm long zircon crystals showing variable internal structures together with rounded corners. Sixty spots were analysed from 60 detrital crystals, including two spots of Permian metamorphic overgrowth dated at 255 ± 4 Ma (1σ) and 263 ± 4 Ma (1σ), respectively. Except for the Permian ages, their U-Pb age distribution pattern is characterized by a few major populations at ~400 Ma, ~1.0 Ga and ~2.5 Ga, and several minor peaks between 400 Ma and 2.5 Ga. This pattern is identical to that of the Devonian Taean Formation widespread along the western Gyeonggi massif. On the other hand, the ~260 Ma overgrowth age in the cobble could represent the zircon fraction formed in the sedimentary protolith of the cobble during the Permo-Triassic collisional orogeny. Zircon crystals from a granitic dyke intruding the SMU generally showed the oscillatory zonation, and yielded the weighted mean 206Pb/238U age of 225 ± 3 (tσ), corresponding to its intrusion age. It is thus likely that the cobble-bearing strata might have deposited during the Triassic prior to 225 Ma. Moreover, the SMU unit may represent post-orogenic sediementary strata filling the extensional basin developing along the suture at the western margin of the Gyeonggi massif.