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

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

The U-Pb isotopic compositions of zircon from ametasandstone cobble and a granitic dyke in Deokjeok Island, westernGyeonggi massif, Korea, were measured using a SHRIMP-IIion microprobe to constrain the depositional age of cobble-bearingmetasedimentary strata. The metasandstone cobble, collected from ametaconglomerate of the slighltly-metamorphosed sedimentary unit(SMU), yielded 50 to 200 μm long zircon crystals showing variableinternal structures together with rounded corners. Sixty spotswere analysed from 60 detrital crystals, including two spots ofPermian metamorphic overgrowth dated at 255 ± 4 Ma (1σ) and263 ± 4 Ma (1σ), respectively. Except for the Permian ages, theirU-Pb age distribution pattern is characterized by a few major populationsat ~400 Ma, ~1.0 Ga and ~2.5 Ga, and several minor peaksbetween 400 Ma and 2.5 Ga. This pattern is identical to that of theDevonian Taean Formation widespread along the western Gyeonggimassif. On the other hand, the ~260 Ma overgrowth age in the cobblecould represent the zircon fraction formed in the sedimentary protolithof the cobble during the Permo-Triassic collisional orogeny. Zircon crystals from a granitic dyke intruding the SMU generallyshowed the oscillatory zonation, and yielded the weighted mean206Pb/238U age of 225 ± 3 (tσ), corresponding to its intrusion age. Itis thus likely that the cobble-bearing strata might have depositedduring the Triassic prior to 225 Ma. Moreover, the SMU unit mayrepresent post-orogenic sediementary strata filling the extensionalbasin developing along the suture at the western margin of the Gyeonggimassif.

The U-Pb isotopic compositions of zircon from ametasandstone cobble and a granitic dyke in Deokjeok Island, westernGyeonggi massif, Korea, were measured using a SHRIMP-IIion microprobe to constrain the depositional age of cobble-bearingmetasedimentary strata. The metasandstone cobble, collected from ametaconglomerate of the slighltly-metamorphosed sedimentary unit(SMU), yielded 50 to 200 μm long zircon crystals showing variableinternal structures together with rounded corners. Sixty spotswere analysed from 60 detrital crystals, including two spots ofPermian metamorphic overgrowth dated at 255 ± 4 Ma (1σ) and263 ± 4 Ma (1σ), respectively. Except for the Permian ages, theirU-Pb age distribution pattern is characterized by a few major populationsat ~400 Ma, ~1.0 Ga and ~2.5 Ga, and several minor peaksbetween 400 Ma and 2.5 Ga. This pattern is identical to that of theDevonian Taean Formation widespread along the western Gyeonggimassif. On the other hand, the ~260 Ma overgrowth age in the cobblecould represent the zircon fraction formed in the sedimentary protolithof the cobble during the Permo-Triassic collisional orogeny. Zircon crystals from a granitic dyke intruding the SMU generallyshowed the oscillatory zonation, and yielded the weighted mean206Pb/238U age of 225 ± 3 (tσ), corresponding to its intrusion age. Itis thus likely that the cobble-bearing strata might have depositedduring the Triassic prior to 225 Ma. Moreover, the SMU unit mayrepresent post-orogenic sediementary strata filling the extensionalbasin developing along the suture at the western margin of the Gyeonggimassif.