The Jurassic Bupyeong caldera in South Korea and its tectonic implication

Abstract:

The Bupyeong circular structure with ~10 km in diameter, near to the west of Seoul in middle Korea, has been considered as the remnant of a Mesozoic caldera, and consists of felsic volcanic and plutonic rocks. However, the ages of the magmatism have not been well constrained. We measured in situ U-Pb zircon ages using LA-ICP-MS technique. Two welded tuffs at the western part of the structure gave 171.3 ± 2.6 (95% confidence level hereafter) Ma (n=18) and 173.4 ± 4.6 Ma (n=3). Four granitic rocks within the structure gave 170.5 ± 5.2 Ma (n=8), 171.3 ± 2.6 Ma (n=18), 172.3 ± 1.9 Ma (n=19) and 172.3 ± 3.7 Ma (n=10). The age data indicate coeval volcanism and plutonism of early Middle Jurassic, supporting the caldera interpretation.

The early Middle Jurassic granitic plutons are widespread in South Korea and form NE-SW or NNE-SSW belts. Only those in the Bupyeong structure are associated with volcanic rocks, indicating that they are shallow level intrusions, while others are deeper level ones. This observation is well supported by aluminum-in-hornblende geobarometric data of previous works, in that granites near the Bupyeong structure have lower emplacement pressure (<4 kb), compared with those in other areas (5-8 kb). In summary, the Bupyeong structure may have formed outside of main orogenic zone during the Daebo (Jurassic) orogeny.