MESOZOIC ARC MIGRATION IN THE EAST ASIAN CONTINENTAL MARGIN INFERRED FROM DETRITAL ZIRCON GEOCHRONOLOGY OF MODERN FLUVIAL SEDIMENTS, SOUTH KOREA

Detrital zircons collected from mouths of seven major southern Korean rivers were analyzed using a laser ablation inductively coupled plasma mass spectrometer (LA-ICPMS) to infer spatial and temporal trends of Mesozoic magmatism of the East Asian continental margin, which has been active since the Mesozoic subduction of the paleo-Pacific plate caused extensive magmatism along the Asian margin throughout the Mesozoic. In South Korea, the NE-trending Mesozoic granitoid batholiths ca. 500 km wide are distributed as a part of the East Asian granitoid belt. They are interpreted to have been formed by orogeny during the Jurassic due to the orthogonal subduction of the oceanic plate and by extension during the Cretaceous because of its oblique subduction. However, due to lack of geological information tectonic situation of the paleo-Pacific plate subduction during the Triassic is not known.

179 concordant analytical results from 700 detrital zircons analyzed yielded various Mesozoic ages (72-249 Ma) indicating continuous magmatic activity throughout the Mesozoic time except for ca. 30 My from 130 Ma to 160 Ma. Considering catchment areas of the studied rivers, these results indicate spatial migration of the magmatic centers in the southern Korean Peninsula; trenchward in both the Triassic and Cretaceous, and cratonward in the Jurassic time. Comparison of these results with previous studies on the magmatic ages in the adjacent regions such as northeastern China and southwestern Japan suggests that the slab roll-back took place in Triassic and Cretaceous time, resulting in the oceanward arc migration, whereas subduction angle of the paleo-Pacific Plate became shallow in Jurassic time, resulting in the landward younging magmatism and consequent magmatic gap along the margin of the East Asian continent.