Abstract
We report a ring dike of Jurassic granite for the first time in Korea. There is a prominent circular structure in Seoul granitic batholith near Uijongbu, north of Seoul. The circular structure consists of large eastern half-circle (ca. 13 km in diameter) and small western half-circle (ca. 6 km in diameter) divided by a north-south-striking fault. A comparison with recently available geologic maps indicates that the geologic structure corresponds to a ring dike (or cone sheet) of garnet biotite granite, which negates an impact origin. We suggest that the Uijongbu structure was formed by differential weathering between resistant garnet biotite granite and easily weathered biotite granite. A ring dike origin for the Uijongbu structure probably associated with a volcanic cauldron subsidence is supported by rather shallow emplacement depth for the Jurassic granite and fast cooling rate (ca. 100°C/Ma) estimated from mineral ages, although no associated volcanics are present in the surrounding region.
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