The Okcheon metamorphic belt (OMB) in central South Korea is surrounded by Middle Jurassic granitoid batholiths that intruded South Korea extensively; the granitic bodies form a complex about 200 km long and 150 km wide as part of a Mesozoic granite belt along the East Asian continental margin. Middle Jurassic magmatism was related to ridge subduction that occurred around 200 to 166 Ma, with the main magmatic period between 175 and 166 Ma; main cooling ages range from 168 to 152 Ma. The magmatism was divided into two stages: (1) a deeper, earlier stage, which resulted in emplacement of diorite, granodiorite and granite as shown in the northeast OMB; and (2) a shallower, younger stage, which resulted in emplacement of granite and two-mica granite as shown in the southwest OMB. Most granitoids are peraluminous to metaluminous I-type granitoids that originated in a volcanic arc; an exception is an S-type two-mica granite. Inherited cores of 998 and 262 Ma U-Pb SHRIMP II zircon ages from the two-mica granite indicate that two-mica granite is reworked crustal material formed by earlier magmatism before the Middle Jurassic event. Together with previous studies on the Middle Jurassic granitoids, the present result indicates that subduction of the Farallon-Izanagi ridge beneath Asia caused widespread igneous activity throughout South Korea, especially during Middle Jurassic ridge subduction.
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