A preliminary CHIME age determination of monazites from metamorphic and granitic rocks in the Gyeonggi Massif, Korea

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The CHIME (Chemical Th-U-Total Pb isochron method) dating was carried out for monazites from kyanite-staurolite-garnet schist of the Yeoncheon Group (Cheolwon area), and from sillimanite-garnet gneiss and two-mica granite in the central Gyeonggi Gneiss Complex (Hwacheon area). Monazites from the kyanite-staurolite-garnet schist yield an age of 255±8 Ma, and date the time of the regional metamorphism for the early-middle Proterozoic (?) Yeoncheon Group. Although the Gyeonggi Gneiss Complex has been believed to be of Archean-early Proterozoic age, monazites from the sillimanite-garnet gneiss yield a CHIME age of 245±3 Ma. Since one monazite grain from the gneiss contains ca. 1700 Ma core of detrital origin, the sedimentation of the gneiss protolith took place in the post-middle Proterozoic. The two-mica granite, intruding the Gyeonggi Gneiss Complex, yields a CHIME monazite age of 172±5 Ma. The present CHIME geochronological study reveals that metamorphic rocks not only in the Yeoncheon Group but also in some part of the Gyeonggi Gneiss Complex were formed through the ca. 250 Ma regional metamorphism. The late Permian-early Triassic metamorphism and Jurassic plutonism were more widespread than has been thought in the Korean Peninsula.

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