U-PB ZIRCON AGES FOR PRECAMBRIAN ROCKS IN SOUTHWESTERN RYEONGNAM AND SOUTHWESTERN GYEONGGI MASSIFS, KOREA

The Korean peninsula is part of the Sine-Korean platform and is compos ed of Precambrian to Holocene rocks. It can be divided into three crat onic blocks, the Nangrim massif of North Korea, and the Gyeonggi and R yeongnam massifs of South Korea. The Precambrian rocks in the Ryeongna m massif are dominantly plutonic, while in the Gyeonggi massif the met asedimentary rocks are dominant. This Precambrian basement is overlain by Phanerozoic rocks and intruded by Mesozoic plutonic rocks. This st udy reports U-Pb zircon ages for Precambrian rocks in the southwestern Ryeongnam and southwestern Gyeonggi massifs. The oldest rocks in both massifs belong to the Gneiss complex which is composed of migmatitic and other gneisses, metasedimentary rocks, and mafic plutonic rocks. I n the Ryeongnam massif the Gneiss complex is intruded by granite and p orphyroblastic orthogneisses. The Kurye granite gneiss has an age of 2 120 and 2113 Ma. The Chailbong granite gneiss is younger, having an ag e of 1923 Ma. The porphyroblastic gneisses were probably emplaced in a t least two episodes. The older ones are dated at 1945, 1935, and 1928 Ma, and a younger porphyroblastic gneiss has an age of 1890 Ma. In th e Gyeonggi massif the Gneiss complex is intruded by two granite orthog neisses. The Seosan granite gneiss has an age of 1766 Ma and the Hongs eong granite gneiss is 687 Ma old. The age for the Hongseong granite g neiss is a lower concordia intercept age, the upper intercept indicate s derivation from a ca. 3 Ga protolith.