U-Pb zircon ages for Precambrian rocks in southwestern Ryeongnam and southwestern Gyeonggi massifs, Korea.

Abstract

The Korean peninsula is part of the Sino-Korean platform and is composed of Precambrian to Holocene rocks. It can be divided into three cratonic blocks, the Nangrim massif of North Korea, and the Gyeonggi and Ryeongnam massifs of South Korea. The Precambrian rocks in the Ryeongnam massif are dominantly plutonic, while in the Gyeonggi massif the metasedimentary rocks are dominant. This Precambrian basement is overlain by Phanerozoic rocks and intruded by Mesozoic plutonic rocks. This study 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. In the Ryeongnam massif the Gneiss complex is intruded by granite and porphyroblastic orthogneisses. The Kurye granite gneiss has an age of 2120 and 2113 Ma. The Chailbong granite gneiss is younger, having an age of 1923 Ma. The porphyroblastic gneisses were probably emplaced in at 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 the Gyeonggi massif the Gneiss complex is intruded by two granite orthogneisses. The Seosan granite gneiss has an age of 1766 Ma and the Hongseong granite gneiss is 687 Ma old. The age for the Hongseong granite gneiss is a lower concordia intercept age, the upper intercept indicates derivation from a ca. 3 Ga protolith.