ABSTRACT

The Ryeongnam massif is composed of Precambrian gneisses, Paleozoic and Mesozoic sedimentary rocks and extensive Triassic-Jurassic plutonic rocks of felsic to mafic composition. In the northeast Ryeongnam massif, the oldest rocks belong to the Sobaegsan gneiss complex, which is composed of orthogneisses, paragneisses and mafic plutonic rocks. U-Pb zircon ages for the felsic and mafic intrusive bodies within the Sobaegsan gneiss complex are: the Icheon granite gneiss, 2357 ± 43 and 2342 ± 47 Ma; the Buncheon granite gneiss, 1963 ± 5 Ma; the Pyeonghae granite gneiss, 1936 ± 21 Ma; the Ogbang amphibolite, 1918 ± 10 Ma; the Imwon leucogranite gneiss, 1826 ± 20 Ma. The Hyeondong biotite schist, which is intruded by the Buncheon granite gneiss and the Ogbang amphibolite, yielded an age of 2271 ± 44 Ma. The Nd-Sm-Pb isotopic data indicate that the felsic plutonic rocks are derived from an older Archean crust. The Nd T_DM ages are Archean, and the εNd values are negative for the felsic rocks and positive for the amphibolite. Common Pb isotope compositions also indicate a crustal source for the felsic intrusives. The U-Pb ages of Precambrian rocks of the Ryeongnam massifs are similar to those in the Gyeonggi massif, and may have a similar crustal evolutionary history. The Precambrian rocks of South Korea could be related either to the North China block or to the South China block, as the isotope ages and patterns are not unique to either block. Similarly, a geologic correlation with Japan, although possible, is tenuous at present.