PALEOPROTEROZOIC OROGENY OF THE YEONGNAM MASSIF IN SOUTH KOREA: COMPARISON BETWEEN SM-ND AND PB-PB GARNET AGES

It is commonly believed that the geology and tectonics of the Korean Peninsula are closely associated with those of the Chinese continent. We obtained Sm-Nd and Pb-Pb garnet ages from six metamorphic rocks of the Yeongnam massif in the southern part of the Peninsula to better understand Precambrian history of the basement rocks. Sm-Nd ages were acquired using garnet, feldspar and whole rock. Pb-Pb ages were obtained by Pb step-leaching method of garnet. Previous studies have shown that the analyzed samples from widely separated parts of the massif experienced upper amphibolite- to granulite-facies (600-880 °ÆC and 3.0-7.5 kb) metamorphism. Sm-Nd garnet ages display a wide range from 952°¾6 (2sd) Ma to 1920°¾240 (2sd) Ma, whereas Pb-Pb garnet ages show a limited variation from 1828°¾67 (2sd) Ma to 1889°¾17 (2sd) Ma. The two ages agree within error for a half of the samples. The Sm-Nd age is younger than the Pb-Pb age for another half of the samples. We interpret that the age difference between the two methods comes from the difference in blocking temperature. Empirical blocking temperature of U-Pb garnet system is estimated to be >800 °ÆC, while both experimental and empirical blocking temperatures of Sm-Nd garnet system have been estimated variably from 600-900 °ÆC. Considering that some of Sm-Nd ages are younger than the Pb-Pb ages, our age data apparently indicate that the blocking temperature of Sm-Nd system is lower than that of U-Pb. We note that those samples with younger Sm-Nd ages tend to show rather poor isochron relationship among individual data. This observation may suggest a possibility of partial resetting (geologically meaningless), which needs to be confirmed by other dating techniques. Our study calls for a caution when the meaning of a Sm-Nd garnet age from high-grade metamorphic terrain is interpreted. Since the blocking temperature of U-Pb garnet system is similar to or higher than the metamorphic temperature of the analyzed samples, we conclude that the Pb-Pb ages represent the timing of metamorphic garnet growth, and that the basement rocks of the Yeongnam massif metamorphosed ca. 1.8-1.9 Ga. The Pb-Pb ages of the Yeongnam massif agree very well with the metamorphic ages of the Central Zone (orogenic belt) in North China Craton, suggesting a possibility of correlation between the two terrains.