Paleoproterozoic orogeny in South Korea: Evidence from Sm-Nd and Pb step-leaching garnet ages of Precambrian basement rocks

Geology and tectonics of the Korean Peninsula have been considered to be closely associated with those of the adjacent Chinese continent. Although Phanerozoic connections between the two areas have been reasonably well worked out, Precambrian ones are less well established. We obtained Sm-Nd and U-Pb garnet ages of 12 metamorphic rocks from widely separated parts of the Gyeonggi and Yeongnam massifs in South Korea to better understand their Precambrian history. The two massifs consist of late Archean to Paleoproterozoic basement rocks and Meso- to Neoproterozoic supracrustal rocks. Previous studies have shown that the two massifs experienced upper amphibolite to granulite facies metamorphism of medium pressure type with clockwise P-T-time paths, suggesting that the two massifs may represent remnants of a collisional orogen. Pb-Pb and U-Pb garnet ages give two distinct age groups: ten Paleoproterozoic (1989 ± 34 to 1835 ± 56 Ma) ages of basement rocks from the two massifs, and one late Permian (265 ± 8 Ma) age of a supracrustal rock from the Gyeonggi massif that corroborates previous estimates of Permo-Triassic (298-245 Ma) metamorphic age for the supracrustal rocks. On the other hand, Sm-Nd garnet ages are less precise and display a wide range of ages from 2257 to 413 Ma for the basement rocks. When we compare the Sm-Nd and Pb-Pb ages from the same sample, the Sm-Nd age is similar to or younger than the Pb-Pb age, suggesting that the blocking temperature of the Sm-Nd garnet system may be lower than that of the U-Pb system (>800°C). We interpret the Pb-Pb and U-Pb ages as representing the timing of metamorphic garnet growth, since the blocking temperature of the U-Pb garnet system is similar to or higher than the metamorphic temperature of the analyzed samples. Apparently, the younger metamorphism, possibly associated with the Permo-Triassic collision between the North and South China cratons, was not strong enough to reset the older metamorphic age. Similar Paleoproterozoic metamorphic ages for basement rocks of the two massifs suggest that they may have come from the same Precambrian terrane. The metamorphic ages of the basement rocks of South Korea agree very well with those of the Central Zone (orogenic belt) in the North China craton, suggesting a possible correlation between the two terranes.