ABSTRACT

Rb-Sr whole-rock isochron ages and Sr and Nd isotopic ratios have been determined for Youngju and Andong granites in the northeastern Yeongnam Massif, Korea. Six samples of Youngju granites yield an isochron age of 267 ± 27 Ma with an initial Sr ratio of 0.71505 ± 0.00026. Seven samples of Andong granites give an isochron age of 361 ± 41 Ma and an initial Sr ratio of 0.70944 ± 0.00011. The isochron ages indicate that Youngju and Andong granites were emplaced in middle Permian and late Devonian of late Paleozoic, respectively. Calculated ε_Sr (T) and ε_Nd (T) values at 267 Ma for Youngju granites are 152.2∼156.4 and -19.7∼-18.0, and those values at 361 Ma for Andong granites are 73.8∼79.7 and -11.4∼-8.0. Model ages of Youngju and Andong granites are 2.4∼3.3 Ga and 1.4∼1.8 Ga, respectively, suggesting that the Youngju granitic magma was derived from highly LREE-enriched lower crust formed in Late Archean, whereas the Andong granitic magma from slightly LREE-enriched lower crust formed in Proterozoic. Initial ⁸⁷Sr/⁸⁶Sr ratios of the granites in Ogcheon Belt and Yeongnam Massif display a broad range, but have two peaks around 0.709 and 0.715 regardless of emplacement age. It is likely that these two kinds of sources which are isotopically different have widely existed in the lower crust of Ogcheon Belt and Yeongnam Massif.