ABSTRACT

K-Ar and Sr isotopic age determinations were made for Mesozoic granitic rocks intruding into Precambrian gneisses and schists in the Inje-Hongcheon district, South Korea. They include hornblende-biotite granodiorite (K-Ar biotite ages of 154 to 165 Ma), porphyritic biotite granite (149 to 157 Ma) and two-mica granite (149 to 155 Ma). The hornblende-biotite granodiorite gives K-Ar hornblende ages of 168 to 180 Ma and a Rb-Sr whole-rock isochron age of 212.0±26.6 Ma with an initial ⁸⁷Sr/⁸⁶Sr ratio of 0.71198±0.00046. These granitic rocks have been derived from crustal melt during late Triassic to early Jurassic time. The cooling rate of the hornblende-biotite granodiorite was slow in early stage and became greater as it approached shallow depths.