ABSTRACT

The Ryeongnam massif of Korea is composed predominantly of crystalline Precambrian rocks, which have been intruded by mafic to felsic plutons of Mesozoic age. This study reports six new U-Pb zircon ages for plutons in the southwestern part of the massif. These ages are: Machon gabbro 223 ± 3 Ma, diorite 210 ± 2 Ma, Tongbok metaporphyry 219 ± 3 Ma, Chahwangsan syenite 197 ± 1 Ma, Yulhyunri foliated granite 195 ± 2 Ma, Sinwon foliated granodiorite 189 ± 3 Ma. The above ages together with existing published zircon ages for other plutons in the region indicate three periods of igneous activity. Foliated mafic and felsic plutons were emplaced between 205–230 Ma and 180–200 Ma and nonfoliated, also mafic and felsic, plutons were intruded at 170–180 Ma. The Honam shear zone, which separates the Ryeongnam massif from the Ogcheon fold belt to the north, developed at ca. 180 Ma. K-Ar and Ar-Ar ages in the area are younger than zircon ages and probably apply to metamorphic activity related to the deformation along the Honam shear zone. Ages for Mesozoic plutons in the Ryeongnam massif are similar to ages reported in the Qinling-Dabie belt of China and indicate coeval igneous activity and possibly similar tectonic evolution.