ADVANCES IN U-PB ZIRCON GEOCHRONOLOGY OF MESOZOIC PLUTONISM IN THE SOUTHWESTERN PART OF RYEONGNAM MASSIF, KOREA

The Ryeongnam massif of Korea is composed predominantly of crystalline Precambrian rocks, which have been intruded by mafic to felsic pluton s of Mesozoic age. This study reports six new U-Pb zircon ages for plu tons in the southwestern part of the massif. These ages are: Machon ga bbro- 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 an d nonfoliated, also mafic and felsic, plutons were intruded at 170-180 Ma. The Honam shear zone, which separates the Ryeongnam massif from t he Ogcheon fold belt to the north, developed at ca. 180 Ma. K-Ar and A r-Ar ages in the area are younger than zircon ages and probably apply to metamorphic activity related to the deformation along the Honam she ar zone. Ages for Mesozoic plutons in the Ryeongnam massif are similar to ages reported in the Qinling-Dabie belt of China and indicate coev al igneous activity and possibly similar tectonic evolution.