ABSTRACT

The Damyang-Geochang area in the southwestern part of the Ryongnam massif, Korea, is composed of Precambrian gneisses that have been intruded by Triassic to Jurassic, felsic to mafic, batholiths and stocks. Since the Triassic this area has been affected by four orogenies. Eight K-Ar, Ar-Ar, and Rb-Sr ages have previously been reported for some of the rocks in the area, and the ages range from 159 to 228 Ma. This study reports the first U-Pb zircon ages for the Korean peninsula. In the Damyang-Geochang area, Mesozoic plutonism appears to span the period from 219 Ma to 176 Ma. The first period of Mesozoic plutonism resulted in the emplacement of the (unnamed) foliated granite, the foliated gabbro, and the Daegang foliated granite, at 219-212 Ma. The second plutonic event occurred at 187-183 Ma, in which a foliated granodiorite and the Sunchang foliated granite were emplaced. In the third plutonic event the Namweon granitic batholith and an associated diorite stock, both non-foliated and post-tectonic, were emplaced at 177-176 Ma. Concordia plots indicate derivation of these plutonic rocks from a Precambrian protolith, 1.9-2.6 Ga in age. Pb isotopic compositions measured on feldspars have an upper crustal signature. A porphyroblastic gneiss from the Precambrian basement yields a zircon age of 1935 Ma.