U-PB ZIRCON AGES OF MESOZOIC PLUTONS IN THE DAMYANG-GEOCHANG AREA, RYONGNAM MASSIF, KOREA

The Damyang-Geochang area in the southwestern part of the Ryongnam mas sif, Korea, is composed of Precambrian gneisses that have been intrude d by Triassic to Jurassic, felsic to mafic, batholiths and stocks. Sin ce 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 t he rocks in the area, and the ages range from 159 to 228 Ma. This stud y reports the first U-Pb zircon ages for the Korean peninsula. In the Damyang-Geochang area, Mesozoic plutonism appears to span the period f rom 219 Ma to 176 Ma. The first period of Mesozoic plutonism resulted in the emplacement of the (unnamed) foliated granite, the foliated gab bro, and the Daegang foliated granite, at 219-212 Ma. The second pluto nic event occurred at 187-183 Ma, in which a foliated granodiorite and the Sunchang foliated granite were emplaced. In the third plutonic ev ent the Namweon granitic batholith and an associated diorite stock, bo th non-foliated and post-tectonic, were emplaced at 177-176 Ma. Concor dia plots indicate derivation of these plutonic rocks from a Precambri an protolith, 1.9-2.6 Ga in age. Pb isotopic compositions measured on feldspars have an upper crustal signature. A porphyroblastic gneiss fr om the Precambrian basement yields a zircon age of 1935 Ma.