K-Ar Ages and Major Mineral Compositions of the Mesozoic Igneous Rocks in the Vicinity of the Geochang Area

거창(居昌)지역에 분포하는 중생대 화성암류에 대한 연령과 주성분 광물의 화학조성

Devonian Geochang foliated granite and Jurassic plutonic rocks intrude Precambrian metamorphic Complex at Geochang area, southern part of the Korean Peninsular. Among them hornblendes from four Jurassic plutonic bodies which have had no trace of metamorphism or deformation since their intrusion were dated by K-Ar method. Hornblende gabbro dike which intruded Anorthosite of unknown age revealed $204{\pm}10Ma$, and hornblende granite and hornblende-biotite granodiorite were $178{\pm}9Ma$ and $181{\pm}9Ma$, repectively. Also, hornblende diorite which partly showing primary foliations were $178{\pm}9Ma$, so igneous activity of Geochang area, northern part of Jirisan, were active about 180 Ma before. Microprobe data of dated hornblends and other major constituent minerals such as plagioclases and biotites were also reported, and their chemical composition showed systematic changes in terms of lithologic types.