Geochronology and Petrogenesis on the Older Granitic Rocks collected across the Ryeongnam Massif, Korea

영남육괴(嶺南陸塊)에 분포(分布)하는 고기화강암질암(古期花崗巖質岩)의 지질연대(地質年代)와 성인(成因)에 대(對)한 연구(硏究)

The older granitic rocks of the study area are composed of mainly orthogneiss and foliated granite. These rocks shows mostly mortar or fiaser structure by strong mylonitization and thermal metamorphism during several orogenies. $^{40}Ar-^{39}Ar$ incremental·release ages of these rocks have been determined for 6 hornblende. 7 muscovite and 4 biotite concentrates separated from rocks collected across the Ryeongnam massif. Most $^{40}Ar-^{39}Ar$ age are discordant with Rb-Sr whole rock age of the same area. These ages range from 1998 to 172Ma. This discordant age is interpreted to indicate that samples were in contact with Daebo granite body that was characterized by large and variable $^{40}Ar-^{39}Ar$ ratios. Such ratios most likely resulted from widespread diffusion of the argon liberated from older granitic rocks during several metamorphic overprint. The general trend of the chemical composition of these rocks suggest that most of them are some series of differentiated products by fractional crystallization.