上東地域 선캠브리아 花崗岩類의 K-Ar 白雲母 年齡測定

K-Ar muscovite dating for precambrian granites in the Sangdong area

The studied Nonggeori and Naedeogri granites in the Sangdong area intruded into the Precambrian metasedimentary rocks of the Yulri Group. The Cambro-Ordovician Choseon Supergroup overlied unconformably upon the Yulri Group. Pegmatitic dykes injected into the Yulri Group and the granites, but not in the Choseon Supergroup. Field relationships suggest approximate ages of the intrusive rocks in the studied area belong to the Precambrian. Extremely pure concentrates of muscovites(40-80#) were obtained from the granites by conventional isodynamic magnetic separators. The contents of K and 40Ar in the muscovites show 8.60-8.78% and 98.52-99.11%, respectively. From the potassium contents of the muscovites and the approximate ages, the sample amounts for argon analyses are average of 0.00371gr. The K-Ar ages on the muscovites were revealed as Proterozoic (1673±22~1802.54±17.5 Ma).