K-Ar whole Rock Ages of the Rhyolitic Rocks at Punggog in the Jangseong Sheet, Taebaegsan Area

太白山 鎭山帶內 長省圖福의 豊谷一帶에서 採取한 두 個의 流紋岩類를 K-Ar 全岩法으로 年齡測定하였다. 그 中, 流紋岩은 62.69±1.15Ma, 流紋岩質凝灰岩은 51.67±6.64Ma로 確認되었다. 一般的으로 火山岩의 K-Ar 全岩年齡은, 地質學的으로 二次的영향을 받지 않았다면, 火山岩의 形成年齡을 提示하나, 여기서 年齡測定한 두개의 流紋岩質岩은 약간 熱水變質받았고, 流紋岩質凝灰岩이 流紋岩의 年齡보다 약간 젊은 것으로 나타났다. 이들 岩石의 野外産出狀態는 整合的인 故로, 적어도 이들 두 流紋岩質岩은 約 60Ma보다 그 以前에 噴出된 것으로 判斷된다.

Two rhyolitic rocks were taken at punggog of the Jangseong sheet in the Taebaegsan mineralized area and isotopically dated by K-Ar whole rock method. One is a rhyolite which gives 62.69±1.15Ma and the other is a rhyolitic tuff which gives 51.67±6.64Ma, respectively. Generally K-Ar whole rock ages of the volcanic rocks can be assumed to be the formation age of them, if there is no geological criterion of secondary effects. But the two rhyolitic rocks were slightly hydrothermally altered and the age the rhyolitic tuff is a little younger than that of the rhyolite. However, there is no geological criterion to show any big hiatus between them in field, yet. Therefore, the age data would be interpreted, as that the rhyolitic rock mass has been probably extruded at about 60 Ma, a little older than 60 Ma, in the area. The ages of them probably appear to be secondary ages after the alteration. This fact well coincides with the K-Ar whole rock age of quartz-porphyry (57.25±0.89Ma) distributed near the 1st Yeonhwa Pb-Zn mine (Park et al., personal comm.), because the quartz-porphyry look to be a product of hydrothermal alteration of the volcanic rock.