海南地域 火山岩類와 납석 및 고령토 鑛床의 K-Ar 年代

K-Ar ages of alunite and sericite in altered rocks, and volcanic rocks around the Haenam area, Southwest Korea

A number of alunite and pyrophyllite deposits occur around the Haenam area where Cretaceous volcanic and volcanogenic sediments are widely distributed. The K-Ar ages of alunite, sericite and whole rocks collected from alunite and pyrophyllite deposits and unaltered rocks representing various stratigraphic horizon of the area were determined and their formation stage was discussed. The ages of volcanic rocks range between 68.6±1.9 and 94.1±2.0 Ma corresponding to Cenomanian-Maastrichtian of upper Cretaceous. Andesitic rock gives 94.1±2.0. Rhyolite and acidic tuffs give 79.47 ±1.7 and 82.8±1.2 Ma corresponding to Campanian. The later stage andesite gives 68.6±1.9 Ma of Maastrichtian. The results suggest that volcanism of the area can be devided into three different stages. The ages of alunite and sericite range 71.8±2.8 to 76.6±2.9 Ma of late Campanian to early Maastrichtian which is rather earlier than the age of granite(67 Ma). It indicates that the alteration ages of these clay mineral deposits appeared to be related with its volcanism rather than the hydrothermal stage of granite of this area.