Abstract
Acid alteration in the western Izu Peninsula reflects the process of the past hydrothermal activity in the district. For the purpose of considering the age of acid alteration in the western Izu Peninsula, K-Ar ages of separated alunite were measured. As a result, the ages of 3.3 ± 0.1 Ma from the Funabara deposits and 0.34 ± 0.05 Ma from the Nishina deposits were obtained. It is considered that the Funabara deposits formed in relation to the late extrusion of Kano andesites. The age of the Nishina deposits and surrounding acid alteration is comparable with intrusion at the northern part of Chokuro mountain. Before 2.0 Ma, hydrothermal attack occurred at Funabara district in Tertiary formation. The extensive acid alteration in Ugusu district formed about 2.0-1.0 Ma. At 1.0-0.3 Ma, subsequent hydrothermal system occurred in Nishina district.
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