伊豆半島西部の酸性変質作用および明礬石の地球化学

抄録

Acid alteration in the western Izu Peninsula, Shizuoka Prefecture, is distributed widely within Miocene and Pliocene formations, and overlain by Quaternary volcanoes. For the purpose of understanding its origins, we examined the acid alteration and the property of alunite from several locations throughout the district.
     The acid alteration zone consists of acid-leached andesite, vuggy silica and massively silicified rocks, surrounded by advanced and intermediate argillic alterations. They are underlain by district-wide propylitic alteration. The advanced argillic alteration is characterized by an assemblage of pyrophyllite, diaspore and alunite. The alunite has cores rich in Sr and PO₄. Oscillatory zoning of K, Na and Ca in alunite, natroalunite and minamiite is found. δ³⁴S values are +18 to +26 per-mil. The oscillatory zoning is ascribed to the variation in the composition of hydrothermal fluid precipitating the alunite crystals, whereas the increased trend in SO₄/PO₄ from core to margin reflects the chemical evolution of hydrothermal fluid. The mineral assemblage and isotopic data suggest that the acid alteration in Ugusu district was formed from higher temperature fluid, which contained reactive magmatic components, while the acid alteration in Shotsubotoge-Nishina district was formed from lower temperature fluid.