愛媛県野村町北部の中期更新世平野火山灰のFT年代および岩石学的研究

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The Hirano volcanic ash in Nomura Town, southwestern Shikoku, Japan, has been characterized by petrography, refractive index and geochemical composition of volcanic glass (major and trace elements), and fission-track age of zircon. The Hirano volcanic ash has abundant vesicular glass shards and phenocrysts of biotite, hornblende and orthopyroxene. The volcanic glass has a rhyolitic composition. LREE (Light Rare Earth Element) profiles of the volcanic glass are steep with La/Sm_N=5.0-5.3 and those of HREE (Heavy Rare Earth Element) are flat with Gd/Yb_N=1.2-1.3. They have a weakly developed Eu anomaly. The Hirano volcanic ash has similar values of some incompatible trace element ratios, such as La/Y, Ba/La and Nb/Zr, with the Yufugawa pyroclastic flow deposit in the Ooita Prefecture. The fission-track age of 0.46±0.09Ma (1σ) was obtained for zircons from the Hirano volcanic ash. The Hirano volcanic ash can be correlated to the Yufugawa pyroclastic flow deposit based on petrographical, geochemical and geochronological data.
2009-12-31