Variance of magnetic properties of the marginal part of Miocene volcanic conduits in northwest of Ehime Prefecture, Shikoku District, Japan

Abstract

    High susceptibility bands of about a few meters were shown at contact zones between ilmeniteseries granite and andesite of Miocene volcanic conduits in northern part of Ehime Prefecture, southwest Japan. A profile of susceptibilities across the Senohana volcanic conduit in Nakajima, Matsuyama City clearly shows high susceptibility bands. Thirty-nine andesite and contacted granite samples were obtained from 11 sites of the Miocene volcanic conduits. Susceptibility, natural remnant magnetization and density were measured of 42 samples. Thermomagnetic and hysteresis measurements were carried out 15 samples of them. Natural remanent magnetization and susceptibility of marginal parts of volcanic conduits tend to be stronger than those of the inner part. Hysteresis parameters suggest that magnetites of marginal parts are smaller than those of inner parts. The granite in contact with andesites exceptionally had high susceptibility and natural remnant magnetization, although ilmenite-series granite had quite low susceptibility and natural remanent magnetization. A lot of fine magnetites were recognized in the granite in contact with andesite by microscope observation. They seem to be formed from biotites in the granite by contact metamorphism with intrusion of volcanic conduits.