2020 年 126 巻 11 号 p. 645-654
Miocene dikes and sills are widely exposed in the eastern Shitara area, Aichi Prefecture, central Japan. Whole-rock chemical composition, grain size, lightness, and the alteration ratio of the intrusive rocks from three outcrops in the Shitara area are described in this paper. The central part of the intrusive rock bodies, commonly dark gray, gradually changes into grayish white toward its marginal part. The lightness of the polished surface of the rocks was measured using the hue, saturation, lightness (HSL) tool in the Inkscape graphics editor. At the white part, plagioclase crystals are fine-grained and severely replaced by secondary minerals such as calcite, whereas plagioclase, at the dark gray part, is much larger than that in the white part and less altered. Turning to chemical composition, common feature cannot be found among three outcrops.