Abstract
The hydrothermal alteration zone (Murasame Zone), where silicification occurred in the Miocene Bessho and Aoki formations, lies to the northeast of Matsumoto City, central Japan, and has a north-south orientation, a width about 2-3km and a length of 80〜90km. We studied the hydrothermal effects in the Bessho and Aoki formations using vitrinite reflectance. Vitrinite reflectance values become higher close to the Murasame Zone with Ro=3.7% at the maximum. The optical anisotropy of vitrinite macerals suggests that the calibration formula (Barker and Pawlewicz, 1994) could be applied and estimate the maximum paleotemperature of the hydothermal alteration in the Murasame Zone as more than 320℃.
