抄録
The Miocene series consisting largely of pyroclastic and volcanic rocks of dacitic to andesitic composition has been subjected to green tuff alteration. In this area, chlorite-chlorite-saponite mixed layer in andesitic pyroclastic rocks and clinoptilolite・mordenite in dacitic pyroclastic rocks have been formed under same conditions. Then, zoning of alteration was examined for the dacitic pyroclastic rocks distributed extensively in the surveyed area. The altered dacitic pyroclastic rocks are divisible into three zones, such as, Zone I (montmorillonite zone), Zone II (chlorite-sericite zone), and Zone III (kaolin zone). Furthermore, Zone I can be subdivided into clinoptilolite-mordenite subzone, mordenite subzone, and analcime subzone on the basis of zeolite mineral content. Zone I and its underlying Zone II have been formed in the Hata Formation of the lower Miocene in the age, and the boundary between them is almost parallel to original depositional surface. At a horizone of this boundary analcime appears sporadically. During the middle Miocene Kuri Formation, Zone I, II, and III have been formed again. This zone II and III are oblique to the bedding planes. Chlorite being a characteristic mineral of Zone II and kaolin coexist within the middle part of Zone III, and petrographic relationships indicate that chlorite is partly replaced by kaolin. Therefore the formation of Zone, III followed that of Zone II.
