PETROGRAPHY OF YABAKEI PYROCLASTIC FLOW DEPOSITS IN THE EASTERN PART OF HITA CITY, OITA PREFECTURE

Abstract

Yabakei pyroclastic flow deposits (YPF), which have been developed in middle Kyushu, especially in Oita prefecture, are described and discussed about two outcrops (Y1, Y2) petrographically and petrochemically. YPF cover over 300km² and their volume is estimated about 25km³. This pyroclastic flow deposits range from augite-hypersthene-hornblende andesite to dacite under the microscopic observation and chemical analysis. YPF are divided into two parts according to their chemical compositions and the degree of welding. The welded part is gradually transitional to that of no welding but some variations in the following chemical compositions (such as SiO₂, Na₂O and Al₂O₃) are recognized. This would be caused mainly by the selective leaching of volcanic glass toward the part of no welding. And it is recognized that it exists negative relation between Na₂O and H₂O (+). This fact may support that selective leaching of volcanic glass in the same part of no welding (Y2).