Abstract
Ontake Volcano is situated at the southern margin of the Norikura volcanic chain, Central Japan. The volcano consists of the Older (0.78—0.39 Ma) and Younger (0.09—0.02 Ma) Ontake Volcanoes with long dormancy between them.
Lithofacies and heavy mineral assemblage of almost all of the tephra (air fall tephra and pyroclastic flow deposits) from the Older Ontake Volcano are described. Petrographic studies of some of them have been further carried out; such features as grain compositions, heavy mineral compositions and chemical compositions of hornblende and pyroxenes. Tephrostratigraphy of the Older Ontake Volcano was established using stratigraphic relations with the dated lavas, and the petrographic features of tephra. Conclusively, the activity of the Older Ontake Volcano was divided into Tephra Stage (before 0.78—0.64 Ma) and Lava Stage (0.64—0.39 Ma) based on the mode of eruption; the Tephra Stage being characterized by effusion of many air fall tephra and pyroclastic flows, whereas the Lava Stage contains many thicker lavas. The Tephra Stage is further subdivided into H Substage (before 0.78 Ma), PH Substage (0.78—0.70 Ma) and OP Substage (0.70—0.64 Ma) by the assemblage of dominant heavy minerals in the air fall tephra as follows: H Substage; green hornblende, PH Substage; pyroxenes and brown hornblende, OP Substage; olivine and pyroxenes. Ages of each stage and substage were inferred from the stratigraphic relations with the dated lavas. Each of the stages and substages is distinguishable not only by heavy mineral assemblages, but also by chemical compositions of hornblende and pyroxenes in the air fall tephra.
