木曽御岳火山周辺の後期更新世の降下火砕堆積物

層序と岩石記載

抄録

The late Pleistocene air fall pyroclastics, distributed around Ontake volcano, are composed of rhyolitic to andesitic pumice and scoria. The air fall pyroclastic deposits are divided into three formations: the Osakada Loam (OLF), the Lower Hata Loam (LHF), and the Upper Hata Loam (UHF) Formations, in ascending order on the basis of obvious unconformity and/or intercalation of paleosoil. These observations show a repose of volcanic activity between the Formations. Members, divided by weathering surfaces, are OLF I, OLF II, LHF I, LHF II, UHF I, UHF II, and UHF III, in ascending order. The degree of weathering, which is somewhat weaker than that of the paleosoil between the Formations, will imply a 'short repose' of volcanic activity. The OLF contains 9 beds of pumice fall erupted from Ontake volcano. The main phenocrysts associated with salic pumice (Pm-1 A, -1 B, --2 A) are of plagioclase (An 50-40), biotite, hornblende, orthopyroxene (En 60-45), magnetite (Tc=430-450℃: Curie Temp.), ilmenite, and rare apatite and zircon. The dacite pumice (Pm-2 B, -3 A, 3 B, -3 C, -3 D) are composed of the minerals plagioclase (An 55-45), hypersthene (En 68-65), augite (En 41 Fs 15 Wo 44), magnetite(Tc:=400℃), ilmenite, hornblende, apatite, and zircon. The mineral assemblage and the chemical compositions of the minerals systematically change from aci dic to rather basic following to the order of eruptions : from Pm-1 A to -2 B and -3 C to 3 D, respectively. An abrupt increase of magma temperature and compositional change from acidic to basic may take place between Pm-2 B and -3 A. Ol-bearing and Ol-free andesitic scoria are the prevailing component of the LHF and the UHF. The Ol-free scoria contains plagioclase, hypersthene (En 73-68), augite (En 40-45 Fs 12-21, Wo 39-48), and magnetite (Tc=220-270℃) as phenocrystic minerals. Plagioclase, olivine (Fo 75-70), augite (En 40-45 Fs 12-21 Wo 48-39), orthopyroxene (En 76-70'En65-60), and magnetite (Tc=180-290℃), are of constituent phenocrystic minerals of Ol-bearing and esite scoria rarely associated with hornblende and apatite. Pyroxene crystals having broad compositional range associated with Mg-rich olivine show complex zoning pattern in the Ol-bearing andesitic scoria. Plagioclase containing dusty inclusion is abundant phenocryst of the scoria. These petrographic features of the Ol-bearing andesitic scoria will suggest disequilibrium relationship between minerals [Sakuyama (1979)]. The LHF II consists of Ol-free scoria, the remaining Members are composed of Ol-bearing scoria. Therefore, it is assumed that the two types of andesitic magma concurrently were active in the latter half of the late Pleistocene beneath Ontake volcano.