Plagioclase zoning in the setogawa ash-flow sheet of the Nohi rhyolite complex, central Japan
✍ Scribed by Tsuyoshi Morohashi; Shohei Banno; Masao Yamasaki
- Publisher
- Springer
- Year
- 1974
- Tongue
- English
- Weight
- 669 KB
- Volume
- 45
- Category
- Article
- ISSN
- 0010-7999
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✦ Synopsis
The zonal structure of plagioclase phenoerysts in essential lenticules as well as other petrographic features of the Setogawa welded ash-flow sheet, a member of the late Cretaceous Nohi rhyolite complex in central Japan, varies from the bottom to the top of the sheet corresponding to the variation in a reverse order within the magma column prior to the ash-flow eruption. Plagioclase phenoerysts from the upper parts of the sheet, i.e. from the lower levels of the magma column, are normally zoned, while those from the lower parts of the sheet and from pumice lumps of underlying ash-fall layers show strongly oscillated zoning. In the middle parts, the degree of oscillation is intermediate.
The origin of the variation in zonal structure is discussed using simplified model of the binary plagioclase system in terms of the shifting of equilibrium relation between the magma and crystals caused by the change of water pressure.
The oscillatory zoned plagioclase crystals were formed in the upper levels of the magma column, where sudden drops and gradual recovering of water pressure were caused repeatedly by intermittent volcanic eruptions prior to the ash-flow eruption. The fluctuation of water pressure were small in the deep levels of the magma reservoir where plagioelase with normal zoning was formed.