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稲城市で見られる根方タフの鉱物学的性質とその噴出源

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稲城市で見られる根方タフの鉱物学的性質とその噴出源

越智 翔士

＊

・藤本 光一郎

＊＊

地学

＊

・環境科学

＊＊

（2006年 5 月26 日受理）

OCHI, S. and FUJIMOTO, K.: The mineralogical nature and the source of the Negata tuff observed in Inagi city, Tokyo, Japan.

Bull. Tokyo Gakugei Univ. Natur. Sci., 58: 163 – 171 (2006) ISSN 1880–4330

Abstract

Inagi city is located in the east of the Tama Hills made of Inagi formation eroded by the Tama River in the early Quaternary Pleistocene. Cliffs at the Anazawatenjinja shrine, at Nishiyama and at Omaru are outcrops which are well known. We can observe blue-gray tephra (Negata tuff) and brown sand layer (Inagi formation) at the back of the Anazawatenjinja shrine. However, the source of the Negata tuff is not identified yet. In this study, we studied the mineralogical nature of tephra, constituents to clarify the distribution, the source of the Negata tuff and a part of geohistory of Inagi city. We collected four tephra samples from different places; Anazawatenjinja shrine (type locality of the Negata tuff), Nishiyama, Omaru and Kokeiji temple, in and near the Inagi city.

Geological settings, heavy mineral composition and chemical composition of volcanic glass indicated that they are identical except Omaru. The Negata tuff was deposited in 1.1±0.1 Ma (FT age of the tephra at the Kokeiji temple after Ito et al., 2002). Shishimuta caldera, Oita prefecture, is candidate of the source of the Negata tuff based on the eruption age and mineralogical features.

Key words : Inagi city, Negata tuff, Inagi formation, Shishimuta caldera, Shishimuta-pink tephra, explosion source, volcanic glass

Department of Environmental Sciences, Tokyo Gakugei University, 4-1-1 Nukui-kita-machi, Koganei-shi, Tokyo 184-8501, Japan

＊・＊＊ 東京学芸大学（184-8501 小金井市貫井北町4–1–1）

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