Petrographic properties of the acidic volcaniclastic deposits in the Omine Belt, western margin of the northern Fossa Magna region, central Japan

摘要

The Omine Belt (Kosaka et al. 1979) is lying from Otari to Matsumoto with 65km in length and 5～7km in width along the Itoigawa-Shizuoka Tectonic Line, which is the western manginal fault of the Fossa Magna region. The Omine Belt is burried up by a large amount of non-marine normal clastic and volcaniclastic sediments which ranges in age from Pliocene to early Pleistocene. Plio-Pleistocene strata in the Omine Belt are divided into the Otari and Omine Formations in ascending order. Volcaniclastic sediments of the Otari Formation are andesitic. On the other hand, most of volcaniclastic sediments of the Omine Formation are acidic. In this paper, acidic volcaniclastic sediments are collected and analised in petrographically. 9 samples of the Otari Formation and 64 samples of the Omine Formation are analised. The results are as follows 1) Acidic volcaniclastic sediments in the Omine Belt are made up largely by pumiceous vitric tuff and welded tuff in lithologically. 2) Most of welded tuff beds are rhyolitic petrologically in which biotite occur as main heavy minerals. 3) The Takagariyama tuff Ⅰ • Ⅱ, which are characterized by a dominance of orthorhombic pyroxene as heavy minerals, are correlative to the Nyukawa Pyroclastic Flow Deposits and Ebisutoge Pyroclastic Deposits in the Takayama Basin, west of Japan Alpus.