山形県田澤地域の新第三紀火山岩

抄録

Tazawa district here mentioned is situated at the central part of the Dewa Mountains between the Shonai and Mogami oil fields, Yamagata Prefecture. According to the present writer's study from 1950 to 1955 the district is chracterized by the frequent volcanic activitis which occured in the Neogene and are represented by flows, pyroclastics, dykes and sills.
The Neogene Tertiary of the district is divided into the Tazawa, Tate yama, Kusanagi and Kitamata formations from lower to upper.
The sequence of the volcanic activities in the district is traced as follows.
1st. stage. Submarine eruption and explosion of tholeiitic basalt(SiO₂=47, 28%-52, 48%) in the Tazawa formation. A part of the basalt changes to spilitic character (NaO₂=3, 63%) by albitization. The great amount of this basaltic reeks was followed by the formation of geosynclinal character.
2nd. stage. Center eruption of Taizosan andesite, which is augite andesite (SiO₂=53, 72%), two pyroxene andesite (SiO₂=56, 12%) and hornblende andesite (SiO₂=58, 68%). This rocks has petrographical chracter which is very coarse grain and rich in plagioclase phenocryst. However, at last stage of this activity, it is very fine grain type.
3rd. stage. Intrusion of olivin basalt (SiO₂=47, 28%) represented by dykes and sills. Parallel dyke swarm of basalt is restricted in the Tazawa and Tateyama formatins and sills occur only in the Kusanagi formation. At the later stage the rocks changes to doleritic chracter.
4th, stage. Subordinate acidic tuff explosion is broadly traced in this area. This tuff is liparitic pumice, and thinly bedded in the boundary of the Kusanagi and Kitamata formations.