南部フォッサ・マグナ地域の新生代火成作用と島弧会合問題

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The history of Cenozoic volcanism in the South Fossa Magna is divided into four phases the first (24-5 Ma), the second (5-2 Ma), the third (2-0.5 Ma) and the fourth (0.5 Ma to the present). The first phase: The first volcanism is most distinctive, and the products attain several thousand meters in total thickness. It is inferred from an occurrence of Lepidcyclina and Miogypsina that the first volcanism in the Tanzawa, Misaka, Koma and Fujigawa areas occurred under shallow sea. Volcanic rocks of the lower part of the Tanzawa, Aikawa, Nishi-yatsushiro and Kushigata-yama Groups are composed of tholeiitic basalt-basaltic andesite with tholeiitic dacite, that is, they are products of bimodal volcanism. In the upper part, calc-alkali andesitic rocks are locally occurred in the northern part of the Misaka and Koma areas. Based on whole-rock and minor chemical composition, the above-mentioned volcanic rocks are considered to have been derived from the island arc type-tholeiiticmagma. On the other hand, alkali rocks erupted in the Takakusa-yama and Fujigawa areas, western part of the South Fossa Magna. Especially the volcanism of the Takakusa-yama area may be inferred to have erupted under deep sea and to have formed a sea mount or oceanic island from an occurrence of benthic foraminiferal assemblage and petrographic characters of volcanic rocks ranging from trachybasalt to trachyandesite and trachyite. Alkali rocks of the Fujigawa area are alkali olivine basalt in different from those of the Takakusa-yama area. Volcanism of the Izu Peninsula occurred under shallow sea. Volcanic rocks of the Yugashima Group are mainly composed of calc-alkali andesite altered to propylite. The second phase: The second volcanism occurred, in smaller scale than the first volcanism, and in the Fujigawa, Ashigara and Izu Peninsula areas. Volcanic rocks of the Fujigawa Group and the lower part of the Ashigara Group are composed mainly of calc-alkali andesite, and partially of tholeiitic basalt and andesite. These Groups contain benthic foraminiferal assemblage showing deep sea environment. Volcanism of the Fujigawa area is thought to have occurred on terrace or scarp bounding the trough-like basin from sedimentological analysis of accompanying flysch and conglomerate. On the other hand, volcanism of the Izu Peninsula occurred under shallow sea. Volcanic rocks of the Shirahama Group and the Umegi Formation are composed mainly of calc-alkali dacite or andesite and partially with basalt. The third phase: The third volcanism is local, and produced the Iwabuchi volcanic rocks and the volcanic rocks of the upper part of the Ashigara Group and the Usami Volcano. The Iwabuchi volcanic rocks which erupted on fan deposits are composed of tholeiitic basalt and andesite and calc-alkali andesite. Volcanic rocks of the Usami Volcano are mainly composed of tholeiitic andesite. The fourth phase: So-called Quaternary Volcanoes such as the Hakone, Fuji, Taga, Yugawara, Amagi and Omuro ones are the products of the fourth volcanism. In regard to the arc junction problem in this region, an hypothesis that the Izu-Bonin arc collided with the Central Honshu has been proposed by MATSUDA (1978) and others. In this hypothesis, it is considered that the main collision took place in the Pliocene as a result of northward movement of the Izu Block. The author intend to briefly discuss on this problem based on the Cenozoic volcanism of the South Fossa Magna. The Tanzawa-Misaka-Koma area joins the Izu Peninsula and both, as a whole form the northern part of the IZU-BOnin arc which is divided from the southern part by the Soofu Tectonic Line. The northern part has a same scale of extension as the Northeast Honshu arc, and shows the NE-SW or NNE-SSW trend of volcanic belts. It is unnatural that the Izu Peninsula is separated from the main part of the South Fossa Magna. On the present relation of position of the Honshu arc and the northern part of the Izu-Bonin arc, the latter is thought to h
日本地質学会の論文
1984-11-30

南部フォッサ・マグナ地域の新生代火成作用と島弧会合問題

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