Volcanic Activity from the Miocene to Pliocene Age in the western Part of Gunma Prefecture, Central Japan

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  • 群馬県西部の中新世から鮮新世にかけての火山活動
  • グンマケン セイブ ノ チュウシンセイ カラ センシンセイ ニ カケテ ノ カ
  • (特集「新生代末期の火成活動」(第29回総会シンポジウム))
  • (Symposium 「Igneous Activities in Late Cenozoic Era in Japan」(Memoirs of the 29th General Meeting of the Association for the Geological Collaboration in Japan))

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Abstract

The voluminous volcanic products of the late Miocene to Pliocene time are widely distributed in the western part of Gunma Prefecture. They are mainly composed of pyroclastic rocks, lava flows and intrusive rocks that range in composition from basalt to dacite. The andesitic lava flows and pyroclastics are most abundant among these volcanic products, which petrographically belong to the hypersthenic rock series. A little amount of siliceous basalts and dacites associated by andesitic rocks also have petrographical character of the hypersthenic rock series. The early stage of the igneous activity of the late Miocene time in this district is characterised by the eruption of a large amount of andesitic lava flows and pyroclastic rocks from many volcanic craters. In the middle stage, many volcanic rocks were intruded in the area around the volcanic craters and they occur in the form of dyke, sheet, volcanic neck and massive body. A remarkable feature in the final stage magmatism is the intrusive activity of the holocrystalline rocks such as diorite, quartzdiorite and porphyrite, which form the so-called volcanoplutonic complexes in some localities. On the other hand, the volcanic activity during the Pliocene time is restricted to the eruption of andesitic lava flows and pyroclastic rocks from the volcanic craters located in the western part of this district. It may be concluded, therefore, that in this district the calc-alkaline magmatism was active during the late Miocene to Pliocene while mode of the igneous activity in the late Miocene was slightly different from that in the Pliocene Age.

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