Low-K2O volcanic rocks in the Ashigara Group and the zoning of K2O content of the post-Miocene volcanic rocks in the southern Fossa Magna region, central Japan

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  • 足柄層群に産出する低カリ質火山岩類と南部フォッサマグナ地域の火山岩のK2O分帯について
  • アシガラソウグン ニ サンシュツ スル テイカリシツ カザンガンルイ ト ナンブ フォッサマグナ チイキ ノ カザンガン ノ K2O ブンタイ ニ ツイテ

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Abstract

The Ashigara Group in the southern Fossa Magna region is divided into six formations; the Doyama, Dai, Seto, Tsuburano, Hatazawa and Shiozawa Formations, in ascending order, which are composed mainly of sedimentary rocks. All formations except the Hatazawa Formation intercalate basaltic to andesitic effusive rocks. The volcanism is considered to have been the most active at the time of deposition of the Tsuburano Formation relative to other ages. The Yamakita Group is deposited in a volcanic depression which was formed after the upheaval of the Ashigara Group. Dikes are divided into the older and the younger ones. The older dikes which is considered to have intruded before the deposotion of the Yamakita Group, consist of two-pyroxene basalt to andesite. The younger dikes which is considered to have intruded after the deposition of the Yamakita Group, consist of hornblende andesite. The Yaguradake quartz diorite complex intrudes into the Hatazawa Formation, relating to the volcanic activity which formed the volcanic depression. The effusive and the dike rocks of the Ashigara Group are identified as low-K2O series defined by Gill (1981), and the hypersthenic rock series defined by Kuno (1966). Most of both the effusive rocks and the older dike rocks are plotted in the "TH series" defined by Miyashiro (1974) because of an enrichment of plagioclase phenocrysts. A spatial zoning is generally recognized in K2O content of the post-Miocene volcanic rocks in the southern Fossa Magna region, Izu Peninsula and Izu islands. The K2O content increases from east to west parallel to a NNE-SSW trend with some exceptions. This zoning could be related to the north to south trending continuity of geological structure of the basement rocks which were formed after the "Green-Tuff Movement" in the southern Fossa Magna region.

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