Transition of Neogene arc volcanism in Central-Western Hokkaido, viewed from K-Ar ages, style of volcanic activity, and bulk rock chemistry.

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  • 北海道中央部～西部の新第三紀火成活動の変遷　　Ｋ‐Ａｒ年代，火山活動様式および全号化学組成から見た東北日本弧北端の島弧火成活動の変遷
  • ホッカイドウ チュウオウブ セイブ ノ シンダイ3キ カセイカツドウ ノ ヘンセン K Ar ネンダイ カザン カツドウ ヨウシキ オヨビ ゼン ガン カガク ソセイ カラ ミタ トウホク ニホン コ ホクタン ノ トウコ カセイカツドウ ノ ヘンセン

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Abstract

Spatial and temporal variations in late Cenozoic volcanism of southwestern Hokkaido at the northern end of NE-Japan arc have been clarified by 261 K-Ar and 76 FT ages including 49 newly determined K-Ar ages, volcanic stratigraphy, physical volcanology and whole-rock geochemistry. Arc volcanism characterized by rocks with low-Ti and Nb, and by across-arc increase in K2O content in these rocks has continued at least since 12 Ma. Based on volcanic stratigraphy, physical volcanology and whole-rock geochemistry, volcanism after 12 Ma can be subdivided into 4 stages, 12-5, 5-1.7, and 1.7-0 Ma. The volcanism from 12 Ma to 5 Ma extended northward widely compared with distribution of Quaternary arc volcanism (1.7-0 Ma). This suggests that the arc trench junction between Kuril and NE-Japan arc's trenches was located about 100 km northward from the present position. Since around 5 Ma until 1.7 Ma, different type of volcanism under local extension field, characterized by a group of monogenetic volcanoes of alkali basalt and shield volcanoes of calc-alkaline andesite, had occurred at northern end of the volcanic region (Takikawa-Mashike region). During and after this volcanism, the northern edge of arc volcanism in the area has migrated southward. This suggests that the trench junction has migrated about 100 km southward since ∼5 Ma. The Quaternary arc volcanism (1.7-0 Ma) has been restricted at the southern part of the region. The volcanism since 12 Ma might be influenced by oblique subduction of Pacific plate beneath Kuril arc, resulting in the formation of local back arc basin at the junction and to southward migration of the trench junction.

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