K-Ar ages of Tertiary intrusive rocks and spatial and temporal transition of magmatism in Central Hokkaido, northern Japanese Islands

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  • 北海道中央部の第三紀迸入岩類のK-Ar年代と火成活動の時空変遷
  • 北海道中央部の第3紀迸入岩類のK-Ar年代と火成活動の時空変遷〔含 討論〕
  • ホッカイドウ チュウオウブ ノ ダイ3キ ヘイニュウガンルイ ノ K Arネン

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Abstract

Based on newly obtained K-Ar whole rock ages of intrusive rocks and previous geological data, the spatial and temporal transition of Tertiary magmatism in Central Hokkaido is summarized as follows: Hidaka magmatism (HM) from 43 (or 53) Ma to approximately 17 Ma; 'Urakawa-Samani'magmatism (USM) at approximately 16-18 Ma; Pipairo-Toyokoro magmatism (PTM) at approximately 15 Ma or from 15 to 12 Ma; andesitic magmatism in northern Central Hokkaido (AMNCH) from 14 to 10 Ma; and Kamishiyubetsu magmatism (KM) at approximately 10 Ma. The following four points are discussed, in relation to the magmatic history mentioned above. (1) Plutonic bodies, emplaced at various levels in the Hidaka Magmatic Zone (HMZ) during the HM, are observed in the Pankenushi-Memuro area with the emplacement level of plutons becoming deeper towards the west. The Nisshoutouge granitic pluton, the shallowest in the area, shows concordant Rb-Sr and K-Ar ages of approximately 17 Ma, it has therefore been concluded that this age represents the time of emplacement. The Pankenushi intrusion, the deepest gabbroic body in the area, exhibits a similar K-Ar age. We note the possibility that deeper level plutonic rocks were rejuvenated by the emplacement of younger granitic rocks at approximately 17 Ma. (2) It is established that the age of the USM in the west of the HMZ partly overlaps the HM, although the tectonic relationship between these magmatisms remains unsolved. (3) The PTM, alkalic and anorogenic magmatism in southern Central Hokkaido, temporaly overlaps the AMNCH. However, the analyzed material indicating an age of 12 Ma is partly altered, therefore it is likely that the PTM is restricted in age to approximately 15 Ma, indicating no overlap between magmatisms. (4) The AMNCH and the KM are gradational in space and time, and the KM probably occurred as the AMNCH migrated southward.

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