北海道北部の中新世火山活動の活動場 : K-Ar年代と主成分化学組成からの推定 [in Japanese] Tectonic setting of the Miocene volcanism in northern Hokkaido, Japan : Speculation from their K-Ar ages and major element chemistry [in Japanese]
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Northern Hokkaido is situated in the back arc region behind the arc-arc junction of the Kurile and Northeast Japan arc. The district is unique because it is located between two back-arc basins, Kurile and Japan basins. Tertiary small-scale shield-like volcanoes (flat lavas) and volcaniclastic formations are widely distributed in this district. 9 K-Ar ages and 148 major element chemistries for these volcanic rocks were determined to clarify their petrological features and tectonic evolution of this district.<br> The newly determined and previously reported K-Ar ages indicate that the vigorous volcanic activity took place during a short period from 14 to 9 Ma in the district. Total volume of the volcanic rocks is more than 650 km<sup>3</sup>. The volume of the volcanic rocks decreases from east (Sea of Okhotsk side) to west (Japan Sea side), suggesting that the volcanism took place mainly in eastern to central part. Most of the volcanic rocks are calc-alkaline and low-K to medium-K tholeiitic andesites, common in island arc setting. However, high TiO<sub>2</sub> andesites and icelandite-like dacites also occur along the Sea of Okhotsk. In addition, the volcanic rocks in northern Hokkaido show no systematic across-arc lateral variation in K<sub>2</sub>O content.<br> The spatial variation in the chemical composition suggests that the Tertiary volcanism in northern Hokkaido was not caused by subduction of the Pacific plate. We presume that the volcanism was due to the episodic uprising of heat source (asthenospheric mantle?) which should be related to the opening of the Kurile basin.
- Journal of Mineralogical and Petrological Sciences
Journal of Mineralogical and Petrological Sciences 90(4), 109-123, 1995-04-05
Japan Association of Mineralogical Sciences