K-Ar年代測定に基づく両白山地の鮮新−更新世火山活動の時空分布

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(3) . . . . ,**0 1 . !" ,**1 + +2 #$. Temporal-spatial variations of Plio-Pleistocene volcanic activity in the Ryohaku Mountains, central Japan : evidences from K-Ar ages. , ῍ ῍, Atsushi TANASE῍, Teruki OIKAWA῍῍, Atusi NINOMIYA῍῍. Shintaro HAYASHI῍῍῍῍ and Koji UMEDA῍῍῍῍῍ Temporal and spatial variations in Plio-Pleistocene volcanism in the Ryohaku Mountains, central Japan, have been investigated by newly obtained K-Ar ages on -2 groundmass samples separated from volcanic rocks. The obtained groundmass ages show small variations and errors and are relatively younger than K-Ar ages from bulk rocks reported by previous studies indicating lesser e#ect of excess argon from phenocrysts. Based on the newly obtained ages and volcano stratigraphic data, we identify activity periods of +.*ῌ*.2 Ma for Kyogatake Volcano, *.2ῌ*.1 Ma for Hoonji Volcano, +.*ῌ*.2 Ma for Toritateyama Volcano, *.2 Ma for Akausagiyama Volcano, -.+ῌ ,.3 Ma and ,./ Ma for Gankyoji-Sannomine Volcano, +./ Ma for Choshigamine Volcano, *.- Ma for Bishamon Volcano, and +., to +.+Ma for Eboshi-Washigatake Volcano. The volcanoes in the Ryohaku Mountains form two volcanic rows of the Kuzuryu and Hakusan Volcanic Chains which have ESE-WNW and N-S alignments, respectively. Early volcanic activity occurred intermittently from -.0 Ma to +./ Ma, and the volcanoes near the junction of the two volcanic chains were active. The volcanic activity in the ESE-WNW trending Kuzuryu Volcanic Chain was restricted in the age range from +., Ma to *.1 Ma with migration from Eboshi-Washigatake Volcano (ESE end) to Hoonji Volcano (WNW end). The N-S trending Hakusan Volcanic Chain was active from *.. Ma to the present. Key words : Ryohaku Mountains, Kuzuryu Volcaic Chain, Hakusan Volcanic Chain, K-Ar age +ῌ ῐ ῎ ῑ ῏. U67>?K'VWXYMZ[9UA7\]^_? `. %&'()*+,-./0'1232456789:. ab" Kimura et al., ,**/" Shimizu and Itaya, +33- ; cd". ;<=7>?@A" B'CDEFGHIJK4L?;M. +33/$e fgh'ij4," 123klmn-./0. ,-./0'1232NO'5P4QRQST4KK. 'o4pqrstu-./0N1232" vwxNij. y++*ῌ***2 z{d|z}~ ,ῌ3ῌ1. y*+*ῌ2/*, ¬¬®¯°£ +ῌ+ t t0$ ¬±¯²³´µ¯g Sumiko Consultants Co. LTD., ,ῌ3ῌ1, Ikenohata, TaitoFaculty of Education and Human Studies, Akita Uniku, Tokyo ++*-***2, Japan. versity, +ῌ+, Tegata-Gakuencho, Akita-shi, Akita *+*ῌ. y-*/ῌ2/01 bz +ῌ+ῌ+ f 1 2/*,, Japan. y/*3ῌ/+*, ¡¢£¤¥ 3/3ῌ-+ $ig Institute of Geology and Geoinformation, Geological $h¦§¨©Sª« Survey of Japan, AIST, central 1, Higashi +ῌ+ῌ+, Japan Atomic Energy Agency, 3/3ῌ-+, Jorinji, Izumi, Tsukuba, Ibaraki -*/ῌ2/01, Japan Toki-shi, Gifu /*3ῌ/+*,, Japan.. : y/*3ῌ/+*, ¡¢£¤¥ 3/3ῌ-+ $h¦§¨©Sª« Corresponding author : Atsushi Tanase Present address : Japan Atomic Energy Agency, 3/3ῌ e-mail : [email protected] -+, Jorinji, Izumi, Toki-shi, Gifu /*3ῌ/+*,, Japan..

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