- Temporal-spatial variations of Plio-Pleistocene volcanic activity in the Ryohaku Mountains, central Japan : evidences from K-Ar ages.
- K Ar ネンダイ ソクテイ ニ モトズク リョウハク サンチ ノ センシン コウシンセイ カザン カツドウ ノ ジクウ ブンプ
Temporal and spatial variations in Plio-Pleistocene volcanism in the Ryohaku Mountains, central Japan, have been investigated by newly obtained K-Ar ages on 38 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 effect of excess argon from phenocrysts. Based on the newly obtained ages and volcano stratigraphic data, we identify activity periods of 1.0-0.8 Ma for Kyogatake Volcano, 0.8-0.7 Ma for Hoonji Volcano, 1.0-0.8 Ma for Toritateyama Volcano, 0.8 Ma for Akausagiyama Volcano, 3.1-2.9 Ma and 2.5 Ma for Gankyoji-Sannomine Volcano, 1.5 Ma for Choshigamine Volcano, 0.3 Ma for Bishamon Volcano, and 1.2 to 1.1Ma 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 3.6 Ma to 1.5 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 1.2 Ma to 0.7 Ma with migration from Eboshi-Washigatake Volcano (ESE end) to Hoonji Volcano (WNW end). The N-S trending Hakusan Volcanic Chain was active from 0.4 Ma to the present.
火山 52 (1), 39-61, 2007