北海道第四紀火山の分布と主成分化学組成の広域変化

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Hokkaido island is situated at the junction of northeastern Japan and Kuril arcs. Recent K-Ar age dating of Cenozoic volcanics in the island revealed that the distribution of Quaternary volcanoes (<1.7Ma) is remarkably different from previously accepted one. On the basis of the newly established distribution of the volcanoes, spatial variations in SiO_2-normalized major element values (SiO_2=60%) of Quaternary andesitic rocks (SiO_2=56-64wt%) are investigated. The Na_2O, CaO and P_2O_5 values vary systematically in the across-arc direction irrespective of the tectonic setting and/or the geological structure. On the contrary, the variations in FeO, Al_2O_3 and K_2O values seem to be largely correlated with the tectonic setting and the Bouguer anomaly. The distribution of the volcanoes and the mode of spatial variations in major element chemistry suggest the presence of three Quatemary volcanic fields, the southwestern Hokkaido (SWH), Taisetsu-Tokachi-Shikaribetsu (TTS) and Akan-Shiretoko (AKS) regions. Modes of the spatial variations in normalized major element values in SWH and AKS regions resemble each other and are nearly the same as those of northern Honshu arc; chemical zonation of volcanoes is sub-parallel to the trench axis in both regions. On the contrary, the spatial variations in FeO and K_2O values in TTS region are different from those in SWH and AKS regions. The mode of the variations in FeO and K_2O values is thus discontinuous in Hokkaido island. The values of Bouguer anomaly at TTS region (-40 to 0 mgal) are much lower than those at other two regions (+40 to +100 mgal). Because crustal thickness under the regions is nearly identical (25-34km), the regional variation in the values of Bouguer anomaly suggests that crustal materials of TTS region are less dense than those of SWH and AKS regions. These different crustal materials, which have been produced under the different tectonic settings, are believed to have resulted in mode of spatial variations in major element chemistry of Quaternary andesites.
特定非営利活動法人日本火山学会の論文
1995-03-10

北海道第四紀火山の分布と主成分化学組成の広域変化

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