北海道中央部馬追丘陵および夕張炭田地域の中新世前火山岩類のK-Ar年代,層序対比および化学組成

抄録

Andesites from the Yubari Coal field and the Umaoi Hill, east of the Ishikari Depression, Hokkaido, were dated by K-Ar method and their major and trace elements and isotope chemistries were determined. The rock spiecemens were collected from the andesitic dyke intruding into the late Eocene Poronai Formation in the Yubari Coal Field, and from the tuff breccia and lava of the Umaoiyama Agglomerate Unit in the Umaoi Hill.
The Umaoiyama Agglomerate Unit have been considered to be middle Miocene in age. All the obtained ages are, however, within the range from 19 to 20 Ma. Thus, the Unaoiyama Agglomerate Unit can be correlated with the lower Miocene strata in the Yubari Coal Field, namely the Takinoue Formation. It can also be correlated with the subsurface andesitic layer at the depth of 2, 465-2, 526m in the westward Ishikari Depression. On such stratigraphical grounds, it is suggested that the andesites from the Yubari Coal field and the Umaoi Hill may represent the eastern end of the volcanics at 19-20 Ma.
The andesites from the studied areas show little difference in major and trace elements, and isotope chemistry from those of the back-arc side of the NE Japan arc at around 20 Ma. The andesites have higher LILE and lower HFSE contents compared with the contemporaneous WPB-type basaltic rock. It suggests that they are not the differentiation products of the WPB-type basalt magma. They might be originated from the metasomatized mantle by the subduction component. Alternatively, crustal component might have participated in the generation of the andesitic magma.