岩石学的特徴とU-Pb年代に基づく函館市街地下の軽石堆積物の給源の検討

抄録

We detected thick pumice layers by observations of cuttings taken from a 1,000 m deep hot spring well at the central Hakodate City, southwestern Hokkaido. In this study, we perform petrological investigations and U-Pb dating for these pumice layers and compare them with the eruptive products from the Quaternary volcanoes at the southern Oshima Peninsula. The pumice layers can be divided into several porphyritic pumice (depth 570-420 m, 310-283 m and 261-66 m) and aphyric pumice (depth 950-660 m, 420-380 m, 343-310 m and 283-261 m) layers. The porphyritic pumice is rich in plagioclase and hornblende phenocrysts and occasionally contains minor amounts of orthopyroxene, clinopyroxene, biotite, quartz and cummingtonite phenocrysts. The U-Pb ages of the lowermost and uppermost parts of the porphyritic pumice layers are 0.93±0.04 Ma and 0.81±0.06 Ma, respectively. Although lithic-rich part is partly observed, these pumice layers mostly consist of pumice clasts. In addition, pumices show variously welded textures at several depths. These features suggest that the thick pumice layers resulted from repeated pyroclastic flows and that reworked deposits are limited. This indicates that the source of the pumice layers should be located adjacent to Hakodate City, whereas the petrological features and U-Pb age of the pumice layers are inconsistent with the known eruptive products of the Quaternary volcanoes at the southern Oshima Peninsula. Therefore, our results indicate that unknown pumice eruptions repeatedly occurred around this area during 0.9-0.8 Ma. Although any pumice eruptions have not been detected in Hakodateyama volcano (1.5 km SW of the hot spring well), the eruption ages and mineral assemblage of the porphyritic pumice are similar to the lavas of the volcano, implying that the source of the pumices was Hakodateyama volcano or its adjacent area.