@article{oai:nagoya.repo.nii.ac.jp:00026264, author = {佐藤, 興平 and 南, 雅代 and 鈴木, 和博 and 柴田, 賢 and Sato, Kohei and Minami, Masayo and Suzuki, Kazuhiro and Shibata, Ken}, journal = {名古屋大学年代測定研究}, month = {Mar}, note = {A large block of andesitic agglutinate at Iwagami in Maebashi was designated as a natural monument in 1938 in appreciation of its massive size and isolated presence in the surrounding flat area. The source of this block has remained an enigma for many years. The block probably derived from Akagi or Asama volcanoes by mudflow. While these two possible sources are hard to discriminate in terms of major element concentrations, our previous report indicated that strontium (Sr) isotope analysis may provide a convincing clue to the origin of the block, as the volcanic rocks of the two volcanoes show remarkably different 87Sr/86Sr ratios. A preliminary Sr isotope analysis of a specimen collected from the block suggests that it was derived from the Asama volcano, not from the Akagi and Haruna volcanoes located near Maebashi. The results of a major element analysis are also consistent with the data for volcanic rocks from the Asama volcano. Additional data on the age and chemistry of volcanic rocks from possible sources will be required for a more definitive conclusion, as older volcanic rocks (ca.6−2Ma) beneath the Onoko and Komochi volcanoes show isotopically similar values to those of the Asama volcano. An investigation of the genesis of the mudflow and transporting mechanism responsible for the movement of the large block over so great a distance will also provide important evidence on the origin of this unique natural monument., 前橋市内の岩神神社には,安山岩質な溶結火砕岩の巨大岩塊があり,1938年に「岩神の飛石」として国の天然記念物に指定された.その起源については指定当初から諸説があり,現在も完全には解明されていないが,赤城火山もしくは浅間火山から泥流で運ばれてきた可能性が考えられている.火山岩の主成分組成でこれら2つの火山を識別することは難しいので,Sr同位体比(87Sr/86Sr)による起源判定とし寸手法を提案した.その後,この岩塊から採取した1試料を入手し87Sr/86Sr比を測定したところ,供給源は前橋に近い赤城火山や榛名火山ではなく,西方に遠く離れた浅間火山らしいことが判明した.この試料の全岩化学分析の結果も,浅間火山の岩石の化学組成と調和的である.ただし,「岩神の飛石」と同様の87Sr/86Sr比は小野子・子持両火山などの下位に産するやや古い火山岩(ca.6-2Ma)についても得られているので,最終的に起源を確定するためには,年代や化学組成のデータを補充して供給源の候補を絞り込むとともに,泥流発生の原因と巨大岩塊運搬のメカニズムについて解明する必要がある.}, pages = {44--50}, title = {火山体崩壊に起因する火山災害軽減のためのパイロット研究 : Sr同位体比から見えてきた巨石の天然記念物の起源}, volume = {1}, year = {2017} }