Correlation between pumiceous tuff beds of the Morozaki Group and the Mizunami Group

Abstract

中部地方の下部中新統である，師崎層群日間賀層および山海層下部と瑞浪層群明世層狭間部層における軽石質凝灰岩の対比について検討した．日間賀層および山海層下部の試料からはそれぞれ，狭間部層の年代と調和的な 17.87 ± 0.75 Maおよび 17.36 ± 0.40 Maのジルコン U–Pb 年代が得られた．また日間賀層の古地磁気は山海層下部や狭間部層と同じく逆極性であった．つまりこれら 3 層はいずれも古地磁気年代のクロン C5Drに相当すると結論できる．EPMA分析から，山海層下部と狭間部層の試料に含まれる火山ガラスは組成変化図において一連のトレンドを示し，3 層に含まれる斜長石組成は類似していた．つまりこれら 3 層の火山砕屑物は同一のマグマ供給系を起源とすると考えられる．以上の結果と古水深から，本研究対象の師崎層群の軽石質凝灰岩は，狭間部層の火山砕屑物が海底を流下して堆積したと考えられる． / Early Miocene sedimentary sequences in the Chubu region of central Japan contain numerous tuff beds, and comparisons and correlations of these beds between sedimentary basins are expected to lead to a better understanding of the stratigraphic architecture of the sequences. In this study, we discuss the correlation of pumiceous tuff beds between the Himaka and lower Yamami Formations of the Morozaki Group in Aichi Prefecture and the Hazama Member of the Akeyo Formation of the Mizunami Group in Gifu Prefecture. We obtained zircon U–Pb ages of 17.87±0.75 Ma and 17.36±0.40 Ma for pumiceous tuff beds from the Himaka and lower Yamami Formations, respectively, and these ages are consistent with the published U–Pb age for the Hazama Member. Paleomagnetic analyses reveal reverse polarity magnetizations in the Himaka and lower Yamami Formations and the Hazama Member. These results suggest that these three units correspond to Chron C5Dr of the paleomagnetic chronology. Chemical analyses of volcanic glasses from pumiceous tuffs in the lower Yamami Formation and the Hazama Member display a single trend in chemical variation diagrams. Plagioclase compositions of pumiceous tuffs in the three units are also consistent. These chemical characteristics indicate that the volcanic glasses and plagioclase of the three units were generated from the same magma plumbing system. The volcaniclastic products of the Morozaki Group examined in this study are considered to have been supplied from the Hazama Member.