ジルコンのフィッション・トラック年代と特徴からみた室生火砕流堆積物と熊野酸性岩類の同時性と類似性(<特集>紀伊半島における中新世火成作用とテクトニクス)  [in Japanese] Simultaneity and similarity of the Muro Pyroclastic Flow Deposit and the Kumano Acidic Rocks in Kii Peninsula, southwest Japan, based on fission track ages and morphological characteristics of zircon(<Special Issue>Miocene igneous activity and tectonics in the Kii Peninsula)  [in Japanese]

紀伊半島北部の室生火砕流堆積物,石仏凝灰岩,古寺凝灰岩および同半島中部〜南東部の大峯花崗岩類と熊野酸性岩類から得た52試料のジルコンを用いたフィッション・トラック(FT)年代測定を行った.これらと既報の玉手山凝灰岩,中奥火砕岩岩脈,潮岬火成複合岩類のジルコンFT年代はすべて15Maに集中した.また大部分の岩体では赤色と白色のジルコンが混在する.室生火砕流堆積物,石仏凝灰岩,玉手山凝灰岩と熊野酸性岩のジルコン試料を調べた結果,赤色と白色のモード比は2:3であった.また,赤色と白色ではそれぞれ{100},{110}面が卓越するという類似性がある.FT年代の同時性とジルコンの特徴から,従来給源の不明であった紀伊半島北部に点在する室生火砕流堆積物と周辺の凝灰岩が,互いに同一の大規模火砕流堆積物として対比され,その起源は熊野酸性岩類と推定される.

Fission track dating was carried out for 52 samples from the Muro Pyroclastic Flow Deposit, the Sekibutsu Tuff and the Furudera Tuff in the northern region of the Kii Peninsula, and from fine-grained granitic bodies of the Omine Granitic Rocks (the Dorogawa, Shirakura, Kose, Asahi, Tenguyama, and Shiratani bodies) and various rock facies of the Kumano Acidic Rocks (the Konogi Rhyolite, the Owase-Shirahama Pyroclastic Rocks, the Kumano Granite Porphyry, and the Kozagawa Dike) in the central to southeastern region of the Kii Peninsula. The resulting ages of all these samples and previously reported ages of the Tamateyama Tuff, the Nakaoku Tuffite Dike and the Shionomisaki Igneous Complex in the peninsula were clustering at 15 Ma. Most of the above igneous bodies contain a mixture of reddish and colorless zircons. Characteristic of the mixed zircons from the Muro Pyroclastic Flow Deposit, Sekibutsu Tuff, Tamateyama Tuff and the north and south units of the Kumano Granite Porphyry is similar in terms of a ratio of reddish and colorless zircons being 2: 3 and dominant crystal planes {100} for reddish and {110} for colorless. Based on the simultaneity and similarity of the mixed zircons, it is strongly suggested that the Muro Pyroclastic Flow Deposit and the adjacent tuffs in the northern Kii Peninsula are correlated with each other as a large-scale pyroclastic flow deposit at 15 Ma, and that its possible source is the Kumano Acidic Rocks in the southeastern Kii Peninsula.