中期中新世テフラの広域対比 : 房総半島Kn-1凝灰岩と紀伊半島室生火砕流堆積物(<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] Widespread correlation of Middle Miocene tephras in Japan : a possible link between the Kn-1 tuff (Boso Peninsula) and the Muro Pyroclastic Flow Deposit (Kii Peninsula)(<Special Issue>Miocene igneous activity and tectonics in the Kii Peninsula)  [in Japanese]

中期中新世前期(15Ma)テフラの広域対比を提案する.記載岩石学的および放射年代学的分析から,房総半島のKn-1凝灰岩は約400km離れた紀伊半島の火砕流堆積物と対比される.Kn-1は木の根層中に挟まる約16m厚の珪長質凝灰岩で,紀伊半島北部の室生火砕流堆積物とそれに対比される火砕流堆積物(玉手山凝灰岩,石仏凝灰岩)と共通した造岩鉱物および火山ガラスをもつ.Kn-1のジルコンのフィッション・トラック年代は約15Maで,紀伊半島の火砕流堆積物の年代と区別できない.玉手山-石仏-室生火砕流堆積物は熊野酸性岩類や大峯花崗岩類を含む紀伊半島外帯火成活動で起きたカルデラ噴火に由来することから,Kn-1凝灰岩も紀伊半島外帯(おそらく熊野)起源の広域テフラと考えられる.

Widespread correlation of early Middle Miocene (15 Ma) tephras is proposed. Mineralogical and geochronological analyses suggest that a felsic tephra on the Boso Peninsula (central Japan) is correlative to an ash-flow deposit on the Kii Peninsula (southwestern Japan), some 400 km distant. The felsic tephra, the Kn-1 tuff in the sedimentary Kinone Formation, is approximately 16 m thick and contains minerals and volcanic glasses that show close affinities with those from the Muro Pyroclastic Flow Deposit and correlative ash-flow units (the Tamateyama and Sekibutsu Tuffs) on the northern Kii Peninsula. Our zircon fission track ages for the Kn-1 tuff are about 15 Ma and indistinguishable from those for the ash-flow deposits. Because the Tamateyama-Sekibutsu-Muro ash-flow deposit originates from a caldera-forming pyroclastic eruption in the Outer Zone (Pacific-side) igneous province including the Kumano and Omine Granitic Rocks, the Kn-1 tuff also comes from that eruption in the Outer Zone (probably at Kumano) of the Kii Peninsula.