紀伊半島北部の室生火砕流堆積物と周辺に分布する凝灰岩の対比およびそれらの給源 : 軽鉱物屈折率を用いたモード分析によるアプローチ(<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] Correlation of the Muro Pyroclastic Flow Deposit and the adjacent tuffs in northern Kii Peninsula, southwest Japan, and their source : an approach from mode analysis using refractive index of light minerals(<Special Issue>Miocene igneous activity and tectonics in the Kii Peninsula)  [in Japanese]

紀伊半島北部に分布する室生火砕流堆積物とその周辺の凝灰岩(石仏凝灰岩,古寺凝灰岩,玉手山凝灰岩)および外帯中新世珪長質岩類について,屈折率を用いた軽鉱物組合せモード分析を行った.その結果,紀伊半島北部の中期中新世珪長質火砕流堆積物の斜長石系列は,すべてオリゴクレース〜ラブラドライトで特徴付けられることから,これら4者は対比された.加えて室生火砕流堆積物は外帯に分布する熊野酸性岩類の流紋岩質凝灰岩の一部と対比できた.これらのことから,室生火砕流堆積物と石仏凝灰岩,古寺凝灰岩,玉手山凝灰岩は15Maの熊野地域のカルデラを給源とする同一の大規模火砕流堆積物であると推定される.また熊野酸性岩類の中のアルバイトで特徴付けられる流紋岩質凝灰岩は,同じ軽鉱物組合せをもつ中奥弧状岩脈を給源とする可能性がある.

Mode analyses using refractive index of light minerals were performed for samples from the Muro Pyroclastic Flow Deposit (MPFD) and the adjacent felsic tuffs (the Sekibutsu Tuff, the Furudera Tuff and the Tamateyama Tuff) in northern Kii Peninsula, and Miocene felsic rocks in southern part of the peninsula. We have confirmed that plagioclase phenocryst of the MPFD and the adjacent three felsic deposits are principally composed of oligoclase to labradorite, and that these four Miocene deposits are correlative each other. The MPFD is also correlative to a certain horizon of the felsic pyroclastic facies of the Kumano Acidic Rocks (KAR) on the southern end of the peninsula. We thus conclude that the MPFD and correlative tephras are an identical large pyroclastic flow deposit sourced from ancient (ca. 15 Ma) caldera volcanoes in the Kumano area. Some parts of the felsic pyroclastic flow deposits of the KAR contain plagioclase phenocryst of principally albite-composition, and could have derived from a different caldera, now represented by an arcuate pyroclastic intrusion (Nakaoku tuffite dike swarm) in the middle part of the peninsula.