Petrology of the volcanic rocks from the Torikabuto Volcano, North Fossa Magna, central Japan−Differentiation process of calc-alkaline volcanic rocks−

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  • 北部フォッサマグナ，鳥甲火山噴出物の岩石学－カルクアルカリ系列火山岩類の形成過程－
  • ホクブ フォッサ マグナ トリカブト カザン フンシュツブツ ノ ガンセキガク カルクアルカリ ケイレツ カザンガンルイ ノ ケイセイ カテイ
  • Petrography of volcanic rocks from the Torikabuto Volcano, North Foss Magna, central Japan – differentiation process of calc-alkaline volcanic rocks –

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Abstract

The Torikabuto Volcano is located at a northeastern end of the “Central Upheaval Zone” in the North Fossa-magna region, central Japan. Most of the volcanic rocks of the Torikabuto Volcano belong to the calc-alkaline rock series, and are composed of basaltic andesite, dacite lavas, and volcaniclastics.</br>Based on their stratigraphic data, volcanic activities in the Torikabuto Volcano are divided into four stages; Maekura Stage, Stage 1, Stage 2, and Stage 3. The rocks at the Maekura Stage are pyroxene-andesite pyroclastic and lava flows. The rocks at the Stage 1 are characterized by large amounts of pyroclastic flows, which are composed of the rocks ranging from basaltic andesite to dacite. The lava flows ranging from basaltic andesite to dacite are widely developed at Stage 2. A small quantity of dacitic pyroclastic flows was formed at the Stage 3. The volcanic rocks at the Stages 1 and 2 contain gabbroic inclusions and in some places they exhibit the layered structure. The K-Ar age dating data indicate that the age of the rocks at the Maekura Stage is ~3.2Ma and the age of those at the Stage 2 is between 0.9-0.8 Ma (Kaneko et al., 1989).<br>Base on the examination of the petrographical, mineralogical, whole-rock geochemical, and Sr-Nd isotopic characteristic of the volcanic rocks from the Torikabuto Volcano, we draws following conclusions.<br>1) Magma mixing is recognized from field observation, petrology of the volcanic rocks and mineralogy of plagioclase after the Stage 1.<br>2) However, fractional crystallization processes of the calc-alkaline magma mainly formed the calc-alkaline volcanic rocks from the Torikabuto Volcano. The fractionated minerals are plagioclase, clinopyroxene, orthopyroxene, Fe-Ti oxides and hornblende. This is supported by the results from mass-balance calculation.<br>3) The Sr-Nd isotopic ratios of volcanic rocks and inclusions from the Torikabuto Volcano, have the same values. This indicates that the volcanic rocks and inclusions formed from the same magma.<br>4) Accordingly, the magma mixing process that is recognized from the volcanic rocks of the Torikabuto Volcano is the “internal mixing” and the inclusions are cumulated rocks.

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