Development of the Middle Miocene Otoge volcanic complex, Shitara district, central Japan

Abstract

Igneous activity of the Middle Miocene Otoge volcanic complex, Shitara district, central Japan, is divided into the cauldron-forming stage with emplacement of the Otoge pyroclastic rocks, and the post-cauldron stage with emplacement of dike and sheet swarm. Crosscutting relationship shows the igneous sequence in the post-cauldron stage : the Kamasawa brecciated intrusions and Kamoyamagawa trachyte dike were formed at the beginning of the post-cauldron stage and the formation of the Otoge cone sheets followed. The Shitara central dike swarm was formed in the final stage. Intrusion structure of these intrusions changed from irregular-shaped peperite to coherent tabular dike reflecting the consolidation of the host Otoge pyroclastic rocks. Most part of the the Otoge volcanic complex consists of alkaline rocks and their compositional change during the post-cauldron stage suggests the repletion of less-fractionated magma into a reservoir beneath the cauldron and progress of crystal fractionation within the reservoir. Input of high-temperature mafic magma to reservoir caused melting of its wall rocks and formed calc-alkaline dacite of the Otoge stocks.