軽石に富む珪長質ハイアロクラスタイトの形成過程

福島県只見地域を例として

抄録

The Managawa lava occurs within the Middle Miocene Oshio Formation, a sequence of rhyolitic submarine volcanic rocks distributed in the southern part of the Northeast Japan. The Managawa lava consists of lava domes and hyaloclastite. The domes, up to 300m in diameter, consist from the center outwards of a central zone of massive lithic rhyolite, a zone of flowed lithic rhyolite, a glassy rind, and a pumiceous rind. The hyaloclastite comprises chiefly of angular fragments of pumiceous and glassy rhyolite, and contains lithic fragments. The glassy and pumiceous rinds of several domes shatter gradually to the surrounding hyaloclastite.
Various lines of evidence indicate that the pumiceous clasts in the hyaloclastite were formed by quench-fragmentation of pumiceous margin of the domes. They include the polyhedral morphology of the clasts, the pumiceous in situ breccia or fractured lava locally present in the hyaloclastite, the absence of vitroclastic matrix, the similarity between the clasts and the pumiceous margin, the low vesicularity of the clasts, and the gradational transition from the pumiceous margin of the Namesawa dome to the surrounding hyaloclastite containing abundant large pumiceous blocks.
Early eruptions of volatile-rich vesicular magma changed into those of less-volatile nonvesicular magma. The lavas formed extrusive domes, and shallow intrusive domes emplaced into the wet hyaloclastite pile resulted from the disruption of other previous lavas. The top of the extrusive domes was auto-brecciated and the lithic fragments were added in the hyaloclastite. Since the excess pressure build-up in the growing bubble in the magma was suppressed, no magmatic explosion occurred. Because of permeable host hyaloclastite and no entrapment, the phreatomagmatic explosion and fluidization of host sediments did not take place.