Geology and Petrology of the Volcano Mashu Hokkaido, Japan

Abstract

The extinct volcano Mashu is situated on the ESE wall of the Kutcharo caldera in eastern Hokkaido; it belongs to the south-western part of the Kurile volcanic zone., In the late Pleistocene, after the piling up of the huge stratovolcano of Kutcharo, a tremendous amount of pumice and welded tuff of dacite was erupted., This was followed by the depression of the Kutcharo caldera (Krakatoan type of WILLIAMS) which measured 26×20km., in diameter., After the depression, the alluvial volcanoes of Mashu, Atosanupuri and Nakajima were formed along the fissure which trends NNW across the caldera., The somma of Mashu was constructed by alternate eruptions of lava and pyroclastic ejecta, which are mafic to intermediate pyroxene-andesites (SiO₂=52., 78∼60., 05%), belonging to the pigeonitic rock series of KUNO., Slightly after the effusion of the somma lavas, an enormous pumice eruption took place., That pumice is felsic augite-hypersthene-andesite (SiO₂=65., 53%)., Following the pumice eruption, the Mashu caldera of the Krakatoan type, measuring 7., 5×5., 5km., in diameter, 36O∼560 m., in depth and 9 km³., in volume, was formed by depression., The volume of the vanished cone was about 13., 5 km³., However, it may be estimated that the actual volume of liquid magma discharged by this eruption was considerably less than that of the vanished cone, similarly to the case of the Crater Lake and other calderas., After the formation of the caldera, a steep-sided cone and a lava dome, named Kamuinupuri and Kamuishu respectively, erupted in the caldera along the NNW fissure above mentioned., The post-caldera volcanoes consist of lavas of felsic augite-hypersthene-andesite and dacite (the latter SiO₂=72., 96%), all of which are included in the hypersthenic rock series of KUNO., The rocks of the volcano Mashu are chemically characterized by poorness in alkalies and MgO, and richness in CaO and FeO+Fe₂O₃., The alkali-lime index is estimated to be 65., 5, indicating a very calcic rock suite., Normative quartz is always calculated as fairly large, up to 39% in the most siliceous lava., Petrological studies lead to the conclusion that the somma lavas of Mashu were formed mainly due to fractional crystallization of the new tholeiitic magma without direct relation to the magma which furnished the pumice and welded tuff of the Kutcharo caldera; while the pumice ejected from the pre-Mashu caldera are considered to be of contaminated origin, similar to the case of the Towada double caldera., A part of the residual magma which supplied the pumice was erupted after the formation of the Mashu caldera, forming a viscous lava dome and a steep-sided cone.,