Deep Structure of Island Arc Volcanoes and its Relevance to the Generation of Silicic Arc Magmas

Eiichi Takahashi, Akihiko Tomiya, Isoji Miyagi

Bulletin of the Volcanological Society of Japan
Vol. 42, Special Number "Magmalogy" (1997)

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Abstract:

  Distributions of basalt, andesite and rhyolite dominant volcanoes
and their close relation with thickness and chemical composition of
the basement crust clearly indicate that the mantle-derived basalt
magmas are being modified to the silicic arc magmas within crustal
magma feeding system.  Systematic relations between liquidus
temperature of hydrous arc magmas with SiO2 = 50 to 75 wt % and water
contents have been established.  Based on the magmatic temperatures
recorded in the volcanic rocks and the above systematics, it is
concluded that the tholeiitic andesite and dacite magmas contain 3 to
5 wt % of H2O which is much higher than previously thought.  On the
other hand, calc-alkaline andesites in most subduction zones are
hybrid products of low temperature silicic melts with 5-8 wt % H2O and 
high-temperature basaltic melts.  The mechanism of generation of the
hybrid magma was studies at the Usu volcano in Hokkaido. The Usu magma 
chamber was originally simple (stratification of evolved basalt and
nearly aphyric rhyolite) but as a consequence of its historic
eruptions, the magma chamber has been evolving into a complex magma
plumbing system with heterogeneous lithology of hybrid volcanic
rocks.  Based on seismological view of the deep structure of Japanese
Quaternary volcanoes, petrologic informations of the arc magmas, and
the geologic cross section of the deep structure of young island arc
exposed in the Hidaka metamorphic belt in Hokkaido, a model on the
crustal magma-feeding system of subduction zone volcanoes is proposed.