Petrological and geochemical investigation of magma system of caldera volcanoes

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We investigated magma system and its genesis of Toya and Shikotsu caldera volcanoes.Main research results are summarized as follows.Caldera-forming stage : 1.Magma mixing process among several types of silicic magmas playd an important role on compositional variations of caldera-forming silicic magma.We found, however, several magma mixing trends in the same geological horizon.This suggests that the trends could not be formed by the equption from single zoned magma chamber.2.In climactic eruption of caldera-forming stage, syn-eruption from multiple zoned magma chambers occurred through several isolated vents, resulting of form caldera collapsa.3.Estimated end-member magmas were not related genetically and should be formed by partial melting of different lower crustal materials.Post-caldera stage : 1.Bimodal magmas basaltic and silicic magmas, have been present during the post-caldera stage and formed zoned magma chambers.The compositional variation in the rocks from the post-caldera stage was formed by magma mixing between the bimodal magmas.The silicic magmas were not genetically related with those in caldera-forming stage.2.There are across arc compositonal variations in the basalts among each centers, suggesting that mafic magma under the caldera volcanoes was subduction related arc-type magmas.Implications for caldera volcanism in subduction system : Although several silicic magmas were formed during the activity, they are not genetically related not only with each silicic magma but also with coexisting basaltic magama.The silicic magamas, therefore, were formed independently by crustal melting due to heat supply of the basaltic magma.Considering temporal variation in volume, temperarute and stagnation deoth of silicic magmas during caldera-forming and post-caldera activity, the mantle diapir model which is discussed in the case of arc-type andesitic startovolcano can explain the magmatism of caldera volcano.