Petrological and geochemical investigation of magma system of caldera volcanoes

本文研究了托亚和四国破火山口的岩浆系统及其成因。主要研究成果如下：破火山口形成阶段：1.几种硅质岩浆之间的混合作用对形成破火山口的硅质岩浆的成分变化起着重要作用。然而，在同一地质层位上，我们发现了几种岩浆混合趋势。这表明这种趋势不是由单一分带岩浆室的方程形成的。2.在破火山口形成阶段的高潮喷发中，多个分带岩浆室通过几个孤立的喷口发生同步喷发。破火山口后阶段：1.破火山口后阶段存在玄武岩和硅质岩浆，形成于破火山口后阶段，形成了带状岩浆房。破火山口后阶段岩石的成分变化是由双峰岩浆之间的岩浆混合形成的。硅质岩浆与破火山口形成阶段的岩石在成因上没有关联。2.玄武岩的成分在各个中心之间存在着跨越弧形的组合变化，俯冲系统对破火山口火山作用的启示：虽然在活动过程中形成了多个硅质岩浆，但它们不仅与各自的硅质岩浆有关，而且与共存的玄武质岩浆也有成因联系，因此，这些硅质岩浆是在玄武岩岩浆供热的情况下，由地壳熔融独立形成的。考虑到破火山口形成过程中硅质岩浆的体积、温度和滞留的时间变化，以及破火山活动后硅质岩浆的体积、温度和滞留的时间变化，本文讨论的弧型和期望型火山的地幔底辟模型可以解释火山口的岩浆作用。

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.