[SCG60-P10] Magma generation process of the Quaternary Yufu Volcano, Northeast Kyushu, Japan: Approach from petrological features of amphibole
Keywords:Yufu volcano, amphibole, magma plumbing system, magma mixing, P-T condition
Based on the chemical composition of amphiboles, two distinct groups of amphiboles are identified as follows; group I: Si-poor (Si = 5.9 - 6.3 apfu) amphiboles and group II: Si-rich (Si = 6.5 - 6.9 apfu) amphiboles. The SiO2melt and FeO*/MgOmelt of group I in andesite of Yufu-dake were 56.0 - 64.5 wt.% and 2.11 - 4.00, respectively, and those of group II were 73.5 - 75.0 wt.% and 2.67 - 3.03, respectively. These results show that group I and II were crystallized in andesitic-dacitic magma and dacitic-rhyolitic magma, respectively, and these were to be coexisted by magma mixing. Although the SiO2melt of group I is similar to SiO2 content of the whole rock of andesite of Yufu-dake, FeO*/MgOmelt of group I is higher than FeO*/MgO of the whole rock of andesite of Yufu-dake. These results suggest that basaltic magma represented by basaltic inclusion, which had lower SiO2 content and FeO*/MgO than group I, is required to explain the bulk composition. Therefore, we suggest that andesite of Yufu-dake was produced by mixing of andesitic-dacitic magma, dacitic-rhyolitic magma and basaltic magma. The temperature and pressure of group I in andesite of Yufu-dake were estimated as 937 - 998 ℃ and 356 - 654 MPa (13.5 - 24.7 km in depth), respectively, and those of group II were estimated as 807 - 836 ℃ and 131 - 188 MPa (5.0 - 7.1 km in depth), respectively. These results show that magma chamber coexisting with group I exists at a depth of 14 – 25 km, and magma chamber coexisting with group II exists at a depth of 5 – 7 km. The chemical composition of amphiboles in basaltic inclusion and the melt composition estimated from amphiboles in basaltic inclusion are similar to those of Yufu-dake. These results suggest that amphiboles in basaltic inclusion were crystallized in same magma which amphiboles in andesite of Yufu-dake were crystallized and were incorporated into basaltic inclusion when magma mixing occurred.