[SCG60-P09] The origin of Quaternary rhyolitic magma from Hime-Shima volcanic group
Keywords:Garnet, Rhyolitic magma, Hime-Shima volcanic group
Lava samples are garnet-bearing rhyolites in Darumayama and Shiroyama volcano. Darumayama lava is mainly composed of garnet, plagioclase and amphibole with groundmass of plagioclase, biotite and quartz. Shiroyama lava is composed of garnet, sillimanite, plagioclase and zircon with glassy groundmass of plagioclase, biotite and quartz. Major element compositions of amphiboles and garnets were determined om an Electro Probe Micro Analyzer.
According to calculated temperature condition (924 - 949 ℃) and SiO2 in coexisting liquid (62 - 64 wt%), amphiboles in rhyolite comes from dacitic magma due to magma mixing. This argument quantitively supports the point of view suggested that garnets in Himeshima rhyolite crystallized in dacitic magma by Itoh (1990).
In Shiroyama lava, we confirmed the part of a lot of garnets coexists plagioclase, sillimanite and zircon. These minerals are subhedral or anhedral. This mineral assemble is like metapelites as xenolith found in dacite in Himeshima volcanic group. The possibility of these minerals are remaining minerals when metapelites melt in rhyolitic magma is high. Chemical compositions of garnets in both lavas are Al-rich and similar. Garnets of rhyolite in Himeshima volcanic group have low CaO (< 4.0 wt%) and high MnO content (5.6-7.0 wt%). Their chemical composition range is like that of garnets in metapelites. Thus, it is estimated garnets of rhyolites in Himeshima volcanic group are xenocrysts from metapelites. The implication is that crustal material such as metapelites influences the genesis of rhyolitic magma in Himeshima volcanic group.