The origin of garnet in Quaternary rhyolites of the Hime-shima Volcanic Group, east Kyushu, southwest Japan Arc

Petrological and chemical analyses were conducted on garnet-bearing rhyolites of the Darumayama and Shiroyama volcanoes in the Hime-shima Volcanic Group (HVG) on Kyushu Island, southwest Japan arc. In the Darumayama rhyolite, garnet occurs at a frequency of approximately one grain per several thin-sections. It is mostly euhedral and dispersed within the groundmass, and locally exhibits resorption textures. Garnet in the Shiroyama rhyolite occurs as multiple euhedral grains that are dispersed within the groundmass and show no distinct resorption features. The almandine, spessartine, and pyrope proportions of the garnets in the Darumayama and Shiroyama rhyolites are nearly identical, indicative of a common origin. The Shiroyama rhyolite contains a plagioclase-rich aggregate containing garnet, sillimanite, and zircon, indicating a metamorphic origin. The garnet in this plagioclase-rich aggregate has a solid-solution composition identical to those outside the aggregate, indicating that all garnet in the HVG rhyolites has a common origin. The high MnO (5.6–7.0 wt.%) and low CaO (0.5–1.2 wt.%) contents of the garnet are indicative of metamorphic origin rather than a magmatic origin. These observations indicate that the garnets in the HVG rhyolites are xenocrysts derived from metapelites rather than magmatic phenocrysts.