Origin and pyrometamorphism of gneissose granitoid xenoliths from the Daisen volcano, SW Japan: Implications for the San’in plutono-metamorphic complex

Xenoliths of gneissose granitoids within Quaternary dacite from the Daisen volcano in southwest Japan were previously correlated with the Hida granitoids.However, zircon U-Pb dating of two gneissose granitoid xenoliths yields a Late Cretaceous igneous age of ~ 78 Ma, suggesting a closer affinity with the Late Cretaceous-Paleogene San'in granitoids.Chemical Th-U-total Pb analysis of thorite in one of the xenolith samples yielded scattered apparent ages, with the oldest values approaching the zircon U-Pb age.These gneissose granitoid xenoliths exhibit a granoblastic texture with quartz dioritic modal compositions.Corroded quartz grains are surrounded by a microcrystalline domain consisting of tridymite + oligoclase / anorthoclase + sanidine, interpreted as the result of partial melting during pyrometamorphism within the Daisen dacitic magma.The absence of hornblende and the presence of pyroxene-rich aggregates (orthopyroxene + augite + plagioclase + magnetite) suggest hornblende dehydration melting at ~ 1000 C.Biotite was not completely decomposed during high-temperature pyrometamorphism, likely due to its elevated fluorine content (~ 5 wt% F at grain rims).These mineralogical features are consistent with low-pressure dehydration melting products observed in experimental studies on calc-alkaline granitoids.The gneissose and granoblastic textures are interpreted as original features of the granitoids predating the pyrometamorphic overprint.The exclusive occurrence of Late Cretaceous gneissose granitoids as xenoliths implies the existence of an unexposed plutono-metamorphic complex beneath the San'in region, representing a deeper counterpart of the exposed San'in Batholith.