Petrology of the Nohi Rhyolite and its related granitoids: A Late Cretaceous large silicic igneous field in central Japan

Abstract

The Nohi Rhyolite (5000–7000 km 3) is one of the largest Late Cretaceous ash-flow fields, related to the Kula/Pacific Plate subduction, in central Japan. It consists mainly of thick, welded ash-flow sheets (a few 100 to > 1000 km 3) infilling cauldrons. Ash-flow tuffs of the Nohi Rhyolite are characterized as crystal-rich rhyodacite to rhyolite, fiammes of which have 30–45 vol.% phenocrysts (quartz, feldspars and minor mafic minerals). The crystal-rich rhyolitic magma sampled by some units of the Nohi Rhyolite formed mainly by fractional crystallization (plagioclase, pyroxene and ilmenite) from a parental crystal-rich rhyodacite. The parental magma of the largest Gero Ash-Flow Sheet (> 2200 km 3) is considered to have had a lower original water content than typical intermediate to evolved magmas produced in subduction zones. The Nohi Rhyolite has similar general petrological properties to the hypabyssal granitoid porphyries and the batholithic Naegi-Agematsu Granite, which is high-K, I-type, ilmenite-series dacitic to rhyolitic in composition and lacking mafic rocks. This petrological similarity, together with a close spatial and temporal relationship indicates that they formed as a series of large silicic magmatic events on the eastern margin of the Eurasian Continent during the Late Cretaceous. There are slight differences in trace element composition within subdivisions of the Nohi Rhyolite and granitoids, which probably relate to the region of magma emplacement rather than a temporal change in magma chemistry.