Correlation of the Muro Pyroclastic Flow Deposit and the adjacent tuffs in northern Kii Peninsula, southwest Japan, and their source: an approach from mode analysis using refractive index of light minerals

Abstract

Mode analyses using refractive index of light minerals were performed for samples from the Muro Pyroclastic Flow Deposit (MPFD) and the adjacent felsic tuffs (the Sekibutsu Tuff, the Furudera Tuff and the Tamateyama Tuff) in northern Kii Peninsula, and Miocene felsic rocks in southern part of the peninsula. We have confirmed that plagioclase phenocryst of the MPFD and the adjacent three felsic deposits are principally composed of oligoclase to labradorite, and that these four Miocene deposits are correlative each other. The MPFD is also correlative to a certain horizon of the felsic pyroclastic facies of the Kumano Acidic Rocks (KAR) on the southern end of the peninsula. We thus conclude that the MPFD and correlative tephras are an identical large pyroclastic flow deposit sourced from ancient (ca. 15 Ma) caldera volcanoes in the Kumano area. Some parts of the felsic pyroclastic flow deposits of the KAR contain plagioclase phenocryst of principally albite-composition, and could have derived from a different caldera, now represented by an arcuate pyroclastic intrusion (Nakaoku tuffite dike swarm) in the middle part of the peninsula.