Occurrence and genesis of Miocene peperites in the Tanzawa area, South Fossa Magna, central Japan

Abstract

Miocene subaqueous volcaniclastic rocks that represent an oceanic island arc belonging to the paleo-Izu-Bonin arc are widely distributed in the Tanzawa area, South Fossa Magna. We identified multiple peperites that formed in a setting of arc volcanism. These peperites developed where intrusive and extrusive mafic magma mingled with unconsolidated subaqueous volcaniclastic and non-volcanic sediments. Peperite is an indicator of in situ subaqueous volcanism within unconsolidated to poorly consolidated sediments. In this paper, we describe four types of peperite (facies A-D) and their locations in paleo-subaqueous volcanoes, and investigate their genesis. Peperite has been commonly classified into blocky and fluidal/globular types. Facies A and B contain both fluidal and blocky juvenile clasts that are not widely dispersed in the host sediments. The host sediments of both types are pumiceous lapilli tuff and volcanic mudstone. Facies C is composed of fluidal juvenile clasts that mingled with tuff breccia. Again, juvenile clasts are not widely dispersed in the host sediments. Facies D is blocky peperite, for which the host sediments are tuff breccia. Juvenile clasts are partly dispersed. It had been assumed that the morphology of peperite (i.e., fluidal vs. blocky) is controlled mainly by the grain size of host sediments; however, this is considered unlikely in the case of the juvenile clasts examined in the present study.