Ca-Al silicates in low-grade metavolcanic rocks from southern Tanzawa Mountains, Central Japan.

The Tanzawa group is composed of Miocene andesitic-basaltic volcanic and volcaniclastic rocks in the southern Tanzawa Mountains, central Japan. It can be divided into the stilbite, laumontite, prehnite-pumpellyite, epidote, epidote-amphibole and amphibole zones. The occurrence and chemistry of Ca-Al silicates are described. The Fe3+ contents of prehnite, pumpellyite and epidote decrease with increasing metamorphic grade. But in one sample or one metamorphic zone, the Fe3+ contents of pumpellyite, prehnite and epidote mainly change with coexisting minerals. Widespread hematite±titanite, grandite garnet and abundant Fe3+-rich Ca-Al-Fe3+ silicates indicate high fo2 of fluid during metamorphism of the Tanzawa group. The high fo2 of fluid controls paragenetic changes between prehnite-pumpellyite and greenschist facies and results in wide distribution of epidote zone and absence of amphibole+prehnite+epidote and amphibole+pumpellyite+epidote assemblages.