Abstract
Early-middle Miocene Tanzawa group and late Mioceneearly Pliocene Ashigara group in the Tanzawa Mountains, central Japan suffered metamorphism ranging from the zeolite facies to the amphibolite facies through the pumpellyite-prehnite facies and the actinolite greenschist facies. This metamorphism occurred by down-sinking of thick pile of submarine volcanic materials, the intrusion of quartz diorite magma and lateral migration of geosynclinal basin during early Miocene to early Pliocene.
Metamorphosed area of the Tanzawa Mountains can be divided into the following five zones by the stability relations of some metamorphic minerals:
Zone V: amphibolite zone
Zone IV: actinolite greenschist zone
Zone III: pumpellyite-prehnite-chlorite zone
Zone II: laumontite-mixed layer chlorite zone
Zone I: stilbite (clinoptilolite)-vermiculite zone
The main phase of higher-grade metamorphism and the invasion of quartz diorite had been almost completed before the deposition of the Ashigara group. The central part of the Tanzawa Mountains occuppied with the Tanzawa group and quartz diorite mass had been exposed above sea-level during the deposition of the Ashigara group and provided huge amounts of materials to the sedimentary basin. The lower-grade metamorphism which formed Zone II and Zone I, however, still continued during and after the sedimentation of the Ashigara group. The Tanzawa area thus provides an example of the lateral migration of the space of metamorphism accompanied by the migration of geosynclinal basin within one continuous tectonic history.
Physical and chemical properties of some metamorphic minerals such as clay minerals, chlorites, pumpellyite, plagioclase, piemontite, calciferous amphiboles and zeolites are described. The occurrence of wairakite and the absence of pumpellyite-actinolite association in metamorphic rocks of the Tanzawa Mountains indicate that the metamorphism in this Mountains was formed under lower pressure conditions than those prevailed in Wakatipu metamorphic belt of New Zealand.
