The metavolcanics of the Miocene Tanzawa Group of the southern Tanzawa Mountains can clearly be divided into six mineral zones ; stilbite, laumontite, prehnite-pumpellyite, epidote, epidote-amphibole and amphibole zones. Four phases of mafic phyllosilicates in the five lower grade zones are identified by systematic X-ray diffraction studies; (1) smectite in the stilbite and laumontite zones, (2) randomly mixed-layer chlorite/smectite mineral which is ubiquitous in the stilbite, laumontite and prehnite-pumpellyite zones, (3) regularly mixed-layer chlorite/smectite in the stilbite zone, (4) chlorite occurs first in the laumontite zone. With increasing metamorphic grade, smectite layers progressively transform to chlorite. Chlorite is the predominant phyllosilicate in the prehnite-pumpellyite zone and is a unique phyllosilicate in the epidote and epidote-amphibole zones.
     Variations in Fe/(Fe+Mg) of chlorite are positively correlated with Fe/(Fe+Mg) ratios of whole rocks, but the other chemical variations are clearly unrelated to whole rock compositions. The content of Si decreases with increasing metamorphic grade.

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Two stages of metamorphism, initial and main stages, are distinguished on the basis of textural features and chemical compositions of sodic amphiboles and associated minerals in the Kamuikotan high-pressure and low-temperature metabasites. The initial stage is detected by sodic amphiboles rimmed by actinolite and enclosed by epidote porphyroblast in the metabasites. These sodic amphiboles are optically and chemically heterogeneous, and range in chemical composition from crossite to glaucophane (Al₂O₃ content: 2.6-10.5 wt.%). The main stage is characterized by the occurrence of epidote, pumpellyite, chlorite, calcite, hematite and jadeite component poor sodic pyroxene in the metabasites. Amphiboles coexisting with these minerals are actinolite, winchite and magnesioriebeckite (Al₂O₃ content: 0-2.9 wt.%).
     These variations of modes of occurrence and chemical compositions of sodic amphiboles suggest that some studied metabasites initially had been metamorphosed under low grade condition with a relatively high ratio of pressure/temperature and were subsequently converted to assemblages of the main stage, correspond to the pumpellyite-actinolite facies, by decline in the pressure/temperature ratio with local effect of deformation and metamorphic fluid.

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