Abstract
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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