Abstract
There widely occur the metamorphic rocks characterized by the stable association of mordenite, clinoptilolite, silica minerals and clay minerals in the Green Tuff (Miocene submarine pyroclastic formation) area of Japan. Many authors studied these rocks have correlated the garde of metamorphism in which these mordenite-bearing rocks were formed to the heulandite-analcime subfacies of the zeolite facies defined by Coombs.
However, mordenite which may have been broken down into heulandite (stilbite)-bearing mineral assemblage under physical conditions prevailed in thick pile of New Zealand geosynclinal deposits was not included within Coombs's definition of the zeolite facies.
There are at least three series of zeolite facies metamorphism as follows:
(1) New Zealand Geosyncline type (low geothermal gradient series)
la: Heulandite-analcime subfacies
lb: Laumontite subfacies
(2) Shinjo Basin type (intermediate geothermal gradient series)
2a: Clinoptilolite-mordenite subfacies
2b: Heulandite-analcime subfacies
2c: Laumontite subfacies
(3) Active geothermal area type (high geothermal gradient series)
3a: Clinoptilolite-mordenite subfacies
3b: Laumontite subfacies
3c: Wairakite subfacies
The occurrence and mineral association of the mineral mordenite must be an important criterion to discuss the physical conditions of the low-grade metamorphism.
