The Unoki Metamorphic Rocks can be divided into two groups, according to their mode of occurrence. The rocks of the first group occur as small tectonic blocks bounded by faults. They are characterized by the hornblende-epidote-sodic plagioclase assemblage. The second group rocks are enclaves in serpentinities. Most of them belong to the amphibolite facies.
Although the Ca and (Na+K) contents of the two groups are comparable, hornblendes of the amphibolite facies have larger amounts of tschermakite molecule as compared to hornblendes of the epidote amphibolite facies. The most important feature in the chemistry of the analysed hornblendes is that they are relatively poor in Ca (1.51-1.72 for 0=23). It is suggested that the glaucophane substitution plays a significant role for lowering Ca content in hornblendes of the epidote amphibolite facies. On the other hand, the riebeckite substitution is important in hornblendes of the amphibolite facies. Subcalcic hornblende, rich in glaucophane molecule, appears to be stable only under restricted physical conditions. However, the mixing of riebeckite molecule in the hornblende solid solution would extend the stability field of subcalcic hornblende.
The minimum content of Ca in hornblendes from central Abukuma, southern Abukuma (Hitachi), S. W. Scottish Highlands and Sanbagawa, respectively, tends to decrease in the order of increasing solid pressure (Fig. 5). These metamorphic terrains have different metamorphic facies series. The chemistry of the hornblendes from the Unoki Metamorphic Rocks may indicate metamorphism of the similar physical conditions to those of the intermediate-pressure type.