Modes of occurrence of sodic amphibole from the Kamuikotan metabasites, west of Asahikawa, central Hokkaido and the metamorphic history.

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.