Compositional variation of calcic amphiboles in Mineoka metabasites, central Japan, and its bearing on the actinolite-hornblende miscibility relationship

Abstract

Calcic amphiboles from the metabasites of the medium-pressure type from Mineoka, central Japan, are chemically continuous from Al-poor actinolite to Al-rich hornblende even in a single grain. Calcic amphibole with the intermediate composition between actinolite and hornblende was confirmed to be a single phase by analytical electron microscope and X-ray diffraction studies. Calcic amphiboles from Mineoka metabasites, transitional between greenschist and epidote amphibolite facies, are characterized by the continuous deficiency of Ca, which is due to the increase of crossite and cummingtonite components from actinolite to hornblende. In metabasites of medium-pressure type, however, calcic amphiboles with low Ca deficiency have the compositional gap between actinolite and hornblende. It is most probable that the miscibility gap in calcic amphiboles of metabasites of medium-pressure type tends to be narrowed with increase of crossite and cummingtonite components.