Two-amphibole assemblages in the system actinolite-hornblende-glaucophane

Electron-probe analyses and optical descriptions are given for 22 amphibole pairs. In actinolite-hornblende pairs, hornblende has the lower Mg/(Mg + Fe) ratio. In actinolite-glaucophane pairs, glaucophane has lower Mg/(Mg + Fe) as well as lower Si/(Si + Al) ratios. In four out of six hornblende-glaucophane pairs, glaucophane has the smaller Mg/(Mg + Fe) ratio, whereas in two, this is reversed. Four pairs, consisting of glaucophane and hornblende-glaucophane, have Mg/(Mg + Fe) and Si/(Si + Al) fractionation patterns very different from other pairs. The majority of textures show complex zonation and patches of two amphiboles in composite grains. These assemblages are evidence for miscibility gaps in the system at relatively low temperatures; the majority occur in greenschist or epidote amphibolite, and glaucophane schist facies, all formed at lower temperatures.