Variation of amphibole composition in high pressure rocks of Franciscan Formation (California, USA)
Dependence of the composition of amphiboles on the composition of protolith of glaucophane and glaucophane-bearing schists of the Franciscan Formation (West California, USA) is discussed in the paper. Amphiboles of only glaucophane composition with rare relics of actinolite occur in apoeclogite schists in association with omphacite, as well as in metachert schists (metasilicitolite) in association with Mn-rich garnet. Garnet amphibolite is characterized by successive evolution of amphibole from the mineral close to pargasite and edenite to amphibole of the tremolite-actinolite series and glaucophane. Edenite and amphiboles of the actinolite-tremolite series are registered in schists from boudines in serpentinite. Amphiboles from metabasalt (?) are mostly characterized by the high Fe concentration; in addition, amphiboles from these rocks are clearly divided into two groups (ferroglaucophane and riebeckite-arfvedsonite) without gradual transitions.