Glaucophane-schist facies metamorphism in the Black Butte area, northern Coast Ranges, California

Mappable units (dominantly quartz-mica-lawsonite schist and phyllite, dominantly metabasalt, and dominantly graywacke and argillite), tentatively assigned to the Franciscan Formation of late Mesozoic age, have been metamorphosed in the glaucophane schist facies. Comparison of chemical analyses of rocks with glaucophane schist facies assemblages to their unmetamorphosed equivalents indicates essentially isochemical metamorphism. Distribution of lawsonite and epidote appears to be controlled by the Fe'''/Al ratio. Close association of greenschist facies mineral assemblages with glaucophane schist facies assemblages suggests the facies are transitional, and regional occurrence of lawsonite-aragonite-albite in contrast to different mineral assemblages of the glaucophane schist facies in other areas suggests that the facies may eventually be subdivided.