K-Ar ages and time span of emplacement of the Boulder Batholith, Montana

Abstract
Six subfacies of the granulite facies are based on variable load pressure, water-vapor pressure, and temperature. Two subfacies of relatively high load pressure are characterized by clinopyroxene-almandite, two intermediate subfacies by orthopyroxene-plagioclase, and two subfacies of relatively low load pressure by cordierite-almandite assemblages in rocks with excess silica. Three subfacies with relatively high water-vapor pressure contain hornblende or biotite, or both, in addition to the characteristic anhydrous assemblages of three subfacies of relatively low water-vapor pressure.