Metamorphic mineral assemblages in metamorphosed mafic volcanic rocks located on and straddling the prehnite (Prh)-pumpellyite (Pmp) to greenschist facies transition in the Flin Flon, Manitoba, area were studied using singular value decomposition. Mass balances obtained for a sample from the transition zone are identical to previously deduced equilibria in the system Na 2 O-CaO-MgO-Al 2 O 3 -SiO 2 -H 2 O and we cannot reject the hypothesis that they equilibrated under conditions of invariant (in this model system) equilibrium. Mass balances obtained between two samples straddling the isograd are in accord with petrological observations across the transition and suggest that it can be modeled by three coincident isograds approximated by: Prh - out , Act - in : Prh + Chl + Ab = Act + Ep + H 2 O , and Pmp - out , Act - in : Prh + Chl + Ab = Act + Ep + H 2 O , where Act is actinolite; Chl is chlorite, Ab is albite, and Ep is epidote. Thermochemical data from various sources cited herein were used to estimate metamorphic conditions at the transition at 2.8–3.4 kbar, 280–290 °C.