Thermodynamic calculations based on addition of mass balance equations to the Gibbs Method (Spear, 1986) are used to model the cordierite-producing reaction in pelitic gneiss from the McCullough Range, southern Nevada. Calculations which treat the model paragenesis as a system open to transfer of H 2 O are consistent with textural relations. Results indicate that cordierite grew by the continuous net-transfer reaction: 0�76 BIO+1�72 SILL+3� 55 QTZ+0�27 PLG+0�005 GRT +0�06Al 2 R2+ −1 Si −1 [BIO]⇌1�02 KSP+0�76 H 2 O +0�30 FeMg −1 [CRD]+0�15FeMg −1 [BIO]+0�0005 FeMg −1 [GRT] +0�005 CaNaAl −1 Si −1 [PLG] with decreasing P , decreasing T , and increasing a H 2 O The steep retrograde d P /d T path for these low-pressure granulites contrasts with isobaric cooling paths typical of higher pressure granulites, and suggests uplift and erosion were active during Proterozoic granulite-grade metamorphism in this area.