Petrology of Biotite-Cordierite-Garnet Gneiss of the McCullough Range, Nevada II. P-T-aH2O Path and Growth of Cordierite During Late Stages of Low-P Granulite-Grade Metamorphism

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 H2O 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 +0·06Al2R2+−1Si−1[BIO]⇌1·02 KSP+0·76 H2O +0·30 FeMg−1[CRD]+0·15FeMg−1[BIO]+0·0005 FeMg−1[GRT] +0·005 CaNaAl−1Si−1[PLG] with decreasing P, decreasing T, and increasing aH2O The steep retrograde dP/dT 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.