Petrology of Biotite-Cordierite-Garnet Gneiss of the McCullough Range, Nevada I. Evidence for Proterozoic Low-Pressure Fluid-Absent Granulite Grade Metamorphism in the Southern Cordillera

Abstract

Proterozoic migmatitic paragneisses exposed in the McCullough Range, southern Nevada, consist of cordierite+almanditic garnet+biotite+sillimanite+plagioclase+K-feldspar+quartz+ilmenite+hercynite. This assemblage is indicative of a low-pressure fades series at hornblende-granulite grade. Textures record a single metamorphic event involving crystallization of cordierite at the expense of biotite and sillimanite. Thermobarometry utilizing cation exchange between garnet, biotite, cordierite, hercynite, and plagioclase yields a preferred temperature range of 590–750°C and a pressure range of 3–4 kb. Equilibrium among biotite, sillimanite, quartz, garnet, and K-feldspar records aH2O between 0·03 and 0·26. The low aH2Otogether with low fO2 (≦QFM) and optical properties of cordierite indicate metamorphism under fluid-absent conditions. Preserved mineral compositions are not consistent with equilibrium with a melt phase. Earlier limited partial melting was apparently extensive enough to cause desiccation of the pelitic assemblage. The relatively low pressures attending high-grade metamorphism of the McCullough Range paragneisses allies this terrane with biotite-cordierite-garnet granulites in other orogenic belts. aosure pressures and temperatures require a transient apparent thermal gradient ofat least 50°C/km during part of this Proterozoic event in the southern Cordillera.