PHASE EQUILIBRIUM MODELING OF PERALUMINOUS GNEISSES FROM THE SAWTOOTH METAMORPHIC COMPLEX, ID: IMPLICATIONS FOR THE MIDDLE-LOWER CRUST

The Sawtooth Metamorphic Complex (SMC), Idaho, lies in an enigmatic region amongst the Archean-Proterozoic basement terranes making up the southwestern Laurentian Craton. The SMC occurs as a pendant largely within the Tertiary (46-47 Ma) Sawtooth Batholith in the Atlanta Lobe of the Idaho Batholith. Preliminary studies suggest that the peraluminous gneisses in the SMC are transitional granulite-facies rocks representing basement crust in this area, where few basement rocks have been found. To petrologically characterize these rocks, a combination of textural analysis and phase equilibrium modeling is used to elucidate the metamorphic condtions of the region.

The SMC peraluminous gneisses contain the assemblage sillimanite + garnet + biotite + alkali feldspar + plagioclase + quartz + ilmenite + monazite + zircon with minor secondary muscovite. Stable biotite and sillimanite define a foliation that wraps around garnet porphyroblasts. This foliation is crosscut by randomly oriented muscovite. Some samples contain leucosomes composed of plagioclase, alkali feldspar, and quartz. The stable assemblage of the peraluminous gneisses indicates peak metamorphism attained condtions above the discontinuous muscovite dehydration-melting reaction, ms + pl + qtz = sil + ksp + liquid, and below the discontinuous biotite-sillimanite dehydration-melting reaction, bt + sil = grt + crd + ksp + liquid. Muscovite dehydration-melting is responsible for the development of leucosomes in the samples, but the biotite + sillimanite continuous melting reaction is contributing factor. Lower temperature rehydration in the muscovite stability field produced secondary muscovite, overprinting the peak metamorphic assemblage.

Phase equilibrium modeling of the peraluminous gneisses using Theriak-Domino constrain the P-T conditions to 7-8 kb and 710-740 °C. Previous studies on SMC peraluminous gneisses yield consistent results of 700-790 °C and 6.2-7.2 kb using garnet-based geothermobarometers. These pressures suggest burial depths of 21-24 km, consistent with the depths of the middle-lower crustal basement terrane of the region. This study provides constraints on the metamorphic conditions that may have occurred during the accretion of southwestern Laurentia.