Metamorphic Conditions of Aluminous Gneisses in the Sawtooth Metamorphic Complex, Idaho, USA: Implications for the Middle-Lower Crust

The Sawtooth Complex (SMC) of central Idaho contains metasedimentary units that elucidate the pressure-temperature conditions and potentially, the evolution of Precambrian crust in the northwestern United States. Petrographic analysis, whole rock geochemistry, and geothermobarometry combined with thermodynamic phase equilibrium modeling record a regional metamorphic pathway for SMC aluminous gneisses characterized by burial to middle-lower crustal levels with at least two deformational events, followed by a retrograde overprint. P-T conditions are generally consistent with the peak assemblage of SMC aluminous gneisses of bt + grt + sil + ilm + zrn + mnz ± pl ± qtz ± kfs ± ap ± xn ± py ± po ± ccp ± gr. This assemblage constrains P-T conditions to be below the biotite dehydration-melting reaction, within sillimanite zone, and above the thermal stability of muscovite. Abundant leucosomes suggest melt. Peak conditions are associated with two deformational events evidenced by crenulation cleavage and rotated inclusions in garnet. Classic geothermobarometry combined with thermodynamic phase equilibrium modeling constrain peak pressure conditions. These pressures decrease from ~11 kbars in the south to ~7.5 kbars in the north at similar temperatures of ~800 °C. Mineral assemblage diagrams are used to identify likely post-peak reequilibration features and used to determine retrograde conditions. SMC samples share a similar retrograde path to conditions near ~4-6 kbars and ~600-650 °C into the stability field of muscovite. Peak pressures from south the north suggest burial depths between 33 and 23 km (3 km /kbar). This range in burial depth indicates that the SMC records different crustal levels that may have been juxtaposed by faulting. Whole rock geochemical data is consistent with a mature shale protolith having undergone significant weathering of feldspar to clay minerals with varying amounts of potassium metasomatism and sedimentary sorting. These shales were likely deposited in a passive margin environment and may represent a deep-water continuation of calc-silicates found adjacent to aluminous gneisses. P-T conditions of SMC aluminous gneisses are consistent with burial to middle crust with subsequent uplift. P-T conditions of SMC aluminous gneisses may elucidate metamorphic conditions along the southwestern margin of the Lauretian craton during the development of Rodinia