METAMORPHISM IN THE SAWTOOTH METAMORPHIC COMPLEX, IDAHO: CONSTRAINTS ON THE MIDDLE CRUST

The Sawtooth Metamorphic Complex (SMC), in central Idaho, is a high-grade metamorphic terrane that provides constraints on crustal evolution of a key area for continental reconstructions involving western Laurentia. The SMC lies near the southwestern margin of Laurentia, in an area that is largely obscured by the voluminous Idaho Batholith and Belt sedimentary cover. Originally mapped as undifferentiated Precambrian metamorphic rocks, recent work demonstrates that a range of rock types exists that can be grouped into three primary units: calc-silicates, quartzofeldspathic gneisses, and peraluminous gneisses. Multi-equilibria thermobarometry of aluminous gneisses at the Thompson headwall yield the highest pressure (P)-temperature (T) conditions of lower-granulite facies ~ 780°C and 7 kbar, above muscovite stability. In contrast, peraluminous gneisses in the Iron Creek area ~ 2 km N, contain fine-grained biotite + sillimanite included in cordierite, suggesting the reaction: biotite + sil = crd + garnet + H₂O; dehydration likely enhanced localized melting. Peak P-T conditions recorded in this assemblage are lower, P ~ 5kbar at T ~750^oC. Subsequent to peak conditions, a hydration event produced muscovite and andalusite that cross cuts the dominant foliation. These data suggest that the Iron Creek and Headwall samples experienced different P-T paths.

Calc-silicate gneisses record at least two metamorphic (M1, M2) and two deformational (D1, D2) events. M1 reached upper-amphibolite to lower-granulite facies at T~ 800^oC at 6 kbar. M2 is characterized by amphibole growth overprinting cpx and retrogressing peak assemblages at T~ 620-740^oC at an assumed P =6.5 kbar. Whole-rock phase diagram calculations have consistent T estimates restricted by the observed assemblage (cpx + qtz + tr + ttn ± K-fsp) to 660-625°C and X(CO₂) from 0-0.2 at an assumed 7 kbar. A later stage involving mylonitic deformation is recorded at T~300-400^oC at ~3 kbar. These data indicate the SMC is composed of tectonic slices that record multiple, distinct metamorphic sequences consistent with a collisional tectonic setting. The SMC, therefore, provides insight into the evolution of a significant section of middle-to-lower crust that places important constraints on the Neopropterozoic and later evolution of the western Laurentian margin.