Metamorphic rocks in the Sawtooth Mountains, Idaho, USA: a window into the Precambrian basement of southwest Laurentia

Abstract

The western North American craton consists of a mosaic of Precambrian basement terranes that developed through episodes of microcontinent collision and magmatic arc accretion. Defining the extent and boundaries of these terranes is critical to deciphering the Precambrian development of Laurentia. The poorly exposed Selway terrane occurs between the Archean Wyoming province and the Neoproterozoic rifted margin of Laurentia. A rare exposure of Precambrian metamorphic rocks, the Sawtooth Metamorphic Complex (SMC), crops out over 33 km2 in the Sawtooth Mountains of Idaho, USA, and potentially provides a window into the deep crust of the region. Field mapping, petrographic analyses, and mineral chemical data characterize the nature of the SMC rocks. Nine rock types occur in the SMC: quartzite, quartzofeldspathic gneiss, calc-silicate gneiss, marble mylonite, titanite-hornblende-biotite gneiss, metagabbro, biotite-amphibolite, peraluminous gneiss, and amphibole-pyroxene gneiss. Cross-cutting relationships and mineral-replacement textures in the Iron Creek peraluminous gneiss indicate the rocks passed through muscovite stability in the sillimanite field during prograde metamorphism, resulting in the reaction: muscovite + quartz  sillimanite + K-feldspar + H2O. Fine-grained biotite and sillimanite are included in cordierite suggesting the reaction: phlogopite + sillimanite  Mg-cordierite + garnet + H2O. These two dehydration reactions contribute to partial melting of the rocks and migmitization at peak metamorphic conditions interpreted at T = 800°C and P = 7 kbar. Tabular muscovite and euhedral andalusite cross-cut the main foliation in Iron Creek peraluminous gneisses, indicating a clock-wise decrease in pressure and temperature during retrograde metamorphism. Thompson Peak rocks share similar prograde, peak, and post-peak conditions, but different retrograde paths. Thompson Peak rocks contain pseudomorphs of kyanite-overprints indicating the rocks remained at relatively high-temperature during retrograde metamorphism. Calculated metamorphic conditions, T = 680 – 750 ±40 °C and P = 5 – 7 ± 0.5 kbar, indicate the rocks are transitional granulites to amphibolites from the mid-lower crust. Clock-wise P-T paths suggest the SMC was metamorphosed in a collisional tectonic setting. However, different retrograde conditions in the Thompson Peak and Iron Creek regions suggest the SMC is composed of multiple tectonic slices from the mid-lower crust.