METAMORPHIC ROCKS IN THE SAWTOOTH MOUNTAINS, IDAHO, USA: A WINDOW INTO THE PRECAMBRIAN BASEMENT OF SOUTHWEST LAURENTIA

The western North American (NA) craton consists of a mosaic of Precambrian basement terranes that developed through episodes of microcontinent collision, magmatic arc accretion and crustal growth. Defining the extent and boundaries of these terranes is critical to deciphering the Precambrian development of Laurentia. The poorly exposed Selway terrane occupies a crucial location between the Archean Wyoming province and the Neoproterozoic rifted margin of ancestral 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 may provide a window into the deep crust of the Selway terrane. Field mapping, petrographic analyses, and mineral chemical data characterize the nature of the SMC rocks. Twenty-three metasedimentary and metaigneous units have been mapped and identified in the Thompson-Williams Peak area. These units can be grouped into the following rock types: quartzites, quartzo-feldspathic gneisses, biotite gneisses, mafic gneisses, aluminous schists, calc-silicate gneisses, marble mylonites, and amphibolites. Several of these mineral assemblages indicate original deposition in a passive margin setting during the Precambrian. Thermo-barometric and mineral chemical data from aluminous gneisses indicate peak metamorphic conditions at T = 765-795°C and P > 7 kbar in the Thompson Peak area and 680-750°C in the Iron Creek area. Amphibolites and garnet-hornblende gneisses record less extreme conditions of T = 600-690°C and P = 6.0 - 6.4 kbar, consistent with amphibolite facies metamorphism. A composite clockwise P-T path indicates these rocks were likely metamorphosed in a collisional tectonic setting and represent a mid-lower crustal regime that is distinctly different from the shallow emplacement of the surrounding Sawtooth and Idaho Batholiths. These data also suggest the SMC is composed of a series of tectonic slices juxtaposed through accretionary processes that occurred along the western margin of Laurentia during the Precambrian.