PRESSURE-TEMPERATURE CONDITIONS FOR MID-TO-UPPER CRUSTAL METAPELITES FROM THE PIONEER CORE COMPLEX AND THE SAWTOOTH METAMORPHIC COMPLEX, IDAHO

Core complexes in the western U.S. expose deeply buried rocks that illuminate mid-to-upper crustal conditions. The Pioneer core complex (PCC) in south-central Idaho, represents the first occurrence of basement rocks north of the Snake River Plain. The footwall of the PCC is divided the Middle and Lower Plate; the base of the Middle plate overlies and was intruded by the 50-48 Ma Pioneer Intrusive Suite, apparently during extensional strain. The protolith age of units in the plate is loosely constrained to be Ordovician to Proterozoic (~1 Ga).The high-temperature strain in the metamorphic rocks is constrained to be 49-47 Ma. Aluminous metapelitic rocks occur in isolated outcrops in the Middle Plate, and locally contain the assemblage sillimanite + garnet + biotite + plagioclase + quartz + ilmenite + graphite + zircon + chalcopyrite + muscovite (secondary). Sillimanite and micas define a well-developed foliation. Sillimanite occurs as coarse prismatic crystals and/or as clusters of sillimanite prisms in a pseudomorph consistent with andalusite morphology. The pseudomorphic texture implies that there was likely “contact” metamorphism producing andalusite followed by a higher-temperature, high-strain metamorphism. Thermobarometric calculations yields T = 700^oC +/- 15^oC and P = 4.4 kbar using garnet – biotite thermometry and GASP geobarometry, respectively. Ti-in-biotite gives T = 655^oC +/- 24C. These P-T conditions as well as mineral textures are distinctly different from those determined from peraluminous gneisses from the Sawtooth Metamorphic Complex (SMC), Idaho, about 160 km to the NW. Metapelites from the Iron Creek area in the SMC record T from 700-775^oC at 4.6 – 5.8 kbar but contain the mineral cordierite and lack apparent andalusite pseudomorphs. Peraluminous gneisses near Thompson Peak produced higher P-T conditions near 730 – 750^oC with P = 6.2 – 7.2 kbar. Metamorphic rocks in the PCC were buried to at least 15 km depth while those in the SMC were buried to ~20 km. Significant uplift exposed segments of the crust in both areas produce information on the mid-crust in the area, but the PCC and SMC are likely exhumed at different times and with different mechanisms.