Metamorphic and geochemical signatures of calc-silicate gneisses from the Sawtooth Metamorphic Complex, Idaho, USA : implications for crustal evolution in western North America

Abstract

The Sawtooth Metamorphic Complex (SMC) of central Idaho contains exposures of metasupracrustal rocks that may provide constraints on Precambrian crustal evolution in northwestern United States. Petrographic textures, whole-rock geochemistry, and thermobarmetry of SMC calc-silicate gneisses, collected at high-resolution, record multiple stages of crustal development, including: contiguous deposition of a sandstone-to-shale sequence derived from post-Archean continental sediments, metamorphism during two medium-high-grade thermal events (M1, M2), and two distinct episodes of deformation (D1, D2). Whole-rock concentrations of Cr (23.97-76.43 ± 3.0 ppm), Ni (6.82-33.10 ± 3.5 ppm), Th (2.11-12.04 ± 1.6 ppm), Sc (3.81-11.53 ± 1.6 ppm), and Zr (322-658 ± 3.9 ppm) in SMC cal-silicates are consistent with derivation from post-Archean, continental sources that have undergone sedimentary maturation and recycling typical of detritus deposited in passive-margin environments. Al-Ti-Zr ratios suggest calc-silicates represent a sequence of calcareous sandstones and marls, with bulk geochemical fluctuations reflecting varying proportions of clay and quartzo-feldspathic detritus. The sandstone-to-shale continuum may reflect a NW-to-SE, deep-to-shallow-water transition in the depositional environment of SMC calc-silicates. Temperature estimates for M1 and M2 are constrained by mineral thermometry and ten isobaric pseudosections. Peak metamorphic M1 conditions, characterized by the assemblage: cpx + qz + pl + kfs + phl + rt, are estimated at temperatures >750-775ºC and oxygenated fluids (XO > 0.35). M1 was followed by widespread D1 deformation at high P and T, resulting in deformation twinning in clinopyroxene. M2 occurred at conditions of ~550-725ºC and H2O-rich fluids (XCO2 ≈ 0.34), resulting in the diagnostic assemblage amph + kfs + qz+ cpx + pl + ttn. Undeformed, poikiloblastic amphiboles overprint deformation twins in clinopyroxene, suggesting M2 was a static, thermal event post-dating M1 and D1. Late-stage, brittle-ductile D2 deformation is characterized by mylonitic lenses of quartz, fractures within M1 clinopyroxenes that cross-cut and offset D2 deformation twins, and localized fracturing of M2 amphiboles. Geochemical signatures of SMC calc-silicates indicative of post-Archean, passive-margin, continental sediments are most consistent with associations with nearby Neoproterozoic and Mesoproterozoic strata, suggesting the SMC may record deposition and multiple metamorphic/deformation events along western Laurentia associated with development of the 1100-750 Ma supercontinent Rodinia.