EXHUMATION AND COOLING HISTORY OF THE SAWTOOTH METAMORPHIC COMPLEX, IDAHO

The metamorphic rocks comprising the Sawtooth Metamorphic Complex (SMC) are found as pendants in the Eocene Sawtooth Batholith (SB) and the southern Atlanta lobe of the Cretaceous Idaho Batholith in the Sawtooth Range, Idaho. In situ U-Pb ages (LA-ICP-MS) of 61 magmatic zircon grains from 3 SB granitic rocks gave an average age of 47.0 ± 0.8 Ma, indicating that the intrusion and crystallization of different phases of the SB occurred at about 47 Ma. Lu-Hf isotopic data from these zircons give an average initial ƐHf value of -25.3±1.0, suggesting derivation from late Paleoproterozoic lower crust. To determine the exhumation and cooling history of the SMC, coexisting muscovite and biotite from two quartzite samples in the central part of the SMC (Williams Peak area), at different stratigraphic levels give ⁴⁰Ar-³⁹Ar ages of 52.1±0.7 Ma and 49.5±0.7 Ma (for the sample SW of Williams Peak), 29.5±2.1 Ma and 26.6±3.5 Ma (for the sample E of Williams Peak), respectively. Muscovite separates from two metasedimentary rocks in the northern SMC and between SMC and SB gave ⁴⁰Ar-³⁹Ar ages of 47.7±0.6 Ma and 47.5±0.7 Ma, respectively. Two quartzofeldspathic gneiss samples from the eastern and southeastern slope of Williams Peak gave ⁴⁰Ar-³⁹Ar ages of 52.7±1.8 Ma (muscovite), and 44.7±1.7 Ma (biotite), respectively. Total fusion ⁴⁰Ar-³⁹Ar ages of biotites from SB granitoids range from about 47 to 21 Ma and generally show discordant age spectra. These ⁴⁰Ar-³⁹Ar data indicate that most of the metamorphic rocks were close to or above muscovite closure temperatures for argon diffusion (about 450˚C) during emplacement of the SB and cooled through muscovite closure immediately after SB crystallization. Rapid cooling through muscovite closure is consistent with the relatively shallow emplacement depth of the SB (<10 km). The scatter of biotite total fusion ages and discordant age spectra are attributed to variable degrees of alteration and excess argon related to post-emplacement hydrothermal system activity. Widespread Eocene fluid flow within and around the SB was previously documented in stable isotopic and K-Ar studies. These results also suggest that the high grades of metamorphism, with peak pressures up to 7 kbars, occurred prior to the Eocene.