EOCENE A-TYPE MAGMATISM IN THE NORTHERN U.S. CORDILLERA – FARALLON ROLLBACK, SLAB WINDOW, OR COLLISION? EVIDENCE FROM THE SAWTOOTH BATHOLITH, IDAHO

The Sawtooth batholith (STB) is part of a widespread, Eocene felsic to ultramafic magmatic suite that includes the Challis, Absaroka, and Montana alkali provinces, and is contemporaneous with the Colville and Kamloops suites. In the Challis magmatic province, many plutons, including the STB, are large, shallow-level, alkali granites intruded into or along the margins of the Cretaceous-Paleocene Idaho batholith. In the STB, compositions range from quartz alkali feldspar granite to syenogranite (two-feldspar or hypersolvus) and are all classified as high K. Miarolitic cavities in the southern part of the batholith indicate shallow emplacement. Thermobarometry based on mineral chemistry and zircon saturation temperatures indicate emplacement from 650 to 750˚C and 2.7 – 3.7 kbar for the northern portion of the STB. HREE values are typically ~10x chondritic and relatively flat, and LREEs are enriched up to ~100x. Most samples display a significant negative Eu anomaly. U-Pb zircon data from 6 samples give ages between about 47 and 43 Ma. These crystallization ages are concordant with K-Ar biotite ages and Ar/Ar muscovite ages from the country rocks, indicating rapid cooling through 450-250˚C. Epsilon Nd values range from -15 to -16 for most samples. Epsilon Hf values of the Eocene zircons give a mean of about -21. These isotopic values are much more homogeneous and less negative than data for the older Idaho batholith. This suggests (1) a different Proterozoic crustal source for the STB or (2) more extensive mixing of asthenospheric and lithospheric derived melts. The second model is consistent with magma genesis over a relatively short time period due to widespread melting of lithosphere previously hydrated by Farallon dewatering, Challis A-type magmatism was likely initiated by asthenospheric upwelling resulting from slab breakoff (or rollback), or opening of a slab window between the Farallon and Kula (or Resurrection) plates.