New Constraints on the Geochronology and Thermochronology of the Sawtooth Batholith, Idaho

The Sawtooth batholith in south-central Idaho is one of the largest Eocene epizonal plutons that intrudes the Cretaceous Atlanta lobe of the Idaho batholith. The batholith is part of a suite of Eocene plutons associated with the voluminous Challis volcanic field in central Idaho. Intense hydrothermal systems within and surrounding this and other Eocene plutons have been likened in scale to the modern Yellowstone hydrothermal systems. The two-feldspar, subsolvus, granites are characterized by pink alkali feldspar, variable amounts of hornblende, biotite, and titanite, a lack of deformation, minor alteration to sericite and/or secondary muscovite, and porosity. The presence of miarolytic cavities and porosity together with additional constraints suggests P-T conditions near 650°C and 1.5 kbar. Previous K-Ar biotite ages suggested that the Sawtooth pluton was intruded at about 47-45 Ma, but no modern geochronology has been performed on the alkali feldspar-rich granites of the Sawtooth batholith.

New U-Pb zircon analyses (LA-MC-ICP-MS) of samples from the Sawtooth pluton have been challenging due to Pb loss and high common Pb likely associated with the hydrothermal alteration. Two spatially separated samples from the northern portion of the batholith, however, yield sufficient concordant zircons with low common Pb to indicate a crystallization age of 47.1 +/- 0.7 Ma and 46.6 +/- 0.6 Ma (2 s.e.m.), respectively. Ar/Ar analyses of biotite and muscovite from the Sawtooth plutons and metasedimentary country rocks yield cooling ages of about 47 to 45 Ma, consistent with very rapid post-crystallization cooling at shallow crustal levels. Other biotite Ar/Ar analyses, however, yield cooling ages as young as about 25 Ma, most of which have discordant age spectra. These samples suffer from chlorite intergrowths associated with the Eocene hydrothermal system and are less retentive than primary biotite. The Miocene cooling ages of the altered biotites are similar to U-Th/He ages of apatite from the Sawtooth Range, and suggest that exhumation associated with displacement on the Sawtooth normal fault commenced in early Miocene time. U-Pb ages of zircon xenocrysts and Lu-Hf isotopic data from Eocene magmatic zircons suggest the source of the Sawtooth magmas is dominated by Proterozoic crustal material.