PRESSURE-TEMPERATURE PATHS AND THORIUM-LEAD MONAZITE DATING IN THE HANGING WALL OF A MAJOR THRUST IN THE HINTERLAND OF THE SEVIER OROGENIC BELT, ALBION MOUNTAINS, SOUTH-CENTRAL IDAHO

The Basin-Elba fault in the Albion Mountains, Idaho, juxtaposes a >2.8 km package of Cambrian and Proterozoic metasedimentary rocks over Paleozoic metasedimentary rocks and is the only thrust preserved through Tertiary extension in the region.  Garnets were collected from garnet schist and garnet-staurolite schist within the schist of Willow Creek middle member in the hanging wall.  In the garnet schist, garnet grew by the reaction Chl + Pl=Grt + Bt ± Qtz ± Ms + H₂O, and in the garnet-staurolite schist by the reaction Bt + St + Pl=Grt + Ms ± Qtz + H₂O.  Garnet-biotite thermometry indicate that garnet growth in the garnet schist began between 506 ± 25 ˚C and 540 ± 25 ˚C and ended between 578 ± 25 ˚C and 600 ± 25 ˚C based on 3 garnets from 3 samples.  In the garnet-staurolite schist, growth began between 546 ± 25 ˚C and 575 ± 25 ˚C and ended between 595 ± 25 ˚C and 598 ± 25 ˚C based on 3 garnets from 3 samples.  Pressures calculated using muscovite-biotite-plagioclase-garnet barometry on the rims of the 6 garnets range from 4.5 to 9.5 kb.  Garnet growth zoning profiles were numerically simulated using the Gibbs method based on Duhem's theorem to determine pressure-temperature (P-T) paths.  Total pressure increases associated with the growth of any one garnet range from 1 to 2 kb.  All garnet profiles contain a distinct boundary between an inclusion-rich core and an inclusion-free rim, correlating with a change to a steeper decline in Mn toward the rim.  This boundary can be explained by an initial growth phase that produced an inclusion-rich garnet, a second phase that consumed the garnet rim, and a final growth phase that formed the inclusion-free rim under increased nucleation density.  Hanging wall P-T paths are complex and different from footwall paths.  Hanging wall paths show episodes of increasing and decreasing pressure with at least one discontinuity in each path.  Th-Pb dating of 24 monazite grains from two samples shows no demonstrable difference between matrix and garnet inclusion ages.  10 monazites from one sample yielded a weighted mean age of 144.6 ± 3.5 Ma and 8 analyses from the other sample yielded a weighted mean age of 120.5 ± 3.4 Ma, which we interpret as maximum ages of garnet growth, consistent with Sevier orogenesis.  The P-T paths suggest repeated episodes of thrust burial and exhumation by faults inferred to lie above the current level of exposure.