METAMORPHIC PRESSURE-TEMPERATURE PATHS AND ISOTOPIC DATING IN THE HANGING WALL OF A MAJOR THRUST, SEVIER HINTERLAND, SOUTH-CENTRAL IDAHO

The Basin-Elba fault (BEF) in the Albion Mountains is the only Sevier thrust in the Albion-Raft River-Grouse Creek metamorphic core complex preserved through Tertiary extension. The fault juxtaposes a >2.8 km package of middle amphibolite facies Proterozoic to Cambrian metasedimentary rocks over lower amphibolite facies Archean to Ordovician metasedimentary rocks. Garnet schist and garnet-staurolite schist was collected from the schist of Willow Creek middle member in the hanging wall. Garnet growth simulations using the Gibbs method based on Duhem's theorem incorporate a change in nucleation density during garnet growth and produce P-T paths showing episodes of increasing and decreasing pressure. A transition from inclusion-rich garnet cores to inclusion-poor rims corresponds to a substantial increase in nucleation density (a garnet nucleation event). Monazite grains from one garnet schist appear homogenous on BSE images, and 24 grains in two samples yield Th-Pb ages (UCLA high resolution ion microprobe) ranging from 101.9 ± 1.6 to 154.3 ± 1.9 Ma with peaks at 124 and 144 Ma. Seventeen monazite grains in a garnet-staurolite schist yield ages ranging from 130.7 ± 1.8 to 229.9 ± 3.3 Ma with peaks at 157, 171, and 185 Ma. In the garnet schists the mean age of monazite inclusions in garnet is 130 ± 12 Ma. P-T paths suggest repeated episodes of thrust burial and exhumation by faults inferred to lie above the current level of exposure. Footwall paths are very different than hanging wall paths, indicating that the footwall was buried by different faults than the hanging wall, and that the hanging wall rocks are most likely far-traveled. Early Cretaceous inclusion ages suggest a maximum age of garnet growth consistent with Sevier orogenesis. If late Early and Middle Jurassic monazite ages record thrust burial, it provides an alternative to the distant Luning-Fencemaker thrust belt as a site of loading to induce subsidence recorded in the accumulation of Early through Late Jurassic strata. Three muscovite ⁴⁰Ar/³⁹Ar age spectra, from quartzite in the hanging wall, are internally discordant with preferred ages of 41.6 ± 1.4 to 47.8 ± 1.6 Ma indicating a Middle Eocene age for hanging wall exhumation, consistent with similar age spectra in the footwall of the BEF, and with exhumation along the Middle Mountain shear zone.