EARLY CRETACEOUS (VALANGINIAN) CRUSTAL THICKENING IN THE SEVIER HINTERLAND FROM GARNET P–T PATHS AND LU–HF GARNET GEOCHRONOLOGY, SOUTHERN IDAHO

The northern Albion Mountains in southern Idaho comprise a thick overturned section of Neoproterozoic clastic metasedimentary rocks in the hanging wall of a major thrust, the Basin–Elba fault. The rocks were buried to 20–25 km and metamorphosed early in the history of the Sevier orogeny. Garnet growth zoning was used to determine a P–T path from four samples of the schist of Willow Creek by modeling one garnet from each sample using a G-minimization approach. The distribution of quartz inclusions, abundant in cores and few to none in rims, combined with well-defined internal resorption features seen on Y maps, establishes the presence of a hiatus in the growth zoning in three of four of the garnets analyzed. The modeled P–T paths are consistent with this interpretation. The four paths are interpreted to record different but overlapping segments of the overall P–T path. The compiled path shows three episodes of isothermal pressure increases cumulatively rising ~2.5 kbar and ~50 °C. Between the second and third episodes, pressure decreases while temperature increases. For three of the garnets, the pressure-decrease segment followed garnet mode contours and crossed the staurolite-in garnet-consuming reaction midway through garnet growth, thus explaining the observed resorption features. Lu–Hf isochron ages from multiple garnet fractions and whole-rock analyses in two samples are 132.1 ± 2.4 Ma and 137.8 ± 0.5 Ma. Garnet rims contain Lu annuli indicating that most Lu is stored in the rims so ages are biased toward rim growth. The P–T paths presented here from the hanging wall to the Basin–Elba fault, which document episodic contraction and exhumation, are similar to previously determined paths from the schist of Mahogany Peaks in the footwall. Additionally, hanging-wall ages are similar to footwall Lu–Hf garnet isochron ages (132.1 ± 5.1 and 138.7 ± 0.7 Ma). Other structures, not preserved in the area, are needed to explain the burial associated with garnet growth. From a regional perspective, the 132–138 Ma ages for thrust-burial metamorphism fill an important gap in the record of retroarc shortening in the Sevier orogen, and are consistent with eastward propagation of initial shortening during growth of the orogenic wedge.