PHASE EQUILIBRIA, GARNET REE GEOCHEMISTRY, AND LASS PETROCHRONOLOGY: CONSTRAINTS ON THE P-T-T HISTORY OF THE PRIEST RIVER COMPLEX, NORTHERN IDAHO

Phase equilibria modeling, garnet REE geochemistry, and monazite/xenotime LASS petrochronology provide new constraints on the P-T-t evolution of the metapelitic, migmatitic Hauser Lake Gneiss (HLG), which is exhumed in the footwall of the Priest River metamorphic core complex (PRC) in northern Idaho. Four HLG samples contain the peak metamorphic assemblage Grt + Ky + Bt + Pl + Or ± Rt ± Ilm + Qtz, corresponding to peak P-T conditions of ~8.9-12.0 kbar and ~750-805°C. A fifth sample from the northern HLG, near the Cretaceous Kaniksu batholith, contains the peak assemblage Ky + Bt + Mu + Pl + Or + Rt + Ilm + Qtz. This sample contains prograde and retrograde Mu, while other samples contain retrograde Mu in trace quantities only, if at all. Peak P-T conditions for this sample are ~8.5-9.6 kbar and ~740-765°C, although mineral chemistry and thermobarometry suggest re-equilibration at lower P-T conditions. Weighted mean ²⁰⁸Pb/²³²Th monazite and xenotime dates for each sample suggest the thermal peak of metamorphism across the PRC occurred from 68.7 ± 0.5 Ma to 64.7 ± 0.4 Ma. Although major element garnet profiles are relatively flat, garnet REE geochemistry reveals increasingly negative Eu anomalies from core to rim in porphyroblasts from most samples. These zoned, strongly negative Eu anomalies (Eu_N/Eu_N* = 0.11-0.14) are consistent with production of peritectic garnet during muscovite dehydration melting at uppermost amphibolite to granulite grade. Monazite REE geochemistry reveals that monazite in most samples becomes HREE-enriched over time, suggesting monazite growth at the expense of garnet. Monazite HREE ratios (Yb_N/Gd_N) vs. age suggests that garnet breakdown followed peak metamorphism, commencing c. 62-54 Ma. The youngest monazite and xenotime analyses are from leucosome inclusions and matrix grains, and constrain the timing of melt crystallization at c. 54-44 Ma. P-T-t paths are constrained by petrographic observations, monazite ²⁰⁸Pb/²³²Th ages and REE geochemistry, as well as garnet and melt modal isopleths on pseudosections. These constraints, and the absence of cordierite or decompression reaction textures, indicate initial rapid isothermal decompression of ~4 kbar. Similar P-T paths along the ~41 km extent of the PRC suggest it was initially exhumed as a coherent package.