Linking thermodynamic modelling, Lu-Hf geochronology and trace elements in garnet: new P-T-t paths from the Sevier hinterland

Major element, trace element and Lu-Hf geochronological data from amphibolite facies pelitic schist in the Raft River and Albion Mountains of northwest Utah and southern Idaho indicate that garnet grew during increasing pressure, interpreted to be the result of tectonic burial and crustal thickening during Sevier orogenesis. Garnet growth was interrupted by hiatuses interpreted from discontinuities in major element zonation. Pressure-temperature paths were determined from the pre-hiatus portions of the garnet chemical zoning profiles and indicate an increase of 2kbar and 50 degrees C in the western Raft River Mountains. Garnet Lu-Hf dates of 150 +/- 1Ma in the western Raft River Mountains and 138.7 +/- 0.7Ma and 132 +/- 5Ma in the southern Albion Mountains indicate the timing of garnet growth. Lutetium garnet zoning profiles indicate that the Lu-Hf ages are biased towards the post-hiatus or outer pre-hiatus segments, indicating that the determined ages likely post-date the recorded P-T path history or date the tail end of the paths. Crustal thickening associated with Sevier orogenesis in the western Raft River Mountains thus began slightly before 150 +/- 1Ma, in the Late Jurassic. This study shows that integrating P-T paths determined from garnet growth zoning with Lu-Hf garnet geochronology and insitu garnet trace element analyses is an effective approach for interpreting and dating deformation events in orogenic belts.