DECIPHERING P-T-T HISTORIES IN THE CLEARWATER COMPLEX, NORTHERN IDAHO, THROUGH INTEGRATED PHASE EQUILIBRIA MODELING, LU-HF GARNET GEOCHRONOLOGY, AND MONAZITE PETROCHRONOLOGY

Precambrian rocks of the Clearwater core complex in northern Idaho record the Proterozoic assembly and modification of northwestern Laurentia. In addition, these rocks preserve a younger metamorphic history during Cretaceous crustal thickening, magmatism, and Eocene extension. Each of these metamorphic events occurred at amphibolite facies conditions, thus interpreting P-T-t histories is hampered by uncertainty in the age of porphyroblast growth relative to each metamorphic event. This study presents results of pseudosection modeling and garnet compositional isopleth thermometry from pelitic rocks across the western extent of the complex in order to decipher the complex metamorphic history preserved in these rocks. These results are integrated with in situ U-Pb monazite geochronology and Lu-Hf garnet geochronology. The polymetamorphic history is best preserved in multi-stage garnet microstructures with diffuse high Ca – low Mn rims, rutile converting to ilmenite, pre-kinematic staurolite, and abundant andalusite and sillimanite after kyanite, particularly in rocks within Eocene shear zones, which preserve 48-46 Ma ages. Lu-Hf garnet ages and monazite armored in garnet cores preserve the Proterozoic event with dominantly ~1.4 - 1.3 Ga ages. Monazite inclusions in kyanite and staurolite porphyroblasts, and matrix monazite record a strong overprinting at 80-60 Ma. Phase equilibria modeling likely reflects Cretaceous metamorphic conditions, with peak conditions of ~7-8 kbar and ~660°C. The results of this research offer insights into the variable nature of the contact of basal Belt Supergroup with Paleoproterozoic basement, in addition to providing resolution of the Mesoproterozoic to Cretaceous metamorphic evolution of the Clearwater complex. Taken together, these results are consistent with previous interpretations of early garnet growth in the Mesoproterozoic, protracted Cretaceous crustal thickening, and a significant overprinting in the high grade core of the complex during core complex exhumation and associated magmatism.