INSIGHTS INTO THE PROTEROZOIC TO CRETACEOUS EVOLUTION OF THE CLEARWATER METAMORPHIC CORE COMPLEX, NORTHERN IDAHO, THROUGH PETROLOGIC AND GEOCHRONOLOGIC ANALYSIS

The Clearwater metamorphic core complex (CMCC), northern Idaho, contains a rare exposure of the contact of the basal section of the Mesoproterozoic Belt Supergroup (BSG) with Paleoproterozoic basement. Marking both the structural and depositional contact is a clean quartzite unit. Prior to this study, this contact has not been well documented. In addition, the rocks within the CMCC preserve a complex history of metamorphism, including both Mesoproterozoic and Cretaceous-Eocene regional events. The objectives of this study are to: 1) establish the nature of the contact of the BSG with the underlying basement through geological mapping, petrographic analysis, and U-Pb detrital zircon analysis, and 2) constrain the P-T-t conditions of metamorphism through U-Pb monazite geochronology and P-T phase equilibria studies. Detrital zircon analysis of five coarsely recrystallized, clean quartzite samples reveal a bimodal age distribution with peaks at around 1800 - 1900 Ma and 2650 Ma. Several grains are younger than 1700 Ma, with the youngest being 1579 ± 16 Ma. These quartzites may correlate to the Laurentian-sourced Neihart formation in Montana, given their stratigraphic position between Paleoproterozoic basement and basal BSG. However, the presence of younger grains, though scarce, may indicate that the depositional ages are younger than the Neihart formation. Three pelites and one orthoamphibolite were selected for P-T-t analysis. U-Pb monazite analysis by LA-ICP-MS was conducted on monazite mineral separates, as well as grains within thin sections to preserve the petrographic context. Monazite armored in garnet cores in each sample preserves ages of ~1350-1400 Ma, whereas matrix monazite is almost uniformly Cretaceous (65-75 Ma). Metamorphism of these rocks occurred at upper amphibolite conditions, within garnet-kyanite-staurolite stability. Phase equilibria modeling likely reflects Cretaceous metamorphism during Sevier crustal thickening, with re-equilibration of these rocks at around 7-8 kbar and ~650°C. The results of this research offer insights into the variable nature of the contact of basal BSG with Paleoproterozoic basement, in addition to providing resolution of the Cretaceous metamorphic evolution of the CMCC.