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

Abstract

The Clearwater metamorphic core complex (CMCC) in northern Idaho contains a rare exposure of the contact of the basal section of the Mesoproterozoic Belt Supergroup with underlying Paleoproterozoic basement. Prior to this study, this contact had 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. Detrital zircon analysis of five coarsely recrystallized, clean quartzite samples at this contact 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 1597 ± 37 Ma. These quartzites may correlate to the Laurentian-sourced Neihart formation in Montana, given their stratigraphic position between Paleoproterozoic basement and basal Belt Supergroup. The difference in detrital zircon signatures is likely a refection of the variability in sediment provenance across the Belt basin. The detrital zircon signature of a pre-Belt, locally derived metapelite yields a primary peak at ~1900 Ma and a secondary peak at~ 2600 Ma, with the youngest grain being 1706 ± 99 Ma. 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 preserves Proterozoic metamorphism with 75 ~1.4 - 1.3 Ga grains and 5 ~1.1 Ga grains. 118 monazite within kyanite and staurolite porphyroblasts, matrix monazite, and monazite separates record a strong overprinting at 80-60 Ma, correlative to a ~65 Ma felsic pegmatite adjacent to a dominantly Cretaceous pelite. Proterozoic metamorphism is best preserved within the orthoamphibolite, whereas the pelites are dominantly Cretaceous. Re-equilibration of these rocks during Cretaceous metamorphism occurred at upper amphibolite conditions. Phase equilibria modeling likely reflects Cretaceous metamorphism during Sevier crustal thickening and associated magmatism, 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 BSG with Paleoproterozoic basement, in addition to providing resolution of the Mesoproterozoic to Cretaceous metamorphic evolution of the CMCC.