THE MONAZITE RECORD OF METAMORPHISM AND CRUSTAL MODIFICATION OF THE WESTERN NORTH AMERICAN CRATON IN THE CLEARWATER COMPLEX OF NORTHERN IDAHO

The Clearwater metamorphic complex in northern Idaho contains rocks that record a wide variety of metamorphic, magmatic, and fluid-flow processes ranging in age from Mesoproterozoic to Eocene. Both the older and younger sequence of events occurred at similar P-T conditions (garnet-staurolite-kyanite grade), thus interpreting P-T histories is hampered by uncertainty in the timing of garnet growth relative to each particular metamorphic event. Using metamorphic assemblage phase diagrams, integrated with in situ laser ablation split stream (LASS) petrochronology of monazite, these complex tectonic histories can be constrained by obtaining U-Pb and REE data simultaneously for each monazite spot analysis. Within the Clearwater complex, all monazite grains that occur as inclusions in garnet are Mesoproterozoic, recording a wide span of ages from 1.48 to 1.30 Ga, and corroborates the Mesoproterozoic garnet ages that have been obtained across the area. The range of dates recorded in garnet inclusions encompasses the time period that includes the deposition, rifting, and subsequent crustal thickening of the Belt Supergroup protolith sediments. West of the complex, exclusively Mesoproterozoic monazite dates are preserved in the hanging wall of the White Rock fault. East of the complex, Mesoproterozoic dates as inclusions in garnet and staurolite porphyroblasts are preserved, however matrix monazite grains record protracted Cretaceous tectonism. Coupled with geochemical and P-T data, these data indicate that a significant and protracted regional Mesoproterozoic metamorphic event affected a large area of rocks along the northwestern margin of the North American craton at 1.38-1.30 Ga. In addition, portions of the complex were strongly overprinted by both Cretaceous regional metamorphism and Eocene extension and associated magmatism. Taken together, these results are consistent with previous interpretations of early garnet and staurolite growth in the Mesoproterozoic related to rift-magmatism metamorphism at ~1.45 and crustal thickening at 1.38-1.30 Ga, followed by protracted Cretaceous regional crustal thickening, and fluid activity related to Eocene plutonism at 46-49 Ma in the core of the complex.