CONSTRAINING THE PROTEROZOIC GROWTH AND MODIFICATION OF THE WESTERN NORTH AMERICAN CRATON IN NORTHERN IDAHO USING MONAZITE AND XENOTIME PETROCHRONOLOGY

Comprehensive understanding of tectonometamorphic processes requires an integrated approach to investigating P-T-t histories. Metamorphic terranes often preserve a variety of mineral microstructures and assemblages as well as dateable accessory phases that provide an opportunity to evaluate the tectonic evolution and timescales of geologic processes for a particular region. However, the common approach of obtaining geochronologic data from mineral separates is often hampered by the absence of a petrographic context, as well as the uncertainty in knowing exactly what event or portion of a P-T path is being dated. How accessory phase growth links to major phase reactions, such as the growth of garnet, is a critical piece of information in unraveling the history of metamorphic rocks with polyphase histories. The Clearwater area in northern Idaho contains rocks that record a wide variety of metamorphic, magmatic, and fluid-flow processes ranging in age from Mesoproterozoic to Eocene. Using isochemical phase diagrams, integrated with in situ laser ablation split stream (LASS) petrochronology of monazite and xenotime, these complex tectonic histories can be constrained. Within the Clearwater complex footwall, all monazite and xenotime grains that occur as inclusions in garnet are Mesoproterozoic, typically ranging in age from 1.48 to 1.30 Ga. This range of dates spans the deposition, rifting, and subsequent burial metamorphism and/or crustal thickening of the Belt Supergroup protolith sediments. Scarce monazite inclusions in kyanite yield 1.1 Ga ages. Within the western hanging wall, exclusively Mesoproterozoic Ar-Ar biotite and U-Pb monazite dates are preserved on the west side of the White Rock fault. The eastern hanging wall preserves Mesoproterozoic dates as inclusions in garnet and staurolite porphyroblasts, but matrix monazite grains record protracted Cretaceous tectonism. Coupled with geochemical and P-T data, these data indicate that a significant and protracted Mesoproterozoic metamorphic event affected a large region of rocks along the northwestern margin of the North American craton at 1.38-1.30 Ga.