GRENVILLE-AGE METAMORPHISM ON THE WESTERN MARGIN OF LAURENTIA, NORTHERN IDAHO: EVIDENCE FROM LU-HF GARNET GEOCHRONOLOGY

Lu-Hf geochronology of garnet-bearing amphibolite facies rocks in northern Idaho reveal a complex multistage metamorphic history. We determined Lu-Hf ages on amphibolite and pelitic schists from the Clearwater complex of northern Idaho and on a pelitic schist more proximal to the Cretaceous Idaho batholith. The core and rim of the garnet amphibolite have Lu-Hf ages of 1149±4 and 118±4 Ma, respectively. Very high Lu/Hf ratios in the garnet core (176Lu/177Hf up to 17.83) result in extraordinary present day 176Hf/177Hf ratios as high as 0.688932 (epsilon Hf= +13,656). Grenville ages of 1056±57 and 1006±5 Ma are recorded in the pelitic schists samples. In contrast, the pelitic schist near the Idaho batholith yields a well-defined age of 89.6±2.6 Ma.

The ~1.1 Ga metamorphic ages combined with polyphase penetrative structures observed in metamorphic tectonites and metasedimentary rocks north of the Idaho batholith strongly suggest that Grenville-age tectonism in the northwestern U.S. Cordillera was widespread and reflects a period of Proterozoic crustal thickening prior to or during the assembly of Rodinia. Thus Early and Middle Proterozoic basins of western Laurentia may record deposition and contraction in an intracontinental setting or may have developed during a history of both passive margin and convergent margin tectonism. The younger Lu-Hf ages are consistent with widely observed evidence for Cretaceous metamorphism in the region.

The presence of domains within a single garnet that record ages differing by over a billion years clearly supports a complex tectonic history for these rocks. This juxtaposition of ages demonstrates the potential of the Lu-Hf isotope system in garnets to see through younger metamorphic overprints and resolve an earlier metamorphic history. Taken together with regional structural analysis, this work illustrates the utility of the Lu-Hf system in dating complex, polymetamorphic histories associated with regional deformation.