T61 Neoproterozoic through Cretaceous Evolution of the North American Cordilleran Margin: Contrasting Tectonics, Paleogeography, and Paleoenvironments

Sunday, 18 October 2009
1:30 PM - 5:30 PM
B110/111/112

EVIDENCE OF GRENVILLE-AGE DEFORMATION AND METAMORPHISM IN BELT SUPERGROUP METAPELITES OF NORTHERN IDAHO

Paper number: T61-2
Program: Technical Sessions
Time: 1:35 PM – 1:50 PM

Authors

Abstract

Metapelites of the Belt Supergroup in northern Idaho preserve a complex polyphase deformational and metamorphic history that in earlier studies was attributed to Cretaceous tectonism due to the close proximity of the Cretaceous Idaho Batholith. Recent Lu-Hf geochronology of garnet-bearing metamorphic rocks, however, indicates a much older (Proterozoic) metamorphism. These ages include recent results from this study of 1084 ± 21 Ma and 1207 ± 48 Ma for pelitic schists near Middle Sister Peak and Junction Peak, respectively, and ages from previous studies ranging from 1006 ± 5 Ma to 1149 ± 4 Ma for metamorphic rocks in the Clearwater Complex and the Clarkia area (Zirakparvar et al., in review). Garnets in samples proximal to the batholith occasionally have Cretaceous overgrowths and, therefore, the timing relations of metamorphism and deformation needs to be reevaluated for the Belt Supergroup.

Microstructural analysis of garnet-fabric relationships combined with Lu-Hf garnet geochronology allows correlation of relative and absolute timing estimates for multiple deformational events. As many as 4 fabrics are observed in areas of exclusively >1.0 Ga garnet growth: 1) S1 compositional layering, possibly after original bedding, 2) the preferred orientation of elongate quartz, S2, preserved as an interior fabric within garnet, 3) S3 defined by the preferred orientation of planar matrix minerals which shares a syn-tectonic (syn-kinematic) relationship with garnet growth, and 4) a poorly developed crenulation cleavage, S4, which deforms S3 and post-dates garnet growth.

Previous studies in northern Idaho linked regional metamorphism and deformation to Cretaceous tectonism because of the Belt meta-sediment’s close proximity to the Cretaceous Idaho Batholith. The presence of regional 1.1 Ga tectonites in northern Idaho contradicts previous assumptions that western Laurentia was a tectonically passive margin during the construction of Rodinia.