GEOCHRONOLOGY OF THE ALBION, RAFT RIVER, AND GROUSE CREEK MOUNTAINS AND IMPLICATIONS FOR THE ARCHEAN HISTORY OF WESTERN LAURENTIA

The Grouse Creek block— a relatively understudied Precambrian province with a small number of known exposures—once represented the western margin of Laurentia. Archean basement rocks of the Grouse Creek block are exposed in the Albion, Raft River, and Grouse Creek mountains of northwestern Utah and south-central Idaho. These ranges are one in a series of metamorphic core complexes that run from Canada to the southwestern United States. These rocks have experienced a complex deformation and magmatic history from the late Cretaceous through the Miocene as a result of Cordilleran orogenic events. While numerous studies have focused on characterizing the more recent deformation history associated with alternating periods of contraction and extension in the Grouse Creek block, few have analyzed the Precambrian cores of these complexes.

In this study, we present 11 new U-Pb and Pb-Pb dates from rocks collected in the Albion, Raft River, and Grouse Creek ranges that will provide additional constraints on the history of the Grouse Creek block. Our results indicate Late Archean ages for samples collected from all 3 ranges and suggest that broad-scale magmatism occurred at ca. 2.6-2.5 Ga. Discordia trends present in all of the samples analyzed in this study indicate that the region suffered a pervasive Pb loss event during the Cretaceous. These results are consistent with previous ages reported in the region ^{[1] [2]} and may be evidence of a substantial heating event. Possible scenarios include the emplacement of a batholith-scale intrusion or a tectonically-driven increase in the basal heat flux. One sample, a schist from the Raft River mountains, produced a concordant 3.4 Ga date from what appears in CL imagery to be a core with a magmatic rim. The presence of this grain and a handful of slightly discordant (<10%) grains with similar ²⁰⁷Pb/²⁰⁶Pb dates from the same sample may indicate magma sourcing from an area containing older crust, but incorporation of material from the country rock during emplacement is more likely.

[1] A. Strickland et al. (2011) American Journal of Science. 311(4); [2] V. Isakson (2012) M.S. Thesis