Abstract

The Sacawee block is a narrow belt of Paleo- to Mesoarchean crust that extends for ∼70 km across the northern Granite Mountains. It is composed of the ∼3.3 Ga Sacawee orthogneiss, additional calc-alkalic and tonalitic orthogneisses, and the ∼2.86 Ga Barlow Gap Group. The Sacawee block basement is characterized by negative εNd values and Paleoarchean Nd crustal residence model ages. A broad east–west-trending zone of Neoarchean high strain, which is part of the Oregon Trail structural belt, transects the Sacawee block and was studied at two locations, the Beulah Belle Lake area and West Sage Hen Rocks. U–Pb analyses of magmatic zircon from a sheared amphibolite within the high-strain zone of the Beulah Belle Lake area constrain the age of the Neoarchean deformation to be later than 2688 ± 5 Ma. At West Sage Hen Rocks, metamorphic zircons in a sheared amphibolite provide a direct date on the shear zone of 2649 ± 2.8 Ma. These data, combined with similar ages of deformation from two other shear zones, are interpreted to suggest that the Neoarchean Oregon Trail structural belt is a pervasive feature of the Sacawee block and may represent a deformation front related to accretion. Multiple east–west-trending shear zones within the Sacawee block are evidence for tectonic modification of the crust between ∼2.65 and 2.63 Ga and horizontal convergence analogous to modern plate tectonics processes. The Sacawee block is either a rare exposure of ancient basement typical of that which originally underlay much of the Wyoming Province or it is an exotic block that was accreted to the core of the Wyoming Province in Neoarchean time.

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