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New U–Pb zircon dates on diabase, diorite, and migmatites within a large magmatic complex in east-central Idaho have refined the age and tectonic setting of the East Kootenay orogeny that affected the Belt basin ca. 1370 Ma. These data show that a large volume of mafic magma was injected into the basin in east-central Idaho and followed shortly thereafter by its own felsic differentiate and granitic plutons and metamorphism of the host sediments ca. 1370 Ma. These data show that the magmatic complex and associated gneisses in east-central Idaho are not pre-Belt basement, but contemporaneous in age with the Belt basin. Nd isotopic analysis of the magmatic rocks establishes that they are not derived from known Proterozoic or Archean basement terranes, but could have formed from the host Yellowjacket Formation or juvenile 1.7 Ga crust. Nd isotopic composition of gneisses and the Yellowjacket Formation are interpreted to support previous correlations between these rocks and the Belt Supergroup. Metamorphic barometry on 1370 Ma migmatites intercalated with the magmatic complex constrain the metamorphism to pressures of 450 MPa (14 km) initially and show that pressure increased to 650 MPa (20 km) before the end of metamorphism, which is consistent with magma intrusion into the bottom of the basin, followed by basin subsidence and sediment loading. We postulate that the East Kootenay orogeny is a pulse of bimodal magmatism, basin rifting, and renewed subsidence and sedimentation that shortly preceded the end of deposition in the Belt basin.

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a crustal-scale extensional horse that was stranded as continued extension moved the underlying metamorphic infrastructure out from beneath it and toward the west along the master detachment. This study shows that large tracts of midcrustal rocks can be translated and stranded as allochthonous fragments during continental crustal extension.

Precambrian basement rocks exposed within tectonic windows in the North American Cordillera help to define the Precambrian crustal structure of western North America and possible reconstructions of the Late Proterozoic supercontinent Rodinia. New geologic mapping and U-Pb dating in the infrastructure of the Priest River metamorphic complex, northern Idaho, documents the first Archean basement (2651 ± 20 Ma) north of the Snake River Plain in the North American Cordillera. The Archean rocks are exposed in the core of an antiform and mantled by a metaquartzite that may represent the nonconformity between basement and the overlying Hauser Lake gneiss, which is correlated with the Prichard Formation of the Belt Supergroup. A structurally higher sheet of augen gneiss interleaved with the Hauser Lake gneiss yields a U-Pb zircon crystallization age somewhat greater than 1577 Ma. The slivers of augen gneiss were tectonically interleaved with the surrounding Hauser Lake gneiss near the base of the Spokane dome mylonite zone, which arches across this part of the Priest River complex. We conclude that the Spokane dome mylonite zone lies above the Archean basement-cover contact and that it was, in part, equivalent to the basal décollement of the Rocky Mountain fold and thrust belt. New U-Pb dates on metamorphic monazite and xenotime reveal peak metamorphism at ca. 72 Ma, compatible with movement along the Spokane dome mylonite zone at that time. The Archean basement could be interpreted as the western extension of the Hearne province, or a new Archean basement terrane separated from the Hearne province by an Early Proterozoic suture. The unique assemblage of 2.65 Ga basement, ~1.58 Ga felsic intrusive rocks, and the Middle Proterozoic Belt Supergroup can be used as a piercing point for the identification of the conjugate margin to Laurentia. Our new dating supports previous correlations of Australia's Gawler craton (2.55-2.65 Ga) and its 1590 Ma plutons with the Priest River complex basement gneisses. The Priest River complex basement may be a piece of eastern Australia stranded during rifting of the supercontinent Rodina in the Late Proterozoic.