Abstract

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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