SALMON RIVER ARCH REVISITED - NEW EVIDENCE FOR 1370 MA RIFTING NEAR THE END OF DEPOSITION IN THE MIDDLE PROTEROZOIC BELT BASIN

New U-Pb zircon dates on diabase, diorite, and migmatites within a lar ge magmatic complex in east-central Idaho have refined the age and tec tonic setting of the East Kootenay orogeny that affected the Belt basi n 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 the reafter by its own felsic differentiate and granitic plutons and metam orphism of the host sediments ca. 1370 Ma. These data show that the ma gmatic complex and associated gneisses in east-central Idaho are not p re-Belt basement, but contemporaneous in age with the Belt basin. Nd i sotopic analysis of the magmatic rocks establishes that they are not d erived from known Proterozoic or Archean basement terranes, but could have formed from the host Yellowjacket Formation or juvenile 1.7 Ga cr ust. Nd isotopic composition of gneisses and the Yellowjacket Formatio n 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 p ressures of 450 MPa (14 km) initially and show that pressure increased to 650 MPa (20 km) before the end of metamorphism, which is consisten t 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 su bsidence acid sedimentation that shortly preceded the end of depositio n in the Belt basin.