IN SITU U-PB ZIRCON GEOCHRONOLOGY OF DEFORMED GRANITOIDS AND ORTHOGNEISSES IN THE MAIN SALMON GNEISS COMPLEX, IDAHO

The Salmon River wilderness in central Idaho contains vast expanses of deformed granitoids and gneisses of uncertain age and affinity. Recent reconnaissance zircon geochronology suggested that the main gneiss unit in the region is Late Cretaceous, comparable in age to the generally undeformed main phase of the Atlanta lobe of the Idaho batholith. Early Cretaceous through Triassic inherited zircons of uncertain origin were also found. Here, we present new laser ablation-inductively coupled plasma-mass spectrometry U-Pb zircon geochronology results for rocks from the eastern and northern portions of this complex.

Deformed granitoids along the Salmon River in the eastern portion of the complex yield crystallization ages between 70 and 80 Ma, but there is a great diversity of inherited zircon components, both with older Cretaceous ages (~80 to 110 Ma) and Mesoproterozoic ages (mostly between 1.3 to 1.4 Ga). The deformed rocks are cut by small relatively undeformed ~50 Ma plutons, indicating that deformation is likely Cretaceous or Paleocene in age.

Deformed granodiorites and tonalities from the northern portion of the complex have crystallization ages between 49 and 51 Ma and almost completely lack Cretaceous age components. Inherited zircon components in these northern samples are Paleoproterozoic and Neoarchean. The Eocene crystallization ages of the deformed plutons indicate that this deformation postdates the deformation that is seen in the east.

The Main Salmon gneiss complex is close to the well-studied Bitterroot metamorphic core complex, and the timing of the younger deformation event in the former may imply a relationship with the latter. Further U-Pb geochronology of zircon and monazite may provide additional insights into the complex structural history of the area. The highly differing and location-dependent zircon inheritance signatures suggest a complex subsurface crustal structure for the area.