TECTONIC AND MAGMATIC EVOLUTION OF CENTRAL IDAHO: PRELIMINARY RESULTS OF THE IDOR PROJECT
The first phases of IDOR have concentrated on geochronology, geochemistry, structural geology, and field mapping; broadband seismometers were installed in summer 2011 and active source seismic studies are planned in summer 2012. Geochronological (U/Pb) analysis of plutons in the region indicates the presence of distinct magmatic events, which occurred in different tectonic environments, with the main phase of the Idaho batholith (peraluminous granites of the Atlanta lobe) forming between 83 and 67 Ma. Geochronology of xenocrystic zircons indicates that the southern Atlanta Lobe came through Archean crust, whereas the northern part came through Proterozoic (?) crust; a younger Eocene dike swarm (Boise Basin dike swarm) occurs along (exploits?) this boundary. Geochemistry on the igneous rocks indicates little or no signature of mantle contribution, and thus this phase of the Idaho batholith may occur solely through crustal melting. The main phase of Idaho batholith construction is pre-dated by arc magmatism (some of which occur simultaneous with movement on the WISZ) and is post-dated by more localized crustal melting in the Bitterroot lobe and the Challis magmatic province, which locally contains a clear mantle component.
Recent field mapping combined with U/Pb zircon geochronology on the WISZ has concentrated in the West Mountains, where the active seismic line crosses the shear zone. Similar fabrics (vertical foliation, vertical lineation, gneissic fabric) occur here as elsewhere in the WISZ, and indicate dextral transpressional deformation. In this locality, onset of the WISZ is constrained by a 101 Ma undeformed pluton located immediately west of the shear zone. We interpret these relations to constrain the WISZ to be active between 101-91 Ma. A syn-deformational tonalitic sill (Payette River tonalite) intrudes in this location as well as along the known extent of the shear zone.