ORDOVICIAN MAGMATISM AND ORPHANED MESOZOIC ACCRETED CRUST? STRANGE BEDFELLOWS ALONG THE SALMON RIVER WEST OF SHOUP, IDAHO

The Salmon River flows ~200 km between North Fork and Riggins, Idaho, through wilderness that has been geologically mapped and characterized only at the reconnaissance level. We mapped three new units and zircons from those units were dated by LA-ICP-MS U-Pb geochronology and analyzed for Hf isotopes.

The megacrystic granite Corn Creek orthogneiss has an Ordovician age of 483 +/- 7 Ma and initial εHf composition around -9. The age is consistent with a belt of Early Paleozoic to Neoproterozoic plutons running through the area (Lund et al., 2010); the Hf composition requires the involvement of Proterozoic or Archean continental crust

In contrast, a sample from the Dwyer Creek gneiss, a mixture of biotite tonalite orthogneiss and subordinate paragneiss, yielded an age peak at 72 Ma and a large number of other Late Cretaceous, Early Cretaceous, Jurassic, and Precambrian grains. The pre-100 Ma Mesozoic ages and positive εHf in Dwyer Creek zircons are found in the accreted Blue Mountains terranes but have no known equivalent in the autochthonous Idaho batholith region where the gneiss is found.

The Rainier Rapids stock, an undeformed biotite granodiorite pluton, contains a main zircon age population at 50.1 +/- 1.1 Ma but also zircons with the same Jurassic ages and Hf isotopic signatures as the Dwyer Creek gneiss. The main Eocene age cluster yields positive εHf values.

The zircon ages and Hf systematics of the Dwyer Creek orthogneiss and the 86 Ma Coolwater Ridge orthogneiss 85 km to the northwest are nearly identical. The latter is a sliver of accreted Blue Mountains crust tectonically inserted into Precambrian North American crust (Lund et al., 2008). The similarities suggest that the Dwyer Creek also is an allochthonous piece of Blue Mountains crust, and the zircon inheritance and Hf data for the Rainier Rapids stock suggest the latter is the product of melting that crust. If this is correct, allochthonous crust 100 km inboard of the Salmon River suture requires a new model for accretion of the Blue Mountains province, perhaps involving greater crustal imbrication similar to that shown in the Canadian Cordillera.