U-Pb geochronology of two augen gneiss terranes, Idaho - new data and tectonic implications.

The augen-gneiss of Red River (Idaho) is interpreted as being approx 1370 m.y. old. U-Pb systematics of cogenetic zircons indicate at least one component of inherited radiogenic lead. Field relationships indicate some intense deformation of the host rocks prior to or synchronous with intrusion of the augen-gneiss of Red River. The augen-gneiss of Priest River is 1576 + or - 13 m.y. old, there being no indication of inherited radiogenic lead. The Priest River augen-gneiss probably forms a pre-belt basement that is either tectonically interleaved with belt strata, unconformably underlies belt strata, or both. In any case, the age of 1576 m.y. is probably a maximum for deposition of the Prichard formation.-J.M.H.