Abstract

Radiometric dating of zircon indicates a 1525± 82 m.y. age of crystallization for augen gneiss that intrudes metasedimentary rocks (near Elk City, Idaho) correlated with the Prichard Formation of the Belt Supergroup. The augen gneiss appears to be late synkinematic with the first deformation and metamorphic event in the metasedimentary rocks. Therefore, the first deformation and metamorphism are also Precambrian in age. Diopside-gneiss-bearing rocks on the Lochsa River, Idaho, correlated to the Wallace Formation of the Belt Supergroup, have a structural history similar to the rocks intruded by augen gneiss. Therefore, the diopside-gneiss-bearing rocks also were metamorphosed at 1525 m.y.

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