GEOLOGIC MAPPING OF NEOPROTEROZOIC TO ORDOVICIAN STRATA IN THE LOST RIVER AND LEMHI RANGES AND SALMON RIVER MOUNTAINS, CENTRAL IDAHO

Thick Neoproterozoic to Ordovician rift and early passive margin strata occur along much of the western margin of Laurentia. Though thin and riddled with unconformities, these rocks are not absent between the eastern Snake River Plain and west-central Idaho, as previously thought. We correlate unfossiliferous quartzites from this region based on regional patterns of detrital zircon (DZ) age populations and baddeleyite geochronology. Stratigraphic relations of DZ populations are consistent south to Pocatello and north to Edwardsburg. Both regional recycled and local first-cycle DZ populations are recognized and provide paleogeographic constraints.

The deepest studied strata are exposed in the Bayhorse anticline, above a 668 Ma tuff. This is the Ramshorn Slate, consisting of 1.5 km of argillaceous and dolomitic rocks with prominent Grenville (Stenian ~1.0-1.2 Ga) DZ populations. The upper Ramshorn was intruded by ca. 600 Ma (Ediacaran) gabbroic sills and overlain by the Clayton Mine Qzte, which we interpret to correlate with the Wilbert and Tyler Peak fms from the southern Lemhi and Lost River ranges. In the base of the Clayton Mine are local major 680-650 Ma DZ populations (e_Hf(i) of -2 to +8), interpreted to be products of remelted ~1380 Ma lithosphere. These grains may have been derived from regional 680 Ma rift-related mafic volcanic rocks (Edwardsburg-Bannock magmatic pulse) or the proximal hypabyssal felsic 650 Ma Big Creek plutonic belt. We place the Precambrian-Cambrian boundary in the middle Clayton Mine Qzte because Grenville-aged zircons disappear above this, as they do in the Tyler Peak, which contains Cambrian trilobites. Paleoproterozoic DZ ages predominate and include a sharp, magmatic zircon peak at 1780 Ma with negative e_Hf(i) of -5 to 0.

In several localities, Ordovician and Upper Cambrian sandstones of the Summerhouse Fm locally contain Grenville-aged grains, a few 660 Ma grains, and locally abundant 480 Ma grains with positive e_Hf(i) of 0 to 7 derived from the Beaverhead Plutonic suite.

The unconformably overlying Kinnikinic Qzte has a completely different DZ age assemblage: there are no local (680-650 and 480 Ma) grains, no Grenville grains, and no 1780 Ma grains. Instead, the youngest grains form a prominent 1860 Ma peak, interpreted to represent provenance far to the north in the Peace River arch.