NEOPROTEROZOIC 695 MA FELSIC ORTHOGNEISS, WILDHORSE CREEK, PIONEER MOUNTAINS, SOUTH-CENTRAL IDAHO: NEW TIE POINT IN RECONSTRUCTION OF RODINIAN RIFTING

In the Big Lost River drainage, E. Idaho, a “ghost” Neoproterozoic (650-740 Ma) detrital zircon population makes up 10% population of zircons in Holocene sands. The origin for the “ghost grains” is a 695 Ma felsic orthogneiss exposed in Wildhorse Creek of the Pioneer Mountains. Three samples yield SHRIMP U-Pb zircon upper concordia intercept ages of 692.3±5.2, 695.7 ± 8.0, and 696.5±9.0, Ma. The zircons analyzed were clear, elongate-prismatic oscillatory zoned grains with an average length of 250 microns that are interpreted to have an igneous paragenesis.

These magmatic zircon grains range from concordant, or within uncertainty of Concordia to ~15% discordant with 206Pb/238U ages as low as ~600 Ma. This range duplicates the spread of ages recorded by detrital zircon in the Holocene sands and explains why some detrital zircon distributions (defined by single 206Pb/238U age analyses) are broader than what we know to be the precise age of their magmatic source.

The Neoproterozoic orthogneiss was previously mapped as Paleoproterozoic based on a Rb-Sr whole-rock age from the Wildhorse gneiss complex of about 2 Ga (Zartman, in Dover, 1983). The orthogneiss intrudes Meso- or Neoproterozoic paragneisses that contain detrital zircons with ages from 1460 to 1850 Ma.

Since the Neoproterozoic orthogneiss is 695 Ma and contains the same metamorphic fabric as Archean orthogneiss, the metamorphism of the entire Wildhorse gneiss complex must be younger. It reached its peak at about 50 Ma, during the Challis magmatic episode, and with the intrusion of the 49 Ma Pioneer Mountains pluton. On many Neoproterozoic zircon grains bright cathodoluminescent rims, less than 5 microns thick, and clear magmatic zircon grains from felsic stringers in migmatitic gneiss represent new Eocene zircons.

The discovery of this Neoproterozoic orthogneiss establishes four locations in Idaho (Pocatello, House Mountain, Edwardsburg, Pioneer Mountains) where Sturtian (750 to 660 Ma) magmatism likely related to Rodinian rifting, is recognized. These plutons provide tie points for reconstructions of the late Neoproterozoic western margin of Laurentia.