ARCHEAN TO PALEOZOIC PROTOLITH AGES FOR WILDHORSE GNEISS, AND HYNDMAN AND EAST FORK FORMATIONS OF THE MIDDLE STRUCTURAL PLATE, PIONEER METAMORPHIC CORE COMPLEX, IDAHO

The Neoarchean to Neoproterozoic Wildhorse Creek gneiss occupies the core of the lowest structural plate in the Pioneer metamorphic core complex of south-central Idaho. The Wildhorse complex contains 3 or 4 locally migmatitic paragneiss units, Neoarchean orthogneiss, and Neoproterozoic orthogneiss. Some quartzites in the structurally lowest paragneiss have multiple concordant populations of detrital zircons as young as 1700 Ma; others have no grains younger than 2600 Ma. This Proterozoic lower paragneiss is structurally overlain by Neoarchean 2600-2700 Ma orthogneiss and the highly strained, heterogeneous middle paragneiss. New U-Pb data from amphibolite in this unit suggest it was emplaced or metamorphosed at ~1830 Ma. Metapsammite contains detrital zircons with rims as young as 1460 Ma. The upper unit is quartzo-feldspathic paragneiss that contains quartzites with zircons as young as 1400 Ma. The middle and upper paragneiss units are intruded by 695 Ma orthogneiss (now structurally concordant lenses), the source of the “ghost” zircons in the Big Lost River. This granitic rock is coeval with Neoproterozioc rift-related volcanic or intrusive rocks elsewhere in Idaho.

The ~48 Ma Pioneer intrusive suite separates the Wildhorse Gneiss below from the overlying middle structural plate of the core complex. The middle plate includes imbricated Hyndman Fm. (unnamed quartzite and pelite units plus Ordovician Clayton Mine quartzite) and overlying East Fork Fm. (Ella, Kinnikinic, and Saturday Mountain units). The Clayton Mine and older units contain 1000-1200 Ma, 1600-1700 Ma, and sparse Archean detrital zircons, requiring a Neoproterozoic or Paleozoic age. Their nearest correlatives are near Bayhorse, 40 km north. The Kinnikinic contains grains > 1800 Ma, as it does elsewhere in ID, with a peak at 1820 -1860 Ma.

We interpret the Neoarchean orthogneiss to be the northern extension of the Grouse Creek Terrane of the Albion and Raft River mountains. Older-over-younger relations suggest the Archean orthogneiss was thrust over the lower paragneiss during the Antler or Sevier orogenies. The upper paragneiss may be the southernmost exposure of Lemhi sub-basin of the Mesoproterozoic Belt Supergroup. Detrital zircon ages from the paragneiss units permit multiple pre-Cretaceous basin stratigraphy configurations.