SHRIMP U-PB AGES FOR ARCHEAN ORTHOGNEISS, MESOPROTEROZOIC PARAGNEISS, AND EOCENE BOULDER CREEK PLUTON, PIONEER MOUNTAINS, SOUTH-CENTRAL IDAHO, PART OF THE 2600 MA GROUSE CREEK BLOCK

We report new SHRIMP U-Pb zircon ages that dictate that the Wildhorse Gneiss Complex of the Pioneer Mountains, Idaho is Archean, and thus likely continuous with the Grouse Creek Terrane south of the Snake River Plain. The Wildhorse Gneiss contains 2600-2700 Ma Archean orthogneiss, and structurally overlying Mesoproterozoic (<1460 Ma) paragneiss intruded by Neoproterozoic (695 Ma) orthogneiss. The gneisses are intruded by 49.00 +/- 0.39 Ma monzonite of the Boulder Creek pluton.

One of the structurally lowest units of the Wildhorse Gneiss Complex is a quartz-biotite-feldspar orthogneiss in the Boulder Creek drainage that yields upper concordia intercept ages of 2608 ± 17 and 2674 ± 9 Ma. These late Archean ages overlap those of the Grouse Creek Terrane to the south of the Snake River Plain, and older than any from the Selway Terrane in southwestern Montana (Foster et al., 2006, CJES). The contact between the Archean Grouse Creek terrane and the Paleoproterozoic Great Falls Tectonic Zone must be north of, or structurally within, the Pioneer Mountains.

Structurally above the Archean orthogneiss are calc-silicate and amphibolitic paragneisses. We obtained SHRIMP U-Pb ages of 1460 to 1850 Ma on detrital zircons from three samples of calc-silicate and quartzitic metasedimentary rocks. Thus, the rocks must have been deposited after 1460 Ma, and be either Meso- or Neoproterozoic in age. This detrital-zircon age mimics the population distribution in the lower Belt Supergroup, the suggested protolith. No Grenville-age (1200 to 1000 Ma) grains are present, as would be expected in a Neoproterozoic sandstone.

In the Boulder Creek drainage a composite monzonite and pyroxenite pluton intrudes all these map units. We obtained a U-Pb SHRIMP mean Pb/U age of 49.0 ± 0.4 Ma for magmatic non-xenocrystic zircons from this rock. This likely represents the age of the Pioneer Mountains pluton, intruded during the Challis magmatic episode.

Since no Cretaceous zircon ages are present either in detrital grains of the Big Lost River drainage or in our U-Pb studies, the Pioneer core complex must have only reached 650 degrees at 50 Ma, despite the existing ~70 Ma K-Ar and Ar-Ar cooling ages.