DETRITAL ZIRCON STUDIES OF THE PIONEER MOUNTAINS CORRECTLY SUMMARIZE GEOLOGIC RELATIONS MAPPED BY BETTY SKIPP AND COLLEAGUES OVER THE LAST 40 YEARS

The Pioneer Mtns in south-central Idaho contain all the elements of the Cordilleran orogenic belt, as delineated by Betty Skipp and USGS colleagues over the last 40 years. . The Pioneers straddle the late Dev. Antler orogenic front, contain the Mississippian Antler Copper Basin clastic wedge and the Pennsylvanian and Permian Sun Valley Gp. of the Oquirrh-Wood River basin. These Pz belts were telescoped by Mesozoic Sevier belt thrusting, intruded on the west by the 80 to 100 Ma Atlanta lobe of the Idaho batholith. All lie in the upper plate of the Eocene Wildhorse detachment fault, which bounds Paleoproterozoic gneiss, Paleozoic quartzite and marble and Eocene granite of the Pioneer Mtns core complex.

U-Pb detrital zircon studies (mostly 60-grain samples) of Holocene and Pleistocene streams, analyzed by SHIRMP at the ANU, provide an accurate summary of geologic relations in the various streams that drain the Pioneers. Specifically, west of the Pioneers, streams (Trail Creek, Big and Little Wood Rivers) draining the Antler allochthon (Milligen Fm.) and overlying Sun Valley Group contain small populations of 380 to 340 Ma grains that must be volcanic contributions from a western Antler arc. Also present are 1050 to 1250 Ma multiply recycled Grenville grains, 1600 to 1800 Ma grains likely recycled from the Belt Supergroup into the Sun Valley Group, and 2300 to 2450 Ma Paleoproterozoic zircons. Streams (Antelope Creek) draining the Antler flysch trough (Copper Basin Fm.) contain 1800 to 1900 Ma zircons recycled from the Peace River arch through Ordovician sands, and Archean zircons as old as 3600 Ma, with unknown ultimate source. The upper Big Lost River contain Paleoproterozoic grains from the gneiss core, a ghost population of 600 to 750 Ma Neoproterozoic grains whose source is unknown, and no Cretaceous grains, confirming the absence of Idaho batholith intrusions in the Pioneer Core complex.

Eocene Challis (52 to 45 Ma) magmatic grains are ubiquitous, again consistent with mapped geology. These studies demonstrate the power of using detrital zircons in streams to define regional geologic terranes.