THE NEOPROTEROZOIC MAGMATIC, RIFT AND GLACIAL HISTORY OF EASTERN IDAHO, FROM DETRITAL ZIRCON GRAINS IN HOLOCENE SANDS

From a database of about 3000 detrital zircon grains in Holocene fluvial systems, Neoproterozoic grains comprise 10% in the Big Lost River, 5% in the Portneuf River and <5% in the composite middle Snake River. Ages were obtained from reconnaissance 4-scan SHRIMP ion microprobe analyses. As only 1-2 microns of the sectioned grains have been ablated by the ion beam, higher-resolution U-Pb analyses of the same grain are possible if a population is of particular interest.

In the Big Lost River system (800 grains analyzed overall) the 80 Neoproterozoic zircons, with ages in the range 650 to 710 Ma, have been traced to the Pioneer Mountains core complex, composed of Eocene plutons and the Wildhorse gneiss, thought to be of Paleoproterozoic age. Arbitrary unmixing of these grains yields significant groupings at ca.655 Ma (16%), ca.675 Ma (22%), ca.705 Ma (30%), and ca.735 Ma (19%). Though the oldest of these ages is about 50 my younger than 780 to 760 Ma Gunbarrel dikes of the Wyoming craton, the ages do overlap with volcanic ages from the Pocatello Formation of SE Idaho (ca.670, 710 and 715 Ma) and the Edwardsburg Formation of central Idaho (ca.685 Ma). These zircons require that a hitherto unknown, and geologically complex, suite of Neoproterozoic metavolcanic or plutonic rocks, spanning multiple magmatic events, exists in the Pioneer Mountains core complex.

In the Portneuf River, which drains Neoproterozoic Pocatello Formation outcrops where known ages of 709 +/- 5 Ma and 667 +/- 5 Ma are found (Fanning and Link, 2004, Geology), single detrital grains in a 60 grain sample correspond exactly to those ages. Further grains of these ages could likely be found, with the analysis of more grains.

In the composite Middle Snake River system (1100 grains), Neoproterozoic grains make up less than 5% of the total, and show no clear relationship with respect to sources. Such grains are found in streams draining the Idaho batholith, suggesting derivation from assimilated wall rocks. Another possible source is recycling through Paleozoic or Mesozoic sandstones.

Neoproterozoic rift-related zirconiferous volcanic or magmatic rocks must have covered an extensive part of central Idaho, linking glacigene diamictites across the Idaho batholith.