WINDERMERE AND ASSOCIATED STRATA IN THE STIBNITE-EDWARDSBURG AREA, CENTRAL IDAHO

Geologic mapping and LA-ICPMS detrital zircon age results reported here build on earlier studies by Lund et al. (2003 GSAB) that identified Neoproterozoic Windermere-equivalent strata in central Idaho. Although complexly folded and faulted, and locally bearing garnet, sillimanite, and andalusite, we estimate that over 4000 meters of Neoproterozoic and lower Paleozoic strata are preserved within this remote region. SW-facing strata near Edwardsburg include a thick lower quartzite (Square Mountain Formation; ~1350 m), higher thin quartzite of the Moores Station Formation (~100 m), and metavolcanic rocks of the 685 Ma Edwardsburg Formation sandwiched in between. A sample from the base of the Square Mountain Formation contains abundant 1770-1650 Ma zircons, a smaller syn-Belt-Purcell population (1480-1400 Ma), and eight 1370-1300 Ma zircons. The upper quartzite is similar, but includes two 700-600 Ma grains. Sources for both quartzites may be Belt-Purcell strata. The age of the lower quartzite is constrained only to 1300-685 Ma so could be Mesoproterozoic. Fifteen km to the south (and up section), the feldspar-bearing quartzite of Profile Creek contains mostly 1200-1000 Ma zircons, along with syn-Belt grains. Similar results, but with an added component of 2600-2500 Ma Archean grains, were obtained from a quartz pebble conglomerate low in the section at Stibnite, 10 km to the SE of Profile Creek. These two quartzitic units share zircon provenance with the Neoproterozoic Caddy Canyon quartzite of the Brigham Group in SE Idaho. In contrast, quartzite high in the section at Stibnite (upper quartzite of Smitherman, 1985) contains mostly 1860-1800 Ma zircons, lacks the 1200-1000 Ma grains, and may be Ordovician (Kinnikinic or Eureka quartzites). The nearby quartzite of Sugar Mountain contains mostly1860-1760 Ma zircons and resembles Cambrian Flathead Quartzite of Montana, the Cambrian portion of the Brigham Group, and the Addy Quartzite of NE Washington. The much greater variation of the detrital zircon data than typical of the Mesoproterozoic Belt-Purcell Supergroup exposed to the east is promising and additional data will assist with regional correlations that have been difficult because of the structural complexity and intrusive history of central Idaho.