MESOPROTEROZOIC(?) TO PALEOZOIC STRATA IN THE STIBNITE-EDWARDSBURG AREA, CENTRAL IDAHO

Geologic mapping and detrital zircon analysis by LA-ICPMS reported here builds on earlier stratigraphic and geochronologic studies by Lund et al. (2003) that identified Neoproterozoic Windermere-equivalent strata in central Idaho. SW-facing strata near Edwardsburg, 60 km NE of McCall, include a thick lower quartzite (Square Mountain Formation; ~1350 m) and, higher in the section, quartzite of the Moores Station Formation (~100 m). Metavolcanic rocks of the 685 Ma Edwardsburg Formation and calc-silicate rocks are present between the quartzite units. A sample from the base of the Square Mountain Formation contains abundant 1770-1650 Ma zircons, a smaller syn-Belt population (1480-1400 Ma), and eight zircons yielding 1370-1300 Ma ages. The upper quartzite is similar, but includes two 700-600 Ma grains. Both quartzite units may represent reworking of Belt strata. The age of the lower quartzite is only constrained to 1300-685 Ma and thus may be Mesoproterozoic. Fifteen km to the south (and up section?), the 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. 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 largely 1860-1800 Ma zircons, lacks the 1200-1000 Ma grains, and may be Ordovician (Kinnikinic Quartzite?). The nearby quartzite of Sugar Mountain contains largely1860-1760 Ma zircons and resembles Cambrian Flathead Quartzite and the Cambrian portion of the Brigham Group. The presence of rare fossils in calcareous strata in the Stibnite area is additional evidence of an early Paleozoic age for at least some of the rocks. Overall, the detrital zircon data show much greater variability here than in the Mesoproterozoic Belt Supergroup exposed to the east in the Salmon area, but similarity to the Bayhorse assemblage near Clayton. Neoproterozoic and younger strata also show a systematic shift to older zircon ages up section, perhaps due to unroofing of progressively older rocks in the source area or a change in source areas over time.