DISTINCTIVE DETRITAL ZIRCON POPULATIONS IN LOWER PALEOZOIC STRATA OF IDAHO (Invited Presentation)

Paleozoic sandstones of the Idaho thrust belt and adjacent North American craton have distinctive zircon age-groupings that record a change from western (Cordilleran) to eastern (Appalachian) provenance in the late Paleozoic. The Transcontinental Arch rose in Early Cambrian time, and supplied 1600-1800 Ma detrital zircons while preventing Appalachian grains from reaching the Cordilleran miogeocline until the Pennsylvanian. Distinctive western Laurentian zircons found in Neoproterozoic and lower Paleozoic strata include: a) 700-660 Ma syn-rift volcanic grains sourced from northern UT, Pocatello, or Edwardsburg, ID; b) 1790 Ma magmatic grains found in the Brigham Gp and Flathead Ss., and with a source not constrained. c) 490 Ma Beaverhead pluton grains found in the Upper Cambrian Worm Creek Mbr of the St. Charles Fm. These 490 Ma grains, found from the western thrust belt to the craton, suggest rejection of the Rubia terrane model (Hildebrand, 2009) for the Idaho Cordillera. d) Most Ordovician sandstones (Kinnikinic and Swan Peak Fms) contain >1850 Ma and 2100 to 2000 Ma grains, with an interpreted source in the Peace River Arch of NW Canada. Two Ordovician sandstones in central ID near Clayton, however, contain Grenville-aged grains and may represent feeders for submarine fan sandstones of the lower Vinini Fm. in NV (Linde et al., 2016). By Middle to Late Devonian time, sinistral transcurrent faulting along western Laurentia resulted in the southwards transport of early Paleozoic arc terranes of northern Caledonian affinity, and perhaps some parts of the Roberts Mountains allochthon, that were broadly part of the Antler orogenic belt. Western-derived strata of the Upper Devonian Milligen Fm contain 430-450 Ma zircons with evolved ε_Hf(t) of -27 to -6, mostly likely sourced from basement units of the newly accreted E Klamath, N Sierra, or Quesnellia terranes. The poorly-constrained late Paleozoic switch from western to eastern Laurentian provenance signatures is first recognized in Pennsylvanian Antler overlap strata of the Sun Valley Gp and Tensleep Ss on the Wyoming Craton. These grains have ε_Hf(t) of 15 down to -10 and are interpreted to have a more juvenile eastern Appalachian source than the early Paleozoic detrital zircons in the western-derived Milligen Fm. This zircon stream breached the Transcontinental Arch.