GHOST NEOPROTEROZOIC (750-600 MA) DETRITAL ZIRCONS WITH NO KNOWN SOURCE: QUATERNARY FLUVIAL AND AEOLIAN SANDS, EASTERN SNAKE RIVER PLAIN, IDAHO

Quaternary fluvial and aeolian deposits of the Big Lost Trough, eastern Snake River Plain, south-central Idaho, contain 8% Neoproterozoic (750-600 Ma) detrital zircons. From sixteen 60-grain random samples (almost 1000 grains analyzed) analyzed by SHRIMP, nearly 80 grains are within this age span and two individual samples contain over 20% Neoproterozoic grains. Groupings of 3 or more grains with 206Pb/238U ages (less than 10% discordant) occur at 740 Ma, 725 Ma, 710 Ma, 695 Ma, 675 Ma, 665 Ma, 655 Ma, and 600 to 570 Ma. The low precision of 4-scan SHRIMP analyses precludes resolution beyond about 5 Ma. Provenance study of all drainage systems in E. Idaho requires that the source for the Neoproterozoic grains be the Big Lost River system. Since the ghost zircons are not present in Antelope Creek, Trail Creek at Sun Valley, or the Salmon and Wood Rivers, the source must be within the East or North Fork of the Big Lost River. Outcropping rocks of the 750-600 Ma age range have not been previously identified in Big Lost River system which is dominated by Early to Middle Paleozoic deposits of the Cordilleran miogeocline, Antler allochthon, Mississippian Antler flysch trough (Copper Basin Group), and the Pennsylvanian-Permian Wood River Basin. If the zircons are first-cycle they must come from an unknown assemblage of Neoproterozoic volcanic rock in the Pioneer Mountains metamorphic core complex. Alternatively, the zircons may be second-cycle, having resided in a Paleozoic sandstone until Neogene uplift and erosion. Cambrian and Ordovician sandstones (Wilbert and Summerhouse formations) in the Borah Peak horst are provenance candidates. These zircons overlap in time and space with recent U-Pb ages and known outcrops of Neoproterozoic volcanic rocks in the Edwardsburg Formation, Gospel Peaks, central Idaho (685 Ma; Lund et al. 2003, GSAB) and Pocatello Formation, southeastern Idaho (717 Ma, 709 Ma, and 667 Ma; Fanning and Link, 2003, GSA Abstracts v. 35, no. 6, p. 389). This range in ages suggests that the fitful rifting of Rodinia (western Laurentia) initiated around 750 Ma and only culminated in continental separation at 580 Ma.