ASSESSMENT OF THE TECTONIC HISTORY RECORDED WITHIN DETRITAL ZIRCONS RECOVERED FROM HIGH GRADE METASEDIMENTARY ROCKS IN THE ELK CITY REGION OF CENTRAL IDAHO

The geologic history of North-Central Idaho is protracted but has lacked adequate geochronology to constrain the occurrence and timing of major geologic events. Recent detailed mapping of high-grade metasedimentary sequences in Elk City region of Central Idaho, combined with new detrital zircon analyses, provides an opportunity to better catalog the tectonic history of the area. The metasedimentary sequences in the Elk City region are attributed to be stratigraphically equivalent to the Mesoproterozoic sedimentary rocks of the Lemhi subbasin of the Belt Supergroup. Eight samples were collected from metasedimentary rocks within the Elk City region for detrital zircon analysis. Sample lithologies vary from predominantly ‘dirty’ biotite feldspathic quartzite, sillimanite biotite schist, and, to a lesser extent, calc-silicate rocks. Zircons were obtained by heavy mineral separation for all samples, annealed and mounted at random in three size fractions and imaged by cathodoluminescence. Zircons from all lithologies have zonations and observable rims consistent with complicated growth histories. U/Pb ages and trace element concentrations were determined on ~150 zircons per sample by laser ablation mass spectrometry. All but one sample contain minor age populations (<10%) of Archean grains and all have a principal mode of 1691-1737 Ma, with another minor population of concordant ages between 1250-1400 Ma. These age characteristics are similar to the age spectra of the Lemhi subbasin and consistent with their attribution to that group of rocks. However, significant numbers of discordant grains, up to 30% within a single sample, and variable Th/U, evidence open system behavior in zircon within all age populations. Specifically, younger tails of concordant analyses in the 1400 and 1700 Ma peaks typically have relatively low Th/U (<0.3), compared to older grains within the same peak. Further, most samples display variable discordance that projects toward the Cretaceous, with concordant zircon age populations in two samples between 80-90 Ma. We investigate whether all discordance can be attributed to Cordilleran orogenesis during this time or if age systematics are more consistent with multiple episodes of metamorphism resulting in open system behavior, and place constraints on the timing of those events.