EVIDENCE FOR METAMORPHOSED BELT-SUPERGROUP STRATA WITHIN THE WILDHORSE GNEISS COMPLEX (PIONEER MOUNTAINS, IDAHO) BASED ON U-PB AND LU-HF ISOTOPIC ANALYSIS OF ZIRCONS

The Wildhorse gneiss complex is in the footwall of the Pioneer metamorphic core complex south of the currently proposed boundary of the Lemhi subbasin of the Belt-Supergroup in Idaho. U-Pb and Lu-Hf isotopic analyses of detrital zircons were completed on 3 metasediments from two units of the Wildhorse Gneiss Complex; two samples were analyzed from the Middle Paragneiss (structurally lower unit) and one from the Upper Paragneiss. The Middle Paragneiss samples are grey to white quartzites with thin bands of mafic minerals. The Upper Paragneiss sample is a clean quartzite. U-Pb ages of detrital zircons from the Middle Paragneiss (n=91) show a peak in a KDE (Kernel Density Estimation) spectrum at 1624 Ma and a range of ages from 1418-1904 Ma (with one grain at 2610 Ma) for grains <10% discordant. U-Pb data from the Upper Paragneiss sample (n=38) shows a normal distribution with a peak of 1660 Ma and range of 1450-1860 Ma of grains <10% discordant. ϵHf_(i) values for all zircons range from -3 to +12 (one grain -9 with crystallization age 1725 Ma), and generally increase for older grains. ϵHf_(i) values are +7 to +8 for grains with ages of ~1660Ma. These detrital zircon results are consistent with the U-Pb and Lu-Hf data from the Belt Supergroup strata of North Central Idaho and Western Montana. These results are dissimilar to data from the nearby Lemhi Sub-Basin, because the spectrum peaks are apparently younger and include a number of grains with ages within the North American Magmatic Gap (1490-1610Ma). Further analyses will be performed on samples from these two units in order to better constrain provenance and maximum depositional age and assess paleogeographic reconstructions of North America and the extent of the Belt-Purcell basin.