DETRITAL ZIRCON POPULATIONS IN THE NEOPROTEROZOIC (CRYOGENIAN) UINTA MOUNTAIN GROUP: THE LAURENTIAN BARCODE

A suite of 494 individual detrital zircon SHRIMP U-Pb ages were obtained from eleven samples of the <770 Ma Uinta Mountain Group and Big Cottonwood Formation, Utah. These include four new samples and seven previously reported (Dehler et al, in press). These data significantly expand the Mueller et al (2007) report based on four Uinta Mountain Group detrital samples (104 grains).

Archean grains cluster between 2600 and 2800 Ma with peaks derived from the “unmix” utility in Isoplot at about 2555 Ma, 2600 Ma, 2680 Ma, and 2790 Ma. These age distributions are noticeably younger than peak Archean ages from Mesozoic Colorado Plateau sandstones, confirming the intuitive latest Neoproterozoic rift-related unroofing of the Wyoming Craton and the ages of late Archean zircon-rich intrusive phases of the Wyoming Province.

Paleoproterozoic grains continuously span 1650 – 1950 Ma with prominent peaks at about 1670 Ma (Mazatzal), 1750 Ma (Yavapai), 1850 Ma (Mojave), and 1970 Ma (Mojave).

Mesoproterozoic grains continuously span 1360 – 1560 Ma and peak at about 1360, 1410, and 1460 (A-type transcontinental granite province), and 1560 (enigmatic! non-North American) Ma. Eight grains, 2% total population, fall within the North American magmatic gap (1508 – 1600 Ma). The ultimate source for these grains may be Asian, though a more proximate source could be reworked lower Belt Supergroup equivalents.

Grains derived from the Grenville orogenic belt continuously span 1000 – 1250 Ma and peak at about 995, 1070, 1155, and 1255 Ma. The continuous age distribution is similar to that reported by Erikson et al (2003) for modern streams draining the Appalachians, but distinctly younger than peak ages reported by Dickinson and Gehrels (2003) for eolian sandstones of the Colorado Plateau, with a more southern source within the Grenville orogen. These age differences suggest provenance change rather than sediment unroofing.

The Neoproterozoic age population is the least prominent yet most significant as it defines a maximum age of 766.4 ±4.8 Ma . These grains were derived from a felsic igneous source via fluvial transport from the north or east, or possibly as ashfall from there or a more distant non-Laurentian source. Detrital zircon geochronology has foiled the “Curse of the Proterozoic Sandstone”.