HAFNIUM ISOTOPE EVIDENCE ON THE SOURCES OF GRENVILLE-AGE DETRITAL ZIRCON DEPOSITED AT THE GREAT UNCONFORMITY

Combined U-Pb ages and Hf isotopic data of detrital zircon (DZ) grains can be used to improve the spatial resolution of provenance determinations where there are multiple potential source regions. In this study, such data are used to refine the sources of DZs in Neoproterozoic-Cambrian sediment deposited across Laurentia, comprising the so-called “Grenville sand sheet.” We compared the U-Pb ages and Hf isotope values for 1.0 Ga to 1.2 Ga crystalline rocks of North America, as a baseline for comparison to four intracratonal basal sandstones from the central U.S. DZ ages from the upper Cambrian Lamotte Sandstone, Missouri, are dominated by ~1.1 Ga ages, with an absence of ca. 1.4 Ga and Archean grains. ε_Hf for these grains has a restricted range of -27.4 to -20.0, which overlaps with sources in the S. Appalachian Mountains. Glauconitic sandstones from drill core in eastern Colorado, potentially of Cambrian age, were deposited on ~1.4 Ga basement, but contain abundant 1.0-1.2 Ga age zircon. These, and zircon from the Neoproterozoic Tava Sandstone (CO) to the west, show a wide range of ε_Hf values from -42.5 to -14.4. Wide ranges in ε_Hf and U-Pb ages suggests mixing of 1.0-1.2 Ga crustal material from both proximal sources, such as the ~1.1 Ga Pikes Peak Batholith (CO), and distal sources, such as Grenville age accretionary terrane of the Llano Uplift (TX). A wider range in ε_Hf of 1.0-1.2 Ga detrital zircon is observed with distance from Appalachian Mountains, suggesting the provenance of these zircon becomes more varied as a function of westward transport distance across the U.S. Low zircon supply from basement rocks indicates that much of the continent had low relief and served as a depocenter for detritus from the south and east. The Upper Flathead (WY) and Sawatch (CO) basal Cambrian sandstones contain no DZs of Grenville age, an indication that Grenville DZ sources to the east or south were shut off. The Flathead is dominated by ~1.7 Ga DZs; although it rests on Archean basement, it contains few Archean grains. The Sawatch rests upon Pikes Peak Granite but yielded no ~1.1 Ga DZs; a dominance of DZs are ~1.4 and ~1.7 Ga.The presence or absence of Grenville derived detrital sediments in the Cambrian sandstones at the “Great Unconformity” may be controlled by external forcing, including irregular transgression of the Sauk Sea.