U-Pb and Hf Isotopic Evidence on the Sources and Sinks of Grenvillian Detrital Zircons in Early Laurentia

Abstract

In situ zircon U-Pb and Hf isotopic data from ∼1.1 Ga intrusive igneous rocks in Mexico and from Grenvillian (0.9–1.3 Ga) detrital zircons in sandstones from the southern midcontinent of Laurentia were used to refine provenance determinations for the Grenvillian detrital zircons delivered to southwestern Laurentia from the Neoproterozoic to the Cambrian and to address the reduction in the Grenvillian detrital zircon abundances documented in Cambrian sandstones from this region. Igneous zircons from Mesoproterozoic anorthosites and granites in northern Sonora have low εHf(0) values (<−22) and could not have been sources of the higher-εHf(0) (>−22), ∼1.1 Ga detrital zircons characteristic of Ediacaran to Terreneuvian sandstones in southwestern Laurentia. Abundant Grenvillian detrital zircons in Cryogenian sandstone injectites from central Colorado have U-Pb ages and high εHf(0) values (>−22) similar to those of zircons in Ediacaran to Terreneuvian sandstones throughout southwestern Laurentia. These zircons were derived from Mesoproterozoic rocks in the Llano uplift and vicinity in Texas and were fluvially transported across southwestern Laurentia from the Cryogenian to the Terreneuvian. In contrast, Cambrian glauconitic sandstones in the subsurface of east-central Colorado and from the Sawatch Sandstone in central Colorado have low Grenvillian zircon abundances, as observed in Cambrian sandstones exposed farther west in Laurentia. The low abundances of Grenvillian detrital zircons in sandstones found both east (Colorado) and west of the strike of a proposed Cambrian “transcontinental arch” suggest that this feature did not disrupt the supply of Llano uplift–derived Grenvillian zircons to southwestern Laurentia. The low Grenvillian zircon abundances instead coincide with the development of marine conditions in south-central Laurentia, suggesting that the progressive encroachment of the Sauk Sea into the continental interior provided a sink for fluvial sediments derived from the Llano uplift that restricted their transport farther to the north and west in the continent.