Tectonic setting and provenance of the Neoproterozoic Uinta Mountain and Big Cottonwood groups, northern Utah: constraints from geochemistry, Nd isotopes, and detrital modes

The Neoproterozoic Uinta Mountain Group was deposited in an east-trending i ntracratonic rift bounded on the north by an active fault system and openin g into a shallow sea on the west where the Big Cottonwood Group was deposit ed in an estuary. Although this rift may have been associated with the earl y stages in the breakup of Rodinia, it was not an aulacogen. Geochemical, N d isotope, and detrital mode studies indicate that Uinta Mountain Group sed iments were derived from mixed Archean and Paleoproterozoic sources with th e former dominating. Big Cottonwood Group sediments appear to have been der ived predominantly from Paleoproterozoic sources. The Archean sediment sour ce is the Wyoming craton, and source rocks comprised dominantly granites en riched in Th, U, Y, Zr, Hf, and REE. The relative abundance of enriched gra nite implied by sedimentary rocks of the Uinta Mountain Group indicates tha t the Wyoming craton is anomalous compared to other Archean cratons. CIA values and A-CN-K relationships in shales of the Uinta Mountain and Big Cottonwood groups indicate high degrees of weathering of sources, probably in subtropical to tropical climates supporting a near-equatorial location for southwestern Laurentia at about 800 Ma. Differences in the Nd isotopic composition between the Big Cottonwood Group and Neoproterozoic sedimentary rocks in western Utah and northeast Nevada suggest a northwest-striking up lift in northwest Utah, possibly ancestral to the Paleozoic Toole-Uinta arc h. (C) 2001 Elsevier Science B.V. All rights reserved.