TECTONIC AND PALEOGEOGRAPIC IMPLICATIONS OF DETRITAL MINERAL AGES FROM THE UINTA MOUNTAIN GROUP. UTAH

The Uinta Mountain Group (UMG) consists primarily of clastic sedimentary rocks of fluvial origin deposited in a Neoproterozoic, fault-bounded basin between the Archean crust of the Wyoming province and the Paleoproterozoic crust of the Yavapai-Mazatzal terranes. The UMG rests unconformably on dominantly Paleoproterozoic metamorphic rocks of the Red Creek quartzite and the Owiyukuts gneisses that appear to be older than adjacent Yavapai-Mazatzal crust (~1.6-1.8 Ga). The ages of detrital zircons from the UMG, however, do not correspond to a simple mixture from these three adjacent Precambrian terranes. Instead, U-Pb ages of detrital zircons from two arkosic sandstones (UMG-1, 2) are dominantly (>90%) 2.5-2.8 Ga and correspond closely to ages of rocks in the southern Wyoming province. Detrital zircons from a quartz arenite and a feldspathic sandstone (UMG-3, 4), however, yield U-Pb ages that are dominantly (>70%) Grenvillian (~0.95-1.35 Ga) and Archean (~20%) with few intermediate ages. In order to further constrain the origin of the Grenville-age zircons, we conducted 1) total laser fusion and limited step-heating experiments on individual white micas from a silty lens within UMG-2 (=2B) and UMG-4 and 2) Lu-Hf isotopic measurements of zircon from UMG-3, 4. Mica ages from UMG-2B range from ~0.6 to 1.6 Ga and are compatible with a late Neoproterozoic depositional age for the UMG and a possible contribution of detritus from Yavapai-Mazatzal rocks. Analyses of micas from UMG-4 (feldspathic sandstone), however, yielded a much different range of ages (~0.3 to 2.45 Ga). Void-filling secondary seritization in UMG-4 suggests the Paleozoic Ar ages may be artifacts of alteration, but does not obviate the co-occurrence of feldspar and Archean grains in these rocks. Epsilon Hf of zircons from UMG-3 and 4 averages about –22, which suggests derivation from older crust reworked during the Grenville orogeny. Overall, these results point to a major change in provenance and sedimentary processes in Laurentia and/or its Neoproterozoic conjugate during deposition of the UMG that included rapid removal of new and recycled Archean zircons from the sedimentary cycle and a substantial flood of Grenville-age detritus to the exclusion of detritus from adjacent older crust.