THE CHEYENNE BELT LIES TO THE NORTH OF THE UINTA AXIS: EVIDENCE FROM THE RED CREEK QUARTZITE, OWIYUKUTS COMPLEX, AND FARMINGTON CANYON COMPLEX, UTAH

Sears et al. (1982) inferred that a crustal boundary existed between the "Proterozoic" Red Creek Quartzite (RCQ) and the "Archean" Owiyukuts Complex (OC) in NE Utah. The axis of the Uinta Mountains has also provided a convenient location through which to draw this boundary (so-called Cheyenne Belt). However, Nelson et al. (2002), based on the U/Pb systematics of a prograde chronometer (monazite), reinterpreted the Farmington Canyon Complex (FCC) to represent initial metamorphism at 1.7-1.8 Ga, requiring the boundary to lie to the north of the western Uinta axis. New geochronologic data suggest it must also lie to the north of the eastern Uinta axis.

⁴⁰Ar/³⁹Ar ages for amphibolites of the RCQ in Beaver Canyon in extreme NE Utah yielded ages of ~1.65 Ga. Only one monazite grain was found in 3 thin sections of OC from Beaver Canyon, but yielded an electron microprobe age of 1.65±32 Ga [n-3] that is concordant with amphibolite. Zircons from the OC in Beaver Canyon have detrital crystal habits and show a normal distribution of U/Pb ages with a mean value near 2.2 Ga, with late Archean and early Proterozoic tails. Although previously interpreted to be a granite gneiss, major element data clearly show that the OC is a paragneiss with an arkosic protolith, consistent with our interpretation that zircon ages record sedimentary provenance rather than age of metamorphism. In short, a strong preponderance of existing evidence suggests that the boundary between Archean and Proterozoic crust, at least at the surface, lies to the north of the Uinta axis.

Independent of the above observations, electron microprobe ages on 2 samples of RCQ from Red Creek Canyon (40 analyses on 11 grains) have a large, distinct node at 1.5 Ga. If this node is verified by further isotopic work, it suggests: 1) that spatially distinct blocks of RCQ may also have temporally distinct metamorphic histories, and 2) that a previously unidentified episode of metamorphism exists.