PB/U AGES AND TH/U RATIOS OF ZIRCONS IN EARLY MESOZOIC SEDIMENTARY UNITS SOUTH AND SOUTHWEST OF THE COLORADO PLATEAU: AGE AND TECTONIC SIGNIFICANCE

Early-Middle Mesozoic sedimentary successions to the west and south of the Colorado Plateau in the southwestern US have been correlated with Plateau rocks based on stratigraphic position and rock types. Two examples of such continental sedimentary rocks are the Buckskin/Vampire group in the eastern Mojave Desert, and unnamed rocks in southern Arizona that overlie Permian carbonate and underlie dated Jurassic volcanic rocks; these two units are important to tectonic reconstruction of the Early Mesozoic Cordilleran arc because they were deposited between the arc and backarc .

New U-Pb detrital zircon ages support correlation of these strata with the Chinle Formation, and suggest refinements of paleogeographic interpretations for Late Triassic time. Tuffaceous sandstone from the upper Buckskin Fm contains a suite of zircons that range from 215 Ma – 235 Ma with a maximum at 220 Ma, and two strongly discordant grains at ~1100 Ma and 1700 Ma. Th/U ratios range from 0.3 – 1.9. Zircons from rocks in southern Arizona range continuously in age from 210 Ma to 275 Ma; other maxima are at 1400 Ma and 1600 Ma, and Th/U ratios in Triassic grains range from 0.12 – 1.8. Th/U ratios are noteworthy, as ratios from the two suites form an overlap between Mojave Desert Permo-Triassic plutons, which have Th/U ratios generally <1.0 and Chinle Formation Triassic zircons, in which Th/U ratios are commonly >1.0.

These two suites provide a link between arc and back-arc. Buckskin Fm rocks were likely derived from proximal volcanoes related to nearby Triassic plutonic rocks. Southern Arizona strata, in contrast, had a more diverse source area, including Mojave Desert arc rocks, potentially the Permo-Triassic Cordilleran arc in northwestern Mexico, and the as-yet-uncertain source of Chinle strata. River systems originating in the Cordilleran arc flowed to the east and northeast. The source of zircons with high Th/U ratios in all the sedimentary units is not constrained, but may have been removed by erosion later in Mesozoic time.