PROVENANCE OF UPPER TRIASSIC STRATA IN SOUTHWESTERN NORTH AMERICA AS SUGGESTED BY ISOTOPIC ANALYSIS AND CHEMISTRY OF ZIRCON CRYSTALS

Upper Triassic strata in southwestern North America include the Chinle Formation on the Colorado Plateau and the El Antimonio and Barranca Groups in Sonora, Mexico. U-Pb isotopic and geochemical analyses of magmatic and detrital zircons by SHRIMP-RG provides new information about the provenance of sedimentary strata and their enclosed igneous clasts, and ages and Th/U ratios can be correlated with proposed source areas in the Permo-Triassic arc in the Mojave Desert of California, and possibly with the Late Paleozoic magmatic arc in northeastern Mexico.

Clasts from the Sonsela Sandstone Member of the Chinle Fm range from ~218 Ma to ~235 Ma; Th/U ratios in magmatic zircons in these clasts range from 0.9 – 1.8. One clast from the Antimonio Fm yielded an age of 280 Ma with average Th/U of 0.3, while zircons from three samples of the nearby Sonoyta pluton yielded a composite age of ~270 Ma and similarly low Th/U. Detrital zircons from the Black Forest Bed (BFB), upsection of the Sonsela Ss, are as young as 200 Ma; individual Th/U ratios range from 0.3 – 2.1. Triassic detrital zircons from strata in the El Antimonio and Barranca Groups have average Th/U ratios of 0.4 – 0.9.

These data shed new light on a variety of sources for Upper Triassic strata. Detrital zircons from Mexican strata correlate well in age and composition with Mojave Desert plutons, which have Th/U ratios <1.1. This contrasts with nearly all Chinle clasts as well as the detrital BFB zircons. Th/U ratios in Chinle zircons, however, match very closely with those obtained from Jura-Cretaceous Winterhaven Formation, exposed in the area of transitional Proterozoic crust between the Early Proterozoic Mojave Province and the Middle Proterozoic Yavapai Province. We propose that a portion of the Triassic arc, as yet undiscovered or eroded, provided the dominant portion of coarse volcanic material to the Chinle Fm. Zircons from the BFB were likely airborne and derived from the Nevada-eastern California portion of the same arc. The well-exposed Mojave arc may have provided zircons to the Mexican depocenters. Sources in the northeastern Mexico arc cannot be discounted, but primary data are not available to assess their likely age more precisely than Penn-Permian. These results do not support an interpretation of movement of Upper Triassic rocks along a later transcurrent fault.