ASSESSING SOURCES OF ZIRCONS IN UPPER TRIASSIC SEDIMENTARY STRATA AND THEIR RELATION TO THE CORDILLERAN MAGMATIC ARC
The Shinarump Member of the Chinle Formation was deposited in the backarc concurrently with eruption of the Cordilleran magmatic arc, but contains surprisingly little volcanic detritus. This suggests that the earliest phases of the arc were either not onshore and/or did not develop substantial topography, and complements traditional observations of paleocurrents from the south and southeast. Zircon populations from Shinarump sandstone samples from Arizona and Utah are heterogeneous and have few discernable geographic patterns. The youngest grains are ~220 Ma and ages are continuous to ~260 Ma. Most samples then have a gap until ~1100 Ma grains, followed by 1400 Ma, 1700 Ma, and in some, 2700 Ma. Th/U ratios of these zircons suggest that even within distinct age ranges, a myriad sources contributed material.
Grains of all ages with Th/U ratios >1.0 make up 7-20% of the samples; these may have derived from Mojave or transitional Mojave-Yavapai crust to the west of the depositional basin, as suggested by the high ratios found in the Mojave crustal province. These ratios are seen in grains ~220-230 Ma, 1400 Ma and 1600 Ma and, to a lesser extent, 1100 Ma. Mesozoic plutons in the Mojave Desert, however, have comparatively low ratios. Zircons younger than ~250 Ma, regardless of Th/U, were likely derived from volcanic eruptions in the arc. The high ratios are matched by those in zircons derived from volcanic clasts within the sandstones, although rarely do clast and detrital zircon values significantly overlap. Permian grains, in contrast, are matched in age by volcanic and plutonic rocks in the Mexican Cordilleran arc, but Th/U ratios are much higher in the Shinarump grains. Th/U ratios in Proterozoic rocks range from “typical” Appalachian or mid-continent (i.e., <1) to much higher. Although the Shinarump Member largely contains detritus from eastern sources, similar-age, but geochemically distinct sources to the west contributed as well.