REGIONAL TRACE ELEMENT GEOCHEMISTRY OF ZIRCONS IN THE JURASSIC VOLCANIC ARC OF THE SOUTHWEST U.S

The geochemistry of zircon provides an important link between magmatic arcs and deposition of sediments in adjacent basins, because zircon can survive erosion and sediment transportation. We are examining longitudinal variations in arc zircon geochemistry in order to better understand the link between intrusive and extrusive magmas and the provenance of detrital zircons in fore-arc and backarc basins. The Jurassic arc of the southwest U.S. formed as a northwestern marine arc and a southeastern continental margin arc across the edge of the North American craton. Zircons from volcanic rock sections of both the marine and continental segments of the arc will help to describe longitudinal differences in arc magma chemistry and characterize the Jurassic volcanic arc upper crust that is now largely eroded and deposited in adjacent basins. A comparison of Jurassic volcanic sections from the Ritter Range and Oak Creek pendants in the eastern Sierra Nevada, the Sidewinder (Fohey-Breting et al., 2010) and Holcomb Creek sections in the Mojave Desert, and the Dome Rock sequence in the Colorado River region (Tosdal and Wooden, 2015) shows that populations of zircons from all three regions have a range of Hf contents (7,000-14,000 ppm), and REE quantities are similar, being HREE-enriched. Middle Jurassic zircons have similar amounts of REE at all sites, but Late Jurassic zircons from the Mojave Desert sites have lower Ce/Gd. Zircons in most samples are high U/Yb, except again those from Late Jurassic within the Mojave Desert that are lower. High Th/U is a characteristic of zircons from all sites - Th/U ranges from 0.5 to 1.5, with Mojave Desert samples having the highest Th/U (up to ~3). These data suggest that high Ce, high Th/U zircons are a distinguishing characteristic of the Middle Jurassic volcanic arc along strike from the central Sierra Nevada to the Colorado River region.