USE OF TH/U IN DETRITAL ZIRCON FOR PROVENANCE ANALYSIS: IMPLICATIONS FOR LATE CRETACEOUS–PALEOGENE TECTONIC EVOLUTION OF THE NORTH AMERICAN CORDILLERA

Detrital zircon U-Pb age dating has proven extremely useful for provenance analysis. However, ambiguity regarding exact sediment source location can arise where zircon is derived from linear magmatic arcs. In such cases, it may be possible to take advantage of regional variations in trace element composition of zircon to constrain source area more precisely than by age alone. An example of the utility of this approach is provided by the North America Cordillera, where a NW-SE–trending Mesozoic arc transects the largely NE-SW grain of Precambrian basement terranes. In southern California (Mojave Desert region) the arc is hosted by Proterozoic rocks with anomalously high Th/U. Mesozoic detrital zircon from the Campanian(?) upper part of the McCoy Mountains Formation, which was sourced from the Mojave Desert, likewise tends to exhibit high Th/U. This trait is particularly evident in the 150–180 Ma grain fraction (average Th/U > 0.80). Similarly high Th/U is also observed in 150–180 Ma zircon from Upper Cretaceous sequences in the Sevier belt of southeast Utah, confirming inferences by previous workers for northeastward transport of sediment along the axis of the Sevier basin from a source area in the Mogollon highlands. In contrast, Upper Cretaceous forearc-basin and subduction-zone sequences in California on the west side of the Cordilleran arc are characterized by Th/U ≈ 0.40–0.50 for 150–180 detrital zircon. This zircon was probably derived from continent-fringing parts of the Jurassic arc, either northwest or outboard of the Mojave Desert. Upper Cretaceous forearc and retroarc sequences also differ in that the former are dominated by arc-derived zircon, whereas the latter show a preponderance of pre-arc (largely Proterozoic) zircon. Paleogene forearc deposits in southern California, however, exhibit a “retroarc” signature with high proportion of pre-arc zircon and Th/U > 0.80 for 150–180 Ma grains. This pattern reflects latest Cretaceous–early Paleogene breaching of the topographic divide between forearc and retroarc areas at the latitude of the Mojave Desert related to Laramide low-angle subduction. Future work will focus on rare earth elements in addition to Th/U and age populations other than 150–180 Ma.