LOW-TEMPERATURE THERMAL RESPONSE OF RADIATION DAMAGED DETRITAL ZIRCON: PROVENANCE AND POST-DEPOSITIONAL THERMAL HISTORY OF LARAMIDE SYNOROGENIC SEDIMENTS, SE ARIZONA

Disagreement regarding patterns of Laramide deformation and basin development in SE Arizona underlies the lack of a clear picture of exhumation and recycling for syntectonic basins in this area. Fission track (FT) ages of zircon from Mesozoic sandstones were analyzed to address the provenance and post-depositional thermal history of the synorogenic strata. Samples from strike-normal transects across the Laramide basin boundary show a complex provenance, with significant recycling from the underlying stratigraphy. FT peak ages from 17 Jura-Cretaceous sandstones include populations between 570-165, 140-82, and 68-42 Ma. Numerous older single-grain ages (600-1000 Ma) are also present, indicating long-term thermal stability for portions of the source area. While most samples show a range of provenance ages, a small fraction have been affected by post-depositional thermal annealing. The distribution of reset ages coincides with proximity to magmatic bodies ranging from 75-40 Ma in age. This setting provides an opportunity to investigate controls on single-grain response to thermal perturbations. The young component of reset ages falls between 68-42 Ma, and is younger than the depositional age of the Jurassic-Paleogene strata in which these ages occur. As such, these strata reached temperatures sufficient to anneal fission tracks in some grains (c. 200-250°C). U-Th-Pb, REE, and Raman microprobe analysis from these samples indicate a correlation between old crystallization ages, high U+Th concentrations, elevated alpha-dose, and young reset FT ages. Resetting occurred in grains with high radiation damage, as determined by FT and Raman microprobe analysis, and total and effective alpha-dose calculations. More crystalline grains have a higher temperature of track retention and therefore retain a primary detrital signature. These data establish criteria for identifying reset grains in detrital populations. Zircon color is related to increasing radiation damage, and can be used for dating different thermal events in both the pre- and post-depositional history of individual zircons.