PREFERENTIAL THERMAL RESETTING OF FISSION TRACKS IN RADIATION-DAMAGED DETRITAL ZIRCON GRAINS: CASE STUDY FROM THE LARAMIDE OF ARIZONA

Fission track ages of detrital zircon from Mesozoic sandstones in SE Arizona were analyzed to understand source rock exhumation and post-depositional thermal history of the strata. Samples show a complex provenance inferred to have included significant recycling from the underlying stratigraphy. FT peak ages from 17 Jura-Cretaceous sandstones include a wide range of grain ages with populations between 570-165 Ma, 140-82 Ma, and 68-42 Ma. While most samples show a range of provenance ages, some are partially reset, with a small fraction of grains that record a post-depositional thermal event. Laramide-associated magmatism, ranging from ~75-40 Ma, is widespread in this area, and the distribution of reset ages coincides with proximity to magmatic bodies. This setting provides an opportunity to investigate how low-temperature heating affects grains with a wide range of single-grain radiation damage. The young component falls between 68-42 Ma, occurs in most samples, and is younger than the depositional age, indicating that these strata reached temperatures sufficient to anneal fission tracks in some grains (~150 to 200°C). This thermal resetting occurred in grains with high radiation damage. Grains with lower radiation damage have a higher temperature of track retention and therefore retain their primary detrital signature. Ion microprobe U-Pb ages of single FT-dated crystals show a correlation between young FT ages and old U-Pb ages. Small decreases in the crystallinity of these zircons, as indicated by Raman microscopy, also indicate a correlation between high U or older crystallization ages and younger FT ages. Partial to complete resetting of FT ages in radiation-damaged zircons presents a problem in interpretation of multi-component age distributions. Based on FT peak ages, U-Pb crystallization ages, and zircon crystallinity, this work establishes criteria for determination of the presence of reset grains within multi-component populations.