STABILITY OF FISSION TRACKS IN RADIATION-DAMAGED ZIRCON FROM THE CORDILLERA HUAYHUASH, NORTHERN PERU

Annealing of fission tracks in zircon is a function of temperature, time, and total accumulated radiation damage. Annealing of radiation-damaged zircon occurs at much lower temperature than annealing of non-damaged zircon that is fully crystalline, and the difference might be as great as 100°C. This difference is most apparent in mixed suites of zircon when sedimentary zircons with a wide range of radiation damage are brought to elevated temperatures (c. 200°C), and then cool. Low-retentive zircon (LRZ), which have a partly disordered crystalline structure, have significant radiation damage and a low temperature of annealing (~180-200°C). High retentive zircon (HRZ), which are nearly crystalline, fully anneal at temperatures of ~250-300°C depending on rate of cooling. Partially reset samples are those where LRZ are reset, HRZ are not reset, and therefore the cooling age is not concordant, but the young population of grain ages records the youngest thermal events. Full resetting of both LRZ and HRZ results in cooling ages that are concordant, or nearly so. The geological significance of different populations of zircon grain ages are highlighted in a suite of samples from the Cordillera Huayhuash in northern Perú. In the Cordillera Huayhuash, Cretaceous quartzites have been heated during intrusion of Miocene plutons along the crest of the Andes. ZFT and ZHe cooling ages near the core of the range indicate moderate to rapid post-intrusive cooling in the Miocene. This work highlights the dramatic difference in effective closure temperature between damaged and undamaged zircon. It underscores the need to understand the role of radiation damage in studies using zircon and the need for well-constrained natural examples to better calibrate the system.