Resumen de Are track length measurements in zircons worth being done?: a positive example from the high pressure - low temperature metamorphic rocks of the Phyllite-Quartzite Unit on Crete, Greece
Contrary to apatite thermochronology, track length measurements are not applied as a routine tool in zircon fission track analysis. The main reasons are the large number ofgrains and the long time often needed to find a sufficient number of suitable tracks. This problem may be reduced by strong etching followed by short repolishing. Data from the high pressure — low temperature metamorphic Phyllite-Quartzite Unit on Crete allow the discrimination oftrue cooling ages from unspecific mixed ages. This is essentialfor the reconstruction o f the thermochronological evolution, mainly because a misinterpretation could result in an overestimation o f the cooling rate after peak metamorphism. As refected by the replacement ofpolymodal track length frequency distributions with mean track lengths shorter than 10 mm by uni modal distributions with larger mean track lengths, final rapid cooling o f Phyllite-Ouartzite Unit zircons in which old tracks were completely annealed at temperatures o f400 ± 50 °C between 24 and 19 Ma ago occurred later than previously thought, i.e. about 15 Ma. In boomerang plots, the track length parameters change only slowly over a broad range o f age reduction, but very fast close to the age o f the thermal overprint.
