Naturally etched tracks in apatites and the correction of fission track dating

Naturally etched tracks have been found in apatites from the rapid cooled, high-level Kunon pluton in the Zhangzhou Igneous Complex, SE China. This is manifested by the fact that the apatite fission track.(FT) age derived from conventional counting of spontaneous and induced tracks yields a result of 140.6 +/- 6.5 Ma, which is much older than the ages determined using other methods on different minerals from the same rock. When tracks are observed after etching the polished inner sections of the apatite grains, the naturally etched tracks characterized by,having hazy boundaries can be distinguished from the normal tracks with sharp boundaries. The age obtained by omitting these-fading-resistant hazy tracks, 76.5 +/- 4.0 Ma, indicates the time of the Kunon pluton cooling down to similar to 100 degrees C. The corrected peak age (73.8 Ma) is consistent with the other apatite FT peak ages (79.2 to 70.2 Ma) of the nearly contemporaneous plutons in the same igneous complex.