THE IMPORTANCE OF APATITE COMPOSITION AND SINGLE-GRAIN AGES WHEN INTERPRETING FISSION-TRACK DATA FROM PLUTONIC ROCKS - A CASE-STUDY FROM THE COAST RANGES, BRITISH-COLUMBIA

Fission track results determined from granitic rocks exposed within the Coast Ranges of British Columbia indicate that significant variations in apatite chemistry may occur in small plutonic rock samples. To test ages determined earlier by Parrish using the population method of fission track dating, new apparent apatite ages were completed using the external detector method so that single-grain ages could be generated. In most cases, the new apatite ages were similar to the earlier ones (within +/-2 sigma). However, in many cases, it was found that individual samples contained significant variations in single-grain apatite ages. Microprobe analyses showed the spread in ages were directly related to variations in apatite chemistry, such that chlorine-rich apatites retained older ages than fluorine-rich grains. Examination of the single-grain age data suggested that the spread in single-grain ages provided information which constrained the thermal history of the sample and that distributions of single-grain ages reflected the temperatures at which they resided during cooling. These results suggest that it is important to generate more than a few single-grain fission track ages when dating apatite from granitic rocks, since variations in the chlorine concentration in apatite grains may exist in kg-sized granitic rock samples. Therefore, it is no longer a valid assumption that each apatite grain separated from a small granite sample will have a similar chlorine concentration, and in future apatite fission track studies of granitic terrains it is important to recognize that the apatite composition might be heterogeneous when interpreting results.