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 th e Coast Ranges of British Columbia indicate that significant variation s in apatite chemistry may occur in small plutonic rock samples. To te st ages determined earlier by Parrish using the population method of f ission track dating, new apparent apatite ages were completed using th e external detector method so that single-grain ages could be generate d. In most cases, the new apatite ages were similar to the earlier one s (within +/-2 sigma). However, in many cases, it was found that indiv idual samples contained significant variations in single-grain apatite ages. Microprobe analyses showed the spread in ages were directly rel ated to variations in apatite chemistry, such that chlorine-rich apati tes retained older ages than fluorine-rich grains. Examination of the single-grain age data suggested that the spread in single-grain ages p rovided information which constrained the thermal history of the sampl e and that distributions of single-grain ages reflected the temperatur es at which they resided during cooling. These results suggest that it is important to generate more than a few single-grain fission track a ges when dating apatite from granitic rocks, since variations in the c hlorine 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 simil ar chlorine concentration, and in future apatite fission track studies of granitic terrains it is important to recognize that the apatite co mposition might be heterogeneous when interpreting results.