It happens that some samples of paragenetic micas show ages, measured by the fossil fission tracks method, which are distributed in time bands. A model is presented which considers the possibility that metamorphic events, with different distribution functions in time, can modify the characteristics of paragenetic minerals. Distribution functions of ages are obtained, under various hypothesis. It is demonstrated that the probability of obtaining the true age of a mineral decreases with time. Methamorphic events of finite duration cause the appearance of time bands. The influence of previous events produces a tail in the distribution of ages in paragenetic minerals. In the limit, distribution function of ages reaches a steady state condition, and cannot furnish information on the age of the minerals. The higher frequency of young ages seems to be not dependent on a higher frequency of methamorphic events but rather to be an effect of the hypothesized distribution function.