“…It is known that the fission track ages of biotite and other minerals may be discordant with the real age of the mineral determined by the K-Ar or Rb-Sr methods (MEIER, 1966;WAGNER, 1973;WELIN et al, 1972;OSBORNE, 1947;LAKATOS and MILLER, 1973;BIGAZziet al, 1971;MILLER and JAGER, 1968;MILLER, 1968;GUPTA et al, 1971). For instance, younger fis sion track ages are caused by thermal annealing of fission tracks (HAACK and POTTS, 1972) in accordance with eq.…”