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Fission track studies and K-Ar dating on three silicate minerals from Sweden have given the following results: Fission track age K-Ar age RM 07249 hornblende, Malmberget, Fragment 1 1255 m.y. 1875 m.y. » 2 450 » RM 620569 biotite, Ingelsbo Fragment 1 305 » 975 » » 2 520 » » 3 355 » » 4 410 » RM 640448-640451 phlogopite,Ainön,Fragment 1 230 » 555 » » 2 240 » It is concluded from this investigation that due to inhomogeneous uranium distribution, recent uranium movement and sensitivity for annealing, the fission track dating technique is suitable only for the study of the last thermal event in areas which have undergone a complex geological evolution.

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