Thirty K–Ar ages are reported for metasediments, metavolcanics, granites, and pegmatites from the Madoc–Bancroft area of the Grenville Province of the Canadian Shield. When these data are compared with dates available in the literature it is seen that K–Ar ages of biotites are in general systematically lower than Rb–Sr whole rock and U–Pb ages for the area. This is interpreted to be the result of a long cooling history for this part of the Grenville Province, during which the Rb–Sr and U–Pb clocks began to record time at least 100 m.y. before the K–Ar clocks. It seems probable that the K–Ar ages are separated in time by an indefinite interval from the event or events they are usually considered to represent. The commencement of argon retention by biotites occurred about 950 m.y. ago whereas U–Pb and Rb–Sr whole rock dating indicate that the major igneous and metamorphic events in the Grenville Province took place1000–1300 m.y. ago.

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