Rb-Sr dating of mylonites from the basal thrust zone of the Kalak Nappe Complex near Børselv, Finnmark, has yielded two distinct isochron ages. One sample series gave an 11-point isochron age of 479 ± 15 Ma. In another series, larger samples taken from nearer the base of the thrust zone, where topto-the-east shear hands are prolifically developed, were sawn into thin stabs parallel to the mylonitic foliation. Two of these subseries of thin stabs produced separate but near-parallel isochron ages of 385 ± 26 and 380 ± 22 Ma, indicating resetting of the Rb-Sr isotope systems on a local scale. Taken at face value the results may appear, at first sight, to lend support to the notion of a two-stage orogenic development for the Kalak metasediments during the protracted Caledonian cycle involving thrustrelated, ductile mylonite generation in Early Ordovician time; and a phase of more brittle, forelanddirected, simple shear close to the base of the Kalak Thrust Zone in the Early to earliest Mid Devonian period. There are, however, certain problems involved in the interpretation of the older (479 Ma) isochron age, in spile of its low MSWD, which call for a measure of reservation.