Muscovite and biotite separated from Moine schists close to the Moine Thrust Zone yield a range of K-Ar ages incompatible with the simple cooling regime previously proposed for the Caledonides of northern Scotland. Differences of only 1 Ma between muscovite and biotite of the same grainsize contrast with differences of 12–13 Ma between the 125–200 μm and 250–500 μm grainsize fractions. This is contrary to the normally accepted dominance of mica type over grainsize effects. Dynamic recrystallization of the micas during early deformation associated with the Moine Thrust Zone is thought to play a role as important as temperature in determining the K-Ar ages. The conclusions drawn have important implications for the use of uplift and cooling rates derived from the absolute K-Ar closure temperatures of micas close to thrust zones.
Although the K-Ar ages from micas with diameters greater than 200 μm are not reproducible, the data can be used to show that deformation associated with thrusting commenced between c. 443 Ma and c. 435 Ma, and that the thrust zone was active until at least 422 Ma ago.