Geochronology of the Little Nahanni pegmatite group, Selwyn Mountains, southwestern Northwest territories

Geochronological and Pb isotopic studies were carried out to constrain the petrogenesis, emplacement age(s), and relationships of the Little Nahanni pegmatite group to other granitoids in the region. A U-Pb age of 81.6 +/- 0.5 Ma for columbite was obtained. K-Ar ages for micas were determined to be 65.4 +/- 4.0 Ma for muscovite and 65.8 +/- 3.4 and 65.4 +/- 3.6 Ma for lepidolite. The U-Pb age is interpreted as the emplacement age for the pegmatites, and the K-Ar ages are interpreted to be the timing of a thermal event responsible for resetting the K-Ar mica system. The U-Pb columbite age is 8-13 Ma younger than the average age of the plutons in the Selwyn plutonic suite and indicates that these pegmatites represent a magmatic event that has not been previously recognized. Likewise, the K-Ar ages point to a later thermal event not previously recognized in this area. The results of our studies demonstrate the utility of the U-Pb columbite geochronometer in dating relatively young pegmatite systems.