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 we re 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 em placement 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 m ica 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 th ese pegmatites represent a magmatic event that has not been previously recognized. Likewise, the K-Ar ages point to a later thermal event no t previously recognized in this area. The results of our studies demon strate the utility of the U-Pb columbite geochronometer in dating rela tively young pegmatite systems.