The Cassiar and Seagull batholiths are mainly post-tectonic felsic intrusions emplaced into the North American miogeocline and Yukon-Tanana Terrane, respectively, near the British Columbia-Yukon boundary. The two bodies range in composition from granodiorite and quartz monzonite to granite. Previous studies reported K-Ar and Rb-Sr dates of ~100 Ma for Seagull batholith and about 110 Ma for Cassiar batholith. Two samples of massive quartz monzonite from the interior of the Cassiar batholith, and a strongly foliated and lineated augen gneiss within a ductile shear zone near the western margin of the batholith, yield overlapping U-Pb monazite and/or zircon ages of 112.3 ± 2.0 Ma, 113.2 ± 2.2 Ma, and 110.2 ± 1.0 Ma respectively, by ID-TIMS methods. Samples of aplitic biotite granite and megacrystic biotite granite from the Seagull batholith give distinctly younger U-Pb zircon ages of 99.3 ± 2.2 Ma and 95.7 ± 2.1 Ma, respectively, using LA-ICP-MS methods.
