A K-feldspar megacrystic biotite granite phase of the Burgeo intrusive suite, southern Newfoundland, has been dated by the Rb–Sr whole-rock method. The isochron age of 412 ± 5 Ma (initial 87Sr/86Sr ratio = 0.7086 ± 0.0004) is consistent with the younger of two U–Pb zircon ages obtained from other phases of the Burgeo intrusive suite. The southern margin of the granite is intensively deformed in a shear zone that also affects amphibolitic gneisses of the Grey River enclave. Data from deformed granite samples plot below the 412 Ma isochron and may have resulted from reaction and partial reequilibration of the samples with a hydrothermal fluid of low initial 87Sr/86Sr ratio during metasomatic reactions accompanying shear zone deformation. However, alternative hypotheses such as crustal contamination or derivation from two separate and isotopically distinct magmas cannot be discounted. Mica ages between 390 and 370 Ma associated with tungsten mineralization at Grey River confirm that the mineralization was unrelated to the Burgeo intrusive suite. They support previous conclusions, based on geological and geochemical data, that the mineralization was related to posttectonic leucogranite intrusions.