Stable isotopic analysis of thirty-five samples from the alkaline Coryell Intrusive Suite (CIS) indicate that huge meteoric-hydrothermal systems affected these plutons during emplacement into the shallow crust near the end of Eocene extension in southeastern British Columbia. Significant lowering of δ ¹⁸O and δD values and ¹⁸O/¹⁶O disequilibrium among mineral assemblages from the Arrow Lake, Renata, and Coryell plutons indicate that hydrothermal activity was intense and was active over a ∼300,000 year duration. The absence of geographic trends in δ ¹⁸O and δD values indicates that this hydrothermal system was pervasive over a large area (∼3500 km²). Quartz δ ¹⁸O values suggest a lower crustal source for the CIS magmas. Melting was triggered by decompression of the lower crust, as it was unroofed during Eocene extension. The overlapping of amphibole and biotite δ ¹⁸O values from the CIS and Penticton Group Volcanics suggest a cogenetic relationship between these two units.