Plutonic igneous rocks of Middle Jurassic, Cretaceous, and Eocene age from southeast British Columbia exhibit a range of initial Nd and Sr isotopic compositions described by ε_Nd(T) = −2.7 to −13.4, and ε_Sr(T) = −13.8 to 69.5. In contrast, Early Jurassic volcanic rocks from the area have ε_Nd(T) values between +2.9 and +6.5, and ε_Sr(T) between −7.1 to 1.7. Cordilleran sedimentary and metasedimentary rocks of Middle Proterozoic to Triassic in age from this area have ε_Nd(at 164 Ma) values between −3.1 and −13.4. These plutonic igneous rocks formed in continental-arc environments and appear to have formed through mixing of mantle-derived magmas and depleted cratonic basement. The presence of 1.8–2.0 Ga old cratonic basement underlying the study area is supported by (1) presence of an old Pb component in zircons from many young igneous intrusives and (2) dating of the middle to upper crustal rocks exposed in the metamorphic core complexes in southern British Columbia. The deeper part of this basement appears to be the most likely crustal end member for assimilation with mantle-derived magmas, and this part has been characterized by its chemical composition (ε_Nd = −16, ε_Sr = 80, Nd ppm = 26, Sr ppm = 400). Similarly, isotopic and trace element compositions of mantle-derived magmas are obtained from those of the mantle xenoliths from the West Kettle River, just west of the study area (ε_Nd = +9, ε_Sr = −25, Nd ppm = 13, Sr ppm = 560). Intermediate isotopic compositions of the granitoid rocks suggest their parental magmas derived from the mantle lithosphere or mafic lower crust must have experienced significant crustal assimilation at deep crustal levels. Simple binary mixing between these end members is inadequate to produce the array of isotopic and trace-element compositions of the granitoid bodies. Coupled assimilation and fractional crystallization process appears to be more effective. Crustal contamination took place generally under high ratios of rate of crystallization to rate of assimilation (r ≥ 0.7) with concomitant crystallization of clinopyroxene and hornblende for the alkalic intrusives, but with plagioclase crystallization for the dominantly calc-alkalic intrusives. Calculations show that amounts of crustal material present in the granitic batholiths in southeastern British Columbia varied from 20% to 50%. Similar isotopic compositions of the spatially related peraluminous parts of the dominantly metaluminous granitoids suggest they were probably fractional crystallization products of the later magmas. Only exceptions are the Cretaceous and Eocene peraluminous magmas with isotopic compositions very similar to those of the cratonic basement. These stocks appear to be products of direct melting of thickened crusts with high thermal gradients.