Ar-40/Ar-39 thermochronometric constraints on the tectonic evolution of the Clachnacudainn complex, southeastern British Columbia

Ar-40/Ar-39 thermochronometry from the Clachnacudainn complex indicates tha t the thermal evolution of the complex was controlled primarily by the intr usion of granitoid plutons in mid- and Late Cretaceous times. Hornblendes f rom the eastern part of the complex cooled below their Ar closure temperatu re (ca. 500%C) shortly after intrusion of the mid-Cretaceous plutons; those from the western part of the complex have latest Cretaceous cooling dates, indicating cooling of these hornblendes after intrusion of the leucogranit e plutons at ca. 71 Ma. Micas from the southern Clachnacudainn complex exhi bit a pattern of progressive cooling toward lower structural levels, where Late Cretaceous and younger intrusions occur. The occurrence of Late Cretac eous - Paleocene mica cooling dates in both the hanging wall and footwall o f the Standfast Creek fault refutes the hypothesis that there has been sign ificant Tertiary extensional exhumation of the Clachnacudainn complex along the Standfast Creek fault. Furthermore, the widespread distribution of Lat e Cretaceous - Paleocene mica cooling ages suggests that an important volum e of Late Cretaceous - early Tertiary intrusive rocks must be present in th e subsurface beneath the Clachnacudainn complex.