Diabase dikes and sills were intruded during a Late Proterozoic phase of extensional tectonics in the northern Canadian Cordillera. This event is strongly expressed in the depositional pattern of formations immediately below and above the base of the Windermere Supergroup. Rb–Sr isochron dates of 766 ± 24 and 769 ± 27 Ma have been obtained for two of the these diabase sheets, which intruded the Tsezotene Formation of the Mackenzie Mountains Supergroup. The emplacement of the diabase sills and dikes may be related to either of two volcanic events in the region, one documented, the other inferred. In either case, emplacement of the diabase at approximately 770 Ma is close to the boundary between the Mackenzie Mountains Supergroup and the overlying Windermere Supergroup, precedes a regional glaciation (Shezal tillite), and reflects a strong accentuation of the west-facing Cordilleran miogeocline.