Ultrabasic–ultrapotassic dykes have been identified in the Toby–Horsethief Creek area of the Purcell Mountains, in southeast British Columbia. The dykes intrude Helikian and Hadrynian strata of the Purcell Anticlinorium, a parautochthonous terrane of folded and faulted sub-lower greenschist-facies metasediments of the ancestral Cordilleran Miogeocline. Structural–stratigraphic relationships indicate that the loci of the dykes are controlled by Helikian to Upper Paleozoic extensional fault systems associated with rifting of the ancestral passive margin.Two dyke types have been distinguished. Group A dykes are typically light green and xenolithic. They may be phlogopitephyric, and consist dominantly of carbonate (commonly pseudomorphing olivine). These dykes are transitional between group II and group III ultrapotassic rocks (rift to active orogen). Group B dykes are dark green, phlogopite- and apatitephyric, with a carbonate, apatite, chlorite, and phlogopite matrix. They are classified as group II (continental rift) ultrapotassic rocks, with kimberlitic affinities.One of the group B dykes yielded a 245 ± 2.4 Ma Rb–Sr phlogopite–apatite mineral-pair age of emplacement, which is contemporaneous with the petrogenetically and tectonically similar Cross kimberlite, in Paleozoic miogeoclinal sediments of the adjacent Rocky Mountain terrane. The presence of these group II ultrapotassic intrusives in the Purcell Mountains suggests that Helikian through Paleozoic rifting of the passive margin continued into the Permo-Triassic. Palinspastic separation of the Cross kimberlite and the Toby–Horsethief Creek dykes indicates a belt of continental rifting at least 150 km wide. An initial 87Sr/86Sr ratio of 0.70707 for the dated group B dyke suggests a (palinspatically restored) stabilized continental lithosphere source approximately 50 km to the west, and supports geophysical evidence that the Hudsonian basement extends west, below the Kootenay Arc.