The Tazin Complex, north of Lake Athabasca, shows widespread development of mylonites associated with northeast trending faults. Hudsonian metamorphism has caused recrystallization of cataclastic rocks with post-crystalline deformation forming narrow zones of later mylonites. Syenitic lamprophyre dikes which, with very few exceptions, are undeformed, form the latest intrusive rocks in the area, filling dilatant fractures developed during crustal uplift. R b S r whole rock dating of eight samples of the dikes in the Beaverlodge area, yield an isochron of 1779 i-72 m.y. (initial ratio = 0.709 0.001, MSWD = 0.39. x("Rb) = 1.39 X 10-U/yr). The late-kinematic phase of the Hudsonian orogeny, therefore, occurred around 1780 m.y. ago, at which time the uplift of the northwestern sector of the Churchill structural province took place. It is also believed that the date is close to the time of final movement of the St. Louis Fault.Le complexe de Tazin, au nord du Lac Alhahasca. montre un dbveloppement Ctendu de mylonites associees B des failles de direction nord-est. Le mktamorphisme hudsonien a causC la recristallisation des roches cataclastiques avec la deformation post-cristalline formant rles zones etroites de mylonites tardives. Des dykes de lamprophyre syknitique qui sont, sauf de rares exceptions, non dCformCs. constituent les roches intrusives les plus tardives dans la rkgion, remplissant les fractures de dilatation dCvelopp6es pendant le soul&vement de la croQte. La datation (roche totale) au R b S r de huit tchantillons des dykes pris dans la rkgion de Beaverlodge, donne un isochrome de 1779 rt 72 millions d'annees (rapport initial = 0.709 2 0.001, MSWD = 0.39, A("Rb) = 1.39 X 10-"/an). La phase tardi-cinkmatique de I'orogine hudsonienne, par conskquent, s'est dkroul6e il y a 1780 millions cl'ann6es environ, au moment oh le soul&venient du sectour nord-ouest de la province structurale de Churchill prenait place. On pense aussi que le mouvement final de la faille de St. Louis s'est produit vers cette date. praduit par le journal]