Geochronology of Neogene alkaline volcanic rocks (Miles Canyon basalt), southern Yukon Territory, Canada: the relative effectiveness of laser ~(40)Ar/~(39)Ar and K-Ar geochronology

摘要

Miles Canyon basalt is an informal term used todescribe numerous exposures of young alkaline olivine basaltflows in southern Yukon. The volcanic rocks are part of theNorthern Cordilleran volcanic province. K-Ar and Ar-Ar whole-rock dates indicate that the Miles Canyon succession of flows atthe Whitehorse Rapids are clearly Late Miocene in age (ca. 8.4Ma). The largest exposure of the Miles Canyon basalt occurs atthe Alligator Lake volcanic complex where two nearlyconcordant Pliocene Ar-Ar dtes indicate eruption at ca. 3.2 Ma.K-Ar analyses form other sites yield dates of 2.4 and 7.1Ma andindicate an episodic Neogene volcanic history for the region.There is no evidence of Quaternary or postglacial volcanism. Thedates are older than assumed by previous workers, and in somecases the K-Ar dates are strongly discordant from Ar-Ardeterminations. More accurate Ar-Ar determinations may resultfrom the method's ability to select smaller amounts of bettermaterial for analysis. Excess ~(40)Ar was not encountered. As aresult, the accuracy of any single or several discordant K-Ardeterminations for Neogene subaerial volcanic rocks, particularlylow-K rocks such as basalts, should be questioned and resultinginterpretations made with caution. Models accounting for theeruption of the Northern Cordilleran volcanic province lavashave typically relied upon extension along north-trending faultsthat were generated by stresses along the continental margin.However, we consider a slab window model which betteraccounts for the initiation and distribution of northernCordilleran Neogene volcanic activity.