Rb-Sr and K-Ar age determinations for muscovite-bearing blueschist in the Yukon Tanana terrane near Ross River indicate a minimum metamorphic age of approximately 250 Ma, and a mid-Paleozoic protolith age. The blueschist is associated with eclogite in a tectonized assemblage of phyllite, quartzite, micaschist and serpentinite. Neither mid-Jurassic metamorphism nor mid-Cretaceous regional plutonism have affected these high-pressure rocks to the point of resetting the isotopic systems. Parts of the Yukon Tanana terrane thus record a Late Paleozoic metamorphism. If the blueschist and eclogite were part of the trench melange of the Lewes River Arc, their age implies that the arc was active in Late Paleozoic time.
