Reconsideration of the age of blueschist facies metamorphism on the Seward Peninsula, Alaska, based on phengite 40Ar/39Ar results

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Abstract
40Ar/39Ar step‐heating and single‐grain laser fusion ages from phengites from the polydeformed and polymetamorphosed blueschist‐greenschist facies Nome Group fall into two groups. Samples from the upper part of the structural section that have experienced a relatively weak metamorphic and deformational post‐blueschist facies overprint and one sample from the Cape Nome orthogneiss yield plateau ages of 116‐125 Ma. More intensely overprinted samples yield hump‐shaped spectra with minimum ages of 123 Ma and maximum ages of 334 Ma. Samples with hump‐shaped spectra are derived from a greater structural depth than most samples with plateau ages. Unreasonably old maximum ages from some of the disturbed spectra suggest that the hump‐shaped spectra result from the incorporation of excess 40Ar. This interpretation conflicts with previous interpretations of similarly disturbed spectra from the Brooks Range, which have been argued to provide minimum ages for blueschist facies metamorphism. Since the maximum temperatures achieved by all samples were probably above the blocking temperature of Ar in phengite, the 116‐125 Ma plateau ages are a minimum age for blueschist facies metamorphism on the Seward Peninsula, Alaska.