Rhyodacite perlite welded tuff exposed near Three Forks, Montana, is one of the oldest glassy volcanic rocks in North America. Its Late Cretaceous age, previously based on inconclusive geologic evidence, has now been confirmed by K-Ar methods: biotite and labradorite phenocrysts are both 74 ± 4 m.y. Glass groundmass is anomalously “young”—three determinations range from 19 ± 2 to 23 ± 7 m.y.—probably owing to simple argon loss.
At least two other comparably old glassy welded tuffs (K-Ar ages on mineral separates range from 71 to 83 m.y.) crop out in western Montana: near Wolf Creek, 80 miles north of Three Forks, and near Maudlow, 27 miles northeast of Three Forks. Although far apart and differing somewhat chemically and petrographically, the three ancient glasses have much in common. They were erupted as thin sheets within thick sequences of now-devitrified welded tuff during the same regional volcanic episode and are still glassy despite intense deformation and high water content.