A fission-track age on zircon from a welded tuff in the Rochester Rhyolite, a part of the Lower Triassic Koipato Group of central Nevada, is 225 ± 30 m.y. This age corresponds with existing lead-alpha dates on rocks considered to be related to the volcanic rocks in the Koipato Group and indicates that the Early Triassic is at least about 225 m.y. old and the Permian-Triassic boundary somewhat older. The age also gives a minimum age for emplacement of the Golconda thrust during the Sonoma orogeny.

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