Potassium-Argon Ages of Volcanic Rocks in Nye and Esmeralda Counties, Nevada

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Abstract
Potassium-argon ages of minerals are reported for 29 specimens of flows and tuffs from volcanic rocks that crop out in Nye and Esmeralda Counties, Nevada. Precision of the age determinations on biotite and sanidine from these rocks is estimated by three sets of data (1) Duplicate analysis for argon and potassium; σ = 0.9 to 3.6 percent. (2) Age determinations of mineral pairs from seven specimens; σ = 1.2 to 3.0 percent. (3) Age determinations from 5, 4, and 2 widely separated localities in three welded tuffs; σ = 2.3 to 6.5 percent, (σ = population standard deviation at 95 percent confidence level.) For purposes of stratigraphie correlation, the precision estimates indicate that in order to be able to distinguish between two rocks at the 95 percent confidence level, their ages must differ by 3.4 to 8.5 percent if each rock is dated by a mineral pair. In contrast to the diversity reported for chemical compositions of the volcanic rocks dated, the potassium concentrations in biotite and sanidine from pyroclastic units in each of the three sequences appear to be remarkably consistent. A check of the published K/Ar ages in Nevada and Utah suggests that this characteristic of minerals in pyroclastic rocks is widespread.

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