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The radiometric dating evidence for the timing and duration of volcanism for the Steens through Wanapum Basalt of the Columbia River Basalt Group is critically reviewed here. K-Ar dates generally underestimate the age of crystallization, though one important exception is detected, where excess argon led to dates that were too old. The 40Ar/39Ar results on whole-rock basalts from 1980 through 2010 are examined for statistical validity of plateau sections, as well as alteration state of the material dated. In most instances, listed ages are shown to be invalid. The 40Ar/39Ar total gas (fusion) ages are, in general, not accurate estimates of the time of formation of these rocks. The 40Ar/39Ar ages on plagioclase separates from basalts yield good estimates of the extrusion age of the lavas. New 40Ar/39Ar ages on whole-rock basalts are presented that are in good agreement with the plagioclase ages. Various forms of the geomagnetic polarity time scale for mid-Miocene time are examined, along with the ages of lavas and their magnetic polarity. The main sections of the Columbia River Basalt Group (Imnaha through Wanapum Basalt) were formed in ~0.5 m.y. between 16.3 and 15.8 Ma. Steens Basalt extrusion occurred about ~0.1 m.y. before the Imnaha Basalt and appears to have been a precursor to the more voluminous volcanism noted in the Columbia River Basalt Group.

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