Two samples from a 1.3 m-thick dike that intrudes the dated pyroclastic unit (Qcup) at Stepanof Cove yielded ages of 1,365 +/- 41 and 1,613 +/- 56 ka (K-Ar). The significantly different ages are due to variations in radiogenic 40Ar content per gram of sample (potassium contents are essentially identical). We conclude that the quickly chilled dike rock contains variable amounts of excess argon and that the younger age probably better reflects the true cooling age of the dike. The younger age is concordant with the minimum age of > or = 1,368 +/- 54 ka for the pyroclastic unit intruded by the dike. These data suggest two interpretations for the age of the pyroclastic (Qcup) unit: (1) the pyroclastic unit may be significantly older than the minimum age of ? Or = 1,368 +/- 54 ka, or (2) the concordant Qcup and younger dike ages may reflect the true cooling age for these units (about 1.4 Ma), and the dike may have been a feeder for the pyroclastic eruption.
Geology and geochemistry of the Geyser Bight Geothermal area, Umnak Island, Aleutian Islands, Alaska
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