Abstract40Ar/39Ar ages on the Hat Creek Basalt (HCB) and stratigraphically related lava flows show that latest Pleistocene tholeiitic basalt with very low K2O can be dated reliably. The HCB underlies ∼ 15 ka glacial gravel and overlies four andesite and basaltic andesite lava flows that yield 40Ar/39Ar ages of 38±7 ka (Cinder Butte; 1.65% K2O), 46±7 ka (Sugarloaf Peak; 1.85% K2O), 67±4 ka (Little Potato Butte; 1.42% K2O) and 77±11 ka (Potato Butte; 1.62% K2O). Given these firm age brackets, we then dated the HCB directly. One sample (0.19% K2O) clearly failed the criteria for plateau-age interpretation, but the inverse isochron age of 26"6 ka is seductively appealing. A second sample (0.17% K2O) yielded concordant plateau, integrated (total fusion), and inverse isochron ages of 26±18, 30±20 and 24±6 ka, all within the time bracket determined by stratigraphic relations; the inverse isochron age of 24"6 ka is preferred. As with all isotopically determined ages, confidence in the results is significantly enhanced when additional constraints imposed by other isotopic ages within a stratigraphic context are taken into account.
Flow direction and source of the Early Miocene andesite lava flows, south of Matsue City, western Japan