40AR/39AR SANIDINE DATING AND PALEOMAGNETIC ANALYSIS OF THE BLUE CREEK FLOW (YELLOWSTONE VOLCANIC FIELD)

While the major eruptions of the Yellowstone Volcanic Field have been extensively studied and dated, minor explosive and effusive eruptions have been examined with less scrutiny. The effusive rhyolitic Blue Creek Flow (BCF) at Big Bend Ridge erupted between the ~2.1 Ma Huckleberry Ridge and ~1.3 Ma Mesa Falls Tuffs, but the temporal and genetic relationship between the BCF, other minor rhyolitic flows, and caldera-forming eruptions that shape the landscape of the greater Yellowstone area remains unclear. Previous K/Ar and paleomagnetic data were obtained on the BCF, but placement of the flow within the chronostratigraphic record of the Yellowstone Volcanic Field was not resolved. Here we present new single crystal ⁴⁰Ar/³⁹Ar incremental heating analyses of potassium-rich feldspars from the BCF to provide precise constraints on the timing of the eruption. Previous K/Ar data suggests an age of 1.77 Ma, whereas the new ⁴⁰Ar/³⁹Ar age is more than 200 ka older. Both the K/Ar and new ⁴⁰Ar/³⁹Ar ages indicate eruption of the BCF during the Matuyama reversed polarity subchron, but published paleomagnetic analyses report a normal polarity direction. Our thorough paleomagnetic analysis of six samples co-located with our petrochronologic samples, however, indicates that the BCF is unambiguously of reverse polarity. Collectively, our results highlight the importance of reassessing pioneering chronostratigraphic data from large volcanic fields with modern laboratory methods to more accurately test the timing, tempo, and character of Quaternary caldera eruptions and geomagnetic excursions.