PALEOMAGNETIC AND ROCK MAGNETIC RESULTS FROM QUATERNARY LAVAS AND TUFFS OF THE YELLOWSTONE PLATEAU VOLCANIC FIELD
We report preliminary paleomagnetic and rock magnetic from selected volcanic flows and tuffs associated with the Yellowstone Caldera within and surrounding Yellowstone National Park. These data were collected in order to understand sources of magnetic variations observed in new high resolution aeromagnetic data recently reported by Finn and Morgan (2002), and to better understand the evolution of the Yellowstone magmatic system. Most paleomagnetic samples are from volcanic rocks of the third eruptive cycle (1.3 Ma to 0.70 Ma), including the ca. 0.639 Ma Lava Creek Tuff, postcaldera rhyolite flows, and contemporaneous marginal or post-caldera basalt flows. Magnetic intensities for samples ranged from 0.12 A/m to 5.9 A/m, with volume susceptibilities of 2.14x10-4 to 1.45x10-3 SI; Koenigsberger ratios (Q) range form 0.67 to 23.8. As expected, most sites yield well-defined paleomagnetic results of north declination and moderate positive inclination consistent with remanence acquisition during the Brunhes polarity chron. However, a few sites from the Rhyolite of Harlequin Lake and Lewis Canyon Rhyolite yield reverse polarity magnetizations consistent with emplacement during the Matuyama chron. Rock magnetic behavior, including high coercivities during AF demagnetization, high laboratory unblocking temperatures, and susceptibility vs. temperature determinations indicate that remanence in these samples is carried by a combination of magnetite and/or hematite. Paleomagnetic results from 39 sites from 21 separate flows yields a grand mean direction with a declination of 352.7° and inclination of 58.7° (k = 36.5, a95 = 5.3°). The observed mean direction is located west (R = -7.3° ± 8.0°) of the time averaged mean direction for the Yellowstone area (dec. = 0.0°, inclination = 63.0°), but is statistically indistinguishable from the expected direction at the 95% confidence level. This result suggests that our sample population averages paleosecular variation, despite the episodic and discontinuous record of volcanic activity within the Yellowstone Caldera.