PALEOMAGNETIC AND ROCK MAGNETIC PROPERTIES OF TERTIARY VOLCANIC AND INTRUSIVE ROCKS OF THE EOCENE ABSAROKA VOLCANIC SUPERGROUP, NORTHEASTERN YELLOWSTONE NATIONAL PARK

We report paleomagnetic and rock magnetic from data from intrusive and volcanic rocks of the 53-42 Ma Absaroka Volcanic Supergroup, exposed along the northeastern margin of Yellowstone National Park in order to understand sources of magnetic variations observed in high resolution aeromagnetic data reported by Finn and Morgan (2002). In general, magnetic anomalies associated with the Tertiary andesitic volcanic and volcaniclastic rocks result in high-amplitude, positive magnetic anomalies, in contrast to the significantly less magnetic, predominantly felsic Quaternary volcanic rocks associated with the Yellowstone Caldera. Magnetic intensities for the Tertiary samples ranged from 2.3 A/m to 0.18 A/m, with volume susceptibilities of 4.5x10^-3 to 1.2x10^-3 SI. Koenigsberger ratios (Q) are low, ranging from 2.36 to 0.30, with lower values usually found in intrusive rocks. Paleomagnetic results from the Lake Butte intrusion reveal that a well-defined reversed polarity remanant magnetization (D = 170.5°, I = -40.3°, k = 293, a₉₅ = 2.3°, N = 15 samples) is characteristic of the intrusion, despite a significant positive aeromagnetic anomaly associated with this feature. Q ratios from the Lake Butte intrusion show wide variation, with many samples yielding values significantly less than unity. Low Q ratios indicate that a significant part of the observed anomaly could result from an induced normal polarity component due to the present day field. Paleomagnetic results from the Crescent Hill Basalt yield dual polarity magnetizations at different sites, indicating that eruption of the flows spanned at least two polarity chrons. The presence of a reverse polarity magnetization contradicts reports by Smedes and Protska (1972) that the Crescent Hill Basalt is composed of normal polarity flows. Paleomagnetic directions from clasts in two volcaniclastic breccias near Slyvan Lake are randomly oriented, indicating that they were emplaced at temperatures below the Curie temperature of magnetite. An in situ grand mean for the volcanic and intrusive rocks yields a declination of 345.2° and inclination of 57.4° (k = 16.2, a₉₅ = 9.4°, N = 16 sites). This result is similar to expected Tertiary reference direction for this region and to paleomagnetic results reported from the Absaroka Volcanic Supergroup by previous studies.