Paleomagnetism, Paleointensity, and Rock Magnetism of the 301 ka andesites of Table Mountain, Mt. Baker, WA

We report results obtained from an initial study of the paleomagnetism and rock-magnetism of andesite volcanics- the andesites of Table Mountain associated with the Mt. Baker composite volcano, in NW Washington State. Arc volcanism in the North Cascades has been long-lived, and many of the volcanic centers associated with the northern Garibaldi Trend of the arc have volcanics that include a variety of compositions including basalts, andesites, and more felsic rocks. The volcanics presently exposed at Mt. Baker range in age from the Holocene to 2.2 Ma, with many units accurately dated using Ar⁴⁰/Ar³⁹ or K-Ar methods. The well-exposed nature, range of compositions, and set of well-determined ages makes these rocks good candidates for improving the database of geomagnetic field variation spanning the past 2 Ma. For this study, oriented block samples were collected from 4 sites from the andesites of Table Mountain. Prior work (Hildreth et al, 2003) reported an age of 301 +/- 5 ka from whole-rock K-Ar methods. Thermal and a.f. demagnetization were successful- yielding well-defined magnetization vectors. The characteristic remanence was removed between 250 and 560 C, or between 20 and 125 mT in the majority of the samples. The samples have normal polarity, and yield a mean direction of Dec = 354, Incl = 57, k = 182, α95 = 6.8. Because these samples have well-behaved magnetic properties, we conducted an initial set of paleointensity experiments using the modified Thellier-Thellier method. The samples were heated in an oven with an Ar atmosphere, using steps from 80 to 560 C for the demagnetization and pTRM steps. The results from 3/3 samples yielded well-defined linear relationships between the NRM demagnetization and pTRM gain, over the range from 250 to 560 C. Using 6 to 7 temperature steps, best-fit lines were used to estimate the paleointensity of the geomagnetic field, obtaining a paleofield of 19.4 +/- 0.5 μT. This results in a VADM for these rocks of 3.06 x10²² Am². Absolute paleointensity results from 250 to 450 ka are relatively sparse, but existing data from the Snake River volcanics, and compilations from other sources including sediments (see Tauxe et al, 2004) suggest lows in geomagnetic field intensity between 250 and 350 ka, which is consistent with the results obtained from the 301 ka andesites of Table Mountain. These data, and previous results from the 10 ka basalts of Sulfur Creek that also yielded quality paleomagnetic and paleointensity results suggest that the volcanic rocks from Mt Baker and other Cascades volcanic centers would be fruitful targets of additional investigations of geomagnetic field variability.