ROCK MAGNETIC STUDIES OF PHYLLITE LAYERS FROM BARABOO INTERVAL ROCKS IN WISCONSIN

Oriented hand samples were collected from phyllitic layers within the Baraboo Syncline for rock magnetic and paleomagnetic studies. We anticipated that the remanent magnetic history of these rocks may provide insights to the metamorphic and heating events in the Baraboo region. The present estimate of the age of folding of the Baraboo Syncline is ~1630 Ma based on mineral cooling dates further north (Holmes et al., 1998, Geology). However, ⁴⁰Ar/³⁹Ar dates from the phyllite layers at Baraboo (Naymark et al., 2001, NCGSA) give a ~1430 Ma age which is associated with the Wolf River age hydrothermal event.

Thermal demagnetization of the paleomagnetic samples indicates a single magnetic direction for all samples that is reduced to zero intensity between 650-750^o C., whereas A.F. demagnetization was ineffective. Coercivity of remanence (Hcr) values derived from hysteresis loops are in the range of 140-180mT. These combined results are indicative of single domain and multidomain hematite as the primary magnetic minerals. The presence of multidomain hematite grains (up to 100µ) suggests some remanent magnetization had to be developed during metamorphism. The remanent magnetic directions at each site are internally consistent but are scattered between sites. Sites with finer grained hematite give magnetic directions closer to those of the unfolded quartzite(Kean and Mercer,1986, Geoscience Wisconsin) and consistent with a 1750 Ma paleomagnetic direction. The two sites with multidomain hematite give similar pole positions before unfolding, but are very distinct from other Proterozoic pole positions of North Amerca. This disparity may be caused by local structural variations, or anisotropy of remanent magnetization produced during metamorphism.