MAGNETIC PROPERTIES OF HEMATITE-ILMENITE AND REHEATING IN THE ECSTALL PLUTON AND IMPLICATIONS FOR THE BAJA-BC HYPOTHESIS

Paleomagnetic inclinations in the Ecstall pluton increase systematically with proximity to the Coast Mountains batholith (CMB) from which it is separated by the Coast Shear Zone (Butler et al. 2002). Hollister et al. (2004) proposed reheating resulting from emplacement and uplift of the CMB, and/or enhanced heat flow from the CSZ, as possible mechanisms for resetting remanence directions via acquisition of lamellar magnetism, which occurs at temperatures <~390°C in exsolving hematite-ilmenite. The low temperature acquisition of lamellar magnetism, <~390°C versus 575°C or 675°C for end-member magnetite and hematite, respectively, may have a profound impact on the timing of magnetization of deep seated plutonic rocks. Using ⁴⁰Ar/³⁹Ar thermochronometry to determine the extent of reheating at several sample locations we compare magnetic properties, both of bulk rock and single hematite-ilmenite crystals, from samples experiencing different thermal histories. Bulk rock samples show a distinct increase in NRM intensity for samples experiencing maximum reheating, i.e. those in closest proximity to the CMB (Brownlee et al. 2006). This relationship may be explained by a greater number of hematite-ilmenite grains acquiring lamellar magnetism during reheating increasing the bulk NRM intensity of the sample. Among the implications of these data are that the anomalously steep inclinations from the eastern part of the Ecstall pluton should be compared with Eocene rather than Cretaceous reference paleomagnetic poles for North America. Thus magnetic data in the Ecstall pluton cannot be explained by structural deformation (i.e. folding, or tilting), and low paleo-latitudes suggested by shallow inclinations on the western margin of the Ecstall pluton cannot be discounted. Therefore, large-scale northward translation cannot be ruled out by paleomagnetic data from the Ecstall pluton.