Comparison of Paleomagnetic and 40Ar/39Ar results from two plutons near Prince Rupert, British Columbia

Paleomagnetic inclinations in the Ecstall pluton increase systematically with proximity to the Coast Mountains batholith (CMB) (Butler et al. 2002). Hollister et al. (2004) propose reheating resulting from emplacement and uplift of the CMB as a possible cause for resetting remanence directions via acquisition of lamellar magnetism, which occurs at temperatures < ~ 390° C in exsolving ilmeno-hematite. 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 interpretation of paleomagnetic data from deep seated plutonic rocks. Partially reset cooling ages from 40Ar/39Ar in hornblende and biotite are consistent with reheating from the east at 58-52 Ma (Brownlee et al. 2005). New 40Ar/39Ar data from hornblende and biotite further define the cooling history of the Ecstall pluton and contrast it with a second pluton adjacent to the CMB, the Butedale pluton. 40Ar/39Ar cooling ages on two size fractions of biotite in the Ecstall pluton, with distinct closure temperatures due to different diffusion radii, are consistent with reheating from the east. Age differences between small and large size fractions appear to increase with distance from the CMB. 40Ar/39Ar cooling ages from biotite in two locations of the Butedale Pluton are 58.7 ± 1.0 Ma and 71.9 ± 0.8 Ma at 7 and 22 Km from the CMB, respectively. These cooling ages are also consistent with reheating from the east, although the Butedale pluton does not appear to have been affected by the same extent of reheating as the Ecstall pluton. Magnetic mineralogy of the Butedale pluton is more heterogeneous than that of the Ecstall pluton and includes zones dominated by multi-domain magnetite and others dominated by exsolved Fe-Ti-oxides. Preliminary paleomagnetic results from the Butedale pluton do not show the same systematic variation as seen in the Ecstall pluton. The combined results indicate that the Butedale pluton experienced a different cooling history than the Ecstall pluton, and experienced some reheating that appears not to have affected magnetic remanence directions.