The paleomagnetic effects of reheating the Ecstall pluton, British Columbia

Abstract

Paleomagnetic data of plutons from the western Canadian Cordillera are currently interpreted to indicate thousands of kilometers of latitudinal transport, in situ tilting, or post-intrusion folding. For the Ecstall pluton of British Columbia, the inclination of magnetic remanence directions steepens to the east from 16° to 81° [Butler et al., J. Geophys. Res. 107 (2002)] between 24 and 12 km west of a thermal boundary with the Coast Mountains batholith (CMB). The CMB was at 700–800°C between 60 and 52 Ma. The stable remanent magnetization is in ilmenohematite grains with exsolved lamellae of ferrian ilmenite. It has the characteristic properties of lamellar magnetism [Robinson et al., Nature 418 (2002) 517–520] which is acquired with exsolution in ilmenohematite below 390°C [Ghiorso, Phys. Chem. Minerals 25 (1997) 28–38]; thus, it is thermal chemical remanent magnetization (TCRM). Our heat flow calculations show that the thermal effects of Eocene CMB on the adjacent 91 Ma Ecstall pluton were enough to reset the remanent magnetization directions. Reheating to the temperatures necessary for TCRM is supported by K/Ar and Ar/Ar cooling dates on hornblende and biotite, which young from west to east towards the CMB in concert with the change in inclinations. Thus, the progressive steepening of magnetization inclinations is attributed to reheating during the Eocene of the 91 Ma Ecstall pluton by the CMB. This reheating followed post-solidification northward latitudinal displacement of the pluton. The amount of translational displacement of the coastal terranes of British Columbia prior to 60 Ma hinges on the interpretation of discordant magnetizations from plutons that may have cooled slowly after emplacement in the mid to lower crust, as was the case for the Ecstall pluton. Many of these plutons contain ilmenohematite as an accessory magnetic phase. Our interpretation for the low temperature acquisition of TCRM in the Ecstall pluton may have far-reaching implications for understanding enigmatic discordant paleomagnetic directions reported from plutons of the western Canadian Cordillera.