IMPLICATIONS OF LAMELLAR MAGNETISM FOR INTERPRETING ANOMALOUS MAGNETIC REMANENCE DIRECTIONS OF DEEP-SEATED PLUTONS

The 91 Ma Ecstall Pluton, which intruded at a depth of 25 km into the Insular superterrane of British Columbia, has a steepening of inclination of magnetic remanence directions from 16 deg to 81 deg (Butler et al., 2002) on approaching the early Tertiary Coast Mountains batholith. Post solidification tilting or folding has been proposed to explain the observed change in inclination, but the proposed tilting or folding is contradicted by geological data. Hollister et al. (2004) considered the possibility that heat from the adjacent Coast Mountains batholith resulted in resetting of magnetic remanence directions that had been initially acquired at a more southerly latitude. Andronicos et al. (2003) showed that the Coast Mountains batholith was at 700 – 800oC between 60 and 52 Ma, when both the CMB and Ecstall pluton were at a depth of about 15 km. Our heat flow calculations show that temperatures above 400oC were reached within the Ecstall pluton where the change in magnetic remanence directions occurs. K/Ar and Ar/Ar cooling dates on hornblende and biotite support this result.

Preliminary data show that the stable remanent magnetization in the Ecstall pluton is in ilmenohematite grains with exsolved lamellae of ferrian ilmenite. It has the characteristic properties of lamellar magnetism (Robinson et al., 2002), which is acquired with exsolution in ilmenohematite below 400oC; thus, it appears to be thermal chemical remanent magnetisation (TCRM).

It remains to be demonstrated that the change in magnetic remanence inclinations of the Ecstall Pluton can be attributed to TCRM or whether it is due to some other unrecognized phenomenon. However, if our preliminary interpretation is confirmed, then the time of acquisition of magnetic remanence directions of deep-seated, slowly cooled, ilmenohematite-bearing plutons is not the time of intrusion of those plutons. An obvious implication is that steep (cratonal) paleomagnetic directions in CMB plutons may be TCRM overprints.