Age and tectonic implications of paleomagnetic results from plutons near Prince Rupert, British Columbia

Abstract

A total of 25 sites (125 cores, 250 specimens) were selected from four plutons along the Skeena River cross-section of the Coast plutonic complex. After af demagnetization a stable remanence direction was isolated for the diorite of the Kasiks pluton, the gneissic diorite and quartz diorite of the Quottoon pluton, and the diorite and quartz diorite of the Ecstall pluton but not for the granodiorite phase of the Ponder pluton. The Kasiks-Quottoon pole (144° W, 76°N (P0.05:7°, 7°)) is concordant with other Eocene poles from cratonic North America, and thus no postemplacement tectonic rotation or translation is indicated for these plutons. The Ecstall pole (47°W, 68°N (P0.05:13°, 16°)) in Greenland is discordant with other Upper Cretaceous poles. This pluton has probably undergone a westerly tilt of ≃20°. The Kasiks and Quottoon plutons most likely cooled quickly within the time span of one normal polarity interval (<½ m.y.) and the Ecstall pluton within the time span of one normal and one reversed interval (<1 m.y.). This result supports the hypothesis that the pole positions and K-Ar ages of 43 ± 5, 48 ± 7, and 75 ± 12 m.y. for the plutons date the age of their emplacement rather than the age of their unroofing or of their thermal metamorphism, which would give different paleomagnetic results. Acceptance of the K-Ar dates as the age of emplacement suggests that the Coast plutonic complex evolved at the western margin of the North American plate as three prolonged episodes of plutonic emplacement to form the western (84–140 m.y.), central (64–80 m.y.), and eastern (40–50 m.y.) K-Ar date zones.