Paleomagnetism, magnetic anisotropy, and mid‐Cretaceous paleolatitude of the Duke Island (Alaska) ultramafic complex

We report paleomagnetic results from layered igneous rocks that imply substantial post mid-Cretaceous poleward motion of the Insular superterrane (western Canadian Cordillera and southeast Alaska) relative to North America. The samples studied are from the stratiform zoned ultramafic body at Duke Island, which intruded rocks of the Alexander terrane at the south end of the southeastern Alaska archipelago at about 110 Ma. Thermal and alternating field demagnetization experiments show that the characteristic remanence of the ultramafic rocks has high coercivity and a narrow unblocking temperature range just below the Curie temperature of magnetite. This remanence is likely carried by low-Ti titanomagnetite exsolved within clinopyroxene and perhaps other silicate hosts. The Duke Island intrusion exhibits a well-developed gravitational layering that was deformed during initial cooling (but below 540°C) into folds that plunge moderately to the west-southwest. The characteristic remanence clearly predates this early folding and is therefore primary; the Fisher parameter describing the concentration of the overall mean remanence direction improves from 3 to 32 when the site-mean directions are corrected by restoring the layering to estimated paleohorizontal. All samples exhibit a magnetic anisotropy that is strong but nonuniform in orientation across the intrusion, and we show that it has no significant or systematic effect on the site-mean directions of remanence. At least some of the anisotropy derives from secondary magnetite formed during partial serpentinization. The mean paleomagnetic inclination (56°±10°) corroborates paleomagnetic results from five coeval silicic plutons of the Canadian Coast Plutonic Complex to the south and southeast and implies 3000 km (±1300 km) of poleward transport relative to the North American craton. Between mid-Cretaceous and middle Eocene time, the Insular superterrane and Coast Plutonic Complex shared a common paleolatitude history, with more poleward transport than coeval inboard terranes.

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