Paleomagnetic evidence for tectonic rotation of the Belchertown Pluton, west central Massachusetts

Abstract

A paleomagnetic study of the Belchertown pluton, a syntectonic Devonian intrusion in the Bronson Hill anticlinorium, Massachusetts, indicates the presence in 23 samples from eight sites in primary quartz monzodiorites of a strong, stable, and consistently oriented remanent magnetization vector (D = 154.9°, I = −13.4°, K = 20.8° and α95 = 12.4°) with a high Q value (7.0). The remanence is inferred to be of thermal and/or thermochemical origin acquired during the original crystallization and cooling of the pluton. Recrystallized equivalents of the primary rocks, which occur as a surrounding border zone, yield weak and inconsistent results (four samples from two sites) due to the absence of opaque minerals. The mean Belchertown vector is discordant with respect to the mean North American Devonian vector (D = 170° and I = +32°) in such a direction as to suggest that the entire pluton has undergone some 30°–60° of rotation to the northwest about a NE-SW axis. The high Q value of the primary rocks and their isolation from magnetic country rocks by the surrounding non-magnetic border zone allow computer modeling of the three-dimensional shape of the pluton using the measured remanent vector and the total field aeromagnetic anomaly. The resulting model is consistent with the proposed amount and direction of tectonic rotation of a once roughly vertically symmetrical funnel-shaped intrusion.