Magnetic fabric of the Barrington Passage pluton, Meguma Terrane, Nova Scotia: a two-stage fabric history of syntectonic emplacement

Abstract

Structural analysis and the anisotropy of magnetic susceptibility (AMS) are used to document the internal, magmatic to high-temperature subsolidus fabrics of the tonalitic Late Devonian Barrington Passage pluton. The AMS fabric orientations compare favorably with mesoscopic-scale fabrics. In general, the K 1– K 2 principal plane of the AMS is close to the observed mineral foliation defined by biotite and plagioclase, and the K 1 axis is aligned parallel to a mineral lineation defined by the same minerals, and to a locally developed, weak extension lineation defined by quartz. Measurement of the anisotropy of anhysteretic remanence (AAR), and image analysis of biotite preferred orientations confirm that the orientation of the AMS is principally controlled by biotite. Locally anomalous AMS orientations are attributed to the influence of fine-grained magnetite crystals, the presence of which is revealed by measurements of hysteresis and coercivity properties. A two-stage fabric history is revealed. First, horizontal fabrics were formed (foliation, compositional banding) during emplacement of a laccolith-like pluton. The second stage involved synmagmatic folding of the horizontal foliation and banding in response to regional contraction associated with the Acadian Orogeny, with development of a lineation that records stretching parallel to the fold axis. The results demonstrate that the Barrington Passage pluton was emplaced during the main phase of the Acadian Orogeny. A model is proposed that depicts the Barrington Passage pluton as the folded base of a larger syntectonic pluton, that is now largely eroded off.