The 370–377 Ma Mooselookmeguntic igneous complex was emplaced at ∼14 km depth into steeply dipping metaturbidites that were folded and metamorphosed ca. 400–405 Ma during the Acadian orogeny. Gravity and drill-hole data, isograd geometry, thermal modeling, and structural measurements all indicate that the eastern aureole of the complex represents an areally extensive roof above the gently east-dipping, tabular-shaped intrusion. This roof preserves a classic example of low-pressure, high-temperature metamorphism caused by the underlying intrusion. Thermal modeling indicates that the roof is no more than 1000 m thick over an area exceeding 100 km2. Unlike other roofs described in the literature, the Mooselookmeguntic igneous complex roof preserves a thick (∼600 m) emplacement-related strain gradient with a deformational fabric that evolved through all stages of crenulation cleavage to become an intense, nondifferentiated foliation approximately parallel to the intrusive contact and perpendicular to the steeply dipping, regional Acadian-age foliation. Fabric evolution through all stages of crenulation cleavage requires the vertical growth of the tabular intrusion to have been largely accommodated by dissolution-precipitation creep, which is a linear viscous deformation mechanism that can occur at lower differential stress than, for example, dislocation creep. As the crenulation cleavage evolves, a stage is reached where further deformation requires a change in deformation mechanism, which may lead to "hardening" of the rock. Development of crenulation cleavage requires a mica-rich foliation at an initially high angle to the emplacement-related flattening plane, and this configuration may be the primary reason why the strain aureole in the Mooselookmeguntic igneous complex roof is so well developed compared to other published examples of midcrustal roofs.
- Received 6 February 2010.
- Revision received 19 July 2010.
- Accepted 19 July 2010.