Coupled deformation and metamorphism in the roof of a tabular midcrustal igneous complex

Abstract

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 orog- eny. Gravity and drill-hole data, isograd geometry, thermal modeling, and struc- tural 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. Ther- mal modeling indicates that the roof is no more than 1000 m thick over an area exceed- ing 100 km2 . Unlike other roofs described in the literature, the Mooselookmeguntic igneous complex roof preserves a thick (~600 m) emplacement-related strain gradi- ent with a deformational fabric that evolved through all stages of crenulation cleavage to become an intense, nondifferentiated folia- tion approximately parallel to the intrusive contact and perpendicular to the steeply dip- ping, 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 differ- ential stress than, for example, dislocation creep. As the crenulation cleavage evolves, a stage is reached where further deformation requires a change in deformation mecha- nism, 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 fl at- tening plane, and this confi guration 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.