THERMAL-MECHANICAL EVOLUTION OF THE ROOF OF A MID-CRUSTAL TABULAR PLUTON: COUPLED DEFORMATION AND METAMORPHISM, RHEOLOGICAL EVOLUTION, AND CONSTRAINTS ON MAGMA CHAMBER GROWTH

Host-rock response to growth, or emplacement, of a pluton involves non-linear coupling of mechanical, thermal and fluid parameters. The inherent nonuniqueness of this system manifests in a wide variety of field and microstructural relations and renders generalized models of emplacement inadequate. Of particular importance is how the roofs and floors of these systems respond to upward magma migration, but mid-crustal exposures of these are rare. Gravity data, isograd geometry and structural measurements all indicate that the eastern aureole of the The Mooselookmeguntic pluton (MP) in western Maine, USA, represents an areally extensive roof overlying the gently east-dipping, tabular-shaped intrusion. This roof preserves a classic example of low-P, high-T metamorphism with metamorphic zones systematically distributed in nearly flat-lying orientations above the intrusion. Unlike other roofs described in the literature, the MP roof preserves an emplacement-related ductile strain gradient with a deformational fabric that evolves through all stages of crenulation cleavage to become an intense, non-differentiated foliation parallel to the pluton contact. The microstructural relations suggest that growth of the peak metamorphic minerals postdated the deformational fabric in all metamorphic zones. Fabric evolution through all stages of crenulation cleavage shows that vertical growth of tabular mid-crustal intrusions can be largely accommodated by dissolution-precipitation creep processes in the roof. Mass transfer associated with this type of fabric development may be assisted by fluid flow in transient microfractures caused by expansion of pore fluids in the thermal gradient. In such a model, fabric development at any point in the aureole should occur prior to that point attaining its peak metamorphic temperature, consistent with our microstructural observations. Coupled mechanical-thermal modeling shows the inadequacy of static conductive solutions and, given the right combination of constraints, provides information about the emplacement history.