RELATIONS BETWEEN DEFORMATION AND METAMORPHISM IN THE AUREOLE OF THE MOOSELOOKMEGUNTIC PLUTON, WESTERN MAINE

The pelitic schists of western Maine are ideal for studying the relations between deformation and metamorphism during pluton emplacement. A regional low-pressure, high-temperature metamorphic event (M2), ca.404 Ma, is characterized by the assemblage staurolite + andalusite + biotite ± garnet and a northeast-trending, steeply-dipping axial-surface foliation. This assemblage was overprinted by a thermal gradient (M3) associated with emplacement of the southern and northern lobes of the Mooselookmeguntic pluton, dated at ~390 Ma and ~370 Ma, respectively. A gently northeast-dipping foliation formed during the emplacement of this pluton, varying from a crenulation cleavage well away from the pluton to an intense foliation at its margin.

At Coos Canyon, very-late syntectonic M2 staurolite porphyroblasts overgrew the regional axial surface foliation resulting in straight inclusion trails that are continuous with the surrounding matrix foliation. At this locality, deformation associated with emplacement of the Mooselookmeguntic pluton is expressed as slight deflections of the regional foliation at the porphyroblast margins. These microstructures are consistent over a large area, and so are used as a "control" to evaluate the structural and metamorphic effects of crenulation cleavage development along transects across the M3 thermal gradient that end at the pluton contact. Specifically, we are evaluating inclusion-trail orientation data to investigate the kinematics of crenulation cleavage development and whether or not the staurolite porphyroblasts rotated relative to the cleavage during its formation. In addition, we are investigating the role of metamorphic reactions in crenulation cleavage development, and the timing of M3 mineral growth relative to emplacement-related foliation development.