CENTIMETER- TO METER-SCALE STRUCTURAL AND PETROGRAPHIC VARIATIONS IN A 3-D EXPOSURE IN THE MIGMATITE GRANITE COMPLEX, SOUTHERN MAINE

Structures in migmatitic rock terranes record magma flow through the crust and their study requires multiple scales of observation. As part of a multi-disciplinary study of rocks of the N Appalachian migmatite-granite belt, we have documented variations at the cm- to m-scale of fabrics and the geometry of granitic bodies in a single large, 3-D exposure located in W. Cumberland, ME. The landowners have removed till from the property, re-exposing glacially-polished 76m x 24m pavement below, and 15m-high cliff faces above on two sides, providing a 3-D opportunity for our study in a terrane that is otherwise not well exposed.

The exposure is in the SE part of the Migmatite-Granite Complex (MGC) in S Maine in contact with the 400 km²Permian Sebago granite pluton, and is W of the dextral Norumbega shear zone system. The MGC is composed of metatexite migmatite and diatexite, with cm- to m-scale bodies of granite with compositions from medium-grained 2-mica granite to pegmatite, and fabrics from unfoliated to augen gneiss and schlieric granite. Fabrics in the MGC are cut by undeformed cm-scale granite dikes.

The exposure is dominated by metasedimentary metatexite migmatite that includes two 3-m-wide concordant amphibolite layers. Structures have a NE-SW trend and moderate SE dips (consistent with regional data). Kinematic indicators of dextral shear are asymmetric tails around porphyroblasts, boudinage and ptygmatic folds of stromatic-structured migmatite. Like the folds, fabrics are in leucosomes, indicating deformation younger than migmatite formation, consistent with regional MGC data. Migmatites are cut by granitic rocks that have grain sizes that are coarse to pegmatitic. Local cm-scale bake zones are found at contacts. The dominant granite bodies are generally cm- to m-thick, 2-mica coarse-grained granite to pegmatite, and concordant to moderately discordant to the migmatite structures. Cm-scale, medium-grained granitic dikes are found throughout the outcrop cutting all structures. All of these data are consistent with rocks found throughout the MGC.