MICROSCOPIC STRUCTURAL AND PETROGRAPHIC VARIATIONS IN A 3-D MEGA-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 km2 Permian 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. Field results were presented previously (Luther et al., 2015, NEGSA). New petrographic analyses have produced microscale tests of the field relatioins from different sections of the outcrop, and has permitted more complete relations to each other and regional structures. From these data, two well-recorded tectonic events are evident, one predating the emplacement of the Sebago pluton (Devonian?), and one roughly co-eval (Permian). Microtextures corraborate field evidence for dextral shearing, likely associated with the sensu lato Norumbega progressive shearing events as most indicators share a similar orientation to the principal strain axes, with shallowly NE-SW-plunging elongation.