DOCUMENTATION OF A PLUTON'S CONTACT ZONE IN THE MIDDLE CRUST: DETAILS AND IMPLICATIONS OF A NEW EXPOSURE OF THE EASTERN CONTACT OF THE SEBAGO PLUTON AND THE EASTERN SEBAGO MIGMATITE DOMAIN, SOUTHERN MAINE

As part of a larger multi-disciplinary study of the N Appalachian migmatite-granite belt, we document variations of mineral fabrics and the geometry of granitic and migmatitic bodies at the mapped eastern contact of the homogeneous, 2-mica granite Sebago pluton (~ 400 km²). The Sebago Migmatite Domain encompasses the pluton on the W, N and E. Our work focused on detailing a recently exposed 2.3 km-long, N-S-oriented road cut in Gray Maine, along the rerouted Rt. 26.

We were fortunate that this new exposure is located at the previously mapped location of the E contact of the Sebago pluton. This mapping was based on the contrast between the pluton's homogeneous granites on the W and the dominantly migmatitic rocks and subordinate, varied granites of the Sebago migmatite domain on the E. Results of our new mapping of the exposure reveals illuminating relations between all of the regional rock types, and we define a ~ 1 km-wide, N-S-trending zone where rock assemblages are distinct from both the pluton and the migmatite domain. Many varieties of granites, mostly very coarse-grained, dominate the exposure, and mostly define consistently-oriented shallowly-N- or S-dipping cm- to 10 m-thick sheet-like bodies. The bodies are sub-concordant to, but encapsulate, pod- to sheet-shaped somewhat migmatitic country rocks. The 2 main types of granite are fine-medium grained fairly homogeneous 2-mica granite, and course-grained to pegmatitic heterogeneous granite. Granite bodies are undeformed, have no evident solid-state fabrics, and some smaller examples cross-cut the exposure for ~ 500 m. Cm-scale bodies found inside the migmatites are deformed. The relation between the granites and the non-granites show structural conformity (ghost stratigraphy) as if preserved during construction of a pluton's contact zone where country rock is progressively displaced. Where granite intruded the migmatite pods, granite bodies are pinched and swelled or boudinage suggesting ongoing deformation. These relations, particularly in contrast to the rocks outside this “contact zone,” suggest rocks at the E contact of the Sebago pluton recorded progressive contact “effects” with no distinct “contact.” These rocks record a progressive assembly of the pluton's boundary, whereas rocks outside the pluton were already deformed and migmatitzed.