THE EASTERN CONTACT ZONE OF THE SEBAGO PLUTON, SW MAINE: IMPLICATIONS FOR TIMING OF EMPLACEMENT

As the next step after mapping and lab analyses of rocks from the northeastern contact region of the Sebago pluton with migmatitic country rocks (near Gray, ME; LaFleur et al., NEGSA07, Nyitrai et al., NEGSA08), new mapping was performed to determine the nature and extent of a projected NNW-SSE-trending pluton contact zone, defined by rocks that are distinctive from both units, both structurally and texturally. To the west, rocks are typical of the Sebago pluton (homogeneous, medium-grained 2-mica granite). To the east, rocks are strongly heterogeneous, consisting of a complex of plastically deformed migmatites and granites (in the migmatite-granite complex, MGC, of southern Maine, formerly the “Sebago Migmatite Domain”).

In contrast, granitic rocks dominate the zone with variable textures. Most rocks are coarse-grained granite, some pegmatitic, and, for the most part undeformed, with local magmatic biotite disjunctive fabrics parallel to local contacts with entrained blocks of migmatitic country rocks. Evidence for solid-state strain is found in thin section (Nyitrai et al., NEGSA09). Cm-scale bodies of granite found inside the migmatite blocks are pinched-and-swelled or boudinage. The blocks themselves are structurally concordant, as are m-scale sheets of the granites, suggesting progressive injection of magma along the regional structure. Zone boundaries are not exposed, and not certain due to poor outcrop density, particularly on the east side. The west side, by the Sebago pluton, the boundary is constrained to within 200m in outcrop, defining a NNE-SSW-trending, abrupt boundary of the contact zone. From these data we can constrain the zone width to 6-8km, but the limits along strike are not known.

Consistent with previous work, these relations suggest contact zone rocks recorded progressive contact effects with no distinct "contact" as the pluton was emplaced within already plastically-deformed (sub-solidus conditions) migmatites and granites. Rocks record a progressive assembly of the pluton's boundary, and, in effect, the structural coherence suggest together that these rocks have recorded the emplacement of the Sebago pluton with 'edge' effects. The MGC may be significantly older than the Sebago pluton, which may mean that the Sebago pluton and the MGC are unrelated, just fortuitously located.