COMPARISON OF GRANITES IN THE SEBAGO PLUTON, ITS CONTACT ZONE, AND IN THE ADJACENT MIGMATITE-GRANITE COMPLEX, SOUTHERN MAINE

Strategic collection of granitic rock specimens was used to test previous mapping and analyses from the Sebago pluton NE contact zone with surrounding migmatite-granite complex country rocks (e.g., LaFleur et al., NEGSA 2008; Nyitrai et al., 2009; Bohlen et al., NEGSA 2010). The contact zone is the center of our study area (near Gray, ME) where the coarse-grained granite is interleaved with migmatite country rocks. To the west is the Sebago pluton (sensu stricto) defined by homogeneous, medium-to coarse-grained 2-mica granite. In contrast to the Sebago pluton, to the east rocks are strongly heterogeneous in a complex of plastically deformed migmatites and granites with varied compositions and textures in the migmatite-granite complex, MGC, of S. Maine, and interpreted to be country rock to the pluton emplacement. Granites of the contact zone are relatively similar to the pluton rocks with discrete finer grained granite dikes.

We selected representative granite specimens along a W-E transect, 1 from each map unit and across the pluton contact. The specimen from the Sebago pluton is from a 300-meter-long roadcut of homogeneous coarse-grained 2-mica granite. The specimen from the contact zone is from a 3-meter-wide dike of medium-grained 2-mica granite that crosscuts the main layers that define the contact zone. The granite from the MGC is from a boudinaged granite layer in migmatite. We collected mineral composition and grain size data for comparison. Thin sections were cut according to any fabrics. We documented any preferred orientations of minerals and grain-shape fabrics.

All specimens are similar in composition but contrast in texture owing to map unit sample location. The Sebago pluton specimen is coarse grained with mosaic equigranular to seriate texture. Undulose extinction in quartz is the only strain microstructure. The contact zone specimen is relatively strain free, similar in texture (finer grained) to the Sebago specimen. In contrast, the MGC specimen is granitic gneiss with significant plastic strain recorded including undulose extinction in both quartz and feldspar, both with serrated grain boundaries, and a strong mica foliation. Consistent with previous work, clearly the Sebago pluton is distinct from the MGC country rocks, yet the contact zone granitic rocks more closely resemble the pluton.