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