MINERAL AND TEXTURAL VARIATIONS IN GRANITES OF THE SEBAGO PLUTON AT ITS EASTERN CONTACT ZONE, SOUTHERN MAINE

LaFleur et al. (NEGSA ‘08) mapped part of the ~1 km-wide, E contact zone of the homogeneous, 2-mica granite Sebago pluton (~ 400 km²) with the Sebago Migmatite Domain (2.3 km-long, N-S road cut, Gray, ME). In the field, the zone is defined by m-scale ‘ghost stratigraphy' of migmatitic deformed rocks interlayered with batch-injected m-scale sub-concordant granite sheets that dominate the exposure. To test timing, we focus on variations in 33 representative granite specimens. Parallel studies focus on absolute timing and comparative geochemistry (McAdam et al., NEGSA '09).

Coarse-grained granite dominates, but 6 granites exist based on composition, grain size and textures: 2-mica granite that is fine-grained (1-2 mm), medium-grained (2-3 mm) and coarse-grained (3-5 mm), fine-grained Bt granite, and two pegmatites separated by grain size. Fine-grained granite is >50% Kfs, equal % Pl and Qtz, and has a weak mica fabric. Medium-grained granite is also >50% Kfs with a weak fabric, ~20% Qtz and relatively low Pl. Coarse-grained granite has a weak mica fabric and equal % Pl, Qtz, and Kfs. Bt granite has penetrative Bt fabric, relatively high %Bt v. 2-mica granite, but ~50% Kfs, 20% Pl and 20% Qtz. Finer-grained (4-10 mm) and coarser-grained (5-20 mm) pegmatites are nearly 50:50 Pl:Qtz. Not apparent in the field, microstructures reveal solid-state strain in all specimens. Qtz has common serrated/sutured edges and strongly-developed subgrain boundaries, some deformation bands or at least distinct undulatory extinction. Pl and Kfs also have these textures in most rocks, including absence of twins in some subgrains. Kfs deformation twins are in most sections; bent Pl twins are in about half. Grain-size reduction of Qtz and feldspar is strongly developed in some samples, but not mylonitic, nor is there an apparent grain-shape fabric, but some samples show local LPOs. Local myrmekitic textures are in one sample of 2-mica granite.

These textures are consistent with strain recorded above greenschist facies, but not higher than amphibolites facies. In contrast, granites in the pluton are not deformed, whereas granites in the migmatite domain are penetratively strongly deformed. Thus, we interpret progressive contact zone assembly by batch injection of granitic magma followed by solid-state strain during final pluton emplacement.