MYLONITES IN COASTAL MAINE: MICROSTRUCTURAL ANALYSIS OF GARNET PORPHYROBLAST-MATRIX RELATIONS

Porphyroblast-matrix relations in the meta-sedimentary rock mylonites of southern coastal Maine have recorded lower Paleozoic deformation episodes in the Norumbega shear zone system. We document microstructures of primarily garnet porphyroblasts and associated matrix mineral fabrics in a corridor of the NE-SW-trending deformation belt, in and surrounding Harpswell Neck, NNE of Portland, coastal Maine, where a complex strain history was recorded up to and including mylonite formation.

Five specimens were selected after mapping. Pairs of perpendicular thin sections were cut from each according to fabrics, one parallel to lineation, and perpendicular to foliation, and the other perpendicular to both (X-Z and Y-Z planes, respectively; 10 sections). Collected data started with mineral composition. Microstructural data include the shape, geometry, and orientation of matrix fabrics (Se), porphyroblast sizes and shapes (n=685), and inclusion trail (Si) geometries and Si-Se pitches (n=142). The mineral content, shape and size of porphyroblast strain-shadow tails are also recorded.

Matrix mineral compositions are similar in all specimens (majority Qtz+Ms+Bt), and define strongly developed, sub-parallel planar and linear fabrics. Garnet porphyroblasts range in shape from equant to strongly inequant (avg. 2:1), and are 0.9-3.7 mm in diameter. Strain shadow tail sizes range 0.1-3.9 mm from porphyroblast boundary to tip (avg. 0.6 mm), and are distinctly oblate-cone-shaped whose flat aspect is along the matrix foliation and long axes along the lineation. Where present, porphyroblasts have straight, dominantly moderately- to steeply-pitching inclusion trails (Si) of mostly Qtz and opaques that are sub-parallel to the long dimension of inequant garnet porphyroblasts. Si orientations in paired thin sections from 2 specimens permitts the determination of Si foliation. Results show Si foliation in each rock are NE-striking and steeply SE-dipping, and 18-55° to the NNE-SSW strike of regional foliation.