PORPHYROBLAST-MATRIX RELATIONS IN MYLONITE IN THE NORUMBEGA SHEAR ZONE SYSTEM, HARPSWELL NECK AREA, SOUTHERN COASTAL MAINE

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 mylonites from 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.

Nine 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; 18 sections). Collected data started with mineral composition. Microstructural data include the shape, geometry, and orientation of matrix fabrics (Se), porphyroblast sizes and shapes, and inclusion trail (Si) geometries and orientations, and Si/Se pitches (n=167 pitches). Mineral content, shape, and size of porphyroblast strain-shadow tails are also recorded.

Matrix mineral compositions are similar (majority Qtz+Ms+Bt), and strongly-developed, sub-parallel planar and linear fabrics (Se). Garnet porphyroblasts are 3.0-20 mm in diameter with straight Qtz-mica inclusion trails (Si). Garnet and their tails are draped by 0.05-0.1 mm-thick matrix minerals. Tails are distinctive and cone-shaped with long axes along the lineation, but oblate with the flat aspect in the matrix foliation (Se). Tail sizes range 0.1-0.6 mm from porphyroblast boundary to tip (most 0.1-0.3 mm). The tail length ratios along the fabric vs. across the fabric are 1.0-2.0, revealing that tails tend longer along the fabrics. Si and their pitches relative to matrix foliation (Se) in all sections are consistent with the shape and the size of garnet porphyroblasts. Inequant garnet crystals have measurable inclusion trails, whereas equant garnet crystals have few to no inclusion trials. Comparing thin section pair Si orientations in 3 specimens permitted the determination of 3D Si, but those foliations show no preferred orientation once placed on our regional map suggesting that either there is no pattern, or mylonite formation was differential in the zone, or both.