17 D3. Petrology, Tectonics, and Structural Geology

Monday, 15 March 2021
1:30 PM - 5:30 PM
Windsor

FABRIC ANALYSIS IN MYLONITE IN THE NORUMBEGA SHEAR ZONE SYSTEM, FREEPORT-HARPSWELL AREA, SOUTHERN MAINE

Paper number: 17-15
Program: Technical Sessions
Time: 4:50 PM – 5:00 PM

Authors

Abstract

Porphyroblast-matrix relations and mineral comparison in similar meta-sedimentary rock mylonites of southern Maine have recorded lower Paleozoic deformation episodes in the Norumbega Shear Zone System (NSZS). 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 Freeport, Maine, NE of Portland, coastal Maine, where a complex strain history was recorded up to and including mylonite formation. We compare data from these samples to the data from rocks analyzed previously in the adjacent area as both an expansion and a test of previous conclusions in the South Harpswell area.

Five specimens were selected from the Freeport, Maine area to be compared to mineral and porphyroblast data from nine specimens previously studied. 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 include 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 (where present), and Si/Se pitches. Mineral content, shape, orientation, and size of porphyroblast strain-shadow tails is also recorded. Results show matrix mineral compositions are noticeably similar to the South Harpswell area rocks (majority Qtz+Ms+Bt), and have strongly-developed, sub-parallel planar and linear fabrics (Se). In contrast to South Harpswell rocks, garnet porphyroblasts in our current study are not as frequent, or as large. Rocks are relatively more quartzo-feldspathic with similar planar and linear fabrics defined by metamorphic minerals. Rocks of the present study are complimentary to rocks of previous work, and we interpret they have recorded strong non-coaxial strain associated with deformation in the NSZS.

Geological Society of America Abstracts with Programs. Vol. 53, No. 1

doi: 10.1130/abs/2021NE-361917

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