Physical Geography and Sciences



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Genna Baldassarre, Geology
Faculty Mentor(s): Professor Gary Solar, Earth Sciences and Science Education

Rocks exposed in southern inland Maine, located in the Freeport area, were formed over 400 million years ago, and then metamorphosed and deformed from about 400 to 280 million years ago. This was within a more than 15 km-wide zone of continental collisional deformation known as the Norumbega shear zone system (NSZS) during the Acadian through Alleghenian Orogenies (mountain building during continent-continent collisions). Evidence of the tectonic plate collisions in question exists at the grain scale, and can be studied and measured through the use of polarized transmitted-light microscopy (petrography) of thin sections. Thin sections were prepared from specimens carefully selected from a suite of rocks collected in the field in the Freeport area. They were then described and measured extensively, with the main focus of the study on mineral composition related to near areas, and characteristic mineral growths resulting from metamorphism unique to this field area. Compared to other similar rocks from surrounding areas, the rocks I have studied from Freeport have a lesser porphyroblast-composed matrix and a more interlocked quartz and twinned plagioclase matrix. Whereas subsequent deformational events may alter the matrix minerals, these inclusion minerals are preserved and reflect the original matrix at time of growth. Mineral data was collected from thin section creation and analysis. Throughout thin sections there is consistent biotite lineation, confirming deformation during metamorphism and the apparent direction of the sample foliations. All of these data are reconciled to form interpretations of the tectonic history as recorded by the rock itself. The data documented shows that the extensive tectonic history within the Norumbega shear zone system was long-lived as new matrix fabrics formed over old ones progressively.

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Structural and Geochemical Documentation of Metamorphic Rocks from Coastal and Inland Maine
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