ORIGIN AND SIGNIFICANCE OF PORPHYROBLAST-MATRIX FABRIC OBLIQUITIES, WESTERN MAINE

Porphyroblast growth during the Acadian orogenic phase in Maine metapelites has traditionally been considered to be post-tectonic. This posit is tested using microstructural analysis of a suite of closely-spaced (metre-scale) samples of Perry Mountain Formation schist at Coos Canyon, Maine. Particular emphasis is placed on the use of porphyroblast-matrix fabric obliquities as a tool for determining relative timing of metamorphism and deformation. Because porphyroblast inclusion fabrics are interpreted to record relict foliation, it has been suggested that the angle made with the matrix fabric is related to the amount of matrix strain accommodated during or after porphyroblast growth. Previously, fabric obliquities have been studied across a km-scale region between the Mooselookmeguntic and Lexington plutons, and it has been proposed that obliquities developed due to matrix flattening as regional folds tightened. According to this model, fabric obliquities in western Maine indicate that Acadian deformation outlasted porphyroblast growth.

Research presented here evaluates whether this model is applicable at outcrop scale, using a high-resolution suite of 45 oriented samples from a continuous strip of outcrop spanning 132 meters across strike. Fabric obliquities in staurolite and garnet porphyroblasts were measured in lineation-parallel and lineation-perpendicular thin-sections from each sample. The resulting data allow comparison of obliquity magnitudes (clockwise or anticlockwise relative to matrix foliation) with respect to mineralogical domains and location on the outcrop, and make it possible to constrain the location of fold axial surface traces based on observed reversals in porphyroblast-matrix fabric obliquity across strike. Models of folding predict different distributions of fabric obliquities as a function of fold kinematics, deformation path, and multi-layer strain partitioning. Distributions observed in samples are compared with predicted distributions in order to characterize strain partitioning and fold kinematics. Orientation of matrix foliation-inclusion foliation intersection lineations is being determined in three dimensions to infer mechanisms of formation of fabric obliquities. Theoretical considerations predict these lineations will have consistent orientations with respect to host folds if obliquities formed by fold-tightening.