QUARTZ CRYSTALLOGRAPHIC FABRICS FROM AN ISOCLINAL FOLD, CUMMINGTON, MA

The large-scale map pattern of the Goshen Formation in western Massachusetts is dominated by approximately 100 m-scale isoclinal folds. This study looks at the microstructures associated with one of these folds, a single well-exposed isoclinal anticline in Cummington, Massachusetts. Here, the Goshen Formation is a thin-bedded (10s of cm) graded micaceous quartzite to phyllite sequence that was metamorphosed to staurolite-kyanite grade.

A roughly 12 meter section of the anticline was sampled with oriented samples taken from both limbs and the hinge. The anticline has a small interlimb angle of no more than approximately 5º. The Goshen Formation has a penetrative foliation that is defined predominantly by the alignment of fine-grained muscovite, but also by the amount of fine-grained quartz and, less commonly, the alignment of biotite. Pressure shadows and inclusions in euhedral to anhedral garnet porphyroblasts ranging from 1 mm to 3 mm indicate garnet growth synchronous with the isoclinal folding. More rarely, staurolite porphyroblasts are present in several samples. Albite is modally abundant in at least one sample.

Quartz content in the samples varies greatly from roughly 20% to 80% modal percentage, depending on whether the sample derives from the more quartz-rich or mica-rich rock of the graded beds. Even in well-foliated samples, quartz grains are anhedral, equidimensional to slightly elongate with straight to gently curved grain boundaries and uniform to only slightly undulose extinction. Quartz lattice preferred orientations (LPO) have been determined by electron backscatter diffraction (EBSD). Preliminary quartz LPOs have relatively weak preferred orientations with a c-axis maximum at a high angle to foliation. The weak quartz grain shape fabric and LPO occur in samples in which large strains are suggested by both the fold morphology and a penetrative fabric.