STRUCTURAL AND PETROLOGIC ANALYSIS OF/IN THE MORETOWN FM, WESTERN MASSACHUSETTS: CONSTRAINING REGIONAL TECTONICS AND ILLUMINATING DEFORMATION PROCESSES

The Moretown Formation in western Massachusetts displays superb crenulation cleavage(s) that can illuminate the tectonic history of western New England and also serve as a laboratory for investigating deformation processes in general. Most rocks preserve a North-striking, steeply dipping crenulation cleavage of varying intensity. This upright cleavage deforms a strong shallowly dipping foliation that itself resulted from the deformation (crenulation and transposition) of one or more earlier cleavages. Garnet inclusion trails have spiral and sigmoid geometries that preserve early stages in crenulation development. The deformation history is particularly well preserved in the km-scale “strain shadow” of the 494Ma Hallockville Pond Gneiss. Moving outward from the gneiss, an early shallowly-dipping foliation is progressively transposed into the regional upright cleavage. High resolution electron microprobe compositional mapping of plagioclase and other matrix phases has been used to constrain the nature and magnitude of mass transfer during deformation. Although previous textural studies have focused on quartz mobility, WDS compositional maps show that plagioclase grains are larger and more abundant in q-domains and macro-fold hinges than in crenulation limbs (p-domains). High resolution maps can be used to distinguish older plagioclase from new dissolution-precipitation plagioclase, and can place constraints on mass transfer during crenulation evolution. Significant amounts of plagioclase were added to q-domains during crenulation formation. The q-domains do not preserve the composition or character of the pre-crenulation fabric, nor do fold hinges or limbs of macroscopic folds. Recent mechanical modeling has been used to clarify the stresses and driving forces during crenulation evolution using the Moretown cleavage as a model. Crenulation development in the Moretown Fm. does not simply involve stress- or strain-induced dissolution-precipitation of quartz, but also a quantifiable, metamorphically-driven change in the mode and composition of all minerals.