High-resolution compositional mapping of matrix phases: implications for mass transfer during crenulation cleavage development in the Moretown Formation, western Massachusetts

High-resolution compositional maps provide a new tool for investigating mass transfer during cleavage formation. The Moretown Formation of western Massachusetts contains a well-developed crenulation cleavage with alternating mica-rich crenulation limbs and mica-poor crenulation hinges. Compositional mapping shows two generations of plagioclase, the second of which was synchronous with the crenulation cleavage. A significant amount of the syn-crenulation plagioclase (10–20% modally) grew in hinge domains. A small amount of syn-crenulation plagioclase (∼1%) and a large amount of phengitic muscovite grew in limb domains. The maps also show that uncrenulated domains experienced mass transfer and reactivation of older cleavages, and thus cannot be used as ‘undeformed’ reference domains for comparison with crenulated regions. Compositional mapping facilitates a new degree of integration between petrologic and structural analysis. Knowledge of the structural context of compositional domains allows better selection of phases and compositions for interpreting metamorphic reactions and linking metamorphism to deformational stages. Knowledge of syntectonic reactions provides new insights into mass transfer and volume change during deformation. In the Moretown Formation, plagioclase- and phengite-producing reactions play a large role in controlling the nature and magnitude of mass transfer, but microstructures control the location of reactants and products within the evolving fabric.