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 (
![](/images/glyphs/BQ1.GIF)
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.