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 mas s transfer during cleavage formation. The Moretown Formation of western Mas sachusetts 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 sync hronous with the crenulation cleavage. A significant amount of the syn-cren ulation plagioclase (10-20% modally) grew in hinge domains. A smalt amount of syn-crenulation plagioclase (similar to1%) and a large amount of phengit ic muscovite grew in limb domains. The maps also show that uncrenulated dom ains experienced mass transfer and reactivation of older cleavages and thus cannot be used as 'undeformed' reference domains for comparison with crenu lated regions. Compositional mapping facilitates a new degree of integratio n between petrologic and structural analysis, Knowledge of the structural c ontext of compositional domains allows better selection of phases and compo sitions for interpreting metamorphic reactions and linking metamorphism to deformational stages. Knowledge of syntectonic reactions provides new insig hts into mass transfer and volume change during deformation. In the Moretow n Formation, plagioclase- and phengite-producing reactions play a large rol e in controlling the nature and magnitude of mass transfer, but microstruct ures control the location of reactants and products within the evolving fab ric. (C) 2001 Elsevier Science Ltd. All rights reserved.