Metasedimentary rock fragments in a breccia at Ascutney Mountain, Vermont, have a complex metamorphic history that involved at least one and perhaps two episodes of Paleozoic regional metamorphism as well as Cretaceous contact metamorphism that resulted from the emplacement of Ascutney Mountain magmas into the Paleozoic country rock. Herein I derive a set of linearly independent net-transfer reactions that represent possible transfers of matter among the minerals in garnet-cordierite schist fragments, a major constituent of the breccia. I use net-transfer reactions to construct three reaction spaces. The reaction spaces define volumes within which an infinite number of reaction paths might have been followed by the schist fragments during metamorphism. I constrained the reaction path by using modal-abundance, textural, and chemical data from the schist fragments in the breccia. The example illustrates the elegant utility of reaction space: for any system, a volume may be defined within which metamorphic reactions occurred during the history of a rock. By examining the modal abundances, textures, and chemistry of the constituent minerals, one may discern and depict the history of complexly metamorphosed rocks.

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