Strict limitations are placed on reacting chemical systems during metamorphism if one or more of the mineral phases is compositionally zoned. An investigation of the volumes of equilibrium in some high-grade metamorphic rocks from the Grenville of southeastern Ontario, therefore, included a study of possible zoning of the garnets. Traverses at 25 micron steps across four garnets, employing a laser-excited spectrochemical technique, revealed no compositional zoning with respect to Fe, Mg, Mn or Ca in three of the grains. High Mn values, however, occur in the central portion of one garnet within an area of quartz inclusions. It is proposed that the garnets of these rocks were originally zoned and that later metamorphic events have eliminated this zoning in most of the garnets, some still showing a relic zoning in their central portions.

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