Metamorphic zones in northern Vermont

Abstract

The metamorphosed pelitic rocks in north-central and northwestern Vermont are strikingly different from those in northeastern Vermont. North-central Vermont is a schistose, regionally metamorphosed terrane, typical of the kyanite-sillimanite facies series of Miyashiro (1961), whereas northeastern Vermont is a hornfelsic, "regional-contact" metamorphosed terrane more typical of the low-pressure intermediate, or Buchan, facies series of Miyashiro (1961). These pronounced differences in the type of metamorphism may be reflections of different times as well as different physical conditions of metamorphism. Higher-grade assemblages in the eastern part of the western metamorphic belt are extensively altered to lower-grade mineral assemblages, although similar higher-grade assemblages somewhat farther west have not undergone such alteration. This observation suggests the possibility that a younger, eastern metamorphic belt has been partially superimposed on an older, more westerly metamorphic belt and that the low-grade adjustment occurred synchronously with higher-grade metamorphism in the eastern belt. The available isotopic dates can be interpreted to indicate that the two belts were not formed simply as stages of a single period of metamorphism, but have substantially different ages. Regardless of their relative ages, distinct differences between the two belts imply that the physical conditions of metamorphism were different. Published assemblages have indicated a sequence of metamorphic reactions in eastern Vermont quite unlike those in western Vermont; leading one to expect that the difference in conditions between the two metamorphic belts might result in different chemical fractionation of Mn, Fe, and Mg between the phases. As will be shown, no differences in the sequence of metamorphic reactions can be demonstrated and the apparent differences between the two belts are due mainly to gross differences in rock composition. Moreover, the temperature dependency of the partition coefficients is so small that it is obscured by other effects.

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