Abstract

Mineral assemblages of fissure veins in pelitic rocks are a consistent function of metamorphic grade within a 1,700 km2 area in and near Dutchess County, New York. The following vein assemblages are cb-served with increasing grade: quartz and quartz-calcite up to just above the staurolite isograd, with limited occurrence of quarrz-albite below the biotite isograd; quartz and quartz-plagioclase (An20 to An50) from the staurolite isograde up to the sillimanite-orthoclase isograd; and quartz, quartz-plagioclase, and quartz-plagioclase-orthoclase above the sillimanite-orthoclase isograd. Extreme deformation makes relative crosscutting relations of veins difficult to determine; however, late quartz veins are fairly common, and late quartz-calcite veins are seen in high-grade rocks near marble contacts.

A tentative conclusion is that quartz and quartz-calcite, quartz-plagioclase, and quartz-plagioclase-orthoclase veins form in pelitic rocks at successively higher metamorphic grades from material derived from the surrounding matrix. This may happen during prograde and retrograde metamorphism and during successive metamorphic events.

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