Granulite facies lithologies from the Adirondack Mountains of New York contain alteration assemblages composed dominantly of calcite ± chlorite ± sericite. These assemblages document fluid infiltration at middle to upper crustal levels. Cathodoluminescence of samples from the Marcy anorthosite massif indicates that the late fluid infiltration is more widespread than initially indicated by transmitted-light petrography alone. Samples that appear unaltered in transmitted light show extensive anastomosing veins of calcite (<0.05 mm wide) along grain boundaries, in crosscutting fractures, and along mineral cleavages. The presence of the retrograde calcite documents paleopermeability in crystalline rocks and is related to the formation of high-density CO2-rich fluid inclusions. Recognition of this process has important implications for studies of granulite genesis and the geo-physical properties of the crust.