Granulite facies metamorphic xenoliths have been found in several Monteregian-age lamprophyric dikes at Ayer's Cliff, Quebec, Canada. From geothermobarometric calculations,it is shown that middle to lower crustal metapelitic assemblages formed at 700-800 °C and 8-10 kbar, whereas mafic assemblages formed near 1200 ±100 °C and 14 ±1 kbar. The metapelitic assemblages are interpreted as Cambrian metasediments similar to those found farther south in Vermont. The mafic assemblages are interpreted as Grenville basement. The occurrence of these lithologies at Ayer's Cliff and in similar dikes in Vermont confirms the seismic interpretation of Grenville basement beneath a large part of the western Appalachians in southern Quebec and northern New England.