Cordierite–gedrite rocks in the southern Grenville Province occur near the base of the Central Metasedimentary Belt boundary thrust zone, interpreted by some as a crustal suture between the 1.29–1.24 Ga Composite Arc Belt and >1.4 Ga rocks of Laurentia. Major and trace-element compositions of these rocks are consistent with volcanic protoliths that range in composition from basalt to dacite. These cordierite–gedrite rocks have low CaO (average 1.2 wt.%) and major element and oxygen-isotope ratios suggestive of hydrothermal alteration before metamorphism. Rare-earth element (REE) compositions also indicate igneous protoliths, although some REE patterns have been modified by local melt extraction. The trace-element compositions of cordierite–gedrite rocks, and neodymium-isotope systematics, are similar to those of metavolcanic rocks in the Composite Arc Belt and are consistent with the extension of the Composite Arc Belt to the base of the boundary thrust zone.