An intrusion breccia exposed along Roaring Brook on the west face of Giant Mountain in the Adirondack Highlands (eastern USA) has been reinterpreted as a magma conduit. It lacks metasedimentary xenoliths, as previously proposed, and field, petrographic, and geochemical evidence point to an igneous origin for the vast majority of the enclaves. A mechanism involving magma mixing is proposed for the enclaves and explains their textural features (e.g., crescumulate and comb textures and mantled feldspars), compositional layering, and geochemical trends. Rock types ranging from charnockite to pyroxenite occur in the exposed section and document the variety of magmatic processes and compositional variation of magmas composing the Marcy Anorthosite massif. The bulk of the enclaves in the intrusion breccia can be explained by a batch of more felsic magma (charnockite-mangerite?) that intruded into a partially crystallized zone of anorthositic, gabbroic, and dioritic rock. This resulted in disruption, partial mixing and hybridization, and chilling of mafic magma within, and against, the more silicic melts.