Information derived from regional geology, styles of magma emplacement, xenoliths, and western Grenville Lithoprobe reflectors is integrated to model the architecture of the Central Metasedimentary Belt in Quebec. The belt comprises a thin-skinned western marble domain that projects structurally to the east above a quartzite domain; both domains are underlain by a series of gneiss domes. In contrast, the seismic reflectors dip systematically to the east. They stem from exposed high-strain zones and tectonic boundaries hosting concordant, 1.7–1.16 Ga, postmetamorphic sheetlike plutons. Though discontinuous, the reflectors reach highly reflective crust segments near the Moho. The reflections are interpreted as series of plutons positioning crustal-scale structures rooted in underplated, felsic, and mafic magmas reservoirs. These structures served as magma conduits and ponding sites. The nonreflective segments of the inferred structures are interpreted as through-going pathways for magmas in gneissose crust. Based on the thousands of felsic to ultramafic xenoliths brought up by a 1.07 Ga minette dyke, the nature of the intermediate and lower crust is interpreted as interleaving of nonexposed quartzite-bearing supracrustal assemblages with mylonites, gneisses, and a series of mafic to ultramafic intrusive bodies, many of them inferred to be underplated material. In western Quebec, crustal growth along the pre-Grenvillian Laurentian margin involved pre-1.2 Ga stacking of allochthons including nonexposed metasedimentary and intrusive components during an early stage of Grenvillian collision. Subsequent, 1.7–1.6 Ga magma emplacement along high-strain zones shaped much of the seismic reflectivity of the belt.