The recent discovery of menzerite-(Y), a garnet containing essential Y, in Mesoproterozoic supracrustal granulites from the southwestern Grenville province has prompted an investigation into the petrogenesis of these rocks, and their Y + HREE-bearing phases, through a high-grade metamorphic cycle. A majority of samples from the supracrustal sequence have bulk compositions that are similar to that of average Proterozoic andesitic and basaltic volcanic rocks. Light REE concentrations in samples from the menzerite-(Y)-bearing locality are similar to that of average upper continental crust, whereas HREE concentrations are considerably higher. Extrapolating from the REE concentrations in detrital zircon cores and bulk Zr content, primary zircon could have contributed only ~6% Y and ~14% Yb to the whole-rock budget. Thus another phase, most likely xenotime, was the main carrier of Y + HREE in the precursor and the source of these constituents for menzerite-(Y). Microstructural relations indicate that menzerite-(Y) crystallized early, in equilibrium with oligoclase, ferrosilite, quartz, clinopyroxene, and iron oxides, then broke down during partial melting, and finally was overgrown by Y + HREE-enriched euhedral almandine as the rocks underwent further heating and burial. Phase-equilibrium modeling constrains conditions for menzerite-(Y) stability to pressures less than 8.5 kbar and temperatures between 550 and 780 °C (at pressures above 5 kbar) for the bulk composition of the host rock. The new constraints on P-T conditions of prograde and peak metamorphism, when combined with other recently acquired data, define a counterclockwise P-T-t path for the PSD that records post-depositional burial of magmatic arc rocks to lower crustal depths between ca. 1250 and 1160 Ma. Peak temperature conditions were followed by a period of isobaric cooling to subsolidus conditions (by ca. 1145 Ma) and subsequent nappe-style transport (at ca. 1100 Ma) onto the Laurentian margin during the Grenville orogeny.