The Thetford Mines ophiolite in southern Quebec is partially underlain by an 800 m thick amphibolite aureole. Rocks in the metamorphic unit grade inward towards the base of the ophiolite from chlorite–muscovite schist, epidote amphibolite, and garnet amphibolite to clinopyroxene-bearing garnet amphibolite closest to the fault contact with the basal tectonized peridotite unit of the ophiolite. The polydeformed metamorphic unit pinches out north and south of Belmina Ridge and is discordant with the overlying tectonized peridotite. Analyses of nine samples show a range of compositions, but the low K2O, P2O5, Rb, Zr, and Sr suggest that the amphibolite protolith was tholeiitic basalt, probably midocean ridge tholeiite.Microprobe mineral compositions determined for coexisting clinopyroxene–garnet–amphibole and garnet–amphibole suggest that amphibolites near the peridotite contact equilibrated at about 850 °C and that the temperatures decreased to about 380 °C 700 m from the contact. Geobarometers based on coexisting clinopyroxene and garnet, and Al and Na partitioning in amphibole yield variable results, but suggest that the pressure during formation of the amphibolite was probably greater than 5 kbar (5 × 105 kPa).Eight conventional K–Ar ages average 488 ± 11 Ma but show some scatter due to inaccuracy of K2O determinations at very low concentrations and, in some samples, possible inherited argon. A single 40Ar/39Ar incremental heating experiment gives concordant plateau and isochron ages of 491 ± 4 and 494 ± 4 Ma, respectively. The best weighted age of 491 ± 3 Ma probably dates the decoupling of the Thetford Mines ophiolite during the initial phase of closing of the Proto-Atlantic Ocean.