An approximately E-W-trending belt of porphyritic peraluminous granitic rocks, metamorphosed and deformed to augen gneiss, is exposed for 400 km across the Yukon-Tanana terrain. Chemical, textural, and isotopic data from large augen-gneiss bodies indicate that these bodies originated as early Mississippian granitic rocks that assimilated, or were anatectically derived from, early Proterozoic crust or metasedimentary rocks. This plutonic belt probably formed in a middle Palaeozoic continental magmatic arc that developed near the edge of a Precambrian craton somewhere along the western margin of North America and was later translated NW to its present location. U/Pb zircon data and concordance of augen-gneiss contacts with metamorphic layering and with probable late-stage sills suggest that regional metamorphism to amphibolite facies of these rocks was synchronous (late kinematic) with intrusion of the porphyritic granitic protolith. -L.di H.