We report the petrological and zircon U-Pb and trace element study of metamafic rocks from the Saint-Joseph massif in the Lyonnais metamorphic complex. The samples are ultrabasic and predominantly composed of garnet–amphibole (up to 75–85%), with plagioclase–quartz (10–15%), titaniferous minerals (5–10%), and clinopyroxene relics (>5%). Some samples show a layering marked by garnet abundance ranging from 0 to 40 %. Plagioclase coats the grain boundaries of garnet and amphibole porphyroblasts, resembling a melt pseudomorph. Thermodynamic modelling of a selected sample indicates that fluid-fluxed melting at 10–12 kbar and 800–900 °C, followed by cooling to ~700 °C, can explain the assemblage garnet–amphibole–plagioclase–quartz–titanite. Separated zircon crystals commonly display a core with oscillatory, or sector zoning, rimmed by domains with complex convolute zoning. This pattern suggests polyphase zircon coupled dissolution-reprecipitation. The analyses of zircon rims show either a flat or a positive slope in the heavy REE pattern, irrespective of the date or intra-grain location. This feature is tentatively attributed to the variable garnet content in the rock layering. Zircon core and rim yield an equivalent smear of apparent ages between c. 360–320 Ma, interpreted as a result of melt-mediated dissolution-reprecipitation. One sample contains an additional zircon zone found as CL-dark cores with oscillatory or featureless zoning, and characterised by an exceptionally high trace-element content, e.g. up to 2000 ppm of U. These cores also yield a smear of apparent, on average, older ages, between c. 425–320 Ma. Nine-spots yield a concordia date of 423.8 ± 4.8 Ma with a MSWD of 0.99 regarded as the age of the magmatic protolith. The occurrence of a Silurian mafic magmatism may be related to mantle melting at the northern Gondwana margin, possibly in a suprasubduction setting. The sample records predominantly a Carboniferous fluid-fluxed partial melting event.