# The zircon record of Carboniferous partial melting in Silurian mafic rocks from the Lyonnais metamorphic complex (French Massif Central)

Luc de Hoÿm de Marien, Simon Couzinié, Marc Poujol, Oscar Laurent, Anne-Cécile Ganzhorn, Jean-François Moyen, Pavel Pitra, Steve Collett

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Luc de Hoÿm de Marien, Simon Couzinié, Marc Poujol, Oscar Laurent, Anne-Cécile Ganzhorn, et al.. The zircon record of Carboniferous partial melting in Silurian mafic rocks from the Lyonnais metamorphic complex (French Massif Central). 29 ème édition de la réunion des sciences de la Terre (RST 2025), Société Géologique de France (SGF), Oct 2025, Montpellier, France. pp.657848. insu-05361258

HAL Id: insu-05361258

https://insu.hal.science/insu-05361258v1

Submitted on 12 Nov 2025

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# The zircon record of Carboniferous partial melting in Silurian mafic rocks from the Lyonnais metamorphic complex (French Massif Central)

Luc De Hoÿm De Marien*¹, Simon Couzinié²,³, Marc Poujol⁴, Oscar Laurent⁵, Anne-Cécile Ganzhorn³, Jean-François Moyen⁶, Pavel Pitra⁴, and Steve Collett¹

¹Czech Geological Survey – République tchèque

²Centre de Recherches Pétrographiques et Géochimiques – Institut National des Sciences de l’Univers, Université de Lorraine, Centre National de la Recherche Scientifique, Institut National des Sciences de l’Univers : UMR7358, Centre National de la Recherche Scientifique : UMR7358 – France

³Laboratoire de Géologie de Lyon - Terre, Planètes, Environnement – Ecole Normale Supérieure de Lyon, Université Claude Bernard Lyon 1, Institut National des Sciences de l’Univers, Université Jean Monnet - Saint-Etienne, Centre National de la Recherche Scientifique – France

⁴Observatoire des sciences de l'environnement de Rennes – Université de Rennes, Institut National des Sciences de l'Univers, Université de Rennes 2, Centre National de la Recherche Scientifique, Institut National de Recherche pour l'Agriculture, l'Alimentation et l'Environnement – France

⁵Géosciences Environnement Toulouse – Institut de Recherche pour le Développement, Université Toulouse III - Paul Sabatier, Institut National des Sciences de l'Univers, Observatoire Midi-Pyrénées, Centre National d'Études Spatiales [Toulouse], Centre National de la Recherche Scientifique, Institut National des Sciences de l'Univers : UMR5563, Centre National de la Recherche Scientifique : UMR5563, Institut de Recherche pour le Développement : UR254 – France

⁶Laboratoire Magmas et Volcans – Institut de Recherche pour le Développement, Institut National des Sciences de l'Univers, Centre National de la Recherche Scientifique, Université Clermont Auvergne, Observatoire de Physique du Globe de Clermont-Ferrand – France

## Résumé

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 ultra-basic 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

<footnote>*Intervenant</footnote>
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.