Two-stage partial melting during the Variscan extensional tectonics (Montagne Noire, France)

Abstract

One of the striking features that characterise the late stages of the Variscan orogeny is the development of gneiss and migmatite domes, as well as extensional Late Carboniferous and Permian sedimentary basins. It remains a matter of debate whether the formation of domes was related to the well documented late orogenic extension or to the contractional tectonics that preceded.Migmatization and magmatism are expected to predate extension if the domes are compression-related regional anticlines, but they must both precede and be contemporaneous with extension if they are extensional core complexes. In the Montagne Noire area (southern French Massif Central), where migmatization, magmatismand the deformation frameworkare well documented, the age of the extensional event was unequivocally constrained to 300-0Ma.Therefore,dating migmatization in this area is a key point for discriminating between the two hypotheses and understanding the Late Palaeozoic evolution of this part of the Variscan belt. For this purpose, a migmatite and an associated anatectic granite from the Montagne Noire dome were dated by LA-ICP-MS (U-Th/Pb onzircon and monazite) andlaser probe40Ar-39Ar (K-Ar on muscovite). Although zircon did not record any Variscan ageunequivocally related to compression (380-330Ma),two age groups were identified from the monazite crystals. A first event, at ca. 319 Ma (U-Th/Pb on monazite),is interpreted as a first stage of migmatization and as the emplacement age of the granite, respectively. A second event at ca. 298-295 Ma, recorded by monazite (U-Th/Pb) and by the muscovite 40Ar-39Ar system in the migmatite and in the granite, could be interpreted as a fluid-induced event, probably related to a second melting event identified through the syn-extensional emplacement of the nearby Montalet leucogranite ca. 295 Ma ago. The agesof these two events post-date the Variscan compression and agreewith an overall extensional context for the development of the Montagne Noire dome-shaped massif.Comparison of these results with published chemical (EPMA) dating of monazite from the samerocks demonstrates that the type of statistical treatment applied to EPMA data is crucial in order to resolve different monazite age populations.