Middle Carboniferous intracontinental subduction in the Outer Zone of the Variscan Belt (Montagne Noire Axial Zone, French Massif Central): multimethod geochronological approach of polyphase metamorphism

Abstract

AbstractIn the migmatitic dome of the Montagne Noire Axial Zone (Variscan French Massif Central), mafic eclogites yield zircon and rutile U–Pb SHRIMP and secondary-ion mass spectrometry (SIMS) ages of c. 315–308 Ma. These ‘young’ dates, obtained in two different laboratories, do not comply with the geological constraints available for the study area that suggest an older age of the high-pressure–low-temperature (M1) metamorphism. Nevertheless, the Sm–Nd age of the same rock at c. 358 Ma appears in better agreement with the geological constraints, and therefore might reflect the age of the high-pressure (HP) event. Similar 357–352 Ma monazite U–Th–Pbtot ages are obtained from relict grains in the Axial Zone kinzigites that represent restites enclosed in migmatites. Furthermore, monazite grains from biotite–garnet–staurolite micaschists from the dome envelope and Axial Zone kinzigites yield U–Th–Pbtot ages in the range 340–320 Ma. These dates are in good agreement with previously documented zircon and monazite ages from the migmatite and anatectic granites that represent a high-temperature–low-pressure (M2) event. The significance of the zircon and rutile ages in the eclogites is discussed in terms of hydrothermal circulations. A crustal-scale model considers: (1) a north-directed intracontinental subduction, responsible for the high-pressure–low-temperature (M1) metamorphism, coeval with kilometre-scale south-vergent recumbent folds in the Palaeozoic non-metamorphic cover, followed by (2) a high-temperature–low-pressure (M2) event, coeval with the development of the Axial Zone migmatitic dome, and upright folding in the Palaeozoic non-metamorphic series.