TIMING AND CONDITIONS OF HIGH-PRESSURE METAMORPHISM IN THE CORE VS. MARGINS OF A PALEOZOIC OROGEN (VARISCAN FRENCH MASSIF CENTRAL)

The Paleozoic Variscan orogen extends across a vast swath of Eurasia and is recognized in exposures of high-grade metamorphic rocks, including migmatite containing eclogite or high-pressure (HP) granulite. The migmatites represent former partially molten crust, much of which crystallized at relatively shallow crustal levels. Mafic inclusions in migmatite are more faithful recorders of the age and maximum depth of both eclogite/granulite and host migmatite and can be used to reconstruct earlier stages of orogeny, including deep-crust metamorphism and flow.

The French Massif Central (FMC) is part of the Variscan orogen and contains abundant, variably retrogressed eclogites that occur in migmatites and in a heterogeneous unit that has been interpreted as an oceanic suture zone. The age and tectonic setting of HP metamorphism has been much debated, with recent studies proposing ages from >360 to 310 Ma during crustal thickening (orogenic) or oceanic subduction. To evaluate the tectonic setting, age (protolith crystallization, HP metamorphism), and deep-crust dynamics, we analyzed 43 eclogites and amphibolites along a N-S transect in the eastern FMC. The sample suite is dominated by migmatite-hosted eclogites with continental geochemical characteristics. These have been analyzed for major- and trace-element composition and representative samples for whole-rock isotopic signatures (Sm-Nd, Rb-Sr, U-Pb) as well as zircon and rutile U-Pb geochronology and trace-element geochemistry. Metamorphic conditions have been evaluated using phase diagram modeling and trace-element thermobarometry.

Zircon cores in migmatite-hosted eclogite record a range of magmatic (protolith) ages, indicating pre-Variscan mafic magmatism between ~500-440 Ma. Zircon rim ages indicate HP metamorphism in the central part of the FMC at ~340 Ma under high-T conditions (>800°C). Eclogite in migmatite at the northern and southern margins of the FMC record younger (~315-310 Ma) and lower T (~750-700°C) HP metamorphism. This difference in eclogite ages and P-T conditions may indicate progressive crustal thickening from orogenic core to margins (e.g., as a result of gravity-driven crustal flow). Eclogite petrology and zircon textures further reveal variations in P-T-t paths within the deep-crust flow system.