Résumé
In exhumed orogens, refractory mafic rocks have the potential to preserve a record of petrogenesis and high-pressure (HP) metamorphism that is commonly obliterated in quartzofeldspathic rocks owing to re-equilibration at high-temperature, low-pressure (HT-LP) conditions. In the Montagne Noire (France) migmatite dome, located in the foreland of the Variscan orogen, eclogite is exposed in both the core and margin of the dome. In this study, we combine in situ U-Pb petrochronology and oxygen-isotope analyses of key eclogite phases to demonstrate that the eclogites had different protoliths and source regions, traveled variable distances laterally in the deep crust, and differentially interacted with surrounding migmatite prior to exhumation. Dome-margin eclogite zircons are small (~40 µm) with well-preserved inherited cores and thin (<15 µm) rims, compared to larger (40-120 µm) neo-and recrystallized dome-core zircons with small relict cores and wide (15-30 µm) recrystallized rims. Protolith and HP metamorphism ages were determined using in situ zircon and rutile petrochronology (LASS-ICP-MS). Both eclogites formed in a continental setting; dome margin protolith zircon cores formed at 442.5 ± 3.4 Ma (steep HREE slope, no Eu-anomaly) whereas zircon cores of the dome-core eclogites yielded scattered dates suggesting protolith crystallization between ~500-400 Ma (steep HREE slope, pronounced Eu-anomaly). Both eclogites experienced HP metamorphism at c. 320-310 Ma in garnet-stable, plagioclase-absent conditions. Most analyzed rutile yielded dates of 307-304 Ma associated with cooling. The record of HP fluid conditions was determined by O-isotope (SIMS) analyses of garnet and zircon. Dome-margin zircon cores and rims have 18 O of ~8.2-8.5 ‰, indistinguishable within uncertainty, in isotopic equilibrium with isotopically unzoned garnet (18 O ~8.0-8.2 ‰). In contrast, zircons in dome-core eclogites have systematically lower zircon-core 18 O values compared to their rims and neocrystallized