Orogenic eclogites record relative magnitude of deep crustal flow and extent of migmatite-eclogite interaction

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 owing re-equilibration at high-temperature, low-pressure (LP–HT) conditions. Montagne Noire (France) migmatite dome, located foreland Variscan orogen, eclogite exposed both core margin dome. this study, we combine situ U–Pb petrochronology oxygen-isotope analyses key phases demonstrate eclogites from two distinct domains had different protoliths source regions, traveled relatively variable distances deep crust, differentially interacted with surrounding prior exhumation. Dome-margin zircons are small (∼40 μm) well-preserved inherited cores thin (<15 rims, compared larger (40–120 neo- recrystallized dome-core relict wide (15–30 rims. Protolith HP ages were determined using zircon rutile (LASS-ICP-MS). Both formed continental setting; dome protolith 442.5 ± 3.4 Ma (steep HREE slope, no Eu-anomaly) whereas yielded scattered dates suggesting crystallization between ∼500–400 pronounced Eu-anomaly). experienced c. 320–310 garnet-stable, plagioclase-absent Most analyzed 307–304 associated cooling. The fluid conditions was by O-isotope (SIMS) garnet zircon. rims δ18O ∼8.2–8.5 ‰, indistinguishable within uncertainty, isotopic equilibrium isotopically unzoned (δ18O ∼ 8.0–8.2 ‰). contrast, systematically lower zircon-core values their neocrystallized grains, major-cation zoned respect oxygen. samples ∼8.6–9.5 ‰ 9.7–10.5 ‰. Based on these results existing fabric data for eclogites, propose (1) gabbro emplaced depths Cambro-Ordovician crustal package; (2) extensively gneiss during burial foreland-vergent flow, dome-margin sourced proximally dome-emplacement location minimal chemical interaction gneiss. At least parts deeply sourced, but more extensive protracted history deep-crustal flow than margin.