Timescales of ultra-high temperature metamorphism and crustal differentiation: Zircon petrochronology from granulite xenoliths of the Variscan French Massif Central

Lower crustal ultra-high temperature (UHT) metamorphism and the resulting production and transfer of granitic magmas represent key processes of intracrustal differentiation. The timescales of these phenomena are debated, due to the complexity of the granulite zircon U-Pb record and because direct links between granulites and granites are difficult to establish. To address these issues, we present the results of zircon petrochronology (coupled U-Pb dating, trace element and Lu-Hf isotopic analyses) for lower crustal felsic and mafic granulite xenoliths from the Variscan eastern French Massif Central and compare them with data from well-characterized mid-/upper crustal migmatites and granites. The felsic and mafic granulites represent pre-Variscan meta-sedimentary and meta-igneous mafic rocks, respectively; which experienced Variscan UHT peak metamorphism at 940–970°C and 8 ± 2 kbar. Zircons from the felsic granulites show U-Pb dates spreading over ∼50 Myr between ∼315 and ∼265 Ma, correlated to Ti-in-zircon temperatures decreasing from 940–970°C to 800°C and REE contents consistent with growth in equilibrium with garnet at (U)HT conditions. This is best explained by continuous crystallization of zircon upon cooling from the thermal peak owing to decreasing Zr solubility of residual melt, despite significant prograde melt loss. Zircons from the mafic granulite only record the last stage of this time-temperature evolution (295–265 Ma; 800–900°C) due to later zircon saturation. Upper crustal granite emplacement started at 340 Ma and culminated at the age of the lower crustal thermal peak of 313 ± 3 Ma (defined by the highest-temperature zircons from felsic granulites), reflecting melt extraction along the ∼27 Myr prograde path of the lower crust. In turn, the crystallization ages of mid-crustal migmatites (315–300 Ma) and the lower crustal granulites (315–265 Ma) are consistent with slow cooling in the presence of melt. These results provide a direct assessment of the timescales of melt production and residence at the crustal scale; and validate the granulite-granite connection in the framework of the melt loss theory in migmatitic systems.