Thermal evolution of a translithospheric diapir, evidence from a high-pressure granulite (French Massif Central)

The Variscan rock record of the French Massif Central (FMC) provides compelling evidence for early Carboniferous crust–mantle interaction during subduction and exhumation of continental high-pressure metamorphic units, but the underlying geodynamic mechanisms remain elusive. The crustal pressure–temperature–time–fluid evolution during this interaction is investigated through a case study of a high-pressure mafic granulite, integrating petrology, phase equilibrium modelling, trace element thermometry, and U–Pb geochronology. Partial replacement of garnet–clinopyroxene–plagioclase–rutile assemblage by amphibole and titanite records hydration and pronounced cooling from ∼900–975°C to ∼700–800°C, under limited decompression from ∼13 to 11 kbar. The age of this event is constrained at 354.6 ± 4.6[8.5] Ma by U–Pb dating of titanite. Subsequent ilmenite–clinopyroxene pseudomorphs after titanite indicate further decompression below ∼9 kbar. The minimum age of this decompression is given by apatite and rutile U–Pb dates of 348.5 ± 2.6[8.5] Ma and 337.9 ± 1.7[7.0] Ma, respectively. The FMC granulites are associated with mantle rocks, and the inferred P–T conditions compare with those of mantle xenoliths from volcanic regions worldwide, suggesting an interaction with the mantle during exhumation. One-dimensional thermal models of crustal translithospheric diapirs consistently predict cooling in the shallow lithospheric mantle within the ranges 700–1000°C and 10–15 kbar. Therefore, the peak conditions and subsequent cooling could result from the thermal interaction of a crustal diapir and the mantle. We propose a new petrogenetic model for the FMC where translithospheric diapirs play a central role.