Prograde and retrograde P-T evolution of a Variscan high-temperature eclogite, French Massif Central, Haut-Allier

The P-T evolution of a mafic eclogite sample from the Haut-Allier was studied in order to constrain the dynamic of the Variscan subduction in the eastern French Massif Central. Three successive metamorphic stages M1, M2 and M3, are characterized by assemblages comprising garnet1-omphacite-kyanite, garnet2-plagioclase, and amphibole-plagioclase, respectively, and define a clockwise P-T path. These events occurred at the conditions of eclogite (M1; similar to 20 kbar, 650 degrees C to similar to 22.5 kbar, 850 degrees C), high-pressure granulite (M2; 19.5 kbar and 875 degrees C) and high-temperature amphibolite facies (M3; < 9 kbar, 750-850 degrees C), respectively. Pseudosection modelling of garnet growth zoning and mineralogy of the inclusions reveal a prograde M1 stage, first dominated by burial and then by near isobaric heating. Subsequent garnet1 resorption, prior to a renewed growth of garnet2 is interpreted in terms of a decompression during M2. High-pressure partial melting is predicted for both the M1 temperature peak and M2. M3 testifies to further strong decompression associated with limited cooling. The preservation of garnet growth zoning indicates the short-lived character of the temperature increase, decompression and cooling cycle. We argue that such P-T evolution is compatible with the juxtaposition of the asthenosphere against the subducted crust prior to exhumation driven by slab rollback.