Timing of detachment and rates of exhumation constrained by in in situ U-Th-Pb dating (monazite, allanite, xenotime): the Gran Paradiso case

International audienceIn the Gran Paradiso Massif (Western Alps), the Money Unit crops out as a tectonic window below the GranParadiso Unit. The Gran Paradiso and the Money Units reached peak pressure conditions at 18-20 kbar, 480-520 C and 17-18 kbar, 500-550 C, respectively. This testifies to continental crust subduction and yields amaximum difference of 9-10 km in the subduction depth reached by these two units during the early Alpinehistory. Thrusting of the Gran Paradiso Unit over the Money Unit led to the conjointly development of the mainfoliation at the same metamorphic conditions (12.5 kbar-14.5 kbar and 530-560 C) in both units. The thrustcontact was subsequently folded and then both units were exhumed together. The relative timing of growth anddissolution of accessory phases has been assessed by combining thermodynamic modelling with inclusion, texturaland chemical (major and trace-element) data from both major and accessory phases. Age of monazite constrainedthe high pressure metamorphism in both the Gran Paradiso and Money Units at 41.5 0.3 Ma and 42.0 0.6 Ma,respectively. Allanite replacing monazite in the matrix has been dated at 32.7 4.2 Ma. Late growth of xenotimeassociated with the crystallization of biotite pseudomorphs at the expense of garnet (at about 10 kbar) has beendated at 32.3 1.0 Ma. Our petrochronological data indicate about 10 m.y. between the peak pressure conditionsand the crystallization of xenotime leading to an exhumation rate of the order of 2.2 to 5 mm/yr