Constraining P-T conditions during thrusting of a higher pressure unit over a lower pressure one (Gran Paradiso, Western Alps)

Identifying higher pressure units overlying lower pressure ones is a first order argument to determine the presence of large-scale thrusting. For the first time, petrology is used to quantify the pressure difference between two stacked units in the Western Alps. In the Gran Paradiso Massif, the Money unit crops out as a tectonic window below the Gran Paradiso unit. The reconstruction of the Alpine evolution of these two units and the history of their tectonic contact has been achieved using a multidisciplinary approach that combines meso- and microstructural analysis and pseudosection calculations. In both units, four stages of deformation and metamorphism have been identified. Stage 1reflects the phase of continental crust subduction and P-T conditions of 18-20kbar, 480-520 degrees Cand of 13-18kbar, 500-530 degrees C have been estimated for the Gran Paradiso and the Money units respectively. This yields a maximum difference of 20km in the depth reached by these two units during the early Alpine history. Thrusting of the Gran Paradiso unit over the Money unit (stage 2) led to the development of the main foliation and occurred in the high-P part of the albite stability field at P-T conditions of 12.5-14.5kbar and 530-560 degrees C, identical in both units. The thrust contact was folded during stage 3 together with the entire Money unit, and then both units were exhumed together (stage 4). During this polyphase evolution, detrital garnet has been partially dissolved, while the earliest Na-bearing phases (glaucophane, paragonite) have been overprinted by the low-P mineral associations. The uncertainties on derived pressures between the two units are unfortunately larger than hoped, and this is attributed to the muscovite solid-solution model not incorporating a pyrophyllite component.