Constraining P–T conditions during thrusting of a higher-pressure unit over a lower-pressure one (Gran Paradiso, Western Alps)
Résumé
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 micro-structural analysis and pseudosection calculations. In both units, four stages of deformation and metamorphism have been identified. Stage 1 reflects the phase of continental crust subduction and P–T conditions of ~18-20 kbar, 480-520 °C and of ~13-18 kbar, 500-530 °C have been estimated for the Gran Paradiso and the Money units, respectively. This yields a maximum difference of ~20 km 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-pressure part of the albite stability field at P–T conditions of ~12.5-14.5 kbar and 530-560 °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-pressure 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.