Peak P-T conditions in the structurally deepest rocks from the Western Alps: the Pinerolo Unit (Dora-Maira Massif)

Abstract The tectonic architecture of a mountain belt is associated to a specific metamorphic architecture, revealing how deep-crustal processes interact during continental subduction and mountain building. Specifically, the Pinerolo Unit in the Dora-Maira Massif (Western Alps) crops out as a tectonic window below the overlying polycyclic ultra-high-pressure units and represents the structurally deepest unit in the Alpine nappe stack. The largest section of the Pinerolo Unit, which is essentially made of graphite-bearing metaconglomerates, metasandstones, and a few metapelites, is exposed in the northern part of the Dora-Maira Massif. Metasandstones and metapelites preserve early, garnet-bearing assemblages that record peak pressure and temperature conditions. Thermodynamic modelling indicates that garnet growth occurs in the quartz stability field, in a narrow temperature range, first during isobaric heating culminating at ⁓1.9-2.0 GPa ⁓500 °C, then at the beginning of decompression. The obtained values are similar to those previously reported for the Pinerolo Unit in the southern Dora-Maira Massif, as well as for the Money Unit in the Gran Paradiso Massif. These results confirm that the internal zones of the Western Alps display an apparent ‘inverted’ pressure gradient, with ultra-high-pressure units overthrusting high-pressure units, resulting from the detachment of subducted crustal slices from the downgoing slab and their stacking during the ensuing continental collision.