Late Collisional Plutons as a Key to Constrain the Exhumation of Continental HP Units: An Example from the Italian Western Alps

Intrusive mechanisms and timing of late orogenic plutons are a key to unravel the late-orogenic evolution of collisional belts. Late-Alpine plutons (Biella and Traversella) are intruded in the innermost part of the Sesia-Lanzo Zone (SLZ), a HP-LT early-alpine metamorphic continental complex, close to the inner boundary of the Alpine metamorphic units (Periadriatic line), which is part of the subducted Adria margin.

These plutons intruded eclogitised continental crust rocks in which generally the pervasive foliation is marked by mineral assemblages indicating HP-LT conditions, related to the Alpine subduction. The last ductile deformation stages are coeval with the regional re-equilibration under greenschist facies conditions recorded during exhumation. Intrusions postdate the ductile deformations of the country rock; only locally syn-intrusive ductile deformation took place. Most of the brittle deformation postdates the intrusion of the plutons, but some fracturing developed during magma cooling.

Thermobarometrical estimates in country and plutonic rocks reveal that both plutons crystallised at a depth between ≈ 3.5 (roof) and ≈ 7 (bottom) km for T ≈ 700 - 750°C. Therefore in the Western Alps, HP-LT complexes are exhumed to shallow crustal greenschist facies conditions before intrusion of late orogenic plutons. Known the P-T estimates and the ages for the eclogitic peak in the SLZ, and the pluton ages, an average pre-intrusive exhumation rate of ≈ 2 km/Ma has been envisaged. During the last 30 Ma the lower exhumation rate of the Biella and Traversella plutons (≈ 0.2 km/Ma) with respect to that of the Bergell pluton (≈ 0.6 km/Ma) may suggest that in the Internal Central Alpine crust a larger volume of the Oligocene intrusive rocks may have been eroded, and, on the contrary, in the Internal Western Alps most part of the Oligocene intrusives could still be buried.