Pseudotachylytes of the Tonale nappe (Italian Alps): Petrogenesis, 40 Ar-39 Ar geochronology and tectonic implications

Pseudotachylyte veins of the Tonale nappe (NE Italy) occur along faults and shear zones which accommodated the deformation that shortened the Adriatic crust to the north of the Peria-driatic Lineament during the late Alpine orogeny. These veins are characterized by thin glass layers showing variable clast content and colours, coupled with chemical variations. They are derived from surrounding gneiss, but mass-balance studies show some quartz missing in the veins (i.e., 1 rock = a vein + b quartz, with b ranging from 0.1 to 0.3). Thermodynamic modelling of the melting process indicates that the energy necessary to melt 1 cm3 of rock is 4-5 kJ. Temperature reached inhomogeneous values between ∼1100°C (total melting of micas) and ∼1700°C (partial melting of the quartz clasts). The modelling of the cooling process accounts well for the microlite blastesis in the glassy groundmass of the veins, provided that the pressure at which pseudotachylytes formed along faults/shear zones ranges betwe...