Permian high-temperature metamorphism in the Western Alps (NW Italy)

During the late Palaeozoic, lithospheric thinning in part of the Alpine realm caused high-temperature low-to-medium pressure metamorphism and partial melting in the lower crust. Permian metamorphism and magmatism has extensively been recorded and dated in the Central, Eastern, and Southern Alps. However, Permian metamorphic ages in the Western Alps so far are constrained by very few and sparsely distributed data. The present study fills this gap. We present U/Pb ages of metamorphic zircon from several Adria-derived continental units now situated in the Western Alps, defining a range between 286 and 266 Ma. Trace element thermometry yields temperatures of 580-890°C from Ti-in-zircon and 630-850°C from Zr-in-rutile for Permian metamorphic rims. These temperature estimates, together with preserved mineral assemblages (garnet-prismatic sillimanite-biotite-plagioclase-quartz-K-feldspar-rutile), define pervasive upper-amphibolite to granulite facies conditions for Permian metamorphism. U/Pb ages from this study are similar to Permian ages reported for the Ivrea Zone in the Southern Alps and Austroalpine units in the Central and Eastern Alps. Regional comparison across the former Adriatic and European margin reveals a complex pattern of ages reported from late Palaeozoic magmatic and metamorphic rocks (and relics thereof): two late Variscan age groups (~330 and ~300 Ma) are followed seamlessly by a broad range of Permian ages (300-250 Ma). The former are associated with late-orogenic collapse; in samples from this study these are weakly represented. Clearly, dominant is the Permian group, which is related to crustal thinning, hinting to a possible initiation of continental rifting along a passive margin

Formation and exhumation of ultra-high-pressure rocks during continental collision: role of detachment in the subduction channel

UHP rocks commonly form and exhume during the transition from oceanic subduction to continental collision. Their exhumation in subduction channels depends on the balance between down-channel shear traction and up-channel buoyancy. Thermal-mechanical upper-mantle-scale numerical models are used to investigate how variations in material properties of the subducting continental margin affect this balance. Changes in shear traction leading to crustal decoupling/detachment are investigated by varying the onset of strain weakening, thermal parameters, and convergence velocity. Variations in buoyancy force are investigated by modifying subducted material density and volume. The model results are interpreted in terms of the exhumation number E, which expresses the role of the pressure gradient, channel thickness, effective viscosity, and subduction velocity. Peak metamorphic conditions, exhumation velocity, and timing of exhumation are temporally and spatially variable and are sensitive to the evolution of E. The models reproduce natural PTt constraints and indicate that neither slab breakoff nor surface erosion is required for UHP exhumation

Deep subduction and rapid exhumation: role of crustal strength and strain weakening in continental subduction and ultrahigh-pressure rock exhumation

The exhumation of crustal ultra-high-pressure (UHP) material depends on temporal and spatial variations in its detachment within the subduction channel. This dependence is investigated using numerical models with variable initial crustal strengths, representing a range of initial crustal compositions, and parameterized strain weakening, representing a range of processes that reduce effective crustal viscosity during deformation. Competition between down-channel shear traction, favoring subduction, and up-channel buoyancy, favoring exhumation, is expressed as the exhumation number, E, which can vary with time and position along the channel. Exhumed lower strength crust, which resists subduction owing to weak down-channel traction, records peak conditions −1. Higher strength crust is efficiently subducted to UHP depths (E 38 kbar. Given sufficient strain weakening, exhumation proceeds at >60 km Ma−1, indicating that buoyancy (E » 1) drives exhumation in these models. In all models, exhuming UHP material forms a deforming ductile plume, with a range of possible structural relationships predicted between exhumed UHP and HP materials